Anthropogenic and natural contributions to the lengthening of the Southern Hemisphere summer season

Abstract

This study provides the first quantitative assessment of observed long-term changes in summer season timing and length in the Southern Hemisphere (SH) and its sub-regions over the past 60 years, enabling a global completeness by complimenting such characteristics previously reported for the Northern Hemisphere (NH). Using an objective algorithm based on temperature indices, relative measures of summer onset, withdrawal, and duration are determined at each land location over the period 1953–2012. Significant widespread summer season lengthening, due to earlier onset and delayed withdrawal, has occurred across the SH, a longer period for extreme heatwave events and wildfires to potentially occur. The asymmetric magnitude (onset versus withdrawal) in summer season lengthening is slightly less over the SH compared to the NH. Contributions of anthropogenic and natural factors to the observed trends in summer season characteristics were investigated using Coupled Model Intercomparison Project phase 5 (CMIP5) multimodel simulations integrated with observed external forcings (anthropogenic plus natural, ALL), greenhouse-gas forcing only (GHG), and natural forcing only (solar and volcanic activities, NAT). Overall, consistent with the NH, increased greenhouse-gases were the main cause of observed changes in the SH with negligible contribution from other external forcings. ALL and GHG simulations also reproduced a slight tendency for earlier summer onset to contribute more to summer lengthening. Proportions of observed regional trends in summer season indices attributable to trends in long-term internal variability in the SH, namely the Interdecadal Pacific Oscillation (IPO) and Southern Annular Mode (SAM), suggests such variability can only explain up to ∼12%, supporting the dominant role of greenhouse-gas forcing.