1. Model and experiment setup The Navy Coastal Ocean Model (NCOM) is used for numerical simulation of the sea level response to tidal forcing. The NCOM is a three-dimensional ocean model but In these simulations, it is run as a barotropic ocean model. The model domain is shown in Figure 1. The horizontal resolution is 1/60°. The model Open Boundaries (OB) are the Straits of Florida and the Caribbean Sea (Figure 1). Flather [1976] OB conditions are applied. The basin is initially at rest and there is no forcing but tides. Only the main four tidal constituents are considered (M 2, S 2, O 1, K 1 ) in this study as they represent 90% of the total tidal bulk [He and Weisberg, 2002]. Only M 2 and O 1 are shown here. Three model experiments are performed: 1) Model forced by a local tidal potential given by equation (1) and derived from Newton’s tidal theory (LTP experiment) (1) 2) Model forced only at OB by tidal barotropic transport and velocities derived from the western Atlantic ADCIRC model [Mukai et al, 2002] (OB experiment) 3) Both forcing mechanisms are combined (LTP&OB experiment) Figure 1: Model domain and tidal gauges Forced Tidal Response in the Gulf of Mexico Motivation Examination of the role of the astronomical forcing in governing the behavior of the tides and new tidal energetic estimates in the Gulf of Mexico Introduction The present study employs a non-assimilative numerical ocean model to understand the nature of tides in the Gulf of Mexico (GoM). A set of numerical experiments is conducted to yield new understanding of the tidal response due to a local tidal potential, tidal signal propagation coming from the Atlantic by considering forcing at the model open boundaries, and the modification of the propagating signal by combining both model forcing mechanisms. Comparison of the model results with observations and previous studies is conducted and new estimates of total tidal power and tidal energy fluxes for semidiurnal and diurnal constituents are produced from the model. Flavien Gouillon 1, Steven Morey 1, Dmitry Dukhovskoy 1, James J. O’Brien 1, (gouillon@coaps.fsu.edu) 1 Center for Ocean-Atmospheric Predictions Studies, Florida State University, Tallahassee, FL, USA Conclusion The LTP forcing alters the tidal signal propagating from the model OB especially for the semidiurnal constituent. The astronomical forcing needs to be taken into account for high resolution numerical studies of the GoM. A careful tidal energetic study provides new estimates of tidal power and tidal dissipation rates within the GoM and confirms that the GoM is acting as a major sink for both semidiurnal as well as diurnal tidal energy. Tidal EnergeticsTidal amplitudes and phases Figure 2: Tidal amplitudes in meters (contoured) and phases in degrees (colored) for all tidal constituents. OB experiment is left panel and LTP&OB is right panel. Figure 3: Comparison of tidal phasesFigure 4: Comparison of tidal amplitudes The sea surface elevation time series from each model experiment is analyzed using the T-Tide [Pawlowiscz et al, 2002] harmonic analysis utility to extract estimates of the phases and amplitude. Resulting maps are shown in Figure 2. Results for semidiurnal constituents are describe in part a) and diurnal constituents description are in part b). Figure 3 and 4 compare amplitudes and phases to observations from the tidal gauges shown in Figure 1. a) The LTP&OB experiment (Figure 2a) shows a good agreement with previous studies. For the semidiurnal constituent the amphidromic point is north of the Yucatan Peninsula (YP). The co-tidal lines become compacted in the central GoM, roughly following a line from the U.S. Gulf Coast to the YP. This is where the tidal pattern changes to become predominantly diurnal. The maximal tidal amplitudes are found in the wide shelves due to resonance phenomena [Clarke, 1995]. Considering only the OB forcing (Figure 2b), the amphidromic point is slightly shifted to the northeast and the tidal wave is traveling faster. b) The LTP&OB experiment for the diurnal experiment (Figure 2c) shows that phases are very uniform into the basin. Both entrances are at the same phase which show the co-oscillating phenomena. Combining the LTP forcing does not change the behavior of the diurnal tides (Figure 2d) except a slight decrease on the overall amplitude which will have an impact on the total diurnal tidal energy in the basin. A set of new numerical experiments is run to calculate tidal energy density maps, tidal power and energy fluxes for M 2 and O 1. Figure 5 shows the sum of the kinetic and potential energy per unit area, averaged over the tidal constituent period. Prominent features include an area of weak energy spatially corresponding with the amphidrome, which naturally has a zero elevation through time. The maximum energy is found to be at the shelf and confirms the tidal shelf amplification theory. Contours show that the two forcing mechanisms work constructively together to increase the tidal signal in some regions as well as destructively interfering to decrease the tidal elevation in other area. For O1, the energy density map is spatially uniform due mainly to the co-oscillating phenomena. Table 1 gives the M 2 and O 1 tidal power within the Gulf for each experiments. Energy fluxes are computed according to Kowalik [1993] and are given by (2) These fluxes are shown in Figure 6. The net tidal energy dissipation and their comparison with previous studies are given by the Table 2 Figure 5: Total energy density maps (colored on a logarithmic scale, units are J.m -2 ). Contours are the ratios of the tidal energy density for the LTP (black solid line) and OB (gray dashed line) over the tidal energy density of the LTP&OB experiment for M 2 (left panel) and O 1 (right panel). Table 1: Tidal power in GWTable 2: Net tidal energy dissipation in GW Figure 6: Energy fluxes in the GoM b) d) Clarke, Allan J., (1995), Northeastern Gulf of Mexico physical oceanography workshop; 417 proceedings of a workshop held in Tallahassee, Florida, April 5-7, 1994. Prepared by 418 Florida State University. OCS Study MMS 94-0044. U.S. Department of the Interior, 419 Minerals Management Services, Gulf of Mexico OCS Region, New Orleans, La. 420 257pp. Flather, R.A., (1976), A tidal model of the northwest European continental shelf. 425 Memoires de la societe Royale de Liege 6 (10), 141-162. Gouillon F., S. M. Morey, D. S. Dukhovskoy, J.J. O’Brien (2007), Forced Tidal response in the Gulf of Mexico, Journ. Geophys. Res, In Review.