7.2                                   Representative sub–Antarctic Islands  

7.2.1                                The Falkland Islands  

7.2.1.1                          Orography and the local environment

The Falkland Islands are situated some 550 km due east of southern Argentina (see Figure 7.2.1.1.1) and are centred near 52º S, 59º W. Consisting of over 600 islands they extend some 220 km east–west and 140 km north–south. For the sake of convenience they are broadly split into West Falkland and East Falkland, separated by Falkland Sound. The area is a mixture of mountains and low moorland, and is heavily indented by sea lochs and inlets (see Figure 7.2.1.1.2).

East Falkland is dominated by the Wickham Heights range of mountains which extend east–west across the whole of the area; there are many peaks above 400 m the highest being Mount Usborne at 705 m. In the south, low moorland and numerous small lakes and ponds dominate the landscape.

West Falkland is generally mountainous with numerous broad valleys. There are many peaks above 400 m, the highest being Mount Robinson at 695 m. Throughout the Falkands, vegetation is mostly of white grass with areas of heather–like scrub. The mountains have large areas of bare rock.

There are numerous landing strips in the islands, but the two airports are both in East Falkland. Stanley Airport, some 25 m above MSL, is situated at 51º 41´ S, 57º 46´ W. Mount Pleasant International Airport (MPIA), some 74 m above MSL, is situated at 51º 49´ S, 58º 27´ W.

7.2.1.2                          Operational requirements and activities relevant to the forecasting process

The Meteorological Office's Forecasting Centre is located at MPIA (MPIAFC). The forecasting centre is staffed continuously and provides meteorological services for a wide range of activities across the Falkland Islands and more widely in the South Atlantic. Services include detailed forecasts for aircraft and ships operating out of the islands, together with general weather forecasts for the public at large as well as specialist advice for sheep farmers.

7.2.1.3                          Data sources and services provided

MPIAFC is connected to the Meteorological Office's facilities at Bracknell, United Kingdom, via two satellite links. These provide routine synoptic and numerical weather prediction data as soft or hard copy and which are accessed through the recently developed NIMBUS visualisation system. NWP data are provided twice daily from The Meteorological Office's global model together with World Aviation Forecasting Centre (WAFC) products, wind wave and swell forecast data. Model data are available up to 120 h ahead. Access to the Internet is also available.

Hourly weather reports are made at MPIAFC, and at the automatic weather stations at Pebble Island, Sea Lion Island and Weddell Island. Upper–air soundings are made at MPIAFC daily at 0000 and 1200 UTC and often also at 0600 UTC and occasionally at 1800 UTC.

MPIAFC also has a satellite data reception facility which provides the downloading of line–of sight IR and visible data from NOAA polar orbiters from near 35º S to around 75º S, together with the capability to receive GOES and METEOSAT imagery. The overall coverage allows forecasters to analyse data on a fine scale, assess cloud top and sea surface temperatures, and determine the general distribution of the southern ocean sea ice in the sector from 20º W to 100º W.

7.2.1.4                          Important weather phenomena and forecasting techniques used at MPIAFC

General overview

The overall weather factor at MPIA is very good and compares favourably with many northern and western parts of the United Kingdom. The weather regime in the Falkland Islands is influenced by the close proximity of the Andes to the west, the surrounding waters, particularly the cold East Falkland current moving northwards, and the presence of the Antarctic Peninsula some 1100 km to the south. Locally the orography of the Falkland Islands, particularly the Wickham Heights, has a direct influence on wind and weather at MPIA. The predominant wind direction is westerly but the particular direction and speed on any given day will have a marked effect on the weather at MPIA. Depressions and their associated fronts frequently move east across the islands at speeds of at least 15 m s^–1 (30 kt). Therefore frontal precipitation is relatively short–lived except when back–bent occlusions follow the preceding warm and cold frontal systems. Synoptic scale weather systems are usually well depicted on satellite imagery and are well handled by NWP model analyses.

Pacific depressions are usually held up near the Chilean coast with the Andes markedly modifying frontal structure, often leaving just cloud above around 12000 ft to move on eastwards. On a short sea track to the islands the dried out low‑level air produces little in the way of low cloud or precipitation at MPIA. However, a depression moving from central Argentina or from further north, and which tracks southeast towards the islands, will pick up moisture and, because of the longer time spent over the sea, extensive low cloud and precipitation are likely especially in northern parts of the islands. This will extend southwards depending on the particular track and other synoptic factors. Depressions forming near Cape Horn and tracking northeast are likely to have extensive cloud, and precipitation is likely. Dependent upon the season, and the exact track of the depression, precipitation may fall as snow even on low ground. To the rear of a v–shaped trough, or ahead of a depression tracking southeast but remaining to the north of the islands, low cloud is likely to become extensive at MPIA for a time.

Anticyclones and ridges of high pressure are usually associated with fair weather. However, when moving only slowly east and when an upstream warm frontal trough is developing and accelerating towards the Falkland Islands, this is likely to lead to very strong northerly winds especially at MPIA and lee wave activity or rotor streaming across the airfield. In winter these northerly winds are likely to advect low cloud and rain or drizzle across MPIA, but in summer cloud amounts are reduced through the effects of insolation, precipitation is unlikely in these situations unless there is medium level instability.

Surface wind and pressure field

The overall pressure distribution is routinely analysed from weather reports in the region, satellite imagery and NWP guidance; thus the general wind and pressure regime is well handled. The mean wind speed tends to be a little higher in summer than winter; nevertheless the maximum recorded gust at MPIA is less than 41 m s^–1 (~80 kt). The effect of the Wickham Heights on a northerly airstream and an unstable south to southwesterly airstream require careful consideration, as do conditions likely to lead to the onset of sea breezes.

Northerly airstreams are generally stable, especially at the lower levels, and are generated most strongly between warm fronts to the northwest of the Falkland Islands and a slow–moving high–pressure system close to the east of the area. In addition, a low level inversion in the northerly flow increases the surface wind speed markedly at MPIA; nocturnal cooling will sharpen the inversion and hence strengthen the surface wind. When the height of the top of the inversion is close to that of Wickham Heights, northerly winds may become very strong and generate marked lee waves or rotor streaming at MPIA; this is closely linked to the degree of stability and low level wind structure. Sudden increases or decreases in wind speed, and/or in direction, are frequent in these synoptic occasions and often lead to severe low level turbulence at MPIA. This is difficult to forecast accurately but when the gradient wind speed is 20 m s^–1 (~40 kt) or more, severe low level turbulence is likely; evidence of this can be seen on the anemogram and, when the air is sufficiently moist, by the presence of cloud with ragged and rapidly changing edges. Sometimes such clouds can be seen to travel as slow–moving eddies over the aerodrome. With gradient wind speeds less than 15 m s^–1 (~30 kt), the low level flow is more likely to be laminar and lee wave activity is likely to be evidenced either, by the familiar clouds, or a smooth top of low cloud spilling over Wickham Heights.

Unstable south–southwesterly airstreams are likely to produce sudden increases in wind speed associated with showers. Gusts may exceed the gradient wind speed.

Sea breezes can occur in summer under a slack pressure gradient; the wind may back to a south–southeasterly direction which, in early summer may lead to stratus being advected from the cold water offshore or developing over the warmer land surface.

Upper wind, temperature and humidity

Mean January and July upper–level wind roses for MPIA are included in Figures A3–9 (a) and A3–9 (b) (in Appendix 3) while mean–temperature profiles for MPIA are shown as Figures A3–1 (a) and (b). The NWP model data are used for providing aviation forecasts and are adjusted as necessary through the analysis of sounding data and satellite imagery. The upper–air products include significant weather forecasts for aircraft operating in the region.

Cloud

The existence and nature of cloud is governed overall by the geographical location of the Falkland Islands and influenced by the proximity of the Wickham Heights to MPIA. Frontal cloud moving east across the Andes is strongly modified, the lowest layers often being completely dried out; as the low level frontal air approaches the Falklands its moisture content will have increased from travelling over the sea, nevertheless precipitation is likely to be slight. At MPIA the front may be dry with the lowest layers drying out again after crossing the islands, its passage being indicated by a shift in wind direction and the presence of cloud at medium or high levels.

