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Monday, November 10, 2008

Equatorial Pacific Warm Water Volume


The Tropical Atmosphere Ocean (TAO) Project Office of the NOAA Pacific Marine Environmental Laboratory has a webpage titled “Warm Water Volume and ENSO”. http://www.pmel.noaa.gov/tao/elnino/wwv/

The text of the page and the referenced papers are well worth reading. The description of the Warm Water Volume (WWV) data linked to the page is: “WWV is defined as the volume of water with temperature above 20°C in oceanic regions of the tropical Pacific between 5°N to 5°S, 120°E to 80°W. Monthly anomalies are computed by removing a mean seasonal cycle for a base period of 1980-2002. Warm water volume and anomalies for the eastern (80°-155°W) and western (155°W-120°E) halves of the basin are also available.”


Read Me File:

Figure 1 shows the Pacific Ocean areas of the WWV data.
Figure 1

Plotting the WWV and SST together for the entire equatorial Pacific would be informative, but it was the data for the eastern and western halves that interested me most. Were there cycles? They should be. Were the cycles in the East and West out of phase? If so, did they remain out of phase during the 28 years of data?


Figure 2 illustrates the Eastern and Western Equatorial Pacific WWV for the period of January 1980 to November 2008. The first thing that stands out is that the Western WWV is approximately twice that of the East WWV, which is consistent with the Pacific Warm Pool and its storage of warm water. The second is that the variations appear to oppose one another, which agrees with the relationships and interactions between indices of SST and subsurface temperature in the Western and Eastern Equatorial Pacific. Third, the Western WWV has increased over the term of the data, while the Eastern WWV has decreased. This appears to result from a shift that took place after the 1997/98 El Nino. This will be more obvious in the East and West anomaly data.
Figure 2


The variations in the Equatorial Pacific WWV are illustrated in Figure 3.
Figure 3

If the WWV is compared to SST for the Equatorial Pacific, it’s easy to see that the change in WWV precedes ENSO events. Refer to Figure 4. This is very similar to the effects of Equatorial Pacific Subsurface Temperature from the preceding post.
Figure 4

Smoothing the data with a 25-month filter, Figure 5, brings out the cyclical nature of the Equatorial Pacific WWV during the 1980s and early 1990s. The cycle then appears to be interrupted in the mid-1990s. This will be clearer in the West and East Pacific WWV data.
Figure 5


[UPDATE: I’ve revised the wording of the following paragraph to make it easier to read.]
Figure 6 is a comparative graph of the East (120E-155W) and West (155W-80W) Equatorial Pacific WWV data. During El Nino events, the WWV from the West is forced to the East where it is upwelled and the heat dispersed. Unless the source of heat to the equatorial Pacific can renew the WWV over a few years, it then appears that much of the warm water is returned to the equatorial Pacific. In the West, this appears to take place during the La Nina phase of an ENSO event, and in the East, it appears to occur as the SSTs rebound from their La Nino levels. Note how, after the 1997/98 El Nino, that the WWV in the West “recharged” during the subsequent La Nina and then remained at a relatively elevated level, a level that is comparable to the peak WWVs during the period before that El Nino.
Figure 6

In Figure 7, NINO3.4 SST anomaly data was added to the comparative graph of East and West Equatorial Pacific WWV. NINO3.4 SST anomaly and the East Pacific WWV correlate quite well.
Figure 7

Figure 8 is a comparative graph of the East and West Equatorial Pacific WWV with the data smoothed with a 25-month filter. During the 1980s and the early 1990s, the East and West WWVs cycle out-of-phase. Then in 1993 there’s an interruption in the cycles, which appears, looking back at Figure 7, to result from the small El Nino that year. The cycles then return, leading to the 1997/98 El Nino. Afterwards, the cycles disappear.
Figure 8


It’s a shame this data set doesn’t extend back further in time, because it would interesting to determine whether the larger multiyear cycles or the lack thereof are the norm.


The source of the Warm Water Volume data is included above in the Introduction.

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

The reasons for smoothing ENSO-related data with a 25-month filter are discussed here:

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