Tuesday, 30 April 2013
North/West Room (Renaissance Seattle Hotel)
In contrast to earlier studies, we describe the persistent deep low-pressure system that exists over the South Pacific sector of the Southern Ocean, referred to as the Amundsen-Bellingshausen Seas Low (ABSL), in terms of its relative (rather than absolute) depth by removing the background area-averaged mean sea level pressure (MSLP) from the monthly MSLP field. In doing so, we remove much of the influence of the Southern Annular Mode (which strongly modulates the absolute depth of the ABSL), allowing a more precise understanding of the influence of the ABSL on the regional climate of West Antarctica. Using ERA-Interim reanalysis fields the annual cycle of relative depth for 1979 to 2011 shows a minimum (maximum) during summer (winter), differing considerably from the earlier studies based on absolute depth which suggests a semi-annual oscillation. The annual cycle of longitudinal position is insensitive to how the ABSL is described, and shows it shifting westwards from ~250ºE to ~210ºE between summer and winter, in agreement with the earlier studies. We demonstrate that ABSL variability (and in particular its longitudinal position) plays an important role in controlling the surface climate of West Antarctica and the surrounding ocean by quantifying its influence on key meteorological parameters. Our investigation of the representation of the ABSL annual cycle in 17 CMIP5 climate models that were run with historical forcing showed that the majority of them have definite biases (which are more pronounced in terms of longitudinal position), and correspondingly a poor representation of West Antarctic climate.
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