The weather and climate of the maritime continent, including the country of Indonesia, is dominated by the seasonal cycle and by the seasonal shift of tropical heating from one side of the equator to the other. This has traditionally been interpreted in terms of a monsoonal mechanism. This is so fundamental to the meteorology that the word for season in the Indonesian language is Musim (or monsoon); and seasons are invariably described in terms of the wet monsoon or the dry monsoon and alternatively as the west and east monsoon. For more than 50 years seasonal forecasts for monsoon onset have been carried out either in experimental or operational mode by the local Indonesian hydrometeorological agency(BMG and its predecessor agencies).

Traditionally the onsets of both the wet and dry monsoons have been defined in terms of in situ rainfall criteria, based on the water requirements for growing rice. As such there is a strong variation across the region with the wet monsoon onset in the western and equatorial part preceding the south-eastern onset by several months.

In this paper we re-examine monsoon onset over Indonesia. Two approaches are taken: the first examines time-longitude sections of Out Going Longwave Radiation (OLR) over various longitudes within the region. Monsoon onset is defined as the sudden transition from a Northen Hemisphere location to a Southern Hemisphere location of the Intertropical Convergence Zone, and conversely at the opposite season. Using this definition it is found that a clear onset occurs over the entire region for the northen monsoon onset in April- May. A clear sudden southern onset occurs also, but over a much more restricted range of longitudes, near eastern Indonesia and northern Australia.

The second, complementary approach, is to define monsoonal structure in terms of a sine-wave seasonal structure in the antisymmetric component (with respect to the equator) of OLR and of zonal wind-component. With this model, a well -defined monsoon is seen to exist over the entire region with the seasonal cycles of antisymmetric low-level wind and of OLR being in phase.

Composites have been carried out for both monsoon definitions for El Nino, La Nina and normal years. Concerning timing of onset the southern onset is strongly influenced by ENSO with a much later onset in the warm-event (El Nino) phase. The timing of the northern onset seems to be little affected, however, by ENSO. In terms of the seasonal monsoon structure (as defined by the antisymmetic component of heating) the monsoon strength is actually enhanced in an El Nino, due to supression of the convection on the winter hemisphere side of the equator.

These planetary scale definitions of monsoon onset have been defined in order to place the local-scale rainfall definitions within a dynamical framework. Ongoing work is focussed on reconciling the various definitions to provide a more comprehensive picture of structure and mechanisms.