[ecco-support] ECCO Mixed Layer Depths criteria

[Menemenlis, Dimitris (329C)]

The Kara et al mixed layer definition unfortunately does not work well in salt-stratified mixed layers, i.e., polar regions.

> On May 2, 2017, at 8:49 AM, Martha Buckley <marthabuckley at gmail.com> wrote:
> 
> Regarding the different criteria in different regions, I thought that using a density equivalent of a temperature threshold took care of this to some degree since the thermal expansion coefficient is smaller at low temperatures.