El Niño, climate prediction, and the

US Southwest

Alexandre S. Gagnon, PhD

Department of Agronomy and Horticulture

New Mexico State University

Las Cruces, New Mexico

January 18, 2004

Outline

• Background information on El Niño, La Niña, and the Southern Oscillation

• The very strong 1997/1998 El Niño: A case study • Worldwide impacts of El Niño • Climatic impacts of El Niño on the United States and New

Mexico• Climate prediction• The Pacific Decadal Oscillation (PDO) and its role in

modulating the El Niño signal• Climate forecasts for the US Southwest• How much confidence can we place in the predictions?• Conclusions

What are El Niño and La Niña?

El Niño: warming of the Pacific Ocean between South America and the Date Line, centered directly on the Equator, and typically extending several degrees of latitude to either side of the equator. Coastal waters near Peru also warm.

• Named by Peruvian fishermen, because of the appearance of warm waters off the coast of Peru around Christmas time

• It occurs every 2 to 7 years (irregular interval)

La Niña: essentially the opposite of El Niño, i.e., anomalous cooling of ocean waters in the eastern equatorial Pacific Ocean

Surface temperature structure of Equatorial Pacific showing typical La Niña, normal and El Niño conditions. From NOAA El Niño website.

How does El Niño work?

• Strong Trade Winds push surface waters to the western Pacific

• Cold, nutrient-rich water occurs at the ocean surface off the coast of Peru. This upwelling supports high biological productivity during normal conditions

• On occasion, normal air pressure pattern breaks down. This causes Trade Winds to weaken and even reverse their direction, dragging warm and nutrient-poor water eastward and initializing El Niño conditions.

a) Normal conditions

b) El Niño conditions

What is the El Niño Southern Oscillation?

• This oscillation of sea surface temperatures (SSTs) in the tropical Pacific Ocean is associated with changes in the tropical Pacific atmospheric circulation

• The Southern Oscillation is the atmospheric component of the El Niño phenomenon

• Thus, the El Niño Southern Oscillation (ENSO) is a coupled atmosphere-ocean phenomenon. In other words, ENSO is an oscillation between the warm El Niño and the cold La Niña phases.

The El Niño Southern Oscillation (cont’d)

• Not all El Niño’s are alike

• One index used to quantify El Niño and La Niña events are the sea surface temperatures (SSTs) in the tropical Pacific Ocean. Positive SSTs refer to El Niño conditions and negative anomalies refer to La Niña conditions in all four regions

Niño 1+2 (0-10S, 80-90W)Niño 3 (5N-5S, 90W-150W)Niño 4 (5N-5S, 150W-160E)Niño 3.4 (5N-5S, 120W-170W)

SSTs in the four El Niño regions 1986-2004

Time/longitude plot of sea surface temperatures since 1986. From NOAA El Niño website

• Typically air rises over the western Pacific and sinks over the eastern Pacific.

• During an El Niño, the ocean warms by 1-2 degrees (0C) above its climatological average. A strong El Niño can result in a 3-4 degrees 0C warming over a large area.

• This warming affects the air pressure patterns. The focus of convection migrates from the Australian/Indonesian region eastward towards the central tropical Pacific Ocean

• Adverse impacts on world’s weather

Atmospheric circulation during El Niño versus normal conditions

Schematic representation of normal versus El Niño conditions. From NOAA El Niño website

The Southern Oscillation Index (SOI)

• Another index used to quantify ENSO events

• Seesaw pattern between pressure in eastern and western tropical Pacific

• Standardized difference in sea level pressure between Darwin (Australia) and Tahiti (French Polynesia)

• A negative SOI refers to low pressure at Tahiti and high pressure at Darwin, indicating El Niño conditions

The Southern Oscillation Index, 1950-2000

Development of the 1997/1998 El Niño(March 1997)

•The 1997/98 El Niño is clearly the strongest event since 1982/83 as indicated by both the temperature data and the Southern Oscillation Index (SOI)

Development of the 1997/1998 El Niño (cont’d) (June 1997)

Development of the 1997/1998 El Niño (cont’d) (September 1997)

Development of the 1997/1998 El Niño (cont’d) (December 1997)

