Some of the mantras
from the past year from myself,
John Monteverdi and many
other responsible meteorologists, have been that each El
Niņo is unique, that the number of past events to compare them to
is quite small, and that there are "no guarantees". (For example see
the current very strong El Niņo continues in the tropical Pacific, we
are indeed seeing signifciant differences from past events; even those
that are categorized as strong and very strong. Consequently, El Niņo
2015-16 is turning out to be the poster child for "Every El Niņo is
Comparisons of Precipitation and Zonal Wind Anomalies by Monthly
Groupings for 2015-2016 with
those for Five Strongest El Niņo Events since 1951, by
Monteverdi and Null.
SEA SURFACE TEMPERATURE ANOMALIES
The Sea Surface Temperature Anomalies (SSTA) for the
past year, when compared to
previous strong and very strong events, are markedly warmer in the western Niņo
regions (i.e., Niņo regions 4 and 3.4), while farther east in Regions 3
anbd 1+2 the current event is in the middle of the pack. (Map
of Niņo regions).
SEA HEIGHT ANOMALIES
This can also be seen in the following Sea Height Anomalies,
comparing this year to 1998. Especially of note are the higher
anomalies centered in the western portion of the El Niņo area of
raised sea level heights with nearer to normal heights along the
South Amercian coast.
The sections above summarize the sea
surface temperature anomalies for the six strongest El Niņo
events since 1951.
In this section, we examine the impact
of those anomalies on aspects of the circulation.
The documented impacts of the temperature anomalies on
the circulation include the creation of stronger than average
zonal flow, basically a subtropical branch of the westerlies, in
the lower subtropics.
The charts below present the differences in the
500 mb zonal wind averaged for November 2015 through January
2016 and that composited for the other five strong or very
strong El Niņo events since 1951, and that for each of the other
five strong El Niņo years since 1951.
Blue fill indicates that the zonal winds were weaker for
the current event.
As you can see, the largest difference
between the zonal wind this year compared to the composited
zonal wind for the other five has been much weaker than normal
westerlies in the lower subtropics.
Essentially, the expected subtropical branch has been
weaker than that experienced in the other years, or even absent.
A comparison of the zonal wind this year with that of each
individual strong or very strong El Niņo year in the subtropical
portions of the Pacific shows that the differences between the
subtropical zonal flow were the greatest between those this year
and those in 1997-98, followed by 1965-66; 1982-83, 1972-73 and
The zonal winds for the period November
2015 through January 2016 were most similar to those for
year there have signficant differences in the precipiation patterns when
compared to previous strong (i.e., Oceanic Niņo Index [ONI] between 1.5 and
2.0 for 1957-58, 1965-66, 1972-73) and very strong (ONI ≥ 2.0) El Niņos
(1982-83 and 1997-98).
In comparing the current event
(2015-16) to the previous strong and very strong El Niņo events,
there is no clear analog. The most striking difference in the
Pacific Northwest where there is a very strong postivie rain
anomaly. There are some consistencies in the eastern half of the
United States, with the Southeast being consistently wetter than
normal except fo 1965-66, but the drier than average Northeast
best matches the winter of 1982-83.
This year's temperature pattern best matches
1982-83 but are consistently warmer than even that year.
But what does it mean? It will take a considerable
analysis to better understand why this year is different from previous
strong and very strong events, but in the short term it remains a
reminder that "All El Niņos are different"!
Send comments and questions to
Jan Null or