The Recent Tropical Pacific Surface Warming Pattern: An Unresolved Puzzle

Changes in the sea surface temperature (SST) pattern in the tropical Pacific modulate radiative feedbacks to greenhouse gas forcing, the pace of global warming, and regional climate impacts. Therefore, elucidating the drivers of the pattern is critically important for reducing uncertainties in future projections. However, the attribution to anthropogenic forcing of observed changes over recent decades, an enhancement of the zonal SST contrast coupled with a strengthening of the Walker circulation, has not yet been successful because of the intricate nature of the problem involving multiple mechanisms at play. This presentation introduces a CLIVAR-CFMIP initiated working group, TROPICS (TROpical PacIfiC SST Warming PatternS). The presentation starts with reviewing existing mechanisms of the forced response based on an energy perspective, which starts from the global/hemispheric energy budget, and a dynamical perspective, which focuses on the tropical atmosphere-ocean coupling. We will then discuss the relative contributions to the past and future SST pattern changes to propose a narrative that reconciles them. Despite uncertainties, the balance of evidence suggests that the mechanisms leading to strengthening the zonal SST contrast have been efficient in the past and those leading to a weakening were less efficient but will become dominant in a future climate. Finally, we present the future plan of TROPICS, focusing on clarifying the timescales of the key mechanisms and resolving the model-observation discrepancy. A few examples will be given.