El Niño

Sea Surface Temperatures

When the water year began on October 1, 2010, the La Niña event was already moderate to strong (Figures 6a−b). Very cool water temperatures—as low as2 degrees Fahrenheit below-average—were present across much of the central and eastern Pacific Ocean. The cool sea surface temperatures (SSTs) helped reinforce already stronger-than-average easterly winds along the equator in the Pacific Ocean basin, which helped maintain La Niña conditions during the winter. In response to the La Niña event, a high pressure system developed along the West Coast of the United States, which consistently steered winter storms into the mainland between central California and the Pacific Northwest. This northern trajectory of storms left much of the Southwest and south-central U.S. in a persistently dry weather pattern between January and March.

Between April and June, SSTs warmed and La Niña’s strength started to wane. NOAA declared an end to the event in June when neutral ENSO conditions returned. The atmosphere, however, lagged behind and enhanced easterly winds, characteristic of La Niña, continued, albeit weak, during the summer. The persistence of stronger-than-average easterly winds signaled neutral conditions likely would be short lived. True to form, the easterly winds helped drag colder-than-average water to the surface in the eastern Pacific through August and September, ushering back weak La Niña conditions. The 2010–2011 water year ended as it started: with the presence of La Niña.   
 

Southern Oscillation Index

Positive values for the Southern Oscillation Index (SOI)—an indicator of the strength of the winds—shows that a moderate to strong La Niña event was well entrenched during most of the 2011 water year. SOI values were very high at 1.8 in October, indicating that the atmosphere had started to shift its large-scale circulation pattern across the entire Pacific Ocean basin at the beginning of the water year. Values continued to climb through the winter as the event became more organized, peaking at 3.2 in December. These high SOI values continued throughout the remainder of the winter season and into the spring, when the event started to weaken. SOI values dropped to 1.9 in April and plunged to 0.4 in May. These values remained low until August and September, when they began to rise again, indicating the atmosphere was again strongly responding to La Niña conditions. 

Click figures to enlarge.