26th Conference on Hurricanes and Tropical Meteorology

Wednesday, 5 May 2004
The role of intraseasonal wave activity in the onset and active-break phases of the Indian monsoon
Richelieu Room (Deauville Beach Resort)
George N. Kiladis, NOAA/AL, Boulder, CO; and M. K. Flatau and P. J. Flatau
Poster PDF (3.8 MB)
Much recent work has empahsized the potential role of the Madden-Julian Oscillation (MJO) in determining the timing of the onset of the Indian monsoon, as well as intraseasonal fluctuations in monsoon activity (active-break cycles). We show that a broad range of other convectively coupled equatorial disturbances also contribute substantially to the pattern of intraseasonal rainfall and circulation over the Indian subcontinent and adjacent regions during northern summer.

The MJO is often implicated in the onset of the Indian monsoon, although we show that monsoon onset can also be tied to the passage of a convectively coupled Kelvin wave. Kelvin waves propagate eastward at a phase speed of around 15 m/s, much faster than the typical phase speed of the MJO (roughly 5 m/s) over the Indian sector, and can typically be traced back to origins over South America or the eastern Pacific. In addition, westward propagating equatorial Rossby (ER) waves originating over the western tropical Pacific or southeastern Asia are also commonly involved in determining the onset timing of the monsoon over southern India, as well as active and break phases. While these disturbances typically make up the larger scale envelope of the MJO, they can occur in isolation as well during the monsoon season and account for a large portion of the observed monsoon variability.

Several cases of so-called "double monsoon onsets" are shown to be due to the precise timing of intraseasonal equatorial modes with respect to the usual seasonal cycle of monsoon onset evolution. A first "bogus onset" typically occurs during early May, which is then followed by a protracted dry spell and heat wave over India prior to the true onset sometime in June. We show that the bogus onsets of 1979 and 1995 were caused by early May occurrences of Kelvin waves passing through the Indian Ocean ITCZ. During other years such as 1986, 1997, 1998, and 2002 and 2003 bogus onsets occur in conjunction with an MJO, however these are preceded by Kelvin pulses propagating off of Africa which appear to "trigger" the formation of the MJO itself. It will also be shown that the usual monsoon onset signal is composed of significant Kelvin, ER, and MJO components, all of which are needed to adequately describe a statistical composite onset scenario, although the roles of each vary from one year to the next.

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