Warm moist air moving south or southeast towards the colder waters of the Falklands is likely to produce a low level inversion, extensive low cloud, and sea fog and mist around the coasts. Windward coasts and upslopes will have very poor conditions with outbreaks of drizzle or rain. The resultant cloud effects due to warm moist air being advected south to MPIA are dependent on the particular wind direction and wind speed, together with the height of the inversion. If the wind is in the northwest quarter, MPIA is unlikely to have stratus below 200 m (~700 ft), but if the inversion is below 150 m (~500 ft) and the wind speed is at or above 15 m s^–1 (~30 kt) then patches of low cloud below 150 m may affect the aerodrome. When the wind is in the northeast quarter, which is relatively infrequent, MPIA is very exposed to very low cloud developing as low as 60 m (~200 ft) and which, together with poor visibility and drizzle, will persist until the associated depression has moved away well to the east.

In a slack south–southwesterly airstream, upslope stratus down to 30 to 60 m (~100 to 200 ft) is likely to form from air stagnating over Lafonia and will cross the aerodrome; together with poor visibility this will persist until there is a shift in wind direction, although in summer cloud amounts should reduce through the effects of insolation.

Figure 7.2.1.1.2     A map of the Falkland Islands. (From http://www.lib.utexas.edu/maps/americas/falkland_islands.gif, courtesy of The General Libraries, The University of Texas at Austin, USA.)

Summer sea breezes may advect stratus from the adjacent cold waters towards MPIA, but it should disperse through insolation.

Wave clouds are common at MPIA in northwesterly wind regimes, both at relatively low and high levels, and are caused by stable air traversing the Wickham Heights and being induced to oscillate. Residual high level wave clouds from fronts moving over the Andes also can be observed occasionally at MPIA. Stratocumulus cloud in south–southwesterly airstreams can also produce significant wave effects on the north side of the Wickham Heights and other mountains in the Falklands; short period oscillations with more than thirty waves in these types of airstreams have been observed from satellite imagery to have been set off by Wickham Heights.

Visibility: particularly precipitation and fog

Visibility in general is very good due to the lack of upstream or in situ sources of industrial pollution. However, haze, smoke from peat fires or volcanic ash from eruptions upwind, and precipitation, all reduce visibility to varying degrees. When the wind direction is in the northwest quarter and strong this is likely to lead in summer to haze caused by lifted dust from Patagonia.

Northerly airstreams, especially when slack, are likely to produce widespread mist or fog conditions particularly over coastal waters north of the islands and north–facing slopes and hills. MPIA is normally well sheltered from these effects, but if a slack low level flow veers to the northeast quarter then the aerodrome will be affected by poor visibility reducing to below 1000 m, especially if the fetch is persistent. A persistent south–southwesterly slack airstream is also likely to lead to poor visibility, or even fog, at the aerodrome due to the advection of moist air from Lafonia.

Radiation fog is rare at MPIA and is most likely to be caused indirectly through the advection of moist air stagnating or fog already formed over adjacent lakes.

Precipitation reduces visibility in the normal way but slight to moderate rain does not appear to lead to the same decrease as in the Northern Hemisphere; this may be due to the air being freer of pollution.

Surface contrast including white–out

No specific information on forecasting has been obtained.

Horizontal definition

No specific information on forecasting has been obtained.

Precipitation

Despite its maritime environment, rainfall in the Falkland Islands in general, and at MPIA in particular, is very low. The average annual rainfall at MPIA is around 625 mm but the station record began only in 1986. (At Stanley itself (51º 42´ S, 57º 54´ W, ~51 m AMSL) rainfall records extend into the 19^th century and from Table 7.2.1.4.1 (in Appendix 2) mean yearly rainfall for Stanley is ~642 mm). There is, however, a wide spread of variability at MPIA with the driest month on record having less than 10 mm ranging to the wettest month recording over 120 mm. The maximum rainfall amount recorded in 24 h is over 35 mm.

As noted earlier frontal precipitation usually moves across the islands quickly so that it lasts normally no more than a maximum of two to three hours. However, a back bent occlusion of the preceding warm front will lead to heavier and more persistent precipitation particularly when the parent depression is slow–moving, or is still deepening.

Showers will occur in unstable south–southwesterly airstreams but usually move very quickly on the wind. In unstable airstreams cumulonimbus clouds, with tops no more than 1800 to 2400 m (~6000 to 8000 ft) in winter are frequent phenomena and often will produce locally heavy but short–lived showers of soft hail or snow.

Medium level instability in the form of altocumulus cloud may be detectable from satellite imagery, and is usually associated with northwesterlies with a source in South America. The associated showers are normally of soft hail or snow, but are usually short–lived.

The critical forecasting issue is whether winter precipitation will be of rain, rain and snow mixed, soft hail or snow. NWP model performance and experience point to the value of examining the 850 hPa wet bulb potential temperature (WBPT) fields together with the 1000–500–hPa thickness fields. When values of 1 to 2º C for the 850 hPa wet bulb potential temperature and 528 dm thickness values are expected then snow is more likely than not especially when ascent is forced over higher ground. Soft hail is likely if the instability extends to a depth of at least 6000 ft and local sounding data points to the instability being around 4ºC lower than the temperature indicated by the environmental lapse rate. Snow may be of granular or flake crystals, resulting in different effects in terms of whether it will lie on the ground or is blown around. Frontal snow from the northwest is unlikely to persist at MPIA before turning to rain. The exception is when a slow–moving depression passes close to the north of the Falkland Islands produces rain but this will turn to snow when there is cold undercutting air on its eastern and southern flanks. These occasions are rare but can produce very poor weather conditions and not only in terms of precipitation.

In winter it is quite common for stratocumulus cloud to be advected by a south–southwesterly airstream, and with cloud tops below 0ºC this will often produce slight or even moderate snow showers. These cloud sheets commonly follow as a ridge of high pressure moves east across the islands and thus cut off deeper convection.

Freezing rain or drizzle are rare phenomena in the Falkland Islands.

Temperature and chill factor

Temperatures are influenced by air–mass characteristics, time of year, and wind direction. Extreme temperatures at MPIA range from a maximum of over 29ºC in summer to a minimum below –9ºC in winter. Table 7.2.4.1.2 (in Appendix 2) shows long term average monthly temperatures for Stanley.

Temperature forecasts are required for a range of users and are predicted using techniques employed in many other forecast offices. Underlining temperature levels in the Falkland Islands is the constant chilling effect of the wind; this is calculated using standard techniques such as that derived by Steadman (for example, Steadman (1971)). Temperature chill factor forecasts are important not only for work outdoors but also for farmers in the sheep shearing season who, in addition, require the very significant effects of precipitation to be taken into account for sheep chill forecasts.

Icing

No specific information on forecasting has been obtained.

Turbulence

Turbulence predictions are provided on a regular basis in locally used forecasts. Whenever moderate or severe low–level turbulence is expected to occur at MPIA, this is included in the routine issues of Terminal Aerodrome Forecasts for the aerodrome. Moderate or severe low level turbulence at the site is normally associated with a strong northerly airstream that has a marked inversion close to the height of the Wickham mountain range. Severe low turbulence occurs regularly at MPIA and can be detected through monitoring of the anemogram, and orographic cloud behaviour south of the mountains together with the varying orientation of windsocks at the aerodrome. Infrequently, the involuntary behaviour of birds in flight demonstrates the existence of sudden and strong downdraughts.

A south–southwest airstream advecting or generating a sheet of stratocumumlus across Wickham Heights may induce a low level and short period wave–train; there are few pilot reports of any associated turbulence. However, given the nature of orography of the Falkland Islands, turbulence is likely to be induced over and to the lee of high ground whenever the gradient speed is in excess of 15 m s^–1 (~30 kt).

Hydraulic jumps

These are rare but can occur under the most severe rotor streaming events; on those occasions pressure changes as much as 3 hPa in ten minutes are possible. They are difficult to predict.

Sea ice

Falkland Island waters are not affected in general by sea ice. However, the MPIAFC monitors the extent of the sea ice and drifting icebergs associated with the Antarctic ice sheet through analysis of satellite imagery together with bulletins from specialist centres. The principal concern is the northern edge of the winter ice pack to the south and the distribution of icebergs in South Georgia waters.

Wind waves and swell

Forecasters at the MPIAFC provide forecasts for use in the Falkland Island waters that cover wind waves and swell. In the Falklands area the generation and decay of wind waves are very responsive to the wind strength, and can reach heights of 7 m (~23 ft) in a very short period of time. Swell is generally more modest, typically in the range 1 to 3 m (~3 to 10 ft) in height with a period frequently of around 10 seconds.

