Time‐dependent, wind‐driven flow over a shallow midshelf submarine bank

Abstract

During summer 2001, high‐resolution hydrographic, velocity, and bio‐optical surveys were conducted over Heceta Bank off central Oregon. North of the bank, upwelling over simple bottom topography exhibited a classic response with a midshelf, baroclinic coastal jet and upwelled isopycnals. The coastal upwelling jet follows the bank topography as it widens offshore before reaching the southern end of the bank, where the shelf break turns almost 90° back toward the coast. The ensuing adjustment involves the offshelf transport of coastal water and the material it contains. The jet meanders anticyclonically in the “lee” of the bank before resuming its equatorward path seaward of the shelf break. A pool of high‐chlorophyll (>15 mg m−3) near‐surface water is found in the low‐velocity “lee” region created by the bank. A low‐temperature, high‐salinity bottom water pool, supplied primarily from the south, lies over the inshore region of the bank. This bottom pool is up to 40 m thick and contains elevated levels of suspended material. By creating an east‐west perturbation in the coastal upwelling front, the flow‐topography interaction introduces an alongshore pressure gradient. When southward upwelling‐favorable winds relax, this alongshore pressure gradient can drive flow back to the north. This leads to northward flow on the inshore side of the bank, ultimately setting up the potential for recirculation around the entire bank complex on a timescale of about 10 days. Strong northward winds during summer lead to downwelling within 15 km of the coast, accompanied by significant (>0.2 m s−1) wind‐driven northward currents.