Comparisons of Precipitation and Zonal Wind Anomalies by Monthly Groupings for 2015-2016
with those for Five Strongest El Niño Events since 1951

John Monteverdi*, CCM

Jan Null**, CCM

Abstract

There is much concern in California about the rainfall to date as of the beginning of February. Chiefly, there is a widespread belief that the El Niño-impacted patterns have failed to materialize, and that the rest of the winter and spring will be dry. We seek to put these concerns in a perspective with a context governed by the climatology of the five strongest El Niño events since 1951. The precipitation anomalies for November through January and for February through May show that four of the five El Niño events did have above normal precipitation from February through May, even if the earlier period was dry. We related this to aspects of the circulation, namely the zonal wind anomalies, normally expected during El Niño events. The results, namely that the patterns examined are least like those in 1997-98 and most like those in 1957-58, are NOT a prediction of what is to come, but a characterization of what has happened in the past for this small sample of years.

A. Charts

The first column on the left shows the November to January precipitation anomalies (departures from long term averages) averaged for the five strongest El Niño events since 1951 at the top, and, below, those anomalies for the November to January period for 2015-16, 1957-58, 1965-66, 1972-73, 1982-83, and 1997-98.

The middle column shows the precipitation anomalies for the remainder of each winter and spring, averaged for the five strongest El Niño events since 1951 at the top, and, below, those anomalies for the November to January period for 1957-58, 1965-66, 1972-73, 1982-83, and 1997-98.

The right hand column shows the November to January zonal wind anomaly averaged for the five strongest El Niño events since 1951 at the top. Below that are the individual differences between the zonal wind anomaly observed November to January 2015-16 (m/s) and that for the composited five strongest El Niño events since 1951. Below that each graphic shows the difference between the zonal wind anomalies for 2015-16 and those for each separate strong or very strong El Niño event, 1957-58, 1965-66, 1972-73, 1982-83, and 1997-98

B. Interpretation

The positive precipitation anomalies achieved higher magnitudes in the late winter and spring of 1983 and 1998. Postive anomalies over the whole state reversed to large negative anomalies in 1966. In 1972-73, wide spread positive precipitation anomalies continued in the second half of the season in the southern two thirds of the state, but reversed in the northern half.

In 1957-58, only the north coast saw weakly positive anomalies in the November to January period, while the rest of the state experienced mostly negative anomalies. The pattern changed to much wetter than average in the whole state in the February to March period.

The zonal anomalies in the lower subtropics in the Pacific can be viewed as a measure of the strength of the subtropical branch of the westerlies that generally develops in El Niño-impacted winters. The warm colors on the the first chart are consistent with the development of the strong zonal flow typically observed in strong El Niño years.

Cold colors indicate that the zonal winds in the lower subtropics for the period November through January 2015-2016 were weaker than observed for the given period. For example, the zonal winds were weaker than those for the composited five other seasons across the entire subtropical region from near the Datelilne to the Central American coast. The difference in zonal wind averaged for November through January for 2016 with that for each individual season indicates that largest difference exists with the zonal winds observed in 1998, and the smallest difference exists with the zonal winds observed in 1957-58.

C. Take Home Message

The take home message from these graphics is that the strong zonal flow expected to develop in 2015-2016 in the lower subtropics has failed to materialize to the extent that it had in prior strong El Niño events. At least for the few strong events that have occurred since 1951, we observe that precipitation anomalies in the February through May period from San Francisco south were positive except for that period in 1966.

We also note that the zonal wind anomalies for 1957-58 compared best with those in 2015-2016. In that year, there was a complete reversal from drier than the composite to wetter than the composite in the latter months of the event.

 

November to January Precipitation Anomalies (in)

February to May Precipitation Anomalies (in)

Composite Zonal Wind Anomaly November - January (m/s)

500 mb Zonal Wind Anomaly Composite, Nov-Jan
1957-58, 1965-66, 1972-73, 1982-83, 1997-98

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Difference of 500 mb Zonal Wind Anomaly Nov-Jan 2015-16 from that of Composite, Nov-Jan 1957-58, 1965-66, 1972-73, 1982-83, 1997-98. Blue shading indicates weaker zonal flow in 2015-16.

Difference of 500 mb Zonal Wind Anomaly Nov-Jan 2015-16 from that of Nov-Jan 1957-58. Blue shading indicates weaker zonal flow in 2015-16.

Difference of 500 mb Zonal Wind Anomaly Nov-Jan 2015-16 from that of Nov-Jan 1965-66. Blue shading indicates weaker zonal flow in 2015-16.

Difference of 500 mb Zonal Wind Anomaly Nov-Jan 2015-16 from that of Nov-Jan 1972-73. Blue shading indicates weaker zonal flow in 2015-16.

Difference of 500 mb Zonal Wind Anomaly Nov-Jan 2015-16 from that of Nov-Jan 1982-83. Blue shading indicates weaker zonal flow in 2015-16.

Difference of 500 mb Zonal Wind Anomaly Nov-Jan 2015-16 from that of Nov-Jan 1997-98. Blue shading indicates weaker zonal flow in 2015-16.

 

* John Monteverdi is Owner and Director of Mayacamas Weather Consultants and a Professor of Meteorology at San Francisco State University.

 

 

**Jan Null is Owner and Director of Golden Gate Weather Services and a Lecturer of Meteorology at San Jose State University