Fast SST error growth in the southeast Pacific Ocean: comparison between high and low-resolution CCSM4 retrospective forecasts

Sea surface temperature errors that develop after 1 week are investigated as a function of resolution using retrospective forecasts from the Community Climate System Model. One version resolves the ocean and atmosphere to approximately 1° while the second version resolves the ocean to 0.1° and the atmosphere to 0.5°. The forecasts are initialized on January 1 from 1982 to 2003. The spatial pattern of the Pacific basin sea surface temperatures errors after 1 week is mostly similar at both resolutions, with the exception of the coast of South America. The coastal ocean surface cools within the higher-resolution simulations but warms within the lower-resolution simulations. The difference in the ocean surface temperature is instead attributed to differing changes in the upwelling. Coastal upwelling increases within the higher-resolution simulation, increasing the lower tropospheric stability and encouraging the cloud cover. In contrast, the upwelling decreases within the lower-resolution simulations at 27°S, allowing the ocean surface to warm in spite of cooling from the atmosphere. In both simulations, the northward winds and surface currents weaken, because the South Pacific sea level pressure high moves westward. The increased oceanic upwelling in the high-resolution simulation is instead attributed to an increase in the westward zonal currents. The high-resolution model resolves the narrow Humboldt current, while the low-resolution model does not. This study demonstrates that the processes responsible for SST errors in eastern upwelling boundary current regions change when the oceanic grid spacing becomes fine enough to allow resolution of the oceanic boundary currents.