Thoughts on El Nino and the Weather



Subject: El Nino
To: ahlgren@gg.caltech.edu (Sara Ahlgren)
Date: Wed, 15 Oct 1997 15:17:15 -0500 (CDT)

> So, a weather question..... Here everything is El nino, El nino
> ..... but in your penultimate post it seemed like it might be
> inappropriate to ascribe certain weather systems to el nino.... what are
> the facts, in layman's terms.

Sara,

Fact 1: El Nino is a warming of the equatorial eastern Pacific Ocean. The atmosphere responds to that warming by producing lower surface pressure above the warm water. That atmospheric response is called the Southern Oscillation. Therefore, when you hear the media talk about how the El Nino will affect you, they really mean how the atmospheric response to the El Nino will affect you.

Fact 2: Whether there is an El Nino going on or not, weather happens. Storms come and go. Floods happen. Droughts happen. Therefore, if southern California has a big rainstorm in February 1998, we have no way of knowing whether it would have occurred were we not having an El Nino. Now, if southern California gets a large number of storms in January through March 1998 and gets washed away, AND, we as atmospheric scientists can say, "the unusually heavy rains in S. CA were due to a jet stream that was farther south than usual because the convection over the equatorial Pacific Ocean . . . (more causative links omitted) . . . which is a direct response to the warm water in that area due to the El Nino", then we have made a link between the heavy rains in a climatological sense and El Nino.

Note added 7/31/99: The excellent paper by Barsugli et al. (1999) in the July 1999 Bulletin of the American Meteorological Society is one way that showing the link between individual weather events and larger-scale processes might be explored.

Just remember, if anyone tries to pin a specific weather event on El Nino or even a general climate anomaly without having done the work (i.e., the causative links), they shouldn't be taken seriously.

Fact 3: The atmosphere has been known to respond to different El Nino episodes differently. Simply put, not every year that has an El Nino has the same weather. AND not every place on the earth is sensitive to El Ninos. Here in Oklahoma, we're split between having a cold winter and a warm winter. My own research (Schultz et al., January 1998, Monthly Weather Review, pp. 5-27) indicates that cold frontal passages in the southern Plains of the U.S. (e.g., Texas, Oklahoma), Mexico, and Central America are about twice as likely to occur during El Nino years than La Nina years (the opposite pattern), but that doesn't necessarily mean colder temperatures on average because the cold air may not last as long. The Pacific Northwest, on the other hand, is very sensitive to the effects of El Nino. Snowfall in the Cascade Mountains can be much below normal because of the diverted jet stream during an El Nino year.

Anyway, that's about it for now. I was actually on a radio talk show here in Norman about two months ago talking about El Nino and tried to make the same points. I've been getting the same questions, so I think I'll make this email into an essay on my web page. Check it out in the near future.

Take care,

Dave

P.S. If you want further information about El Ninos, check out the following web pages:
NOAA's Climate Prediction Center and NOAA's Pacific Marine Environmental Laboratory.


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