El Niño / La Niña and Extreme Daily Precipitation and Streamflow Values.


This work performed jointly by Dan Cayan (Scripps Institution of Oceanography), Kelly Redmond (Western Regional Climate Center), and Larry Riddle (Scripps). Figures supplied by Dan Cayan.

Method. A few hundred daily climate and daily stream gage records were selected from around the West. For each day of the year, various percentiles were found for each site. For example, the value which is exceeded on 50 percent, or 75 percent, or 90 percent of the days centered on December 15 (say) over all past years, was determined. This was repeated for every day of the year from October through April. When all years are considered together, regardless of the status of El Nino or La Nina, a certain value will be exceeded only 10 percent of the time. This is the 90th percentile. During La Nina or El Nino years, however, this same value may occur with greater or lesser frequency, depending on the form of the relationship between climate and the Southern Oscillation.

A group of La Nina and El Nino years were selected from the data period (1949-1995). The selection criteria was a June-November average SOI of -0.50 or less ("El Nino") or +0.50 or more ("La Nina"). The number of times that values for a given day of the year exceeded the 90th percentile for that same day of the year was then determined for each year, selecting only days during the winter (October-April) period. The number of such extreme values was determined separately for the El Nino and for the La Nina years, and averaged over only those sets of years. Systematic differences were then sought between the number of such occurrences (of very wet days, or of very high streamflow) between El Nino years, La Nina years, and all years lumped together. The difference in the number of such days between El Nino and La Nina years was determined, and divided by the average number of days for all years. The resulting ratios were found for every station, and mapped for the entire western United States.

Figure 1 shows several features.

The left hand diagram compares the number of times that median precipitation values occurred, for the months of October thru April. In the reddish areas, daily precipitation exceeding the long-term median daily value occurred more often in El Nino years than in La Nina years, as seen in the Southwest. Such days occurred more than twice as often in the lower Colorado River Basin. In the northern West (bluish colors), there were fewer wet days during El Nino years.

The right hand diagram compares the number of times that unusually wet days occurred (those days normally in the top 10 percent wettest of all days). The reddish areas show where heavy precipitation days are more likely during El Nino years than La Nina years, and blue areas show where La Nina brings more of the wettest days. There is a clear preponderance of the wettest days during El Nino years, compared with La Nina years, for the southern third of the West. Similarly, in the northern portion of the West, there is a clear preponderance of the wettest days in La Nina years, and a marked tendency toward fewer wettest days in El Nino years.

Figure 2 compares the likelihood of the wettest days (those usually in the top 10 percent) for different seasons of the year between El Nino and La Nina.

The top left diagram shows that the number of wet days for November-December-January is greater during El Nino years in the desert Southwest, and lesser in the Pacific Northwest.

The top right diagram shows that the number of wet days for February-March-April is similarly much greater during El Nino years in the U.S. Southwest, and is less in the northern Cascades and Rocky Mountains.

The bottom left diagram shows that the number of wet days in May-June-July is higher in parts of California and in the interior West, for years when El Nino was prevalent the previous autumn and winter. Note that there is more local structure, the differences are not as great, and that in some parts of the region (such as southern and coastal California) a "wet day" during this time of year is much drier than a "wet day" in the heart of winter.

The bottom right diagram shows that the number of wet days in the August-September-October period following years with an El Nino prevalent during the previous autumn and winter is greater than the number of days in La Nina years, over the monsoon region centered on the Four Corners area of the American Southwest (that is, the reddish areas). The purpose of this diagram is to see if there might be lingering effects. In general, though, the patterns are not as regionally coherent as in the winter months.

Figure 3 compares El Nino / La Nina differences for winter between precipitation and streamflow, for two different definitions of "wet days."

The lower left diagram compares the number of days that are in the top half (wet half) of the daily precipitation distribution, for the period October through April. Days that are at least mildly wet occur more frequently in El Nino years than in La Nina years, in the U.S. Southwest. Days that are at leas mildly wet occur less frequently in El Nino years than in La Nina years, in the northern tier of western states.

The lower right diagram compares the number of days that are in the top half (wet half) of the daily streamflow distribution, for a different set of stations that measure streamflow. The period is shifted from October-April (used for precipitation) to January-July, because winter precipitation falling as snow does not run off until the spring and early summer snowmelt season. The same pattern is seen as for precipitation, except that the differences are much more pronounced for streamflow. Red areas have more days with at least mildly high streamflow in El Nino winters, and blue areas have fewer days with at least mildly high streamflow in El Nino winters. The converse is true for La Nina winters (more high-flow days to the north, fewer high-flow days to the south).

The upper left diagram compares the number of unusually wet days (in the top 10 percent) for the same October through April portion of the year. Reddish areas have many more very wet days during El Nino winters (the southwest states), and bluish areas have fewer very wet days during El Nino years (the northern states in the West).

The upper right diagram compares the number of unusually high streamflow values (in the top 10 percent) seen in the period January through July in El Nino years versus La Nina years. The period is shifted from October-April (used for precipitation) to January-July, because winter precipitation falling as snow does not run off until the spring and early summer snowmelt season. The reddish areas show a much greater incidence of high-flow days throughout the Southwest in El Nino years, and a much reduced incidence of high-flow days in Washington, Oregon, Idaho, Montana, and northwest Wyoming. Conversely, La Nina brings much higher daily streamflows to the northern West, and much lower daily streamflows to the southern West. The differences in the U.S. Southwest are as great as 1000 percent at some stream gaging stations.

Main Conclusions

In winter in the U.S. Southwest,

Mildly wet days are more frequent in typical El Nino years.
Very wet days are even more frequent in typical El Nino years.
High streamflow values are much more frequent in typical El Nino years.
Streamflow patterns are accentuated versions of precipitation patterns.

In winter in the Pacific Northwest and northern Rockies,

Mildly wet days are less frequent in typical El Nino years.
Very wet days are even less frequent in typical El Nino years.
High streamflow values are much less frequent in typical El Nino years.
Streamflow patterns are accentuated versions of precipitation patterns.