SS: Metocean: Toward an inertial current forecast system for the Gulf of Mexico

Abstract

Abstract Inertial currents (also known as inertial oscillations or inertial waves) are a well known ocean phenomenon commonly occurring in the Gulf of Mexico (GoM) as well as in other parts of the world. Inertial currents are driven primarily by wind and characterized by constantly changing direction with a rotation rate around 15 degrees per hour. Inertial currents routinely reach 1.5-2 kts amplitude (over 7 kts in hurricane conditions) and have a potentially deliterious impact on current-sensitive offshore drilling, anchoring, diving, and ROV operations. Presented in this paper is our recently developed operational Inertial Current Forecast System (ICFS) designed to predict the location and amplitude of inertial currents in the GoM. The accuracy of the ICFS was validated using hindcasts of inertial currents generated by hurricanes Katrina and Rita (2005) as well as several strong wind events in the GoM. Observational data for inertial currents during modeled events were obtained from a network of GPS-tracked drifting buoys routinely deployed in the GoM. A discussion of the validation results is presented. Introduction Inertial currents (also known as inertial oscillations or inertial waves) are a well known ocean phenomenon commonly occurring in the Gulf of Mexico (GoM) as well as in other parts of the world. Inertial currents are driven primarily by wind and characterized by constantly changing direction with a rotation rate around 15 degrees per hour. Inertial currents routinely reach 1.5-2 kts amplitude and have a potentially deleterious impact on current-sensitive offshore drilling, anchoring, diving, and ROV operations. In hurricane conditions the amplitude of inertial currents can exceed 7 kts. Furthermore, interaction of inertial currents with warm-core eddies typically leads to their accelerated downward propagation resulting in strong currents (in excess of 3 kts) at depth below 600 feet (see accompanying paper by Sergei Frolov). Currents of this magnitude not only disrupt offshore operations but also pose a significant threat to offshore structures. Horizon Marine, Inc. (HMI) and WeatherPredict Consulting (WPC) have been working on creating an ocean forecast system capable of predicting inertial current events. The system is built as an extension to the existing Eddy Forecast System that has been running operationally in partnership with WPC since 2004. The ocean model is extended to accurately simulate the effect of the wind forcing and dynamical processes occurring in the surface mixed layer. This extension is based on well-established technology previously developed by the University of Rhode Island, NOAA, and now in operational use by the National Weather Service. At present the system is primarily geared toward predicting inertial currents following hurricane events. This is because the wind forcing associated with hurricanes is relatively well known and also because hurricane-driven currents pose significant danger to offshore operations. Over the past few months, however, we have been working on implementing an operational system for forecasting inertial currents on a daily basis with a forecast horizon of 1-7 days.