A dynamical description of fall and winter mean current profiles over the northern California shelf

Fall and winter mean current profiles from a midshelf (water depth similar to 90 m) northern California site exhibit a similar vertical structure for several different years. The alongshelf flow is poleward with a maximum vel ocity of 5-10 cm s(-1) in the middle or upper water column. There is an off shore flow of about 2 cm s(-1) in the upper 20-30 m, an onshore flow of abo ut 2 cm s(-1) in the interior (depths 35-65 m), and an offshore flow of abo ut 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 midshe lf sites along northern California and two sites off Peru also have a simil ar vertical structure. The vertical shear in the mean alongshelf flow is geostrophic throughout th e water column, that is, in thermal wind balance with the cross-shelf densi ty gradient. For timescales of a week or longer the thermal wind balance ex tends to within 1 m of the bottom and reduces the mean near-bottom alongshe lf flow to 1 cm s(-1) or less. These observations support recent theoretica l work suggesting that, over a sloping bottom, adjustment of the flow and d ensity fields within the bottom boundary layer may reduce the bottom stress . The alongshelf momentum balance is less clear. Weekly averages of offshor e transports in the upper and lower water column, relative to the interior onshore flow, are correlated with the surface and bottom stresses, suggesti ng Ekman balances. However, both the surface and bottom stresses are genera lly too small by a factor of 2-3 to account for the offshore transports. Li mited data suggest that alongshelf buoyancy gradients, estimated over scale s of 15 km or less, can be a significant component of the alongshelf moment um balance within both the upper and lower water column.