The Layers of Meaning in Levitus

Guest Post by Willis Eschenbach

There have been a lot of electrons sacrificed on the altar of the discussion of the Levitus ocean heat content data. The oddity seems to be that the deep ocean is gaining heat faster than the upper ocean. Here’s a typical graphic showing the issue:

Figure 1. Changes in the ocean heat content for two layers, 0-700 metres and 0-2000 metres. Values are pentadal (5-year) centered averages. SOURCE: NOAA/NODC

This week I got to ruminating about this graphic, and a number of similar graphics I’d seen. And yesterday I realized that it wasn’t showing what I thought it was showing. Let me illustrate what I mean.

I’ll start with an overview of the oceanic heat content (OHC) of the three layers that are provided by NOAA. These cover 0-100, 0-700, and 0-2000 metres depth. Figure 2 shows that data.

Figure 2. Annual changes in oceanic heat content for the 0-100, 0-700, and 0-2000 metre layers. 

Now, my problem was that when I looked at graphs like Figures 1 & 2, I thought that the deepest layer was gaining heat the fastest. And there’s been a lot of discussion about how that could be, and much speculation about the reason for the big increase in the deeper layers from 2001 onwards.

But yesterday I thought hey, wait a minute … those layers of the ocean overlap! They are not separate layers, they all extend to the surface.  So what we’re seeing in the deep 2000 metre level data is to some extent affected by what’s happening in the other levels. Yeah, I know, I should have seen it earlier, but I’m not gonna pretend.

The good news is that we’re measuring ocean heat content (OHC), so it’s very different from temperature. We can simply subtract the changes in the 700 metre level OHC from the 2000 metre level OHC changes, and what is left is the change in heat content for the layer from 700 metres down to 2000 metres. Can’t do that with temperature. Figure 3 shows the same OHC data as in Figure 2, except split out into distinct and separate layers, at the same scale. as Figure 2.

Figure 3. Changes in oceanic heat content. The exact same data was used as in Figure 2, except it was split into three separate layers rather than three overlapping layers.

I was quite surprised by this result. Once I split the information up so that I could see the changes in each of the layers separately, much of the apparent change post-2001 disappeared. In Figure 2 there’s not a lot of change in 2001.

I also found it interesting that for most of the time covered by the study, all three layers picked up about the same amount of heat. Only in the last decade has the middle layer (100-700 m) picked up a bit more heat than the other two layers. I hadn’t expected them to warm and cool generally in unison as we see above.

Finally, I calculated the change in temperature for each of the levels. The nice thing about the ocean is that the temperature and heat content are mathematically related by the fact that it takes about 4 megajoules to warm a tonne of water by 1°C. This lets us convert from heat content to temperature and back as needed.

Remember that the three layers have very different volumes. So a terajoule of energy added to the shallow 0-100 metre layer will warm it more than the same terajoule of energy added to the more voluminous 700-2000 metre layer. Fortunately, NOAA also provided the ocean depths on a 1° x 1° grid, so we can calculate the volume of each of the layers. Once we know the volumes, we can calculate the temperature changes. Figure 4 shows the same data as in Figure 3, except expressed as a temperature change rather than as a change in heat content.

Figure 4. Measurements of ocean temperatures at the surface and three sub-surface layers.

There are several interesting things about this plot of the temperature measurements.

First, as one might hope, we have relatively good agreement between the sea surface temperature (SST) and topmost layer (0-100 m). However, the annual and inter-annual swings in the upper 100 metres are larger than those at the surface … which seems somewhat strange to me. I’d have expected the surface to change more than the bulk.

Next, as we’d expect, the nearer to the surface, the greater the changes in temperature.

Finally, I’ve marked the eruptions of El Chichon and Pinatubo for future reference. I’ll come back to this in a subsequent post. For now, note that there is no visible effect of the volcanoes on any of the five different measurements of ocean temperatures. As near as we can tell from these measurements, the effect of the volcanoes on the ocean was below the limit of detection.

So … now that we have had a better overview of what’s happening to the various layers under the sea, are the changes in ocean heat content surprising?

I’d say not particularly. Yes, the middle layer (100-700 m) started warming in 1995. And yes, the lower layer (700-2000 m) followed suit starting in about 2001. But neither of these seem particularly surprising. I don’t have any explanation for them, but they do not seem to be unusual. It is possible, for example, that they represent the sub-surface changes associated with the gradual shift of the Pacific Decadal Oscillation from the positive to the negative phase. We know very little about the ocean depths, including how much we’d expect them to vary. And the records are short, too short to even show two PDO changes. It is clear, however, that the changes in heat content are not caused by CO2, at least directly.

Finally, we have to consider that the changes in the deeper layers may be an artifact. One obvious possible source is the integration of the Argo data into the Levitus analysis. The first Argo floats went in during the early nineties, and were added progressively over the next two decades. It is at least conceivable that some or all of the recent changes in the deeper layers are an artifact of the change in measuring methods.

Best regards to all,

w.

NOTES:

The data is from NOAA , except the ERSST and HadISST data, which are from KNMI.

The NOAA ocean depth data is here.

The R code to extract and calculate the volumes for the various Levitus layers is here.