Large-Scale Redistributions of Atmospheric Mass

Seven years of daily global surface pressure (Ps,) analyses derived from European Centre for Medium Range Forecasts (ECMWF) data are examined to describe more fully interhemispheric mass exchanges and intraseasonal variability. Extreme events in hemispheric mean Ps are determined, and composited grid point differences show that hemispheric anomalies are mainly determined by pressures in the North Pacific, western North Atlantic, northern Asia and the Southern Hemisphere (SH) circumpolar trough. Seasonal differences in the composites indicate that the regional anomalies occur farther poleward in the winter hemisphere, and the tropical anomalies tend to have the same sign as that of the summer hemispheric mean anomaly. Long-lasting, localized, extreme Ps anomalies are identified in 18 significant events of hemispheric mass imbalance, and are found to be highly favored when the hemispheric mean departs significantly from normal. The result implies that regionally persistent anomalies are related to global-scale mass redistributions, rather than being totally the result of more localized redistributions. The global atmospheric angular momentum exhibits significant changes during interhemispheric mass imbalances that exceed one standard deviation (about 0.4 mb). There is a strong tendency for the hemisphere in which a deficit of mass occurs to experience, on average, a 5% increase in hemispheric angular momentum. Zonal complex empirical orthogonal functions are used to describe the Ps cos ϕ anomalies, filtered for 30–75 day fluctuations. Dominant modes are found in which each hemisphere, independently, produced intrahemispheric exchanges between polar and temperate latitudes. An interhemispheric mode indicates exchanges of mass between the midlatitudes of the Northern Hemisphere and the entire tropics plus the SH subtropics. The interhemispheric mode displays a southward propagation of anomalies from the tropical belt into the SH.