1. Modeling the M 2 and O 1 Barotropic and Baroclinic Tides in the Gulf of Mexico Using the HYbrid Coordinate Ocean Model (HYCOM) Flavien Gouillon 1 ; B. Vanniere 2 ; A. Bozec 1 ; E. P. Chassignet 1 ; (gouillon@coaps.fsu.edu) 1 Center for Ocean-Atmospheric Predictions Studies, Florida State University, Tallahassee, FL, USA 2 Ecole Normale Supérieur de Lyon, Lyon, France 1)Objectives of the study Validation of the M 2 and O 1 barotropic tidal model implementation in the Gulf of Mexico using the HYbrid Coordinate Ocean Model (HYCOM) Investigate the importance of the tidal conversion process as a contributor to the barotropic tidal dissipation for a realistic stratified ocean 2) Model and Experiment Setup HYCOM is used to conduct all numerical experiments. It is run in fully isopycnal mode to avoid spurious diapycnal mixing. The model domain is shown in Figure 1. The horizontal resolution is 1/25°. The barotropic tides in a semi-enclosed basin are originating from a local tidal potential (celestial forces) described by Newton’s theory and also from the model open boundaries, by tidal barotropic transport and velocities derived from the Egbert and Ray model (2005). Flather (1979) conditions are applied at the model boundaries. The model is run 8 days after spin-up. Output are saved every hour. For the barotropic case, the model is run as a 1-layer model. For the baroclinic case, we use a mean GDEM3 climatology for the Gulf of Mexico from the month of January with 25 layers. Figure 1: Model Domain. Open boundaries are the dashed line, solid black line denotes the location of cross-vertical sections. 5) The baroclinic simulation of the M 2 tide The same configuration is run with a realistic stratification (Figure 4). The tidal signal is very similar to the barotropic configuration. 7) Conclusion The M 2 and O 1 barotropic tides in the Gulf of Mexico are accurately simulated using HYCOM. Over the whole basin, the baroclinic tidal energy is negligible. Baroclinic energy can be locally important (i.e. West Florida Shelf, Yucatan Peninsula). 6) The M 2 Baroclinic Energetics The basin baroclinic power is computed from Khatiwala (2003): P corresponds to 1/30,000 of the basin barotropic total power. Baroclinic energy is negligible. Internal wave structures exist in a stratified Gulf of Mexico (Fig. 5). The baroclinic velocities at the West Florida Shelf can reach 1 m.s -1. Baroclinic energy cannot be neglected in some locations. The tidal conversion is computed following Llewellyn Smith and Young (2001): The black solid dots in Figure 6 denote where C is maximum and topography is rough. Figure 6 shows the signature of the internal wave propagation within the Gulf of Mexico. Figure 5: Cross-Vertical section of zonal/meridional baroclinic velocities Figure 6: Depth integrated vertical velocity and max tidal conversion locations Figure 3: Barotropic tidal amplitudes and phases for M 2 a) and c), and O 1 b) and d) for HYCOM (left panel) and GOT99 (right panel) 3) Validation of the barotropic M 2 and O 1 tides using tidal gauges ~60 tidal gauges located along the Gulf of Mexico coast are used. Tidal amplitudes compare well against observations. A circular statistic approach is used in order to compare the phases. Circular correlation coefficients (ρ) are computed and are shown in Figure 2b. Figure 2: Scatter plot of the tidal amplitudes a) and histogram of the circular distances and phase bias b) Figure 4: M 2 Baroclinic tidal amplitudes and phases Internal wave beam 4) Validation of the barotropic M 2 and O 1 tides using GOT99 A comparison against GOT99 (observation by satellite altimetry measurements) is shown in Figure 3. The modeled barotropic tides compare well against the realistic ones. The M 2 modeled tidal amplitudes are slightly overestimated. The O 1 modeled tidal amplitudes are well simulated. Tidal phases are close to the observations however: The modeled M 2 tide leads the observed tide The modeled O 1 tidal wave is late compared to observation The dissipation that occurs through bottom friction at the shelf compares well with previous studies (~1.1 GW, Gouillon et al, 2008). Tidal energy fluxes at the open boundaries agree with previous studies: 300 m isobath With h the bathymetry, p’ the baroclinic pressure, U the baroclinic velocity, ω 0 is the tidal frequency With p B the baroclinic pressure, U the baroclinic velocity, ω 0 is the tidal frequency, ρ 0 the reference density Tidal ConstituentGouillon et al. (2008) (GW)This study (GW) M2M2 2.452.52 O1O1 0.720.21