Northern Summer Tropical Circulations During Drought and Normal Rainfall Months

Abstract

We contrast the 200 mb flow regimes during a drought year (1972) with those during a normal rainfall year (1967) over the global tropics for the northern summer months. It is shown that the deficient rainfall over central India and western Africa during 1972 may be related to the following: 1) warm sea surface temperatures over the equatorial Pacific; 2) excessive number of typhoon days over the western Pacific; 3) strong east-northeasterlies over the equatorial eastern Indian ocean (related to upper level outflows from typhoons); 4) weaker tropical easterly jet; 5) weaker meridional pressure gradient over India; 6) weaker Tibetan high; 7) a southeastward shift of the major circulation patterns as well as of several dynamical parameters; 8) weaker vertical wind shear and a weaker measure of the combined barotropic-baroclinic instability over West Africa; and 9) weaker westward steering for rain-producing disturbances over India and a consequent stronger influence of the mountains. A sequential in... Abstract We contrast the 200 mb flow regimes during a drought year (1972) with those during a normal rainfall year (1967) over the global tropics for the northern summer months. It is shown that the deficient rainfall over central India and western Africa during 1972 may be related to the following: 1) warm sea surface temperatures over the equatorial Pacific; 2) excessive number of typhoon days over the western Pacific; 3) strong east-northeasterlies over the equatorial eastern Indian ocean (related to upper level outflows from typhoons); 4) weaker tropical easterly jet; 5) weaker meridional pressure gradient over India; 6) weaker Tibetan high; 7) a southeastward shift of the major circulation patterns as well as of several dynamical parameters; 8) weaker vertical wind shear and a weaker measure of the combined barotropic-baroclinic instability over West Africa; and 9) weaker westward steering for rain-producing disturbances over India and a consequent stronger influence of the mountains. A sequential in...