Contrasting meteorological structure of intra-seasonal wet and dry spells in Zimbabwe



The meteorological structure of contrasting wet and dry spells in Zimbabwe, south-east Africa is studied using European Centre for Medium Range Weather Forecasts composites. Diagnostic examination of composite synoptic-scale features indicates that lower northerly and upper easterly flow is enhanced during wet spells, corresponding with a tropical low over Zambia and an anticyclone off the south-east coast of Africa, respectively. Dry spells exhibit an increase in mid-latitude cyclones off the south-west coast of Africa and tropical cyclones near Madagascar. Mid-tropospheric troughs are located over the east and west coasts of Africa near 25°S and serve to increase anticyclonic vorticity over Zimbabwe in dry spells. Specific humidity fields reveal that the maximum shifts southward 5° and eastward 10° in the wet spells. Cyclonic vorticity and low geopotential heights occur on two or three days in the wet spell, coincident with intense cumulus convection and high rain rates. The eddy covariances of zonal wind and specific humidity are dominated by westward (eastward) fluxes in wet (dry) spells; while poleward moisture fluxes contributed by transient eddies are largest over the south Mozambique Channel (south-east Atlantic) in wet (dry) spells. The eddy fluxes are shifted 15–20° longitude eastward in the wet spells. The relative contribution of thermodynamic and kinematic inputs are evaluated in the mean flows and transient eddies for contrasting wet and dry spells. Much of the difference between wet and dry spells is explained by the mean flow, while the transient flux in the mid-latitudes assumes a subordinate role.