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Thursday, October 2, 2008

Tropical SST Anomalies Revisited – West Pacific Ocean


Tropical SST Anomalies Revisited – Introduction

Tropical SST Anomalies Revisited – Atlantic Ocean

Tropical SST Anomalies Revisited – East Pacific Ocean


While this post does deal primarily with Tropical West Pacific SST anomalies, it also presents the Pacific Warm Pool, located in the Tropical West Pacific, and its relationship with ENSO.


Figure 1 illustrates all but two of the geographic areas included in this post. They include the:
- Tropical Northwest Pacific (0-20N, 100-180E)
- Tropical Southwest Pacific (0-20S, 130-180E)
- Northwest Pacific (0-65N, 100-180E)
- Southwest Pacific (0-60S, 145-180E)
- Kuroshio Extension (32-38N, 142-180E)

The areas not illustrated are:
- Tropical Northeast Pacific (0-20N, 90-180W)
- Tropical Southeast Pacific (0-20S, 70-180W)
Refer to Figure 1 in the above link for the post on the “Tropical SST Anomalies Revisited – East Pacific Ocean”.

(Note: I apologize for the size and lack of clarity in Figure 1, but I tried to reduce it in size in Microsoft Office Picture Manager and all that did was decrease the clarity. Tried it here on the blogger dashboard, and the same thing happened. )
Figure 1


The long-term (January 1854 to August 2008) SST Anomalies for the Northern and Southern Tropical West Pacific are shown in Figure 2. The data has been smoothed with an 85-month running average filter. The two data sets parallel one another, as could be expected, but the Southern Tropical West Pacific has greater variations.
Figure 2

This greater variability in the Southern Tropical West Pacific is confirmed in the short-term data (not smoothed), Figure 3. This is consistent with the eastern portion of the Southern Tropical Pacific data from the prior post. Refer to link above.
Figure 3


Figure 4 provides a comparison of the Northern Tropical Pacific, East and West, from 1854 to 2008, where the data has been smoothed with an 85-month filter. It difficult to believe the two data sets share the same ocean. ENSO events have a much clearer impact on the East, while, in the West, it seems to have little influence. The smoothing and the length of the data hide the relationship between East and West. This will be discussed later in this post.
Figure 4

Figure 5 illustrates SST anomalies for the Northwest Pacific and for the Northern Tropical West Pacific. The overall changes in trend agree with one another. This is logical since ocean currents do run from South to North along the East Asian coast, so the Tropical Northwest Pacific waters do feed the Northwest Pacific.
Figure 5

There are differences in the amplitude of the decadal variations, which are illustrated by subtracting the Tropical Northwest Pacific from the Northwest Pacific data. See Figure 6. The difference appears to be due to the additional thermohaline circulation/meridional overturning circulation component in the North Pacific. Or it could result from the pooling of warm or cool waters after ENSO events in the Northwest Pacific. Or it could be a combination of both.
Figure 6

I’ve provided a graph of the SST anomaly for the Kuroshio Extension to show the similarity to the graph of the difference between the Tropical Northwest Pacific from the Northwest Pacific (Figure 6). Refer to Figure 7. Also refer to Figure 1 for the coordinates and location of the Kuroshio Extension used in this graph.
Figure 7


The Southern Tropical Pacific, East and West, from 1854 to 2008 are compared in Figure 8. Again, the data has been smoothed with an 85-month filter. As with the Northern Tropics comparison, Figure 4, the smoothing and the length of the data hide the relationship between East and West. This will be discussed later in this post.
Figure 8

The SST anomalies for the Southwest Pacific and for the Southern Tropical West Pacific are compared in Figure 9.
Figure 9

Like their Northern counterparts, there are differences between the two. This difference is illustrated in Figure 10. Based on the shape of that curve, the Southern Ocean has an obvious influence on the South Pacific.
Figure 10


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 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-term 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


Is a La Nina a separate event or is it just a continuation of an El Nino, completing a cycle? During La Nina events, are the SSTs of the East Pacific simply overreacting before returning to “normal”? There are multiple theories, and, in my opinion, I haven’t found one that appears dominant.


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 about the Pacific Warm Pool are available from the Center for Research on the Changing Earth System (CRCES) at:http://www.crces.org/presentations/dmv_ipwp/
The map of Pacific Ocean currents (Figure 13) was cropped from an image with the title “Ocean currents 1943 (borderless).png” found here:

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