PNNL

Abstract

An abundance of evidence indicates that the tropics are expanding under climate change. Despite many attempts to decipher the cause, the underlying dynamical mechanism driving tropical expansion is still not entirely clear. Here, based on observations, multi-model simulations from the Coupled Model Intercomparison Project phase 5 (CMIP5) and purposefully designed numerical experiments, the variations and trends of the tropical width are explored from a regional perspective. We find that the width of the tropics closely follows the meridional displacement of subtropical front. In a warming climate, the subtropical ocean experiences more surface warming because of the mean Ekman convergence of the anomalously warm water. The enhanced subtropical warming, which is independent of the natural climate oscillations, such as the Pacific Decadal Oscillation, leads to poleward advance of subtropical front and drives the tropical expansion. Our results, supported by both observations and model simulations, imply that global warming may have already significantly contributed to the ongoing expanding tropics, especially over the ocean dominant Southern Hemisphere.