WMO El Niño/La Niña update (July)

Summary

La Niña conditions prevailed in the tropical Pacific during the first half of 2007.

Movement toward a typical La Niña condition was unsteady during the March-June period. A strong break in the pathway toward La Niña occurred during May and early June, increasing uncertainty over whether a La Niña event would develop in 2007.

Nonetheless, cool waters continue to be found beneath the central equatorial Pacific, and forecast models continue to predict at least some surface cooling in the central and eastern equatorial Pacific, with most dynamical models continuing to strongly favour La Niña development.

While the fluctuating conditions in the equatorial Pacific over the last few months imply a possibility of neutral conditions continuing through the remainder of 2007, a La Niña event is considered, albeit slightly, more likely. Development of an El Niño in 2007 is considered very unlikely.

While it is possible that La Niña conditions may develop in the next one to three months, the timing and magnitude of such an event continue to be uncertain, with no indications of a particularly strong event in terms of sea-surface temperatures.

In view of the above assessment, regions typically impacted by La Niña events are advised to take note of the continued enhanced risk of such an event this year.

The situation in the tropical Pacific will therefore continue to be carefully monitored. More detailed interpretations of regional climate fluctuations will be generated routinely by the climate forecasting community over the coming months and will be made available through the respective National Meteorological and Hydrological Services.

Climate patterns in the Pacific Recent research 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, they become lower than normal. These temperature changes are strongly linked to major climate fluctuations around the globe and, once initiated, can last for 12months 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 events change the likelihood of particular climate patterns around the globe, 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 event and its intensity, there is always potential for an event to generate serious impacts in some regions irrespective of its intensity. 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. The exchange and processing of the data are carried out under programmes coordinated by WMO.

See Info Note No. 33

http://www.wmo.int/pages/prog/wcp/index_en.html

http://www.wmo.int/pages/prog/wcrp/AP_CLIVAR.html