Tropical Expansion Driven by Poleward Advancing Midlatitude Meridional Temperature Gradients

An abundance of evidence indicates that the tropics are expanding. Despite many attempts to decipher the cause, the underlying dynamical mechanism driving tropical expansion is still not entirely clear. Here, based on observations, multimodel 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 displacement of oceanic midlatitude meridional temperature gradients (MMTG). Under global warming, as a first‐order response, the subtropical ocean experiences more surface warming because of the mean Ekman convergence of anomalously warm water. The enhanced subtropical warming, which is partially independent of natural climate oscillations, such as the Pacific Decadal Oscillation, leads to poleward advance of the MMTG 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 tropical expansion, especially over the ocean‐dominant Southern Hemisphere. Plain Language Summary Both observations and climate simulations have shown that the edges of tropics and associated subtropical climate zone are shifting toward higher latitudes under climate change. The underlying dynamical mechanism driving this phenomenon that has puzzled the scientific community for more than a decade, however, is still not entirely clear. A number of investigations argued that the atmospheric processes, in the absence of the ocean dynamics, lead to the tropical expansion. For example, increasing greenhouse gases, decreasing ozone and increasing aerosols are suggested to be the dominant factors contributing to expanding the tropics. However, these investigations are mostly based on model simulations, and observations show a much more complex evolution of expanding tropics. By examining the tropical width individually over each ocean basin, in this study, we find that the width of the tropics closely follows the displacement of oceanic midlatitude meridional temperature gradients (MMTG). Under global warming, as a first‐order response, the subtropical convergence zone experiences more surface warming due to background convergence of surface water. Such warming induces poleward shift of the oceanic MMTG and drives the tropical expansion. Key Points Evidence sh