7.2.2                                South Georgia 

7.2.2.1                          Orography and the local environment

South Georgia is situated some 1500 km east–southeast of the Falkland Islands (see Figure 7.2.1.1.1) and is centred near 54º S, 37º W. The island is south of the Antarctic Convergence Zone and lies within the maximum northern limit of sea ice associated with Antarctica. The island is around 150 km long and roughly 30 km wide, and is orientated in a northwest–southeast direction. South Georgia has a few offshore islands but is treated here as a single land mass (see Figure 7.2.2.1.1). It is composed mostly of steeply rising mountains and glaciers; the highest peak is Mount Paget at over 2960 m. The coastline is heavily indented with fjords and bays, the most sheltered of which are on the eastern coast. Vegetation is relatively sparse and limited to coastal fringes.

The administrative centre is at Grytviken on the east coast at the head of King Edward Cove, which is an inlet from Cumberland East Bay. Nearby, at King Edward Point, synoptic observations are made several times each day.

Figure 7.2.2.1.1     A map of South Georgia. (Courtesy of Paul Carroll.)

7.2.2.2                          Operational requirements and activities relevant to the forecasting process

Weather forecasts from the forecast centre in the Falkland Islands are provided in support of British military forces operating in the area.

7.2.2.3                          Data sources and services provided

Surface observations are performed on South Georgia at Grytviken (WMO No. 88903) and the South African Weather Bureau operates an AWS (WMO No. 88986) on South Thule in the South Sandwich Island group (located some 500 – 600 km to the southeast of South Georgia).

There are no forecasting facilities on South Georgia. At the Mount Pleasant International Airport Forecasting Centre the forecasting area of responsibility and data coverage extends eastwards to include South Georgia and the South Sandwich Islands on a routine basis. Synoptic analyses, together with NWP model data and satellite imagery provide the basis for all forecasts required by personnel on the ground and British aircraft or ships in the area. Forecasts are sent via communications links with the Falkland Islands.

7.2.2.4                          Important weather phenomena and forecasting techniques used at the location

General overview

Due to its location and terrain South Georgia experiences a wide range of frequently hostile weather. Prevailing winds blow from the west so that the western side of the island is particularly exposed to low cloud and severe weather conditions at all times of the year, whilst the sheltered eastern side may experience quiet and clear weather. Travelling depressions usually move east very quickly so that deteriorations in weather can be sudden; severe gales and blizzards can occur at any time of year. Depressions, which engage relatively warm, low–level air are likely to produce extensive low cloud, and fog, particularly when winds are light.

Sunny conditions occur most frequently on the eastern side of the island. However cloudy spells lasting several days are common across the island. Temperatures range from over 20ºC in sheltered low lying eastern areas in summer to well below –10ºC in winter.

Surface wind and the pressure field

Whenever a deep depression passes close to the island gales can be expected. The severity and direction may be markedly modified by the orography of the particular area of concern. Local winds can be dominated by katabatic flow coming off adjacent glaciers, and thus may be contrary to the gradient direction and speed.

Katabatic winds occur in the King Edward Cove area. The predominant wind direction there is between north and west. Easterly winds are usually most common in association with a depression to the north or northeast of the island, and in general are less strong than westerlies.

Upper wind, temperature and humidity

NWP model data are used extensively for all forecasts for South Georgia. Forecast upper–air profiles are very useful tools, especially for predicting low level weather conditions important for aircraft and shipping operations. Satellite imagery interpretation provides crucial information to enhance short period model data.

Cloud

By virtue of its location, South Georgia lies close to the track of travelling depressions so the island is very exposed to cloudy systems. Warm air moving south towards the island is cooled at low levels as it travels over the cold waters; this results in extensive low cloud, and especially poor conditions when the surface gradient is northeasterly. South to southwesterly airstreams are cold and, travelling over an increasingly warm sea surface, often lead to the formation of extensive sheets of stratocumulus; on reaching the western coasts these cloud systems often thicken through forced ascent of high ground. However, the mountains act as a barrier to this direction of low–level flow; satellite imagery frequently shows a dam effect with clear conditions on the eastern side of South Georgia, though cloud may spill around the extreme north and southern parts of the island.

Wave clouds are common phenomena. Satellite imagery shows wave clouds to be most frequent at or above 3000 m (~10000 ft). Occasionally, cirrus is generated by flow over the Mount Paget area, such that it appears as a long plume of dense cloud 100 km (~50 nm) or more in length with its forcing source clearly observed. Substantial wave–trains can also be induced by the South Georgia mountains, and satellite imagery shows there are occasions when there are wave–trains comprising twelve or more waves with a wavelength of around 30 km (~ 5 nm); this is close to the average width of the island.

The prediction of amounts and height of cloud is extremely difficult and forecasters rely heavily on satellite imagery and NWP model data.

Visibility: snow and fog

Visibility is governed by air–mass considerations and geographical location. In active cloud systems, precipitation will reduce visibility in the normal way and, given that snow is the most common form of precipitation, this will be a frequent cause of very poor visibility. In areas sheltered from the prevailing wind, visibility is generally very good; it follows that the eastern side of the island often experiences these conditions.

Fog and poor air–mass visibility arises most frequently when northeasterly winds advect moist air. Thus sea fog and hill fog can occur most readily on the eastern side of the island when it is exposed to this synoptic type.

Precipitation

Precipitation is highly variable in distribution because of the nature of the island's orography, but is most commonly of snow, though hail can occur in association with cumulonimbus clouds. Snowfall is often heavy, and frequently gives rise to blizzard conditions either from frontal systems or mesoscale features. The latter are always difficult to predict. Table 7.2.2.4.1 (in Appendix 2) shows the mean‑monthly rainfall for Grytviken.

Temperature and chill factor

Temperatures recorded range from over 20ºC in sheltered eastern parts to extremely low values both there and elsewhere. Since most of the island is very mountainous and glaciers are numerous, frequently inland areas are likely to experience extremely low temperatures. Winds underline temperature values and will lead to dangerously low chill factors. There are no reliable methods of predicting temperatures and chill factors in South Georgia, but the Grytviken area provides perhaps one of the least hostile temperature environments due to its sheltered location. Table 7.2.2.4.2 (in Appendix 2) shows the mean–monthly temperature for Grytviken.

Turbulence

Turbulence is a feature of the island, and its prediction is important for aviation forecasts. Moderate to severe turbulence may occur on the lee side of the island in an unstable air mass even when the surface gradient flow is no more than 13 m s^–1 (~25 kt). However, a stable flow of 18 to 20 m s^–1 (~35 to 40 kt) may not give rise to noticeable turbulence on the lee side. Forecasters rely heavily on NWP model data for assessing the air–mass stability, which together with routine synoptic analysis, provide the most practical approach.

Hydraulic jumps

There is no information available on these phenomena, and it is not possible to determine their likelihood.

Sea ice

The waters surrounding South Georgia experience sea ice. In a severe winter the Antarctic ice pack may extend as far north as the island. However, the normal hazard facing shipping is one of drifting ice that has broken away from Antarctica or from the Island's own glaciers. Icebergs drifting north towards the island are apparently generally largest to the west; however the eastern waters also have frequent icebergs, some of which may be large.

Forecasters rely on information from specialist ice centres and satellite imagery for determining the ice edge and the presence of any giant icebergs in the region.

Wind waves and swell

The region is very exposed to wind waves and swell but there are few observations. Both phenomena may produce mountainous seas. Forecasts of wind waves and swell are based on The UK Meteorological Office's global wave model products available to forecasters in the Falkland Islands.

7.2.3                                Gough Island 

7.2.3.1                          Orography and Local Environment

Gough Island is located within the roaring forties and has a moist temperate climate. The island is orientated north–west to south–east and measures 73 km² in area (approximately 13 km long and 7 km at its widest) (see Figure 7.2.3.1.1). Due to the island’s volcanic origin, the terrain is very rugged and rises steeply from the coast to a maximum height of 910 m (~3000 ft). Cliffs as high as 300 m (~1000 ft) and boulder beaches are found along the coastline, with few suitable spots for beaching boats. The meteorological station is located on the south–eastern portion of the island (40º 21´ S; 9º 53´ W, 54 m AMSL) and protected from the prevailing westerly wind and swell.

Most of the climatic data provided in this section were obtained from the South African Weather Bureau climate archives for the period 1961 to 1990.

