An Investigation of the General Circulation Associated With Extreme Anomalies in Hemispheric Mean Atmospheric Mass
Christy, John Raymond
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https://hdl.handle.net/2142/70924
Description
Title
An Investigation of the General Circulation Associated With Extreme Anomalies in Hemispheric Mean Atmospheric Mass
Author(s)
Christy, John Raymond
Issue Date
1987
Doctoral Committee Chair(s)
Trenberth, Kevin E.
Department of Study
Atmospheric Sciences
Discipline
Atmospheric Sciences
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Atmospheric Science
Abstract
Significant events in terms of hemispheric mass anomalies are identified using surface data from the European Centre for Medium-Range Weather Forecasts (ECMWF) for 1979-1982. The time scale of the anomalous fluctuations in hemispheric mean atmospheric mass is found to have considerable variance in the 30-80 day period band.
The horizontal mass distribution reveals that significant events in hemispheric mean mass are accompanied by a significant increase in the presence of extreme localized stationary surface anomalies in the mid and upper latitudes of both hemispheres. The implication is that extreme cases of localized anomalies in the mass field are dependent to some degree on the global distribution of mass.
An analysis of the ECMWF pressures is performed by means of standard and complex empirical orthogonal functions (EOF). In general, there is found a strong tendency for large anomalies in the winter hemisphere midlatitudes to be offset by anomalies in the tropical and summer hemisphere midlatitude regions.
The zonal mean circulation is examined for northern winter and differences between events of excessive hemispheric mass and deficient mass are found. The method presents several zonal mean fields contained in the ECMWF data as well as some derived fields, e.g., diabatic heating rates determined as residuals of the thermodynamic equation. Hemispheric mean values of these rates for January are (K day$\sp{-1}$) SH, +0.05 and NH (Northern Hemisphere), $-$0.03 for a global mean of +0.01. There is found a positive correlation between the hemispheric atmospheric mass anomaly and the hemispheric anomaly in heating rate. The heating anomaly is due primarily to meridonal shifting of the zonal mean Hadley cell.
Significant differences in the midlatitude circulation are also detected during periods of anomalous hemispheric mean atmospheric mass. When the NH mass is less than normal, a circulation pattern resembling the Pacific/North American mode (PNA) with extended east Asia jet (i.e., negative pressure anomalies in the northern Pacific) is indicated. The opposite pattern emerges when the NH experiences a surplus in mass. In the region affected by the PNA, storm tracks are shown to be enhanced in areas of positive zonal wind anomalies. The results from anomalies in geopotential height and poleward heat fluxes point to the equivalent barotropic nature of the regional perturbations that occur in association with hemispheric mass anomalies.
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