2025/08/04 by Merlis, Timothy M.
#Atmospheric and Oceanic Physics (physics.ao-ph) #FOS: Physical sciences
paper · doi:10.48550/arxiv.2508.02892
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. A characteristic magnitude of zonal asymmetry in a surface energy tendency that can bring changes in line with observed trends is ≈ 3 \mathrmW 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 \mathrmW m-2, a more amplifying surface radiative feedback in the west than the east with a contrast of ≈ 4 \mathrmW 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 % \mathrmK-1, or a more zonally asymmetric wind speed that increases by ≈ 16 % \mathrmK-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.