Double dip for La Niña this winter
December 2011 - Sea surface temperatures are currently below normal over much of the central and eastern tropical Pacific Ocean. In conjunction with stronger trade winds, suppressed rainfall in the central Pacific and a large scale decrease in equatorial sub-surface ocean temperatures, this indicates that La Niña has returned for a second consecutive winter.
Following last year's unusually strong La Niña, sea surface temperatures had warmed to near normal during summer. However, around this time the Met Office seasonal forecast model, Met Office seasonal prediction system: GloSea4 (Arribas et al. 2011), began to indicate that there was a possibility of a return to La Niña conditions by late autumn. The Tropical Pacific sea surface temperature forecasts indicates that the current La Niña is likely to continue through northern-hemisphere winter 2011/12. Following this there is a gradual rise in sea surface temperature anomalies so that by April 2012, the end of our forecast outlook, there is a 60% chance of continued La Niña and a 40% chance of a return to normal conditions. The WMO consensus, issued in November, is that the present La Niña is likely to be considerably weaker than the La Niña of 2010-2011.
How does La Niña affect worldwide climate?
La Niña and its warm counterpart El Niño are natural fluctuations in sea surface temperature that occur every few years in the tropical Pacific and are part of the ENSO (El Niño Southern Oscillation) cycle. The Southern Oscillation describes changes in atmospheric surface pressure between the eastern and western tropical Pacific, that accompany El Niño and La Niña events in the ocean. The ENSO cycle involves close interaction between the atmosphere and ocean.
During ENSO the changes in the Pacific Ocean are coupled to changes in atmospheric winds and in temperature and rainfall. Through teleconnections, these atmospheric changes extend well beyond the tropical Pacific region and neighbouring countries. The Atlantic and Indian oceans are affected, which further extends and prolongs the impacts. The impacts vary considerably with location: for example, during a La Niña the westward shift of rainfall in the western Pacific region tends to cause extra rainfall in the Philippines, Indonesia, northern and eastern Australia (for example, remember the severe Queensland floods early in 2011), while central Pacific islands experience a rainfall deficit. By averaging the effects of many events we can see the main impacts. Note however that each individual event is different (in severity and timing), and impacts are also modified by the state of the climate at the time: even the sign of the anomalies (excess or deficient rainfall for example) may vary from one event to another for particular locations. Predictions of the effects of the current La Niña are best inferred from forecasts that take into account the present climate state, rather than using 'typical' impact maps.
In the European region the effects of ENSO are relatively weak and variable and there is a tendency for an intra-seasonal switch between early and late winter. During La Niña the tendency in late winter is for a positive North Atlantic Oscillation pressure pattern, with mild conditions in northern Europe and the UK and cold conditions in southern Europe (Brönnimann et al., 2007; Moron and Gouirand, 2003). The opposite signal - cold late-winter conditions for the Northern Europe region - is associated with El Niño years. Of course, in any individual year the observed climate anomaly over a region will likely be a complex combination of many factors, of which ENSO is just one.
Arribas, A and coauthors, The GloSea4 Ensemble Prediction System for Seasonal Forecasting, MWR, 139, doi: 10.1175/2010MWR3615.1, 2011
Brönnimann, S. and coauthors, ENSO influence on Europe during the last centuries. Clim. Dyn., 28, 181-197 doi: 10.1007/s00382-006-0175-z, 2007
Moron, V. and Gouirand, I. Seasonal modulation of the ENSO relationships with sea level pressure anomalies over the North Atlantic in October-March 1873-1996. Int. J. Climatol., 23, 143-155, 2003