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Regional and interannual variations of the rainy season over Indonesia are investigated using daily rainfall data during 1961-90. Pentad-mean rainfall data, with a relatively better continuity have been obtained for 46 stations, and the annual and semi-annual cycles of rainfall at these stations have been objectively analyzed by harmonic analysis. The onset of southern-hemispheric spring/summer (SON/ DJF) rainy season starts from the Indian Ocean side of Jawa in the middle September, and propagates northward (in Jawa) and eastward (to Nusa Tenggara in middle December). Another route of rainy seaCorresponding author: Jun-Ichi Hamada, Frontier Observational Research System for Global Change (FORSGC), Japan Marin Science and Technology Center (JAMSTEC), Yokohama, Japan. E-mail: hamada@jamstec.go.jp ( 2002, Meteorological Society of Japan son propagation from Irian Jaya to Nusa Tenggara is observed. The withdrawal of the rainy season starts from western Nusa Tenggara in March, and goes eastward (to eastern Nusa Tenggara), and westward (to Jawa), until late May.Rainy season onset comes later (earlier) in El Niñ o (La Niñ a) years than the average at most stations (particularly in the south-eastern part of Jawa). Correlations between rainfall amounts at those stations, and the southern oscillation index in SON, are significantly high. However, the rainfall amount throughout a rainy season is not dependent upon the length of the rainy season (between onset and withdrawal) in many areas (except southern Sulawesi). The dominant time scale of interannual variations of rainy season onset during 1961-90 is 2-3 years, which looks somewhat shorter than that during 1910-41.

This paper describes a diurnal cycle in systematic cloud system migration observed with the GMS IR1 sensor over Sumatera (approximately 1,500 km in length) from May 2001 to April 2002. Convective clouds developed over mountainous areas in the afternoon, and migrated westward and/or eastward for several hundred kilometers (@500 km) from midnight to morning. Westward migration occurred in almost every month except August over southernmost Sumatera Island. Eastward migration occurred when lower-tropospheric winds were westerly and/or when super cloud clusters moved eastward along the Intertropical Convergence Zone (ITCZ), which moves northward and southward with an annual cycle.

Precipitation measurements from the Tropical Rainfall Measuring Mission satellite indicate that annual rainfall over the sea in the vicinity of western Sumatra Island is among the highest on the earth, and most of this rainfall occurs during nighttime. Surface meteorological observations at Tabing on the western coast of the island show frequent occurrences of sudden offshore winds accompanied by an abrupt drop in surface temperatures in the late afternoon and evening. Model simulations for a 1-month period during the rainy season of the region successfully simulate the satellite-observed regional distribution and diurnal variation of rainfall. The simulation results show that convection develops across a wide area over the mountainous areas of the island at similar times in the afternoon with the development of thermally induced local circulations. At these times of the day, convection over the sea along the western coast of the island is suppressed by the thermally and topographically induced diurnal changes in the boundary layer flow. When convection over the mountains of the island dissipates in the late afternoon and evening, a zone of cold surface outflow along the western coast results from the mountain convection breaking out to the sea. Meanwhile, the convective inhibition offshore is reduced in the evening, and the offshore flow causes regular occurrences of convection over the sea near the coast. The triggered convective systems propagate offshore and westward in multicell cluster storms during nighttime, bringing heavy rainfall over the sea off the western coast of the island. Sensitivity experiments with a flat-topography model demonstrate that the frequent occurrence of offshore flow in the late afternoon and evening on the western coast is caused by the mountainous topography of the island and its induced afternoon convection. The mountains on the island and the resultant thermally and convectively induced local circulations can play an important role in the formation of nocturnal abundant rainfall over the sea west of Sumatra Island.

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During the Monsoon Asian Hydro−Atmosphere Scientific Research and Prediction Initiative (MAHASRI; 2006-16), we carried out two projects over the Indonesian maritime continent (IMC), constructing the Hydrometeorological Array for Intraseasonal Variation−Monsoon Automonitoring (HARIMAU; 2005-10) radar network and setting up a prototype institute for climate studies, the Maritime Continent Center of Excellence (MCCOE; 2009-14). Here, we review the climatological features of the world's largest "regional" rainfall over the IMC studied in these projects. The fundamental mode of atmospheric variability over the IMC is the diurnal cycle generated along coastlines by land−sea temperature contrast: afternoon land becomes hotter than sea by clear-sky insolation before noon, with the opposite contrast before sunrise caused by evening rainfall-induced "sprinkler"-like land cooling (different from the extratropical infrared cooling on clear nights). Thus, unlike the extratropics, the diurnal cycle over the IMC is more important in the rainy season. The intraseasonal, seasonal to annual, and interannual climate variabilities appear as amplitude modulations of the diurnal cycle. For example, in Jawa and Bali the rainy season is the southern hemispheric summer, because land heating in the clear morning and water vapor transport by afternoon sea breeze is strongest in the season of maximum insolation. During El Niño, cooler sea water surrounding the IMC makes morning maritime convection and rainfall weaker than normal. Because the diurnal cycle is almost the only mechanism generating convective clouds systematically near the equator with little cyclone activity, the local annual rainfall amount in the tropics is a steeply decreasing function of coastal distance (e-folding scale 100-300 km), and regional annual rainfall is an increasing function of "coastline density" (coastal length/land area) measured at a horizontal resolution of 100 km. The coastline density effect explains why rainfall and latent heating over the IMC are twice the global mean for an area that makes up only 4% of the Earth's surface. The diurnal cycles appearing almost synchronously over the whole IMC generate teleconnections between the IMC convection and the global climate. Thus, high-resolution (<< 100 km; << 1 day) observations and models over the IMC are essential to improve both local disaster prevention and global climate prediction.

An extreme precipitation/flood event that occurred in the Indo nesian capital of Jakarta on Java Island in the middle of January 2013 coincided with an active phase of the MaddenJulian Oscil lation (MJO) with the enhanced convective phase centered on the western Pacific. Analysis of upperair sounding data showed that strong to moderate upper westerly to northwesterly winds persist ed over the island prior to and during the heavy rain event, which were caused by the active phase of the MJO, while northwesterly winds occurred near the surface. Meteorological radar observa tions indicated regular genesis of convection at night over the sea to the northwest of the island, and southeastward propagation over the island from the nighttime to early morning. The movement of the precipitation systems was dominated by the upper north westerly winds. The results suggest that the eastward propagation of an active phase of the MJO exerted a strong influence on the formation of extreme heavy rain over western Java Island.

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