1. Wet and Dry Cold Surges Over the Maritime Continent.
- Author
-
Tan, I., Reeder, M. J., Singh, M. S., Birch, C. E., and Peatman, S. C.
- Subjects
MADDEN-Julian oscillation ,RAINFALL ,PHASE oscillations ,CONTINENTS ,WESTERLIES ,BUOYANCY ,FLOODS - Abstract
Cold surges are synoptic weather systems that occur over the Maritime Continent during the boreal winter. They are characterized by the strengthening of prevailing low‐level northerly to north‐easterly winds, temperature falls of a few degrees over several days, and in some cases, heavy prolonged rainfall and flooding. We investigate the synoptic structure and development of cold surges through composites of dry, moderate and wet surges. Each surge category is defined by the distribution of precipitation averaged within a specified domain over the equatorial South China Sea. Dry surges are characterized by cross‐equatorial flow and positive mean sea‐level pressure anomalies that reach the Southern Hemisphere, and enhanced descent or weaker ascent. Wet surges are characterized by limited cross‐equatorial flow, low‐pressure anomalies over the equator, and enhanced moisture and ascent. The local diurnal precipitation patterns are consistent with the convection being controlled by the mid‐tropospheric buoyancy of an idealized entraining plume. This buoyancy diagnostic suggests that wet surges are characterized by a moister free troposphere because this reduces the effect of entrainment and allows deep convection to develop. On the large scale, the environment in which wet or dry cold surges form is closely related to the Madden‐Julian Oscillation (MJO) phase. Dry and moderate surges occur predominantly in the suppressed phases of the MJO, while wet surges often herald the eastward propagation of the MJO convective envelope. Finally, convectively active phases in the Australian monsoon are more likely following dry cold surges, and are sometimes associated with the MJO. Plain Language Summary: Cold surges are large‐scale monsoon weather systems that bring strong north‐easterly winds, colder air, and sometimes very heavy rainfall and flooding over the Maritime Continent region between November and March of each year. We investigate the structure and development of dry, moderate, and wet cold surges. The three categories are defined based on rainfall averages over a domain located near the equator. The rainfall, on which the categories are defined, is primarily influenced by the Madden‐Julian Oscillation (MJO). The MJO has a major influence on tropical weather on weekly to monthly timescales. Of the three categories, dry surges are characterized by the highest mean sea‐level pressure anomalies, and by north‐easterly winds that cross the equator into the Southern Hemisphere and then turn to become westerly over Java and northern Australia. The westerly winds are often associated with activity in the Australian summer monsoon. In contrast, wet surges are characterized by low‐pressure anomalies over the equator, and weak westerly or easterly winds over southern Java and northern Australia. Key Points: Cold surges occur on approximately 18% of days over a 20‐year analysis period, and account for approximately 30% of the total rainfallLocally, wet cold surges are characterized by a moister free troposphere, which allows convection to penetrate the lower troposphereRegionally, dry (wet) surges usually form in the suppressed (active) Madden‐Julian Oscillation phases and are sometimes linked to the Australian summer monsoon [ABSTRACT FROM AUTHOR]
- Published
- 2023
- Full Text
- View/download PDF