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Wednesday, October 29, 2008

Atlantic and Pacific SST Dipoles


In the controversial paper “Advancing Decadal-Scale Climate Prediction in the North Atlantic Sector”, Keenlyside et al used the Atlantic SST dipole index as an indicator of Thermohaline Circulation/ Meridional Overturning Circulation (THC/MOC), as opposed to the Atlantic Multidecadal Oscillation (AMO). http://www.usclivar.org/Pubs/2May08Keenlyside.pdf

They define it as: “Atlantic SST dipole index (60–10W, 40–60N minus 50–0W, 40–60S SST area averages), which is constructed to isolate MOC forced SST fluctuations from radiatively forced variations.” The areas of the North and South Atlantic used by Keenlyside are illustrated in Figure 1, as are the areas of the Pacific used in the latter part of this post.
Figure 1

Keenlyside et al cite the 2005 Latif et al paper “Is the Thermohaline Circulation Changing?” as reference.
http://luv.dkrz.de/publications_2005/pub_291_329.pdf Note that the answer to the title question is no. They explain in the abstract: “Indications of a sustained THC weakening are not seen during the last few decades. Instead a strengthening since the 1980s is observed.” Latif et al further discuss the use of the dipole as, “On these multidecadal timescales, the dipole SST anomaly pattern can thus be used as a fingerprint to detect changes in the MOC (Latif et al., 2004).”

If the intent of the Keenlyside paper was to predict future trends in global temperature, does the use of the Atlantic SST dipole in place of the AMO in the Keenlyside paper seem appropriate? The Keenlyside paper determined the changes in various Northern Hemisphere area temperatures that result from the MOC signal in the North Atlantic, not a small segment of the North Atlantic as illustrated in Figure 1. Or does it make any difference? Is the Atlantic SST Dipole significantly different than the AMO?

Is there a multidecadal pattern in the Pacific dipole that would illustrate the presence of MOC in the Pacific?

Note 1: All graphs in this post include SST anomaly data from January 1854 to September 2008. The data have been smoothed with 37-month running-average filters.

Note 2: Past posts of the North Pacific Residual (NPR) calculated the NPR by subtracting Global SST Anomalies from the SST anomalies of the North Pacific, North of 20N. In this post, Global SST anomalies are subtracted from the SST anomalies of the ENTIRE North Pacific (0-65N). The differences in the NPR are illustrated at the end.


Figure 2 illustrates the SST anomalies of the two areas chosen by Keenlyside and Latif for use in creating the Atlantic SST Dipole. The North Atlantic data shows the typical decrease in temperature from the late 19th Century to the early 20th, followed by the rebound to the late 1930s. It also illustrates a continuation of the cycle, though dampened, from the 1930s to present, ending in a peak that is approximately the same temperature as the peak in the 1930s. The South Atlantic data show evidence of the Southern Ocean influence.
Figure 2

Subtracting the SST anomalies of the segment of the South Atlantic from the segment of the North Atlantic creates the Atlantic SST Dipole. Refer to Figure 3. It’s similar in appearance to the AMO, but how similar?
Figure 3

Figure 4 is a comparative graph of the Atlantic SST Dipole and the AMO. The AMO data was created by subtracting global SST anomalies from the North Atlantic (70W-10E, 0-75N) SST anomalies. It is clear that the Atlantic SST Dipole has a greater variation. Scaling the AMO by multiplying the data by 2.5, Figure 5, reveals that the signals correlate well from around 1900 to present.
Figure 4
Figure 5

Pacific SST Dipole

To provide as reasonable comparison of Atlantic and Pacific SST Dipoles, I used the same latitudes (40 to 60N and 40 to 60S) for the Pacific SST Dipole data sets. Figure 6 shows the SST anomalies of the North and South Pacific areas used to create it. The North Pacific SSTs dropped significantly from its maximum in the early 1880s to minimum about 1910, then rebounded to a peak of lesser SST in the 1940s. Since the 1940s, the SST anomalies of the area selected in the North Pacific have varied, but never again reached the values in the 1880s. And like the South Atlantic, the SSTs of the area in the South Pacific show the influence of the Southern Ocean.
Figure 6

In Figure 7, the Pacific SST Dipole is shown. It was created by subtracting the SST anomalies of the section of the South Pacific from the SST anomalies of the section of the North Pacific. The Pacific SST Dipole does show variability consistent with THC/MOC.
Figure 7

A comparison of the Pacific SST Dipole and the North Pacific Residual (Figure 8) shows that the Pacific SST Dipole has variations that are much greater in magnitude. Scaling the North Pacific Residual (Figure 9) illustrates that the two indices do correlate well with one another.
Figure 8
Figure 9


Figure 10 is a comparative graph of the Atlantic and Pacific SST Dipoles. The variations in the Atlantic SST Dipole are clearly greater in magnitude. This indicates a stronger MOC influence in the Atlantic, but does not rule out an MOC signal in the North Pacific. The two indices appear to parallel one another at times, but then diverge at others, amplifying or dampening their impacts on Northern Hemisphere and Global temperatures.
Figure 10


When I originally plotted the North Pacific Residual, it was for use in a comparison with the Pacific Decadal Oscillation (PDO). Since the PDO is calculated from North Pacific SST anomalies of the area North of 20N, I used the same area for the NPR. This post, as noted earlier, used a North Pacific Residual that was calculated from the SST anomalies of the entire North Pacific. The data sets are different, as illustrated in Figure 11.
Figure 11

Figure 12 illustrates the difference between the “cycles” of the Atlantic Multidecadal Oscillation and the North Pacific Residual when the NPR is calculated using SST anomaly data for the area North of 20N. As discussed in earlier posts, the two indices are in synch during some periods and out of synch during others, which would reinforce or partially offset their combined impact on global temperatures.
Figure 12


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

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