Weather types across the Maritime Continent: From the diurnal cycle to interannual variations

Multi-scale interactions over the Maritime Continent (MC) may cause island-scale seasonal rainfall anomaly dipoles during the rainy season in association with ENSO: El Niño events are found to accompany increased frequency of a quiescent large-scale daily circulation type. These large-scale quiescent conditions in turn allow stronger island-scale diurnal land-sea breeze circulations to develop; the latter result in more rainfall over the mountainous regions of Java and increased subsidence over the coastal plains. A similar phenomenon occurs over Borneo, where rainfall is enhanced in the coastal regions, in which sea breezes head against off-shore synoptic-scale low-level winds.

This contribution builds on our previous work to consider the impacts of both the Madden-Julian Oscillation (MJO) and ENSO, in addition to the seasonal cycle, on daily weather types (WTs); here WTs are identified using cluster analysis over the MC, throughout the entire austral summer season. We investigate how strongly ENSO and MJO control WT frequency versus their modifying the circulation patterns of the WTs; both will modify air-sea interactions through changes in surface fluxes. Robust control of WT frequency is found, with the seasonal cycle being the strongest of the three effects, ENSO the intermediate one, and the MJO being least important. In terms of circulation patterns, ENSO is found to modify the circulation patterns themselves to some extent while the MJO does not.

We argue that these findings have important implications for sub-seasonal to seasonal predictability at local scales: predicting the changes in frequency of occurrence for a small number of weather types is potentially more tractable than the full, much higher-dimensional prediction problem. They may also allow a better understanding of air-sea interactions involving modulations of wind-driven surface fluxes associated with the seasonal cycle, ENSO and the MJO.