[OANNES Foro] Return of La Niña

[Mario Cabrejos]

Nature
Published online 16 September 2010 
doi:10.1038/news.2010.477 

As El Niño's cooler sister rolls round again, Nature probes the environmental pros and cons.
Return of La Niña
Adam Mann 
La Niña, the climatic event in which swathes of the equatorial east-central Pacific cool, strengthened through August, according to reports from the US National Oceanic and Atmospheric Administration (NOAA). Across all latitudes monitored by the NOAA, the ocean surface cooled by 1.3-1.8 ºC. Models predict that La Niña will persist until at least early 2011, and could, according to the NOAA, cool further during the coming winter. Nature explores its potential effects on the global environment.

What is the scientific definition of La Niña? 

La Niña is a natural 3-6-year cycle, and the cold stretch of a periodic thermal oscillation in ocean surface temperatures that occurs throughout the tropical Pacific. Along with the better-known warming event, El Niño, it involves a difference from average water temperatures of more than 0.5 ºC. La Niña can persist for 1-3 years, as seen, for example, in 1998-2000. 

What are its key global impacts? 

Ocean cooling affects tropical Pacific rainfall from Indonesia to South America, says Gerry Bell, a NOAA climate scientist at the Climate Prediction Center in Camp Springs, Maryland. Some places, such as northern Australia, experience wetter than average seasons. "The changes are so large that it affects wind, too," adds Bell, "and right downstream of this area is the Atlantic." 

Does this have implications for the Atlantic hurricane season? 

La Niña reduces variations in wind speed and direction throughout the atmosphere, which makes Atlantic hurricanes likelier, Bell says. This year may be particularly active, he adds. On top of La Niña, Atlantic water temperatures have remained unusually high over the past 15 years, and wind patterns that have been in place since 1995 - such as weak easterly trade winds and high pressure in the upper atmosphere - are particularly conducive to a strong hurricane season. NOAA expects 8-12 Atlantic hurricanes this season, of which it predicts 4-6 will be major hurricanes (that is, with sustained wind speeds of more than 178 km h-1).

Will La Niña help to lower global temperatures after recent record highs? 

Average temperature over land outside the tropical regions of the Northern Hemisphere reached a new high in July 2010, according to a report by the European Centre for Medium-Range Weather Forecasts. In addition, the global average over land during the previous 12 months ranks as the second highest on record. La Niña conditions have offset these highs, but have mainly affected tropical Pacific waters. Global temperatures have been rising since the turn of the twentieth century, most notably during the past 30 years, but La Niña is part of a regular cycle that is simply overlaid onto that pattern.

How strong is this La Niña event compared with previous ones? 

So far, La Niña has been relatively strong this summer. "That's not unheard of," says Bell, adding that similar conditions in 1998 made headlines. The oscillation usually begins to strengthen during the autumn, so it remains to be seen how much more powerful this year will be compared with previous years.

Why is it important to keep track of these events? 

Knowledge of how La Niña affects global weather patterns can help governments and people to plan for drought or heavy rainfall. For instance, researchers know that the Pacific Northwest is likely to be wetter than normal in the late autumn and early winter of La Niña years, which helps to refill parched aquifers in the wake of dry periods. Predicting global-temperature oscillations is crucial to water management, as well as to farmers seeking to mitigate potential losses. 
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