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

Abstract 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 are 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 the observed trends. A characteristic magnitude of zonal asymmetry in a surface energy tendency that can bring changes in line with observed trends is ≈3 W m −2 . Strengthened zonal SST gradients can arise from a more zonally asymmetric ocean heat flux that increases by ≈20% K −1 using that implied by ERA5’s surface fluxes, a spatially varying radiative forcing with a west–east contrast of ≈3.3 W m −2 , a more amplifying surface radiative feedback in the west than in the east with a contrast of ≈4 W m −2 K −1 , a surface-air relative humidity (RH) contrast that increases RH in the west and decreases it in the east by ≈0.5% K −1 , or a more zonally asymmetric wind speed that increases by ≈16% K −1 . The “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. Significance Statement What is the magnitude of changes in energy fluxes in the tropical Pacific that could provoke the recent several decades of observed sea surface temperature trends? This urgent question of climate science is addressed using surface energy balance calculations with the various input terms perturbed in an idealized manner. These calculations provide an essential order-of-magnitude baseline for evaluating how novel mechanisms that are absent or weak in comprehensive climate model simulations might affect the pattern of historical trends.