Gadgil, S. and S. Sajani and Participating Modelling Groups
of AMIP, 1998: Monsoon precipitation in the AMIP runs, Climate
Dynamics, 14, 659-689.
We present an analysis of the seasonal precipitation associated with the African, Indian and the Australian-Indonesian monsoon and the interannual variation of the Indian monsoon simulated by thirty atmospheric general circulation models during the Atmospheric Model Intercomparison Project (AMIP) under the AMIP Subproject 26.
The seasonal migration of the major rainbelt observed over the African region is reasonably well simulated by almost all the models. However, the Asia-west Pacific region is more complex because of the presence of warm oceans equatorward of heated continents. Whereas some models simulate the observed seasonal migration of the primary rainbelt, in several others this rainbelt persists over the equatorial oceans in all the seasons. Thus the models fall into two distinct classes on the basis of the seasonal variation of the major rainbelt over the Asia-west Pacific sector- the first (class I) comprising models with a realistic simulation of the seasonal migration and the major rainbelt over the continent in the boreal summer; and the second (class II) comprising models with a smaller amplitude of seasonal migration than observed. It turns out that such a classification is also useful for interpreting the simulation of the interannual variation of the Indian monsoon.
The mean rainfall pattern over the Indian region for July-August (the peak monsoon months) is even more complex because, in addition to the primary rainbelt over the !ndian monsoon zone and the secondary one over the equatorial Indian ocean, another zone with sign)ficant rainfall occurs over the foothills of Himalayas just north of the monsoon zone. Thus in some models this orographic rainbelt and/or the oceanic rainbelt appear to be stronger than the primary rainbelt over the monsoon zone. However, eleven models are able to simulate the mean rainfall pattern over the Indian region, reasonably realistically (category A). In models of class II of this category (such as COLA or MOO), the rainfall over the Indian monsoon zone is associated with a localised system which has smaller longitudinal extent than obsenred. The rainfall is primarily over the equatorial ocean (category C1) in the simulation of nine models, and primarily over the orographic one to the north of Indian monsoon zone in one (category C~). In the intermediate category are models (category B) which simulate a primary rainbelt over the Indian region, but somewhat southward of the observed. While the vast majority of the models of class 1 belong to categories A and B (twelve out of thirteen), almost half the