Poster Presentation AMOS Annual Meeting and International Conference on Tropical Meteorology and Oceanography

Importance of Zonal vs. Meridional flow for climate extremes in the Southern Hemisphere (#1028)

Ghyslaine Boschat 1 , Julie Arblaster 1 , Damien Irving 2
  1. Monash University, Clayton, VICTORIA, Australia
  2. Oceans and Atmosphere, CSIRO , Hobart, Tasmania, Australia

The aim of this project is to examine the seasonal relationship between extratropical atmospheric variability and climate extremes in the Southern Hemisphere (SH). But how can we objectively define extratropical variability? The Southern Annular Mode (SAM) has been recognised as the leading mode of variability in the SH, describing the north-south movement of the westerly wind belt that circles Antarctica. However, although this annular flow is dominant in summer, this is not necessary true for other seasons when considerable zonal asymmetry appears in many basins, comparable to a zonal wave 3 (ZW3) structure. So what configuration of the atmospheric flow can be more favourable for the development of extreme heat or rainfall in the SH? To answer this, we compare the influence of a meridional flow measured by ZW3 with the zonality of SAM, through regressions and composites with temperature and rainfall.

Our results show that for both ZW3 and SAM, the teleconnections with extremes appear as an amplification of their teleconnection with mean fields, with relatively weak influence from ENSO. A positive phase of SAM is associated with cooler temperatures over Antarctica and Australia, but different regions are affected during different seasons, and an opposite relation is found for ZW3 in autumn and winter. We also compare the meridional transport of heat and moisture during negative SAM and positive ZW3 seasons, to highlight when extreme temperatures or rainfall can be linked to a regional or more hemispheric signal. Finally, we explore changes in the characteristics of fronts and cyclones during extremes phases of SAM and ZW3, to better understand how the zonality or meridionality of the large-scale flow can impact or interact with processes at the synoptic scale to produce extreme climate conditions.