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El Nino-Southern Oscillation (ENSO)

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Overview

El Nino, the most powerful control on weather across the planet is rearing its ugly head as we speak. The telltale signs are building and atmospheric scientists are predicting one of the most powerful El Nino events in history, certainly the strongest since 1998. This actually could be very good news for drought-stricken California as El Nino typically delivers a lot of rain and snow to the state. In the case of California, the drought has been so severe that rains last winter caused massive mudslides. And the rain caused so much new vegetation to grow and this served as fuel for the fires in Summer 2017. Southern Australia becomes very dry in El Nino years and the drought causes great hardship for farmers and can often lead to very dangerous bush fires. And in Peru, El Nino comes with changing ocean currents that lead to the collapse of fisheries and great financial hardships. In this section we learn about ocean circulation and how it impacts climate on a global scale.

One of the clearest examples of the influence of the oceans on climate is the El Nino Southern Oscillation (ENSO). This is the strongest inter-annual (i.e. periodic) variation in the modern climate system. The ENSO cycle lasts between two to seven years (average of 4 years) and corresponds to sea surface and thermocline temperatures and atmospheric pressure changes in the Pacific Ocean that ultimately have effects on weather and surface ocean conditions across the globe. The ENSO cycle consists of two main climatic intervals, the El Nino interval and the La Nina interval. The El Nino stage begins with an increase in sea surface temperatures across the eastern Pacific as warm waters spread from the west.

Two images show ocean temp distribution characterizing El Nino and La Nina episodes in the Pacific - two images show changes from mean temps
Ocean temperature distribution (top panel) that characterize El Nino (left) and La Nina right episodes in the Pacific Ocean. Changes from mean temperatures are shown in the bottom panel.
Credit: NOAA

Typically, there is a very large east to west SST gradient across the Pacific, but El Nino virtually erases this gradient. For reasons that are not fully understood, the El Nino stage is characterized by high atmospheric pressure in the western Pacific. The opposing La Nina stage is characterized by relatively cool waters in the eastern tropical Pacific and low atmospheric pressure in the western Pacific. The westward trade wind flow piles up warm water in the western Pacific and this causes sea level to be higher by about 20 cm in the western compared to the eastern Pacific (so sea level is, relatively speaking, higher in the eastern Pacific during El Nino). To compensate for this windflow, the thermocline slopes down upwards towards the eastern Pacific, resulting in greater stratification in the east. The slope of the thermocline shallowing changes markedly during ENSO. As El Nino piles up warm water in the eastern Pacific, the thermocline deepens in that location whereas the La Nina part of the cycle corresponds to a very shallow thermocline in the eastern Pacific.

The following animation shows how an El Nino event in 1997 developed in the tropical Pacific Ocean:

Video: Visalizing El Nino (2:28). This video is not narrated.

As you may recall from Module 2, there is a good correlation between El Nino events and warmer temperatures on a global scale; in fact, a good part of the temperature variation superimposed on the general warming trend of the last 60 years can be explained by ENSO variability. This can be seen in the figure below (from Module 2).

Graph to show global temperature anomaly.
Global Temperature Anomaly
Credit: D. Bice

During an El Nino event (red parts of the ENSO curve), more water vapor is added to the atmosphere, making it warmer, and during a La Nina event (blue parts of the ENSO curve), less water vapor makes it into the atmosphere, so the global temperature drops. Remember that water vapor is a greenhouse gas, and it also transports heat to the atmosphere when the vapor condenses to form water droplets (clouds and rain).

ENSO has major implications on weather across the globe. For example, drought conditions in 2012 in the Midwest are a direct result of the ENSO cycle. El Nino intervals correspond to increased precipitation in California and eastern South America due largely to the ability of warm water to hold more moisture than cold water. The warm surface water in the eastern Pacific also increases surface ocean stratification and decreases upwelling offshore Peru and Ecuador. The implications of diminished upwelling are severe for coastal fisheries in these countries. Warm water also increases the occurrence of bleaching in Caribbean and eastern Pacific corals. The position of the high-pressure region in the Pacific affects the tracks of Pacific cyclones. In the Atlantic, increased wind shear during El Nino intervals inhibits the formation of hurricanes. El Nino intervals also correspond to cooler and wetter winters in Florida and the southeastern US and warmer and drier winters across the northern tier of the US and in Canada. The La Nina phase corresponds to the opposite conditions from El Nino, most notably an increase in the frequency in Atlantic hurricanes as conditions become more favorable for their formation (i.e. less shear in the tropical Atlantic).

Check Your Understanding

ENSO question:

Which is a key component of the El Nino phase?

 Increased precipitation in the northern Pacific
 Piling up of warm water in the eastern Pacific
 Piling up of warm water in the western Pacific
Increased precipitation in the southern Pacific


Click for answer.

ANSWER:
The El Nino phase corresponds to piling up of warm water in the eastern Pacific