In this study, a coupled atmosphere-ocean wave modeling system is used to simulate air-sea interaction under high wind conditions. This coupled modeling system is made of three well-tested model components: The Pennsylvania State University-National Center for Atmospheric Research regional atmospheric Mesoscale Model, the University of Colorado version of the Princeton Ocean Model, and the ocean surface gravity wave model developed by the Wave Model Development and Implementation Group. The ocean model is initialized using a 9-month spinup simulation forced by 6-hourly wind stresses and with assimilation of satellite sea surface temperature (SST) and altimetric data into the model. The wave model is initialized using a zero wave state. The scenario in which the study is carried out is the intensification of a simulated hurricane passing over the Gulf of Mexico. The focus of the study is to evaluate the impact of sea spray, mixing in the upper ocean, warm-core oceanic eddies shed by the Gulf Loop Current, and the sea surface wave field on hurricane development, especially the intensity. The results from the experiments with and without sea spray show that the inclusion of sea spray evaporation can significantly increase hurricane intensity in a coupled air-sea model when the part of the spray that evaporates is only a small fraction of the total spray mass. In this case the heat required for spray evaporation comes from the ocean. When the fraction of sea spray that evaporates increases, so that the evaporation extracts heat from the atmosphere and cools the lower atmospheric boundary layer, the impact of sea spray evaporation on increasing hurricane intensity diminishes. It is shown that the development of the simulated hurricane is dependent on the location and size of a warm-core anticyclonic eddy shed by the Loop Current. The eddy affects the timing, rate, and duration of hurricane intensification. This dependence occurs in part due to changes in the translation speed of the hurricane, with a slower-moving hurricane being more sensitive to a warm-core eddy. The feedback from the SST change in the wake of the simulated hurricane is negative so that a reduction of SST results in a weaker-simulated hurricane than that produced when SST is held unchanged during the simulation. The degree of surface cooling is strongly dependent on the initial oceanic mixed layer (OML) depth. It is also found in this study that in order to obtain a realistic thermodynamic state of the upper ocean and not distort the evolution of the OML structure during data assimilation, care must be taken in the data assimilation procedure so as not to interfere with the turbulent dynamics of the OML. Compared with the sensitivity to the initial OML depth and the location and intensity of the warm eddy associated with the loop current, the model is found to be less sensitive to the wave-age-dependent roughness length.

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The development and implementation of a real-time ocean forecast system based on the Regional Ocean Modeling System (ROMS) off the coast of central California are described. The ROMS configuration consists of three nested modeling domains with increasing spatial resolutions: the US West coastal ocean at 15-km resolution, the central California coastal ocean at 5 km, and the Monterey Bay region at 1.5 km. All three nested models have 32 vertical sigma (or terrain-following) layers and were integrated in conjunction with a three-dimensional variational data assimilation algorithm (3DVAR) to produce snapshots of the ocean state every 6 h (the reanalysis) and 48-h forecasts once a day. This ROMS forecast system was operated in real time during the field experiment known as the Autonomous Ocean Sampling Network (AOSN-II) in August 2003. After the field experiment, a number of improvements were made to the ROMS forecast system: more data were added in the reanalysis with more careful quality control procedures, improvements were made in the data assimilation scheme, as well as model surface and side boundary conditions.The results from the ROMS reanalysis are presented here. The ROMS reanalysis is first compared with the assimilated data as a consistency check. An evaluation of the ROMS reanalysis against the independent measurements that are not assimilated into the model is then presented. This evaluation shows the mean differences in temperature and salinity between reanalysis and observations to be less than 1 °C and 0.2 psu (practical salinity unit), respectively, with root-mean-square (RMS) differences of less than 1.5 °C and 0.25 psu. Qualitative agreement is found between independent current measurements and the ROMS reanalysis. The agreement is particularly good for the vertically integrated current along the offshore glider tracks: the ROMS reanalysis can realistically reproduce the poleward California Undercurrent. Reasonably good agreement is found in the spatial patterns of the surface current as measured by high-frequency (HF) radars. Preliminary results concerning the ROMS forecast skill and predictability are also presented. Future plans to improve the ROMS forecast system with a particular focus on assimilation of HF radar current measurements are discussed.

