Data Assimilation Experiments in the Gulf Stream Region: How Useful Are Satellite-Derived Surface Data for Nowcasting the Subsurface Fields?

Abstract

Satellite-derived surface data have become an important source of information for studies of the Gulf Stream system. The question of just how useful these datasets are for nowcasting the subsurface thermal fields, however, remains to be fully explored. Three types of surface data-sea surface temperature (SST), sea surface height (SSH), and Gulf Stream position (GSP)-are used here in a series of data assimilation experiments to test their usefulness when assimilated into a realistic primitive equation model. The U.S. Navy's analysis fields from the Optimal Thermal Interpolation System are used to simulate the surface data and to evaluate nowcast errors. Correlation factors between variations of the surface data and variations of the subsurface temperature are used to project the surface information into the deep ocean, using data and model error estimates and an optimal interpolation approach to blend model and observed fields. While assimilation of each surface data source shows some skill in nowcasting the subsurface fields (i.e., reducing errors compared to a control case without assimilation), SSH data reduce errors more effectively in middepths (around 500 m), and SST data reduce errors more effectively in the upper layers (above 100 m). Assimilation of GSP is effective in nowcasting the deep Gulf Stream, while the model dynamics produce eddies that are not included in the GSP analysis. An attempt to optimally combine SST and SSH data in the assimilation shows an improved skill at all depths compared to assimilation of each set of data separately.

A comparative study of satellite-based operational analyses and ship-based in-situ observations of sea surface temperatures over the eastern Canadian shelf

“…The satellite-derived SST data, for example, were used in assisting the tuna fishing industry along the south-western coast of Australia (Myers and Hick, 1990), improving the performance of ocean circulation models (Ezer and Mellor, 1997), and specifying lower boundary conditions for numerical weather forecast models (Brasnett, 2008). In comparison to the in-situ SST measurements established from ships or buoys, a major advantage of the satellite-based SST is the global coverage and near real-time availability.…”

The California Current System: A multiscale overview and the development of a feature-oriented regional modeling system (FORMS)

“…In this work we chose the terrain-following coordinate system for this implementation of the Princeton Ocean Model -POM (Ezer and Mellor, 1994;Mellor, 1987 andMellor, 1997 The model bathymetry is based on the ETOPO5 data set. The depth range is 5 to 6000 m.…”

“…Synoptic short-term simulations require three-dimensional specifications of initial fields of T, S, u and v. However, the standard practice for basin-scale numerical modeling exercises is to specify T and S only, with zero velocity (Ezer and Mellor, 1994;Ezer and Mellor, 1997;Mathew and McClean, 2005). The basin-scale approach requires models to develop the inertia fields in a one-to-ten years long "spin-up'' period.…”

“…The strategy for setting up POM in the CCS region was carried out using climatological temperature and salinity initial fields following Ezer and Mellor's (1997) methodology, with the additional implementation of geostrophic velocity as initial condition for the baroclinic velocities, balanced with initial T-S field. Specifically, we held the T and S fields fixed over the first five days of simulation in the diagnostic mode, an approach described by Ezer and Mellor (1997). These authors used this diagnostic mode, available in POM, to dynamically adjust the momentum field with the underlying mass field and to the topography.…”

Assimilation of sea-level anomalies and Argo data into HYCOM and its impact on the 24 hour forecasts in the western tropical and South Atlantic

“…The CHM/REMO system is based on HYCOM with 1/4°and 1/12°of horizontal resolution over the whole Atlantic and the South Atlantic Ocean, respectively, and 21 vertical hybrid layers. The CHM/REMO system assimilates SST and sea-level anomaly (SLA) from HYCOM+NCODA analyses with an OI scheme that rely on fixed co-variances between the surface and subsurface according to Ezer and Mellor (1997). These systems are in permanent development.…”