Trajectories of extratropical cyclones formed in the Antarctic Peninsula

The Antarctic continent has a huge importance to global climate system, since it is a sink of heat and energy from lower latitudes. The low pressure belt around the Antarctic continent creates conditions for development of extratropical cyclones (EC) over the Southern Ocean (SO) (Owens and Zawar-Reza, 2015). They’re the main contribution of atmospheric moisture flux in the mid and high-latitudes (King and Turner, 1997), hence, they’re one of the most studied meteorological features.

Early studies attempted to describe the EC’s distribution over the SO (e.g. van Loon 1965; Taljaard, 1967). These authors were able to achieve relatively well distributed results, including the description of a maxima cyclogenesis around Antarctica’s coastline (Taljaard, 1967). Afterwards, similar results were found by several authors, using more sophisticated approaches, such as automated tracking algorithms (e.g. Simmonds and Keay, 2000; Hoskins and Hodges, 2005; Grieger et al., 2018).

The present work is a contribution to AnTarctic Modeling Observation System (ATMOS) project led by the National Institute for Space Research (INPE) which is part of Antarctic Brazilian Program (PROANTAR). Here we investigated EC formed near the Antarctic Peninsula and their trajectories to lower latitudes. We focus on EC that moved towards the central part of South Atlantic Ocean and also to the polar meteo-oceanography cyclogenesis conditions. EC were identified and tracked based on their relative vorticity at 850 hPa, calculated from Climate Forecast System Reanalysis (CFSR/CFSv2) U and V components of wind, using TRACK algorithm (Hodges, 1994, 1995). EC were tracked at interval of six hour, from 1982 to 2019 and the ocean influence was assessed using NOAA Optimum Interpolation Sea Surface Temperature (OISST) dataset (May et al,. 1998), and Mean Sea Level Pressure from CFSR/CFSv2. To be considered a valid EC case, it must has minimum local vorticity of -1x10-5s-1, lasted for 2 days and, displaced at least 1000 km within the area of interest (15° S to 85° S and 110° W to 10° W).

After, four sub-regions within the main area were defined. Three around the Antarctic Peninsula and one near the extreme southern tip of South America (Argentina) and EC trajectories were analyzed from each one of them. The Sea Surface Temperature (SST) anomalies values, genesis density and mean intensity of cyclones (TRACK outputs), were used as choice criteria.

The TRACK algorithm found a total of 1,600 EC over the 38 years. Anomalous displacements towards north/northeast were registered in all four sub-regions, regardless of the season. The sub-region of cyclogenesis located eastern of the Antarctic Peninsula had EC displacing over South America, mostly the south coast of Argentina and Chile. Whilst the EC formed at the north of the Peninsula might have an indirect influence on South America’s southeastern coast, impacting the oceanic swell for instance. The third Antarctic sub-region over the Weddell Sea, had no influence on South America’s climate or oceanic conditions.

The highest number of EC that crossed 35° S were formed on the fourth sub-region and during winter it was observed one cyclone that reached longest displacement, approximately 3,530 km (from 52° S to 21° S) lasting for three days. These results could be linked to Antarctic sea-ice (ASI). Simulations done by Parise et al. (2015) showed the lower-atmosphere cooling under ASI extremes conditions, where the cold air temperature anomalies extended from high to mid-latitudes over the Atlantic ocean. Another explanation could rely on the baroclinicity, since it is the main source of available potential energy to the cyclones at extratropics (Holton, 2004). Machado et al. (2020) using CFSR and another reanalysis dataset from 1979 to 2011 found the highest values of baroclinic instabilities between the latitudinal bands of 60° S and 20° S, especially in winter.

The TRACK’s trajectories agree with previous studies (Simmonds and Keay, 2000, Hoskins and Hodges, 2005, Reboita et al,. 2015), both with the path pattern (mostly eastward and poleward), and with the cyclogenetic areas near the Antarctic Peninsula. The anomalous trajectories found here raise new information about southern EC behavior, since previous studies (e. g. Simmonds and Keay, 2000, Hoskins and Hodges, 2005) did not focus on similar trajectories.

Concluding, few EC moved northward directly influencing South America's southeastern coast. This scenario suggests that teleconnections between Southern Ocean and central sector of the South Atlantic Ocean are possibly more expressive on winter season, which is when cold air masses incursions occur and when there are larger thermal contrasts between mid‐latitudes and the polar region (King and Turner, 1997).

Acknowledgments

This research was funded by the Brazilian agency CNPq, through Antarctic Modeling and Observation System Project (CNPq/PROANTAR 443013/2018-7). Mariana Gandra was also funded by ATMOS (372300/201907) with research grant. We also thank Dr. Kevin Hodges for his assistance with the TRACK algorithm.