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 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 $0.5 \, \mathrm{K}$ and about $0.4 \, \mathrm{K}$ more west Pacific warming than east Pacific warming, based approximately on observed trends. A characteristic magnitude of zonal asymmetry in a surface energy tendency that can bring changes in line with observed trends is $\approx 3 \, \mathrm{W \, m^{-2}}$. Strengthened zonal SST gradients can arise from a more zonally asymmetric ocean heat flux that increases by $\approx 20\% \, \mathrm{K}^{-1}$ using that implied by ERA5's surface fluxes, a spatially varying radiative forcing with a west--east contrast of $\approx 3.3 \, \mathrm{W \, m^{-2}}$, a more amplifying surface radiative feedback in the west than the east with a contrast of $\approx 4 \, \mathrm{W \, m^{-2} \, K^{-1}}$, a surface-air relative humidity (RH) contrast that increases RH in the west and decreases it in the east by $\approx 0.5 \% \, \mathrm{K^{-1}}$, or a more zonally asymmetric wind speed that increases by $\approx 16 \% \, \mathrm{K^{-1}}$. 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.