7.2.3.2                          Operational Requirements

A relief vessel (SA Agulhas) brings fresh supplies to the base once a year (during September and October): 5-day forecasts of wind, weather, visibility and sea-state are provided for this vessel using NWP output from various models. Helicopters are used extensively to cargo sling supplies from the ship, while rafts are sometimes used to transport supplies to the coast where they are lifted ashore by a crane. The annual supply of diesel fuel is pumped ashore directly from the ship. All the aforementioned tasks are highly dependent on the weather and sea conditions in the vicinity of the base.

Figure 7.2.3.1.1     A map of Gough Island. (Courtesy of Paul Carroll.)

7.2.3.3                          Data Sources and Services Provided

Dedicated forecasts are generally only issued during take–over and during voyages to and from the island. The forecasts are provided up to three days ahead to assist with the logistics and planning. When compiling a good forecast, an accurate surface analysis is crucial and surface charts for the southern Atlantic are analysed at six–hour intervals. The surface pressure field is analysed using observations from land, ships and drifting weather buoys, as well as the UKMO “first guess” pressure fields and METEOSAT images. The METEOSAT imagery is particularly useful when determining the position of depressions and their corresponding frontal systems. Surface winds are forecast using the UKMO prognostic winds out to 48 hours; beyond this, winds are forecast using gradient winds inferred from the ECMWF prognostic surface pressure fields. Other forecast parameters include the expected weather and visibility. The 12–hourly ECMWF accumulated precipitation fields are used for determining the onset and duration of precipitation associated with depressions and frontal systems. In situ surface and upper–air data are available from the GTS and are taken into consideration when issuing short term forecasts. Sea conditions (swell period and height, as well as total sea) are forecast for days 1 and 2 using the UKMO swell model data available very 12 hours (out to 120 hours).

During the year several fishing vessels operate around the island, but use the high sea forecasts (available on the GTS) issued by the South African Weather Bureau.

There is also a Local User Terminal (LUT) for drifting buoy reception. Present communication limitations preclude the transfer of raw buoy data direct to Argos (the data processed on the island (and on Marion Island) are considered insufficiently reliable for direct inclusion on the GTS). The same equipment (there is an identical system on Marion Island) - is capable of receiving AVHRR imagery but unfortunately the limited communication bandwidth precludes its transmission to Pretoria.

7.2.3.4                          Important Weather Phenomena and Forecasting Techniques

General overview

The weather is strongly influenced by the regular procession of depressions and frontal systems past the island. The formation, movement and decay of these systems are generally handled very well by the models, but due to their coarse resolution, the models cannot take into account the significant impact the island’s orography has on the local weather regimes. Weather conditions on Gough Island are notorious for changing rapidly and can vary significantly depending on one’s location. This is particularly true for precipitation, cloud cover, wind and sea conditions. As a result, issuing forecasts for Gough Island is extremely difficult, especially if one is not familiar with the island. For the purpose of this section, we will assume the climate data and forecasting techniques are only valid for the base and Transvaal Bay where ships anchor during resupply operations.

Surface Pressure and Winds

Gough Island lies on the southern periphery of the sub–tropical high–pressure belt with a mean sea–level pressure of approximately 1015 hPa. The pressure, however, can vary over a wide range from 980 hPa to 1040 hPa. Mean–monthly pressures for Gough Island are shown in Table 7.2.3.4.1 (in Appendix 2).

Due to its sheltered location, winds on the south–east coast tend to be lower than over the open sea and exposed western parts of the island. The winds blow predominantly from the west, with gales observed on about 10% of days in winter and 5% in the summer. Gales at the base tend to blow from directions between north and south–west, i.e. anti–clockwise northerly through south–westerly. With the approach of each depression the wind follows a remarkably similar sequence. Ahead of the depression winds blow from the north–east or north and intensify. As the system moves closer, the winds continue to back and swing rapidly to westerly or south–westerly with the passage of the cold front. On occasions when depressions pass to the north of the island, the east coast can be buffeted by gale–force easterly or south–easterly winds.

When the wind has a westerly component, the model–derived winds are often too strong when applied to the east coast and should be used with caution. The station is relatively well exposed to winds from between north–easterly and south–easterly and under these conditions the model winds can be used with confidence. Model evaluations of the UKMO and ECMWF surface winds conducted by the authors during voyages through the South Atlantic indicate that the model winds display significant skill and can be used with confidence when forecasting winds.

A phenomenon unique to the east coast is the funnelling of winds down the steep glens as strong westerly winds cross the interior. This funnelling is clearly indicated by rotating columns of sea spray moving away from the coast in the vicinity of the glens: vortex shedding off the edge of major orographic features are probably also contributors to these effects.

Upper wind, temperature and humidity

No specific information on forecasting has been obtained.

Clouds

The island is often blanketed by cloud (especially on the windward slopes), with an average cloud cover of 6 oktas. The maximum cloud cover is generally observed in the late afternoon and evening. Cumulonimbus clouds are rarely observed at Gough Island, with stratocumulus, stratus and nimbostratus predominating. Warm fronts are associated with nimbostratus and stratus, with bases as low as 90 m (~300 ft). Cloud bases are higher behind cold fronts and on such occasions the clouds are typically towering cumulus, with bases higher than 450 m (~1500 ft).

Visibility: fog

The annual variation in the occurrence of fog is summarized in Table 7.2.3.4.2 (in Appendix 2). Fog occurs most frequently during the second half of summer and early autumn (January‑April), with an average of 23 days being reported at the station annually. Weak winds from the north–eastern quadrant or stagnant airflow within a moist air mass (such as in the warm sector of slow–moving depressions) are conducive for the formation of fog.

Precipitation

The precipitation statistics are summarized in Table 7.2.3.4.3 (in Appendix 2). The meteorological station receives an average of 3,154 mm of rain annually, with precipitation (>0.1 mm) observed on 293 days. July is the wettest month with an average of 28.8 days of rain, while February is the driest with 19.3 days. Snow is observed on an average of 8 days per year, and is most common between July and September. Ice pellets are frequently observed during cold outbreaks. Thunderstorms are observed an average of only 3 times per year and are most likely in May.

During the passage of frontal systems (associated with a well–defined upper–low), torrential rain (in excess of 100 mm) is possible in 24 h. It is very likely that these amounts could be substantially higher on the western side of the island and over the interior.

Temperature

The temperature statistics are summarized in Table 7.2.3.4.4 (in Appendix 2). Temperatures at the station are mild, with a mean annual temperature of 11.7ºC  and wind chill is rarely a concern when issuing forecasts. Temperatures below 0ºC have only been recorded in July and August. There is a marked seasonal variation in the mean–monthly temperature, with the warmest average temperatures occurring in February (14.5ºC) and the coldest in August (8.9ºC).

Sea ice

Not relevant at this location although icebergs of Antarctic origin might infrequently reach the area (see Figure 7.2.5.4.1).

Wind Waves and Swell

Due to the location of the station, issuing forecasts for swell and total sea is problematic. Transvaal Bay is generally sheltered from the westerly and north–westerly swells, but any swells having a southerly component are likely to influence the conditions within the bay. The worst sea conditions are observed when the swell is running between north–easterly and south–easterly. During the winter months, 8–10 m (~26–33 ft) swells with an easterly component are occasionally observed, with swells of 4–5 m (~13–16 ft) occurring frequently. During the summer, conditions are more settled, but large south–westerly or southerly swells generated by systems far to the south occasionally reach the island.

Evaluations of the UKMO swell model have revealed that where actual wave measurements have been available, the UKMO model is seen to predict total sea (i.e. the combined effect of wind and swell waves) relatively accurately. This model, however, tends to keep the total wave energy for too long in the wind wave component – i.e. it will predict extreme wind waves even well after the predicted wind has dropped and a heavy swell has moved into the area. This distinction between sea and swell is important when one is trying to predict vessel motion during off-loading – or, where relevant, the likelihood of a northerly swell, needed to break up the pack ice.

7.2.4                                Bouvetoya  

Bouvetoya (54º 24' S, 3º 25' E), formerly known as Bouvet Island is the southern–most island of the mid–Atlantic Ridge and consists of a single volcanic cone with a wide indented crater and attaining an elevation of 780 m (2,560 ft) at Olav Peak at the centre of the island. The area of the island is 54 km². It was placed under Norwegian sovereignty by a Norwegian Royal Decree of 23 January 1928.

