CMIP7 diagnostics t17d and t20d

[adcroft]

Two new diagnostics:

• t20d : Depth of 20 degree Celsius Isotherm airtable
• t17d : Depth of 17 degree Celsius Isotherm airtable

Both are requested monthly but t20d is also requested daily. These would best be done online so need new code.

[geoffstanley]

Hello @adcroft!

We're also adding these at CCCma, and the question arises of what to do when the isotherm exists at multiple depths (temperature is non-monotonic with depth). Curious if you'll use the shallowest or deepest (or...) isotherm depth? Is there a standard practice?

We use NEMO which has coded it for the deepest isotherm. Papers often don't say what they do, though I found Palmer and Haynes (2009) use the deepest depth also. My instinct is to use the shallowest depth, to more usefully measure thermocline depth.

[adcroft]

Hi @geoffstanley,

I'm also inclined to search for the deepest. Although unlikely for 17oC and 20oC, I was imagining diurnal forcing could cause the depth to jump back and forth between deep and shallow, and I'm assuming the interest in the diagnostic is about the longer term behavior. Given the two examples you gave, I'll think this can be considered decided!

[geoffstanley]

Ah, when you put it that way, of course we should use the deepest isotherm. The shallower ones are more likely to be the ephemeral ones. Thanks!