The Maritime Continent and Its Role in the Global Climate: A GCM Study

Abstract

The Maritime Continent, with its complex system of islands and shallow seas, presents a major challenge to models, which tend to systematically underestimate the precipitation in this region. Experiments with a climate version of the Met Office model (HadAM3) show that even with a threefold increase in horizontal resolution there is no improvement in the dry bias. It is argued that the diurnal cycle over the islands and the complex circulation patterns generated by land?sea contrasts are crucial for the energy and hydrological cycles of the Maritime Continent and for determining the mean climate. It is shown that the model has substantial errors in its simulation of the diurnal cycle over the islands, which can rectify onto the seasonal mean climate. It is further argued that deficient rainfall over the Maritime Continent could be a driver for other systematic errors, such as the excess precipitation over the western Indian Ocean. To demonstrate the sensitivity of global systematic model errors to the heating in this region, two experiments have been performed, one with the existing distribution of islands and a second where the island grid points are replaced by ocean grid points. In the absence of the islands of the Maritime Continent, the local precipitation increases by 15%, reducing the existing dry bias and bringing the model closer to observations. In response to this improved heating distribution, precipitation decreases over the west Indian Ocean and South Pacific convergence zone, reducing the systematic wet bias in these regions. This supports the hypothesis that tropical systematic errors are often related through vertical (Walker) circulations. The extratropical response to changes in the Maritime Continent heat source is also well demonstrated by these experiments. The enhanced heating and, hence, divergent outflow generates Rossby waves, which have a significant impact on the winter circulation and surface temperatures across much of North America and the northeast Eurasian region. These changes are such as to substantially reduce model systematic error in these regions. These results reinforce the critical role played by the Maritime Continent in the global circulation. It emphasizes the need for better representation of convective organization over regions of complex land?sea terrains and the importance of considering the global context of model systematic errors in which biases in the Tropics may be a key factor.