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The Impact of a Changing Southern Hemisphere Annular Mode on Antarctic Peninsula Summer Temperatures,

This paper presents the first strong evidence linking climate change in the Antarctic Peninsula with human activity. Work at BAS and elsewhere had already demonstrated that the observed increase in the strength of the circumpolar westerly winds since the mid-1960s is largely a result of anthropogenic forcings — ozone depletion and greenhouse gas increases in particular. In this paper, we show that the increase in the strength of the westerlies (driven by human activity) has been responsible for the observed rapid rise in summer temperatures in the northeast of the Peninsula. It is this summer warming that has almost certainly caused the rapid collapse of ice shelves in this region.

The mechanism that we have identified as responsible for amplified warming in this region — interaction of the westerlies with the topography of the Peninsula — is most effective in the northern part of the Peninsula. This may explain why the Prince Gustav and Larsen A (but not Larsen B) ice shelves were not present during the mid-Holocene. The paper also paves the way to producing better predictions of future climate in the Peninsula region. A press release issued at the time of publication generated significant media interest.

Abstract Since the mid-1960s rapid regional summer warming has occurred on the east coast of the northern Antarctic Peninsula, with near-surface temperatures increasing by more than 2°C. This warming has contributed significantly to the collapse of the northern sections of the Larsen Ice Shelf. Coincident with this warming, the summer Southern Hemisphere Annular Mode (SAM) has exhibited a marked trend, suggested by modelling studies to be predominantly a response to anthropogenic forcing, resulting in increased westerlies across the northern Peninsula. We utilize observations and reanalysis data to demonstrate that the changing SAM has played a key role in driving this local summer warming. We propose that the stronger summer westerly winds reduce the blocking effect of the Peninsula and lead to a higher frequency of air masses being advected eastward over the orographic barrier of the northern Antarctic Peninsula. When this occurs, a combination of a climatological temperature gradient across the barrier and the formation of a föhn wind on the lee side typically results in a summer near-surface temperature sensitivity to the SAM that is three times greater on the eastern side of the Peninsula than on the west. SAM variability is also shown to play a lesser role in determining summer temperatures at stations west of the barrier in the northern Peninsula (~62°S), both at the surface and throughout the troposphere. This is in contrast to a station farther south (~65°S) where the SAM exerts little influence.

BAS Press Release: First direct evidence that human activity is linked to Antarctic Ice Shelf Collapse

Find link to the full paper on the NERC Open Research Archive

Authors

Marshall, G.J., Orr, A., van Lipzig, N.P.M. and King, J.C.

Publication

Journal of Climate , 19, No. 20, 5388-5404.