Thursday, December 27, 2007

La Niña brings some rain, as Global Climate hots up

One of the important weather factors in the Pacific basin affecting global weather but particularly Pacific Rim countries is the El Niño/Southern Oscillation (ENSO). El Niño and La Niña are caused by a change in sea surface temperatures across the Pacific Ocean which affects trade wind patterns which bring rain and drought to regions around the Pacific and further afield. A La Niña condition has been developing since February 2007 and is likely to last to April 2008.

Related: El Nino-Southern Oscillation (ENSO)

La Niña operating until April 2008

In November/December 2007 we are experiencing La Niña with sea surface temperatures in the eastern tropical Pacific below average, and temperatures in the western tropical Pacific above average. La Niña brings rain to Indonesia and Australia and drought to the southwest of the United States, western Mexico, Peru and Chile. Effects are typically opposite to those associated with El Niño.

The Southern Oscillation has a global impact. NASA describes the effect as:

"Globally, La Niña is characterized by wetter than normal conditions west of the equatorial central Pacific over northern Australia and Indonesia during the northern hemisphere winter, and over the Philippines during the northern hemisphere summer. Wetter than normal conditions are also observed over southeastern Africa and northern Brazil, during the northern hemisphere winter season. During the northern hemisphere summer season, the Indian monsoon rainfall tends to be greater than normal, especially in northwest India. Drier than normal conditions are observed along the west coast of tropical South America, and at subtropical latitudes of North America (Gulf Coast) and South America (southern Brazil to central Argentina) during their respective winter seasons."

Australia: Higher than average temperatures and rainfall predicted

The widespread rains in Victoria and New South Wales over the last week were probably partly induced by La Niña. The POAMA model run daily at the Australian Bureau of Meteorology predicts "La Niña conditions to remain over the coming months, persisting until around April 2008." Read more about Australian rainfall patterns during La Niña events.

The Bureau of Meteorology is predicting higher than average temperatures over much of southern Australia from January to March 2008. The temperature on New Years Eve in Melbourne is predicted to be 38 degrees. There is also a greater chance for higher than the median rainfall "in a large area extending from southeast Queensland across both the northern inland and east of NSW. The southwest of WA also has similar chances, although it's a seasonally dry time of the year in this part of the country."

For North and South America "La Niña is the diva of drought,” explained Bill Patzert of NASA’s Jet Propulsion Laboratory. “It keeps the Pacific jet stream farther north, toward Oregon, Washington, and Canada, so the U.S. Southwest and Southeast get less moisture.”

Warmest Decade on Record

The World Meteorology Organisation announced in a press release on 13 December that the decade of 1998-2007 is the warmest on record. "The global mean surface temperature for 2007 is currently estimated at 0.41°C/0.74°F above the 1961-1990 annual average of 14.00°C/57.20°F." The statement also detailed "the record-low Arctic sea ice extent...., a relatively small Antarctic Ozone Hole; development of La Niña in the central and eastern Equatorial Pacific; and devastating floods, drought and storms in many places around the world."

"2007 global temperatures have been averaged separately for both hemispheres. Surface temperatures for the northern hemisphere are likely to be the second warmest on record, at 0.63°C above the 30-year mean (1961-90) of 14.6°C/58.3°F. The southern hemisphere temperature is 0.20°C higher than the 30-year average of 13.4°C/56.1°F, making it the ninth warmest in the instrumental record since 1850."

According to the WMO statement "Australia recorded its coldest ever June with the mean temperature dropping to 1.5°C below normal." and "while conditions were not as severely dry as in 2006, long term drought meant water resources remained extremely low in many areas. Below average rainfall over the densely populated and agricultural regions resulted in significant crop and stock losses, as well as water restrictions in most major cities."

Sea Level Rise and Greenland Ice melt Accelerating

The WMO highlighted that sea level continues to rise "substantially above the average for the 20th century of about 1.7 mm per year." According to satellite measurements "global averaged sea level has been rising at about 3 mm per year" since 1993. This comes hot on the news that the melting of the Greenland icecap is accelerating according to study by the Steffen Research Group from the University of Colorado at Boulder. "The amount of ice lost by Greenland over the last year is the equivalent of two times all the ice in the Alps, or a layer of water more than one-half mile deep covering Washington, D.C." said lead climate scientists Konrad Steffen. "The more lubrication there is under the ice, the faster that ice moves to the coast," said Steffen. "Those glaciers with floating ice 'tongues' also will increase in iceberg production." Greenland ice melt currently contributes to global sea levels by about 0.5 millimeters per year. Some climate scientists like James Hansen believe the Greenland ice sheet may disintegrate within 100 years leading to a sea level rise of several metres.


* NASA Earth Observatory La Nina for November. See also from February 2007 El Nino May Be Morphing to La Nina and in October 2007 La Nina Strenghtens in Autumn 2007
* NASA Earth Observatory La Niña Factsheet
* Bureau of Meteorology El Nino-Southern Oscillation (ENSO)
* Bureau of Meteorology Dec 17, 2007 - Warmer season favoured in southern Australia
* Bureau of Meteorology Dec 17, 2007 - Mixed March quarter rainfall outlook
* World Meteorological Organization Dec 13, 2007Press Release No 805