Sources of baroclinic tidal energy in the Gaoping Submarine Canyon off southwestern Taiwan


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The vertically integrated Fbc (arrows) and Ebt2bc (colors) averaged over seven days of simulation from (a) the control run, (b) Exp1, (c) Exp2 and (d) Exp3. The blue rectangle enclosing the GPSC denotes the area for Ebt2bc and baroclinic energy integration. The blue open arrow and the number inside it indicate the sectional area-integrated Fbc in MW and the associated direction across the interface.

A three-dimensional model driven by tidal constituents O1, K1, M2 and S2 was adopted to evaluate the sources of baroclinic tidal energy in the Gaoping Submarine Canyon (GPSC) off southwestern Taiwan. The model domain covered the probable primary generation sites, including the Luzon Strait (LS) and the southeastern Taiwan Strait (TS). The simulated baroclinic tides agreed with the observations of tidal current velocity, isotherm vertical displacement, and baroclinic tidal energy flux (Fbc) in the GPSC. The depth-integrated, seven-day-averaged Fbc computed from the model result was 2.2 kW m-1 in the GPSC, and the corresponding area-integrated Fbc reached 189.4 MW. The results obtained from the model suggest that the baroclinic tides lead to strong turbulent mixing near the canyon head with a vertical diffusivity of 3.5×10-3 m2 s-1. Baroclinic tidal energy in the GPSC is mainly generated on the western ridge in the LS and on the steep topography in the southeastern TS. The local generation of baroclinic energy only accounts for 4.4% of the total value. The other 95.6% of the baroclinic tidal energy is remotely generated at the LS and the southeastern TS of which 31.9% and 8.8% are directly emanated, respectively, into the GPSC. The northwestward and southeastward baroclinic energy beams radiating from the LS and the southeastern TS, respectively, meet each other and form internal partial standing tides outside the GPSC. The transverse baroclinic energy from the internal partial standing tides accounts for the remaining 54.9% of the baroclinic tidal energy in the GPSC.

Chen, S.N., W. R. Geyer, D. K. Ralston, and J. A. Lerczak (2011) Estuarine exchange flow quantified with isohaline coordinates: contrasting long and short estuaries. Journal of Physical Oceanography, accepted.

We use the isohaline coordinate method to compare the exchange flow in two contrasting estuaries, the long (with respect to tidal excursion) Hudson and short Merrimack River, using validated numerical models. The isohaline method averages fluxes in salinity space rather than in physical space, yielding the isohaline exchange flow that incorporates both subtidal and tidal fluxes and precisely satisfies the Knudsen relation. The isohaline method can be consistently applied to both subtidally and tidally dominated estuaries. In the Hudson, the isohaline exchange flow is similar to results from the Eulerian analysis, and the conventional estuarine theory can be used to quantify the salt transport based on scaling with the baroclinic pressure gradient. In the Merrimack, the isohaline exchange flow is much larger than the Eulerian quantity, indicating the dominance of tidal salt flux. The exchange flow does not scale with the baroclinic pressure gradient but rather with tidal volume flux. This tidal exchange is driven by tidal pumping due to the jet-sink flow at the mouth constriction (Stommel and Farmer 1952), leading to a linear dependence of exchange flow on tidal volume flux. Finally, to characterize the exchange processes among different systems, we propose a tidal conversion parameter Qin/Qprism that measures the fraction of tidal inflow (Qprism) that is converted into net exchange (Qin). We also demonstrate that the length scale ratio between tidal excursion and salinity intrusion provides a characteristic to distinguish estuarine regimes.