Upper Tropospheric Wave Disturbances Observed in the Arabian Sea during the Summer MONEX

Abstract

The upper tropospheric disturbances propagating westward with a period of about 4 days and a wavelength of 2000km-3000km are detected over the equatorial Arabian Sea by the stationary polygon ship observations during the period from 18 May to 30 May 1979. The disturbances are concentrated in the shallow layer between 300mb and 150mb with the strongest meridional wind fluctuations (-5ms-1 amplitude) and relative vorticities (-2×10-5s-1) near 200mb. Convergence occurs on and ahead of the cyclonic circulation center and divergence behind the center. Temperature anomalies due to the disturbances are quite small.
Analyses of 200-mb wind fields using the ECMWF FGGE level III-b data show that these disturbances are present in the equatorial Arabian Sea region (50°E-80°E) during the limited short period (middle May-middle June). Wind maps for individual disturbances reveal that these disturbances are initiated by the deepening of the extended troughs from middle latitudes which are located temporarily over the central Arabian Sea during the premonsoon period.
There is some indication that the upper tropospheric disturbances have links with the lower tropospheric disturbances.