Influence of upwelling-season coastal currents on near-bottom dissolved oxygen concentrations over a submarine bank Public Deposited


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  • Wind-driven coastal upwelling brings subsurface water onto the central-Oregon shelf after the spring transition each year. This cold and salty source water is oxygen-poor, yet above the hypoxic threshold, dissolved oxygen < 1.4 ml l⁻¹. Once on the shelf, dissolved oxygen (DO) concentrations of upwelled near-bottom waters are modified by physical and biological shelf processes, such as advection, mixing and microbial respiration. The influences of shelf processes on near-bottom DO concentrations on tidal, event and seasonal time scales are investigated using moored continuous time series and underwater glider cross-shelf transects over the Heceta and Stonewall Bank complex (HSBC) off central Oregon. A linear, seasonal decline rate of ~0.01 ml l⁻¹ day⁻¹ is observed from moored near-bottom continuous time series on the mid shelf over HSBC. This seasonal decline rate is only 30% of the expected draw down from calculated respiration rates over the shelf (0.026 ± 0.013 ml l⁻¹ day⁻¹). The severity of low-oxygen concentrations in a given year is a combined result of the biological consumption rate, the physical replenishment rate due to advection and mixing and the length of the season. Along the Newport Hydrographic (NH) line (44.65°N), cross-shelf variability of hypoxic measurements sampled in over 100 underwater glider cross-shelf transects (2006 - 2012) is investigated. Prevalent near-bottom hypoxic areas are identified on the mid (50-80-m isobaths) and the outer (120-150-m isobaths) shelf regions. The gap in between the two regions is just north of Stonewall Bank, an area of enhanced mixing where higher-DO water is likely mixed into the bottom mixed layer. A seasonal change in near-bottom currents is observed on the mid shelf. In early July of 2011, equatorward along-shelf currents weaken and near-bottom cross-shelf currents change from strongly onshore to weakly onshore and offshore. This change is likely due to the offshore movement of the coastal jet during the upwelling season. Consequently, bottom Ekman transport decreases significantly throughout the upwelling season. Low along-shelf and cross-shelf flow, or flushing, of near-bottom shelf waters increases the risk for hypoxia. Cross-shelf advection cannot account for the large decrease in outer-shelf DO observed in a sequence of 10 glider lines in late-summer 2011. This decrease is attributed to an along-shelf DO gradient of -0.72 ml l⁻¹ over 2.58 km, or 0.28 ml l⁻¹ per km, such that equatorward near-bottom flow brings low-DO water south past the NH line. The effect of the Heceta and Stonewall Bank complex on along-shelf variability of shelf and slope waters is investigated during the 2008 upwelling season by comparing the NH glider line with a glider line located just south of the Bank (Umpqua River - UR, 43.7°N). Spicy, subsurface poleward flows, characteristic of the California Undercurrent (CU), are observed along both lines in September 2008. Spice values are proportional to a salinity anomaly and thus can be positive and negative. High spice values indicate transport from the southern CCS via the CU. A poleward core of 0.05 m s⁻¹ is observed against the continental slope of the NH line, however the spiciness of this poleward flow is diffuse and reaches a maximum of -0.1 kg m⁻³. The UR-line poleward core, 0.1 m s⁻¹, and spice maximum, 0.05 kg m⁻³, are much stronger signals. The average spice field from 64 summertime Seaglider lines collected along the NH line from 2008 to 2012, also show a wide, diffuse subsurface spice signal of magnitude -0.1 kg m⁻³. Mixing of CU water induced by flow around the Bank is likely resulting in the weak signal observed on the NH line. To investigate the along-shelf difference in shelf water properties, spice-density and DO-density relationships over the 300-m isobath are investigated. Spice along isopycnals which upwell onto the shelf, 26.4 - 26.6 kg m⁻³, are much higher on the UR line. DO concentrations along isopycnals are similar on the NH and UR lines. This indicates that spicier source water does not equate to lower DO concentrations on the UR line. Near-bottom shelf waters, 10 m above the 100-m isobath, are also spicier on the UR line than the NH line in 2008. A spice-DO relationship for shelf waters indicates the influence of source water variability on shelf DO concentrations is small in comparison to the shelf respiration signal on shelf DO concentrations.
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