The 40-50 day oscillation of the low-level monsoon circulation over the Indian Ocean.

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 about 20oN. 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. Concludes that 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.-from Authors