Impacts of Different Warm-Layer, Cool-Skin Parameterizations on Seasonal-Length Coupled UFS Simulations

NOAA is in the process of developing a new global coupled Earth system model for seasonal-length (1-year) forecast horizons; this new modeling system will use the Unified Forecast System (UFS) framework. The current configuration of the ocean model has a vertical resolution of 2 meters near the surface, which can only partially resolve the near-surface diurnal warm layer. Properly representing the effects of both the diurnal warm layer, and the interfacial “cool skin”, of the upper ocean is essential to obtain the best representation of sea-surface temperature (SST). Because this variable is used for atmospheric calculations of surface fluxes of sensible/latent heat and radiation, SST is critical for driving large-scale circulations of the Earth system (including, but not limited to, ENSO).

The present research introduces a new parameterization scheme for diagnosing SST within a coupled atmosphere/ocean model, and compares it to the current default (control) scheme (which was originally designed for uncoupled, atmosphere-only models). Specifically, the two SST schemes are compared head-to-head through 10 sets of runs (initial conditions every May 1 from 2010 through 2019) integrated out to lead times of 12 months. Preliminary results have already shown that the new parameterization scheme provides a more physically realistic relationship between the uppermost prognostic ocean layer temperature and SST, relative to the control. The impact of changing SST parameterization scheme will be looked at in the context of SST biases, ability to simulate ENSO, and other variables including but not limited to precipitation.