arXiv:2508.02892v1 Announce Type: new
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.