7.3.5                                Trinity Peninsula  

7.3.5.1                          Orography and the local environment

The Trinity Peninsula is at the very tip of the Antarctic Peninsula and separates the Bransfield Strait from the waters of the Weddell Sea (see Figure 7.3.1 and Figure 7.2.1.1.1). The peninsula is aligned northeast to southwest and extends for several hundred kilometres. On the Weddell Sea side, James Ross Island is separated from the peninsula by the Erebus and Terror Gulf, and the Prince Gustav Channel. For many years it was not possible to circumnavigate James Ross Island by ship, but in the late 1990s the fast ice disintegrated as a result of the warming that has taken place across the region in recent years.

There are several research stations around the Trinity Peninsula:

·                         The Argentine Marambio Station (64o 14’ 42” S, 56o 39’ 25” W, 200 m AMSL) is located on Seymour Island, a small island just east of James Ross Island.

·                         The Argentine Esperanza Station (63º 23' 42" S, 56º 59' 46" W, 24 m AMSL) is located in San Martin Land on a solid rock surface 30 m from the Antarctic coastline.

·                         The Chilean General Bernardo O’Higgins Station (63° 19' S, 57° 4' W, 12 m AMSL) is located on the western side of the Trinity Peninsula on solid rock and about 50 m from the Antarctic coastline. The station was opened in 1948.

·                         The Bulgarian St Kliment Orchridiski Station (62° 38' 06'' S, 60° 21' 07'' W) is located near the tip of the Trinity Peninsula.

·                         The Argentine Petrel Station (63° 28´ S, 56° 17´ W, 18 m AMSL) is located on Dundee Island about 400 m from the coast. It is 37 km from Esperanza Station.

·                         The Argentine Primavera Station (64º 09' S, 60º 57' 50' W, 50 m AMSL) is on Cape Primavera on the western coast of the Antarctic Peninsula. The station is located about 500 m from the coast and is built on rock.

7.3.5.2                          Operational requirements and activities relevant to the forecasting process

·                         At Marambio there is a 1,200–yard runway (see Figure 7.3.5.2.1) on a raised plateau operated by the Argentine air force that can take wheeled aircraft, including Hercules C–130 cargo planes and DHC–6, plus helicopters. The base has the only airstrip in Antarctica where planes can land year–round without skis, so it is of considerable importance within the region. Although Marambio is a military base it gives logistical support to the scientific community, with most activity taking place during the summer months. The base has about 30 staff in winter and an additional 50 temporary staff in summer, plus about 60 scientists visiting during the summer. Weather forecasts are required for flights to and from the station, base activities and scientific parties operating in the area.

·                         Esperanza base operates all year. There is an unmarked snow landing area approximately 2.8 km (~1.5 nm) from the base (see Figure 7.3.5.2.2).

·                         General Bernardo O’Higgins Station operates all year, including a year–round observations programme. There is a skyway located 2,000 m from the station at an elevation of 300 m. The station was opened in 1948.

·                         St Kliment Orchridiski Station is a summer–only station.

·                         Petrel Station is a summer–only station but has meteorological records extending back to 1967.

·                         Primavera is an another summer–only station.

7.3.5.3                          Data sources and services provided

·                         At Marambio there is a surface meteorological observing programme that provides 6–hourly data that are put on the GTS. The Marambio Meteorological Antarctic Center is operational year round and provides briefings for all air operations. Current weather and forecasts are also provided by HF and VHF. All information is in Spanish or English. Fax charts are obtained from the Meteorological Bureau in Buenos Aires. Satellite imagery from the NOAA polar–orbiting (APT) and geostationary GOES satellites are received at the station.

·                         O’Higgins maintains a surface meteorological observing programme.

·                         Petrel maintains a surface meteorological observing programme.

7.3.5.4                          Important weather phenomena and forecasting techniques used at the location

General overview

This area is close to the latitude of the circumpolar trough and is therefore affected by many active depressions throughout the year in the Bellingshausen Sea and the Drake Passage.

Figure 7.3.5.2.1     A sketch map of Marambio showing the runway.

The western side of the area has a more maritime climate, while to the east of the Peninsula the weather is more continental and generally colder. The colder air temperatures make the region more prone to fog forming as warm air masses move south.

Stations on the western side are strongly affected by depressions in the Bellingshausen Sea and fronts that move down over the area from the northwest bringing cloud and precipitation. However, lows can also develop over the southern Bellingshausen Sea and track northeastwards over the area.

Stations on the eastern side are affected by lows passing through the Drake Passage bringing strong winds, low cloud and extensive precipitation. However, as these lows move to the east of the area they can be affected by south or south easterly winds that can bring cold, showery conditions from the Weddell Sea. Lee cyclogenesis events occur occasionally to the east of the Antarctic Peninsula.

Surface wind and the pressure field

Stations on the western side are strongly affected by the climatological low–pressure in the Bellingshausen Sea and have predominantly north to northwesterly winds. Esperanza is reported to be prone to strong winds coming from the northeast that at times exceed 55 m s–1 (~108 kt).

