Contraction of the Northern Hemisphere, lower tropospheric, wintertime cold pool over the past 66 years

Employing reanalysis data sets, several threshold temperatures at 850 hPa are used to measure the wintertime (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 -5°C air.

Composite differences in lower tropospheric temperature, middle tropospheric geopotential height and tropopause-level jet anomalies between the 5 coldest and 5 warmest years are considered. Cold years are characterized by an equatorward expansion of the jet in the Pacific and Atlantic sectors of the hemisphere and by invigorated cold air production in high latitude Eurasia and North America. Systematic poleward encroachment of the -5°C 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 cold Northern 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-year time series.