A Dynamical Description of Fall and Winter Mean Current Profiles over the Northern California Shelf

Steve Lentz Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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John Trowbridge Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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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.

Corresponding author address: Steve Lentz, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.

Email: slentz@whoi.edu

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.

Corresponding author address: Steve Lentz, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.

Email: slentz@whoi.edu

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