Distribution and Characteristics of Mesoscale Cyclones in the Antarctic: Ross Sea Eastward to the Weddell Sea

Jorge F. Carrasco Direccion Meteorologica de Chile, Santiago, Chile

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David H. Bromwich Polar Meteorology Group, Byrd Polar Research Center, and Atmospheric Sciences Program, Department of Geography, The Ohio State University, Columbus, Ohio

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Andrew J. Monaghan Polar Meteorology Group, Byrd Polar Research Center, and Atmospheric Sciences Program, Department of Geography, The Ohio State University, Columbus, Ohio

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Abstract

The mesoscale cyclone activity observed in the portion of Antarctica that faces the South Pacific Ocean and Weddell Sea area is summarized from a study of 1991. In general, area-normalized results reveal much greater mesoscale cyclonic activity over the Ross Sea/Ross Ice Shelf and southern Marie Byrd Land than on both sides of the Antarctic Peninsula. More than 50% of the observed mesoscale vortices are of the comma cloud type. The average diameter of mesoscale vortices is approximately 200 km near Terra Nova Bay, 270 km near Byrd Glacier, and 280 km near Siple Coast. Near the Antarctic Peninsula, the average diameter is about 370 km over the Bellingshausen Sea and 380 km on the Weddell Sea side. The largest percentage of deep vortices occurs over the Bellingshausen Sea sector (38% of all cases), where convective instability frequently occurs. Over the Ross Sea/Ross Ice Shelf and Weddell Sea sectors the majority of the mesoscale vortices are low cloud features that probably do not exceed the 700-hPa level due to the prevailing lower-atmospheric stability. The areas identified as sources of mesoscale vortices concur with the locations of enhanced katabatic winds.

A synthesis of the available literature leads to some general characteristics of mesoscale cyclone formation and development. Mesoscale cyclogenesis is associated with areas of warm and/or cold air advection, low-level baroclinicity, and cyclonic vorticity resulting from the stretching mechanism. Subsequent intensification depends on the presence of upper-level support. Spatial and temporal variability in mesoscale cyclone formation is often related to the behavior of synoptic-scale cyclone tracks. Mesoscale cyclones can generate precipitation and severe weather conditions and thus present a critical forecasting challenge.

Corresponding author address: Dr. David H. Bromwich, Polar Meteorology Group, Byrd Polar Research Center, The Ohio State University, 1090 Carmack Rd., Columbus, OH 43210. Email: bromwich@polarmet1.mps.ohio-state.edu

Abstract

The mesoscale cyclone activity observed in the portion of Antarctica that faces the South Pacific Ocean and Weddell Sea area is summarized from a study of 1991. In general, area-normalized results reveal much greater mesoscale cyclonic activity over the Ross Sea/Ross Ice Shelf and southern Marie Byrd Land than on both sides of the Antarctic Peninsula. More than 50% of the observed mesoscale vortices are of the comma cloud type. The average diameter of mesoscale vortices is approximately 200 km near Terra Nova Bay, 270 km near Byrd Glacier, and 280 km near Siple Coast. Near the Antarctic Peninsula, the average diameter is about 370 km over the Bellingshausen Sea and 380 km on the Weddell Sea side. The largest percentage of deep vortices occurs over the Bellingshausen Sea sector (38% of all cases), where convective instability frequently occurs. Over the Ross Sea/Ross Ice Shelf and Weddell Sea sectors the majority of the mesoscale vortices are low cloud features that probably do not exceed the 700-hPa level due to the prevailing lower-atmospheric stability. The areas identified as sources of mesoscale vortices concur with the locations of enhanced katabatic winds.

A synthesis of the available literature leads to some general characteristics of mesoscale cyclone formation and development. Mesoscale cyclogenesis is associated with areas of warm and/or cold air advection, low-level baroclinicity, and cyclonic vorticity resulting from the stretching mechanism. Subsequent intensification depends on the presence of upper-level support. Spatial and temporal variability in mesoscale cyclone formation is often related to the behavior of synoptic-scale cyclone tracks. Mesoscale cyclones can generate precipitation and severe weather conditions and thus present a critical forecasting challenge.

Corresponding author address: Dr. David H. Bromwich, Polar Meteorology Group, Byrd Polar Research Center, The Ohio State University, 1090 Carmack Rd., Columbus, OH 43210. Email: bromwich@polarmet1.mps.ohio-state.edu

1. Introduction

Many subsynoptic-scale cyclone studies in the high latitudes of the Southern Hemisphere were carried out during the last two decades of the twentieth century. These cyclonic systems, which are less than 1000 km in horizontal diameter, were first intensively studied in the Northern Hemisphere. They were identified as polar lows, although in the current meteorological literature they are referred to by a multitude of terms, including subsynoptic-scale (or mesoscale) cyclones (or vortices), mesocyclones, mesovortices, arctic or Antarctic polar depressions, arctic hurricanes, mesoscale instabilities, etc. (Rasmussen 1992). At the Fifth Meeting of the European Geophysical Society's Polar Lows Working Group (held in 1994) a polar low was defined as an intense maritime cyclonic vortex that develops poleward of a polar front, whose horizontal scale does not exceed 1000 km, and has a surface wind speed over 15 m s−1. According to this definition, the vast majority of the subsynoptic-scale cyclones in the Southern Hemisphere are not true polar lows, even if they undergo intense development. This is because the formation and development can take place far south of the open ocean near the coastal margin of Antarctica, and not necessarily on the poleward side of a polar front.

Observational and numerical simulation studies in the Southern Hemisphere reveal that cyclonic activity at the subsynoptic scale can occur throughout the year at all latitudes and longitudes around the Antarctic continent. They can form and develop near the continental coast, on the ice shelves, over and near the northern margin of the sea ice zone, and over the ice-free Southern Ocean. These cyclonic perturbations have a horizontal diameter that ranges from a few hundred to 1000 km, but it is typically between 100 and 500 km. To distinguish these cyclonic perturbations that are commonly observed in the Southern Hemisphere from the “true” polar lows, they are herein referred to as mesoscale cyclones or vortices.

