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Friday, June 27, 2008

El Nino Effects on Global Equatorial SST

The original intent of this post was to illustrate the time lags from the East Equatorial Pacific (NINO3 plus most of NINO4) to the West Equatorial Pacific (Pacific Warm Pool), the Indian Ocean, and the Atlantic Ocean, but the absence of an expected rise in the West Pacific reminded me of the origin of an El Nino’s heat: The Pacific Warm Pool.

Prior to reading the rest of this post, please watch a 2.5 minute film from the NASA Scientific Visualization Studio titled “Visualizing El Nino”. http://svs.gsfc.nasa.gov/vis/a000000/a000200/a000287/a000287.mpg
Source: http://svs.gsfc.nasa.gov/vis/a000000/a000200/a000287/index.html

It has great graphics and provides informative explanations of the processes of the 97/98 El Nino.

Figure 1 illustrates the Equatorial SST Anomalies in the Atlantic, Indian, and East and West Pacific Oceans, from January 1996 to December 2000, catching the impacts of the 97/98 El Nino. The monumental spike in the East Pacific SST anomaly is the stand-out feature. Notice, however, that initially, from February to March 1997, both the Indian and East Pacific Oceans rise in unison, after which time the Pacific skyrockets and the Indian Ocean climbs much more subtly. After the East Pacific and Indian Ocean anomalies rise above zero in March 1997, the Atlantic responds with its increase in temperature. This initial Atlantic temperature increase appears, based on timing, as if it would have to be the reaction to increased atmospheric temperatures, and the later jump in Atlantic SST in mid-1999 (21 months later peak-to-peak) would most likely be an oceanic response, where the wave of elevated SST finally made it to the equatorial Atlantic.

Figure 1

Much more subtle, and the reason for asking that you watch the video, is the interaction between the Equatorial West and East Pacific. The West Pacific temperature anomaly decreases as the East Pacific increases. Then, around July or August 1998, the West Pacific anomaly begins to rise slowly. It continues to increase at that snail’s pace until the East anomaly drops below the West; then the West increases much more quickly. From that point on, it appears the two areas of the equatorial Pacific are transferring heat back and forth, with a small change in the West Pacific causing an amplified change in the East.

Monthly SSTs for the same period and variables are illustrated in Figure 2. Looking at the East Pacific curve, the not only did the 97/98 El Nino elevate the summer peaks, it also eliminated the winter trough.

Figure 2

In Figure 3, the SST anomalies for the 82/83 El Nino for the same areas of the equatorial oceans are illustrated. The reactions of the East and West Pacific are significantly different, due in part, at least, from the explosive volcanic eruption of El Chichon. The secondary 21-month response lag of the Atlantic is the same as in the 97/98 El Nino. Based solely on the reaction of the Atlantic following both major El Ninos, there appears to be a consistent 21-month lag in Atlantic SST to a change in the East Pacific.

Figure 3

Figure 4 shows the SSTs for the 82/83 El Nino.

Figure 4

In the last two graphs, Figures 5 and 6, the 06/07 El Nino and 07/08 La Nina effects on equatorial SSTs are illustrated. What I find most interesting about this graph is the fall in West Pacific SST since January 2007, most visible in Figure 5. If the West Pacific Warm Pool supplies the energy for El Ninos, the heat source is dwindling.

Figure 5

Figure 6


Sea Surface Temperature Data is Smith and Reynolds Extended Reconstructed SST (ERSST.v2) available through the NOAA National Operational Model Archive & Distribution System (NOMADS).

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