405 - The new face of El Niño
El Niño is changing. This climatic troublemaker is increasingly appearing in a form known as Modoki, Japanese for 'similar but different'. The heart of the phenomenon is moved from the eastern tropical Pacific towards the centre of the Pacific basin. New research conducted by the IRD and its partners from the Legos( 1) laboratory details the biological aspects of Modoki in the equatorial zone. Such events reduce the levels of phytoplankton in the central Pacific area: Satellite images analysed between 1997 and 2010 show the ocean to be less green during the events of 2002-2007 and 2009-2010. This colouring demonstrates reduced levels of surface marine algae, synonymous with a low level of biological activity.
Another study conducted with Peruvian( 2) researchers and those from the Locean( 3) laboratory has revealed that Modoki, inversely to its larger cousin, might be responsible for upwelling( 4) the length of the South-American coastline. A high resolution oceanic model, linked to satellite and historic data from Imarpe( 5), indicates that recent episodes have seen an increase in this rising cold water, rich in nutrients. The predicted increase in frequency of Modoki( 6) events could thus influence fishing in the zone.
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.
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.
Colder, richer waters to the east
Another recent study, carried out in partnership with researchers from Peru( 2), has recently demonstrated that Modoki type events might encourage the upwelling phenomenon which occurs in this case along the South-American coastline.
As part of their research, the team examined the surface temperature of the ocean, again observed from space and simulated using a high-resolution oceanic model. To achieve this, researchers analysed satellite images from all across Peru since 1981, and a high-resolution simulation stretching back to 1958. They thus demonstrated a cooling of the sea nearer the Peruvian coastline, corresponding to an increase in upwelling, linked to an increase in the frequency of Modoki events.
This cold water rising from the deep is rich in the nutrients that support new life in the region’s seas. The new face of El Niño could thus have an effect on halieutic resources around the coasts of South America.
Initially viewed by scientists as a new phenomenon, Modoki has since been proved by research to be a variant of El Niño. Indeed, Modoki is not a recent phenomenon. Researchers have found evidence of it in climate records dating back 120 years(6). However, even though a link to climate change has not yet been firmly established, its frequency may increase five times by 2050(6). Classic major episodes of El Niño from 1982-3 and 1997-8 led to a drop in fish stocks, particularly in Peru. What will happen with Modoki? The extent to which it will influence fishing resources is still to be determined.
(4) An ‘upwelling’ is a zone of cold and nutrient-rich water rising to the surface. Around the equator as a result of the Coriolis effect, masses of water moved by the trade winds are pushed towards the poles and replaced by deeper water.
(6) Nature , 2009, 461 (7263), p. 511-514. fdi:010048207( http://www.documentation.ird.fr/hor/fdi:010048207)
By Gaëlle Courcoux