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La Niña is back

Research conducted over recent decades has shed considerable light on the important role played by interactions of the atmosphere and ocean in the tropical belt of the Pacific Ocean in altering global weather and climate patterns. During “El Niño” events, for example, sea-surface temperatures in the central and eastern tropical Pacific Ocean become substantially higher than normal. In contrast, during “La Niña” events, the sea-surface temperatures in those regions become lower than normal. These temperature changes are strongly linked to major climate fluctuations around the globe, which, once initiated, can last for 12 months or more. The strong El Niño event of 1997/1998 was followed by a prolonged La Niña phase that extended from mid-1998 to early 2001. El Niño/La Niña events change the likelihood of particular climate patterns around the globe and have a tendency to recur in an irregular cycle of 2-7 years, but the outcomes of each event are never exactly the same. Furthermore, while there is generally a relationship between the global impacts of an El Niño/La Niñaevent and its intensity, there is always potential for an event to generate serious impacts in some regions, irrespective of its intensity.

flood

Photo: Associated Press

The forecasting of Pacific Ocean developments is undertaken in a number of ways. Complex dynamical models project the evolution of the tropical Pacific Ocean from its currently observed state. Statistical forecast models can also capture some of the precursors of such developments. Expert analysis of the current situation adds further value, especially in interpreting the implications of the evolving situation below the ocean surface. All forecast methods try to incorporate the effects of ocean-atmosphere interactions within the climate system.

The meteorological and oceanographic data that allow El Niño and La Niña episodes to be monitored and forecast are drawn from national and international observing systems. Many groups, research institutes and National Meteorological and Hydrological Services (NMHSs) are making concerted efforts to probe deeply the El Niño and La Niña phenomena, to improve prediction and to clarify variability and impacts. The exchange and processing of the data are carried out under programmes coordinated by WMO. The WMO Commission for Climatology (CCl) and World Climate Programme (WCP) proactively engage these groups to develop partnerships/collaboration, thereby enriching global achievements and supporting synergy of effort.

The “WMO El Niño/La Niña Update” is a consensus report prepared in collaboration with the International Research Institute for Climate and Society (IRI) and with contributions from NMHSs, regional and global prediction/research centres and individual experts. It is issued on a quasi-regular basis once every three months. While this WMO update provides a global consensus on the current situation and outlook for the next couple of seasons, the country- or region-specific seasonal climate outlooks, as produced by NMHSs or related regional entities, provide detailed information on expected impacts, after consideration of other factors that influence the regional climate. In considering response strategies, it is therefore important to consider regional climate outlooks and not to rely solely on the presence of El Niño or La Niña.

figure 2

Colder-than-average sea-surface temperature in the tropical Pacific during July, August and September 2007,
showing the development of a La Niña episode
(Source: NOAA/NCDC)

The El Niño/La Niña Update of 20 July 2007 said “a La Niña event could, more likely than not, develop in the second half of 2007”. Subsequently, several climate prediction centres noted the establishment of La Niña conditions (see figure above). WMO and IRI are now in the process of developing the next update, which is expected to be released in late October 2007. Some possible impacts of La Niña have already been reported. Japan experienced continuous extremely hot conditions from August to September. Most parts of South Asia, South-East Asia, southern China and Japan and western Africa have received heavy rainfall, leading to severe flood situations. So far this Atlantic hurricane season (June-November), there have been three named storms (Andrea, Barry, and Chantal), which is slightly above average for June and July. The September 2007 climate over tropical Pacific islands was marked by suppressed convection over western and eastern Kiribati and further east, and record rainfall in Vanuatu, New Caledonia and parts of Fiji; well below average rainfall in Kiribati, Tuvalu, northern Cook Islands and Marquesas Islands; warmer than normal in the subtropical South Pacific, record high temperatures in parts of Fiji. For example, New Caledonia recorded 305 per cent of the normal monthly rainfall amount (three times the normal) in September, and it ranked in the top three wettest Septembers for the 1951-2007 period. While many of these are climatic anomalies typically associated with La Niña, current conditions have been rather atypical over eastern Africa (unusually heavy rains), and Australia (dry conditions). Conditions over the Indian Ocean, such as the presence of a positive phase of the Indian Ocean Dipole mode, are believed to have some role in such atypical regional impacts. This underscores the need for detailed regional evaluations of prevailing conditions, combining expected El Niño/La Niña influences with other influences as well, to arrive at the best estimates of the weather patterns to expect regionally and locally.

 

2007 Sahel flooding: possible linkage with La Niña

The Sahel rainy season of 2007 was remarkable, not only because of the damage it inflicted on the fragile populations, but also for its wide geographical extent, stretching from Senegal in the west to Sudan and Kenya in the east. The flooding caused some 250 deaths and made more than 650 000 homeless. Studies carried out in the mid-1990s established a significant statistical linkage between the El Niño-Southern Oscillation (ENSO) phenomenon and rainfall in various parts of the African continent. For the Sahel belt, the La Niña event was found to be closely linked with the wet season July-September). This might have been the case for the summer this year: conditions show development of La Niña off the South American coast to the central tropical Pacific.

 

figure 1

La Nina composite maps of precipitation indicating high probability for a wet season to occur during July-August-September, 1950-1995 over western and central Africa
Grey areas indicate dry season (seasonal average <5 cm and annual average <15 cm).
Cold El Niño years (increasing magnitude): 1956, 1967, 1961, 1964, 1975, 1955, 1988,
1954, 1973, 1970

(source:IRI)

 

 

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