Emerging once every two to seven years in the equatorial Pacific, El Niño causes disorder across the globe and for the global economy. But in the past ten years, it has changed its face. It is increasingly taking the form of Modoki, 'similar but different' as it was baptised by the Japanese team who first discovered this less tumultuous cousin that provokes droughts in India and Australia.
Emerging once every two to seven years in the equatorial Pacific, El Niño causes disorder across the globe and for the global economy. But in the past ten years, it has changed its face. It is increasingly taking the form of Modoki, 'similar but different' as it was baptised by the Japanese team who first discovered this less tumultuous cousin that provokes droughts in India and Australia. Recent research has described the physical manifestations of this El Niño variant, which is centred in the central pacific, unlike its eastern relative. The impact on marine biology and its probable effects on fishing still need to be examined. To achieve this, IRD researchers and their partners from the Legos(1) and Locean(3) laboratories have studied its effects on the very first links in the food chain.
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Less aquatic life in the centre of the Pacific
In the same manner as its physical mechanisms, from a biological point of view, Modoki episodes are also characterised by displacement of phenomenal effects. There is not the same widespread decrease in phytoplankton that is seen in the eastern Pacific during classic El Niño events, rather a more localised version in the central zone of the basin.
Oceanographers have observed the colouring of the Pacific using 'water colouration' satellite images taken between 1997 and 2010. The blue or green colour of the ocean seen from space actually reflects variations in the surface levels of chlorophyll. Scientists have thus observed low levels of chlorophyll in the centre of the Pacific basin (under 0.1mg per m3) during Modoki events that took place between 2002-3, 2004-5, 2006-7 and 2009-10. The concentration is an indicator of the biomass of phytoplankton at the ocean surface. In this way, the levels recorded in the central area of the basin during recent El Niño Modoki events convey the scarcity of aquatic nutrients necessary for plant development and thus for marine life forms.
When El Niño fools with the weather
Under 'normal' conditions above the Pacific, the trade winds blow strongly from east to west. They accumulate, thus confining the vast warm waters with little surface activity known as the 'warm pool' to the western part of the ocean. This enormous warm water reservoir with a temperature of over 28°C feeds the flows of warmth and humidity that affect the vast majority of the Earth's atmosphere, rather like a planetary heat pump. When a classic El Niño episode occurs, the trade winds experience a brutal drop in force, and the upwelling(4) phenomenon that is triggered all around the equator by the Coriolis effect slows down. The huge reservoir spreads out into the Pacific towards the east and the water becomes depleted.
But when Modoki occurs, the trade winds hardly drop in force. The result is a minimal slowing down of equatorial upwelling and a blockage of water from the 'warm pool' in the central basin, which in turn explains the depletion being localised in the centre of the ocean.
Article continues at Science Daily
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