An oceanographic nowcast/forecast system for the Red Sea

We describe the application of a nowcast/forecast system for three-dimensional currents, temperature, and salinity to the Red Sea. The modeling system is constructed around a high-resolution (6×7 km) primitive-equation numerical circulation model with complete thermodynamics and an imbedded turbulence closure submodel. It is coupled to near-real-time databases containing meteorological forecasts and remotely sensed and in situ temperature and salinity data. The temperature and salinity data are ingested into the model daily using a nudged objective analysis technique. Because the Red Sea is a relatively narrow basin bounded by typically high and complex orography, a single-layer atmospheric boundary layer submodel has been used to increase the effective resolution of the original 40-km resolution meteorological fields by taking into account orographic steering of the low-level winds. In order to validate the modeling system, two very complete hydrographic surveys of the Red Sea were undertaken, and their results are described. Both the surveys and the modeling system nowcasts demonstrate that the circulation pattern of the Red Sea is variable and often composed of a series of eddies or subgyres, mainly anticyclones. Immediately before one survey, the winds tended to be along axis, while just before the second, they tended to be cross axis. The Red Sea circulation was much more eddy-like when the winds were cross-axis. By forcing the modeling system with and without orographically steered winds we were able to establish that the complex eddy structure is a response to a wind field which can be highly structured and vorticity rich because of interaction with the high and variable adjacent orography. When the winds are more along axis, there is less interaction with the adjacent orography, and consequently, there are fewer eddies in the Red Sea.