Outline

Further Reading: Chapter 07 of the text book

- terminology

- characteristics of ENSO

- impacts

Natural Environments: The AtmosphereGE 101 – Spring 2007

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MyneniLecture 20: ENSO

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Introduction

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MyneniLecture 20: ENSO

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• Today:

– We want to look at another source of variability in the atmosphere and ocean circulations, namely ENSO (El Nino Southern Oscillation)

– ENSO represents one of the best examples of ocean/atmosphere feedbacks in the climate system

– Also has impacts outside of the tropical Pacific due to teleconnections (linkage between changes in atmospheric circulation occurring in widely separated parts of the globe) within the general circulation of the atmosphere

• Key Properties:

– Pseudo cyclic: occurs every 2-7 years

– Varies in intensity: 1997-98 Very strong; 1992 Weak

– Largest effects occur during hemispheric winter

Terminology

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• El Nino– Warming of the sea surface temperatures in the eastern tropical Pacific

• La Nina– Cooling of sea surface temperatures in the eastern tropical Pacific

• Southern Oscillation– Changes in surface winds/atmospheric circulations over the equator associated

with El Nino

• ENSO (El Nino/Southern Oscillation)– The coupled response between ocean and atmosphere circulations over the

tropical Pacific

Normal Conditions

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Exceptionally intense trade winds come together in the Intertropical Convergence Zone (ITCZ), and the huge convective zone is to the west of the date line, where sea surface temperatures exceed 28C.

The eastward surface current between 3N and 10N, which is known as the North Equatorial Countercurrent because it flows counter to the prevailing winds, is relatively weak.

The westward South equatorial current is extremely strong, especially near the equator, where divergent motions cause intense upwelling and hence low sea surface temperatures.

The thermocline, the layer of large vertical temperature gradients that separates the warm surface waters from the cold waters at depth, slopes steeply to the west, where its depth is approximately 150 meters

Natural Environments: The AtmosphereGE 101 – Spring 2007

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MyneniLecture 20: ENSO

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El Nino Conditions

The trade winds have collapsed, to be replaced in the west by westerly winds.

The eastward movement of the convective zone is associated with –

- an eastward expansion of warm surface waters,

- a thermocline that is depressed in the east,

- an intensified eastward North Equatorial countercurrent,

- a weakened westward South Equatorial Current that is replaced by an eastward equatorial jet in the west,

- very weak equatorial upwelling

Natural Environments: The AtmosphereGE 101 – Spring 2007

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La Nina Conditions

• We find that the waters over the eastern Pacific cool down• This increases the pressure gradient from east to west• Hence the trade winds intensify• This pushes more warm water to the west• The western Pacific warms up• Convection intensifies• Hence the trades intensify even more, resulting in more upwelling in

the eastern Pacific and a further cooling of the temperatures.

Natural Environments: The AtmosphereGE 101 – Spring 2007

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Southern Oscillation Index (SOI)

The Southern Oscillation Index (SOI) is defined as the difference in surface level pressure Between Tahiti (in central Pacific) and Darwin (Australia).

During an El Nino there is high pressure at Darwin and low pressure area near Tahiti, in the mid-equatorial Pacific. The SOI is therefore negative.

Likewise, during the La Nina events, the SOI is positive.

LH L H

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Recent ENSO Events

• More warm events since the mid 70s• 97-98: strongest event on record• 82-83: second strong event• 90-95: series of successive warm events• 88-89: strong cold event – severe drought in the US• Warm event strength, persistence, periodicity changed – is this due to global change?

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Movement of Tropical Convection

– As the water moves east, the region of convection moves east

– In addition, the rainfall associated with the convection moves east -> find droughts over the western Pacific

– At the same time, rainfall increases over the eastern Pacific -> floods in Peru

Natural Environments: The AtmosphereGE 101 – Spring 2007

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Impacts-El Nino

In the Tropics, El Niño episodes are associated with increased rainfall across the east-central and eastern Pacific and with drier than normal conditions over northern Australia, Indonesia and the Philippines. Elsewhere, wetter than normal conditions tend to be observed 1) during December-February (DJF) along coastal Ecuador, northwestern Peru, southern Brazil, central Argentina, and equatorial eastern Africa, and 2) during June-August (JJA) in the intermountain regions of the United States and over central Chile. Drier than normal conditions generally observed over northern South America, Central America and southern Africa during DJF, and over eastern Australia during JJA.

El Niño episodes also contribute to large-scale temperature departures throughout the world, with most of the affected regions experiencing abnormally warm conditions during December-February. Some of the most prominent temperature departures include: 1) warmer than normal conditions during December-February across southeastern Asia, southeastern Africa, Japan, southern Alaska and western/central Canada, southeastern Brazil and southeastern Australia; 2) warmer than normal conditions during June-August along the west coast of South America and across southeastern Brazil; and 3) cooler than normal conditions during December-February along the Gulf coast of the United States.

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Impacts-La Nina

During La Niña episodes rainfall is enhanced across the western equatorial Pacific, Indonesia and the Philippines and is nearly absent across the eastern equatorial Pacific. Elsewhere, wetter than normal conditions tend to be observed during December-February (DJF) over northern South America and southern Africa, and during June-August (JJA) over southeastern Australia. Drier than normal conditions are generally observed along coastal Ecuador, northwestern Peru and equatorial eastern Africa during DJF, and over southern Brazil and central Argentina during JJA.

La Niña episodes also contribute to large-scale temperature departures throughout the world, with most of the affected regions experiencing abnormally cool conditions. Some of the most prominent temperature departures include: 1) below-normal temperatures during December-February over southeastern Africa, Japan, southern Alaska and western/central Canada, and southeastern Brazil; 2) cooler than normal conditions during June-August across India and southeastern Asia, along the west coast of South America, across the Gulf of Guinea region, and across northern South America and portions of central America; and 3) warmer than normal conditions during December-February along the Gulf coast of the United States.

The impacts of El Niño upon climate in temperate latitudes show up most clearly during wintertime. For example, most El Niño winters are mild over western Canada and parts of the northern United States, and wet over the southern United States from Texas to Florida. El Niño affects temperate climates in other seasons as well. But even during wintertime, El Niño is only one of a number of factors that influence temperate climates. El Niño years, therefore, are not always marked by "typical" El Niño conditions the way they are in parts of the tropics.

Natural Environments: The AtmosphereGE 101 – Spring 2007

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El Nino Impacts - USA

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ENSO Impacts - Highlights

ENSO impacts are the effects (positive and negative) of El Niño- and La Niña-related climate conditions on society and the environment.

Perhaps the most famous impacts of El Niño events are:

• The damages from floods and landslides caused by very high rainfall in Peru and southern California • The forest fires in Indonesia that have caused serious air pollution problems • The crop failures and sometimes famine from droughts in southern Africa and Australia• The collapse of the Peruvian anchovetta fisheries because of warmer coastal waters.

But ENSO events (in particular to the strong El Niño events of 1982–83 and 1997–98) have been connected to impacts on almost every aspect of human life:

- disease outbreaks, low and high agricultural yields, natural disasters, availability of water resources, energy demand, disruption to hydropower generation, price fluctuations, fishery catch fluctuations, animal movements, forest fires, the economic well-being of nations …

Natural Environments: The AtmosphereGE 101 – Spring 2007

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ENSO On The Web

Current state of ENSO

50 years of ENSO

NOAA ENSO theme Page

More about ENSO