Distinct roles of high and low clouds in modulating the tropical pacific SST warming pattern from 1985 to 2014

Abstract The pattern of sea surface temperature (SST) warming in the tropics modulates global climate variability. However, the key physical processes that govern this pattern remain poorly understood. This study investigates the influence of high- and low-cloud radiative effects on the tropical SST warming pattern from 1985 to 2014. Observational analysis reveals a spatially heterogeneous trend in SST: significant warming in the western Pacific (WP) contrasted with cooling in the central Pacific (CP). Associated with this SST warming pattern, the high cloud fraction (HCF) increased over the WP and decreased over the CP, while the low cloud fraction (LCF) increased over the northeast Pacific (NEP) and southeast Pacific (SEP). For high clouds, the longwave and shortwave effects largely cancel at the TOA, rendering the TOA net cloud radiative effect (CRE) insensitive to HCF changes. At the surface, increased (decreased) HCF in the WP (CP) enhances (reduces) shortwave reflection, offsetting local warming (cooling). The high cloud CREs at the surface constitute a negative feedback on local SST trends. For low clouds, increased LCF in the NEP and SEP exerts a cooling effect via reflecting more shortwave radiation both at the TOA and surface, amplifying surface cooling and providing a positive feedback on local SST trend. The TOA net CRE trend pattern is predominantly governed by low clouds, whereas the surface net CRE trend pattern is dominated by high clouds. Evaluation of 32 Coupled Model Intercomparison Project Phase 6 (CMIP6) models shows that, despite a large inter-model spread in the simulated SST and cloud trends, the simulations produce SST-cloud relationships consistent with the observations. Besides, the high-cloud (surface) versus low-cloud (TOA) partition also explains the inter-model spread in CMIP6. Furthermore, simulated tropical zonal wind trends correlate strongly with SST warming patterns, with intensified easterlies associated with a strengthened zonal SST gradient, underscoring the significant role of coupled ocean-atmosphere interactions in shaping tropical SST trends.