The Reliability of Antarctic Tropospheric Pressure and Temperature in the Latest Global Reanalyses

In this study, surface and radiosonde data from staffed Antarctic observation stations are compared to output from five reanalyses [Climate Forecast System Reanalysis (CFSR), 40-yr ECMWF Re-Analysis (ERA-40), ECMWF Interim Re-Analysis (ERA-Interim), Japanese 25-year Reanalysis (JRA-25), and Modern Era Retrospective-Analysis for Research and Applications (MERRA)] over three decades spanning 1979-2008. Bias and year-to-year correlation between the reanalyses and observations are assessed for four variables: mean sea level pressure (MSLP), near-surface air temperature (T-s), 500-hPa geopotential height (H-500), and 500-hPa temperature (T-500). It was found that CFSR and MERRA are of a sufficiently high resolution for the height of the orography to be accurately reproduced at coastal observation stations. Progressively larger negative T-s biases at these coastal stations are apparent for reanalyses in order of decreasing resolution. However, orography height bias cannot explain large winter warm biases in CFSR. JRA-25, and MERRA (11.1 degrees, 10.2 degrees, and 7.9 degrees C, respectively) at Amundsen-Scott and Vostok, which have been linked to problems with representing the surface energy balance. Linear trends in the annual-mean T-500 and H-500 averaged over Antarctica as a whole were found to be most reliable in CFSR, ERA-Interim, and MERRA, none of which show significant trends over the period 1979-2008. In contrast JRA-25 shows significant negative trends over 1979-2008 and ERA-40 gives significant positive trends during the 1980s (evident in both T-500 and H-500). Comparison to observations indicates that the positive trend in ERA-40 is spurious. At the smaller spatial scale of individual stations all five reanalyses have some spurious trends. However, ERA-Interim was found to be the most reliable for MSLP and H-500 trends at station locations.