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Researchers have characterised a missing piece in the global climate puzzle, helping to explain how moisture on land affects the atmosphere. Their findings, published in Science today (20 August), could lead to significantly more precise and longer-term weather forecasts.

This would help farmers improve yields and reduce costs by planning growing seasons and the timing of irrigation and fertiliser use.

Accurate seasonal weather forecasts depend on our ability to predict how the atmosphere will respond to slow changes in the oceans and landmasses. But while the role of the oceans in generating patterns rain, snow and other forms of precipitation is relatively well understood, the way changes on the ground affect the atmosphere is poorly understood.

Part of the reason why seasonal forecasts are unreliable is that different climate models produce different results. To overcome this, an international team of climate scientists, led by Randal Koster at NASA's Goddard Space Flight Centre, compared the results of 12 different models running the same experiment.

It emerged that the 12 models were in agreement about certain geographical areas, where soil moisture and precipitation appeared to be closely linked. These 'hot spots' were the Sahel region south of the Sahara desert, equatorial Africa, the plains of North America, and across northern India. All are 'transition zones', where the climate is neither very wet, nor very dry.

"We modelled soil moisture around the world and compared it with modelled rainfall," says Christopher Taylor, of the UK Centre for Ecology and Hydrology, and a participant in the study. We found that in very wet or very dry areas the impact of changes in soil moisture on rainfall was negligible. But in the areas in between it was a different story. Here it seems, moisture and rainfall are connected."

The scientists say that these regions are the land equivalents of the well-understood oceanic 'hotpots' that are already used in to forecast climate patterns up to a year in advance because of strong, predictable responses of the atmosphere to changing water temperature. The eastern Pacific is one such oceanic 'hot spot' and plays a key role in the El Niño – La Niña cycle.

Less intense 'hot spots' were identified on the landmasses of South America, central Asia and China.

The 'hot spots' are areas where evaporation of soil moisture has a strong effect on rainfall. The researchers suggest they identify areas where routine monitoring of soil moisture using both ground-based and satellite technologies will be most useful for creating long-term forecasts for the months of June to August.

"There is still a great deal of uncertainty but the study shows that many independent models agree on these hot spots as the areas where soil moisture does affect rainfall," says David Lawrence, a member of the team who is based at University of Reading, United Kingdom.



Link to full paper by Randal D. Koster et al in Science

Reference: Science 305,1138 (2004)

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