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Abstract

We define the cold airmass (CAM) as airmass of lower than a designated potential temperature and quantitatively explore the geographical distributions of polar cold airmass (CAM), negative heat content (NHC) and their horizontal fluxes. CAM and NHC are adiabatically conservative materials confined in the lower troposphere of the extratropics. The threshold is specified to beθ=280K in DJF by considering the mean-meridional isentropic mass circulation in the extratropics. The cold airmass is generated by radiative cooling over the high-latitude continents, flowed out toward lower latitudes, and extinguished mainly by the surface heat fluxes near the storm tracks over the ocean. The CAM trajectories can be traced in a Lagrangian sense, by means of vertical mean flux velocity. In the Northern Hemispheric winter, CAM has two main streams: East Asian stream and the North American stream. CAM amount and flux are good indicators of extremely cold events which intermittently occur in the favorable synoptic situation. CAM gradually disappears over the warm ocean. Its front head sometimes proceeds southward, reaches the tropics and initiates heavy precipitation. Interannual variability of seasonal mean cold airmass stream is under significant control of the Pacific Ocean SST. Rossby waves generated by anomalous deep convections propagate toward the extratropics and affect synoptic situations related to CAOs. The interannual decreasing trend in hemispheric total amount of CAM is a good indicator of global warming. It is significant that decreasing rates are increasing by decreasing the threshold potential temperature.