Monotonic decrease of the zonal SST gradient of the equatorial Pacific as a function of CO2 concentration in CCSM3 and CCSM4

Abstract The west‐east sea surface temperature (SST) gradient in the equatorial Pacific Ocean is a key feature of Earth's climate. How this gradient responds to varying climatic forcing is a challenge to both climate theory and climate modeling. Using the coupled atmosphere‐ocean general circulation models, Community Climate System Model version 3 (CCSM3) and version 4 (CCSM4), we show that the zonal SST gradient is an almost monotonically decreasing function of atmospheric CO2 concentration (pCO2) across a wide range from 17.5 to 4576 ppmv. As pCO2 is increased, the optical depth of clouds over the western and central Pacific increases significantly, reflecting more insolation back to space, suppressing surface warming in this region and thereby reducing the zonal SST gradient. Ocean adjustment to a weakening of surface zonal winds is characterized by relaxations of the equatorial thermocline tilt, zonal surface currents, and ocean upwelling.