The eastern side of the peninsula has winds that are predominantly from the south to southwest and reflect the barrier wind flow on the eastern side of the Peninsula. However, on a high proportion of occasions winds from the northwest are experienced as depressions enter the Drake Passage from the west. Marambio often receives strong winds that can cause problems for planes landing at the airstrip. The summer season (November – March) wind rose for Marambio is shown in Figure 7.3.5.4.1 (in Appendix 2). Mean–monthly and annual station level pressures for Marambio are shown in Table 7.3.5.4.1 (in Appendix 2) and similar MSLP averages for General Bernardo O’Higgins Station are shown in Table 7.3.5.4.2 (in Appendix 2).


              Figure 7.3.5.2.2     A sketch map of the snow landing area at Esperanza.

Upper wind, temperature and humidity

Upper–air conditions are forecast using prognoses obtained from Buenos Aires or other centres.

Clouds

The tip of the Antarctic Peninsula is a very cloudy region as a result of the many depressions in the area and extensive frontal and non–frontal cloud. Marambio reports about 70% cloud cover throughout the year, which is probably typical for the region.

As indicated earlier, frontal cloud band arriving from the northwest are a major factor on the western side of the peninsula, but cloud can also arrive from the west. When a low‑pressure centre is located over the Antarctic Peninsula or the western Weddell Sea a cold, showery airstream can affect the area with convective cloud being present.

At stations such as Marambio the many depressions in the Drake Passage bring extensive cloud, but less cloudy conditions are also experienced during the brief periods when anticyclones are present in the area or when drier continental air masses arrive from the south. At Marambio significant protection from low cloud in west or north–westerly winds is afforded by the high ground of the Peninsula. However, a veer to the north or northeast may well bring in extensive low cloud.

Visibility: fog

Fog can be a problem at all the stations in this area since relatively warm, moist air masses can be advected southwards over the cold ocean. The area of the strong gradient in sea surface temperature across the Antarctic Circumpolar Current is particularly prone to fog on occasions and this can affect the more northerly stations. However, the generally colder conditions on the eastern side of the Peninsula Visibility make fog a great problem in this area than to the west. Visibility is very variable at Marambio with good visibility being experienced when continental air masses arrive over the station, but also extensive fog being reported. When the station is to the south west of a depression, low cloud may produce fog on the airfield.

Surface contrast including white–out

The large amounts of cloud across the area can give poor contrast on occasions but the extensive areas of exposed rock improve matters towards the end of summer.

Horizontal definition

Not generally a problem across the area.

Precipitation

This area has some of the greatest amounts of precipitation in the Antarctic with over 1 m water equivalent being recorded. However, conditions are quite different on the eastern and western sides of the Peninsula. To the west the area is exposed to the full force of the many depressions and these bring extensive and frequent precipitation. This can be in the form of rain or snow at any season, although snow (rain) is obviously more common in winter (summer). Orographic uplift gives greater precipitation amounts at higher elevations.

The stations on the eastern side of the Peninsula generally get less precipitation since they are in a rain/snow shadow region. However, northeasterly flow can bring relatively warm air and moderate/heavy falls to the area. Lee cyclones can also give significant falls. Mean–monthly and annual precipitation amounts for General Bernardo O’Higgins Station are shown in Table 7.3.5.4.3 (in Appendix 2).

Temperature and chill factor

Temperatures in this area are the highest in any part of the Antarctic and are above freezing point for extensive periods in summer. Primavera has reported a maximum temperature of 13ºC giving an idea of the warm air masses that can affect the area. Mean January temperatures at O'Higgins are often above freezing and were +2oC in the warm month of January 1963. On other occasions in summer they can average around –1oC (see Table 7.3.5.4.4 (in Appendix 2)).

The colder temperatures on the eastern side are typified by Marambio where the mean–monthly temperatures range from just above –2ºC in December and January to around –15oC in winter, with the annual mean temperature being around –9.0oC. However, in summer the temperature can rise to +10 C or higher (see Table 7.3.5.4.5 (in Appendix 2)).

Mean–monthly and annual temperatures for Esperanza are shown in Table 7.3.5.4.6 (in Appendix 2) and similar averages for Petrel Station are shown in Table 7.3.5.4.7 (in Appendix 2).

Icing

The highly variable temperatures and the availability of large amounts of cloud liquid water make icing a potential problem in the area. It can be forecast using knowledge of tropospheric temperatures from model or radiosonde ascents and cloud information from satellite imagery.

Turbulence

The strong winds experienced on occasions at Marambio can give severe turbulence as aircraft approach the runway.

Hydraulic jumps

Hydraulic jumps have not been reported at stations in this area.

Sea ice

In summer the ocean around Marambio is covered in loose pack ice drifting northwards in the prevailing ocean current. However, sea ice is less of a problem on the western side of the peninsula.

Wind waves and swell

Wind waves and swell can be predicted using the output of wave models, which is available on the GTS.