This paper synthesizes studies by Carrasco and Bromwich (1996a,b) and Carrasco et al. (1997a,b) of 1 yr (1991) of mesoscale cyclone behavior using satellite imagery. It provides for the first time an integrated, high-resolution, satellite-based evaluation of mesoscale cyclone activity over a large fraction of Antarctica. The regions of emphasis are the Ross Sea/Ross Ice Shelf region, Marie Byrd Land, and the Antarctic Peninsula (Fig. 1). The mechanisms facilitating mesoscale cyclogenesis in these regions are investigated. Finally, a review is presented on the relationship of mesoscale cyclones with Antarctic weather.

2. Data

A survey of mesoscale cyclones is performed by examining 1 yr (1991) of digital Advanced Very High Resolution Radiometer (AVHRR) satellite imagery collected in situ from the High Resolution Picture Transmission data stream at the U.S. McMurdo and Palmer stations (Van Woert et al. 1992; Whritner et al. 1998). Pass imagery from the two locations collectively covers an area that spans 160°E eastward to 10°W, including the southern polar oceans and a vast portion of the Antarctic continent. At least two images per day are available most of the time. The satellite images are processed via the Terascan software package in a digital infrared format using channel 5 (11.50–12.50 μm) with a spatial resolution of 3.3 km centered over the Ross Sea, Marie Byrd Land, or the Antarctic Peninsula. This resolution allows coverage of large areas for identification of vortex cloud signatures and tracking of mesoscale cyclones. Identification of mesoscale vortices is based upon the recognition of cloud signatures following the general patterns described in previous studies (e.g., Forbes and Lottes 1985; Carleton and Fitch 1993; Carrasco and Bromwich 1994). Table 1 describes the cloud pattern and classification scheme used by Carrasco and Bromwich (1996a,b) and Carrasco et al. (1997a,b) in comparison to those used by Forbes and Lottes (1985) and Carleton and Fitch (1993). In general, the interpretation of the cloud signatures is similar. However, in the case of Forbes and Lottes (1985) and Carleton and Fitch (1993) additional types are included according to the stage of development of the vortex cloud.

3. Observational satellite studies

a. Spatial and temporal distribution

1) Findings from this study

Through the 1-yr (1991) satellite survey of mesoscale vortices, it is found that the most active area is the southwestern corner of the Ross Sea. Figure 2 shows the annual normalized distribution of all mesoscale vortices initially detected for the entire region under investigation. The annual number of mesoscale vortices is normalized to an equal unit area (=10 000 km2) to overcome, in part, the distortion of the polar stereographic projection. Results reveal that 5.0–6.0 mesoscale vortices (10 000 km2)−1 yr−1 are observed within the defined area near Terra Nova Bay (region 1 in Fig. 1). The second and third most active areas are located near Byrd Glacier [region 2; 3.0–4.0 vortices (10 000 km2)−1 yr−1] and the southernmost part of Marie Byrd Land [region 3; 2.0–2.9 vortices (10 000 km2)−1 yr−1], respectively. The activity per unit area decreases farther eastward over the Ross Sea and Ross Ice Shelf. The mesoscale cyclonic activity on both sides of the Antarctic Peninsula is more homogeneously distributed than over the Ross Sea area. The annual normalized distribution suggests a slightly higher activity just to the northwest of the peninsula, and offshore from the Filchner–Ronne Ice Shelf and Coats Land. By normalizing these regions to the same unit area used over the Ross Sea/Ross Ice Shelf, it can be seen that, in general, much greater mesoscale cyclonic activity occurs over the Ross Sea/Ross Ice Shelf and southern Marie Byrd Land than on both sides of the Antarctic Peninsula, at least during 1991. While 58 mesoscale cyclones are observed within region 1 near Terra Nova Bay (approximately 100 000 km2) during the year, a maximum of 9–10 mesoscale vortices per equivalent area are observed near the Antarctic Peninsula for the same period.

Figure 3 shows the annual time series of mesoscale vortex occurrences found in the satellite imagery during 1991 over the Ross Sea/Ross Ice Shelf region (ROSS), southern Marie Byrd Land (MBL), Bellingshausen Sea (BSS), and Weddell Sea (WSS) sectors. The monthly number of mesoscale cyclones is normalized by taking into account the number of days with available satellite images. No homogenization is done this time to overcome the incongruities in spatial distribution introduced by the map distortion. Therefore, the monthly results of each sector are not directly comparable to each other. In general, mesoscale cyclonic activity tends to be less during the winter (i.e., May–September) than during the spring and summer seasons (i.e., October–March), at least in 1991. The observation of maximum activity during summer concurs with results found by Heinemann (1990) and Turner et al. (1996). On the other hand, Carleton and Song (1997) find a high frequency of mesoscale cyclogenesis during the transition months (April and October). However, they do not include the summer months in their study, so no conclusion as to the actual seasonal behavior of mesoscale cyclones can be derived from their work.

The prevalent mesoscale vortex tracks in the regions under consideration are schematically presented in Fig. 4. The main trajectories indicate that mesoscale cyclones move away from the southwestern corner of the Ross Sea in a northeastward or east-southeastward direction. Over the Ross Ice Shelf the dominant trajectory is toward the northwest, parallel to the Transantarctic Mountains, revealing mesoscale vortices propagating away from southern Marie Byrd Land. Near Byrd Glacier, mesoscale cyclones seem to remain nearly stationary, although a subtle northeastward tendency is suggested. Over the southeastern South Pacific Ocean, mesoscale cyclone tracks indicate a preferential eastward progression toward the Drake Passage. Some of the mesoscale vortices follow a northeastward course toward the southern tip of South America. Many of these mesoscale cyclones are well-developed systems and correspond to the type studied by Lyons (1983); these are indicated by the broad arrows in Fig. 4. Over the Weddell Sea, the main trajectory is toward the northeast, with many mesoscale vortices initially observed over the Filchner–Ronne Ice Shelf. Eastward movement of mesoscale cyclones is also found over the southern Indian and Pacific Oceans (Carleton and Song 1997).