Development of the 1997/1998 El Niño (cont’d) (April 1998)

History of El Niño

• El Niño is not a recent phenomenon

• coral evidence indicate El Niño's have been occurring for the last 2 million years

• detailed knowledge from 1500 to present

• 1525 - 1987   115 El Niño's  (Quinn & Neal, 1992)

Typical influence of El Niño • As an El Niño develops,

the first impacts are confined to the tropical Pacific, where Indonesia begins to experience drier-than-normal conditions

• El Niño’s shift temperature and precipitation patterns in many different regions of the world

From: International Research Institute for Climate Prediction (IRI)

Typical influence of La Niña

•Impacts of La Niña are generally opposite to those of El Niño

From: International Research Institute on Climate Prediction (IRI)

Other impacts of El Niño

• El Niño became a well known issue when the Peruvian anchovy industry collapsed after the 1972-73 El Niño. The warm El Niño waters inhibit the upwelling of colder nutrient rich waters thus limiting the food supply.

• Birds (esp. guano): population decreased due to lack of fish for food • Hurricanes: more in eastern Pacific and less in the Caribbean/Atlantic region

• Health impacts: mosquito-transmitted diseases such as dengue fever and malaria

Climatic impacts of El Niño on the United States and New Mexico

•The impacts of El Niño on the US climate show up most clearly in the winter

•New Mexico typically experiences cooler winter temperatures (DJF) during El Niño

Maximum temperature anomalies during El Niño

Climatic impacts of El Niño on the United States and New Mexico (cont’d)

Precipitation anomalies during El Niño in the winter

•New Mexico typically experiences wetter winters (DJF) during El Niño

El Niño and New Mexico in more details

Las Cruces

From: NOAA-CPC website

•Normal versus El Niño-average precipitation by climatic region

•For the southern desert, there is almost 2 times more winter (DJF) precipitation during a typical El Niño event

•Winter temperature is on average 1.3 degree cooler during an El Niño in the same climate division

• With El Niño, the period October through March tends to be wetter than usual. Precipitation frequency and intensity are increased.

• El Niño winters can be 2-3 times wetter than La Niña winters in this region.

• Cooler temperatures during El Niño, particularly over New Mexico

• Large percentage of streamflow in the West is produced by melting of snow in the spring

• Cooler winters and a strong tendency toward wet winters imply a deeper snowpack in higher elevation and therefore greater streamflow in spring and summer in the Southwest during El Niño years.

Overview of climatic impacts of El Niño on the US Southwest

• Opposite of El Niño, thus dry winters during La Niña years

• The La Niña signal on the climate of the Southwest is more reliable than the El Niño signal. Not all El Niño’s bring wet winters while there are no exceptions to the climatic consequences of La Niña during the past 65 years in the region.

• Although El Niño has received considerably more attention than La Niña, the climatic consequences associated with La Niña winters have serious consequences on the New Mexican economy

• The impacts of both El Niño and La Niña are less clear in the summer and there are contradictory results from research on the relationship between ENSO and monsoonal rains (Sheppard et al., 2002)

Climatic impacts of La Niña in the US Southwest

Climate prediction

• What is the difference between weather and climate?

-Weather is the condition of the atmosphere at any particular time and place.

-Climate is "averaged weather", the weather in some location averaged over some long period of time

• Some might wonder how we make climate predictions if we can’t predict the weather for next week

• We can’t predict the weather for next season but under certain conditions we can determine whether the next season will be warmer/cooler and wetter/drier than under normal climatic conditions

• What we need to know in order to forecast next season’s climate are variables that affect the atmosphere and change slowly with time such as the sea surface temperatures (SSTs).

• Ocean temperatures change relatively slowly and can be very different from one year to the next, especially when there is an El Niño, as explained earlier.

• El Niño is therefore important for long-range forecasting

• Other atmospheric phenomena that affect the year-to-year variations in the climate are the North Atlantic Oscillation (NAO), which shifts rainfall and temperature patterns over eastern North America and Europe, and the Pacific Decadal Oscillation (PDO), which, like ENSO, is more important for the US Southwest.

