North and equatorial Pacific Ocean circulation in the CORE-II hindcast simulations

Highlights: • Mean circulation patterns are assessed and Kuroshio transport is underestimated. • Water massdistribution is compared and analyzed within COREII models. • Main biases of deep MLDs result from the inaccurate Kuroshio separation. • Reasonable modeled tropical dynamics but a discrepancy from the surface wind. Abstract: We evaluate the mean circulation patterns, water massdistributions, and tropical dynamics of the North and Equatorial Pacific Ocean based on a suite of global ocean-sea ice simulations driven by the CORE-II atmospheric forcing from 1963-2007. The first three moments (mean, standard deviation and skewness) of sea surface heightand surface temperature variability are assessed against observations. Large discrepancies are found in the variance and skewness of sea surface heightand in the skewness of sea surface temperature. Comparing with the observation, most models underestimate the Kuroshio transport in the Asian Marginal seas due to the missing influence of the unresolved western boundary currentand meso-scale eddies. In terms of the Mixed Layer Depths (MLDs) in the North Pacific, the two observed maxima associated with Subtropical Mode Waterand Central Mode Waterformation coalesce into a large pool of deep MLDs in all participating models, but another local maximum associated with the formation of Eastern Subtropical Mode Watercan be found in all models with different magnitudes. The main model bias of deep MLDs results from excessive Subtropical Mode Waterformation due to inaccurate representation of the Kuroshio separation and of the associated excessively warm and salty Kuroshio water. Further water massanalysis shows that the North Pacific Intermediate Watercan penetrate southward in most models, but its distribution greatly varies among models depending not only on grid resolution and vertical coordinate but also on the model dynamics. All simulations show overall similar large scale tropical current system, but with differences in the structures of the Equatorial Undercurrent. We also confirm the key role of the meridional gradient of the wind stresscurl in driving the equatorial transport, leading to a generally weak North Equatorial Counter Current in all models due to inaccurate CORE-II equatorial wind fields. Most models show a larger interior transport of Pacific subtropicalcells than the observation due to the overestimated transport in the Northern Hemisphere likely resulting from the deep pycnocline