2.5                                   Mesocyclones

Mesocyclones are high latitude, sub synoptic–scale depressions that have a horizontal length scale of less than about 1000 km and generally exist for less than 24 hours. With their limited horizontal scale they can often pass undetected between synoptic reporting stations and are usually identified via visible or IR satellite imagery. In fact these lows were hardly known in the pre–satellite era and have only been studied since about the early 1980s. Despite their small scale, these lows can be very important in the forecasting process in some sectors of the Antarctic since they can have winds of greater than gale–force and can bring moderate or heavy snowfall to coastal locations. In this section we provide a brief introduction to mesocyclones. Further information on these systems can be found in the meteorological literature (Turner and Row, 1989; Turner et al., 1993; Turner et al., 1991; Heinemann and Claud, 1997; Rasmussen and Turner, 2003) and in Section 6.5, which deals with the specific problems involved in forecasting these vortices. There is a bewildering variety of names applied to these systems in the two polar regions including polar lows, Arctic hurricanes, polar depression, mesocyclones and cold air vortices. However, these terms all refer to mesoscale depressions in the polar regions and we will refer to them here simply as mesocyclones. It should be noted that the term polar low is often used to refer to the more intense vortices that have surface wind speeds of greater than gale force.

2.5.1                                The general characteristics of mesocyclones

Mesocyclones are cold air vortices that are usually found south of the main polar front and well away from the main frontal cloud bands. Since they tend to form in cold air outbreaks they are often found to the west of large synoptic–scale lows when very cold continental air pushes northwards over the Southern Ocean. A typical example of a mesocyclone–synoptic scale low passing through the Drake Passage to the north of the Antarctic Peninsula (see Figure 7.2.1.1.1) is shown in Figure 2.5.1.1.

As discussed above, most mesocyclones have a horizontal length scale of less than 1000 km, although some vortices can be as large as 1,500 km, since there is really a spectrum of low‑pressure systems around the Antarctic and it is not possible to specify any firm boundary separating the different types of lows. However, satellite studies (Turner and Thomas, 1994) have shown that the vast majority of mesocyclones are less than 500 km in horizontal extent. The satellite imagery has also shown that most mesocyclones are quite short lived with most existing for less than 12 hours (Heinemann, 1990).

The satellite imagery has also provided much useful information on the cloud patterns associated with mesocyclones. What distinguishes mesocyclones from frontal cyclones is that they tend to have a single band of cloud around the low centre, rather than the warm, cold and occluded fronts of a classic baroclinic wave. This cloud can take the form of a single comma or can be wrapped two or more times around the low centre in the form of a spiral cloud band. Although many papers refer to comma and spiral systems it is not clear whether there is any fundamental differences in the mechanisms involved in their development. However, spiral‑form systems tend to be found in synoptically quiet regions while comma–type lows are more usually found when the background flow is stronger, such as to the west of a deep depression. A third type of mesocyclone that has been noted is the “merry–go–round” type of development discussed below.

 Figure 2.5.1.1     A typical example of a mesocyclone–synoptic scale low passing through

 the Drake Passage to the north of the Antarctic Peninsula.

New forms of satellite data, such as passive microwave imagery and winds from the scatterometers, have provided information on the surface wind field associated with mesocyclones. These data have shown that most mesocyclones are quite weak and often have no more than a trough in the surface pressure field. When in situ data have been available they have suggested that most mesocyclones have a surface pressure perturbation of less than 5 hPa. However, a number of deep systems with winds at gale–force or stronger have been observed.

As with other types of depression, mesocyclones are local maxima of cyclonic vorticity. This vorticity can increase for a number of reasons associated with the formation and development of the low's circulation. Our understanding of the formation of mesocyclones is far from complete but the following factors seem to be important in the development of some systems:

·                         Baroclinic instability. Small scale lows with the appearance of classic baroclinic waves have been observed to form on some frontal bands, for example near the ice edge where a significant horizontal thermal contrast can be found, especially when the flow is parallel to the ice edge. It has been suggested that the small horizontal scale of the lows is a result of the shallow depth of the baroclinic zone.

·                         Barotropic instability. This is a shearing instability that can be responsible for the development of some vortices were there is a pronounced horizontal wind shear without any major thermal gradient. An example of this type of development occurs in Prydz Bay, at the mouth of the Lambert Glacier, where the phenomenon has become known locally as the “Prydz Bay Low”. Figure 2.5.1.2 is an example of this type of low: in this image the structure of the pack ice north of the Amery Ice Shelf reflects the strong south to southwest continental outflow from the Lambert Glacier with a small low having formed on the eastern shear zone of the outflow. Low–level convergence (see below) may also play a role here.

·                         Upper–level troughs and cold pools. Upper troughs can be responsible for significant cyclonic vorticity advection into an area resulting in the spin–up of mesocyclones. This can take place to the west of synoptic–scale lows where the mesocyclone can be associated with minor surface and upper–air troughs. In addition, the presence of an upper cold pool can be an important factor in generating instability through a layer of the atmosphere.

·                         The decay of large synoptic–scale lows. As large lows decline and become slow–moving they can often decay into a number of mesoscale centres of circulation that rotate around a common centre (Figure 2.5.1.3). Such systems tend to be rather rare around the Antarctic.

·                         Surface fluxes of heat and moisture. The ocean currents around the Antarctic tend to be quite zonal and warm, mid–latitude water is rarely carried to high latitudes. The flux of heat into the atmosphere therefore tends to be quite small, although it is important for triggering low–level convection and large fields of convective cloud can be observed during cold air outbreaks over the ocean. Mesocyclones can be observed to form in some of these cumulus fields on occasions. In contrast to the Northern Hemisphere, the layer of instability thus generated is usually quite shallow, and so vigorous convection does not occur. This limits the vertical development of such systems and so the associated weather conditions are not generally severe. Although not severe, conditions may be hazardous, particularly to aviation operations, with winds up to 18 m s^–1 (35 kt), sustained heavy snowfall and low cloud. Heavy icing may be encountered.

·                         Low–level convergence. Some parts of the Antarctic are prone to frequent convergence at low level, such as on the large Ronne and Ross Ice Shelves where flow down from the surrounding mountains takes place. This flow pattern can result in the spin–up of mesocyclones, especially during the winter when the katabatic flow is most pronounced.

Of course more than one of these factors can be responsible for the development of an individual mesocyclone. For example, over a coastal polynya there may be strong fluxes of heat and moisture into the lowest layers destabilising the atmosphere, with the arrival of an upper–level trough triggering the spin–up of a mesocyclone.

  Figure 2.5.1.2     An example of a coastal mesoscale low formed as

  a result of shear in a continental outflow. (The image is from the NOAA–14

    Figure 2.5.1.3     A “merry–go–round” group of mesocyclones

    (indicated by the letters A to E) over the northern Weddell Sea that

    formed from a disintegrating synoptic–scale low.

2.5.2                                Spatial distribution

Most mesocyclones are found over the ocean areas around the Antarctic and there are very few vortices over the high plateau areas of the continent. Since many mesocyclones develop in cold air outbreaks and there is an almost continuous band of frequent depression activity around the continent it is not surprising that mesocyclones are found in all sectors of the Southern Ocean. In some areas where depressions become slow–moving, such as the Bellingshausen Sea, mesocyclone developments are particularly common. Similarly, when there is frequent off–shore flow over open water caused by katabatic winds (for example, near Terra Nova Bay (see Section 7.12.2) or the broad–scale circulation (the eastern Weddell Sea) then mesocyclones are commonly found.

The low–lying ice shelves also have many mesocyclone developments because of the low–level convergence and supply of moist air from passing synoptic depressions. Studies on the Ross Ice Shelves based on AWS data have shown that vortices can occur that have no cloud present.

Although mesocyclones are rare on the plateau some systems do occur here and these lows can be a problem for fieldwork in the interior. Such lows move with the broad–scale flow and can be quite long–lived because of the lack of synoptic activity to cause their dissipation. An example of this type of development is shown in Figure 2.5.2.1.

Figure 2.5.2.1     A mesocyclone over Dronning Maud land high on the Antarctic plateau as observed in AVHRR channel 3 imagery. The ice surface appears black and the cloud composed of supercooled water droplets white.

2.5.3                                Temporal variability

Mesocyclones are a feature of all seasons but year–round investigations in the Antarctic Peninsula region (Turner et al., 1996b) have shown a peak of activity during the summer months when more open water is present and there is a greater supply of moisture. However, many systems also occur in winter when the lows can be seen over the sea ice in the coastal zone. Since the development of mesocyclones is so closely associated with synoptic–scale activity we can expect a link with the semi–annual oscillation and longer–period climatic cycles, although such considerations are not of importance in forecasting these lows.