Long‐term trend of cold air mass amount below a designated potential temperature in Northern and Southern Hemispheric winters using reanalysis data sets

This study shows that the total hemispheric cold air mass (CAM) amount below a designated potential temperature of 280 K serves as a good indicator of long‐term climate change trends in the polar region. We conduct quantitative analyses of the warming trends of Northern Hemisphere (NH) and Southern Hemisphere (SH) winters using five reanalysis data sets (Climate Forecast System Reanalysis, ERA‐Interim, Japanese 55 year Reanalysis Japanese 25 year Reanalysis, JRA‐25, and National Centers for Environmental Prediction–National Center for Atmospheric Research) and JRA‐55‐related data sets (JRA‐55 Conventional reanalysis and JRA‐55 Atmospheric Model Intercomparison Project type simulation). Total hemispheric CAM amount for the NH winter shows a significant decreasing trend for most of the reanalyses at a rate of approximately −1.37 to −0.77% decade −1 from 1959 to 2012 and at a rate of approximately −1.57 to −0.84% decade −1 from 1980 to 2012. The CAM amount trends show very different geographic patterns during the two periods, which suggests that geographical patterns of the trends estimated for these periods are largely controlled by internal low‐frequency dynamics. The CAM flux across 45°N, which serves as an indicator of hemispheric scale cold air outbreaks, has so large interannual variability that its trend becomes insignificant. In the SH winter, trends of total hemispheric CAM amount and its equatorward flux across 50°S are not consistent among the new reanalyses for 1980–2012. Although total hemispheric CAM amount decreases for 1959–2012, sparse observations and changes in systematic bias, due to the implementation of assimilation of satellite observation data in 1979, compromise the reliability of the trends. Improvements to the reanalyses are desirable as a means of monitoring historical changes in CAM for the SH winter.