Mid-Level Dry Air Intrusions over the southern Maritime Continent

Abstract

Patterns in extreme precipitation across the Maritime Continent in Southeast Asia are known to be modulated by many processes, from large-scale modes of variability such as the Madden-Julian Oscillation and planetary waves, to finer-scale processes such as the diurnal cycle. Transient mid-level dry air intrusions are an example of a process not extensively studied over the Maritime Continent, which has the potential to influence rainfall patterns.   Through Lagrangian trajectory and event composite analyses, we use a humidity metric which identifies mid-level dry air intrusions. These intrusions originate from upper-level disturbances along the subtropical jet. Mid-level cyclonic circulation anomalies northwest of Australia from December-February (DJF) intensify westerlies in the southern Maritime Continent, advecting dry air eastward. In contrast, mid-level anticyclonic circulation anomalies northwest of Australia from June-August (JJA) intensify southern Maritime Continent easterlies, advecting dry air westward. The resultant transport direction of associated air parcels is also dependent on the seasonal low-level monsoon circulation, and potentially convective entrainment.   Dry air intrusions are found to be important in influencing low-level wind circulations and rainfall patterns in the southern Maritime Continent. Dry air suppresses rainfall over seas near to the southern Maritime Continent in both seasons. Further suppression matches intrusions trajectories, such as over southern Maritime Continent islands in DJF, and the Indian Ocean in JJA. In both seasons, there is enhanced rainfall to the east of the intrusion, where there is moist return flow to the extratropics.