It is relevant to discuss the synoptic situation for 1991 with respect to the climatology, as mesoscale cyclogenesis is a function of synoptic variability (e.g., Carleton and Song 1997, 2000). Figure 5 shows the sea level pressure (SLP) anomalies for 1991 for the Southern Hemisphere taken from the European Centre for Medium-Range Weather Forecasts Tropical Ocean Global Atmosphere (ECMWF/TOGA) operational archives (the upper-level geopotential fields are similar). The most prominent feature is the strong positive anomaly over the Amundsen and Bellingshausen Seas (approximately +5 hPa), reflecting the onset of a warm El Niño–Southern Oscillation (ENSO) event beginning in February 1991. This is in agreement with Carleton and Song (2000), who cite other authors and note that during a warm ENSO event, the time-averaged synoptic-scale low pressure system in the Amundsen Sea is weaker. They find that this results in a decrease in mesoscale cyclones occurring in the Amundsen and Bellingshausen Seas. In the Weddell Sea, there is a very small negative SLP anomaly in the western basin, and a small positive SLP anomaly in the eastern basin. However, in general, the synoptic conditions in the Weddell Sea in 1991 are near normal. In the Ross Sea, the SLP anomaly is nearly zero in the western basin and over the Ross Ice Shelf (the areas of prominent mesoscale cyclogenesis). This suggests that with respect to synoptic forcing, the mesoscale cyclone activity for 1991 is not far from climatological values. This is in agreement with Carrasco and Bromwich (1996a), who compare the mesoscale activity in the Ross Sea for 1991 with that of 1985 and 1988, and find that 1985 actually has the highest number of “significant” mesoscale cyclones, suggesting that 1991 is not an anomalously active year in terms of mesoscale cyclogenesis. Seasonal analyses of SLP and upper-level anomalies for 1991 are similar to the annual findings shown in Fig. 5, and are available in the Australian Bureau of Meteorology Climate Monitoring Bulletins, as well as seasonal climate summaries for the Southern Hemisphere (Gaffney 1991; de Hoedt 1992; Beard 1993).

2) Findings from related studies

Turner et al. (1996) perform a similar study of 1 yr of satellite imagery (March 1993–February 1994) in the vicinity of the Antarctic Peninsula. They also find a spatial distribution similar to that shown in Fig. 2, with a maximum activity of mesoscale cyclones just to the northeast of the Amundsen Sea, and another offshore from Coats Land (their Fig. 12). Heinemann (1990), Turner and Thomas (1994), and Carrasco and Bromwich (1997) also find the latter maximum. In a subsequent study of 1 yr (March 1993 to February 1994) of cyclogenesis in the area bounded by 50°–90°S, 0°–100°W, Turner et al. (1998) find a maximum of 0.48 events (10 000 km2)−1 yr−1 in the Bellingshausen Sea. In addition, they find a secondary maximum in the lee of the Antarctic Peninsula. While the study focuses on synoptic-scale events (including synoptic-scale events that start or end as “mesocyclones”), the results are similar to those shown in Fig. 2. In addition, Carleton and Song (1997), using Geostationary Meteorological Satellite infrared (GMS IR) images, study mesoscale cyclone activity over the Australian sector of the southern Indian and Pacific Oceans during the autumn–spring period of 1992, north of the Antarctic coastline. They detect mesoscale cyclogenesis throughout their study area in the Southern Ocean, with a maximum activity area to the southwest of Australia (centered about 50°S, 80°E), and another to the south of New Zealand. They also show mesoscale cyclone activity over the southeastern corner of the South Pacific Ocean, including the offshore sector of southern South America. Their analyses indicate that cyclogenesis takes place within colder air masses. It is noteworthy that the above-mentioned studies analyze satellite imagery over short periods (1 yr or less), and results must be interpreted in the light of interannual variability.

b. Satellite characteristics of mesoscale vortices

1) Type and size

The satellite characteristics of mesoscale vortices are summarized in Table 2. Overall more than 50% of the observed mesoscale vortices are of comma cloud type. The dominance of this type has been found in similar studies conducted by Carleton and Carpenter (1989), Carleton and Fitch (1993), Turner and Thomas (1992), and Carrasco and Bromwich (1994). The average diameter of mesoscale vortices in proximity to katabatic wind confluence zones (Parish and Bromwich 1987) is approximately 200 km near Terra Nova Bay, 270 km near Byrd Glacier, and 280 km near the Siple Coast. Adjacent to the Antarctic Peninsula, the average diameter is about 370 km over the Bellingshausen Sea and 380 km over the Weddell Sea. About 35% (59%) of the vortices fall within the 300–399-km (200–399 km) diameter size. Turner et al. (1996) also find a dominant diameter modal range of 300–399 km in the Antarctic Peninsula region. Carleton and Song (1997) determine an average diameter of 354 km from their examination of GMS IR images. This compares well with the 370-km diameter obtained over the Bellingshausen Sea sector, implying that the open ocean provides favorable conditions for the development of larger cloud structures associated with mesoscale cyclones.

The largest percentage of deep vortices (defined as those that show middle/white cloud signatures associated with them on a grayscale satellite image) occur over the Bellingshausen Sea sector (38% of all cases), which is significantly larger than elsewhere in the study region. Over the Ross Sea/Ross Ice Shelf and Weddell Sea sectors, the majority of the mesoscale vortices are low cloud features that probably do not exceed the 700-hPa level. These sectors are characterized by (in contrast to the Bellingshausen Sea sector) the presence of a huge ice shelf and an extended area of sea ice. In addition, numerical model results (Parish and Bromwich 1987, 1991; Carrasco 1994; Guo et al. 2003) indicate that these sectors are affected by katabatic drainage. The outflow horizontally propagates northward across the ice shelves, after converging into the area through glaciers that dissect the coastal margin of Antarctica. This creates a more stable environment over the Ross Sea/Ross Ice Shelf and Weddell Sea sector than over the Bellingshausen Sea sector.

