Why do different climate models give different answers? An analysis of 21st century wind changes over the Amundsen Sea, West Antarctica, in the CMIP5 climate models

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• Dr. Thomas Bracegirdle
• Wednesday 17 September 2014, 11:00-12:00
• British Antarctic Survey, Room 330B.

If you have a question about this talk, please contact Dr. Pranab Deb.

All welcome. If external to BAS, please email the organiser in advance to gain access to the building.

The influence of changes in winds over the Amundsen Sea has been shown to be a potentially key mechanism in explaining rapid loss of ice from major glaciers in West Antarctica, which is having a significant impact on global sea level. Here, Coupled Model Intercomparison Project Phase 5 (CMIP5) climate model data are used to assess 21st century projections in westerly winds over the Amundsen Sea (UAS). For a given scenario of greenhouse gas concentrations, differences between different model projections can be split into contributions from internal climate variability and model uncertainty. Model uncertainty comes from differences in the way different climate models are constructed and internal variability of the climate system is a further contributor. These sources of uncertainty are quantified in projections following Representative Concentration Pathway (RCP) 4.5 and RCP 8 .5 scenarios.

For the decade 2090-2099 the CMIP5 models show ensemble mean 21st century changes in annual mean UAS of 0.3 and 0.7 m s-1 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 UAS (more skilful models show larger UAS 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.

[All welcome. If external to BAS , please email the organiser in advance to gain access to the building.]

This talk is part of the British Antarctic Survey series.

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