or collective redistirbution of any portion article of any by of this or collective redistirbution Th THE INDONESIAN SEAS articleis has been in published M2 Oceanography , Volume 18, Number journal of Th 4, a quarterly , Volume BAROCLINIC permitted only w is photocopy machine, reposting, means or other TIDES in the Indonesian Seas BY ROBIN ROBERTSON AND AMY FFIELD 2005 by Th e Oceanography Copyright Society. Baroclinic, or internal, Ocean. We used a tidal model to simulate in depths less than 2000 m where the tides play a signifi cant the barotropic and baroclinic tides in the internal tidal wavelength ranges from role in mixing in the deep Indonesian seas to verify model perfor- ~20–50 km. Grid cells of 4–5 km or of Th approval the ith ocean and in shallow mance against observations and to pro- fi ner are required to resolve the internal seas (Munk and Wunsch, 1998; Garrett, vide examples of baroclinic tidal activity. wavelengths in shallow water (Holloway, 2003). In a stratifi ed ocean, when the ver- Although internal tides have been ob- 2001). In a study for Fieberling Guyot gran e Oceanography is Society. All rights reserved. Permission tically uniform horizontal velocities of served in the Indonesian seas, the baro- (Robertson, submitted), a resolution of or Th e Oceanography [email protected] Society. Send to: all correspondence barotropic tides (tides in which surfaces clinic tidal fi elds are not well known. In 1 km was required to accurately predict of constant pressure are parallel to sur- particular, relatively few full-depth, long- mean currents and major axis ampli- faces of constant density) interact with period current observations suitable for tudes. In the Fieberling Guyot study, rough topography, disturbing isotherms accurately quantifying the baroclinic model performance improved with high- and isopycnals, they generate baroclinic tidal fi elds exist. More complete coverage er-resolution (< 1 km) bathymetry and tides, for which the velocities are not ver- has been provided by modeling stud- more vertical layers; however, a resolu- tically uniform. Currents, internal waves, ies; good replication of the barotropic tion of ~5 km was suffi cient for indicat- and heaving isotherms resulting from fi elds has been obtained by Mazzega and ing where baroclinic tides occur. baroclinic tides affect forces on structures Bergé (1994), Hatayama et al. (1996), In this study, our goal is to estimate ted to copy this article Repu for use copy this and research. to in teaching ted and vessels. Vigorous internal tides have and Egbert and Erofeeva (2002) us- the baroclinic tidal fi elds for the Indo- been observed in the Indonesian seas ing two-dimensional simulations. For nesian seas to provide baroclinic tidal e Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. with isotherm excursions up to 90 m in the baroclinic tides, three-dimensional information for future observational the Ceram Sea during 14-hour yo-yo sta- simulation is required to include vertical programs and focused regional modeling tions (Ffi eld and Gordon, 1996). Tides variability. Schiller (2004) and Simmons studies. Computing limitations require also affect the generation of mean cur- et al. (2004) simulated the baroclinic a balance between geographic coverage rents and mixing. Mixing processes mod- tides in the Indonesian seas on coarse and model performance, which is reso- ify the ocean’s hydrography, or physical grids (~0.5° in latitude and longitude, or lution dependent. For our overview, we characteristics, leading to density-driven ~50 km) using a z-level model (Modular selected a resolution of 5 km in order to fl ows. In the Indonesian seas, mixing Ocean Model [MOM]) and an isopyc- include the entire Indonesian seas re- blication, systemmatic reproduction, reproduction, systemmatic blication, transforms the Pacifi c infl ow waters into nal model, respectively. However, the gion, realizing that this would be a more Indonesian throughfl ow waters (Gordon, grid cell sizes of these simulations are qualitative estimate than an accurate this issue) before export to the Indian inadequate to resolve the internal tides quantitative estimate. 62 Oceanography Vol. 18, No. 4, Dec. 2005 ...our goal is to estimate the baroclinic tidal fi elds for the Indonesian seas to provide baroclinic tidal information for future observational programs and focused regional modeling studies. Oceanography Vol. 18, No. 4, Dec. 2005 63 TIDALLY INDUCED EFFECTS box in Figure 1a) covers much of the included in the domain and much of the Interactions of the tides with bottom to- Indonesian Seas with a horizontal resolu- region adjacent to the boundaries will be pography affect circulation and mixing, tion of 5 km (Figure 1b) and 24 vertical ignored, due to the infl uence of model both as mean and oscillating effects. Our levels. Realistic bathymetry was taken boundary effects. focus is the latter. In a stratifi ed ocean, from Smith and Sandwell (1997) using internal waves can be generated when the 0 m isobath as the coastline; water TIDAL OBSERVATIONS the tide interacts with topography and depths