Multi-scale atmospheric and oceanic coupled processes influencing Maritime Continent rainfall

The Maritime Continent (MC) archipelago in southeast Asia experiences heavy rainfall all year round, modulated by processes over a range of spatial and temporal scales. Scale interactions between these processes, combined with complex coastlines and orography, make understanding the regional meteorology more difficult. Limitations to our process knowledge mean errors persist in weather and climate models for the MC, with these errors propagating globally. In this thesis, several understudied atmospheric and oceanic coupled processes, on meso-to-synoptic scales, are analysed.

The first novel finding is that mid-level dry air intrusions, which are transient phenomena of extratropical origin, suppress convection over the southern MC, while enhancing rainfall on their eastern margin towards northern Australia, by influencing low-level wind circulations between the tropics and extratropics. Secondly, the characteristics of mesoscale ocean eddies are derived across the entire MC through usage of satellite altimetry data. Eddy-induced sea surface temperature and surface heat flux anomalies lead to minimal imprint on the atmosphere, an unexpected result given the significant eddy-associated signatures observed elsewhere. Lastly, high-resolution convective-permitting simulations show that Sumatra squalls, observed over the western MC, have dynamical structures as seen more broadly for tropical squall lines. Compared to previous studies, the diurnal cycle, vertical wind shear, density currents, moisture flux convergence and low-to-mid-tropospheric winds are shown to more directly influence Sumatra squall evolution.

Further interactions between processes investigated in this thesis, and the more extensively studied large-scale modes of variability, include how the Madden-Julian Oscillation may influence dry air intrusion occurrence, and how equatorial waves modulate Sumatra squall propagation. Improvements to our knowledge of these key processes and associated interactions which influence rainfall variability will positively impact our ability to model, forecast, and predict severe weather over the MC.