EGU General Assembly 2020
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the Creative Commons Attribution 4.0 License.

Structure and variability of the Antarctic coastal easterly winds

John King and Thomas Bracegirdle
John King and Thomas Bracegirdle
  • British Antarctic Survey, Cambridge, United Kingdom of Great Britain and Northern Ireland (

The belt of climatological easterly (westward) winds that lies to the south of the circumpolar trough of low pressure surrounding Antarctica has received less attention than the westerlies to the north of the trough, yet it plays a crucial role in atmosphere-ocean-cryosphere interactions in the near-coastal region. The westward-directed wind stress associated with the easterly winds drives a coastal westward ocean current and a westward transport of sea ice around the continent. Easterly winds also inhibit the flow of warm water masses from intermediate depths onto the continental shelves, thus protecting coastal ice shelves from enhanced basal melt. We use the ECMWF ERA-Interim reanalysis to study the mean structure and variability of the coastal easterly winds. The surface component of the easterlies generally extends no more than 200 km to the north of the coast. The easterlies are quite shallow (~ 1-2 km) and are relatively weak (generally < 3 m s-1 at the surface in the annual mean) over the ocean but become both deeper (~ 2-3 km) and stronger (~ 7 m s-1) over the steep coastal slopes of the continent. While persistent katabatic flow down these slopes is a source of easterly momentum (through the action of the Coriolis force), the primary driver of the easterlies appears to be the large-scale baroclinicity of the flow, which is enhanced in the coastal region where isentropes are forced to follow the steep coastal orography. Variability of the easterlies on monthly and longer timescales is related to variations in the strength and latitude of the circumpolar trough. On shorter (synoptic) timescales, large variations in the strength of the easterlies at coastal locations are forced by cyclones that move south from the circumpolar trough and decay in the coastal region.

How to cite: King, J. and Bracegirdle, T.: Structure and variability of the Antarctic coastal easterly winds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5127,, 2020

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Display material version 1 – uploaded on 29 Apr 2020
  • CC1: Comment on EGU2020-5127, Étienne Vignon, 01 May 2020

    Dear John,

    thank you very much for this very nice presentation. May I quickly ask you what would be the connection between your results - particularly those concerning extreme easterly flows - and barrier jets when the upper level geostrophic flow is oriented towards the ice sheet (I am particularly thinking of the study by van den Broeke and Gallée 1996, QJRMS).

    Best wishes,


    • AC1: Reply to CC1, John King, 02 May 2020

      Dear Étienne, Thank you for your interest in my presentation. I agree with you that barrier jets formed by blocking of a northerly airstream by the Antarctic orography may be a contributor to the coastal easterlies. As you say, this mechanism may be particularly important in the cases of extreme easterlies that I have studied. The composite anomaly maps on slide 16 clearly show quite strong northerly winds ahead of the cyclone that drives the extreme winds and these northerlies may well be driving a barrier jet. My analysis so far has been largely climatological but I do intend to look more closely at the dynamics driving these extreme events. The cross-section in slide 18 shows a double-jet structure in the easterlies for these extreme events and the low-level maximum at the coast could well be a barrier jet. Regards, John.