Regional Characteristics of Tropical Expansion and the Role of Climate Variability

Abstract:

Radiosonde-based tropical expansion rate estimates for six continental-centred regions of the globe are presented. The analysis technique builds on earlier work from Lucas et al [2012]. Since 1979, 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 Antarctic ozone depletion acting as a co-driver 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 correlation and 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 suggests that the PDO accounts for up to 50% of the observed tropical expansion over Asia, while SAM accounts 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.

Coupled climate models simulate average rates of expansion of 0.1- 0.2° decade^-1, less than that observed. Removing these effects of climate variability makes the observed tropical expansion more globally uniform, and brings the observed values in tropical expansion more into line with the estimates from coupled models. In short, values of observed tropical expansion over the period of record (i.e. from 1979) appear to be inflated due to coincident changes in modes of climate variability. Further, these climate modes have regional signatures which make tropical expansion non-uniform around the globe, potentially acting to modulate the observed impacts.