Observed Structure and Propagation Characteristics of Tropical Summertime Synoptic Scale Disturbances

Abstract

The three-dimensional structure and propagation characteristics of tropical synoptic scale transients during the northern summer we studied with twice daily ECMWF global gridded analyses for the 1980?1987 period. Regions of enhanced variability in relative vorticity at 850 mb are identified in the western Pacific, eastern Pacific, Bay of Bengal/northern India and eastern Atlantic/western Africa sectors. Dominant spectral peaks with time scales ranging from 3 to 8 days are noted in the power spectra for these locations. The lag-correlation and regression statistics of tropical fluctuations with synoptic time scales are examined. Strong teleconnectivity and temporal coherence are found over all of the active sites with enhanced vorticity variance, as well as over the western Atlantic/Caribbean and the Indochinese Peninsula. These results indicate that a substantial amount of synoptic scale variability in the tropics is associated with propagating wavelike disturbances that remain coherent over several days. The disturbances in all active regions tend to travel west/northwestward. The eastern portion of each active site is characterized by rapid growth of the disturbances, whereas decay typically occurs in the western portion. The transient behavior throughout the tropics is also investigated using Extended Empirical Orthogonal Function (EEOF) techniques. The sites of activity thus identified coincide with the locations inferred from the lag-correlation analyses. Using time series of the EEOF coefficients as a reference, the temporal evolution as well as the horizontal and vertical structure of the disturbances occurring in each active region are delineated by composites of selected meteorological variables. Well-defined in vorticity, vertical velocity, temperature and humility at various tropospheric levels, as well as convective activity (deduced from the outgoing longwave radiation field), are discernible in the disturbances at various sites. Phase relationships among different variables are interpreted in terms of dynamical and physical processes operating within the disturbances. The horizontal phase tilt of the fluctuations and their positions relative to the ambient mean circulation suggest a tendency for kinetic energy transfer from the quasi-stationary flow to the transient eddies. Most of the findings reported here are in accord with previous investigations based on different analysis tools and more limited datasets. Whereas considerable similarities are noted among disturbances occurring over various active maritime sites, the perturbations over central and western Africa exhibit structural characteristics that are unique to that region. Specifically, two propagation tracks are identified in the African sector. The northern track along southern Sahara consists mostly of eddies commonly found over arid zones, with ascent of warm and dry air over surface troughs. The southern track is collocated with the climatological rainfall maximum at about 10°N, and is associated with moist convective systems.