>PREVIOUS POSTS IN SERIES
Tropical SST Anomalies Revisited – Introduction
Tropical SST Anomalies Revisited – Atlantic Ocean
Figure 1 illustrates the geographic areas included in this post and their coordinates. They include the:
-Tropical Northeast Pacific (0-20N, 90-180W)
-Tropical Southeast Pacific (0-20S, 70-180W)
-Northeast Pacific (0-65N, 90-180W)
-Southeast Pacific (0-60S, 70-180W)
-Northeast Pacific Upwelling Area (32-48N, 116-132W)
-Southeast Pacific Upwelling Area (16-48S, 70-78W)
NORTHERN AND SOUTHERN TROPICAL EAST PACIFIC
The long-term (January 1854 to August 2008) SST Anomalies for the Northern and Southern Tropical East Pacific are shown in Figure 2. The data has been smoothed with an 85-month running average filter. Note how the sign of the difference between the two data sets shifts with time. Before the 1920s and after 1976, the Southern Tropical East Pacific anomalies are higher in temperature than the Northern Tropical East Pacific, and between the 1920s and 1976, the Southern Tropical East Pacific anomalies are lower in temperature than the Northern Tropical East Pacific.
In Figure 3, I’ve subtracted the Southern Tropical East Pacific anomalies from the Northern Tropical East Pacific anomalies. I’ll have to examine oceanic SST dipoles in a future post.
Figure 4 (raw data) shows the short-term (January 1978 to August 2008) SST Anomalies for the Northern and Southern Tropical East Pacific. The Southern Tropical East Pacific shows the greater variation.
OFF TOPIC – THE GREAT PACIFIC CLIMATE SHIFT OF 1976
Refer back to Figure 2. Note the sharp rise in SST anomalies at 1976 in both Northern and Southern Tropical East Pacific data sets. Is that an indicator of the Great Pacific Climate Shift of 1976? Normally, that shift is discussed in terms of changes in the Pacific Decadal Oscillation and in El Nino frequency. There are a few papers, however, that discuss the actual shift in basin-wide Pacific SST. In Figure 5, I’ve provided a short-term graph of the Northern and Southern Tropical East Pacific SST anomalies from January 1960 to December 1985. Note how the trends before and after 1976 are different.
To help illustrate the change in trend and the jump in SST, I’ve shown only the Southern Tropical East Pacific SST anomalies and added linear trend lines for two periods, from January 1960 to December 1975 and from January 1977 to December 1955, in Figure 6. Based solely on the end and start points of the two trend lines, the Southern Tropical East Pacific SST anomalies stepped up almost 0.5 deg C during 1976. When I’m finished with this series, I will examine the Pacific Ocean on a basin wide basis to determine the total impact of the Great Pacific Climate Shift.
IS THE SOUTHERN OCEAN INFLUENCIAL?
In Figure 7, I’ve compared Southern Ocean SST anomaly data to Northern and Southern Tropical East Pacific SST anomalies. There appears to be little relationship between the Southern Ocean and the Tropical Pacific.
However, Smith and Reynolds provided a much updated representation of the Southern Ocean in their latest version of SST data, ERSST.v3. In Figure 8, I’ve replaced the ERSST.v2 data for the Southern Ocean with the ERSST.v3 data. The Southern Ocean definitely influences the Eastern Tropical Pacific Ocean.
Subtracting the Southern Ocean SST anomalies from the Northern and Southern Tropical East Pacific SST anomalies creates two curves with an apparent long-term oscillation. Refer to Figure 9. I found the overall shape of the curve in Figure 9 and the points at which the trends changed interesting. (Note that the ERSST.v3 data is only available, in simple format, in specific latitude bands and only from January 1880 to April 2008. They stopped updating it in April. For this reason, the span of the graph has been reduced.) Don’t concern yourself with the spikes at 1994, three years before the 97/98 El Nino. It’s simply a product of the smoothing.
If the smoothing is changed from 85 months to 12 months, Figure 10, the El Ninos and La Ninas appear at the correct times for the most part.
In Figure 11, I’ve returned to a straight comparison of monthly data for the Southern Ocean and the Northern and Southern Tropical East Pacific SST anomalies for the years 1965 to 1985. The data is raw. The Southern Ocean data appears not to be the cause of the 1976 climate shift.
COMPARISON TO NINO3.4
As one would expect, SSTs in the NINO3.4 area are a major influence on the SSTs of the Tropical East Pacific. Refer to Figure 12. But there are differences in the underlying trends. These differences would be caused by the additional influences of the North and South Pacific.
The magnitudes of the variations caused by ENSO make a simple cause and effect evaluation difficult, which will be seen in the following, much more difficult than that of the Tropical Atlantic in the prior post.
NORTHERN TROPICAL EAST PACIFIC
Figure 13 illustrates SST anomalies for the Northeast Pacific and for the Northern Tropical East Pacific. The overall changes in trend agree with one another. There are differences in the overall amplitude of the decadal variations, which may be due to the additional thermohaline circulation/meridional overturning circulation component in the North Pacific, or due to the reductions in the effects of NINO3.4 as the perturbations are spread out over a large area, or a combination of both.
Figure 14 compares the SST anomalies of the North East Pacific Upwelling area and for the Northern Tropical East Pacific.
Ocean currents do run from North to South along the North American coast, so the Upwelling area waters do feed the tropics. But ENSO events also distribute changes Northward through Rossby waves. These ENSO fed variations should then be returned over time to the tropics, creating a delayed feedback to the tropics. The North East Pacific Upwelling area is also smaller in area than the Northern Tropical East Pacific so it should reflect the variations more.
Note: I could but will not attempt to extract the base NINO3.4 component from the other signals. Maybe a future post.
SOUTHERN TROPICAL EAST PACIFIC
The SST anomalies for the Southeast Pacific and for the Southern Tropical East Pacific are compared in Figure 15. Like their Northern counterparts, there are differences between the two. However, the Southern Ocean would also have an influence on these data sets.
The Southern Tropical East Pacific and the coastal upwelling area off the Southwest coast of South America are compared in Figure 16. Again, like the Northern Tropics and Upwelling area, the influences of ENSO on both these data sets, along with the feedbacks due to ocean currents, would make it difficult to isolate the variables using simple tools.
While I had expected to find a clearer relationship between the North and South American west coast upwelling areas and the Tropical East Pacific, I was not disappointed with this post. The relationship between the Southern Ocean (ERSST.v3) and the Tropical East Pacific (Figure 8) was revealing, especially when the negative trend in the Southern Ocean SST over the last 20+ years is considered. Will the Tropics follow? The other find I was pleased with was the 1976 SST shift that’s easily visible in the North and South Tropical East Pacific data. Refer again to Figures 2, 5, and 6.
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 more recent version of the Smith and Reynolds Extended Reconstructed SST (ERSST.v3), along with land surface temperature and combined (land + ocean) surface temperatures, are available in various latitudinal bands at:
Don’t let the PDO in the address confuse you. There’s much more there. The overview for the update is here: