Guest Post by Willis Eschenbach
When thick, the surface layer of the ocean acts as a buffer to extreme marine heating–but a new study from the University of Colorado Boulder shows this “mixed layer” is becoming shallower each year.
Those who know me know that I take the UK Royal Society’s motto, “Nullius In Verba”, very seriously. So I went off to find out what I could about the mixed layer.
I figured that my best bet was the data from the Argo floats. These are amazing man-made ocean-dwelling subsurface creatures. They sleep down 1000 metres under the surface. Then every nine days, they wake up, sink down another 1000 metres, then slowly rise to the surface collecting data. Once on the surface, they call home like ET, report their findings, and then sink down again and go back to sleep.
Now, when I looked at the Argo float mixed-layer data, what I found was a curious thing. I hadn’t known that there are no less than four different methods for calculating the mixed layer depth— the temperature threshold, temperature algorithm, density threshold, and density algorithm methods. Always more to learn. The threshold methods are described here, and the algorithm methods are described here.
With that as a preface, here are the global results for the two threshold methods.
And here are the results for the two algorithm methods.
I note that the algorithmic methods give shallower average mixed layer depths than the threshold methods. And the range between the four methods is about ten metres out of sixty or so, very large.
So … is the mixed layer getting shallower? Well, it depends on which method you prefer. Here are the trends for threshold methods.
Note that before about 2004, there weren’t enough Argo floats deployed to give us good data. Looking at the post-2004 data, neither one of the threshold methods shows any trend. Next, here are the algorithm methods.
Ya pays yer money and ya takes yer choices … the algorithmic methods both show a slight decrease in the mixed layer depth of about 250 mm/year. However, after allowing for autocorrelation neither of the algorithmic methods shows a statistically significant change.
Next, is the claimed shallowing of the mixed layer due to the slight rise in temperature? Unknown. The authors of the study say:
… it is clear that large internal climate variability complicates the detectability of the forced MLD signal in observations
And speaking of “large internal climate variability” here is another look at the density algorithm method, this time with a gaussian smooth instead of a trend line:
You can see that the mixed layer depth got slightly deeper from 2004 to 2011, shallowed until 2016, and has deepened slightly since then … internal variability indeed.
Net result? I’d say that we don’t have enough data to say either that the mixed layer is shallowing, or that the cause is the slight ongoing global warming.
Lovely rain here, best of a wonderful world to everyone.
PS: As is my wont, I ask that when you comment you quote the exact words you are discussing, so we can all know both who and what you are talking about.
via Watts Up With That?
February 2, 2021 at 12:12PM