Stable conditions inhibit deep vertical development of mesoscale vortices. Individual case studies in these stable regions (Carrasco and Bromwich 1993a; Turner et al. 1993; Heinemann 1996a; Bromwich et al. 2003) show that well-developed mesoscale vortices coincide with well-defined synoptic-scale upper-level support. Over the Bellingshausen Sea sector the maximum northward extent of sea ice lies to the south of 65°S, with open ocean to the north. A strong low-level thermal contrast exists at the sea ice edge, with cold air over the sea ice and warmer conditions over the open ocean. Cold air outbreaks into this area can quickly reach the sea ice front and the warmer open ocean to the north. The large air–sea temperature contrast over the open ocean favors convection and then vertical development of mesoscale vortices. The enhanced area of activity north of the Bellingshausen Sea occurs near the sea ice edge where formation of mesoscale cyclonic perturbations is favored within a relatively unstable environment. Similar conditions have been found in the Northern Hemisphere in the winter season over the Norwegian Sea, Barents Sea, and Gulf of Alaska.

2) Detection of dry vortices in satellite data

Automatic weather station (AWS) data are available with relatively good spatial coverage over the Ross Ice Shelf and western Ross Sea, more so than any other region of Antarctica. The pressure, wind, and temperature data from this network are useful information when applied in conjunction with satellite imagery, often allowing quantification of the magnitude of satellite-observed mesoscale features. In addition, AWS sites are frequently the only means of resolving cyclones when cloud signatures are absent from satellite imagery, especially in the (absolutely dry) winter season (e.g., Bromwich et al. 2003; Heinemann and Klein 2003). In this study, these data are used to construct regional sea level pressure charts twice a day for the entire year. The weekly frequency of mesoscale cyclones obtained from the satellite imagery is 1.5 near Terra Nova Bay and 0.6 near Byrd Glacier. Combined analyses of satellite imagery and the sea level pressure charts indicate that the weekly frequencies of mesoscale cyclones in these areas are 2.5 and 1.8, respectively. This implies that approximately 40% (70%) of the mesoscale cyclones resolved near Terra Nova Bay (Byrd Glacier) do not develop a well-defined cloud signature and/or they are cyclonic dry features. As previously suggested by Bromwich (1989) and Carrasco and Bromwich (1994) the lack of moisture due to the Ross Ice Shelf and the presence of sea ice inhibits cloud formation, a situation most common during the winter months. This indicates that the results obtained only from satellite imagery may be underestimating the mesoscale cyclonic activity over the Ross Sea, Ross Ice Shelf, and Marie Byrd Land.

4. Mesoscale cyclone formation and development: A discussion

a. Warm and/or cold air advection

Bromwich (1989) and Carrasco and Bromwich (1993a) show that warm-air advection plays an important role in mesoscale cyclogenesis near Terra Nova Bay and Byrd Glacier. Similarly, a case study near the Siple Coast (Bromwich and Carrasco 1995) associates the development of an intense mesoscale cyclone with warm-air advection into southern Marie Byrd Land. Thus, mesoscale cyclones over the Ross Sea/Ross Ice Shelf and Marie Byrd Land regions form when a warmer synoptic-scale circulation affects the area. The high concentration of mesoscale cyclones in combination with the warm synoptic environment suggests that mesoscale cyclogenesis occurs by the interaction between cold katabatic airflow and warmer air offshore from the continent and/or advected into the interior. The associated warm 1000–500-hPa thickness pattern is not statistically different (95% confidence level using a two-tailed t-test) from the seasonal average (Carrasco and Bromwich 1996a; Carrasco et al. 1997a). Similarly, a warmer environment is also suggested over the Weddell Sea on some occasions (Carrasco et al. 1997b).

In contrast, mesoscale cyclones on both sides of the Antarctic Peninsula are typically associated with a colder environment (statistically significant). These events are most likely related to cold-air outbreaks from Antarctica, with northward cold-air advection near the Antarctic Peninsula providing the conditions for mesoscale cyclogenesis over the Bellingshausen Sea sector. This suggests that the main role of the synoptic-scale circulation over the Ross Sea/Ross Ice Shelf is to provide southward warm-air advection that interacts with cold katabatic winds to establish boundary layer fronts. The opposite may be true in the Antarctic Peninsula region, where synoptic-scale systems most likely provide northward cold-air advection that interacts with the (relatively) warm maritime air. Both cases imply that events near Antarctica associated with synoptic-scale storms are important factors modulating the mesoscale cyclonic activity over the southern South Pacific Ocean.

b. Low-level baroclinicity

Gallée (1995, 1996) conducts idealized regional studies of mesoscale cyclogenesis over the southwestern Ross Sea sector using a 5-km resolution model. His limited-area model domain is restricted to the Ross Sea/northern Ross Ice Shelf and immediate surrounding areas. The findings for the late summer case (1995) and the winter case (1996, polar night), neither having any large-scale wind forcing, are similar. Katabatic airflow induces the formation of a mesoscale boundary layer front and a subsequent mesoscale cyclone to the south of Terra Nova Bay. Simulated sensible heat flux from ice-free ocean (and/or polynyas and leads) further supports the formation and deepening of the simulated mesoscale cyclone. The author indicates that baroclinic processes associated with the boundary layer front appear to be the main mechanism for mesoscale cyclogenesis.

