The seasonal cycle of redistribution of atmospheric mass between continent and ocean in the Northern Hemisphere

Using NCEP/NCAR and ERA-40 reanalyses, we studied the seasonal cycle of redistribution of air mass between continents and oceans over the Northern Hemisphere. Our results demonstrate that air mass in the Northern Hemisphere shifts clearly between continents and oceans when the season cycles. In July, the air mass reaches its lowest over Eurasia and its highest over the Pacific, and the opposite occurs in January. However, a different scenario is observed over the north Atlantic; the accumulated air mass reaches its maximum there in May. The maintenance of the accumulation or loss of air mass in a region is found to be related to the areal mean air mass flux divergence and the difference between precipitation and evaporation in an air column. The zonal-vertical circulations change with season, with the air ascent and decent reversed between land and sea. Besides, there also exists a noticeable difference of water vapor content of the air between continents and oceans, and this difference is season-dependent. Physically, the vapor content is able to significantly affect the atmosphere in absorbing solar short- and earth’s long-wave radiations, hence influencing atmospheric thermal conditions. The land-sea thermal contrasts inclusive of the diabatic heating rate changes their signs with season going on, resulting in the reversal of orientations of the temperature gradient. These thermal forcings not only facilitate the formation of the monsoons but also indirectly induce the seasonal cycle of the air mass exchanging over regions between continents and oceans.