Optimizing coupled boundary layer process studies in the tropical Pacific using high resolution models and in situ observations

We propose to identify and study the physical mechanisms that play a key role in the evolution of the coupled boundary layer and air-sea interaction in the tropical west Pacific region (eastern edge of the warm pool [WPEE]). We will identify optimal strategies to observe these processes by isolating the coupled and uncoupled processes using short-term high resolution coupled and uncoupled regional model experiments and observing system simulation experiments (OSSEs). Current forecasts of the MJO and ENSO experience a systematic error (climate drift) that results in sustained biases of the model tropical western Pacific climatology. Salt-stratified barrier layers are persistent in the warm-fresh pool of the western Pacific. Correct modeling of the presence, location, and thickness of the barrier layer in the WPEE is critical for getting coupled air-sea processes right on MJO and ENSO time scales. This is because barrier layers can mediate the intensity of SST-wind-precipitation coupling in the region by inhibiting entrainment of cooler water from below and trapping solar radiation and wind momentum into the thin surface layer. This changing background state redefines the anomalous state, which is the target for subseasonal and interannual forecasts. The time scale over which this error develops depends on the model framework and the physical parameterizations. We propose to identify key processes and structures/characteristics that need increased observations (and at what temporal, horizontal, and vertical scales) to help improve our simulation of the coupled boundary layer in the region. Some of the processes we will focus on are:
● Barrier layer formation mechanisms (tilting, advection, stretching, buoyancy production vs shear-driven mixing) and their time/space evolution
● Understanding momentum, heat, and salinity budgets in the WPEE
● Coupled air-sea fluxes (including convection) east/west of SST front at the WPEE.
We will first survey existing observations and reanalyses to compute statistical analyses of the different time periods and air-sea interaction regimes (e.g. wind and precipitation conditions, subsurface oceanic stratification, etc), to identify the optimal conditions under which to observe the processes of interest. We will then perform analysis of the CMIP6 models and observation sensitivity experiments using the ECMWF Forecast System to study these regimes of interest in the WPEE region and focus on the processes of interest. We will then configure and run a high resolution (~10 km) and very high resolution (~1 km) nested coupled model in the Tropical Pacific basin to sample (OSSE) the region just as an observationally-based process field campaign would with virtual observational assets.
The proposed activity will help to address fundamental questions at the heart of the CVP call. Where do we need to know the ocean vertical structure versus being able to rely on surface conditions measured by satellites? Where do we need the highest density of Argo floats and gliders (that provide stratification) to detect barrier layers? We will work closely with the rest of the NOAA CVP and TPOS process study teams to utilize the tools we develop and optimize the design of the process study for success.