On the role of the Asian monsoon in the angular momentum and kinetic energy balances of the tropics

The balance conditions of relative angular momentum and time-mean kinetic energy and their annual variations are studied for the Northern Hemisphere tropical belt. The belt is divided into two equal size parts; the monsoon, and the extramonsoon regions. The data used consist of available daily rawinsonde reports from the world aerological network for the two five-year periods 1958-1963 and 1968-1973. In winter, the trade winds in the monsoon and extramonsoon regions are both sources of westerly relative angular momentum for the middle latitude circulation; however, the angular momentum gained in the extramonsoon region of the Tropics is mostly destroyed by a net southward flow of mass in that region, and becomes regenerated in the monsoon region by a net northward flow of mass there. This excess of angular momentum, with the angular momentum picked up locally in the monsoon region, is almost all exported across its northern boundary. In winter, the Tropics are also an important source of mean kinetic energy for middle latitudes. Again, almost all export of kinetic energy takes place across the northern boundary of the monsoon sector. Most of this energy must be generated through the pressure gradient term inside the monsoon region itself, since the transformation from transient eddy kinetic energy is small. The proper evaluation of the pressure gradient is the main stumbling block in the study, preventing the authors from estimating the generation and therefore, as a residual, the frictional dissipation in the two regions. In summer, the extramonsoon region remains a source of angular momentum, but the monsoon region with its surface westerlies acts as a sink, leading to a sharp reduction (and even a midsummer reversal) of the export into middle latitudes. The export of mean kinetic energy almost vanishes in summer, except for a small southward transfer across the Equator. The calculations for two five-year periods give similar estimates, and, thereby, show the reliability of the results.