Long coastlines of maritime continent governing the global climate

The Indonesian maritime continent (IMC) is known as the region of the most active convective clouds and their producing the largest rainfall on the earth, which has been explained by the warmest seawater surrounding the IMC. Indeed, on one hand, if clouds are once generated, convection is developed spontaneously by so-called conditional instability, and larger evaporation from warmer sea surface may make more active convective cloud and larger rainfall. However, the largest rainfall does not occur over the open ocean. On the other hand, clouds must appear as a result of convection, which may be generated much easier on hotter land surface, but the largest rainfall does not occur on the true continents. In order to settle these paradoxical issues, here we shall point out the longest coastlines of the IMC are essential to generate the most active clouds there. Frequency spectral analysis of 14-year hourly satellite-observed cloud top temperature shows that the diurnal cycle (sea-land breeze circulation) near a coastline is the almost unique mechanism to generate convective clouds systematically near the equator almost free from any cyclone activities. An empirical formula between mean regional annual rainfall and coastline length divided by land area is obtained, which seems satisfied by several equatorial regions over the world. This implies that a climate model needs to resolve the equatorial coastlines with a scale sufficiently smaller than 100 km. This "coastline hypothesis" is consistent to seasonal and interannual variation of IMC rainfall. The seasonal variation is dependent on solar radiation during clear period of the diurnal cycle. The correlation between IMC rainfall and surrounding SST (varying with ENSO and IOD) is explained by diurnal-cycle actitivity dependent on land-sea temperature difference.AOGS 11th Annual Meeting, July 28 - August 1, 2014, Sapporo, Japa