Last year was shaping up to be the hottest on record, and then La Niña came along. Cold surface currents in the eastern Pacific contributed to conspicuous and in some cases catastrophic weather, from record snowfall in the United States to drought in eastern Africa and massive flooding in Australia.

Now the US National Oceanic and Atmospheric Administration (NOAA) is reporting that La Niña is going for a double-dip.

This doesn't necessarily mean that we are in for a repeat of last year, but ocean temperatures do affect weather patterns in statistically significant ways and can thus provide some guideposts when agencies like NOAA are trying to cast forward into the chaos.

As illustrated above, La Niña is an upwelling of cool waters in the eastern equatorial Pacific that occurs as warmer surface waters move west; the opposite, a concentration of warm waters in the east, is known as El Niño.

The Pacific typically cycles back and forth every several years, and these temperature signals — or their absence — serve as the foundation of seasonal weather forecasting. NOAA will issue its outlook for winter weather in October, so stay tuned.

As for global temperatures, if La Niña comes through in a big way, it could depress temperatures yet again in 2011 and into 2012. This effect may have been just enough to knock last year out of the top slot, but even so, 2010 tied with 2005 for the warmest year on record.

(Nature, 09/09/2011)