2. Department of Geosciences and the National Weather Service
A Presentation of the
Department of Geosciences
and the
National Weather Service

4. El Niño and California Precipitation
National Weather Service
Technical Attachment No
Today’s Presenter:
John Monteverdi
Professor of Meteorology
San Francisco State University
John Monteverdi
San Francisco State University
Jan Null
National Weather Service
Rock Slides
Landslides
Coastal Erosion
Floods

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

8. 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.

11. 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.

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

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

15. Three “sub-types” of El Niño
Type 1 El Niño
Type 3 El Niño
SST Anomalies Dec 1997
Type 2 El Niño

16. El Niño is NOT:
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).

17. Into Proper Perspective
Putting the El Niño
Into Proper Perspective
Sea-surface Temperature Patterns
How Large Was It?

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

20. 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?

21. 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.

22. 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.

23. Is this borne out by the historical record? Conceptually, the
El Niño-influenced
jet stream position
should be associated
with wet conditions
central and Southern
California, and dryness
in the Pacific Northwest.

24. Precipitation Anomalies by Climatic Division for
Seven Type 1 El Niño Events Since 1951

25. 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).
The blue line represents the 70th Percentile rainfall amount. (This means that rainfalls exceeding this value are greater than 70% of the values observed).

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

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

28. 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.

29. But WereThese Precipitation Anomalies Statistically Significant?
To What Extent Can They Be Used To “Foreshadow” Precipitation In California?

30. 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.

31. 45%, statistically significant at the
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.

33. 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.

34. Conclusions
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.

35. All El Niños Regardless of Sub-type
Fast Forward to 2002
What can we expect?

36. Type 2 El Niño

37. Type 3 El Niño

38. Current Sea Surface Temperature

39. Sea Surface Temperature Evolution

40. Forecast Sea Surface Temperature
Feb, Mar, Apr
May, Jun, Jul
Aug, Sep, Oct

41. 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 Aeronautics and Space Administration, Jet Propulsion Lab
National Centers for Environmental Prediction
National Oceanographic and Atmospheric Administration
National Weather Service, San Francisco Bay Area, Forecast Office