WESTERN REGION TECHNICAL ATTACHMENT
NO. 97-37
NOVEMBER 21, 1997

El Niño and California Precipitation


John Monteverdi Professor of Meteorology
Department of Geosciences, San Francisco State University 
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Jan Null Lead Forecaster
National Weather Service Forecast Office, San Francisco Bay Area

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Send comments or corrections to the Site Manager, John Monteverdi

Final Update June 24, 1998

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Table of Contents

This WebSite Is Now Complete!

El Niño Event Retrospective

Climatology provided accurate assessment of precipitation in the West for the winter of 1997-98 (but not for the Mississippi River Valley nor the mid-Atlantic States).

La Niña Information 

As a look ahead, we provide some historical information on the relation of La Niña to California Rainfall  .

Last Update of SST Analysis
7-Day Average Centered on 6/17/98
(Click To See Large Image)

Initial newspaper, television and radio reports (in summer 1997) on the issue of El Niño and its impact on California rainfall in many cases were misleading. For example, early (summer 1997) attempts at computer modeling of impacts of El Niño patterns on California precipitation did indeed produce a unified view that heavier than normal precipitation would occur, particularly in Southern California.

However, some of the computer runs produced extreme outlier "foreshadowings" of 300-400% normal precipitation or greater. General reports in the media focused on these extreme numbers (without providing the context) and not the consensus view of meteorologists. Misinterpretations of such results of computer modeling predictions (that really applied to Southern California) then were reported widely as applying to the whole state and also attributed to meteorologists in general.


(Note: 300% to 400% of normal rainfall would translate out to 62" to 82" for San Francisco and 44" to 59" for Los Angeles. See yellow box at upper right).
In addition, much confusion was apparent in the press on the issue of what El Niño actually was and what the impacts of El Niño would be. All of this has led to a domino effect of poorly drawn conclusions and overreactions.

The purpose of this site is to dispel some of this confusion to show why the authors felt when this site was prepared in October 1997 (and still feel) that these early predictions were overstated and not supported by the evidence in the recent precipitation history of California.


Read the box at right for our point of view. If you would like to see the reasoning and some excellent graphical illustrations, proceed to the information below.


Final Rainfall Totals 1997-98 Rainfall Season

1997-98 Seasonal Rainfall, Downtown San Francisco: 47.19", 230% of Normal Seasonal Rainfall

By February 4,1998 the seasonal total in San Francisco had exceeded normal (defined as the 70th Percentile--see below); hence, now 7 out of 9 Type 1 El Niños since 1950 have had greater than normal precipitation.

By February 17, the seasonal total in San Francisco had exceeded 170% of the long term seasonal normal; hence, now 4 out of 9 Type 1 El Niños have had much greater than normal precipitation.


1997-98 Seasonal Rainfall, Downtown Los Angeles: 31.01", 210% of Normal Seasonal Rainfall

By February 23,1998 the seasonal total in Los Angeles had exceeded normal (defined as the 70th Percentile--see below); hence, now 9 out of the 9 Type 1 El Niños since 1950 have had greater than normal precipitation.

By March 15, the seasonal total in Los Angeles had exceeded 170% of the long term seasonal normal; hence, now 5 out of 9 Type 1 El Niños have had much greater than normal precipitation.

Summary

The summary below was prepared by the authors in October 1997.

What Past Type 1 El Niño Events Tell Us About San Francisco Rainfall In The Winter  

a. AS THE PRESENT EL Niño PERSISTS THROUGH THE WINTER, IT IS REASONABLE TO EXPECT GREATER THAN NORMAL PRECIPITATION. 6 out of 8 past Type 1 El Niños (75% of an admittedly small sample) had greater than normal precipitation. Hence, the concerns that the upcoming winter (1997-1998) will be wetter than normal appear reasonable.

b. ALSO, AS THE PRESENT EL Niño PERSISTS THROUGH THE WINTER, IT IS REASONABLE TO PREPARE FOR MUCH HEAVIER THAN NORMAL PRECIPITATION  (defined as greater than roughly 170% of the normal rainfall--35.70"). 3 out of the 8 past Type 1 El Niños (82-83, 72-73 and 57-58) had about 170% (or greater) of the normal precipitation. Again, this is a small sample upon which to make generalizations. However, the history of these recent events suggests that San Franciscans should be prepared for a winter potentially much wetter than normal.


