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Abstract

The Northeastern Pacific climate system is featured by a large-scale low-cloud deck off California on the southeastern flank of a subtropical high, which accompanies intense northeasterly trade winds and relatively low sea-surface temperature (SST), especially in the warm season. The climatic impacts of the low cloud deck are investigated by turning low-cloud radiative forcing on and off only within the subtropical Northeastern Pacific in a coupled atmosphere-ocean model. The radiative forcing lowers SST locally by as much as 3ºC, and the low-cloud impacts on the North Pacific subtropical high are found to maximize in summer under the seasonally enhanced insolation. In combination with the seasonally shallow ocean mixed layer, the SST decrease by low clouds off California peaks in summer, suppressing deep convective precipitation that would otherwise occur in the absence of the low-cloud deck. The resultant anomalous diabatic cooling induces a surface anticyclonic response in summer and autumn as a baroclinic Matsuno-Gill pattern. The consequent enhancement of the trade winds on its equatorward flank leads to further SST lowering that extends southwestward through the wind-evaporation-SST feedback. The enhanced trade winds accompany intensified upper-tropospheric westerlies, strengthening the vertical wind shear that, together with lowered SST, acts to shield Hawaii from powerful hurricanes. On the basin scale, the surface anticyclonic wind response accelerates the North Pacific subtropical ocean gyre to speed up the Kuroshio by as much as 30%. Thus, SST increases along the Kuroshio and its extension, intensifying upward turbulent heat fluxes from the ocean with increased precipitation.