Introduction

Current and temperature measurements obtained on a long-term mooring in 450 m water depth on the upper continental slope off northern California during STRATAFORM in 1997 reveal energetic tidal and higher frequency internal waves whose intensity and structure have considerable temporal variability. We describe these data and discuss implications of internal tidal wave dynamics for continental slope sedimentation and the generation of bottom and intermediate nepheloid layers in this region. A global model for the interaction of internal semi-diurnal tides and sediment deposition on continental slopes is presented in the context of these new results.

Evidence for intensified internal tidal flows over sloping oceanic boundaries has been mounting over the past 30 years. Time-series measurements of currents, temperature and salinity have documented significant energy levels of these flows over sloping topography, and field experiments have shown the intensification of these motions near the seafloor (Ericksen, 1982; Holloway and Barnes, 1998). The amplification of across-slope velocities associated with shoaling internal waves was originally proposed theoretically by Wunsch (1969), and more recently elucidated through numerical models that include non-linear and viscous effects (Slinn and Riley, 1996; Holloway and Barnes, 1998). Early laboratory studies of progressive internal waves over a linear bottom slope showed upslope amplification of the wave forms and near-bottom velocities for cases when the energy rays or characteristics were reflected upslope or were aligned parallel to the bottom slope (Cacchione and Wunsch, 1974). Later laboratory studies confirmed these results, and further elaborated on the nature of the turbulent boundary layer flows produced by the reflecting waves over the slope (Ivey and Nokes, 1989; Taylor, 1993).

Cacchione and Southard (1974) discussed the potential significance of shoaling internal waves for causing sediment movement on continental shelves and slopes. They proposed a simple model that predicted entrainment of natural sediment on shelves and slopes by internal waves. Laboratory experiments confirmed that shoaling interfacial waves could generate ripples and larger bedforms in artificial sediment (Southard and Cacchione, 1972). More recent studies also have found that internal waves of various types are potentially capable of resuspending and transporting sediment (Bogucki, et al., 1998).

Cacchione and Drake (1986) proposed a conceptual model for the generation and maintenance of bottom and intermediate nepheloid layers above continental shelves and slopes by turbulent shear caused by shoaling internal waves. This idea was also suggested by Dickson and McCave (1986) based on an analysis of transmissometer profiles on the continental margin west of Ireland. The latter study proposed that well-defined intermediate and bottom nepheloid layers which emanated from bottom slopes in 400 to 600 m depths were caused by bottom erosion under internal tides and higher frequency internal waves. They calculated that the bottom gradient was aligned with the slope of the energy ray for the semi-diurnal internal tide, leading to increased velocities and erosion of the bottom sediment. No corroborating data from direct observations or current measurements were available to support this conclusion.

Results

An instrumented mooring was deployed on the upper continental slope in 450 m water depth in the STRATAFORM field area off northern California. The mooring site is about 15 nm NW of Eureka, CA. Instrument clusters including temperature, salinity, current, and light transmission sensors were located on the mooring at 60, 180 and 435 m water depths. This mooring has been maintained at this site since September 1995. Locally the bottom slope has a gentle gradient of about 0.05 (2.8o), and the bottom is mantled with fine silt. The shelf break in this area is at 150-m depth. The analysis presented here uses data collected from January 18 (Day 18) until April 15 (Day 107), 1997.

Internal semi-diurnal currents measured at about 15 m above the seafloor during this period occasionally exceed 35 cm/s; these strong currents are correlated with considerable mixing above the seafloor as indicated by concurrent temperature records. During these periods of enhanced internal tidal flows, downslope-directed currents persist for longer durations than upslope currents, leading to net downslope transport over many tidal cycles. The most striking internal tidal event occurred during the latter ten days of the deployment period. Upslope flows were peaked and of shorter duration than downslope flows, leading to net downslope motion over this period. Hourly upslope speeds reach 35 cm/s; downslope maximum speeds are about 40 cm/s. Hourly temperature data from the three bottom tmeperature sensors indicate intense periods of mixing (periods of temperatures coalescing). Times when temperatures are falling are correlated with upslope flows, suggesting movement of deeper cold water associated with internal tidal motion. The general structure of the velocity and temperature records suggests bore-like propagation of the internal tide upslope. Holloway and Barnes (1998) have described these types of internal tidal motions above a sloping bottom recently from numerical simulations and other field data.

Discussion

We propose that the strong internal tidal currents and associated turbulent mixing retard the settling of fine-grained materials onto the seabed, thereby inhibiting deposition along this section of the slope as observed in surficial sediment samples. Net downslope flow provides a mechanism for transport of suspended materials into deeper water or into intermediate nepheloid layers. It is not known whether the episodic, strong internal tidal currents resuspend the local bottom sediment along this portion of the slope.

Based on CTD profiles taken in this region during the mooring deployments, the slope of the characteristics for the internal M2 tide are approximately critical for bottom slopes at the shelf break and in slightly deeper water (about 500 m). Power spectra of the cross slope component for the middle and lowest current meters indicate that the energy at M2 is largest near the bottom, and that substantially elevated energy levels are found throughout the frequency band from M2 to M4. The latter overtide is approximately critical at the 450 m site, and also is substantially more energetic in the across-slope flows at the bottom current sensor.

Conceptual Model

Semi-diurnal internal tidal currents are likely major factors in shaping continental slopes. Continental slopes are generally narrow physiographic seafloor features that mark the transition from shallow continental to deeper oceanic domains. Continental slopes span a depth range of about 2500 m, with the shallow edge averaging about 120-150 m. The regional gradients of continental slopes generally fall in the range of 0.5o - 5o, but locally these gradients can be much steeper.

As the sediment deposition through time progrades the surface of the continental slope, a gradual reduction in the steepness of the slope surfaces might result. The bottom shear and energy dissipation along the slope associated with the semi-diurnal internal tide will likely increase as the slope surface approaches "critical." The turbulent mixing and shear associated with this process will inhibit deposition of the fine-grained suspended materials, creating intermediate and bottom nepheloid layers, and will cause the regional slope to reach an equilibrium gradient.

References

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