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Friday, October 10, 2008

The Pacific Warm Pool vs ENSO


The following discussion was contained in my recent post about the Tropical West Pacific.

I felt it important enough to warrant a post of its own. I have not changed the numbering of the figures, which is why the illustrations begin with Figure 11.


The Pacific Warm Pool (PWP) is defined as the area in the Western Tropical Pacific enclosed by the 28.5 deg C isotherm. Refer to Figure 11. I’ve also provided a link to a Quicktime video that shows the modulation in PWP size from 1950 to 2001. Its carry over into the Indian Ocean explains why it’s also known as the Indo-Pacific Warm Pool.

Figure 11

Link to PWP Quicktime movie:

Since the definition of the PWP is area based, most graphs of the PWP illustrate just that, area. Refer to the upper graphs in Figure 12. Others show the warmest annual SST in the Warm Pool, the lower graphs in Figure 12, not annual mean or average. The choice of variables use to illustrate the PWP does little to help expose the relationship between ENSO and the PWP.
Figure 12

I had originally intended to show that relationship between the Western and Eastern Tropical Pacific using the NINO3 and NINO6 and the NINO 4 and NINO5 areas--yes, there are NINO5 and NINO6 areas--but comparison graphs of the Western and Eastern Tropical Pacific SST anomalies illustrate the phenomenon better.

But first: Figure 13 shows the currents for the Pacific Ocean. Keys to this discussion are the North and South Equatorial Currents, which travel East to West, and the Equatorial Counter Current, which opposes the North and South Equatorial Currents, running between them and travelling from West to East.
Figure 13

Most discussions of the PWP use trade winds to explain why warm waters pool in the Western Tropical Pacific. Occasionally, a change in Pacific trade winds causes the collection of warm PWP waters to shift, following a Kelvin wave, and travel east. The subsurface warm waters in the PWP are also shifted to the East and are upwelled in El Nino events. This is explained and illustrated (much better than I can) in the following film from the NASA Scientific Visualization Studio titled “Visualizing El Nino”. http://svs.gsfc.nasa.gov/vis/a000000/a000200/a000287/a000287.mpg
That video shows the heat moving in only one direction, from West to East; it doesn’t show how the surface waters cycle the heat back and forth between El Nino and La Nina events. That is illustrated in Figures 14 and 15. The short-tem data for east and west data of the North Tropical Pacific (Figure 14) and South Tropical Pacific (Figure 15) have not been smoothed. They also have not been scaled, which may or may not have provided a better illustration of the effect. Note how, in both graphs, the West and East portions of the North and South Tropical Pacific are out of phase. Keying off the El Nino and La Nina events in the Eastern Pacific data, the SST in the East rises when the SST in the west drops. The east responds at a much greater amplitude during ENSO events, but between those events, the amplitudes of the out-of-phase changes can be comparable in magnitude. This is very clear in Figure 14, during the period that extends from mid-1991 to mid-1995. The 1997/98 of course may also provide an exception to the out-of-phase relationship. Refer to the Southern Tropical data, Figure 15. Note how, after the 97/98 El Nino, both data sets decrease in 1998 and 1999 and then, afterwards, begin to vary in opposition. http://i34.tinypic.com/r715j6.jpg
Figure 14
Figure 15


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

Figures 11 and 12 and the Quicktime movie are available from the Center for Research on the Changing Earth System (CRCES) at:
The map of Pacific Ocean currents was cropped from an image with the title “Ocean currents 1943 (borderless).png” found here:

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