Signals of ENSO in the atmospheric circulation around the Antarctic Peninsula

It is important to understand how the signals of the ENSO cycle affect the Antarctic so as to be able to interpret correctly the climatic data locked within ice cores. In addition, climate models must be able to simulate correctly the poleward transmission of ENSO signals if we are to be able to trust the high latitude predictions of these models when run for the coming decades and centuries.

The work of Karoly (1989) suggested that during El Niño events there was a Rossby wave train emanating from the central, tropical Pacific, with an anticyclonic anomaly present at upper levels. Since the Karoly study only covered three El Niño events we have recomputed the anomalies for the eight El Niño events for the period 1968-1999 using NCEP re-analysis data. We find the same general anomaly patterns (present at the surface and upper levels) for the El Niño phase of ENSO, but major variations throughout the events. The anticyclonic anomaly gives a reduction in the mobility of the circulation over the Amundsen-Bellingshausen Sea (ABS), although the location of the blocking high is crucial in determining the areas of cyclone development and track. With a feed of relatively warm mid-latitude air southwards on the western side of the block there is increased baroclinicity in the Antarctic coastal region and greater cyclogenesis south of the circumpolar trough. There is also greater cyclogenesis just to the west of the Antarctic Peninsula and fewer depressions approaching the Peninsula from the northwest. During El Niño periods there are a greater number of precipitation reports at Faraday Station, probably because of the larger number of active depressions forming just to the west of the Peninsula.

For La Nina events there is a climatological negative anomaly at the surface and upper levels over the ABS. This translates into very mobile conditions within the circumpolar trough and a succession of depressions arriving at the Antarctic Peninsula from the west. More lows tend to arrive over the ABS from mid-latitudes and there is less cyclogenesis in the immediate Antarctic coastal regions. Blocking episodes tend to be short-lived and the anticyclones are found at relatively northerly latitudes. Although there are many depressions over the ABS, many of these are mature lows and overall during La Nina periods there is only an average number of precipitation reports at Faraday Station.

References

Karoly,D.J. (1989), Southern Hemisphere circulation features associated with El Niño-Southern Oscillation events, J. Clim. 2, 1239-52.