1984 Volume 62 Issue 3 Pages 440-468
Based on FGGE Level IIIb data, the structural features of 40-50 day oscillations over ann extensive region (30°S-30°N, 30°E-150°W) during the 1979 summer are detailed. The analysis confirms earlier investigations that these low frequency modes are primarily associated with the zonal wind oscillations. These 40-50 day perturbations propagate northward and eastward, which is most clearly defined over the monsoon region north of the equator from 60° to 150°E. The monsoon region is characterized by prominent spectral peaks in the 850mb meridional winds with periods shorter than 10 days, probably reflecting the activities of monsoon disturbances. However, the local Hadley circulation, as defined by averaging the meridional component of the wind between 60° and 150°E, exhibits a distinct spectral peak in the period range of 40-50 days. Similarly, the square of the meridional winds, which is a measure of synoptic-scale disturbance activity, also changes with a period of 40-50 days. These features, which are similar to the group velocity phenomena, are pronounced only over the central monsoon region (10°-20°N, 60°-150°E). The low frequency modes propagate northward and become most intensified near 10°-20°N through mutual interaction between synoptic-scale disturbances, the local Hadley circulation, and the tonal mean flows over the monsoon region.
At the equator, the 40-50 day zonal wind pertubations propagate systematically eastward (500km/day) and upward (0.7km/day). In the equatorial region, the low frequency oscillations owe their existence to a lateral geopotential wave-energy flux from the monsoon region, which represents the major energy source for 40-50 day perturbations via the conversion from potential to kinetic enegy.
Compared to the equator, the phase propagation of zonal wind perturbations along 15°N, although moving eastward, is not as systematic. At this latitude, zonal wind perturbations are pronounced in the lower troposphere over the monsoon region, and also in the upper troposphere over the western Pacific. As an integral part of E-W interaction between these two regimes, there occurs downward progression of westerly (or easterly) perturbations over to the Arabian Sea region. The downward phase of westerly (easterly) modes corresponds to the commencement of "active" ("break") monsoons over South and Southeast Asia.