"This Is Important to Validate These Models Because The IPCC Future Climate Projections Are Based on Them"
22 Jun, 2007 05:02 pm
Gael Alory answers scitizen's questions on his research on the changes occuring in the Indian Ocean due to climate change.
Temperature observations collected over the last 40 years show a general surface warming in the Indian Ocean. This warming is the largest (about 2°C) in the subtropical latitudes, around 40°S-50°S, where it extends down to 1000 m depth. On the contrary, in the tropics, there is a cooling at 200 m depth, below the surface warming.
What are the direct causes of these changes?
We used climate models to understand the mechanisms responsible for these temperature changes. We found that the deep-reaching subtropical warming is due to the ocean currents moving south in that area. These changes in the oceanic circulation are related to the observed strengthening of the westerly winds (the roaring forties) over the last 40 years. In the tropics, the subsurface cooling is also observed in the western Pacific where it is due to weakening trade winds in the Pacific, and is transmitted to the Indian Ocean by oceanic currents through the Indonesian seas. Tropical and subtropical changes in wind, which cause the temperature changes, have been related to human influence through greenhouse gases and ozone depletion.
How do these changes affect region land masses?
The sea surface temperature is a key parameter for ocean-atmosphere coupling, and has especially a strong impact on rainfall. For example, as the oceanic circulation is moving south in the subtropical Indian Ocean, storms forming over this oceanic region are moving south too. These storms, which used to bring winter rainfall over south-western Australia, now tend to bring rainfall over the ocean south of the Australian continent, which explains the worrying decrease in rainfall observed on land since the 1980s. Also, in the tropics, the Indian Ocean Dipole, which is a climatic phenomenon similar to El Nino in the Pacific, is due to a strong ocean-atmosphere coupling. This coupling is reinforced by the sea surface warming so we expect changes in the Indian Ocean Dipole, which is known to affect rainfall in Australia, East Africa, and also the Indian Monsoon.
The climate models were able to reproduce the observed changes in ocean temperatures, when forced with past greenhouse gases emissions. This is important to validate these models because the IPCC (Intergovernmental Panel for Climate Change) future climate projections are based on them. Also, further research is needed to understand better the local impacts of the large scale changes occuring in the oceans.
Interview by: Christopher Le Coq
Gael Alory is a researcher at theAustralia's Commonwealth Scientific and Industrial Research Organization.
Alory, G., et al. (2007), Observed temperaturetrends in the Indian Ocean over 1960-1999 and associated mechanisms, Geophysical Research Letters, 34, L02606.