On the Role of the Tropics in the General Circulation of the Atmosphere

Abstract

Present knowledge on the role of the tropics in the general circulation is reviewed in comparison with our knowledge twenty years ago. Increasing amounts of upper‐air data in the tropics since that time plus the advent of rawin measurements have placed many calculations of the mean seasonal exchange of heat, moisture and angular momentum between the tropics and the extratropical regions on a fairly firm basis. The large troughs seen extending from higher latitudes to the heart of the tropics even with late World War II observations have been confirmed in their role as the principal exchange mechanism. On the other hand, the mean meridional circulation can account for the upward transport of angular momentum which is generated with easterlies within the tropics and also for the net generation of kinetic energy south of 30°N. About 50 per cent of the kinetic energy required north of 30°N in winter to maintain the circulation against frictional dissipation, is imported from the tropics by means of the standing waves of the subtropical jet‐stream. Hence it is clear that the tropics help to sustain the kinetic energy of the westerlies and that the tropical circulation is not driven from higher latitudes as often claimed. The sense of total energy transport by the mean meridional circulation inside the tropics is not easily calculated and may even reverse sign. Complications on total energy export also have arisen through the nimbus ii albedo observations which have shown lower albedo (less cloudiness), hence also a larger energy source, for the tropics than previously estimated. It appears certain, merely, that the equatorial trough migrates seasonally so that energy very likely does not pass through it, hence that it constitutes a true “meteorological equator”. No evidence concerning important coupling between the hemispheres across the equator has come to light; however, the new data systems of the next decade may well alter this conclusion. Because of the sensitive computational situation concerning energy transport by the mean meridional circulation, it may be that successful operational application of cloud seeding techniques, such as demonstrated in Project Stormfury in the Caribbean, could exert a subtle influence on the general circulation. This apart, the present status of observational networks is judged to be so far advanced that fluctuations of quantities such as the mean meridional circulation within a season, and in the same month from year to year, can shortly be calculated, possibly shedding light on the variability of the general circulation and precipitation excesses and deficits in tropical regions.

Tropical Cyclones

“…There seems to be a paradox in the fact that while the Hadley cell is most intense in winter, the frequency ot oceanic tropical disturbances, which one would expect to be an important contributor to its rising branch, is greatest in summer." Even if temporally and longitudinally varying eddy transfer associated with pressure troughs, rather than the zonally symmetric mean meridional (Hadley) cell, is the primary mechanism by which moisture and angular momentum are transported from the tropics to the midlatitudes (Riehl, 1954, chapter 12;Riehl, 1969a (Palmen and Newton, 1969, p. 572…”

“…The tropical ambient must be close to moist adiabatic for cumulonimbi to be significant; this is in fact the case (Riehl, 1969a(Riehl, , 1969b). If the ambient were moist adiabatic, cumulonimbi would be unnecessary because the ambient would be too unstable (there would be gross convective lifting).…”