Abstract
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.
Highlights
Tropical rainfall in the global climate Recognition of rainfall as one of the most important quantities for classifying climate and its global quantitative description was established by Köppen (1918), who divided the tropical climate (A; classified by temperature in Yamanaka et al Progress in Earth and Planetary Science (2018) 5:21rainfall with a scale somewhat larger than that supposed by Köppen on the basis of vegetation.Köppen classified low latitude regions of South America and Africa and most of the “East Indies” as Af climate regions defined by a minimum monthly rainfall ≥ 60 mm/month, but carefully excluded the inland areas of Sumatera, Kalimantan and Papua,1 as well as those of SouthAmerica and Africa so that his “idealized” land had Af along the equator including inland areas
When we proposed a project related to MAHASRI over the Indonesian maritime continent (IMC), the Japanese Government established the Japan Earth-Observation System Promotion Program (JEPP) contributing to the Global Earth Observation System of Systems (GEOSS; see Group on Earth Observations (GEO) 2007), which emphasized societal benefits
In this article, we review the concentration of tropical rainfall around coastlines and why the world’s largest regional rainfall is distributed over the IMC that has the longest total length of coastline
Summary
Tropical rainfall in the global climate Recognition of rainfall as one of the most important quantities for classifying climate and its global quantitative description was established by Köppen (1918), who divided the tropical climate (A; classified by temperature in Yamanaka et al Progress in Earth and Planetary Science (2018) 5:21rainfall with a scale somewhat larger than that supposed (seasonal−annual and 102–103 km) by Köppen on the basis of vegetation.Köppen classified low latitude regions of South America and Africa and most of the “East Indies” as Af climate regions defined by a minimum monthly rainfall ≥ 60 mm/month (total annual rainfall >> 720 mm/year), but carefully excluded the inland areas of Sumatera, Kalimantan and Papua,1 as well as those of SouthAmerica and Africa (except for the Amazon and Congo River Basins) so that his “idealized” land had Af along the equator including inland areas (see Fig. 1d).
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