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Marco L. Carrera
and
John R. Gyakum

Abstract

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.

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Marco L. Carrera
and
John R. Gyakum

Abstract

A recent study of significant events of atmospheric mass depletion from the Northern Hemisphere (NH) during the extended boreal winter indicated that Southeast Asian pressure surges were an important physical mechanism that acted to channel the atmospheric mass equatorward out of the NH on a rapid time scale. This study builds upon this finding and examines both the direct and indirect roles of Southeast Asian pressure surges for a particular event of dry atmospheric mass depletion from the NH. The focus of this study is on the enhanced interhemispheric interactions and associated Southern Hemisphere (SH) tropical and extratropical responses resulting from the pressure surges.

First, this study examines the conservation of dry atmospheric mass (i.e., the relationship between the dry meridional winds and the area-integrated dry air surface pressure) in the NCEP reanalysis for the 25 significant events of dry atmospheric mass depletion from the NH. Results indicate that the NCEP dry meridional winds are able to qualitatively capture the dry atmospheric mass evacuation from the NH. In a quantitative sense there is very good agreement between the wind and pressure data in the extratropics of both hemispheres. A distinct negative or southward bias in the NCEP vertically and zonally integrated dry meridional winds is apparent between 5° and 17.5°N. This southward bias was not present in the ECMWF Re-Analysis. The source of the southward bias in NCEP appears to result from a weaker analyzed ITCZ.

The particular case of dry atmospheric mass depletion from the NH examined in detail is associated with an intense pressure surge over Southeast Asia. A significant enhancement of convection in the monsoon trough region of northern Australia occurs roughly 4 days after the peak intensity of the Siberian high. A low-level westerly wind burst develops in response to this enhanced zonal pressure gradient caused by the pressure surge as part of the onset of an active phase of the Australian summer monsoon. This study shows that three prominent anticyclonic circulations intensify in the SH extratropics, stretching from the south Indian Ocean to the South Pacific, beneath regions of upper-tropospheric dry atmospheric mass convergence, originating partly from the monsoon convection outflow. These anticyclonic circulations are regional manifestations of the dry atmospheric mass increase in the SH.

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