El Niño

Sea Surface Temperatures

At the beginning of the water year, an El Niño was already brewing as above-average sea surface temperatures (SSTs) were emerging in the eastern Pacific Ocean. In October 2009, the pattern and amount of warm water pooling in the eastern Pacific indicated that at least a weak El Niño was in the process of forming. Through November and December, trade winds blowing east along the equator slackened in response to the weak El Niño event, allowing more warm water to accumulate near the coast of South America. This coupling between the ocean and atmosphere intensified the El Niño event and by the end of December, El Niño was classified as moderate to strong. SSTs peaked in late January and early February at more than 2 degrees Celsius above average in the central Pacific. The El Niño event induced a strong subtropical jet stream over the northern mid-latitudes that delivered numerous and strong winter storms to the desert Southwest. By March, SSTs were decreasing and the 2009–2010 El Niño event began to wane. El Niño and La Niña events often peak in the early winter and fade soon thereafter.

ENSO-neutral conditions returned by May but didn’t last long. SSTs entered La Niña territory by mid-summer, marking a rapid transition from El Niño to La Niña conditions. By September, a strong La Niña event had materialized. It continues to gain strength, and most models suggest that the event will persist through at least early winter.

Southern Oscillation Index

The Southern Oscillation Index (SOI) reflected the roller coaster ENSO ride experienced during the 2010 water year. SOI values were very negative (<-1.5) in October 2009 and were persistently negative through the winter season, indicating a strong atmospheric response to the warming SSTs. SOI values reached their lowest level in February, which was consistent with a strong teleconnection pattern that shifted the western U.S. winter storm track south over Arizona and New Mexico.

As the El Niño event began to wane in March and April, SOI values responded quickly and rose to positive values by the end of April. Values continued to increase for the rest of the water year, while SSTs continued to cool. SOI values climbed to more than 2.5 in the last month of the water year, indicating a strong La Niña event was in place.

Click figures to enlarge.