Consistent with previous studies on the onset of the Australian monsoon, NCEP re-analyses indicate that it is generally characterised by a sudden, large scale strengthening and deepening in tropical westerly winds. However the analyses also indicate that this is preceded by up to a 7-day period of tropospheric moistening.

Analysis diagnostics suggest that the onset process may be associated with major cyclogenesis over high latitudes in the southwest Indian Ocean. The process that eventually affects the monsoon is an eastward and equatorward propagating wave train emanating from the high latitude cyclogenesis.

To test this hypothesis, idealised, dry simulations have been made using the BMRC Limited Area Prediction System, LAPS, with periodic boundary conditions. Balanced, zonally-averaged initial states of (a) a baroclinically-unstable polar front jet at 400S, (b) a weak monsoon at 100S, and (c) a subtropical jet at 250S, are systematically introduced. Results indicate that high latitude cyclogenesis in the presence of a subtropical jet can influence the meridional location, zonal structure, vorticity and divergence of the monsoon. Similar to the analyses, the simulations reveal interesting upper trough-ridge structures extending to very low latitudes. In accordance with theoretical studies, the subtropical jet seems to act as a duct for the northeastward propagation of wave trains into the tropics.

Comparison with previous studies, implications for tropical NWP, and planned future work will be briefly discussed.