Decoding the Anthropogenic Influences on Pacific Warming Patterns

The observed lack of surface warming in the Southeast and central equatorial Pacific sharply contrasts with climate model projections, which consistently simulate an enhanced equatorial warming pattern. A recent assessment suggests that the zonal sea surface temperature gradient has historically been controlled by strengthening mechanisms but is projected to shift toward dominance by weakening mechanisms in the future (Watanabe et al., 2024). A pressing question remains: When will the weakening of the equatorial zonal sea surface temperature gradient emerge?

 

To address this question, I will review recent work from my group and collaborators, focusing on identifying the fast and slow components of sea surface temperature pattern responses to anthropogenic aerosols, stratospheric ozone, and greenhouse gases via idealized step-function experiments. Our findings suggest that the superposition of fast and slow responses to these forcings can sustain the equatorial cooling trend for longer than anticipated. Contrary to the interannual and decadal variability literature, which primarily emphasizes wave dynamics, we highlight the critical roles of spatial patterns in the atmospheric energy budget (moist static energy budget) in driving the initial adjustments of Hadley and Walker circulations. The fast components, along with the associated cloud radiative effects, initiate a series of air-sea interactions that set the stage for the slower components. Possible explanations for the discrepancies between model projections and observations will also be discussed.