Modeling internal tides over Fieberling Guyot: resolution, parameterization, performance

Abstract

Terrain-following ocean models are being used to simulate baroclinic tides and provide estimates of the tidal fields for circulation and mixing studies. These models have successfully reproduced elevations with most of the remaining inaccuracies attributed to topographic errors; however, the replication of barotropic and baroclinic velocity fields has not been as robust. Part of the problem is the lack of an adequate observational dataset in the simulated regions to compare the models. This problem was addressed using a dataset collected during the Flow over Abrupt Topography initiative at Fieberling Guyot. To evaluate the capability of the Regional Ocean Model System (ROMS) to simulate baroclinic tidal velocities, the combined tides for four constituents, M 2 , S 2 , K 1 , and O 1 , were modeled over Fieberling Guyot. Model inputs, numerical schemes, and parameterizations were varied to improve agreement with observations. These included hydrography, horizontal resolution, and the vertical mixing parameterization. Other factors were evaluated but are not included in this paper. With the best case, semidiurnal baroclinic tides were well replicated with RMS differences between the model estimates and the observations of 1.85 and 0.60 cm s −1 for the major axes of the tidal ellipses for M 2 and S 2 , respectively. However, diurnal K 1 baroclinic tides were poorly simulated with RMS differences of 4.49 cm s −1 . In the simulations, the K 1 baroclinic tides remained bottom-trapped unlike the observed fields, which had free waves due to the contribution of the mean velocity to the potential vorticity. The model did not adequately simulate the mean velocity, and the K 1 tides remained trapped. A resolution of 1 km most accurately reproduced the major axes and mean velocities; however, a 4-km resolution was sufficient for a qualitative estimate of where baroclinic tidal generation occurred. Nine vertical mixing parameterizations were compared. The vertical mixing parameterization was found to have minor effects on the velocity fields, with most effects occurring over the crown of guyot and in the lower water column; however, it had dramatic effects on the estimation of vertical diffusivity of temperature. Although there was no definitive best performer for the vertical mixing parameterization, several parameterizations could be eliminated based on comparison of the vertical diffusivity estimates with observations. The best performers were Mellor-Yamada and three generic length scale schemes.

Interactions between tides and other frequencies in the Indonesian seas

“…As previously mentioned, these mean currents represent only the tidal contribution to the mean and not the true mean currents resulting from the other driving forces or the observed mean currents. A prior evaluation of the effect of horizontal resolution on velocities shows that at the 5-km resolution used in these simulations, mean velocities are often overestimated (Robertson 2006), probably due to topographic differences and sigma-coordinate considerations. Accurate estimation of the mean velocities requires a horizontal resolution of 1 km, which is beyond the present capabilities when covering the entire region.…”

“…Vertical mixing is determined according to the generic length scale parameterization of Umlauf and Burchard (2003) using their original generic option (with m=1, n=−0.67, and p=1; Warner et al 2005). These options were selected based on the results of a sensitivity study on model operating parameters for a baroclinic tidal simulation of the region around Fieberling Guyot (Robertson 2006). Since ROMS is hydrostatic, internal solutions are not simulated.…”

“…3-D tides are simulated globally by Schiller (2004), Simmons et al (2004), and Simmons (2008) and regionally by Niwa and Hibiya (2001) for the Pacific Ocean and by Hatayama et al (1996), Hatayama (2004), and Ffield (2005, 2008) for the Indonesian seas. Since internal tidal wavelengths can be on the order of 20 km in this region, models should have a resolution finer than 5 km to resolve the internal tidal signals (Robertson 2006). Schiller's (2004) simulation was global with eight tidal constituents and intended to resolve barotropic tides, but not baroclinic tides, despite being 3-D. Simmons et al (2004) developed a global simulation to evaluate the conversion of M 2 barotropic tidal energy to baroclinic tidal energy.…”

Tidal effects on ichthyoplankton aggregation and dispersion in the Southern Gulf of Mexico

“…The Regional Ocean Model System (ROMS) has been used to model internal tides and to estimate tidal fields for studying circulation and mixing. According to Robertson (2006), the semidiurnal baroclinic tides are well simulated with ROMS. In this study the ROMS was set up following Moore et al (2004).…”

Studies of internal tide generation at a slope with nonlinear and linearized simulations: Dynamics and implications for ocean acoustics

“…The horizontal grid increments and the quantity of terrainfollowing vertical layers are known to affect the model fields [7]. Convergence studies were performed The majority of the energy is in the fundamental M2 harmonic, but the sensitivity of the overall full-bandwidth response and the resultant internal wave field that appears on the continental slope and shelf is somewhat sensitive to model parameters.…”