Carrasco (1994), using a 20-km resolution model for the whole continent and a large portion of the surrounding open ocean, also studies mesoscale cyclogenesis near Terra Nova Bay (and other areas) during the polar night. Similar to the work of Gallée (1995, 1996), the simulation is initiated from a state of rest (no synoptic forcing), ensuring that any wind forcing results from katabatic winds generated by the radiative cooling in the model. In contrast to the work of Gallée (1995, 1996), no lateral boundary conditions are imposed. It is also noteworthy that the simulation is run with no sea ice cover to enhance the air–sea temperature difference. Parish (1992) shows that the presence or absence of sea ice has little effect on the simulated katabatic wind regime. Carrasco's results show that the first mesoscale cyclone forms after 42 h of model integration and takes place to the south of Terra Nova Bay. This area is to the south of the low-level katabatic jet stream that blows offshore from Terra Nova Bay. Similar to the findings of Gallée (1995, 1996), the katabatic winds facilitate accelerated development of an offshore baroclinic zone, which appears to be the trigger mechanism for formation of the mesoscale cyclone. Because the mesoscale cyclogenesis takes place on the south side of the katabatic airflow, where cyclonic shear occurs, Carrasco (1994) suggests that barotropic instability may be the initial trigger mechanism for formation of the mesoscale vortex. However, the incipient cyclonic circulation shown by the simulated streamlines (before the actual cyclone forms) and the characteristics of the cloud signatures revealed by case studies indicate that baroclinic instability takes over immediately after the initial stage of formation for subsequent development. The simulated cyclonic circulation does not extend beyond the third sigma level (below 700 hPa). This concurs with the satellite observations, which usually indicate low cloud signatures associated with the mesoscale vortices near Terra Nova Bay. Three mesoscale cyclones form during the 10-day model integration and move eastward from the southwestern corner of the Ross Sea. These results coincide with the observations, which show that, on average, two to three mesoscale cyclones form each week in this area (Bromwich 1991; Carrasco and Bromwich 1994, 1996a), with many of these propagating eastward.

c. Other forcing mechanisms

Engels and Heinemann (1996) and Heinemann (1996a,b) use the Norwegian Limited-Area Model (NORLAM) with 25-km resolution to simulate three cases of mesoscale cyclogenesis over the Weddell Sea sector during the summer. They find that the production of cyclonic vorticity by the stretching mechanism is responsible for the formation of mesoscale cyclones. This takes place when synoptically assisted katabatic airflow descends the steep coastal slopes of the Antarctic continent. Carrasco and Bromwich (1995) also find that cyclonic vorticity (via the stretching mechanism) contributes to the development of a major cyclone over the southern Ross Sea/Ross Ice Shelf sector from a mesoscale cyclone over the East Antarctic plateau. In a simulated study of this case, Heinemann and Klein (2003) find the same results.

Klein and Heinemann (2001) use NORLAM with 25-km resolution to examine mesoscale cyclogenesis in the eastern Weddell Sea for various initial conditions during summertime. They find that the cyclone is forced by an interaction of several mechanisms at different stages of development. First, a strong topographic gradient gives rise to katabatic winds. As they move downslope, the winds stretch vertically, producing cyclonic vorticity. Often, nearby synoptic-scale support strengthens the vertical stretching, and prevents the flow from dissipating as it reaches the bottom of the coastal slope. Next, the existence of a coastal polynya or open water near the coast provides an environment for enhanced sensible and latent heat fluxes, creating low-level baroclinicity between the open water and the continent. This allows for the release of latent heat and subsequent warming and moistening of the atmosphere over the polynya, contributing to further cyclone development.

Finally, Heinemann and Klein (2003), employing NORLAM, simulate mesoscale cyclone formation and development for observed case studies in the Weddell Sea sector, and for the case studies of Carrasco and Bromwich (1993a, 1995) in the Ross Sea sector. Their general conclusions are that the Antarctic topography plays an essential role in mesoscale cyclogenesis, mainly in those places where the convergence of katabatic airflow provides cyclonic shear for the initial formation (i.e., Terra Nova Bay and Byrd Glacier). However, subsequent development of the mesoscale vortex requires upper-level synoptic-scale support to become a major subsynoptic (or even synoptic) system.

5. A review of mesoscale cyclones and antarctic weather

a. Mesoscale cyclones and large-scale climatological features

Parish (1992; also see Parish and Bromwich 1991) indicates that the katabatic wind regime is responsible for the establishment of a polar vortex, which in turn decays the katabatic drainage. The same results are obtained earlier by James (1989) and Egger (1985, 1992). As mentioned by these authors, the fact that the horizontal pressure gradient associated with the vortex weakens the katabatic airflow suggests that the descending cold air should be a transient phenomenon and not persistent, as is actually observed throughout the winter. This implies that certain mechanisms must weaken the polar vortex so that the horizontal pressure gradient associated with the vortex is less effective in opposing the katabatic drainage. James (1989) and Parish (1992) suggest that the midlatitude synoptic-scale cyclones that spiral toward and decay near the Antarctic continent can remove the excess vorticity from the polar vortex, thereby weakening the horizontal pressure gradient and maintaining the katabatic drainage.

The asymmetry of the continent with respect to the geographic pole and gravity wave drag may also provide mechanisms that weaken the polar vortex (James 1989). One mechanism is related to Rossby wave generation that advects cyclonic vorticity toward lower latitudes from the polar region. Another has to do with small-scale gravity waves generated by the Antarctic topography that may enter the upper troposphere and lower stratosphere, decreasing the westerly winds (James 1989). James (1989) also mentions a mechanism that might be associated with mesoscale cyclones that form around the edge of Antarctica. If they form frequently and are deep enough, they may contribute to weakening the polar vortex by extracting energy, allowing the persistence of the drainage flow and/or its reestablishment. Based on the statistics for deep vortices listed in Table 2, this mechanism could only be active in the Bellingshausen Sea sector.

b. Mesoscale cyclones and precipitation

General circulation modeling studies (Murray and Simmonds 1991; Tzeng et al. 1993) resolve a subpolar convergence zone that may be associated with the mesoscale cyclogenesis that takes place near the Antarctic coastline. In the latter study, this convergence zone plays an important role in simulated snowfall generation over the Antarctic coastal slopes, indicating that mesoscale cyclone activity can significantly contribute to the total precipitation in some coastal zones. This significant contribution of mesoscale cyclone activity to coastal precipitation is discussed below for two active regions.

