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Wednesday, June 25, 2008

Equatorial SST Comparison

Figure 1 is a global Mercator projection that shows how the data sets were divided in the following comparison. The primary reason for the illustration is to show where I split the Indian and Pacific Oceans.

Figure 1

Figures 2 and 3 illustrate equatorial SST anomaly and SST data for the Atlantic (Blue), Indian (Green), and Pacific (Red) Oceans from January 1854 to May 2008. All three data sets were smoothed with an 85-month running average filter. In Figure 2, the Pacific Ocean anomaly has greater year-to-year perturbations, as expected, primarily a result of ENSO, while the overall trend of the Indian Ocean varies more than the other two oceans. I would’ve expected the Atlantic to be the ocean with the greatest variation at the equator since the Indian Ocean is so rarely discussed in papers, but that could just be the way I select reading material. The SSTs illustrated in Figure 3 didn’t come as a surprise, considering the differences in hemispheric ocean areas.
Figure 2

Figure 3

The problem with illustrating the Equatorial Pacific as one data set is that it fails to account for the significant differences between the East and West. The Western Equatorial Pacific is part of the Pacific Warm Pool, which registers the highest ocean temperatures. The Eastern Equatorial Pacific is well known for its El Nino/La Nina activity, but it is also one of the global ocean areas of upwelling of cool subsurface waters. In Figures 4 and 5, I’ve divided the Pacific into two data sets, East (Bronze) and West (Purple) of 180 deg longitude. I’ve left the Indian and Atlantic Oceans as references. Figure 4 shows that the Western Equatorial Pacific is placid, its yearly variations and overall trends varying comparatively little, while in the Eastern equatorial Pacific, changes in trends are as large as the Indian and Atlantic Oceans and its annual variations dwarf the others. Recall that those data sets have been smoothed with 7-year+ filters. In Figure 5, the middle-of-the-road Pacific SSTs from Figure 3 are now the two extremes. The Western Equatorial Pacific is approximately 3 deg C warmer than the Eastern. What really stand out to me in the Eastern Equatorial Pacific (Bronze) curve are the 11-year (approx) jaggedly topped cycles, but then I’m always looking for solar influences. I’ll have to do a comparison with solar in another post.
Figure 4

Figure 5

In Figures 6, 7, and 8, the time span of the data has been shortened to the last 30 years and the smoothing has been eliminated. The size of the annual changes in SST and SST anomaly of the Eastern Equatorial Pacific makes the temperature scale of the data so large that the changes in the other data sets become insignificant. Figure 6: It’s barely possible to make out the lags in the responses of the other oceans to the major ENSO events in the Eastern Equatorial Pacific. After the clearly identifiable 82/83 and 97/98 El Nino peaks in the Eastern Pacific data, the Equatorial Atlantic portrays a small spike 21 months later. The timing is consistent in both instances. In Figure 7, the vertical bronze lines mark the peaks of the two major El Ninos. The lags in the Indian, Atlantic, and Western Pacific Oceans to the 82/83 and 97/98 El Nino events are much clearer when the Eastern Pacific data is removed from the graph. What are also quite clear are the magnitudes of the drops in temperature that have recently occurred in the equatorial segments of the Indian and Western Pacific Oceans. Will they continue to decline now that the La Nina, visible in the Eastern Pacific data of figure 6, has started to rise, or will they follow? Their histories indicate that they should follow. The lowest Eastern Equatorial Pacific anomaly of the recent La Nina was reached in November 2007. Will the Equatorial Atlantic respond 21 months later, in August 2009?

Looking again at Figure 7, the rises in the Atlantic, Indian, and Western Pacific data after 2000 appear to be an aftereffect of the 97/98 El Nino. Could it have taken 10 years to dissipate the heat discharged into the surface of the world oceans by that one El Nino? There were reports in 1994 that the oceanic Rossby waves generated by the 82/83 El Nino were still visible after 12 years. The upwardly bowed responses in temperature from 2000 to 2008 are most evident in the annual minimums of the Equatorial Eastern Pacific, Indian, and Atlantic data sets in Figure 8. And look at the period of the oscillation in the annual minimum temperature of the Eastern Equatorial Pacific data. While there’s only 30 years of data, those look like 10 to 11 year cycles. I’ll look at it in another post.

Figure 6

Figure 7

Figure 8


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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