Summary
Large-scale zonal asymmetries in the climatological spatial pattern of atmospheric variables such as geopotential height, temperature, and trace gas concentration are primarily dictated by the characteristics of quasi-stationary Rossby waves (QSWs). These waves are mainly forced by spatial inhomogeneities in surface temperature (particularly associated with land-sea contrasts), orography, and large-scale latent heat release, and are in turn affected by transient eddies and changes in the meridional and vertical gradient of temperature. Here we examine meteorological reanalyses and climate model simulations to quantify variability and change in the QSW patterns in the troposphere and stratosphere of the Southern Hemisphere extratropics. The primary motivation for this study is to examine how well climate models simulate the climatological phase and amplitude of the low-order QSW modes, particularly in light of potential model biases in the temperature of the Southern Ocean and the distribution of Antarctic sea ice, and long-term changes in greenhouse gases and stratospheric ozone. We use atmosphere-ocean (AO) climate model simulations from the Climate Model Intercomparsion Project (CMIP) and chemistry-climate model simulations from the SPARC Chemistry-Climate Model Validation Activity (CCMVal) and the SPARC/IGAC Chemistry-Climate Model Initiative (CCMI). In reanalysis data (e.g. ERA-Interim) of the satellite era (from 1979), we find significant trends in the longitude and amplitude of QSW wave-1 and wave-2 patterns in the Antarctic lower stratosphere during spring and summer, that appear in part related to ozone change. Climate models with interactive stratospheric chemistry generally perform better at reproducing the observed trends than AO models. We discuss the observed trends and performance of the climate models for the southern extratropics, and diagnose characteristics of the models that result in the best agreement with the observed QSW behaviour.
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