La Niña and California Precipitation

Long range ocean temperature forecasts indicate the a major cool episode will be occurring in the
eastern tropical Pacific in the upcoming winter.  

This animation of Scripp's graphics  (click to see movie) illustrates the evolution from
El Niño to La Niña that is in the process of occurring.

See Our La Niña and California Precipitation Page for more background information on this.

See Jan Null's La Niña Page for more background information on La Niña.


Signatures in the California Precipitation Record for Northern and Southern California

Historical rainfall data indicate that Southern California probably is in line for subaverage precipitation this upcoming winter, with 7 out of 8 La Niña rainfall seasons since 1949 having experienced subaverage rainfall.

The pattern is less clear in north-central California. Although the average rainfall at San Francisco for La Niña episodes is around 99%, several of the cold episode winters had above average rainfall.

There is a clearer signature at Eureka with an indication of at least average rainfall this upcoming winter.

The graphic at right shows the normalized rainfall at San Francisco Civic Center and Los Angeles Civic Centers for the last eight La Niña episodes. The horizontal axis is the Multivariate Index (MEI), discussed below. Negative MEIs indicate La Niña conditions.

Click To See Full Size Version

Note that for small MEI, San Francisco rainfall was actually greater than average, whereas Los Angeles Rainfall was below average for all but two events. The blue solid and red dashed lines are the best-fit linear regression lines fitted to the data for San Francisco and Los Angeles, respectively.

Click To See Full Size Version
Eureka Rainfall
(Blue Bars La Niña; Red Bars Type 1 El Niño)

Average La Niña Rainfall: 110% 

Click To See Full Size Version
San Francisco Rainfall
(Blue Bars La Niña; Red Bars Type 1 El Niño)

Average La Niña Rainfall:  99%

Click To See Full Size Version
Los Angeles Rainfall
(Blue Bars La Niña; Red Bars Type 1 El Niño)

Average La Niña Rainfall:  79%


Climatology Provided An Excellent Foreshadowing of the Precipitation in the West for the Winter 1997-98
Composite Averages for Recent Type 1 El Niño Events Generally Anticipated Winter 1997-98 Precipitation in the West

When constructing this Web Site, the authors believed that simple compositing of the patterns and rainfall associated with past Type 1 El Niño events could provide a reasonable insight to the range of precipitation values one might expect for San Francisco during the winter of 1997-98. This procedure showed that Type 1 El Niños typically have been associated with rainfall at San Francisco in the range of 140-180% of the long term normal (discussed below). These estimates seemed small at the time, in the light of predictions headlined in the media last summer.   These cautious estimates were closer to the actual values than the widely misquoted1"forecasts" of 300-400% normal seasonal rainfall for California.

It is important to note that even if precipitation  for 1997-98 at San Francisco exceeded the greatest ever observed (approximately 49 inches or 235% of the long term normal) the authors continue to believe that composite patterns discussed here provided reasonable guidance for this winter's precipitation.

1See above.

Click To See Full Size Version
1. What Actually Happened
(Courtesy: NOAA CPC)

The chart (1) above (from NOAA's Climate Prediction Center) shows the average precipitation anomalies (inches) for the 48 conterminous states. The deep blue colors represent average anomalies of 8" or more relative to the long term average.

Click To See Full Size Version
2. What Actually Happened
(Courtesy: NOAA CPC)

The chart (2) above (from NOAA's Climate Prediction Center) shows the the average rainfall anomalies (from (1)) as a fraction of the standard deviation. Deep blue colors represent positive departures of greater than two standard deviations from the long term mean. Note that extreme wetness occurred in California and the southeastern portion of the United States.


