This post illustrates an inverted ENSO signals contained within the Global SST anomaly dataset, using SST anomaly residuals for the East Indian and West Pacific Oceans. SST anomaly residuals in this post are defined as the SST anomalies for specific areas of the globe minus global SST anomalies.
This inverted ENSO signal contained within the East Indian and West Pacific Ocean dataset confirms the dipole effect between the Eastern and Western Tropical Pacific that extends into the East Indian Ocean that was discussed in two other posts about the multiyear aftereffects of ENSO events:
More Detail On The Multiyear Aftereffects Of ENSO – Part 2 – La Nina Events Recharge The Heat Released By El Nino Events AND…
More Detail On The Multiyear Aftereffects Of ENSO – Part 3 – East Indian & West Pacific Oceans Can Warm In Response To Both El Nino & La Nina Events
EAST INDIAN-WEST PACIFIC SST RESIDUALS SHOW AN INVERTED ENSO SIGNAL
Figure 1 compares Global SST anomalies and the SST anomalies for the East Indian-West Pacific Ocean, a dataset with the coordinates of 60S-65N, 80E-180. It’s well understood that the Global dataset includes an ENSO component. Global SST anomalies increase when Eastern and Central Tropical Pacific SST anomalies rise in response to an El Nino, and Global SST anomalies fall when a La Nina causes Eastern and Central Tropical Pacific SST anomalies to drop. The East Indian and West Pacific SST anomalies in Figure 1 follow the basic rises and falls of the Global dataset, but there is additional variability, indicating the East Indian and West Pacific SST anomalies are impacted by something other than the “normal” ENSO signal.
In Figure 2, the Global SST anomalies (with its ENSO component) have been subtracted from the East Indian and West Pacific SST anomalies, leaving the East Indian and West Pacific SST anomaly residual. Again, this residual illustrates the difference between Global SST anomalies and the East Indian-West Pacific SST anomalies. Recognize the curve?
In Figure 3, I’ve scaled the NINO3.4 SST anomaly data and inverted it by multiplying it by a factor of -0.15. As illustrated, the curves of the East Indian and West Pacific SST anomaly residuals and the inverted and scaled NINO3.4 SST anomalies are remarkably similar. They correlate well for the entire term of the data, but they do diverge slightly at times.
THE CAUSE OF THIS OPPOSING EFFECT
A much-simplified version: during a significant El Nino, warm water from the Western Pacific Warm Pool (to depths of 300 meters) sloshes to the east and spreads across the surface of the central and eastern tropical Pacific. Warm water that was below the surface of the Pacific Warm Pool and excluded from the Sea SURFACE Temperature measurement is now included, raising SST anomalies in the central and eastern tropical Pacific.
A number of things happen during the La Nina that follows. Trade winds increase in the tropical Pacific, and with strengthened Equatorial Currents, the warmer-than-normal water in the central and eastern tropical Pacific is carried back to the western tropical Pacific. Some of the warm water helps to recharge the warm water in the Pacific Warm Pool, and some of it is carried by ocean currents north into the Northwest Pacific, and some of it is carried south by ocean currents into the Southwest Pacific, and some of it is carried into the eastern tropical Indian Ocean by a current called the Indonesian Throughflow. The stronger-than-normal trade winds during the La Nina also cause a decrease in cloud cover in the tropical Pacific, which causes an increase in Downward Shortwave Radiation (visible light). This additional Downward Shortwave Radiation warms the surface and subsurface waters of the tropical Pacific, and the trade winds and ocean currents carry the warm water to the west, where it is transported into the Northwest and Southwest Pacific and into the eastern tropical Indian Ocean, as described above, by ocean currents.
The result is a visible dipole (seesaw-like) effect between the central and eastern tropical Pacific and the western Pacific as an ENSO event goes from El Nino to La Nina. Refer to Figure 4.
THE “REST OF THE WORLD” SST ANOMALY RESIDUALS
Someone was bound to ask, What does the rest of the world oceans look like? So I’ve added this section to the post.
Figure 5 is a comparison graph of global SST anomalies and the SST anomalies for the area between the latitudes of 60S and 65N that is not included in the East Indian and West Pacific Ocean dataset. The coordinates for the “Rest of the World” SST anomaly data are 60S-65N, 180-80E. As illustrated, the variations in the two datasets mimic one another, with the “Rest of the World” subset varying more than the global data.
The same process was used to create the residuals. That is, the Global SST anomalies (which have a strong ENSO component) are subtracted from the “Rest of the World” SST anomalies. The residual illustrates how that dataset differs from the global data. The result was not unexpected. It shows that the “Rest of the World” data has yet another, but smaller, ENSO component. In Figure 6, I’ve scaled NINO3.4 SST anomalies by a factor of 0.05 for comparison to the “Rest of the World” SST anomaly residual.
KEEP IN MIND THIS POST WAS ABOUT SST ANOMALY RESIDUALS
Again, the intent of this post was to provide another means of illustrating the east to west Pacific SST dipole effect, by showing the inverted signal within the residuals of the East Indian-West Pacific dataset. Recall that the SST residuals contain the positive trend of the global SST anomaly curve. This changes perspective. Also, if we look at the SST anomaly data of East Indian through the Eastern Pacific Oceans, it is clear that the positive ENSO signal dominates. Comparing it to scaled NINO3.4 SST anomalies, Figure 7, there is little lag, at least on the leading side of most of the major variations. This appears to indicate that the opposing East Indian and West Pacific dataset only suppresses the global response to the primary ENSO signal being produced in the Central and Eastern Tropical Pacific. Then, when looking for a secondary ENSO signal in the global dataset, it should be a lagged positive signal.
The data used in this post is available through the NOAA NOMADS website: