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.

中文翻译：

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Poleward推进中纬子午温度梯度推动的热带扩张

大量证据表明热带地区正在扩张。尽管进行了许多尝试来解释原因，但驱动热带扩张的潜在动力机制仍不完全清楚。在这里，基于观察结果，耦合模型比较项目第5阶段（CMIP5）的多模型模拟和有目的设计的数值实验，从区域角度探讨了热带宽度的变化和趋势。我们发现，热带的宽度紧随海洋中纬度子午温度梯度（MMTG）的位移。在全球变暖下，作为一阶响应，由于异常暖水的平均Ekman收敛，亚热带海洋经历了更多的表面变暖。亚热带变暖加剧，部分与自然气候波动无关，例如太平洋年代际振荡，导致MMTG向极进，并推动热带扩张。我们的结果得到观测和模型模拟的支持，表明全球变暖可能已经对正在进行的热带扩张做出了重要贡献，特别是在海洋为主的南半球。