Ocean dynamics and tropical Pacific climate change in ocean reanalyses and coupled climate models

The role of ocean dynamics in tropical Pacific climate change is studied using an ensemble run of Simple Ocean Data Assimilationsparse input version 1 (SODAsi.1) and the Coupled Model Intercomparison Project Phase 5 (CMIP5) historical runs. An eight-member ensemble of ocean reanalyses (SODAsi.1) from 1871 to 2008 is produced by using forcing from eight ensemble members of an atmospheric reanalysis. The long-term trends of tropical Pacific surface temperature, wind stress, subsurface temperature, and strength of the subtropical cells (STCs) are analyzed. The ensemble reanalysis shows that there is a slight cooling trend of surface and subsurface temperature in the central tropical Pacific due to enhanced tropical Pacific circulation. The STCs, which consist of equatorial upwelling, Ekman transport, extratropical subduction, and pycnocline transport from the subtropical to the tropical region, strengthen from 1900 to 2008. When the STCs are accelerated, equatorial upwelling increases bringing cold water from the subsurface that cools the surface. An increasing trend in convergence transport is mainly from the Southern Hemisphere. In contrast with the reanalysis most of the CMIP5 models have warming trends at the surface and the transport of the STCs has a decreasing trend. The CMIP5 models also underestimate tropical Pacific Ocean circulation relative to the reanalysis that is mostly due to differences in wind forcing.