Contraction of the Northern Hemisphere, Lower-Tropospheric, Wintertime Cold Pool over the Past 66 Years

Employing reanalysis datasets, several threshold temperatures at 850 hPa are used tomeasure the wintertime [December–February (DJF)] areal extent of the lower-tropospheric, Northern Hemisphere, cold-air pool over the past 66 cold seasons. The analysis indicates a systematic contraction of the cold pool at each of the threshold temperatures. Special emphasis is placed on analysis of the trends in the extent of the 258C air. Composite differences in lower-tropospheric temperature, midtropospheric geopotential height, and tropopause level jet anomalies between the five coldest and five warmest years are considered. Cold years are characterized by an equatorward expansion of the jet in the Pacific andAtlantic sectors of the hemisphere and by invigorated cold-air production in high-latitude Eurasia and North America. Systematic poleward encroachment of the 258C isotherm in the exit regions of the storm tracks accounts for nearly 50% of the observed contraction of the hemispheric wintertime cold pool since 1948. It is suggested that this trend is linked to displacement of the storm tracks associated with global warming. Correlation analyses suggest that the interannual variability of the areal extent of the 850-hPa cold pool is unrelated to variations in hemispheric snow cover, the Arctic Oscillation, or the phase and intensity of ENSO. A modest statistical connection with the East Asian winter monsoon, however, does appear to exist. Importantly, there is no evidence that a resurgent trend in coldNorthern Hemisphere winters is ongoing. In fact, the winter of 2013/14, though desperately cold in North America, was the warmest ever observed in the 66-yr time series.