CMIP5 Projections of 21st Century Westerly Wind Changes over the Amundsen Sea, West Antarctica: Long-Term Predictability and the Role of Tropical Linkages

Abstract:

Winds over the Amundsen Sea, West Antarctica, are a key driver of variability and change in regional ocean-ice interactions and therefore an important consideration in assessing past and future ice loss from glaciers over West Antarctica, which is an important component of global sea level change. The predictability of 21^stcentury climate change in this region is affected significantly by the large atmospheric internal variability, driven in part by external tropical influences.

In this talk an assessment of 21^st century projections of westerly winds over the Amundsen Sea (U_AS) will be presented, based on output from the Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model ensemble. The focus will be on quantifying the importance of model uncertainty and internal climate variability in RCP4.5 and RCP8.5 scenario projections and identifying potential sources of model uncertainty. In general the CMIP5 ensemble-mean response exhibits increases in U_AS over the 21^st century, with the Amundsen Sea being positioned on the poleward flank of large-scale westerly wind increases across mid-to-low latitudes. In terms of the annual mean, increases of 0.3 and 0.7 m s^-1are simulated following the RCP4.5 and RCP8.5 scenarios respectively. However, as a consequence of large internal climate variability over the Amundsen Sea, it takes until approximately 2030 (2065) for the RCP8.5 response to exceed one (two) standard deviation(s) of decadal internal variability.

In all scenarios and seasons the model uncertainty is large. However the present-day climatological zonal wind bias over the whole South Pacific, which is important for tropical teleconnections, is strongly related to inter-model differences in projected change in U_AS (more skilful models show larger U_AS increases). This relationship is significant in winter (r = -0.56) and spring (r = -0.65), when the influence of the tropics on the Amundsen Sea region is known to be important. Horizontal grid spacing and present day sea ice extent were not found to be significant sources of inter-model spread. Possible physical/dynamical explanations for the above correlations will be discussed, in particular considering the role of tropical-Antarctic teleconnections and including current work extending the analysis to the Atlantic and Indian sectors.