National Weather Service Technical Attachment No John Monteverdi San Francisco State University Jan Null National Weather Service Today’s Presenter: John Monteverdi Professor of Meteorology San Francisco State University

Separating “Hype” from Science  What is El Niño?  What impact (if any) does it have on California precipitation?

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

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.

Pacific Sea Surface Temperatures (SST) in non-El Niño Situation Walker Circulation

El Niño Pacific Sea Surface Temperatures (SST) Trade Winds weaken Or reverse Water “Sloshes” Eastward, Bringing Warm Water and Higher Sea-levels To South American Coast

Three “sub-types” of El Niño Type 2 El NiñoType 3 El NiñoSST 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 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 have on California Precipitation?  How and why does El Niño effect 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, the El Niño-influenced jet stream position should be associated with wet conditions central and Southern California, and dryness in the Pacific Northwest. Is this borne out by the historical record?

Precipitation Anomalies by Climatic Division for Seven 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 El Niño Year would be wetter than normal in California? Very great chance in Southern California decreasing To a “reasonable” chance in north-central California, with The most likely category for Northern California being “Normal” rainfall.

To What Extent Can They Be Used To “Foreshadow” 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 eight out of eight cases, rainfall was greater than normal (amounts exceeded 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 El Niño year? 65%, statistically significant at the 95% 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 El Niño year? 45%, statistically significant at the 95% level.

The pattern depicted for the composite is similar to that which occurred in the winter of , with important differences. The composite did not portray the dry conditions in the Mississippi River 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 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-type Fast Forward to 2002 What can we expect?

Type 2 El Niño

Type 3 El Niño

Current Sea Surface Temperature

Sea Surface Temperature Evolution

Forecast Sea Surface Temperature Feb, Mar, Apr May, Jun, Jul Aug, 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