1) The Ross Sea region

Several studies specific to the Ross Sea region associate meteorological conditions with mesoscale cyclones. Rockey and Braaten (1995) suggest that about 38% of the precipitation at McMurdo station is associated with mesoscale vortices that form and/or develop nearby. In fact, the maximum precipitation at McMurdo station occurs in March, which coincides with the period of maximum mesoscale cyclone activity found in the southwestern corner of the Ross Sea in 1991 (Fig. 3). Trajectories reveal that these cyclonic perturbations also contribute to the snowfall along the Transantarctic Mountains and over the Siple Coast. O'Connor et al. (1994) find that mesoscale cyclones moving along the Transantarctic Mountains can set up conditions for the development of barrier winds that result in gale force winds at McMurdo station. Smith et al. (1993) describe a blizzard (with sustained winds of 28 m s−1, gusting over 35 m s−1) encountered by a team of meteorologists in 1992 during a field campaign in southern Marie Byrd Land. In a subsequent study, Bromwich and Carrasco (1995) reveal that this intense storm was associated with the development of a mesoscale cyclone that moved northwestward parallel to the mountains.

2) The Antarctic Peninsula

Adverse weather conditions caused by mesoscale cyclones are also observed in the vicinity of the Antarctic Peninsula. Lyons (1983) was the first to study the characteristics of intense mesoscale depressions in the region. He found that cyclones developing and moving toward the southern tip of South America can cause moderate and severe weather conditions. A study of the origin of the precipitation that affects the southern tip of South America in 1992 (recorded at the Punta Arenas and Puerto Williams stations) reveals that during some months 30%–50% of the total precipitation is associated with subsynoptic-scale cyclone perturbations (Flores 1996). An analysis of the annual precipitation in 1991 at Eduardo Frei station, located at the northern tip of the Antarctic Peninsula, reveals that approximately 40% of the precipitation events are not associated with passing frontal systems. The trajectories of the mesoscale cyclones detected on satellite images during 1991 indicate that Frei station is affected by some of the vortices, suggesting that at least a fraction of the nonfrontal precipitation may be associated with mesoscale cyclone activity. In contrast, Turner et al. (1995), during a study of 1 yr of AVHRR imagery (March 1992–February 1993), find that none of the precipitation at Rothera (67°34′S, 68°08′W; Fig. 1) is attributable to “mesocyclones.” Rather, most of the precipitation is associated with synoptic-scale cyclones, 50% of which form south of 60°S. This high number of cyclones forming in situ suggests that some might be diagnosed on the mesoscale level in the early stages of development. Thus, in comparison to studies such as this, there may be some overlap between the mesoscale and the synoptic scale.

c. Mesoscale cyclones and synoptic events

Studies (e.g., Streten and Troup 1973; Carleton 1979; Sinclair 1994, 1995) show two main trajectories of synoptic-scale storms over the South Pacific Ocean: one from the northeast of New Zealand toward Drake Passage and the other from south of Australia toward the Amundsen Sea region. High latitudes in the southern Pacific Ocean are confirmed as active regions of synoptic-scale cyclone movement, including a high frequency of decaying synoptic-scale cyclones located near the Antarctic Peninsula and Amundsen Sea. The magnitude and frequency of storms along these tracks would suggest adequate support for mesoscale cyclogenesis, via mechanisms such as decay, advection of warm and/or cold air, upper-level support, and the initiation (and subsequent sustenance) of katabatic wind events. What happens then, when one of the synoptic-scale storm tracks prevails? Does, on average, the mesoscale cyclonic activity increase (decrease) if a storm track strengthens (weakens) in a given region?

This question is addressed in Carleton and Fitch (1993), who note an interannual spatial variation in mesoscale cyclonic activity over the Ross Sea sector and southeastern South Pacific Ocean (in their winter analysis). They find more mesoscale cyclonic activity over the Ross Sea region than near the Antarctic Peninsula for the winter of 1988. The situation is reversed for winter 1989. The enhanced activity over the Ross Sea (Antarctic Peninsula) in 1988 (1989) is associated with negative anomalies in the sea level pressure field in the Ross Sea (Antarctic Peninsula) and positive anomalies near the Antarctic Peninsula (northern Ross Sea), which can be related to the interannual variability of the synoptic-scale storm tracks. In a related study, a numerical simulation using the 1988 pressure field is conducted by Bromwich et al. (1994), which shows that an acceleration of the katabatic winds in Terra Nova Bay and Byrd Glacier takes place. Carrasco and Bromwich (1996a) investigate several years of mesoscale cyclone activity in the Terra Nova Bay, Byrd Glacier, and Marie Byrd Land regions. They conclude that increased synoptic-scale support implies more vigorous mesoscale cyclones and that katabatic winds provide the primary forcing for mesoscale cyclogenesis. Other studies, without implicitly relating katabatic winds to mesoscale cyclogenesis, have also demonstrated the influence of synoptic-scale weather on katabatic wind fields in the Ross Sea/Ross Ice Shelf region (Bromwich et al. 1992, 1993, 1994; Carrasco and Bromwich 1993b). The findings of these studies suggest that the primary role of synoptic-scale systems is to modify the katabatic winds, which in turn provide ideal environments for mesoscale cyclogenesis.

