The Underestimated Responses of the Pacific Walker Circulation to ENSO in CMIP5 Models

The tropical Pacific Walker circulation is an important component of local air-sea interaction and is tightly related to ENSO on interannual timescale. This work examines the responses of the Pacific Walker Circulation to the niño34 SST during 1979-2005 in CMIP5 (the fifth phase of the Coupled Model Intercomparison Project). Five variables (sea level pressure, mass stream function, zonal wind, precipitation, sea surface temperature) are analyzed in observation, reanalysis, 29 climate models in AMIP (Atmospheric Model Intercomparison Project) and 45 climate models in CMIP5. The responses of sea level pressure gradient to the niño34 in AMIP and CMIP5 show a better performance. However, the amplitudes of mass stream functions’ responses in AMIP and CMIP5 drop approximately three quarters and half of that in Reanalysis. On lower easterlies responses, AMIP fall below three fifths and CMIP5 is less than half of that in observation. On upper westlies responses, AMIP5 are nearly equal to Reanalysis but CMIP5 are still much less than Reanalysis. In terms of individual models, these weaker responses are significantly except upper westlies in AMIP. Overall, responses of Pacific Walker circulation to the niño34 SST change, just two thirds and a half of the amplitude could be reproduced in AMIP5 and CMIP5. After analyzing the responses of precipitation to the niño34 SST, we find that the weaker responses are mainly caused by two reasons. One is that parameters of the convection parameterization scheme is insensitive in AMIP5. Other is that ENSO pattern are not simulated successfully in CMIP5. These results imply that underestimated responses of the Pacific Walker Circulation to ENSO are important and deserve further investigation and discussion.