The slopes of the central cone terminate on all sides in precipitous cliffs or glaciers, descending abruptly to sea level. Glaciers cover 93% of the island and prevent landings on the south and east coasts, while steep cliffs as high as 490 m block access to the north, west and southwest.

Bouvetoya is the most isolated island on earth. The nearest substantial land mass is more than 1600 km away. And as the island is rarely visited and relevant forecasting information unavailable the normal format used for describing other stations in this handbook has not been followed. Reference to the island is included purely because of its isolation. However, it should be noted that being so isolated the AWS on Bouvet provides a very important sea level pressure observation for input into global models. The area to the west of the island is a preferred area for explosive cyclogenesis that is largely responsible for the more severe sea/swell events in this portion of the Southern Ocean.

7.2.5                                Marion Island 

7.2.5.1                          Orography and the local environment

Marion Island is one of the Prince Edward Islands, a pair of islands that are under the sovereignty of the Republic of South Africa and are located about 1,600 km southeast of that country (see Figure 7.2.6.1.2). These islands are the peaks of a submerged volcano and are separated from each other by 17 km. Prince Edward Island, the more northeastern of the pair, has an area of about 100 km²; Marion Island has an area of 210 km² and rises to an elevation of 1,186 m (3,890 ft) at Jan Smuts Peak (see Figure 7.2.5.1.1). The island is bounded by rocky cliffs that are, in general, low on the east side and high on the west side. At the northeast end Transvaal Cove is the site of the meteorological station and depot (46º 53' S, 37º 52' E; 20 m AMSL).

7.2.5.2                          Operational requirements and activities relevant to the forecasting process

The station on Marion Island opened on 29 December 1947 and now supports a maximum of 24 people over the summer and an average of 12 people during winter. The following science activities are carried out on the island:

·                         environmental monitoring (since 1996);

·                         meteorological observations (since 1947);

·                         offshore marine biology (since 1982);

·                         onshore geology/geophysics (since 1965);

·                         terrestrial biology (since 1965).

To support these activities there are only one or two ship visits per season with no intercontinental flights possible. On the island there are around 10 helicopter flights per season. Use of wheeled vehicles is not possible on the mire conditions on the island. An outboard Zodiac is ship–based and used only during the re–supply period in April/May.

7.2.5.3                          Data sources and services provided

Meteorological observations have been made on a regular basis at Marion Island since 1947. There is also a LUT for drifting buoy reception. Present communication limitations preclude the transfer of raw buoy data direct to Argos (the data processed on the island (and on Gough Island) are considered insufficiently reliable for direct inclusion on the GTS). The same equipment (there is an identical system on Gough Island) - is capable of receiving AVHRR imagery but unfortunately the limited communication bandwidth precludes its transmission to Pretoria.

7.2.5.4                          Important weather phenomena and forecasting techniques used at the location

General overview

The weather is generally cloudy or dull with rain or snow on most days of the year. Snow occurs in all months, varying from 2 days per month in summer to 10 days in winter. In summer the snow line varies from 300 to 900 m. Fog and very low cloud are rather common, especially on the west coasts from February to April but the frequency decreases to about three days per month for the rest of the year.

Annual means for Marion Island are: pressure 1007 hPa, temperature maximum 8ºC minimum 3ºC, wind 9 m s^–1 (18 kt), rain–days 305.

Surface wind and the pressure field

No specific information on forecasting has been obtained on forecasting procedures. It is likely that conventional forecasting methods for mid–latitude meteorology would apply. Table 7.2.5.4.1 (in Appendix 2) shows the monthly mean sea level pressure for Marion Island.

Upper wind, temperature and humidity

No specific information on forecasting has been obtained on forecasting procedures. It is likely that conventional forecasting methods for mid–latitude meteorology would apply.

Clouds

No specific information on forecasting has been obtained on forecast procedures but it is noted that low cloud is common. It is likely that conventional forecasting methods for mid–latitude meteorology would apply.

Figure 7.2.5.1.1     A map of Marion and Prince Edward Islands. (Courtesy of Paul Carroll.)

Visibility: precipitation and fog

No specific information on forecasting has been obtained on forecast procedures but it is noted that fog is common.

Surface contrast including white–out

No specific information on forecasting has been obtained on forecast procedures. It is noted that even in summer the snow line may be as low as 300 m and so there may be some instances where white out and surface contrast may be a problem.

Horizontal definition

No specific information available on forecast procedures.

Precipitation

No specific information available on forecast procedures. Table 7.2.5.4.2 (in Appendix 2) shows the monthly mean–monthly rainfall for Marion Island.

Temperature and chill factor

No specific information available on forecast procedures. Table 7.2.5.4.3 (in Appendix 2) shows the monthly mean–monthly rainfall for Marion Island.

Icing

No specific information available on forecast procedures.

Turbulence

No specific information available, although being in the belt of strong westerlies lee wave and mechanical turbulence should be considered.

Hydraulic jumps

Hydraulic jumps are unlikely at this location.

Sea ice

Not relevant at this location although icebergs of Antarctic origin can reach the area (see Figure 7.2.5.4.1).

Wind waves and swell

No specific information on forecasting has been obtained although the numerical wave output from NWP models should be of some assistance.

Figure 7.2.5.4.1   An iceberg aground off Marion Island, 7 March 2002. (Courtesy of Sarette Slabber and the South African Weather Bureau).

7.2.6                                Crozet Islands  

7.2.6.1                          Orography and the local environment

The Crozet Archipelago extends between the longitudes of 50º E and 53º E at a latitude of 46º S (see Figure 7.2.6.1.1 and Figure 7.2.6.1.2). The volcanic islands splits into two groups 100 km apart. Possession Island has an area of about 150 km². The highest point is called Pic du Mascarin and reaches 934 m (~3,060 ft). The permanent base lies on the eastern side of a plateau at an elevation of 140 m (~450 ft).

Figure 7.2.6.1.2      A map of the location of the Crozet Islands relative to the Antarctic. (Courtesy of Paul Carroll.)

7.2.6.2                          Operational requirements and activities relevant to the forecasting process

No specific information has been obtained.

7.2.6.3                          Data sources and services provided

Meteorological observations have been made on a regular basis at Crozet since 1973.

7.2.6.4                          Important weather phenomena and forecasting techniques used at the location

General overview

The climate of Crozet Island is of a cold oceanic type being windy and very wet (see Tables 7.2.6.4.3 and 7.2.6.4.3 (in Appendix 2)). The precipitation is the most remarkable meteorological element; the number of days with more than 10 mm of precipitation varies between five in February and nine in August; the mean number of days in the year above this threshold is 86.

The prevailing wind direction is 220 to 340 degrees, which accounts for 75% of the observations. This percentage reaches 85% for average wind speeds greater than or equal to 16 m s^–1 (~ 1 kt), which are observed for 317 days a year on average. Those reaching 24 m s^–1 (47 kt) are also frequent (194 days on average) with a minimum of 10 days in February and a maximum of 20 days between June and September.

At mid–latitudes in the Indian ocean, depressions and ridges track from the west and cross the Crozet Islands and the Kerguelen Islands area. This flow regime is interrupted two or three times each month by high–pressure centres located close to the Crozet Islands.

The lows (about 10 per month on average) affecting the Crozet Islands come from:

·                         The Atlantic Ocean (about 50% of the systems). They appear not to be very active at Crozet. Despite the fact that they are located south of 50º S, they can lead to a strengthening of westerly winds

·                         The area between South Africa and Crozet (30 to 40% of lows)

·                         The remainder coming from the Indian Ocean south of 50º S.

Surface wind and the pressure field

Table 7.2.6.4.1 (in Appendix 2) shows monthly average wind speeds while Table 7.2.6.4.2 (in Appendix 2) shows monthly mean surface pressure at Crozet. Surface values of air pressure higher than 1025 hPa are recorded every month, but the highest pressures occur in winter (the highest value measured was 1040.4 hPa).

Low–pressure systems generally do not cause the air pressure at Crozet to fall below 990 hPa. Rapid deepening (40 to 50 hPa in 24 hours), as well as deep low–pressure systems, are not common (less than 10 instances per year on average, mainly between April and January (the lowest pressure recorded was 959.8 hPa).

High–pressure systems centred between 25°S and 35°S move eastwards from the Atlantic Ocean to over the Indian Ocean and the following behaviour has been noted:

·                         Sometimes an area of high pressure slows down at it moves eastwards and then strengthens between 40° E and 60° E.

