clipping sea ice tendencies

[szy21]

Purpose

In Holloway and Manabe 1971 it says

If the solution to this heat balance equation results in a value of T* above freezing, this value is set back to freezing with the implicit assumption that the excess heat is used for melting pack ice.

The current code sets the tendency to zero if it results in T above freezing, which differs from the paper. I change it to clip the tendency to (T_freeze - T) / dt, which should match the paper more closely. I also remove a comment about sublimation which doesn't seem to be true in the code.

Edit: One out of four simulations break with dt 240 in the latest nightly build: https://buildkite.com/clima/climacoupler-coarse-nightly-amip/builds/73. So I change dt back to 180 in this PR, and change t_end to one and a half years.

To-do

I will revert the change in nightly pipeline before merging.

Content

---

• I have read and checked the items on the review checklist.

[szy21]

Update: One out of four simulations break with dt 240 in the latest nightly build: https://buildkite.com/clima/climacoupler-coarse-nightly-amip/builds/73. So I change dt back to 180 in this PR, and change t_end to one and a half years.