Click To See Full Size Version
3. What Climatology Told Us In October 1997 Was "Likely" To Happen

The chart (3) above shows the composite average rainfall anomalies for Type 1 El Niño Events (not including the present event) since 1950 and should be compared with chart (1) above.  

Fig. 3 is also included in the body of the Web Site below and was prepared by the authors in October 1997 to provide an historical context for our view that early summer predictions of precipitation amounts for California and the West were exaggerated.  

The pattern depicted for the composite is similar to that which has occurred so far this winter, 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.

Click To See Full Size Version
4. What Climatology Told Us In October 1997 Was "Likely "To Happen

The chart (4) above shows the composite average rainfall anomalies for Type 1 El Niño Events (not including the present event) as a fraction of the standard deviation and was also prepared in October 1997 by the authors (and included below). This should be compared to chart (2) above. 

Each of these charts shows the distribution of a different measure of precipitation anomalies for the period considered. However, an examination of all figures for the region west of the Great Plains, shows that the historical anomalies did in fact provide an accurate "foreshadowing" of the pattern of precipitation anomalies in the West that did occur in the winter of 1997-98.

The composite failed miserably in the Mississippi River Valley and in the Atlantic States in providing a reasonable estimate of precipitation anomalies this past winter.


What is El Niño ?
Brief Definition of "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.

Link to NOAA El Niño Explanation Page

Link to Environmental News Network El Niño Explanation Page

At right find a progression of NASA images which actually show deviations of sealevel from average. Where sealevel is higher, the ocean is warmer than normal (yellows, reds, and whites). Blues and purples indicate depressed sealevels (colder than normal). The latest images shows that the region of warmer than normal ocean temperatures in the eastern Pacific has contracted but still cover a substantial portion of the eastern tropical Pacific..

Although this suggests that this El Niño event may have peaked, it is important to keep in mind that this is consistent with the forecasts discussed below (that indicate that El Niño conditions should be present but waning through the next several months).  In addition, although the fraction of the Pacific experiencing warmer than normal ocean temperatures may have decreased, the fraction of the eastern Pacific experiencing very warm conditions is still extremely large (see below).  It is important to keep in mind the fact that El Niño conditions will be with us throughout the winter.

Link to NASA Discussion Of These Images

Click On Each Image To See Larger Versions


Quicktime Movie showing the progression from a La Niña last November to the current El Niño conditions.

The Present El Niño

El Niño events are characterized by sudden onset of large temperature deviations (on the order of 2oC, or about 4oF 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 Oscillation, a cyclical change in the pressure and temperature distribution along the Equator. 

In the current El Niño Southern Oscillation (ENSO) event, ocean temperatures have risen over 5oC (almost 11oF) in that area.

The images (showing average sea surface temperatures and anomalies for the 7-day periods centering on December 10, 1997 and June 17, 1998) at right show that the portion of the eastern Pacific with a greater than 5oC anomaly (deep red) has disappeared.  In addition, the fraction of the Pacific with temperatures greater than 28oC (light orange--implying great exchange of heat and water vapor to the atmosphere) has decreased as well.  El Niño's impact on the atmosphere also has waned.

(Note: the term ANOMALY  is a synonym for DEVIATION FROM NORMAL).

Link to More on Development of Present Pattern from NOAA/NCEP


Patterns Show Developing Cold Anomalies

How Can Sea Surface Temperatures Increase When Sea Surface Temperature Anomalies Associated with El Niño Are Decreasing?

The graphs at top right illustrate the abating of the current El Niño. Greater than 5oC anomalies in October have decreased to around 2oC.  This verifies the forecasts made by CPC in October that El Niño anomalies would wane during the spring.

The graph at bottom right shows normal seasurface temperature (bottom line), seasurface temperature anomaly (top line) and the threshold temperature for marked exchange of heat and moisture with the atmosphere (28oC) for the Niño 3 (eastern tropical Pacific) region.  Note that although the anomalies were greatest (space between the two lines or width of orange area) in December and are waning currently, the deviation of the seasurface temperature from the threshold temperature is increasing due to the incremental effects of seasonal warming.