·                         But sometimes a situation evolves in the following way. From a quasi‑stationary anticyclone, located west of South Africa, a ridge of high pressure, generally located around 35º S, progressively extends to the Indian Ocean. In the eastern part of this high–pressure system a new anticyclone builds and can remain stationary for several days in the area between 30º S–40º S (or even further south) and 30º E–40º E.

·                         In summer, slow, eastward–moving anticyclones strengthen in the centre of the Indian Ocean and radiate towards the south (over the Kerguelen Island' area) or southeast (over the Crozet Islands' area.

When an anticyclone is located north of Crozet Islands the following situation occurs frequently. The cold front, associated with a low located southeast of Kerguelen Island, exhibits a strong cyclonic curvature on its leading edge and is orientated zonally south of the anticyclone, at mid–latitudes between 70°E and 40º E. The front might then extend towards the northwest (South Africa)). In this situation the weather at Crozet can remain mild and rainy for up to 48 h with a mean westerly wind that can reach 20 m s^–1  (~40 kt). Often a wave forms in the frontal cloud band, very close to Crozet, and the precipitation increases. At Kerguelen when the cold front has passed the weather becomes variable with westerly winds and showers.

When an anticyclone is centred close to Crozet Islands it splits the sub–Antarctic area into two zones.

·                         A southwest to southerly flow affects Kerguelen Islands with numerous snow showers.

·                         The areas just west of Crozet in the meantime benefit from an advection of warm and humid air, which has been cooled by tracking over a colder sea and characterised by advection fog and low clouds. Depressions coming from the west come up against high pressure and are deviated towards the southeast leading to the formation of a zone of strong pressure gradient, which induces strong wind gusts from the northwest to north at Crozet. It is then common to measure a mean wind speed reaching 20–25 m s^–1 (~40–50 kt) with gusts over 40 m s^–1 (~80 kt) from the northwest during a 12 to 24 h period, followed by weak wind from the southwest behind the depression. With a westerly flow the wind speed can be of the same order in the northwest and southwest sectors.

Different types of evolution follow this:

·                         The anticyclonic system shifts either towards the northeast or slowly towards the south: the westerly airflow therefore lies temporarily north of 50º S and at high latitudes the anticyclone can remain several days.

·                         Another type of evolution involves a shifting of the high–pressure centre towards the east with, most frequently, a weakening of the centre.

Upper wind, temperature and humidity

No specific information on forecasting has been obtained.

Clouds

Average cloudiness is high throughout the year. Mean cloud cover is rarely lower than 2 oktas.

Visibility: blowing snow and fog

Fog is frequent at Crozet with 6–9 occurrences per month. Fog is most frequent in summer when it’s frequently the base of clouds, which lower beneath the altitude of the station when storms are passing.

Surface contrast including white–out

No specific information on forecasting has been obtained.

Horizontal definition

No specific information on forecasting has been obtained.

Precipitation

Table 7.2.6.4.3 (in Appendix 2) shows the mean–monthly precipitation amounts for Crozet. The monthly mean number of days of precipitation varies between 27 and 29, except for January (25) and February (22). On average 97 days with solid precipitation are recorded each year. Solid precipitation is highly likely throughout the year and a third of the precipitation events are a result of southwesterly showery airstreams behind lows.

Satellite imagery shows that the cloudy systems that form over the Indian Ocean, west of Crozet, are large and their cold fronts sometime extend up to 20º S. Images show large cloud bands that possibly merge with groups of convective clouds or even tropical cyclones in their declining stages. The maximum amount of precipitation measured in 24 h varies between 50–70 mm, except in February, March and April when they are between 84–102 mm.

Temperature and chill factor

Table 7.2.6.4.4(in Appendix 2) shows the mean–monthly temperature at Crozet. There are, on average, 66 days of frost occurring between April and October. The temperature has varied between an extreme high of 23.1ºC to the lowest on record of –5.4 ºC.

Icing

No specific information on forecasting has been obtained.

Turbulence

No specific information on forecasting has been obtained.

Hydraulic jumps

Hydraulic jumps have not been observed in this area.

Sea ice

Sea ice is not relevant to this area although icebergs may move into these latitudes (see Figure 7.2.5.4.1).

Wind waves and swell

No specific information on forecasting has been obtained.

7.2.7                                Kerguelen Islands

7.2.7.1                          Orography and the local environment

The Kerguelen archipelago is located at 49^o S, 70^o E (see Figure 7.2.6.1.2) and is dominated by the main island, the island of Grande Terre, which has a very broken coastline, composed of numerous peninsula and fjords, extending a considerable way inland (see Figure 7.2.7.1.1). The main island is surrounded by about 300 smaller islands. The surface area is about 7,000 km². The Cook Ice Cap extends across the western part and is reminiscent of a giant Quaternary ice sheet, with a dome reaching 1,040 m  (~3400 ft) elevation. Mt. Ross, in the southern part of the island, reaches 1,850 m (~6070 ft). The permanent station, Port aux Français, is situated in the lee of the highest part of the orography, where the land is flatter.

7.2.7.2                          Operational requirements and activities relevant to the forecasting process

No specific information has been obtained.

7.2.7.3                          Data sources and services provided

Meteorological observations and radiosonde launches have been carried out at Kerguelen since 1951.

7.2.7.4                          Important weather phenomena and forecasting techniques used at the location

General overview

Located 1,300 km east-southeast of the Crozet Islands, the Kerguelen archipelago has a windy oceanic climate, but it is slightly colder, and more importantly, drier than Crozet. The temperature variability from the long–term mean is smallest in the summer, and has a maximum in June (1.6^oC), when there are19 days of frost, compared to 5 days at Crozet.

Surface wind and the pressure field

The prevailing direction at Kerguelen Station is between southwest and northwest in 80% of cases. Easterlies, on the other hand represent less than 5% of observations. The windy nature of the climate may be seen from Table 7.2.7.4.1 (in Appendix 2) where the mean annual wind speed is reported as 9.8 m s^–1 (~19 kt). Wind speeds greater or equal to 16 m s^–1 (~31 kt) occur on 300 days per year while speeds greater or equal to 28 ms^-1 (~ 54 kt) occur on around 82 days per year.

The general westerlies are interrupted during short blocking periods by high–pressure systems that may reach 1010 hPa in winter and 1025 hPa in summer. The variability in the pressure fields is discussed in the similar section on Crozet Islands above . Table 7.2.7.4.2 (in Appendix 2) shows the variation in mean–monthly surface air pressure at Kerguelen.

Upper wind, temperature and humidity

No specific information on forecasting has been obtained.

Clouds

The mean cloud cover is 6 oktas but is not representative of the cloud experienced by the whole of the Kerguelen Archipelago. This is because of the Foehn winds experienced.

Visibility

The mean visibility, excluding all precipitation events, is rarely less than 30 km.

Surface contrast including white–out

No specific information on forecasting has been obtained.

Horizontal definition

No specific information on forecasting has been obtained.

Precipitation

Table 7.2.7.4.3 (in Appendix 2) shows the mean–monthly precipitation totals for Kerguelen. Precipitation occurs on an average of 285 days per year, of which 23 have a total of greater than 10 mm. 103 days of snowfall are recorded for the year. Precipitation is more important on the west coast, however, the accumulation difference between Crozet and Kerguelen is important. This difference comes from the fact that advected air masses of hot and tropical origin are transported directly to Crozet, but appear indirectly at Kerguelen via a southwesterly direction. Satellite imagery allows the identification of rapidly occluding depressions during their eastward passage. When they reach the Kerguelen Island area depressions are often mature and the warm sector is then of quite limited extent and the cold front is only a narrow cloud band.

Temperature and chill factor

The average annual temperature on Kerguelen is 4.5ºC (Table 7.2.7.4.4 (in Appendix 2)). The absolute temperature extremes recorded are +23.0° C and –9.4° C. There are 116 frost days on average per year, and five days per year, on average, when the temperature does not rise above freezing point.

Icing

No specific information on forecasting has been obtained.

Turbulence

No specific information on forecasting has been obtained.

Hydraulic jumps

No specific information on forecasting has been obtained.

Sea ice

No specific information on forecasting has been obtained although icebergs may move into these latitudes (see Figure 7.2.5.4.1).

Wind waves and swell

No specific information on forecasting has been obtained.

