Significant Events of Interhemispheric Atmospheric Mass Exchange: Composite Structure and Evolution

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 subseasonal 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 surface pressure, significant events when the Northern Hemisphere (NH) loses dry atmospheric mass on subseasonal time scales during the boreal winter from 1968 to 1997 are identified. A total of 25 events is found, with a preferred time scale of 9 days from the time of maximum to minimum NH dry atmospheric mass. The linear correlation coefficient between the dry atmospheric mass anomalies for the NH and Southern Hemisphere (SH) is —0.91 for the 25 events, indicating very strong interhemispheric compensation and increasing confidence in the suitability of the NCEP-NCAR reanalysis 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 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 out of the NH extratropics 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, two noted regions of atmospheric blocking.