Mid‐level dry air intrusions over the southern Maritime Continent

• Published in: Quarterly journal of the Royal Meteorological Society 2024-01, Vol.150 (759), p.727-745
• Main Authors: Aslam, Ashar A., Schwendike, Juliane, Peatman, Simon C., Birch, Cathryn E., Bollasina, Massimo A., Barrett, Paul
• Format: Article
• Language: English
• Subjects: Air; Air parcels; Anomalies; Anticyclonic circulation; Circulation anomalies; Cyclonic circulation; deep convection; Diurnal cycle; Dry air; dry air intrusions; Easterlies; Extreme weather; Madden-Julian oscillation; Maritime Continent; Monsoon circulation; Precipitation; Rainfall; Rainfall patterns; Return flow; Seasons; Subtropical circulation; Tropical environments; tropical–extratropical interactions; Westerlies
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1002/qj.4618

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, to finer‐scale mechanisms such as the diurnal cycle. Transient mid‐level dry air intrusions are an example of a feature not extensively studied over the Maritime Continent, which has the potential to influence rainfall patterns. Here, we show that these dry air intrusions originate from upper level disturbances along the subtropical jet. Mid‐level cyclonic circulation anomalies northwest of Australia from December to 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 to 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. Dry air intrusions are important in influencing low‐level wind and rainfall patterns, suppressing rainfall over seas near the southern Maritime Continent in both seasons, as well 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. This study highlights the importance of synoptic‐scale extratropical features in influencing meteorological patterns in the Tropics. Many processes influence rainfall patterns observed across the Maritime Continent in southeast Asia. Dry air intrusions (a tropical–extratropical interaction) arriving in the southern Maritime Continent are due to upper level disturbances along the subtropical jet. This precursor mechanism and resultant intrusion trajectories show differences across seasons. However, dry air intrusions predominantly suppress rainfall, regardless of the time of year, and can modify low‐level moist flow and lead to enhanced rainfall anomalies to the east.