Seasonal to Multi-Decadal Variability of the Width of the Tropical Belt

May 13, 2013

Nick Davis

Committee: Thomas Birner (advisor), Dave Thompson, Karan Venayagamoorthy (Civil and Environmental Engineering)

Abstract

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 robust diagnostics for the width of the tropical belt to explore their seasonality, interannual variability with respect to the Quasi-biennial and El Niño-Southern Oscillations, and multi-decadal trends.

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 and Northern Hemisphere edge latitudes based on the latitudes at which the vertically-averaged streamfunction vanishes, a measure of the Hadley circulation's poleward extent, lags those of the other diagnostics by approximately one month. A disparity is found in the reanalyses in the rate of tropical widening over the 1979-2012 period between the streamfunction diagnostic and the subtropical jet and bulk stability diagnostics, providing further evidence of a possible decoupling.

Finally, GPS radio occultation observations are used to assess the ability of the reanalyses to reproduce the tropical belt width, finding that they better situate the latitudes of maximum bulk stability versus those of the subtropical jets.