February 2, 2011 | It's been a noticeably dry and gusty fall and winter thus far along the Front Range, with wildfires, wind storms, and wave clouds filling the skies. Meanwhile, early-season snowpack in Colorado’s mountains has measured well above average in many locations. The likely cause of this weather? La Niña, El Niño’s sister.
La Niña occurs when cooler-than-normal sea surface temperatures form in the eastern and central Pacific Ocean off the coasts of Peru and Ecuador. The cooler water temperatures are caused by an increase in easterly sea surface winds that force cold water from below the ocean’s surface to the top. The cooler water, in turn, chills the overlying air and helps reinforce the La Niña pattern.
If what goes around comes around, as the saying goes, nowhere is this more evident than with atmospheric patterns such as La Niña. A handful of major weather and climate patterns with names that range from the fanciful (Pineapple Express) to the more technical (Pacific Decadal Oscillation) stretch thousands of miles across the atmosphere and shape weather and climate in disparate places. These circulation patterns arise because of heating contrasts between the poles and equator, modulated by the changing seasons, and the rates at which land and water absorb and release heat. The result is an ever-changing patchwork of warmer, cooler, wetter, and drier regions.
At NCAR, scientists in NESL/CGD and other divisions are working to better understand and predict these patterns, as they have considerable effects on regional temperature and precipitation. They’re also looking at the interplay between atmospheric patterns and climate change.
Children of the tropics
Better known for the El Niño and La Niña patterns it produces, ENSO (El Niño–Southern Oscillation) can cause extreme weather around the world, including floods and droughts. It is characterized by variations in sea surface temperatures in the eastern tropical Pacific, with the warm phase called El Niño and the cool phase La Niña, along with a seesaw in atmospheric pressure in the tropical western Pacific known as the Southern Oscillation.
South American fishermen gave El Niño its name (Spanish for “The Boy”) in reference to the Christ child, because the periodic warming of Pacific waters off Peru and Ecuador is often noticed around Christmastime. The neutral phase of ENSO, during which the atmosphere and ocean are neither unusually warm nor cold, is sometimes humorously referred to as La Nada ("The Nothing").
Rather than being mirror images, El Niño and La Niña display significant differences in their spatial structures and seasonal evolution. New research in CGD has reproduced some of the key differences using climate models. The study, led by Yuko Okumura, has important implications for the prediction of ENSO and its global influences.
Yuko and Clara Deser analyzed two datasets of monthly sea surface temperatures spanning different periods: 1900–2008 and 1982–2008. They confirmed a robust asymmetry between El Niño and La Niña throughout the record, especially during strong ENSOs. Both phases typically begin in late spring or summer. Most El Niños terminate rapidly after peaking in December or January, but many La Niñas persist through the following spring and summer and re-intensify in winter, some even lasting through a third year. Modeling experiments suggest that this asymmetry can be explained by the different evolution of surface wind anomalies over the far western Pacific during El Niño and La Niña.
The surface wind anomalies associated with the current La Niña show a pattern consistent with Yuko and Clara’s analysis, as NOAA’s Climate Prediction Center reports that the present La Niña is expected to continue well into spring 2011 in the Northern Hemisphere.
The ability of climate models to capture ENSO got a boost in April 2010 with the release of CCSM4, which is being used for an ambitious set of climate experiments that will be featured in the IPCC’s next report, due in 2014.
CCSM4 depicts ENSO better than previous versions of the model, according to CGD’s Rich Neale. The model is more sensitive to tropospheric humidity, making it better able to represent deep convection in the tropics. It’s also more sensitive to vertical wind shear. In addition, the model reproduces the observed asymmetry in the durations of El Niño and La Niña, which was not captured by the earlier versions of CCSM.
The north Pacific
Related to ENSO is the Pacific Decadal Oscillation (PDO) in the north Pacific. During the PDO’s positive phase, sea surface temperatures tend to be above average along the west coast of North America and in the eastern tropical Pacific, while across the central north Pacific they are cooler than average. The opposite pattern occurs during the negative phase. Each phase typically persists for 10–30 years. A warm phase predominated from the late 1970s to around 2000, but the PDO has alternated between cold and warm phases since then.