A blended blended layer – watts with it?

Guest contribution by Willis Eschenbach

In a recently published article on WUWT entitled “Claim: Heat waves in the ocean are becoming more intense and frequent”, reference was made to a study:

When the surface layer of the ocean is thick, it acts as a buffer for extreme ocean warming. However, a new study from the University of Colorado Boulder shows that this “mixed layer” is getting shallower from year to year.

Those who know me know that I take the British Royal Society’s motto “Nullius In Verba” very seriously. So I went out to see what I could do about the mixed shift.

I thought my best bet was the data from the Argo floats. These are amazing man-made underground creatures that live in oceans. They sleep 1000 meters below the surface. Then they wake up every nine days, descend another 1,000 meters, and then slowly rise to the surface to collect data. Once on the surface, like ET, they call home, report their results, and then sink back down and go back to sleep.

Now when I looked at the Argo Float Mixed Layer data, I found an odd thing. Little did I know that there are no fewer than four different methods for calculating the mixed layer depth – the methods for temperature threshold, temperature algorithm, density threshold and density algorithm. To learn more and more. The threshold methods are described here and the algorithm methods are described here.

With that foreword, the global results for the two threshold methods are listed here.

And here are the results for the two algorithm methods.

I find that the algorithmic methods give shallower average mixed layer depths than the threshold methods. And the range between the four methods is about ten meters by about sixty, very large.

So … is the mixed layer getting shallower? Well it depends which method you prefer. Here are the trends for threshold methods.

Note that before about 2004, not enough argo floats were provided to give us good data. For the post-2004 data, none of the threshold methods shows a trend. Next here are the algorithm methods.

You pay your money and you make your choice … both algorithmic methods show a slight decrease in the mixed layer depth of around 250 mm / year. However, after taking autocorrelation into account, none of the algorithmic methods shows a statistically significant change.

Next is the alleged flatness of the mixed layer due to the slight temperature rise? Unknown. The study’s authors say:

… It is clear that a large internal climate variability makes it difficult to detect the forced MLD signal during observations

And when we speak of “large internal climate variability” here, this is another look at the density algorithm method, this time with Gaussian smoothing instead of a trend line:

You can see the mixed layer depth got a little deeper from 2004 to 2011, was shallow until 2016, and has deepened slightly since then … internal variability indeed.

Net result? I would say we don’t have enough data to say that the mixed layer is shallow or that the cause is slightly sustained global warming.

Nice rain here, the best of a wonderful world for everyone.

w.

PS: As is my custom, when you comment, I ask you to quote the exact words you are talking about so that we can all know who and what you are talking about.

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