Understanding the Future Changes of Equatorial Pacific Trade Winds in the CMIP5

The weakened Walker circulation has been suggested to be one of the most robust features due to anthropogenic greenhouse gas (GHG) forcing, which is usually measured by the changes of zonal sea level pressure (SLP) gradients or the vertical motion change differences between the western and eastern tropical Pacific. The changes of equatorial Pacific trade winds are dynamically linked to the SLP and vertical motion changes while they cannot provide the spatial picture of trade winds changes. Based on 19 Coupled Model Intercomparison Project Phase 5 (CMIP5) models and one coupled model--POEM, it is revealed that the projected weakened trade wind changes are only robust in the equatorial eastern Pacific (EEP, 150oW-80oW) but highly uncertain over the equatorial central-western Pacific (ECWP, 140oE-150oW) although the multi-model-ensemble (MME) results exhibit weakened trade winds prevailing from the western Pacific to the far eastern Pacific. Different from the vertical motion change, the horizontal trade wind changes may not be able to fully explained from the energy balance perspective between the radiative cooling and condensational heating. Over the EEP, the robust westerly wind changes are tightly linked to a pronounced cyclonic circulation driven by the mean subsidence that advects the strong upper-tropospheric warming downward to warm the lower-troposphere so as to form an anomalous warming and low pressure center over the southeastern Pacific. In the ECWP, however, 9 out of 19 models produce intensified trade winds in contrasting to the MME results. The large uncertainty is prominently associated with the magnitude of the relative SST over the equatorial Pacific deviating from the tropical mean. Of particular interest is that the ECWP trade wind changes have a significant relationship with the mean winds over east of dateline. Stronger mean winds indicate stronger dampening effect over the equatorial SST due to both dynamic and thermodynamic effects, favoring the formation of enhanced trade winds over the ECWP. We further advocate that the MME results may provide a relatively accurate estimate for future projection given the comparable mean winds between MME of CMIP5 and observations. The results also have important implication on the tropical SST pattern formation.