The 40–50 Day Oscillation of the Low-Level Monsoon Circulation over the Indian Ocean

Abstract

It has been observed that the low-level monsoon circulation, especially the Somali jet, exhibits a 40?50 day oscillation and obtains its maximum intensity in this oscillation when the migrating transient monsoon trough approaches ?20°N. The data generated by the FGGE III-b analyses of the European Centre for Medium Range Weather Forecasts for the northern summer were used to explore the air mass source of this oscillation and to explain energetically and synoptically the intensification and decay of the low-level monsoon circulation in association with the 40?50 day oscillation. A synoptic analysis of the divergent wind fields suggests that the convergence induced by the intertropical convergence zone and the deepening of the monsoon trough over northern India supplies the air mass to the 40?50 day oscillation. The energetics analysis shows that the 40?50 day oscillation of the low-level monsoon circulation is essentially described by the rotational mode. The 40?50 day oscillation of this flow field is maintained both by the generation of the 40?50 day rotational kinetic energy, i.e., the work done by the cross contour rotational mode [G(kR?)], and by the interaction between the 40?50 day divergent and rotational modes with the former process of primary importance. It is inferred from the dominant role of [G(kR?)] that the intensity of the Somali jet is highly related to the gradients of height field over the southwest fringe of the monsoon trough over northern India. This trough is deepened when the transient migrating monsoon trough reaches ?20°N. It is clear that the concerned gradients of height field and the monsoon trough over northern India must develop synchronously. This synoptic relationship between them explains why the Somali jet intensifies or decays with the same pace as this monsoon trough.