Hanh Nguyen

An observational analysis of Southern Hemisphere tropical expansion

[1]xa0Historical radiosonde data are analyzed using the tropopause height frequency method to investigate the variation of the Southern Hemisphere tropical edge from 1979/80–2010/11, independently of reanalysis-derived data. Averaged across the hemisphere we identify a tropical expansion trend of 0.41xa0±xa00.37xa0deg dec−1, significant at the 90% level. A comparison with four reanalyses shows generally consistent results between radiosondes and reanalyses. Estimated rates of tropical expansion in the SH are broadly similar, as is the interannual variability. However, notable differences remain. Some of these differences are related to the methodology used to identify the height of the tropopause in the reanalyses, which produces inconsistent results in the subtropics. Differences between radiosondes and reanalyses are also more manifest in data-poor regions. In these regions, the reanalyses are not fully constrained, allowing the internal model dynamics to drive the variability. The performance of the reanalyses varies temporally compared to the radiosonde data. These differences are particularly apparent from 1979 to 1985 and from 2001 to 2010. In the latter period, we hypothesize that the increased availability and quality of satellite-based data improves the results from the ERA Interim reanalysis, creating an inconsistency with earlier data. This apparent inhomogeneity results in a tropical expansion trend in that product that is inconsistent with the radiosonde-based observations. These results confirm the need for careful evaluation of reanalysis-based data for use in studies of long-term climate variability.

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 Nino–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.

The impact of the Southern Annular Mode on future changes in Southern Hemisphere rainfall

A robust positive trend in the Southern Annular Mode (SAM) is projected for the end of the 21st century under the Representative Concentration Pathway 8.5 scenario, which results in rainfall decreases in the midlatitudes and increases in the high latitudes in the Southern Hemisphere (SH). We find that this SAM trend also increases rainfall over the SH subtropics in austral summer but not in winter, leading to a pronounced wintertime poleward expansion of the subtropical dry zone. These dynamically driven rainfall changes by the SAM appear to oppose the thermodynamically driven projected rainfall changes in the SH subtropics and midlatitudes, whereas the two components reinforce each other in the high latitudes. However, we show that most climate models fall short in capturing the observed SAM component driven by the El Nino–Southern Oscillation and associated rainfall in the austral warm seasons, which limits our confidence in quantifying the contribution of the SAM to projected rainfall changes.

Variability of the extent of the Hadley circulation in the southern hemisphere: a regional perspective

In order to understand the regional impacts of variations in the extent of the Hadley circulation in the Southern Hemisphere, regional Hadley circulations are defined in three sectors centered on the main tropical heat sources over Africa, Asia-Pacific (Maritime Continent) and the Americas. These regional circulations are defined by computing a streamfunction from the divergent component of the meridional wind. A major finding from this study is that year-to-year variability in the extent of the hemispheric Hadley circulation in the Southern Hemisphere is primarily governed by variations of the extent of the Hadley circulation in the Asia-Pacific sector, especially during austral spring and summer when there is little co-variability with the African sector, and the American sector exhibits an out of phase behavior. An expanded Hadley circulation in the Southern Hemisphere (both hemispherically and in the Asia-Pacific sector) is associated with La Niña conditions and a poleward expansion of the tropical wet zone in the Asia-Pacific sector. While La Niña also promotes expansion in the American and African sectors during austral winter, these tropical conditions tend to promote contraction in the two sectors during austral summer as a result of compensating convergence over the Americas and Africa sectors: a process driven by variations in the Walker circulation and Rossby wave trains emanating from the tropical Indian Ocean.