The width of the tropical belt on seasonal to multi-decadal timescales

An expansion of the tropical belt has been extensively reported in observations, reanalyses, and climate model simulations, but there is a great deal of uncertainty in estimates of the rate of widening as different diagnostics give a wide range of results. This study critically examines objective diagnostics for the width of the tropical belt to explore their seasonality, interannual variability, and multi-decadal trends in reanalyses, augmented by the novel use of GPS radio occultation observations. The width based on the latitudes of the maximum tropospheric dry bulk static stability, measuring the difference in potential temperature between the tropopause and the surface, is found to be closely coupled to the width based on the subtropical jet cores on all timescales. In contrast, the tropical belt width based on the latitudes at which the vertically-averaged streamfunction vanishes, a measure of the Hadley circulation's poleward edges, lags the other diagnostics by approximately one month, with evidence that the Northern and Southern Hemisphere Hadley cells and their associated jets exist in significantly different dynamical regimes. A disparity is found in the reanalyses in the rate of tropical widening between the streamfunction diagnostic and the subtropical jet and bulk stability diagnostics, confirming previous studies of similar diagnostics.