Thus, El Niño impacts on the atmosphere now are even stronger than those observed earlier with the larger SST anomalies.

Seasurface Temperatures Still Increasing As El Niño Wanes

Not All El Niño Events Are Alike

The present El Niño is a so-called "Type 1" El Niño (Fu et al. 1986).Type 1 events have the strongest SST anomaly (>2.0 C) and extend from approximately 160E to 80W. During the period of 1949-1993, there were eight Type 1 ENSO events: 1951-1952,1957-1958, 1965-1966, 1968-1969, 1972-1973, 1977-1978, 1982-1983, and 1991-1992 (Schonher and Nicholson 1989).

Link To Discussion of Three El Niño Types

Fu, C., Diaz, H. F. & Fletcher, J. O., 1986: Characteristics of the Response of Sea Surface Temperature in the Central Pacific Associated with Warm Episodes of the Southern Oscillation. Monthly Weather Review, 114, 1716-1738.

Schonher, T. & Nicholson, S. E., 1989: The Relationship between California Rainfall and ENSO Events. Journal of Climate, 2, 1258-1269.

Schematic Chart Showing Portion of the Pacific With Positive Temperature Anomalies in Type 1 El Niños

El Niño is NOT:

  • a series of catastrophic flood producing weather events in California or the West
  • 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).


Is This El Niño Event Bigger Than/Smaller Than/The Same Strength As Other Type 1 Events?

The present event is dissimilar to other Type 1 Events in that it grew more rapidly after its initial onset than any other observed. Once mature, the present pattern seemed to rival that of 1982-83 in strength and size.

Now, there has been much in the news lately regarding the relative strength of the present event, especially as the "strongest of the 20th century.". Once again, we caution against "hype." The present event is as big, slightly bigger or slightly less significant than the 1982-83 event, depending upon the particular measure used to characterize it. It seems pointless to try to argue that it is "the El Niño event" of the century, especially since our ability to characterize events earlier than 1950 is greatly limited by lack of data.

Link To NOAA El Niño Evolution Page

Link to El Niño Index Page from Climate Diagnostics Center

Link to International Research Institute for Climatic Prediction

(Image Courtesy of Klaus Wolter and Michael Timlin, NOAA-CIRES, CDC)

About the Same as 1982-83


(Image Courtesy of International Research Institute for Climatic Prediction)

Bigger than 1982-83


Much Less Significant than 1982-83


Click on each graph above to see a READABLE version of it.


The multivariate index (top) is a combination of other indices (pressure differences across the tropical Pacific etc.).  The more positive the index, the greater the likelihood of El Niño conditions in the eastern tropical Pacific. The steep decrease in the index in October has been replaced by a gradual rise in the last several months (with generally lower values than those observed during the 1982-83 event). 

The sea surface temperature anomalies (middle) for the Niño3 region (5S-5N; 150W-90W) in the eastern tropical Pacific show that, despite the early peak in the multivariate index, this El Niño episode was continuing to intensify through November. Although the anomalies exceeded those observed for 1982-83, suggesting that the present El Niño event the strongest of the 20th century, the values of the Southern Oscillation Index (difference in pressure between Tahiti and Darwin, Australia; negative numbers generally indicate a warm episode ENSO)(shown at bottom), often used to indicate strength of El Niño, are of lesser magnitude than those of the 1982-83 El Niño. 

Given these slightly disparate views, it is not yet possible to characterize this present El Niño as the largest of the 20th Century.

Predictions through the Early Fall 1998

Will This El Niño Event Persist Through The Summer?

Here is the latest series of forecasts  from the National Centers for Environmental Prediction (NCEP). The figure at right shows sea surface temperature (SST)  anomaly forecasts by 3-month period through early Fall 1998.  

The temperature anomalies associated with El Niño are forecast to decrease through the spring. Thus, it appears that spring storm patterns in California will be affected by the present El Niño.

Note the appearance of normal temperatures along the South American coast by late summer.

