Analysis of recent circulation and thermal advection change in the northern Antarctic Peninsula

Changes in circulation at 500 and 850 hPa and horizontal thermal advection between these two levels are examined at Bellingshausen station in the northern Antarctic Peninsula between 1969 and 2000. A synthesis of radiosonde observations and reanalysis data is employed, the latter being 'corrected' to remove any bias and trend versus the observations. These data only reveal statistically significant increases in the annual zonal wind component; although the mean direction of the thermal advection is 339degrees there is no indication of increasing northerlies coincident with the remarkable regional surface warming. Seasonally, the zonal wind is most highly correlated with Surface temperature in autumn and winter, the seasons showing the greatest temperature rise. This Suggests that the increasing westerlies have played a role in the recent warming, and several authors have Postulated that this results from a trend towards the positive phase of the Southern Hemisphere annular mode (SAM). However, a comparison between a regional zonal circulation index derived independently from observations and the reanalysis data used in past Studies of the SAM indicates long-term increases in the latter may be an artefact caused by errors in the early reanalysis data. While observations do indicate an increase in the zonal wind since the late 1970s, coincident with a decline in the strength of the semi-annual oscillation, there is no clear evidence of circulation changes driving the long-term Antarctic Peninsula surface warming.