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Wednesday, September 24, 2008

Tropical SST Anomalies Revisited - Introduction


In their recent paper “Limits on CO2 Climate Forcing from Recent Temperature Data of Earth”, Douglass and Christy state in their summary remarks, “An underlying temperature trend of 0.062±0.010 deg K/decade was estimated from data in the tropical latitude band. Corrections to this trend value from solar and aerosols climate forcings are estimated to be a fraction of this value. The trend expected from CO2 climate forcing is 0.070g degC/decade, where g is the gain due to any feedback. If the underlying trend is due to CO2 then g~1. Models giving values of g greater than 1 would need a negative climate forcing to partially cancel that from CO2. This negative forcing cannot be from aerosols.” The if statement, “If the underlying trend is due to CO2 then g~1,” drives the conclusion about the underlying trend. Suppose there are other easily observable variables that would reduce the CO2 contribution.

The intent of this post is not to define the values of the forcings, but to identify any oceanic oscillations that can be used to explain all or part that warming. I will post hemispheric tropical data for individual oceans and attempt to identify the sources of their variations in subsequent posts.


Figure 1 illustrates monthly Global and Tropical SST Anomaly data from January 1854 to August 2008. The data have been smoothed with 85-month running-average filters. There is a period of substantial difference between 1945 and 1975. During that time, tropical SST anomalies dipped well below global SST anomalies. Other than that, the two data sets correlate well.

Figure 1

In Figure 2, I’ve divided the data at the equator, creating Northern and Southern Tropical SST anomaly data sets. The dip from 1945 to 1975 was most prevalent in the Southern Hemisphere.
Figure 2

Recent short-term Tropical SST Anomaly data for both hemispheres is illustrated in Figure 3. Note that the Southern Hemisphere responses to the two major El Nino events are larger than the Northern Hemisphere. Does this indicate a difference in how the two El Nino events originated? Or does it reflect a difference in how the two hemispheres react?
Figure 3


In the tropics of the Northern Hemisphere, the Atlantic and Pacific Oceans are separated by Central America. Refer to Figure 4. And in the tropics for the Southern Hemisphere, the border between the Indian and Pacific Oceans is normally represented to include Sumatra, the Lesser Sunda Islands, then on to Australia.
Figure 4

These two borders do not lend themselves to being separated at single longitudes, which is required by the NOAA NOMADS system, the source of the data used in these Smith and Reynolds SST posts. Are the differences between the data between those oceans significant enough for concern? Or does a similarity with only one of the two oceans suggest that those areas be included in one but not the other data set? Or are should they be excluded so not to unduly influence either data? Only one way to find out.

Figure 5 shows SST anomalies for the Tropical North Atlantic, for the Tropical Northeast Pacific, and for Mixed Atlantic-Pacific Data, the area that separates the prior two. At this stage, the mixed data was not included in the Atlantic or Pacific data. There is enough difference between the three data that I’ve elected to exclude the Mixed Atlantic-Pacific Data completely. I’ll just have to consider it if an unusual bias unveils itself in subsequent posts.
Figure 5

I went through the same process for the Indian, West Pacific, and the Mixed Indian-Pacific data sets for the Southern Tropics. Refer to Figure 6. While the Mixed data has greater variations than the Southern Tropical Indian Ocean data, a result possibly of the small geographic area, it’s clear that the Mixed data does follow the underlying trends of the Indian Ocean data. This is further illustrated by the short-term data in Figure 7. I will include the Mixed data with the Indian Ocean.
Figure 6

Figure 7

Figure 8 is just a graphic with notes to document those conclusions.
Figure 8


The last illustration, Figure 9, is the global Mercator projection that shows the areas to be looked at in the following posts and their coordinates.
Figure 9


Sea Surface Temperature Data is Smith and Reynolds Extended Reconstructed SST (ERSST.v2) available through the NOAA National Operational Model Archive & Distribution System (NOMADS).

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