Perturbing the Surface Energy Balance to Emulate the Historical Pattern of Tropical Pacific Sea Surface Temperature Trends

The strengthening of the zonal sea surface temperature (SST) gradient observed in the tropical Pacific in recent decades is a regional climate change signal that may be outside the range of historical simulations with comprehensive climate models. Given the important role that this change has on other aspects of climate, a series of idealized surface energy balance calculations with imposed parameters is performed to build a baseline understanding of the sensitivities that govern these changes. I quantify the requisite magnitudes of five perturbations that reach a new equilibrium with a mean-SST warming of about 0.5 K and about 0.4 K more west Pacific warming than east Pacific warming, based approximately on observed trends. Strengthened zonal SST gradients can arise from a more zonally asymmetric ocean heat flux that increases by about 20% per K, a spatially varying radiative forcing with a west--east contrast of about 3.3 W m^-2, a more destabilizing surface radiative feedback in the west than the east with a contrast of about 4 W m^-2 K^-1, a surface-air relative humidity (RH) contrast that increases RH in the west and decreases it in east by about 1% per K, or a windspeed change that decreases in the west and increases in the east by about 16% per K. These "storylines" of forced surface energy budget change identified here are valuable in determining the plausibility of mechanisms that may be absent or underestimated in coupled climate model simulations.