Can Current Reanalyses Accurately Portray Changes in Southern Annular Mode Structure Prior to 1979?

Abstract

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 1950–1979 (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.

Spatially heterogeneous effect of the climate warming on the Arctic land ice

“…The latter is constant at approximately r = 0.99 from 1950 to the present day. The analysis nevertheless reveals a slight increase of the correlation coefficient in the 2m temperature, from 0.96 in 1955 to 0.97 in 1985 as a results of satellite datasets assimilation (in particular sea ice cover (SIC) and sea surface temperature (SST)) after 1979: this strongly influences the reconstructions elsewhere, mostly where observational data was scarce (Marshall et al, 2022). However, the good amount of older observations in the Arctic (compared to the Southern Hemisphere) explains the good performance of MAR forced by ERA-5 before 1979 (Hersbach et al, 2020); and even before 1957 the International Geophysical Year (see Table S1).…”

“…Because our study goes up to 1950, it is worth evaluating the precision as a function of time: the further we go back, the fewer observations available. For example, ERA-5 prior to 1979 has been shown to be performing poorly above the Antarctic, because of the scarcity of satellite observations over the continent (Marshall et al, 2022).…”