7.2.8                                Heard and McDonald Islands  

7.2.8.1                          Orography and the local environment

Heard Island is roughly circular in shape and slopes steeply upward forming an impressive and almost symmetrical mountain known as "Big Ben" (see Figure 7.2.8.1.1) at a general height of between 2,200 to 2,400 m (~7,200–7,800 ft) (see Figure 7.2.8.1.2 and Figure 7.2.6.1.2). The mountain is of volcanic origin, and a number of cones, the highest Mawson Peak 2,827 m (~9,274 ft), project above the general level of the old crater. Subdued volcanic activity still persists and smoke and steam have been noticed issuing from fissures in Mawson Peak on many occasions (see, for example Figure 3.4.3.3). At the north end of the island, Laurens Peninsula and Rogers Head Peninsula are connected to the main body of the island by a low–lying sandy isthmus. A long narrow sand and boulder spit at the southeast end of the island extends seawards for about 6 km, ending in shoal waters. Permanent ice covers most of the island, and glaciers, descending to the sea on all sides of Big Ben, terminate in ice cliffs from 15 to 30 m high. The northern limit of the Antarctic pack–ice lies to the south of Heard Island but during calm weather in winter pan–cake ice has been observed in Atlas Cove (the bay opposite West Bay in Figure 7.2.8.1.2).

The McDonald Island group (53º 3' S, 72º 35' E), lie 37 km west–southwest of Heard Island. There are three islands in the group: McDonald Island, Flat Island, and Meyer Rock.

Figure 7.2.8.1.2     A map of Heard Island. (Courtesy of Paul Carroll and based on data courtesy of the Australian Antarctic Division.)

7.2.8.2                          Operational requirements and activities relevant to the forecasting process

An Australian National Antarctic Research Expedition station was set up on 11 December 1947 on the Rogers Head Peninsula near the shores of Atlas Cove (53º 01' S, 72º 23' E), and operated continuously up to 1954. Synoptic observations are archived from 1 February 1948.

In more recent times there have been occasional visits for scientific work. Much of the forecast information presented here comes from an expedition held in 1980.

7.2.8.3                          Data sources and services provided

Australia operates two AWS on Heard Island, one (WMO 94997) is at "The Spit" at 53º 06´ 24″ S, 73º 43´ 15″ E, at an elevation of 12 m, the other (WMO 95997) is in Atlas Cove at 53º 01´ 12″ S, 73º 23´ 24″ E, at an elevation of 3 m. Forecasting for Australian Antarctic Programme voyages often depends on the nature of activities being undertaken, and may range from not being needed, to being provided by ship–based forecasters, or from one of Casey or Davis (if there are forecasters on station) or from Hobart, Australia.

7.2.8.4                          Important weather phenomena and forecasting techniques used at the location

General overview

Table 7.2.8.4.1 (in Appendix 2) gives climate statistics for Heard Island. Gales are frequent in all seasons and arise with astonishing rapidity. They are generally westerly, but gales from the east and north are not uncommon. Shaw (1955) has considered the representativeness of wind observations from Heard Island for broad–scale analysis. Gradient winds from east–southeast to southeast observed at the station are poorly correlated with the synoptic pressure gradient.

"Big Ben" acts as a conical obstruction to the wind flow. Loewe and Radok (1955) reported on persistent downstream lenticular clouds from 700 to 7,000 m on an otherwise cloudless day of anticyclonic weather.

Cloud photographs often show lee waves and wave vortex streets near Heard Island (see, for example, Figure 7.2.8.4.1and Figure 7.2.8.4.2). Brighton (1978) provided a model for strongly stratified flow past a cone with downstream waves, cowhorn eddy and vortex streets that might be applicable. So frequently is the island obscured by cloud that the first view of it from an approaching ship is of forbidding cliffs or abrupt headlands looming out of the mist. On occasions, however the icy summit of the island is visible, rising above the low cloud, from the immense distances. Big Ben's dome seems to float in the sky above the horizontal murk, making one of the most dramatic sights of the stormy southern ocean.

Figure 7.2.8.4.2    Banner cloud streaming from Big Ben on Heard Island.

Surface wind and the pressure field

The two main windy areas in westerly air streams are:

·                         firstly, the north side of the island, (the Compton and Brown Glaciers and Skua Beach area) is undoubtedly the windiest area of all with a combination of very strong squalls and a steady blast running down the mountainside and several kilometres out to sea to the northeast. These winds are turbulent and exceed 50 m s^–1 (~100 kt) fairly frequently.

·                         secondly, the Fifty–one Glacier, Lambeth Bluff and Cape Lockyer area to the east of Long Beach has frequent strong winds, and being close to the cliffs, turbulent winds often above 25 m s^–1 (~50 kt). These winds appear to sweep down the Fifty–one Glacier and divert easterly along the coast to Winston Lagoon. In the beach area blowing sand can be quite painful to the traveler.

·                         a third area, which also shows a great deal of squally weather is Atlas Cove. The cove is frequently whipped up by squalls descending the cliffs of the Laurens Peninsula and sweeping across the inlet of Walrus Beach. Fortunately, the full strength of these squalls rarely hits the station but when they do they can be quite devastating. Wind generated cracking and booming was reported in the Jacka Valley.

It has been often found that areas at the western end of the Laurens Peninsula (Macey Cone etc.) and the coast of the main island between the Vahsel Glacier round to Long Beach would have relatively light winds of 7 to 12 m s^–1 (~15 to 25 kt), while a full gusty gale would affect the four bays area and particularly the Nullabor Plain. The existence of huge ventifacts at the neck between West Bay and Walrus Bay attest to this being a very windy area also. In short, weather‑wise Atlas Cove is not a suitable place to build a base or from which to operate aircraft.

If the winds back to the south with the passage of a cyclone the compression zone appears to swing around the island to Atlas Cove, Saddle Point area on the northwest and to Spit Point on the east side. Cloud may disperse completely depending on the water vapour in the air, which is generally colder and drier.

Upper wind, temperature and humidity

Forecasts would be based largely on NWP output.

Clouds

The cloud generated by the uplift of the moist air as it hits Big Ben and the Laurens Peninsula forms a spectacular and omnipresent cloud cap over the northwest/southwest side of the island extending many kilometres out to sea. This standing wave cloud is repeated at intervals of 10 to 20 km downwind and can be picked out clearly on many days on the satellite pictures as a herringbone pattern with Heard Island at its apex. As a result it is quite common for both ends of the island to have totally different weather, for example Atlas Cove could have 15 to 20 m s^–1 (~30 to 40 kt) wind, rain and low cloud while Spit Point would be cloudless except for some lenticular or orographic cloud on the mountain, bright sunshine and calm conditions. The roll cloud generated by the mountain and rough seas off Skua, Fairchild and Compton Glacier beaches could be clearly seen from Spit Point. Attempts to fly aircraft at Spit Point with a search and rescue (SAR) cover from Atlas Cove were frequently thwarted by those conditions of unflyable weather at Atlas Cove.

Visibility: blowing snow and fog

In the vicinity of Heard Island fog is frequent, especially with north and northwest winds, but with westerly winds visibility is often good. Fog may prevent all visual or physical ship to shore communication for several days at a time. On the other hand in 1980 it was reported "on the couple of clear days that we experienced, flying was a real pleasure. The cold air and unlimited visibility allowed the helicopter to operate at its most efficient altitudes with power to spare".

Surface contrast including white–out

No specific information on forecasting has been obtained.

Horizontal definition

No specific information on forecasting has been obtained.

Precipitation

No specific information on forecasting has been obtained.

Temperature and chill factor

No specific information on forecasting has been obtained.

Icing

Two dangers mostly likely to cause a forced aircraft landing on the islands are bird strikes and airframe icing, which can occur with frightening speed. Rain or sleet squalls can appear suddenly around a cliff or down a valley and can make the Perspex bubble look like frosted glass in seconds.

Turbulence

Strong winds and resulting turbulence present the greatest problems for aircraft operations. During some of the vertical photography sorties in a 1980 expedition, at altitudes up to 3,000 m (~10,000 ft) the winds were around 40 m s^–1 (~80 kt) and the turbulence in the lee of Big Ben was severe.

Turbulence is created by the numerous volcanic cones like Crater and Corinth Head. Even in light winds these cones can create turbulence like "mini–cyclones". On one occasion during the 1980 expedition turbulence caused the rotors to lose speed so badly that the "engine out" warning came on momentarily.

