El Niño

Sea Surface Temperatures

La Niña dominated the headlines again this water year, and the event sent shockwaves across the atmosphere that helped bring drier-than-average conditions to the Southwest. It was the second consecutive winter in which La Niña was present; back-to-back La Niña events often occur if the first one is relatively strong, like it was in the winter of 2010–2011.

The 2012 water year began with weak La Niña conditions returning to the equatorial Pacific Ocean in September after brief hiatus between May and August (Figures 6a–b). During the early winter months, sea surface temperatures (SSTs) in the eastern Pacific Ocean along the equator continued to cool and the event peaked in February, reaching  weak to moderate strength. The La Niña began to dissipate thereafter and officially ended in April. Even though this La Niña was relatively weak compared to the previous year’s event, it had a substantial impact on weather patterns across the western U.S. throughout the winter. During the January–March period, for example, the winter storm track took on a more northerly route, dumping above-average precipitation on the Pacific Northwest but leaving much of the Southwest with below-average rain and snow, which is the typical pattern during La Niña events. This was the second winter in a row with below-average precipitation, leading to worsening drought conditions across the region.

ENSO-neutral conditions in April and May gave way to borderline El Niño conditions in June as SSTs warmed. This rapid increase in temperature gave rise to forecasts that an El Niño event would emerge during the late summer. By August, however, the eastern Pacific  Ocean began to cool. The water year ended with ENSO-neutral conditions in control and the prospects of a developing El Niño event uncertain.

Southern Oscillation Index

The Southern Oscillation Index (SOI), a measure of the atmospheric response to El Niño or La Niña conditions, showed positive values that are characteristic of La Niña events for the duration of the event. SOI values rose to moderate levels as La Niña conditions developed early in the water year, then peaked in December and quickly decreased throughout the winter and early spring as the La Niña event weakened. SOI values returned to close to zero by May and even became slightly negative in early summer, when SSTs warmed slightly in the eastern Pacific Ocean. This was the first sign that the atmosphere was being shifted by a developing El Niño event. That El Nino event never fully materialized and the water year ended with SOI values reflecting ENSO-neutral conditions.

Click figures to enlarge.