In the Weddell Sea and Bellingshausen Sea sectors, cold-air outbreaks are supported on the western side of synoptic-scale cyclones passing across the Antarctic Peninsula and/or Drake Passage, or decaying over or to the east of the Weddell Sea. Heinemann (1990) observes that mesoscale cyclones over the Weddell Sea are usually associated with synoptic-scale cyclones centered to the east of or over the Bellingshausen Sea or to the east of the Weddell Sea. With synoptic cyclones to the east of the Bellingshausen Sea cold air outbreaks from the interior of East Antarctica can be supported. With synoptic cyclones over the Bellingshausen Sea, warm-air advection as well as lee cyclogenesis can occur. Klein and Heinemann (2001), using a mesoscale model, demonstrate that a synoptic low located in the northeastern Weddell Sea is favorable for mesoscale cyclogenesis in the Weddell Sea. However, when they place the same low 1500 km to the west, mesoscale cyclogenesis does not occur. These findings strongly suggest that spatial variability in mesoscale cyclone formation is associated with the preferential movement of synoptic-scale cyclones, modifying areas of cyclogenesis and cyclolysis. The importance of this relation has been highlighted in recent studies (e.g., Jones and Simmonds, 1993; Turner et al. 1995, 1998; Carleton and Song 1997, 2000; Simmonds et al. 2003), which have shown that, in addition to cyclones that form in the midlatitudes and track south and east, a significant percentage of synoptic-scale storms originate in the Antarctic region (south of 60°S). It is likely that many of these systems grow from mesoscale cyclones and likewise, when mature, contribute to subsequent mesoscale cyclogenesis.

6. Conclusions

From the spatial frequency distribution of mesoscale vortices and their trajectories during 1991, the Terra Nova Bay and Byrd Glacier sectors are confirmed as mesoscale cyclogenetic regions. Southern Marie Byrd Land is also confirmed as a source of mesoscale cyclones. To the west of the Antarctic Peninsula, the source areas of mesoscale cyclones are not clearly resolved. The initial appearance of many vortices suggests that the area just to the north of the Bellingshausen Sea may be a cyclogenetic region where the formation and/or development of mesoscale cyclones occurs near the northern edge of the sea ice. The few mesoscale cyclones moving away from the Amundsen Sea may reveal another region of cyclogenesis. Over the Weddell Sea, two areas seem to be cyclogenetic: one offshore from the Filchner–Ronne Ice Shelf and the other approximately 200 km north of Coats Land.

In Fig. 6, areas of maximum annual normalized distribution of mesoscale vortices (consistent with areas of maximum mesoscale activity shown in Fig. 2) are superimposed on the katabatic wind drainage of Antarctica simulated by Parish and Bromwich (1987). The areas identified as sources of mesoscale vortices over the Ross Sea and Ross Ice Shelf coincide with the locations of the katabatic wind confluence zones near Terra Nova Bay, Byrd Glacier, and the Siple Coast. Over the Amundsen Sea the source is near the katabatic wind confluence zones affecting Walgreen Coast. The source located offshore from the Filchner–Ronne Ice Shelf is located in an area affected by cold air outbreaks, probably associated with katabatic airflows propagating northward from the ice shelf. The proximity of these areas of maximum cyclone formation to katabatic wind confluence zones suggests a strong connection between the two.

A significantly larger number of deep vortices are detected over the Bellingshausen sector than in any other area of the study region (Ross Sea/Ice Shelf, Marie Byrd Land, and Weddell Sea sectors). This is due to an unstable environment favoring the formation and development of mesoscale cyclonic perturbations. The greater instability in the Bellingshausen Sea sector is, in part, attributable to the northern limit of the sea ice pack being constrained to high latitudes, leaving a large year-round open ocean area to the north (Carleton and Song 2000). The subsequent development of larger mesoscale vortices requires synoptic-scale upper-level support, as has been revealed by individual case studies.

Over the southwestern corner of the Ross Sea and over the Ross Ice Shelf, where there is a network of automatic weather stations, mesoscale sea level pressure analyses are constructed twice a day for 1991. Results reveal greater mesoscale cyclonic activity near Terra Nova Bay and Byrd Glacier than is obtained by examination of satellite imagery alone. This indicates that a number of mesoscale cyclones do not develop a cloud signature due to the lack of moisture, mainly during winter. This may also be true for other sectors of Antarctica, but cannot be confirmed due to the sparse observational network.

The evidence suggests that the Ross Sea/Ross Ice Shelf region is the most active mesoscale cyclogenesis region in the study area, which ranges from the Ross Sea eastward to the Weddell Sea. The likely reasons for this are the high frequency of katabatic wind events coupled with the synoptic activity (at both the surface and upper levels) associated with the midtropospheric circumpolar vortex that is centered just to the northeast of the Ross Ice Shelf.

Mesoscale cyclones contribute to the annual amount of precipitation in many coastal areas of Antarctica. Occasionally, they can develop into major features causing moderate and severe weather conditions. Mesoscale cyclonic circulation near the Transantarctic Mountains may set up conditions for the development of barrier winds. In addition, interactions with the katabatic wind regime and synoptic-scale systems indicate the important role mesoscale cyclones play in the dynamics of the high-latitude circulation. The misrepresentation that some numerical models show in simulating the atmospheric circulation in the southern polar region can be, in part, attributable to the resolution of models, which do not capture the mesoscale cyclonic activity. Studies using high-resolution models reveal that mesoscale cyclones can be simulated, although the intensity tends to be underestimated (Heinemann and Klein 2003). Further studies are needed in this regard to better understand and improve regional and global atmospheric models, especially those used for numerical weather prediction.

The conditions associated with synoptic-scale cyclones often provide mechanisms that support mesoscale cyclogenesis (i.e., decay, advection of warm/cold air, upper-level support, and the initiation/sustenance of katabatic wind events). As such, the spatial and temporal variability in mesoscale cyclone formation is often related to the behavior of synoptic-scale cyclone tracks and to the occurrence of individual synoptic events. Many satellite-based studies (including this one) cover periods of 1 yr or less, and the effect of the unremoved interannual variability may diminish their climatological significance.

Acknowledgments

This research was supported by the National Science Foundation, Office of Polar Programs Grant OPP-9117448, and published with funding from NASA Grant NAG5-9518. Satellite images were obtained from Mr. Robert Whritner of the Arctic and Antarctic Research Center at Scripps Institution of Oceanography. Automatic weather station data were obtained from Charles R. Stearns of the Antarctic Meteorology Research Center at the University of Wisconsin—Madison. ECMWF/TOGA data were obtained from the National Center for Atmospheric Research.

