TerraMaris: The Maritime Continent - Driver of the Global Climate System

The Maritime Continent (MC) is the archipelago of tropical islands that lies between the Indian and Pacific Oceans, with a population of over 400 million. It comprises large (Sumatra, Java, Borneo, and New Guinea) and many smaller islands, with high mountains. High solar input warms the surrounding seas, which supply an abundance of moisture to the atmosphere, turning the whole region into an atmospheric "boiler box". Deep convective clouds rise up over the islands every day, leading to average rainfall rates in excess of 10 mm per day, approximately three times the rainfall rate over the UK. As well as supplying local agriculture, rain that falls over the MC has a far-reaching, global effect on weather and climate. Tremendous heat energy is released by condensation into the atmosphere in these convective clouds. This heat source drives giant, overturning circulations in the atmosphere: the Hadley and Walker cells, which feed into the jet streams and lead to weather and climate changes far downstream, even over the UK. For example, the origins of the infamous cold winter of 1962/63 and the recent very cold March of 2013 have been traced to atmospheric convection over the MC. For these reasons, the MC has been described as the engine room of the global climate system.

Due to the complex nature of the distribution of the islands, and fundamental inadequacies in current models of the atmosphere (mainly related to their representation of convection), both climate predictions and weather forecasts show serious errors over the MC, particularly in their simulation of rainfall. Up until now, these errors have been extremely difficult to address, as there has been a lack of suitable observations over this region. Computing power, and the atmospheric modelling expertise to harness the advances in computing resources, has been inadequate to run computer models with sufficient detail to resolve the convective processes and their interactions, which are the building blocks of atmospheric circulation, for long enough to allow interactions with larger scales.

However, we now stand on the cusp of transforming our understanding of atmospheric processes over the MC. Computer power and modelling expertise have progressed to the point where we have the capability to run simulations of the atmosphere at sufficient resolution to accurately capture the complex distribution of islands, and to accurately model the convective processes themselves. In response to this, the international Years of the Maritime Continent (YMC) field experiment (2017-2020) will make the measurements of the atmosphere and ocean at the very small scales that are needed to evaluate and understand the outputs of these new model simulations.

Through TerraMaris the UK will take a leading role in YMC, by making observations of convective processes over the MC using the UK meteorological research aircraft, atmospheric radars, balloon and land-based measurements on the islands, and observing the surrounding seas using autonomous underwater and surface vehicles. This unprecedented suite of coordinated observations will complement measurements being taken by our international partners. The UK and the TerraMaris research team has led the way in developing high-resolution atmospheric modelling over recent years. We will apply the skills and knowledge learned to understand the complex mechanisms behind the multiple scales of convection and atmospheric circulations that have made the weather over the MC such a tough problem to crack. This knowledge will enable ground-breaking advances in atmospheric modelling, to improve weather forecasts and climate prediction over the MC region, with direct benefit to the substantial regional population. The downstream effects will see these benefits extend to the far corners of the globe, improving global and regional medium-range weather prediction, and climate projections.

Due to the integral nature of the MC and the Madden-Julian Oscillation (MJO) within the global weather and climate system, this work will significantly impact forecasting on scales ranging from the short-range process-level understanding to climate prediction. Weather forecasters, including the UK Met Office and Indonesian BMKG (project partners), will benefit from TerraMaris through scientific advances in understanding and modelling the interactions of convection with atmospheric dynamics and ocean thermodynamics and the impact of these processes on severe and hazardous weather, including UK weather. This will feed into improved measures of uncertainty in weather forecasts, and into the process of developing the next generation of convective parameterisation schemes through the ParaCon project.

The MC provides a unique test bed for the new convective parameterisation schemes being developed in ParaCon, within which no new field measurements could be funded. The performance of parametrisations over the MC is critical to global modelling. Improved convection schemes will lead directly to improved rainfall forecasts, not only over the MC, but by improving the MC convection, across the world. The MC mosaic of steep, high, mountainous islands set among shallow warm seas, will provide a challenging and distinct regime in which to evaluate these schemes compared with, e.g., convection over open ocean or over continental regions (e.g., Great Plains of North America, Sahel region of Africa). TerraMaris will deliver a comprehensive, state-of-the-art set of case studies and statistical observations, a hierarchy of model simulations and detailed understanding of convective processes within this regime to validate convective paradigms developed in ParaCon, and to benchmark quantitatively ParaCon numerical simulations of convection and its interaction with atmospheric dynamics. TerraMaris will develop these products in close consultation with ParaCon, through Co-I's Woolnough, Marsham and Birch.

An improved understanding of the MJO and its propagation through the MC will lead to better constraints on the confidence of global medium-range forecasts (up to 15 days) at the UK Met Office, the European Centre for Medium Range Weather Forecasts, and other national meteorological agencies. The MC is a critical region for the El Nino-Southern Oscillation (ENSO), the major source of climate predictability on seasonal to inter-annual time scales; hence improved understanding of MC processes will feed into ENSO prediction and analysis of biases in ENSO forecasts. These will improve sub-seasonal and seasonal forecasts. On climate time scales (decadal and longer), the identification of the causes of regional model errors over the MC will lead to better constraints on regional climate projections, benefitting the IPCC and end users of climate projections, such as national and regional governments, and stakeholders in agriculture and water supply sectors.

Indonesian researchers in climate science, meteorology and oceanography will benefit from the advances described above, as well as the new techniques that TerraMaris will develop and apply (e.g., coordination of aircraft, radiosonde, land and ocean based measurements to measure the diurnal cycle of convection and land-sea breezes and gravity waves). The ocean observations will also include elements of biogeochemistry, so our results will benefit those working on fisheries management, carbon sequestration and ecosystem modelling. TerraMaris will benefit the International Indian Ocean Expedition 2 (IIOE-2), by improving understanding of ocean-atmosphere coupling in the Indian Ocean region. Scientists in the developing countries of the MC region will benefit by learning how to operate radiosondes and take surface flux measurements, and how to deploy, operate and pilot Seagliders and Wavegliders.