Link to NCEP ENSO Forecast Page

NCEP Prediction Issued 2/25/98

How Has this El Niño Event Affected The Pacific Storm Track?

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) should extend from somewhere near or north of the latitude of Hawaii (approximately 20o to 35oN) to the southern (or central) coast of California.

Although such storms may actually circulate tropical moisture from the El Niño-impacted eastern tropical Pacific to California, this is not a "required" part of the El Niño pattern.

If the storm track is not far enough south to entrain the moist air (i.e., not associated with traditional "low latitude" or "pineapple connection type" storms), it still may be stronger than normal and locked at the latitude of California. In either case, the position of the subtropical branch determines which portion of California and/or the West receives the heaviest precipitation.1

The schematic chart at upper right should be compared with the chart at bottom right which shows the actual 200 mb flow in January 1998.  Note the very active subtropical branch and the split in the storm tracks about 2000 miles west of San Francisco.  Click on this chart to also see the NORMAL January flow and you will see that the westerlies along the California coast (orange colors) are anomalous.

The pattern that actually occurred this winter did not favor the "low latitude type" ("pineapple connection") storms that give dramatic cloud plumes extending from the tropics.  Yet the pattern was the same sort of active subtropical branch that occurred in the Type 1 El Niño Event of 1982-83.2


Schematic Diagram Showing Midwinter Upper Tropospheric Features Common During Type 1 El Niño Events (Compare With January 1998 200 mb Flow Shown in Image Below)

200 mb Flow, January 1998

Click on the chart to see version that also has a side-by-side comparison with the normal 200 mb flow.

Click on Images To See Full Size Graphics


1Another impact on precipitation can occur when advancing middle latitude type cyclones intercept the moister air (due to higher sea-surface temperatures) along the California coast during the mature phase of El Niño. Heavier rainfall and a higher incidence of thunderstorms occurs in association with such storms.

2It should be pointed out that "low latitude type" cyclones ("pineapple connection storms") are a normal feature of the California climate even in non-El Niño years. (For example, the storms of December 1996 and January 1997 were "low latitude"type.)  

A Word About Flooding in California

Not all flooding events in California occur during El Niño years, and not all El Niño years produce widespread flooding.  For example, last season (a non El Niño year) saw seasonal rainfall totals near normal throughout most of California. However because most of last season's precipitation fell during December and January (instead of being spread throughout the season) (see graphic at upper right, which shows the rainfall totals from December 20, 1996 through January 3, 1997) there were record floods in many parts of the state. Some other historic floods in California that have occurred during non-El Niño Years include Christmas 1955, December 1964, January 1982 and February 1986.

The graphic at bottom right shows the low latitude storm type associated with the flooding rains in late December 1996 and early January 1997, a non El Niño winter season.

The type of situation that most often leads to flooding in California is when a succession of low latitude storms impact an already saturated region in a short period of time. The probability of this occurring is greater during El Niño events because of the shift in the storm track, but even a day or two break between weather systems can make a large difference in the flood potential.  

Click on Image To See Full Size Graphics


The bottom line for individuals and public agencies is that parts of California do flood during both El Niño years and non El Niño years.

Current Climate Prediction Center Forecast for the Spring and Summer

Here are the current temperature and rainfall forecasts for the upcoming six months or so from NOAAs Climatic Prediction Center (CPC).  

Green to dark green colors on precipitation forecast charts generally indicate high probability of Above Normal Precipitation (meaning rainfall greater than about 70% of the observed annual rainfalls in the past).  Note that El Niño-impact precipitation anomalies are expected to disappear by the summer.

Link to CPC ENSO WebSite

Click on Image To See Full Size Graphic

California Rainfall for Recent Type 1 El Niños

Based Upon Statistical Analyses of Historical Rainfall Data

How Have Type 1 El Niño Events Historically Affected the General Precipitation Patterns Over the West?

Type 1 Events have a very strong signal in the precipitation record of the West.  The graph at right shows the composite precipitation anomalies (inches)  by climatic division across the conterminous United States for Type 1 Events.