shallower than 50 m were deep- Model performance evaluation requires the tidal frequency [ω] falls between the ened to 50 m. Smoothing was applied us- observations of both elevation (i.e., sea inertial frequency [f] and the buoyancy ing repeated Gaussian weighting in areas level) and velocities at multiple depths. frequency [N] (i.e., equatorward of the that exceeded the steepness tolerance of Thirteen TOPEX/Poseidon (T/P) satel- critical latitude). The critical latitude is the model. Hydrographic fi elds, potential lite crossover observations of elevations defi ned as the location where the iner- temperature, and salinity for the domain (yellow crosses in Figure 1a and Table 1) tial frequency equals the tidal frequency: 28–30o latitude for the diurnal constitu- o ents and 74.5–86 latitude for the semidi- For more accurate quantitative estimates, higher urnals. According to linear internal wave theory, the strength of internal-wave gen- horizontal and vertical resolutions are required. eration is dependent on the stratifi cation characterized by the buoyancy frequency, the inertial frequency, and the steepness were optimally interpolated from obser- and four moorings with velocities ob- of the bathymetry with the strongest vational profi les in the region obtained servations at multiple depths (red stars generation, both in amount and magni- from the National Oceanographic Data in Figure 1) were used. The T/P cross- tude, occurring when the steepness of the Center. Tidal forcing focused on a single over points of elevation observations bathymetry is roughly equivalent to the constituent, the M2 tide with a period were provided as amplitudes and phase slope of the internal wave characteristics: of 12.42 hrs, and was implemented by lags from the PATHFINDER data base prescribing the M elevations along the (contact R. Ray, [email protected]. ω2 − 2 2 f open boundaries using TPXO6.2 results nasa.gov). Current-meter mooring data N 2 −ω2 (Egbert and Erofeeva, 2002). Flather ra- from the two Makassar moorings were In the Indonesian seas, this criterion diative boundary conditions were used analyzed for tides using the T_Tide soft- predicts internal tides generation by for the normal 2-D velocities, advective ware (Pawlowicz et al., 2002), which steep topography at both diurnal and conditions for the 2-D tangential veloci- also provided estimates of the observa- semidiurnal frequencies over much of ties, Martinsen and Engedahl (1987) fl ow tional uncertainty. the domain. relaxation over four cells for the 3-D velocities, and relaxation over four cells ELEVATIONS TIDAL MODEL for the tracers. There were no surface or Model estimates of the M2 tidal eleva- We simulated tides using a primitive- bottom fl uxes of tracers and the volume tion amplitudes range from 20–50 cm equation, terrain-following coordinate was unconstrained. A fuller description model, the Regional Ocean Model System of the model, boundary conditions, and Robin Robertson ([email protected] (ROMS) (http://marine.rutgers.edu/po/ forcing can be found in Robertson et al. bia.edu) is Doherty Associate Research Sci- index.php?model=roms). This model has (2003). The model was run for fi ve days entist, Lamont-Doherty Earth Observatory, been used for simulating tides for various before data were saved for analysis, which Palisades, NY, USA. Amy Ffi eld is Senior regions (Robertson et al., 2003; Robert- was suffi cient for the energies to stabilize. Scientist, Earth & Space Research, Upper son, 2005a, 2005b). The domain (yellow The small portion of the Sulu Sea that is Grandview, NY, USA. 64 Oceanography Vol. 18, No. 4, Dec. 2005 Figure 1. (a) Th e bathymetry of the Indonesian Seas with some of the major features identifi ed. Th e model domain is indicated by a yellow dashed box and the locations of TOPEX/Poseidon crossover elevation observations by yellow crosses. Current meter mooring locations in the Makassar Strait and Maluku and Halmahera Seas are marked with red stars. (b) Th e bathymetry over the model domain. Transect locations of Figure 4 are indicated by yellow dashed lines with a red cross at the origin. Table 1. M2 elevation amplitude and phase lags comparison between T/P crossover observations and model estimates at the locations shown as yellow crosses in Figure 1a. Th e diff erences and rms of the diff erences are given for both the amplitudes and phase lags. Site Latitude Longitude M2 Amplitude (cm) Phase Lag (o) Obs. ROMS Diff . Obs. ROMS Diff . 1 5° 56’ N 121° 53’E 59.6 66 +6 290 298 +8 2 2° 9’ N 120° 28’E 58.9 71 +12 290 300 +10 3 2° 2’ N 123°17’ E 57.2 65 +8 291 297 +6 4 2° 1’ N 128°58’ E 49.3 46 -3 288 279 -9 5 2° 3’ S 127° 33’E 25.6 21 -5 160 165 +5 6 2° 9’ S 119° 3’ E 46.4 46 0 277 298 +21 7 5° 54’ S 131° 49’E 58.5 51 -8 139 154 +15 8 5° 58’ S 114° 48’E 13.4 19 +6 140 126 -14 9 9° 45’ S 127° 33’ E 62.2 51 -11 120 125 +5 10 9° 45’ S 119° 3’ E 83.1 91 +8 54 66 +12 11 9° 46’ S 133° 13’E 43.5 101 +57 189 189 0 12 13° 30’S 126° 8’E 84.7 68 -15 62 34 -28 13 13° 31’S 120° 28’E 89.7 94 +4 56 66 +10 Rms differences: all sites 18 13 Rms differences: excluding site 11 8 13 Oceanography Vol.