Trends in Antarctic geopotential height and temperature: A comparison between radiosonde and NCEP-NCAR reanalysis data
A comparison of 40-yr (1960-99) trends in Antarctic geopotential height and temperature from quality controlled radiosonde observations and NCEP-NCAR reanalysis (NNR) data is undertaken. Observations from four Antarctic stations-having sufficiently long-term and consistent datasets-at four pressure levels (850, 500, 300, and 100 hPa) are utilized. The NNR reveals substantial negative trends in tropospheric geopotential height at high southern latitudes with no significant trends seen in the lower stratosphere above Antarctica. In contrast, observations indicate only minor negative trends in tropospheric height, while statistically significant decreases in height in the lower stratosphere have occurred over East Antarctica. However, both NNR and observations show a consistent, significant warming (similar to1degreesC) in the lower troposphere (>500 hPa) above coastal Antarctica. At higher altitudes, trends derived from the two datasets diverge; the NNR fails to capture the marked cooling in the lower stratosphere associated with seasonal ozone loss. Differences in the trends are principally caused by NNR errors prior to the assimilation of satellite sounder data, which coincides with significant jumps in NNR upper-air temperatures. A rapid drop in NNR tropospheric geopotential height across East Antarctica as recently as 1993 is traced to the introduction of automatic weather stations in the region. Errors in the model height of these surface pressure data cause a significant climate jump in the NNR not observed in comparable models. Such spurious jumps considerably diminish the usefulness of the NNR for climate change studies in Antarctica.