Near-surface air temperatures on the western side of the Antarctic Peninsula have risen more over the last 50 years than anywhere else in the Southern Hemisphere. In parallel with the temperature rise, there has been the disintegration of several major floating ice shelves on both the eastern and western sides of the Peninsula. At Faraday/Vernadsky station winter season temperatures have increased at a rate of +0.56 deg C per decade over 1951-2000, a figure that is statistically significant at <5% level. Most of the temperature increase occurred between 1950, when the temperature record from the station began, and about 1970, with temperatures since that time showing little change. The area of major winter warming is quite limited and appears to be restricted to the central and southern parts of the western side of the Peninsula. Data for recent years shows a high correlation between near-surface air temperatures and sea ice extent over the Bellingshausen sea to the west of the Peninsula. This suggests that in the 1950s and 1960s there was more extensive winter sea ice over the Bellingshausen Sea.

At the tip of the Peninsula there has been a significant summer season warming, with Esperanza station showing a temperature increase during that season of +0.43 deg C per decade over 1961-2000, which is significant at the 5 percent level.

A major problem in understanding the reasons for these changes is the lack of reliable surface and upper air fields for the 1950s and 1960s. The NCEP and ECMWF re-analysis projects produce fields extending back to 1958, but comparison with the Antarctic station data shows that their quality is poor before the 1970s because of the lack of data over the ocean areas. We have therefore attempted to gain insight into the atmospheric circulation around the Antarctic Peninsula using the available synoptic reports. Surface reports from island stations allow the computation of the Southern Hemisphere Annular Mode (SAM) back to about 1958. Analysis of the Esperanza surface and upper air data suggests that the summer temperature increases have been associated with an increase in the northerly component of the wind. The changes in the SAM towards its positive phase have resulted in stronger westerlies, but it is the increase in lee cyclogenesis that have resulted in the more northerly flow.

On the western side of the Peninsula we have used the reports of precipitation days from Faraday/Vernadsky to gain insight into the broad-scale synoptic conditions over the Bellingshausen Sea during the 1950s and 1960s. We show that the number of precipitation days is a good proxy for mean sea level pressure over the ocean areas to the west of the Peninsula. The number of precipitation days has increased at a statistically significant level since the 1950s indicating that the mean sea level pressure was higher in the middle of the Twentieth Century; conditions which allowed the sea ice to be more extensive during the winter period.