I posted an interesting comparative graph of SST anomaly residuals of the North Pacific and Indian Oceans in Individual Ocean SST Anomalies In Perspective. Refer to Figure 1. The two curves opposed one another.
WHAT THE RESIDUALS REPRESENT
Figure 2 compares Global SST anomalies (red curve) with the SST anomalies of the North Pacific (blue curve) and Indian Ocean (green curve). The North Pacific and Indian Oceans SST anomalies follow the global dataset with minor deviations. The residuals represent these minor differences, calculated as Global SST anomalies subtracted from the respective SST anomaly dataset.
The comparison of the North Pacific and Indian Ocean residuals from 1978 to present, Figure 3, also shows that the two datasets oppose one another in the short term. (The data in Figure 3 has been smoothed with a 12-month running-average filter.) I’ve highlighted 1998, about the time of the peak of the 1997/98 El Nino, and the periods that would have been impacted by the El Chichon and Mount Pinatubo eruptions. Note the responses of the North Pacific and Indian Ocean residuals after the 1997/98 El Nino. The North Pacific residual exhibits a multiyear dip and rebound associated with the 1998/99/00 La Nina, while the Indian Ocean shows a rise and multiyear decay in the anomalously high residuals over the same period. In terms of NINO3.4 SST anomalies (not illustrated), the 1982/83 El Nino was nearly the same magnitude at the 1997/98, but the responses of the North Pacific and Indian Ocean residuals were not nearly as great. The eruption of Mount Pinatubo appears to have suppressed the responses to the 1982/83 El Nino. And the Mount Pinatubo eruption overpowered the reactions to the 1991/92 El Nino.
But what causes the North Pacific residual to drop after an El Nino event? And what causes the Indian Ocean residual to rise?
To answer those questions, we’ll refer to a short-term graph of Global, North Pacific, and Indian Ocean SST anomalies (not residuals), Figure 4. Starting early in 1997, all three datasets rose in response to the 1997/98 El Nino. Note how the North Pacific SST anomalies dip well below those of the global SST anomalies during the 1998/99/00 La Nina. The North Pacific SST anomaly response is exaggerated at that time because it fully follows the variations in equatorial SST anomalies. This, in turn, causes the North Pacific residual to drop. On the other hand, while temperatures are increasing in 1997, the Indian Ocean response to the 1997/98 is also slightly more than the global dataset, but as SST anomalies drop in response to the resulting La Nina, the SST anomalies of the Indian Ocean do not fall as far as the global SST anomalies. This causes the rise in the Indian Ocean residual.
LOOSE LONG-TERM CORRELATIONS WITH OTHER DATASETS
There is a very loose correlation between the North Pacific and North Atlantic Residuals, Figure 5. Do the SST anomaly variations caused by Thermohaline Circulation/Meridional Overturning Circulation in the North Atlantic impact SST anomalies in the North Pacific? Or does the North Pacific influence the North Atlantic? Most likely, they both have an influence on the other.
And it appears that the Indian Ocean Residual may be impacted by the Southern Ocean SST anomalies (not residuals) for the area south of the Indian Ocean. Refer to Figure 6. Note, however, that there is a long-term decline in the Southern Ocean subset from the 1880s to the 1930s and more sudden rise during the 1940s that is not present in the Indian Ocean Residual data.
ERSST.v3b data is available through the KNMI Climate Explorer website: