| Summary: | The various modes of atmospheric mass redistribution characterize the principal variations of the general circulation of the atmosphere. Interhemispheric exchanges of atmospheric mass occur with considerable regularity on intraseasonal time-scales. Observational evidence from previous studies indicates that anomalous and persistent regional atmospheric mass distributions (e.g., atmospheric blocking) may often be related to interhemispheric atmospheric mass exchange. Using the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis, we identify significant events when the northern hemisphere (NH) loses dry atmospheric mass on subseasonal time-scales during the boreal winter from 1968 to 1997. A total of 25 events is found, with a preferred time-scale of 9 days. The linear correlation coefficient between the dry atmospheric mass anomalies for the NH and southern hemisphere (SH) is -0.91 for the 25 significant events, indicating very strong interhemispheric compensation and increasing our confidence in the suitability of this dataset for the study of interhemispheric dry atmospheric mass exchange. Positive sea-level pressure anomalies are found over northern Eurasia, the North Pacific and the North Atlantic prior to the onset of the composite NH dry atmospheric mass collapse event. Over northern Eurasia the positive atmospheric mass anomaly associated with the building of the Siberian high is found to be a statistically significant precursor to the events. The breakdown of NH dry atmospheric mass occurs in association with the decay of the positive atmospheric mass anomaly in the North Pacific as a cyclone deepens explosively in the Gulf of Alaska. Pressure surges over Southeast Asia and North America, associated with statistically significant positive atmospheric mass anomalies, are mechanisms that act to channel the atmospheric mass equatorward on a rapid time-scale (~4 days). The dry atmospheric mass increase in the SH is manifested as enhanced surface ridging over the South Pacific and South Indian Oceans. Preferential interhemispheric interaction is found in the region between 100°E and 130°E, and over the central Pacific in the vicinity of the dateline. A prominent channel of southeastward dry atmospheric mass flux, emanating from the Australian continent, combined with a southward channel from the equatorial central Pacific, contribute to the atmospheric mass buildup over the South Pacific. The role of a Southeast Asian pressure surge was examined for a representative event. A large evacuation of atmospheric mass from northern Eurasia occurs as the atmospheric mass surges equatorward and into the SH. Along the west coast of Australia, a southerly pressure surge extends equatorward and converges with the northerly surge to create a pronounced near equatorial zonal pressure gradient. A low-level westerly wind burst develops in response to this enhanced zonal pressure gradient as part of the onset of an active phase of the Australian summer monsoon. We show that three prominent anticyclonic circulations intensify in the southern hemisphere extratropics, stretching from the South Indian Ocean to the South Pacific, beneath regions of upper tropospheric dry atmospheric mass convergence, originating from the monsoon convection outflow. These anticyclonic circulations are largely responsible for the dry atmospheric mass increase in the SH.
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