No abstract

An updated assessment of uncertainties in ''observed'' climatological winds and temperatures in the middle atmosphere (over altitudes ϳ10-80 km) is provided by detailed intercomparisons of contemporary and historic datasets. These datasets include global meteorological analyses and assimilations, climatologies derived from research satellite measurements, historical reference atmosphere circulation statistics, rocketsonde wind and temperature data, and lidar temperature measurements. The comparisons focus on a few basic circulation statistics (temperatures and zonal winds), with special attention given to tropical variability. Notable differences are found between analyses for temperatures near the tropical tropopause and polar lower stratosphere, temperatures near the global stratopause, and zonal winds throughout the Tropics. Comparisons of historical reference atmosphere and rocketsonde temperatures with more recent global analyses show the influence of decadal-scale cooling of the stratosphere and mesosphere. Detailed comparisons of the tropical semiannual oscillation (SAO) and quasibiennial oscillation (QBO) show large differences in amplitude between analyses; recent data assimilation schemes show the best agreement with equatorial radiosonde, rocket, and satellite data.

The global climate during 1998 was affected by opposite extremes of the ENSO cycle, with one of the strongest Pacific warm episodes (El Nino) in the historical record continuing during January-early May and Pacific cold episode (La Nina) conditions occurring from July-December. In both periods, regional temperature, rainfall, and atmospheric circulation patterns across the Pacific Ocean and the Americas were generally consistent with those observed during past warm and cold episodes.

A quantitative comparison of six meteorological analyses is presented for the cold 1999/2000 and 1995/1996 Arctic winters. Using different analyzed data sets to obtain temperatures and temperature histories can have significant consequences. The area with temperatures below a polar stratospheric cloud (PSC) formation threshold commonly varies by ∼25% between the analyses, with some differences over 50%. Biases between analyses vary from year to year; in January 2000, Met Office analyses were coldest and National Centers for Environmental Prediction (NCEP) analyses were warmest, while NCEP analyses were usually coldest in 1995/1996 and NCEP/National Center for Atmospheric Research Reanalysis (REAN) were usually warmest. Freie Universität Berlin analyses are often colder than others at T ≲ 205 K. European Centre for Medium‐Range Weather Forecasts (ECMWF) temperatures agreed better with other analyses in 1999/2000, after improvements in the assimilation system, than in 1995/1996. Temperature history case studies show substantial differences using Met Office, NCEP, REAN, ECMWF, and NASA Data Assimilation Office (DAO) analyses. In January 2000 (when a large cold region was centered in the polar vortex), all analyses gave qualitatively similar results. However, in February 2000 (a much warmer period) and in January and February 1996 (comparably cold to January 2000 but with the cold region near the polar vortex edge), distributions of “potential PSC lifetimes” and total time spent below a PSC formation threshold varied significantly between the analyses. Largest peaks in “PSC lifetime” distributions in January 2000 were at 4–6 and 11–14 days, while in 1996 they were at 1–3 days. Different meteorological conditions in comparably cold winters have a large impact on expectations for PSC formation and on the effects of discrepancies between different meteorological analyses. Met Office, NCEP, REAN, ECMWF, and DAO analyses are commonly used in modeling polar processes; the choice of analysis can strongly influence the results of such studies.