el nino

[mjb]

im seeing reports that an el nino pattern is or has started. people
talk about an el nino summer a few years back that sounds pretty
legendary. how long do these last and can awesome surf be expected
from it during the winter?

[Nathan Cool]

Hi MJB,

That's a loaded question, but there are some speculative answers.
Although El Niño or La Niñas can last anywhere between 2 and 7 years
(typically), each one is different, both on weather and surf. There
are though "trends" that can be expected, with El Niño having
(usually) an impact on bringing bigger winter surf to California;
this, due to the jetstream lowering and driving northern hemi storms
more towards California than Alaska, Washington, etc.

To see if a trend is likely, we have to know the "signal" of an El
Nino -- which can be thought of as its strength. This can be found in
the Climate Prediction Center's reports, which the latest can be found
here:
http://www.cpc.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.html

Per their latest report: "El Niño is expected to strengthen and last
through the Northern Hemisphere winter 2009-2010."

...so we know an El Niño is imminent. The signal (strength) of the El
Niño is measured by what's known as the Oceanic Nino Index, or ONI. A
history of this can be seen here:
http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml

As shown, not all El Nino or La Nina years had equal signal, and were
not as noticeable. Note one of the strongest El Ninos in 1997-1998,
when the ONI was a powerful 2.5 for October 1997 through January 1998
(OND and NDJ are months, e.g. OND is October, November, December).
That was likely the most notable El Nino in recent history.

To see now this year's El Nino compares to the past, we have to go
back to the NCEP's assessment report mentioned earlier. From this, we
can estimate the ONI from anomalous SSTs -- above normal water temps
in the eastern Pacific. We can see these temps graphically here:
http://www.cpc.noaa.gov/products/analysis_monitoring/enso_advisory/figure1.gif

...which shows lots of warm water around the equator in the eastern
Pacific. Taking a look at just how warm those temps are above normal,
we can judge it in the SST anomaly graphs, here:
http://www.cpc.noaa.gov/products/analysis_monitoring/enso_advisory/figure2.gif

As these show, we're seeing temps nearing 1 degree C above normal, or,
an ONI signal that is around 1 degree. Bear in mind, that the
1997-1998 El Nino had a signal, at its peak, at 2.5 degrees, so we're
not quite half way there. Furthermore, the 1997 ONI for this time of
year (ASO, for August September, October), was at 2.2. Taking this
into account, we'd still only be at about 50% of the signal of the
1997 event.

Lastly, we can take a (subjective) look into the future by seeing what
various climate models are predicting. Various model forecasts for
anomalous SST's/ONI, can be seen here:
http://www.cpc.noaa.gov/products/analysis_monitoring/enso_advisory/figure5.gif

The mean value for the peak of these models seems to point to an ONI
of about 1 to 1.5, with the greatest outliers on the extreme showing
something above 2, and something below 1. Mean though is around 1 to
1.5.

So, long story short, it looks like this year's El Nino would be
moderate. Every El Nino though is different, and being moderate does
not necessarily mean we'd see above normal swells this winter. This
does though increase the likelihood for California to see more NW
swells, and from those influenced by El Nino's positioning of the
jetstream: NW swells from lower latitudes, with a more WNW than NW
angle, resulting in less loss of energy once reaching the California
coast. Lower latitude storms though increase the chances of
precipitation over SoCal in particular, so bigger and more frequent
surf can also equate to sloppier, rainier conditions.

Being a moderate El Nino though, it is quite likely that the jetstream
influence won't ride at such a lower latitude, and possibly we may see
merely just a slightly above normal swell season this winter, and with
storms not pile driving directly into our region.

Either way, it's a tough call, especially given that the ONI, so far,
for this year, is not even at 50% of what we saw in 1997.


Nathan Cool
http://www.NathanCool.com