Abstract

Fall and winter mean current profiles from a midshelf (water depth ∼90 m) northern California site exhibit a similar vertical structure for several different years. The alongshelf flow is poleward with a maximum velocity of 5–10 cm s⁻¹ in the middle or upper water column. There is an offshore flow of about 2 cm s⁻¹ in the upper 20–30 m, an onshore flow of about 2 cm s⁻¹ in the interior (depths 35–65 m), and an offshore flow of about 1 cm s⁻¹ within 20 m of the bottom. Profiles are similar for averages over timescales from weeks to months. Mean current profiles at other midshelf sites along northern California and two sites off Peru also have a similar vertical structure.

The vertical shear in the mean alongshelf flow is geostrophic throughout the water column, that is, in thermal wind balance with the cross-shelf density gradient. For timescales of a week or longer the thermal wind balance extends to within 1 m of the bottom and reduces the mean near-bottom alongshelf flow to 1 cm s⁻¹ or less. These observations support recent theoretical work suggesting that, over a sloping bottom, adjustment of the flow and density fields within the bottom boundary layer may reduce the bottom stress. The alongshelf momentum balance is less clear. Weekly averages of offshore transports in the upper and lower water column, relative to the interior onshore flow, are correlated with the surface and bottom stresses, suggesting Ekman balances. However, both the surface and bottom stresses are generally too small by a factor of 2–3 to account for the offshore transports. Limited data suggest that alongshelf buoyancy gradients, estimated over scales of 15 km or less, can be a significant component of the alongshelf momentum balance within both the upper and lower water column.

Abstract

Observations from 18 near-bottom pressure sensors and 8 current meter moorings provide a characterization of the barotropic tides over the North Carolina continental shelf between Chesapeake Bay and Cape Hatteras. The largest tidal constituents in this region are the M ₂ (sea level amplitude 47 cm), N ₂ (11 cm), and S ₂ (10 cm) semidiurnal tides and the K ₁/P ₁ (7 cm) and O ₁ (5 cm) diurnal tides. The barotropic tidal currents are small, less than 3 cm s⁻¹, except for the M ₂ current which reaches 10 cm s⁻¹ at mid shelf. Cross-shelf currents increase linearly from approximately zero near the coast to a maximum over the mid and outer shelf. Alongshelf currents are nonzero near the coast and increase to roughly twice the coastal value over the mid and outer shelf. While bottom friction is weak over most of the shelf, in water depths of less than 10 m bottom friction results in a rapid phase decrease toward the coast in the semidiurnal alongshelf velocities. This phase difference is not observed in the K ₁/P ₁ alongshelf flow, possibly because diurnal wind forcing causes an opposing phase increase toward the coast.

Alongshelf variations in tidal amplitudes and phases near the coast are much larger than expected based on variations along the shelf break inferred from basin-scale altimeter estimates. The shelf width in this region increases from 25 to 100 km over an alongshelf distance of about 150 km. Results from an analytic, flat-bottom, two-dimensional model indicate that widening of the shelf toward the north causes the observed alongshelf increase in the semidiurnal amplitude and phase, but does not explain alongshelf variations in the weaker diurnal constituents.

Abstract

Low-salinity water from Chesapeake Bay forms an intermittent buoyant gravity current that propagates more than 100 km southward along the coast. During five events when wind and surface gravity-wave forcing were weak, the buoyant coastal current 90 km south of Chesapeake Bay was less than 5 km wide, was 5–10 m thick, and propagated alongshore at ∼50 cm s⁻¹. The density decreased 2–3 kg m⁻³ over a few hundred meters at the nose of the buoyant coastal current, which was located about 1 km offshore in ∼8 m of water. Water up to 4 km ahead of the advancing nose was displaced southward and offshore (maximum velocities near the nose of 20 and 10 cm s⁻¹, respectively). The southward alongshore current increased abruptly to ∼50 cm s⁻¹ at the nose and continued to increase to a supercritical maximum of ∼70 cm s⁻¹ about 1 km behind the nose. An onshore flow of between 5 and 15 cm s⁻¹, which extended at least 5 km behind the nose, supplied buoyant water to the onshore region of weak, subcritical alongshore flow. The observed flow structure is qualitatively similar to theoretical predictions and laboratory measurements of buoyant gravity currents propagating along a sloping bottom.