An assessment of objective diagnostics for the width of the tropical belt with GPS radio occultation observations

Climate model simulations of global warming predict an expansion of the Hadley circulation, with recent studies suggesting that the tropical belt may have already widened beyond that predicted for the end of the century. However, many existing diagnostics for the tropical belt edge latitudes are subjective, in some cases with trends in the tropical belt width sensitive to the value of the parameters chosen. Further, it is unknown if reanalysis products reproduce the observed tropical belt width and edge latitudes, and the scarcity of upper air observations in the tropics presents significant difficulty in validating wind-based diagnostics. This study introduces a new temperature-based, objective diagnostic for the tropical belt edge latitude: the latitude of the maximum tropospheric dry gross stability. Observations from Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) and Challenging Mini-Satellite Payload (CHAMP) are used to demonstrate that modern reanalyses capture the seasonal cycle and interannual variability of the tropical belt width. Of two objective, wind-based diagnostics, the latitude of the subtropical jet core and the latitude at which the mean meridional streamfunction vanishes, the jet diagnostic correlates most strongly with the new stability diagnostic. Trends in the tropical belt width obtained from the objective diagnostics are lower in magnitude than those found in previous studies, although all still indicate an expansion of the Hadley circulation. These objective diagnostics are then examined in the context of a simple two-layer model of the Hadley circulation, finding that the model's predicted seasonal cycle and sensitivity to changes in static stability are in broad agreement with those observed in GPS-RO data and reanalyses. This suggests that the projected increases in static stability due to global warming will likely lead to a continued expansion of the tropical belt.