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No El Niņo, No La Niņa, No Idea

Last year was classified as having the worst storms ever recorded in history of the United States. In 2011, La Niņa disappeared in early January and was not replaced by El Niņo, resulting in an untamed jet stream that wreaked havoc on the United States. The spring was plagued with severe storms, tornadoes, hail and lightening that killed hundreds of people.

 

Climatological conditions known as El Niņo and La Niņa develop when abnormally warm or cool sea surface temperatures (SST) in the eastern Pacific interact with the atmosphere. An El Niņo occurs when the SST rise above normal (3 month average >0.5 Celsius) following months of faltering trade winds. The warm ocean surface water adds moisture to the air above it, forming low-pressure systems. A La Niņa occurs when the trade winds push the warmer surface waters away from the equator. The warm water is replaced by cooler, nutrient-rich water rising from deeper in the ocean, a process called upwelling. The cooler water impedes the formation of rain-producing clouds, leading to dry conditions near the equator in the Pacific Ocean. These two phenomenon provide structure to the climate system for the entire winter allowing climatologists to make fairly decent long range forecast.  

 

During an El Niņo, weather conditions between October and March tend to be wetter than normal in the southern United States. The opposite is observed in the northern United States, with drier winters resulting in reduced rain and snowfall (Figure 1). The climatic effect of a La Niņa in these regions is almost the opposite compared to El Niņo. The southern region experiences drier than normal conditions while the northern region experiences wetter than normal conditions (Figure 2).

 

Although El Niņo generally brings more rainfall to the southern region, there are some exceptions. La Niņa appears to be the more reliable of the two, almost always bringing dry conditions to the southern region. Climate effects in the northern region, on the other hand, are not as reliable and harder to predict.   While La Niņa generally brings wetter conditions, the effects of El Niņo appear to be less predictable in this region.  

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Last week we had the opportunity to observe spring-run Chinook salmon ascending the Parrot-Phelan diversion dam on Butte Creek near Chico. Currently the largest naturally spawning spring-run Chinook population in California, adult returns in Spring ChinookButte Creek have increased tenfold over the past decade. The increase has been attributed to habitat restoration, including the removal of four dams, the addition of eight adult passage ways and the installation of screened barriers to prevent juvenile fish from being diverted with water destined for irrigation... Read more >

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Figure 1. Sea surface temperatures (°C) and climate conditions during an El Niņo.  

 

The ocean-atmosphere interaction, known as the El Niņo-Southern Oscillation (ENSO), affects the location of the jet stream, altering rainfall patterns across the United States and throughout the tropics. The shifting of the jet stream alters the location and frequency of severe storms and hurricanes in the Atlantic and Pacific Oceans. La Niņa tends to lead to wind patterns that favor the formation of hurricanes in the Atlantic while reducing the number in the Pacific. Whereas, El Niņo tends to reduce the number of hurricanes in the Atlantic, but increase the number of hurricanes in the eastern Pacific.


Figure 2. Sea surface temperatures (°C) and climate conditions during an La Niņa. 

La Niņa pushes the jet stream northward, forcing cold Arctic air away from the lower United States, while El Niņo pushes the jet stream south bringing more storms with it. In the absence of a La Niņa or El Niņo (i.e., ENSO-neutral), the jet stream is unconstrained resulting in an unpredictable weather pattern as the jet stream is allowed to meander wildly through the United States. In 2012, La Niņa dissipated last month and has not been replaced by an El Niņo. Climate models indicate it is unlikely that the La Niņa will re-develop later in the summer, but there is uncertainty as to whether ENSO-neutral or El Niņo conditions will prevail. We could be in for another crazy year of weather if the jet stream is left untamed. 
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