Hydraulic jumps

There is no record of occurrence of hydraulic jumps on Heard Island.

Sea ice

No specific information on forecasting has been obtained although icebergs may move into these latitudes (see Figure 7.2.5.4.1).

Wind waves and swell

Anchorage may be obtained 1.5 km northwest of the head of Atlas Cove: it is exposed to the prevailing westerly swell rolling in from the north–northwest.

If the wind swings to the northeast a heavy swell can be expected into Atlas Cove making it a very precarious anchorage against a lee shore; at other times it is also subject to sudden and violent squalls. Compton Lagoon is much too windy and dangerous for boats, though its bar might be deep enough for some small craft and Winston Lagoon closes and opens its entrance to the sea at regular intervals.

7.2.9                                Macquarie Island  

7.2.9.1                          Orography and the local environment

Macquarie Island (54º 25' S, 158º 58' E) with an area of 120 km² is 34 km long and up to 5 km wide, and lies in a north–south direction (see Figure 7.2.9.1.1). Most of the island consists of a plateau at a general elevation of 250 to 350 m, rising in places to low rounded spurs and hills of 400 to 470 m. The edge of the plateau falls away abruptly to the sea or to narrow beaches. In the north of the island, Wireless Hill (102 m) is joined to the main mass of the island by a low, flat isthmus. The pioneer ANARE party arrived at the island on 7 March 1948 and the station was established on the isthmus near the old Australian Antarctic Expedition (1911–14) hut, which was still standing, though in poor condition. Observations are archived from April 1948. The highest point is Mount Hamilton at 433 m, 6 km north of Hurd Point, the southern most extremity.

7.2.9.2                          Operational requirements and activities relevant to the forecasting process

Macquarie Island Station has a winter complement of staff of about 19 people: this number increases to a maximum of 40 during the summer. The following science activities are carried out at the station: environmental monitoring; human biology; ionospheric/auroral observations; terrestrial biology; and meteorological observations.

Main re–supply of the station generally occurs during summer. Re–supply of the various huts around the island by helicopter or by small boat is another important activity that benefits from the forecasting service.

7.2.9.3                          Data sources and services provided

The meteorological observing programme includes surface three–hourly synoptic observations. The upper–air sounding programme provides upper–air data at 12–hourly intervals, including computation of upper winds, as well as measuring pressure, temperature and moisture at upper levels of the atmosphere to a height of 22 km. Total ozone is also measured daily.

Forecast services for the station are, in the main, provided by the Regional Forecasting Centre in Hobart, Tasmania, which provides twice daily general forecasts for the island's activities, and provides aviation forecasts when helicopters are in operation during station/hut re–supply. Forecasts are received at the station by business facsimile. The station also has access to Internet facilities through which it obtains much of its weather information from theAntarctic Division.

7.2.9.4                          Important weather phenomena and forecasting techniques used at the location

General overview

Table 7.2.9.4.1 (in Appendix 2) shows climate statistics for Macquarie Island, which is affected by the persistent westerly belt of winds that sweep over the Southern Ocean. The island is ice–free and has no permanent snow cover, although the upper levels of the island are covered with wet snow for most of the year. The characteristic features of this island's climate are the small variation in temperature both seasonally and diurnally, and the persistent strong westerly and northwesterly prevailing winds. The windiest time of year is generally between August and October, when the north–south pressure gradient is strongest. The island's weather is also characterised by overcast skies and a large number of days with precipitation, the island often being obscured by mist. Wind chill is a major hazard for people living on the island because of the precipitation and strong winds, combined with near freezing temperatures.

The warmer months are from December to April, and July is the coldest month. The mean relative humidity ranges between 85 and 90 percent, making it the Australian station with the highest mean relative humidity. Mean–monthly rainfall is slightly higher during the warmer months.

Surface wind and the pressure field

As Macquarie Island lies in the regime of the mid–latitude westerlies, throughout the year winds are predominately from a direction between west and northwest. The mean wind speed throughout the year is 8 m s^–1 (~16 kt). The windiest month is September with an average speed of 8.6 m s^–1 (~16.7 kt) while the least windy month is December with an average speed of 6.9 m s^–1 (~ 13.5 kt). The highest recorded gust occurred in September and was 51.4 m s^–1 (~100 kt).

The wind summaries show that 70% of all wind directions blow from between west and northwest throughout the year at an average speed of about 10 m s^–1 (~20 kt). The station winds are not always representative of the winds elsewhere around the island. In strong southwesterlies the station often reports lower wind speeds than elsewhere around the island. In a northwest airstream winds can arrive in Buckles Bay as north to northeasterlies due to flow around Wireless Hill. The elongated ridge of the island intercepts the wind flow from all directions and this results in local lee vortices particularly in the region of Buckles Bay. Here the orographic effects of the detached peak of Wireless Hill, the low lying isthmus and the northern end of the main island ridge result in variable lee wind patterns in the prevailing westerly flow. This often causes difficult conditions during ship to shore operations.

Upper wind, temperature and humidity

Forecasts of upper parameters are based on nowcasting techniques, using radiosonde data, and on NWP. Mean January and July upper–level wind roses for Macquarie Island are included in Figures A3–9 (a) and A3–9 (b) (in Appendix 3) while mean–temperature profiles for this station are also shown in Appendix 3 as Figures A3–7 (a) and (b).

Clouds

Typical of a mid to high–latitude maritime environment the average cloud amount at Macquarie Island is 7 oktas.

Visibility: mist and fog

The island is often obscured by mist and/or very low cloud. Prolonged periods of northerly air–flow, often due to slow moving highs over the Tasman Sea, bring moist warm air from lower latitudes, causing vast areas of fog over the waters surrounding the island. Fogs are also common in northerly airflow ahead of frontal changes. Fogs are observed at Macquarie Island on average about 70 days each year and are distributed fairly evenly in all months. Plateau visibility in fog is often less than 50 m.

K.C. Hines (who participated in the first ANARE in 1948) described the island's weather as: "For days on end the upper plateau would be shrouded in mist and a great wall of fog would block out the sea on all sides. A fine misty rain would be falling continuously and the inevitable wind sweeping in past the apparently impenetrable fog barrier."

Surface contrast including white–out

These are not generally a problem due to the vegetation and exposed rock.

Horizontal definition

This is not generally a problem.

Precipitation

Snowfall occurs throughout the year with a maximum frequency in spring. During September, falls can be expected on 7 days reducing to 1 day a month during summer. The upper levels of the island's plateau are covered with wet snow for most of the year, particularly from May to October, with depths in level areas between 0.5 and 1.0 m by the end of the season. The average annual rainfall total is 909 mm, varying little throughout the year. The wettest month is January (85 mm) and the driest July and August (65 mm). Rain can be expected on 305 days of the year.

Forecasting precipitation is based on airstream–weather concepts, together with satellite imagery and NWP output.

Temperature and chill factor

Mean daily maximum temperatures range from 9º C in January to 5º C in July. Mean minima range from 5º C in January to 1º C in September. This relatively small temperature range is typical of oceanic islands at these southern latitudes and is very similar to those recorded at Campbell Island, Kerguelen Island, Marion Island and the Falkland Islands. The highest temperature on record is 14º C (recorded in December), the lowest on record is –9º C (recorded in both July and August). Although temperatures seldom fall far below freezing, wind chill is a major hazard.

Icing

Forecasting icing is based on airstream–weather concepts together with satellite imagery, NWP and radiosonde data.

Turbulence

Although lee waves occur, mechanical turbulence is a main concern and is forecast on the basis of predicted low–level winds.

Hydraulic jumps

Hydraulic jumps are not observed on Macquarie Island.

Sea ice

Sea ice is not relevant to Macquarie Island although icebergs although icebergs may move into these (see Figure 7.2.5.4.1).

Wind waves and swell

The most important anchorage is Buckles Bay (Figure 7.2.9.1.1), followed by Hasselborough Bay (not shown) directly opposite a separating isthmus. Being close together, a vessel can, in the event of a complete change of wind, obtain shelter by proceeding from one to the other of these anchorages by passing round the northern–most promontory.

There are short spells of east to southeast winds that reach gale–force at times and affect Buckles Bay in particular, these are generally accompanied by large waves from that direction. Swells can be critical to ship unloading, in particular on ships where the hatch covers cannot be stacked if there is a roll of more than a few degrees.