Subsidence near the Equator

Abstract

ACCORDING to the classical concept of general circulation of the Earth's atmosphere, two tropical Hadley cells give a large scale vertical upward motion on both sides of the equator. Contrary to this concept, there were some early observations, and many more satellite cloud observations in the past few years, which suggest subsidence somewhere very close to the equator. These observations are briefly summarized as follows. (a) Pilots crossing the equator during the Second World War frequently reported relative clearance of clouds over the equator compared with two regions of cloudiness on either side of it. On the basis of these reports, Fletcher¹ attempted to build up a hypothesis of two equatorial indirect cells, one on each side of the equator, with a common limb of subsidence over the equator. Rossby² attributed the well known semi-desert climate of Christmas Island in the Pacific at 2° N. to subsidence of this type. (b) Radiosonde ascents at Gan Island (00° 41′ S., 73° 09′ E.) and a few available upper air observations at mid-oceanic equatorial locations in the Indian Ocean during the International Indian Ocean Expedition of 1963–64 show characteristic dryness suggestive of subsidence³. (c) Recent daily satellite cloud photographs over the equatorial Indian Ocean regularly received here generally confirm the lack of much cloudiness in the mid-ocean. Sadler's monthly mean cloud data (personal communication) suggest that this clearance of the equator in mid-oceans is even more marked in the Atlantic and the Pacific than in the Indian Ocean. (d) The well known non-formation of tropical cyclone centres in a belt of 3°–4° latitude on either side of the equator appears to be connected with some persistent feature of subsidence in the region.