Spatial and temporal temperature trends in the lower stratosphere during the extended boreal winter from reanalyses

Abstract

Temperature anomalies in the lower stratosphere (namely, at 50-hPa) poleward of 30°N (hereafter, T50) are analysed from reanalysis daily products (i.e. ERA-40 and NCEP/NCAR) in order to detect those regions with statistically significant trends of T50 during the common 1957–2002 period. We also analyse radiosonde data in order to validate the reanalyses results. The analyses are conducted for the extended polar winter (i.e. from November to May) in order to relate T50 changes to the northern polar vortex variability. In relation to the previous literature, the novelty of this study is double: first, temporal evolution is considered according to regions with similar T50 temporal variability in the Northern Hemisphere; second, trend analyses of stratospheric temperatures are obtained with the use of running windows with variable width of each principal component series computed from monthly mean values. Two main stratospheric regions were identified from both reanalyses, with a statistically significant cooling within the study period: one over the high latitudes and a second one tracing a subtropical ring. The first one was detected in November, December and May during several decades, particularly at the beginning and in the middle of the study period (i.e. 1957–1985). The second region (i.e. the subtropical ring) showed overall cooling for all months along the study period. In addition, the lower stratospheric temperature over northern Europe exhibited an outstanding cooling in May for the whole study period. The results obtained from both reanalyses are practically identical, a fact that provides robustness thereto. Radiosonde data confirm the above-mentioned results, but the magnitude of the trends given by reanalysis products is substantially overestimated in the winter months over high latitudes.