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Early reanalyses are less than optimal for investigating the regional effects of ozone depletion on Southern Hemisphere (SH) high-latitude climate because the availability of satellite sounder data from 1979 significantly improved their accuracy in data sparse regions, leading to a coincident inhomogeneity. To determine whether current reanalyses are better at SH high-latitudes in the pre-satellite era, here we examine the capabilities of the European Centre for Medium-range Weather Forecasts (ECMWF) fifth generation reanalysis (ERA5), the Twentieth Century Reanalysis version 3 (20CRv3), and the Japanese Meteorological Agency (JMA) 55-year reanalysis (JRA-55) to reproduce and help explain the pronounced change in the relationship between the Southern Annular Mode (SAM) and Antarctic near-surface air temperatures (SAT) between 1950 and 1979 (EARLY period) and 1980–2020 (LATE period). We find that ERA5 best reproduces Antarctic SAT in the EARLY period and is also the most homogeneous reanalysis across the EARLY and LATE periods. ERA5 and 20CRv3 provide a good representation of SAM in both periods with JRA-55 only similarly skilful in the LATE period. Nevertheless, all three reanalyses show the marked change in Antarctic SAM-SAT relationships between the two periods. In particular, ERA5 and 20CRv3 demonstrate the observed switch in the sign of the SAM-SAT relationship in the Antarctic Peninsula: analysis of changes in SAM structure and associated meridional wind anomalies reveal that in these reanalyses positive SAM is linked to cold southerly winds during the EARLY period and warm northerly winds in the LATE period, thus providing a simple explanation for the regional SAM-SAT relationship reversal.

 

Abstract. This study quantifies differences among four widely usedatmospheric reanalysis datasets (ERA5, JRA-55, MERRA-2, and CFSR) in theirrepresentation of the dynamical changes induced by springtime polarstratospheric ozone depletion in the Southern Hemisphere from 1980 to 2001.The intercomparison is undertaken as part of the SPARC(Stratosphere–troposphere Processes and their Role in Climate) ReanalysisIntercomparison Project (S-RIP). The reanalyses are generally in goodagreement in their representation of the strengthening of the lowerstratospheric polar vortex during the austral spring–summer season,associated with reduced radiative heating due to ozone loss, as well as thedescent of anomalously strong westerly winds into the troposphere duringsummer and the subsequent poleward displacement and intensification of thepolar front jet. Differences in the trends in zonal wind between thereanalyses are generally small compared to the mean trends. The exception isCFSR, which exhibits greater disagreement compared to the other threereanalysis datasets, with stronger westerly winds in the lower stratospherein spring and a larger poleward displacement of the tropospheric westerlyjet in summer. The dynamical changes associated with the ozone hole are examined byinvestigating the momentum budget and then the eddy heat and momentumfluxes in terms of planetary- and synoptic-scale Rossby wave contributions.The dynamical changes are consistently represented across the reanalysesand support our dynamical understanding of the response of the coupledstratosphere–troposphere system to the ozone hole. Although our resultssuggest a high degree of consistency across the four reanalysis datasets inthe representation of these dynamical changes, there are larger differencesin the wave forcing, residual circulation, and eddy propagation changes compared to the zonal wind trends. In particular, there is a noticeabledisparity in these trends in CFSR compared to the other three reanalyses,while the best agreement is found between ERA5 and JRA-55. Greateruncertainty in the components of the momentum budget, as opposed to meancirculation, suggests that the zonal wind is better constrained by theassimilation of observations compared to the wave forcing, residualcirculation, and eddy momentum and heat fluxes, which are more dependent onthe model-based forecasts that can differ between reanalyses. Lookingforward, however, these findings give us confidence that reanalysis datasetscan be used to assess changes associated with the ongoing recovery ofstratospheric ozone. 

The Southern Annular Mode (SAM) is the leading mode of extratropical Southern Hemisphere climate variability, associated with changes in the strength and position of the polar jet around Antarctica. This variability in the polar jet drives large fluctuations in the Southern Hemisphere climate, from the lower stratosphere into the troposphere, and stretching from the midlatitudes across the Southern Ocean to Antarctica. Notably, the SAM index has displayed marked positive trends in the austral summer season (stronger and poleward shifted westerlies), associated with stratospheric ozone loss. Historical reconstructions demonstrate that these recent positive SAM index values are unprecedented in the last millennia, and fall outside the range of natural climate variability. Despite these advances in the understanding of the SAM behavior, several areas of active research are identified that highlight gaps in our present knowledge.

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Paleoclimates and Current Trends > Earth System Behavior