Climate prediction (cont’d)

Pacific Decadal Oscillation (PDO)•El Niño-like pattern of Pacific climate variability, •PDO "events" persist for 20-to-30 years, while typical ENSO events persist for 6 to 18 months •Causes for the PDO are not currently known. Likewise, the potential predictability for this climate oscillation is not known •Index is positive (negative) when Northeastern Pacific Ocean temperatures are warm (cool)

a) Positive ‘warm’ phase b) Negative ‘cool’ phase

From: NASA Jet Propulsion Laboratory

• Scientists believe that we entered the 'cool' phase in 1998/99

• During the cool phase, the PDO causes the jet stream to move further north over the western United States resulting in decreased precipitation over the Southwestern states

• El Niño is stronger during positives phases of the PDO and La Niña is stronger during negative phases of the PDO

Pacific Decadal Oscillation (cont’d)

Monthly values for the PDO index: Jan 1900-Aug 2004. From JISAO.

•Conversely, El Niño is weaker during negative phases of the PDO and La Niña is weaker during negative phases

• Since there is some ability to predict ENSO cycles, there is the possibility of making long-range forecasts– NOAA successfully predicted the major El Niño of 1997-1998

six months in advance

• Numerical models forced with current sea surface temperature (SST) data in the tropical ocean basins, and model predictions of how the oceans will likely evolve during the next several months, are used to predict mean seasonal patterns of temperature and rainfall

• The IRI provides a Net Assessment Forecast based on the predictions from several computer models

(http://iri.columbia.edu/climate/forecast/net_asmt/

• These forecasts tell us the probability of above normal, normal, or below normal precipitation and temperature

Climate prediction (cont’d)

What are the climate forecasts for the US Southwest?

•Climate forecast from the IRI are available for January - June 2005.

•These climate forecasts are based on SST data, notably on the weak El Niño conditions that currently exist and that are predicted to continue but will gradually weaken during the forecast period (six months for the IRI).

ENSO forecast for 2005From: IRI website

What are the precipitation forecasts for the US Southwest?

IRI multi-model probability forecast for precipitation February-March-April 2005, issued December 2004

<- same as above but for April-May-June 2005

Enhanced probabilities for above normal precipitation are forecast for the FMA period over New Mexico

What are the temperature forecasts for the US Southwest?

IRI multi-model probability forecast for temperature February-March-April 2005, issued December 2004

<- same as above but for April-May-June 2005

Climate Predictions from NOAA-CPC

Precipitation forecast for Mar-May 2005 (left) and Apr-Jun (right)

Precipitation forecast for Jan-Mar 2005 (left) and Feb-Apr (right)

•Increased chances of above-average precipitation are predicted for the US Southwest for the next few months

What was the precipitation forecast for the strong 1997-1998 El Niño?

•There was a 55% probability that precipitation was going to be above average over New Mexico and Arizona in October-December 1997

•Was it successfully forecast?

•Validation: precipitation anomalies during this period were 120-150% of the median value in southern and eastern New Mexico

How much confidence can we place in the predictions?

• All forecasts are fallible. In the Southwest, not all El Niños bring wet winters

• The climate predictions for January - June 2005 are dependent on the accuracy of the SST predictions. There is some uncertainty concerning the evolution of SSTs: small chance that El Niño could strengthened to moderate intensity but also a chance that it will weaken back to normal conditions

• Note that even if perfectly accurate SST forecasts were possible, there would still be uncertainty in the climate forecast due to chaotic internal variability of the atmosphere

• These uncertainties are reflected in the probabilities given in the forecast

• These climate forecasts do not fully account for all factors that influence regional climate variability and are relevant only to seasonal time scales and relatively large areas

Conclusions

• Our ability to predict El Niño has led to the development of climate forecasts

• Weak El Niño conditions are currently observed and are predicted to last through winter and spring

• Climate models predict enhanced probabilities for above normal precipitation and warmer temperatures for winter and spring

• Will increased precipitation improved drought conditions? • The PDO is currently in its negative phase and it may decrease

the impact of El Niño over the US Southwest• The PDO cycle is 20-30 years in duration; will the PDO offset

part El Niño-generated precipitation until 2020 (or longer)?

• Will the increased frequency of El Niño and decreased frequency of La Niña events benefit New Mexico and the US Southwest?