National Weather ServiceTechnical AttachmentNo. 97-37

John MonteverdiSan Francisco State University

Jan NullNational Weather Service

Today’s Presenter:

John MonteverdiProfessor of MeteorologySan Francisco State University

Separating “Hype” from Separating “Hype” from ScienceScience

What is El Niño?What is El Niño? What impact (if any) does it have on What impact (if any) does it have on

California precipitation?California precipitation?

What is El Niño?What is El Niño?The term El Niño refers to a rapid, dramatic warming of The term El Niño refers to a rapid, dramatic warming of the sea-surface temperatures in the eastern tropical the sea-surface temperatures in the eastern tropical Pacific, chiefly along the north-central coast of South Pacific, chiefly along the north-central coast of South America and westward. America and westward.

QuickTime™ and aCinepak decompressor

are needed to see this picture.

El Niño events are characterized by sudden onset of large temperature deviations (on the order of 2C, or about 4F or greater) usually peaking in December(hence the name which refers to Christmas--"El Niño” = "the Christ Child").

Warmings occur rather frequently (on the order of once every four years or so) as part of the Southern (aka Biennial) Oscillation, a cyclical change in the pressure and temperature distribution along the Equator that takes place every two years or so.

PacificPacific Sea Surface Sea Surface Temperatures (SST) in non-El Temperatures (SST) in non-El

Niño SituationNiño Situation

Walker Circulation

El NiñoEl NiñoPacific Sea Surface Pacific Sea Surface Temperatures (SST)Temperatures (SST)

Trade Winds weakenOr reverse

Water “Sloshes”Eastward, BringingWarm Water andHigher Sea-levelsTo South American Coast

Three “sub-types” of Three “sub-types” of El NiñoEl Niño

Type 2 El NiñoType 3 El Niño SST Anomalies Dec 1997Type 1 El Niño

• a series of catastrophic flood producing weather events • a hurricane • a period of drought in California or the West • a series of storms

Any of these MAY occur as an impact of El Niño (or be associated with it coincidentally).

Putting the 1997-98 El Niño Into Proper Perspective

• Sea-surface Temperature Patterns

• How Large Was It?

Southern Oscillation Index:Pressure (Tahiti - Darwin)Mean SST Anomaly Eastern Tropical Pacific

What impact does El Niño What impact does El Niño have on California have on California

Precipitation?Precipitation?

How and why does El Niño effect How and why does El Niño effect California storms and precipitation?California storms and precipitation?

What do climatology and meteorology tell us regarding the resulting precipitation patterns associated with El Niño in the past?

The unusually warm sea surface temperature pattern in the eastern tropical and subtropical Pacific is expected to create a southern branch of the storm track.

This southern branch (green arrow) extends from somewhere near or north of the latitude of Hawaii (approximately 30 to 40N) to the southern (or central) coast of California.

Conceptually, theEl Niño-influencedjet stream position should be associatedwith wet conditions central and SouthernCalifornia, and drynessin the Pacific Northwest.

Is this borne out by the historical

record?

Precipitation Anomalies by Climatic Division forSeven Type 1 El Niño Events Since 1951

The blue line represents the 70th Percentile rainfall amount. (This means that rainfalls exceeding this value are greater than 70% of the values observed).

Red bars indicate seasonal rainfall for El Niño type 1 years, greenbars indicate rainfall for El Niño Type 2 events and yellow bars indicate rainfall for El Niño Type 3 events.

The zero line represents the long term or 30 year average rainfall (inches) (also called “Normal” rainfall).

For San Francisco, six out of eight Type 1 El Niño years were wet.

Note that for Eureka, four Type 1 El Niño years had greater than normal rainfall, and four had lesser than normal rainfall.

Based upon past statistics, what were the chances that

the 1997-1998 El Niño Year

would be wetter than normal in California?

Very great chance in Southern California decreasingTo a “reasonable” chance in north-central California, with

The most likely category for Northern California being“Normal” rainfall.

To What Extent Can They Be To What Extent Can They Be Used To “Foreshadow” Used To “Foreshadow”

Precipitation In California?Precipitation In California?

But WereThese Precipitation Anomalies Statistically Significant?

What are the chances that any one Type 1 El Niño Year

would be wetter than normal?

In other words, what are the chances that a sample of eight years represents the true mean and distribution of the values of the population?

Given the size of the sample, and the fact that for eightout of eight cases, rainfall was greater than normal (amountsexceeded 70 percentile value),what were the chances that the precipitation at Los Angeles would be greater than normal (in the top 30 percentile)in the 1997-98 El Niño year?

65%, statistically significant at the95% level.

Given the size of the sample, and the fact that for six out of eight cases rainfall was greater than normal at San Francisco, what are the chances that the precipitation at San Francisco would be greater than normal (in the top 30 percentile) in the 1997-98 El Niño year?

45%, statistically significant at the95% level.

• The pattern depicted for the composite is similar to that whichoccurred in the winter of 1997-98, with important differences.

• The composite did not portray the dry conditions in the MississippiRiver Valley nor the extreme wetness in the mid-Atlantic states.

• On the West Coast, the composite captured the pattern in California well, but underestimated rainfall in the Pacific Northwest.

The 1997-98 El Niño was one of the largest such events of the 20th century.

The impact on Pacific storm tracks was more or less as expected.

Historical composites of precipitation provided an accurate foreshadowing for California’s rainfall.

El Niño signatures are strong in the California rainfall record; this suggests that accurate precipitation forecasts a season in advance are possible for El Niño events.

All El Niños Regardless of Sub-All El Niños Regardless of Sub-typetype

Fast Forward to 2002What can we expect?

Type 2 El NiñoType 2 El Niño

Type 3 El NiñoType 3 El Niño

Current Sea Surface Current Sea Surface TemperatureTemperature

Sea Surface Temperature Sea Surface Temperature EvolutionEvolution

Forecast Sea Surface Forecast Sea Surface TemperatureTemperature

Feb, Mar, AprFeb, Mar, Apr

May, Jun, JulMay, Jun, Jul

Aug, Sep, OctAug, Sep, Oct

Graphics and Images for this presentation provided courtesy of:

California Regional Weather Server, Department of Geosciences, San Francisco State University

International Research Institute for Climatic Prediction

National Centers for Environmental Prediction

National Aeronautics and Space Administration, Jet Propulsion Lab

National Weather Service, San Francisco Bay Area, Forecast Office

National Oceanographic and Atmospheric Administration