What Controls the Mean East-West Sea Surface Temperature Gradient in the Equatorial Pacific: The Role of Cloud Albedo

The mean east-west sea surface temperature gradient along the equator is a key feature of tropical climate. Tightly coupled to the atmospheric Walker circulation and the oceanic east-west thermocline tilt, it effectively defines tropical climate conditions. In the Pacific, its presence permits the El Nino-Southern Oscillation phenomenon. What determines this temperature gradient within the fully coupled ocean-atmosphere system is therefore a central question in climate dynamics, critical for understanding past and future climates. Using a comprehensive coupled model [Community Earth System Model (CESM)], the authors demonstrate how the meridional gradient in cloud albedo between the tropics and midlatitudes (Delta alpha) sets the mean east-west sea surface temperature gradient in the equatorial Pacific. To change Delta alpha in the numerical experiments, the authors change the optical properties of clouds by modifying the atmospheric water path, but only in the shortwave radiation scheme of the model. When Delta alpha is varied from approximately -0.15 to 0.1, the east-west SST contrast in the equatorial Pacific reduces from 7.5 degrees C to less than 1 degrees C and the Walker circulation nearly collapses. These experiments reveal a near-linear dependence between Delta alpha and the zonal temperature gradient, which generally agrees with results from the Coupled Model Intercomparison Project phase 5 (CMIP5) preindustrial control simulations. The authors explain the close relation between the two variables using an energy balance model incorporating the essential dynamics of the warm pool, cold tongue, and Walker circulation complex.