Mesoscale Structure of the Low-Level Flow near the Equatorial East African Coast

Abstract

A field experiment called MONSOON 77 was organized in June-July 1977 to delineate the kinematics and thermodynamics of the low-level flow near the equator over the East African coast. The National Center for Atmospheric Research's Electra flew the aerial missions in a sawtooth pattern and collected Meteorological data which were subsequently processed. Out of the 11 missions flown from Nairobi, Kenya, the trade-dominated missions of 11 June and 29 June and the Mozambique Channel dominated flights of 15 June and 4 July were selected for a detailed study. Both the eastbound and westbound flights for each day were analyzed. Meteorological fields of v (meridional wind), u (zonal wind), T (temperature), and q (mixing ratio) were displayed along equatorial cross selections from 37 to 44°E. These cross sections extended from the surface to 700 mb. For brevity, only a few of these cross sections are discussed. Three of the eastbound (westbound) cross sections are described for 0800 (1100) local lime. The eastbound (westbound) cross section on 29 June 1977,. however, represents 2000 (2300) local time. The highlights of these cross sections are: 1) pronounced diurnal variation of v- and u-components over land; 2) synoptic variation of the low-level meridional wind maximum, both over land and over water, 3) the existence of stable stratification over water between 850 and 750 mb extending typically about 200 km horizontally; 4) the presence of a dry layer (2 g kg-1) near 700 mb, usually associated with westerlies., and 5) vertical buildup of clouds, apparently in response to the divergence-convergence patterns, modifying the existing temperature and moisture fields. The cross-equatorial water vapor transport, based on the analyzed v- and q-fields, registered less than 10% diurnal variation and about 17% synoptic variation, with respect to the mean. Furthermore, this calculation also showed that earlier, similar calculations have underestimated the transport in the western Indian Ocean because of lack of data. A calculation of the momentum transport revealed that the cross-equatorial flow is promoted or inhibited depending on whether south-southeasterlies or south-southwesterlies exist on these cross sections. The observation of south-southeasterlies or south-southwesterlies, in turn, is contingent on whether the flight track is south or north of the shear line that is commonly present in this geographical area. The shear line itself is associated with an equatorial synoptic system, called the Southern Equatorial Trough.