The NRL Layered Global Ocean Model (NLOM) with an Embedded Mixed Layer Submodel: Formulation and Tuning*

A bulk-type (modified Kraus-Turner) mixed layer model that is embedded within the Naval Research Laboratory (NRL) Layered Ocean Model (NLOM) is introduced. It is an independent submodel loosely coupled to NLOM's dynamical core, requiring only near-surface currents, the temperature just below the mixed layer, and an estimate of the stable mixed layer depth. Coupling is achieved by explicitly distributing atmospheric forcing across the mixed layer (which can span multiple dynamic layers), and by making the heat flux and thermal expansion of seawater dependent upon the mixed layer model's sea surface temperature (SST). An advantage of this approach is that the relative independence of the dynamical solution from the mixed layer allows the initial state for simulations with the mixed layer to be defined from existing near-global model simulations spun up from rest without a mixed layer (requiring many hundreds of model years). The goal is to use the mixed layer model in near-global multidecadal simulations with realistic 6-hourly atmospheric forcing from operational weather center archives. A minimum requirement therefore is that there be no drift in yearly average SST over time. The dynamical layer densities are relaxed to climatology as a standard part of the NLOM model design, and this ensures that the temperature just below the mixed layer provided to the mixed layer submodel does not drift. The density relaxation below the mixed layer does not significantly dampen anomalies even on interannual timescales because the anomalies are largely defined by layer thickness variations. When combined with calculating the latent and sensible heat flux using model SST, this is sufficient to keep SST on track without any explicit relaxation to the SST climatology.