Wetter than normal conditions are suggested for California.  The Pacific Northwest has experienced generally normal to slightly subnormal precipitation during Type 1 El Niños, with Washington state experiencing dry conditions.  

Click on Image To See Full Size Graphic

Chart showing the anomalies above as a fraction of the standard deviation for each climatic division in the US.

How do the anomalies shown in the last section relate to the normal annual rainfall?

The map at right shows the precipitation history in California for Type 1 El Niño events. It was computed by dividing the anomalies shown in the last section by the mean rainfall for each climatic division.  In coastal California, the the greatest departures from normal have occurred from San Francisco south.

See the charts we have constructed for

Click on Image To See Full Size Graphic

What Sort of Rainfall in the San Francisco Bay Region Has Characterized Type 1 El Niño Events?


For the eight well-documented Type 1 winter El Niño events since 1950 (rainfall seasons of 1951-52, 1957-58, 1965-66, 1968-69, 1972-73, 1977-78, 1982-83 and 1991-92), rainfall at San Francisco averaged about 37% greater than normal, with a mean anomaly of about 7.5 inches at
San Francisco1 (thus, implying an average rainfall for Type 1 El Niño events of around 28.5" compared to the 30 year average of 21" or so).  Note also that several Type 1 seasons were very wet (>170%), suggesting that there should be a reasonable concern for such amounts in the winter of 1997-98.


1 The official NWS rainfall measuring site is in downtown San Francisco. Everyone should realize that there is tremendous areal variability of rainfall amounts in the SF area, from as little as 14" per year in the Santa Clara Valley to nearly 60" per year in parts of the Santa Cruz Mts., the north Bay and Marin County. The increases at these locations will be proportional.

The chart above shows the deviation from the normal seasonal rainfall at downtown San Francisco from the1949-50 rainfall season to present time. The red bars represent the rainfall for Type 1 El Niños.

The zero line here represents the long term or 30 year average rainfall (inches) (also called the "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). Red bars indicate seasonal rainfall for El Niño type 1 years, green bars indicate  rainfall for El Niño Type 2 events and yellow bars indicate rainfall for El Niño Type 3 events.


Can we expect more days with rain and will the rain on those days be very heavy?

(Based on work of Jan Null of the National Weather Service. See his El Niño page which has a compilation of most if not all links to El Niño sites around).  Also, see Prof. Jerry Steffen's careful statistical analyses of the rainfall during Type 1 El Niño winters at a number of California sites for an overview of the statistical backing for our opinions.

The Los Angeles Area Has Experienced PROPORTIONALLY More Precipitation Than North-Central California In Type 1 El Niño Events

Note that for Los Angeles Civic Center, EVERY Type 1 El Niño was associated with heavier than average precipitation. In fact, statistical analyses suggest that it is likely that Southern California will experience a wetter than normal winter this season. The average deviation for the 8 recent Type 1 Events was about 75% above the normal mean annual rainfall, giving an average rainfall for Type 1 El Niños at LA Civic Center of around 26 inches.

Rainfall In The Eureka Area Is As Often Less Than Normal as it is Greater Than Normal During Type 1 El Niños

Note that for Eureka, four Type 1 El Niño years had greater than normal rainfall, and four had lesser than normal rainfall. The average deviation was about 10% above the average, allowing us to anticipate that far northern California will be affected the least during the upcoming winter.

Did This El Niño Event Impact The 1997 Hurricane Season in the Eastern Pacific?

Actual and anomalous sea surface temperature patterns in the eastern Pacific during the Fall (not shown) indicated that the portion of the eastern Pacific experiencing ocean temperatures favorable for hurricane formation was much expanded, with the 27C isotherm extending to Point Eugenia in Baja California. In general, temperatures favorable for hurricane formation encompassed twice the area normally experiencing such conditions.

