Guest post by Willis Eschenbach
Bob Tisdale has discussed a variety of issues with the hemispheric and basis-by-basin Levitus summary of the ARGO data in his excellent post here on WUWT. I wanted to take a larger look at at the global ocean data, to provide it with some context. After following a variety of blind paths and dry holes, I arrived at Figure 1. This shows the Levitus data, along with several datasets for both the sea surface temperature (SST) and the land surface air temperatures.
Figure 1. Three land surface air temperature datasets, three ocean surface temperature datasets, and two measures of the deeper ocean. Datasets from KNMI, except the Levitus ocean data. Levitus data have been converted from ocean heat content values to temperature values.
Unfortunately, we only have pentadal (5-year centered average) data for the deeper ocean layer from 1955, no annual data for the deeper layer is available before 2005. So I have shown the other data as pentadal averages as well, so that they can be compared directly. Now, I have no profound conclusions from this, but there are some curiosities about these results.
The most obvious oddity involves the sea surface temperature data. Two of the datasets, the ERSST and the HadISST, are quite similar, but are offset over much of their length by an amount that varies in an odd stepwise fashion. In particular, they diverge significantly in the period 1997-2001. They have the same small peaks and valleys, but the land is going up while the sea temperature is dropping. Figure 2 shows this in more detail:
On the other hand, Figure 2 also shows that the ICOADS dataset runs almost exactly in parallel with the HadISST data up until 1997, when it shifts gears and goes up to agree with the ERSST dataset.
I have absolutely no idea what happened in 1997, or in earlier periods, which allowed the three datasets to vary so much. Always more questions than answers.
The next oddity involves the changes in the land and ocean surface temperatures. Figure 3 is like Figure 1 but leaves out the sub-surface temperatures:
The oddity here is two-fold: what happened around 1977 or so, and what happened around 2002 or so? From 1977 on, after twenty years with no significant land or sea temperature change, both land and sea temperatures start to rise. In addition, the land temperature rises much more rapidly than the SST. Then around 1998, SSTs start to level off, and they peak in 2002 and start to fall. The land temperatures start falling shortly thereafter. Why, on all counts?
Now, I don’t know the answer to these questions either. However, in this modern world there’s a term called a “WAG”, which means a wild-assed guess. However, scientists don’t provide those. Instead, there is a refined version called a “SWAG”, or a scientific wild-assed guess.
So here’s my SWAG about what we’re seeing here. I think at least some of this is the effect of the Pacific Decadal Oscillation, or PDO.
The PDO is a large-scale reorganization of the flow of the North Pacific Ocean. I think that, like other large-scale patterns such as the El Nino/La Nina alterations, these all function to regulate the temperature. It is obvious that one of the two major patterns (positive or negative PDO) must be more efficient than the other one at cooling the planet. The transfer of heat via the oceans from the equator to the poles is one of the two major pathways by which the tropics moves heat (the other is via the atmosphere). One of the two PDO states must be better at cooling the planet.
I hypothesize that this alteration between PDO states is one of the ways by which the planet maintains such a regular temperature. Unfortunately, I have no idea how to support that hypothesis using observational data.
A final oddity is the decoupling of the land temperatures from the ocean temperatures. Figure 4 shows a longer version of the same data, this time extending back to the 1870’s:
The oddity to me in Figure 4 is that since about 1974, the land temperatures have been rising much faster than the sea surface temperatures. The SST records and the land temperature records run quite close to each other up until about 1975 or so, but after that land temperatures have been rising much more quickly, while the sea surface temperatures (and to a greater extent the sub-surface temperatures) have not followed suit.
Once again, I’m out of explanations for the decoupling of the temperatures. Clearly it’s not CO2. Urban heat islands? Aerosols reducing the clouds over the land? I have no idea. All suggestions welcome. That’s the beauty of settled science, there are always unexplained questions to
Best regards to everyone,
Levitus provides two different global pentadal datasets. One covers the layer from 0 to 700 m depth, and the other covers a thicker layer, from 0 to 2000 m depth. I wanted to see what was happening in the “missing link”, which is the data covering the deeper layer, from 700m to 2000m depth.
The good news is that since we are dealing in oceanic heat content, the heat content of the lower layer is simply the difference between the two datasets.
My spreadsheet with the results is here.