Tropical SST Anomalies Revisited – Introduction
Figure 1 illustrates the geographic areas included in this post and their coordinates. They include the:
– Tropical North Atlantic (0-20N, 78W-10E)
– Tropical South Atlantic (0-20S, 50W-15E)
– North Atlantic (0-70N, 70W-10E)
– South Atlantic (0-60S, 70W-20E)
– North Atlantic Upwelling Area (12-28N, 14-24W)
– South Atlantic Upwelling Area (12-30S, 4-14E)
NORTHERN AND SOUTHERN TROPICAL ATLANTIC
The long-term (January 1854 to August 2008) SST Anomalies for the Northern and Southern Tropical Atlantic are shown in Figure 2. The data has been smoothed with an 85-month running average filter. The Southern Tropical Atlantic has the greater swing in SST over the term of the data, with much of that variation caused by a significant drop in SST from the late 1890s to 1906 and the rebound from 1906 to the mid-1920s.
Figure 3 (raw data) shows the short-term (January 1978 to August 2008) SST Anomalies for the Northern and Southern Tropical Atlantic. Again, the Southern Tropical Atlantic shows the greater variation. I found two differences between the two data sets interesting. In 1984, the Southern Tropical Atlantic almost appears to be reacting to an El Nino when none took place that year; to the contrary, 84/85 included a La Nina. Also, there was a dip in Southern Tropical Atlantic SST early in 1997, before the 97/98 El Nino.
Smoothing the short-term data with a 12-month filter, Figure 4, helps illustrate two things. That early 1997 dip in temperature appears it might lose its novelty and looks like it might have been part of a possible periodic oscillation. The second noticeable effect revealed by the smoothed data is the shift (rise) in the Southern Tropical Atlantic SST trend after the 97/98 El Nino.
NORTHERN TROPICAL ATLANTIC
Figure 5 is a comparative graph of SST anomalies for the North Atlantic and for the Northern Tropical Atlantic. While the overall changes in trend agree with one another, it is clear that the one of the two data sets had an additional driver prior to the mid-1960s. They agree remarkably well afterwards. I have no explanation.
Figure 6 is the short-term data for the North Atlantic and for the Northern Tropical Atlantic. In recent years the two data sets coincide well. The Tropical data has the greater variation, which is expected for an oceanic data set with a smaller area.
In Figure 7, the Northern Tropical Atlantic data is compared to the coastal upwelling area off the Northwest coast of Africa. Those two data sets agree over the term of the data, with the upwelling area having exaggerated swings in SST. Since the Canary Current is from North to South along the West coast of Africa, the North Atlantic Upwelling area appears to drive the temperature of the Tropical North Atlantic. It must be recalled however that SSTs of those areas “upstream” of the upwelling area also impact the Tropical North Atlantic, as do the surface waters passing from the South Atlantic to the North Atlantic as part of the overall thermohaline circulation circuit.
The short-term comparison of SST anomalies for the Northern Tropical Atlantic and the North Atlantic Upwelling area is illustrated in Figure 8. Again, the signals in the smaller geographic region are not dampened by area, so the variations in the Upwelling area are greater. Note that the anomaly (spike) at 2002 is greater than the value at 1998 even though the 2002/2003 El Nino was significantly smaller than the 97/98 El Nino. Interesting.
SOUTHERN TROPICAL ATLANTIC
The SST anomalies for the South Atlantic and for the Southern Tropical Atlantic are compared in Figure 9. There are enough differences between the two data sets to suggest that the South Atlantic does not drive the Southern Tropical Atlantic.
The Southern Tropical Atlantic and the coastal upwelling area off the Southwest coast of Africa are compared in Figure 10. Those two data sets agree over the term of the data, with the upwelling area having exaggerated swings in SST. The Benguela Current would carry the waters from the upwelling area northward along the Southwest coast of Africa to the Tropical Southern Atlantic. The South Atlantic Upwelling area appears to drive the temperature of the Tropical South Atlantic. Again, it must be remembered that SSTs of those areas “upstream” of the upwelling area also impact the Tropical South Atlantic.
Figure 11 illustrates the short-term SST anomalies for the Southern Tropical Atlantic and the South Atlantic Upwelling area. Again, the signals in the smaller geographic region have greater variations. But note that the significant drop in SST immediately before the 1997/98 El Nino also appears in the upwelling data. As opposed to being a part of a possible oscillation as noted in the discussion of Figure 4, this time the plunge in SST appears anomalous. I’ll have to keep an eye out for that in other upwelling areas.
For information purposes only, the last graph, Figure 12, provides a comparison of the SST anomalies for the upwelling areas for the North and South Atlantic.
Sea Surface Temperature Data is Smith and Reynolds Extended Reconstructed SST (ERSST.v2) available through the NOAA National Operational Model Archive & Distribution System (NOMADS).