Regional characteristics of tropical expansion and the role of climate variability

Radiosonde‐based tropical expansion rate estimates for six continental‐centered regions of the globe are discussed. New results from the Northern Hemisphere are presented, complementing previous Southern Hemisphere (SH) results using an identical methodology. Expansion rates are largest over Asia and Australia‐New Zealand (ANZ). Other regions show more modest rates of expansion, and there is no statistically significant expansion over North America. In the hemispheric average, expansion rates are slightly larger in the SH. This asymmetry, although not statistically significant, is consistent with ozone depletion acting as a codriver of tropical expansion since 1979. The picture of regional expansion here is different from that in other studies or that derived solely from reanalyses. The relationships between tropical expansion and modes of climate variability are explored using partial regression techniques. Interannually, a relationship is seen with the El Niño–Southern Oscillation (ENSO) across most of the globe. Regionally, the Pacific Decadal Oscillation (PDO) has a significant impact over Asia, while the Southern Annular Mode (SAM) affects expansion over ANZ. Considering the longer‐term changes in the climate variability indices since 1979, it suggests that the PDO may account for up to 50% of the observed tropical expansion over Asia, while SAM may account for 20–30% of the SH expansion, particularly over ANZ. Decadal changes in ENSO may account for a further 20–30% of the global and regional trends. Accounting for the effect of climate variability helps to reconcile the differences in observations and model‐based simulations of tropical expansion.