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Accurate real time estimates of the state of an oceanic regionare useful to many, including the offshore industry. The advent of precision altimetry, combined with the readily available remotely sensed data from altimeters and radiometers has made real-time oceanic nowcast/forecasts a reality, and realistic coupled ocean-atmosphere model simulations a possibility. The fact that such real-time capability was not on hand even a year ago testifies to the rapid advances made in this field in the past year or so, and to the unqualified success of the NASA TOPEX/Poseidon (T/P) altimetric mission. With Jason mission followon to T/P, it is likely that this capability will be available into the foreseeable future. Here, in this paper, we will present results of hindcasts and real-time nowcast/forecasts for the Gulf of Mexico, based on a comprehensive ocean model driven by numerical weather prediction (NWP) winds and assimilating multichannel sea surface temperature (MCSST) and altimetric sea surface height (SSH) anomalies. The examples discussed include eddy Aggie in 1995, eddies Deviant and El Dorado in 1997 and ongoing real-time high resolution nowcast/forecasts in 1998–99. These examples illustrate that there now exists a capability that could be of great potential utility to offshore operations in the Gulf, and by extension, elsewhere around the world. Introduction The current decade has seen enormous strides in our ability to observe and monitor the oceans. Advanced telemetering in-situ devices, such as PALACE floats, autonomous underwater vehicles (AUV) and long-term moorings, are providing us with the capability for intense but limited sampling of the oceans. On the other hand, satellite-borne sensors are giving us the ability to monitor continuously many surface properties over most of the global ocean. Of these, altimeters, radiometers and color sensors have proven especially useful. As is evident from the recent 1997-98 El Nino, the combined suite of satellite and in-situ sensors has been very helpful in interpreting and even forecasting the evolution of such important oceanic processes. Nevertheless, estimating and monitoring the state of the global ocean including its interior, requires appealing to comprehensive ocean models assimilating these in-situ and remotely sensed data, and it is here that we will see rapid advances in the coming decade. Here, we provide an example of the hindcast/nowcast/forecast capabilities that ocean models and observing systems together are providing. The region is the Gulf of Mexico, from which nearly half of the US domestic oil is extracted, and where real-time nowcast/forecasts of currents in the water column are a valuable aid to the US offshore industry. Model-based Ocean Monitoring & Forecast System An important component of a numerical-model-based ocean monitoring and forecast system is the ocean model itself. The hope is that if properly done, dynamics will provide a means of interpolating and extrapolating observations temporally and spatially. The centerpiece of our nowcast/forecast system is a regional, relocatable, three-dimensional, primitive-equation-based, sigma-coordinate baroclinic circulation model, the CU version of Princeton Ocean Model1 (CUPOM). It includes free surface dynamics essential to littoral applications.

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Oceanographic conditions in the Gulf of Mexico have been extensively studied from the summer of 1999 through the fall of 2000. These studies included cradle to grave surveys of Juggernaut Eddy; surveys in the Caribbean by several consortia; oceanographic surveys in support of hurricane research in the Gulf; measurements of inflow/outflow through the Yucatan Strait; and regional nowcasts and hindcasts of circulation. Several interesting phenomena have been observed during this period, including energetic bottom currents in excess of 2 knots and bottom furrows on the continental slope; the intrusion of one of the strongest eddies in a decade into the north-central Gulf leases; and strong midwater-column currents in SE Ewing Bank (~140 cm/s). This paper describes the observational data available and summarizes ongoing efforts to understand the oceanographic conditions during this period. Eventually, it is hoped that these efforts will lead to a better estimation of offshore structure design currents; an understanding of the dynamical causes of the strong mid-water-column currents; determining whether there is a link between the strong bottom currents at the base of the Sigsbee Escarpment and the Loop Current; and assessing the skill of the regional forecast/nowcast/hindcast oceanographic models.