Actual hurricane tracks for the period August 1 through mid-November are depicted top right. The tracks suggest that the 1997 hurricane season featured a greatly expanded region of tropical cyclone activity, with four named storms moving west of 140W longitude, compared to a normal of one such storm. In addition, 6 storms became hurricanes, 1 storm became a "super" hurricane (Linda), and several intact storms moved north of 30N latitude. Finally, remnants of several storms, including Ignacio in August 1997, passed as far north as 37N, producing anomalous warm season rains in California.

The primary impact of the heating associated with El Niño in the climatologically-favored hurricane-genesis region of the eastern Pacific is to raise tropopause heights, decrease north-south pressure gradients south of the subtropical highs, decrease easterly trades and, thus, decrease surface to upper troposphere vertical wind shear. Also, in mature El Niño conditions, ocean temperatures >27C (80F) generally extend about 5 latitude further north than the climatological normal.

Hurricane formation in the eastern tropical Pacific is encouraged during times of weakened easterly flow, minimal vertical wind shear and ocean temperatures greater than 27C (80F).

Click on Image to See Full Sized Image

Actual Hurricane Tracks From August 1 Through November 1, 1997


Vertical Shear in the Tropics, 1 August to 8 November 1997

As would be expected, the sea surface temperature pattern discussed above was reflected by weaker easterlies and by much decreased vertical shear in the eastern Pacific. The chart above shows the mean vertical wind shear and anomaly (m s-1) in the eastern Pacific for the period August 7 though November 1, 1997. Shaded areas on the top diagram indicate shear of 7 m s-1 or less, normally considered favorable for hurricane formation. Orange and red shaded areas on the anomaly chart indicate greatly weakened shear; these areas are located in the portion of the eastern Pacific with the greatest positive temperature anomalies (not shown). Note that both figures suggest suppressed hurricane incidence in the Caribbean and Atlantic.



Things To Keep In Mind

Produced October 1997

  • both predictions (based upon computerized models of the linked physics of the oceans and the atmosphere) and statistical analyses of the past rainfall record (summarized above) suggest that wetter than normal conditions for north-central California should occur in the winter of 97-98. However, two of the eight recent Type 1 El Niños were associated with subaverage precipitation in north-central California. The greatest positive anomaly occurred for the Type 1 El Niños of 1982-83 (182%) and 1957-58 (175%).

  • the present El Niño is one of the strongest (if not the strongest) of the 20th century. Despite the unprecedented rapid growth to maturity of the warm temperatures by late Spring, this warm episode will not disappear and both current indications and predictions suggest that this event WILL persist through summer 1998

  • there is absolutely NO  evidence in the record to support contentions that El Niño events can be (or ever were) associated with "300% to 400% of normal rainfall" in north-central California. 300% to 400% would produce rainfall of 60" to 80" in San Francisco.  The greatest rainfall total ever recorded at San Francisco (season of 1861-62) was on the order of 49" or so, about 230% of the long term average .  

  • flooding events in California relate to timing and intensity of the rainfall systems that affect the state.  It is true that storms on a greatly-strengthened subtropical storm track are more frequent in El Niño years. However, the phasing of such storms and other factors such as the saturation of the soils and tides determine whether or not flooding and mudsliding will occur even in an El Niño year in which the yearly precipitation is substantially greater than normal

The upshot here is that there is evidence (both CPC forecasts and statistical analyses of the California rainfall record for Type 1 El Niños) to support a reasonable concern for a wetter-than-normal winter for most of California and even a much wetter-than-normal cool season in the state.  There may in fact be flooding problems when heavy rainfall events occur. There is no reason, however, for panic or hysteria nor is their any evidence to support the overreactions stimulated by early news reports this past summer.

Graphics and Images on these pages are courtesy of:

Department of Geosciences, San Francisco State University

International Research Institute for Climatic Prediction

National Centers for Environmental Prediction, El Niño Page

National Aeronautics and Space Administration, Jet Propulsion Laboratory

National Weather Service, San Francisco Bay Area, Forecast Office

NOAA/CIRES Climate Diagnostics Center

NOAA Climate Prediction Center ENSO WebSite

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