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Fig. 1.
Fig. 1.

Location map of the Pacific sector of the Antarctic continent. Enclosed regions are areas used for study of mesoscale cyclones

Citation: Monthly Weather Review 131, 2; 10.1175/1520-0493(2003)131<0289:DACOMC>2.0.CO;2

Fig. 2.
Fig. 2.

Annual area-normalized distribution of all mesoscale vortices initially detected for the region under investigation in 1991. [Units = vortices (10 000 km2)−1 yr−1]

Citation: Monthly Weather Review 131, 2; 10.1175/1520-0493(2003)131<0289:DACOMC>2.0.CO;2

Fig. 3.
Fig. 3.

Seasonal variation of mesoscale vortex occurrences found during 1991 over the Ross Sea/Ross Ice Shelf region (ROSS), southern Marie Byrd Land (MBL), Bellingshausen Sea (BSS), and Weddell Sea (WSS) sectors

Citation: Monthly Weather Review 131, 2; 10.1175/1520-0493(2003)131<0289:DACOMC>2.0.CO;2

Fig. 4.
Fig. 4.

Schematic depiction of the mesoscale vortex trajectories observed during 1991

Citation: Monthly Weather Review 131, 2; 10.1175/1520-0493(2003)131<0289:DACOMC>2.0.CO;2

Fig. 5.
Fig. 5.

Mean sea level pressure anomaly for 1991 in relation to the 1985–98 average for the Southern Hemisphere using ECMWF/TOGA data. The contour interval is 1 hPa

Citation: Monthly Weather Review 131, 2; 10.1175/1520-0493(2003)131<0289:DACOMC>2.0.CO;2

Fig. 6.
Fig. 6.

Areas of the maximum annual normalized distribution of mesoscale vortices superimposed on the katabatic wind drainage of Antarctica as simulated by Parish and Bromwich (1987)

Citation: Monthly Weather Review 131, 2; 10.1175/1520-0493(2003)131<0289:DACOMC>2.0.CO;2

Table 1. 

Description of the cloud pattern for mesoscale vortices as observed in satellite imagery from Forbes and Lottes (1985), Carleton and Fitch (1993), and Carrassco and Bromwich (1994)

Table 1. 
Table 2. 

Summary of the satellite characteristics of mesoscale vortices adjacent to the Pacific coast of Antarctica and the Weddell Sea sector: TNB; Terra Nova Bay; RS, Ross Sea; BG; Byrd Glacier; RIS, Ross Ice Shelf; SC, Siple Coast; BSS, Bellingshausen Sea sector; and WSS, Weddell Sea sector. Deep vortices are mesoscale cyclones that showed middle/high cloud associated with them in satellite images

Table 2. 

*

Byrd Polar Research Center Contribution Number 1242.

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  • Beard, G. S., 1993: Seasonal climate summary southern hemisphere (spring 1991): A maturing El Niño–Southern Oscillation (ENSO) episode. Aust. Meteor. Mag., 42 , 8186.

    • Search Google Scholar
    • Export Citation
  • Bromwich, D. H., 1989: Subsynoptic-scale cyclone developments in the Ross Sea sector of the Antarctic. Polar and Arctic Lows, P. F. Twitchell, E. A. Rasmussen, and K. L. Davidson, Eds., A Deepak, 331–345.

    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
  • Bromwich, D. H., and J. F. Carrasco, 1995: An investigation of an intense mesoscale cyclone over southern Marie Byrd Land, West Antarctica. Preprints, Fourth Conf. on Polar Meteorology and Oceanography. Dallas, TX, Amer. Meteor. Soc., 238–243.

    • Search Google Scholar
    • Export Citation
  • Bromwich, D. H., J. F. Carrasco, and C. R. Stearns, 1992: Satellite observations of katabatic wind propagation for great distances across the Ross Ice Shelf. Mon. Wea. Rev., 120 , 19401949.

    • Search Google Scholar
    • Export Citation
  • Bromwich, D. H., J. F. Carrasco, Z. Liu, and R. Y. Tzeng, 1993: Hemispheric atmospheric variations and oceanographic impacts associated with katabatic surges across the Ross Ice Shelf, Antarctica. J. Geophys. Res., 98 , 1304513062.

    • Search Google Scholar
    • Export Citation
  • Bromwich, D. H., Y. Du, and T. R. Parish, 1994: Numerical simulation of winter katabatic winds from West Antarctica crossing Siple Coast and the Ross Ice Shelf. Mon. Wea. Rev., 122 , 14171435.

    • Search Google Scholar
    • Export Citation
  • Bromwich, D. H., A. J. Monaghan, J. G. Powers, J. J. Cassano, H. L. Wei, Y. H. Kuo, and A. Pellegrini, 2003: Antarctic Mesoscale Prediction System (AMPS): A case study from the 2000–01 field season. Mon. Wea. Rev., 131 , 412434.

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  • Fig. 1.

    Location map of the Pacific sector of the Antarctic continent. Enclosed regions are areas used for study of mesoscale cyclones

  • Fig. 2.

    Annual area-normalized distribution of all mesoscale vortices initially detected for the region under investigation in 1991. [Units = vortices (10 000 km2)−1 yr−1]

  • Fig. 3.

    Seasonal variation of mesoscale vortex occurrences found during 1991 over the Ross Sea/Ross Ice Shelf region (ROSS), southern Marie Byrd Land (MBL), Bellingshausen Sea (BSS), and Weddell Sea (WSS) sectors

  • Fig. 4.

    Schematic depiction of the mesoscale vortex trajectories observed during 1991

  • Fig. 5.

    Mean sea level pressure anomaly for 1991 in relation to the 1985–98 average for the Southern Hemisphere using ECMWF/TOGA data. The contour interval is 1 hPa

  • Fig. 6.

    Areas of the maximum annual normalized distribution of mesoscale vortices superimposed on the katabatic wind drainage of Antarctica as simulated by Parish and Bromwich (1987)

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