1.5
Two complementary views of the atmospheric general circulation: the zonal-mean and zonally asymmetric perspectives
Ngar-Cheung Lau, NOAA/GFDL, Princeton, NJ
A review is offered on the contributions of Gene Rasmusson to a comprehensive documentation of the zonally averaged general circulation of the observed atmosphere, and to our understanding of the maintenance of this circulation.
Subsequent works by other investigators, which focus on the nature of departures of the time-mean circulation from zonal symmetry, are then described. By compiling the three-dimensional distribution of circulation statistics on the basis of gridded synoptic weather charts produced at operational weather centers, the latter studies highlight the longitudinal dependence of various circulation features. Atmospheric diagnostics under this zonally asymmetric framework have yielded considerable insights in the nature of local interactions between the time-mean and time-varying (transient) components of the circulation, as well as between transient fluctuations of different time scales. Mapping of the pertinent meteorological signals in the full three-dimensional domain also facilitates the analysis of feedbacks between the atmospheric circulation and boundary forcings in various geographical locations, such as those associated with influences of sea surface temperature (SST) anomalies, orography and ground hydrology.
Notwithstanding the demonstrable utility of the approach emphasizing longitudinally varying aspects of the general circulation, still more recent investigations indicate that many prevalent modes of atmospheric variability are characterized by a high degree of zonal symmetry. Examples of atmospheric patterns with a distinct zonal mean component include annual modes with large vertical extent in the middle- and high-latitude zones of both hemispheres, upper tropospheric response of the tropical geopotential height to SST forcing related to El Nino-Southern Oscillation, mid-latitude atmospheric anomalies occurring in prolonged drought episodes, as well as circulation shifts accompanying past and projected climate changes. The prevalence of these structures suggests that zonal mean diagnoses remain to be a powerful tool for examining certain facets of the general circulation.
The above survey illustrates the complementary nature of the zonal-mean and zonally asymmetric views of the general circulation. The applicability of a particular approach is contingent upon the phenomenon or scientific issue at hand.
Recorded presentationSession 1, Oral Session Part I
Thursday, 18 January 2007, 8:20 AM-12:00 PM, 217C
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