>Figures 1 and 2 are comparative graphs of Tropical (20S to 20N) and Global SSTs. Figure 1 is of long-term data, smoothed with an 85-month filter. Figure 2 is of raw data from January 1978 to present. Referring to Figure 1, between its early peak in the 1860s to its minimum near 1910, Tropical SSTs dropped approximately 0.42 deg C. Then from trough (near 1910) to peak (about 2003), Tropical SSTs rose approximately 0.77 deg C. Figure 2 shows that, excluding the rise and fall of the 97/98 El Nino, Tropical SSTs have been dropping since 2003, though the decrease is erratic.
In Figure 3, another comparison, the Tropics have been divided into the Atlantic (70W to 15E), Indian (35 to 100E), East Pacific (70W to 180), and West Pacific (100E to 180). The greatest variation occurs in the Atlantic and the least variation in SST is in West Pacific. The magnitude of the changes in the Atlantic skews the perspective of other data sets.
Figure 4 illustrates the short-term oscillations the SSTs for the Tropical East Pacific that are attributable to ENSO and the long-term oscillations that result from thermohaline circulation/meridional overturning circulation. This curve is typical of NINO data that’s been smoothed with an 85-month filter. It reveals an overall increase in East Pacific SSTs that peaked in 1995, two years prior to the major El Nino of 97/98. The rise in Tropical East Pacific SST is difficult to determine due to the variability. Adding a 6th order polynomial curve to the data (Figure 5) illustrates the underlying trends over the term. Suffice it to say, without the ENSO variations, the overall rise in Tropical East Pacific SSTs would be less than the rise in Tropical SST for the globe of 0.77 deg shown in Figure 1.
The long-term Tropical West Pacific SSTs are shown in Figure 6. Its dip from the late 19th to the early 20th centuries preceded the dip in the Tropical SST for the globe but the rebound lagged, waiting until the late 1920s before stating to rise. That rise from the 1920s to approximately 2003 is just under that of the data for the Tropical SST for the globe shown in Figure 1. The recent downturn in Tropical West Pacific SST is visible even with the 85-month filter. Refer to Figure 7 for the short-term raw data.
The Tropical Indian Ocean SST anomalies are illustrated in Figure 8. Its rise of approximately 0.87 deg C exceeded the rise in Tropical SST for the globe of 0.77 deg C.
Off Topic Note: The dip in temperature from 1942 to 1960 does not appear to be a result of “bucket adjustments”; it looks more like a combined effect of the multiple-year El Nino in the early 40s and a THC/MOC signal. Refer again to Figures 4 and 6.
The greatest long-term variation in tropical SSTs is present in the Atlantic, with a whopping 1.16 deg C rise from 1905 to present. Refer to Figure 9. Although it’s over a slightly different time frame, that’s over 50% greater than the rise in the Tropical SST for the globe (1.16 deg C/0.77 deg C = 1.506). It surely is the driver of the Tropical SST data for the globe. Let’s examine it further to see what it’s comprised of.
Comparing the Northern and Southern Tropical SSTs for the Atlantic, Figure 10, the Southern data shows the greater variation of approximately 1.27 deg C, where the Northern data shows a 1.08 deg C increase.
In Figure 11, the North Atlantic and the Tropical North Atlantic SST comparison illustrates that Tropical North Atlantic SSTs are a function or blend of North Atlantic SSTs and Tropical South Atlantic SSTs.
Comparing the South Atlantic SSTs and the Tropical South Atlantic SSTs illustrates something entirely different. Refer to Figure 12. The variation in South Atlantic SSTs is less than that of the Tropical South Atlantic. Since the Tropical South Atlantic SSTs should also be effected by the Tropical North Atlantic SSTs, and since the variations in Tropical South Atlantic SSTs are greater than the Tropical North Atlantic SSTs, Figure 10, I would have expected that whatever is driving the Tropical South Atlantic SSTs would have a greater variation than the Tropical South Atlantic. But the South Atlantic does not.
Instead of segmenting the South Atlantic data more, let’s say into mid-latitude data, I first looked at the upwelling area that runs along the West Coast of Southern Africa. The Benguela Current that runs along the West Coast of Africa should transport the upwelling of water there into the Tropical South Atlantic. See Figure 13.
Figure 14 illustrates the SSTs of the Tropical South Atlantic and the upwelling area off the West Coast of Africa (0 to 35S, 0 to 20E), what I have identified as the South Atlantic Upwelling SST data. It has a substantially greater SST variation, highlighted by the 1.0 deg C plunge from 1900 to 1905. The upwelling area then appears likely to be the driver of the sizable variation in the Tropical South Atlantic SST.
To confirm this, I isolated the Tropical and the upwelling area data from the remainder of the South Atlantic, using the coordinates 20 to 60S and 0 to 70W and compared the remaining South Atlantic data to the Tropical South Atlantic data. Refer to Figure 15. The shallower decline in SSTs during the late 19th to early 20th centuries and the plateau reached in the 1970s indicate that Tropical South Atlantic is not impacted greatly by that area. The Eastern South Atlantic upwelling area off the African coast does drive Tropical South Atlantic SSTs along with Tropical North Atlantic SSTS.
Note that the plateau in the cyan colored curve (Mid-Latitude South Atlantic Excluding the Upwelling Area) in Figure 15 is due to the influence of the Southern Ocean.
Sea Surface Temperature Data is Smith and Reynolds Extended Reconstructed SST (ERSST.v2) available through the NOAA National Operational Model Archive & Distribution System (NOMADS).