Anthony R. Kirincich

Emergence of Anoxia in the California Current Large Marine Ecosystem

Eastern boundary current systems are among the worlds most productive large marine ecosystems. Because upwelling currents transport nutrient-rich but oxygen-depleted water onto shallow seas, large expanses of productive continental shelves can be vulnerable to the risk of extreme low-oxygen events. Here, we report the novel rise of water-column shelf anoxia in the northern California Current system, a large marine ecosystem with no previous record of such extreme oxygen deficits. The expansion of anoxia highlights the potential for rapid and discontinuous ecosystem change in productive coastal systems that sustain a major portion of the worlds fisheries.

Delayed upwelling alters nearshore coastal ocean ecosystems in the northern California current

Wind-driven coastal ocean upwelling supplies nutrients to the euphotic zone near the coast. Nutrients fuel the growth of phytoplankton, the base of a very productive coastal marine ecosystem [Pauly D, Christensen V (1995) Nature 374:255–257]. Because nutrient supply and phytoplankton biomass in shelf waters are highly sensitive to variation in upwelling-driven circulation, shifts in the timing and strength of upwelling may alter basic nutrient and carbon fluxes through marine food webs. We show how a 1-month delay in the 2005 spring transition to upwelling-favorable wind stress in the northern California Current Large Marine Ecosystem resulted in numerous anomalies: warm water, low nutrient levels, low primary productivity, and an unprecedented low recruitment of rocky intertidal organisms. The delay was associated with 20- to 40-day wind oscillations accompanying a southward shift of the jet stream. Early in the upwelling season (May–July) off Oregon, the cumulative upwelling-favorable wind stress was the lowest in 20 years, nearshore surface waters averaged 2°C warmer than normal, surf-zone chlorophyll-a and nutrients were 50% and 30% less than normal, respectively, and densities of recruits of mussels and barnacles were reduced by 83% and 66%, respectively. Delayed early-season upwelling and stronger late-season upwelling are consistent with predictions of the influence of global warming on coastal upwelling regions.

Northern Monterey Bay upwelling shadow front: Observations of a coastally and surface-trapped buoyant plume

(1) During the upwelling season in central California, northwesterly winds along the coast produce a strong upwelling jet that originates at Point Ano Nuevo and flows southward across the mouth of Monterey Bay. A convergent front with a mean temperature change of 3.77 ± 0.29C develops between the warm interior waters and the cold offshore upwelling jet. To examine the forcing mechanisms driving the location and movement of the upwelling shadow front and its effects on biological communities in northern Monterey Bay, oceanographic conditions were monitored using cross-shelf mooring arrays, drifters, and hydrographic surveys along a 20 km stretch of coast extending northwestward from Santa Cruz, California, during the upwelling season of 2007 (May-September). The alongshore location of the upwelling shadow front at the northern edge of the bay was driven by: regional wind forcing, through an alongshore pressure gradient; buoyancy forces due to the temperature change across the front; and local wind forcing (the diurnal sea breeze). The upwelling shadow front behaved as a surface-trapped buoyant current, which is superimposed on a poleward barotropic current, moving up and down the coast up to several kilometers each day. We surmise that the front is advected poleward by a preexisting northward barotropic current of 0.10 m s � 1 that arises due to an alongshore pressure gradient caused by focused upwelling at Point Ano Nuevo. The frontal circulation (onshore surface currents) breaks the typical two-dimensional wind-driven, cross-shelf circulation (offshore surface currents) and introduces another way for water, and the material it contains (e.g., pollutants, larvae), to go across the shelf toward shore.

Alongshelf Variability of Inner-Shelf Circulation along the Central Oregon Coast during Summer

The spatial and temporal variability of inner-shelf circulation along the central Oregon coast during the 2004 upwelling season is described using a 70-km-long array of moorings along the 15-m isobath. Circulation at three stations located onshore of a submarine bank differed from that of a station north of the bank, despite the relatively uniform wind forcing and inner-shelf bathymetry present. During upwelling-favorable winds, strong southward alongshelf flow occurred north of the bank, no alongshelf flow occurred onshore of the northern part of the bank, and increasing southward flow occurred onshore of the southern part of the bank. During downwelling-favorable winds, strong northward flow occurred in the inner shelf onshore of the bank while weak flow occurred north of the bank. These alongshelf differences in inner-shelf circulation were due to the effects of the bank, which isolated the inner shelf onshore of the bank from the regional upwelling circulation that was evident at the northernmost station. As a result, circulation onshore of the bank was drivenprimarily by local wind forcing, whileflow north of the bank was only partiallydriven by local winds. A secondary mode of variability, attributed to the movement of the regional upwelling jet due to remote forcings, contributed the bulk of the variability observed north of the bank. With the time-dependent wind forcing present, acceleration was an important term in the depth-averaged alongshelf momentum equation at all stations. During upwelling, bottom stress and acceleration opposed the wind stress north of the bank, while bottom stress was weaker onshore of the bank where the across-shelf momentum flux and the alongshelf pressure gradient balanced the residual of the acceleration and stresses. During downwelling, waters onshore of the bank surged northward at magnitudes much larger than that found north of the bank. These spatial variations developed as the season progressed and the regional upwelling circulation intensified, explaining known variations in growth and recruitment of nearshore invertebrate species.

Wave-Driven Inner-Shelf Motions on the Oregon Coast*

Recent work by S. Lentz et al. documents offshore transport in the inner shelf due to a wave-driven return flow associated with the Hasselmann wave stress (the Stokes‐Coriolis force). This analysis is extended using observations from the central Oregon coast to identify the wave-driven return flow present and quantify the potential bias of wind-driven across-shelf exchange by unresolved wave-driven circulation. Using acoustic Doppler current profiler (ADCP) measurements at six stations, each in water depths of 13‐15 m, observed depth-averaged, across-shelf velocities were generally correlated with theoretical estimates of the proposed return flow. During times of minimal wind forcing, across-shelf velocity profiles were vertically sheared, with strongervelocitiesnearthetop of themeasuredportionofthe watercolumn,andincreasedin magnitudewith increasing significant wave height, consistentwith circulation due to the Hasselmann wave stress. Yet velocity magnitudes and vertical shears were stronger than that predicted by linear wave theory, and more similar to the stratified ‘‘summer’’ velocity profiles described by S. Lentz et al. Additionally, substantial temporal and spatial variability of the wave-driven return flow was found, potentially due to changing wind and wave conditions as well as local bathymetric variability. Despite the wave-driven circulation found, subtracting estimates of the return flow from the observed across-shelf velocity had no significant effect on estimates of the across-shelf exchange dueto along-shelfwind forcing at these waterdepths along the Oregoncoast during summer.

Calculating Reynolds Stresses from ADCP Measurements in the Presence of Surface Gravity Waves Using the Cospectra-Fit Method

Recently, the velocity observations of acoustic Doppler current profilers (ADCPs) have been successfully used to estimate turbulent Reynoldsstresses in estuaries and tidal channels. However,the presence of surface gravity waves can significantly bias stress estimates, limiting application of the technique in the coastal ocean. This work describes a new approach to estimate Reynolds stresses from ADCP velocities obtained in the presence of waves. The method fits an established semiempirical model of boundary layer turbulence to the measured turbulent cospectra at frequencies below those of surface gravity waves to estimate the stress. Applied to ADCP observations made in weakly stratified waters and variable significant wave heights, estimated near-bottom and near-surface stresses using this method compared well with independent estimates of the boundary stresses in contrast to previous methods. Additionally, the vertical structure of tidal stress estimated using the new approach matched that inferred from a linear momentum balance at stress levels below the estimated stress uncertainties. Because the method makes an estimate of the horizontal turbulent length scales present as part of the modelfit, these results can also enable a direct correction for the mean bias errors resulting from instrument tilt, if these scales are long relative to the beam separation.

Eulerian and Lagrangian Correspondence of High-Frequency Radar and Surface Drifter Data: Effects of Radar Resolution and Flow Components

AbstractThis study investigated the correspondence between the near-surface drifters from a mass drifter deployment near Martha’s Vineyard, Massachusetts, and the surface current observations from a network of three high-resolution, high-frequency radars to understand the effects of the radar temporal and spatial resolution on the resulting Eulerian current velocities and Lagrangian trajectories and their predictability. The radar-based surface currents were found to be unbiased in direction but biased in magnitude with respect to drifter velocities. The radar systematically underestimated velocities by approximately 2 cm s−1 due to the smoothing effects of spatial and temporal averaging. The radar accuracy, quantified by the domain-averaged rms difference between instantaneous radar and drifter velocities, was found to be about 3.8 cm s−1. A Lagrangian comparison between the real and simulated drifters resulted in the separation distances of roughly 1 km over the course of 10 h, or an equivalent separati...

Improving HF Radar Estimates of Surface Currents Using Signal Quality Metrics, with Application to the MVCO High-Resolution Radar System

AbstractEstimates of surface currents over the continental shelf are now regularly made using high-frequency radar (HFR) systems along much of the U.S. coastline. The recently deployed HFR system at the Martha’s Vineyard Coastal Observatory (MVCO) is a unique addition to these systems, focusing on high spatial resolution over a relatively small coastal ocean domain with high accuracy. However, initial results from the system showed sizable errors and biased estimates of M2 tidal currents, prompting an examination of new methods to improve the quality of radar-based velocity data. The analysis described here utilizes the radial metric output of CODAR Ocean Systems’ version 7 release of the SeaSonde Radial Site Software Suite to examine both the characteristics of the received signal and the output of the direction-finding algorithm to provide data quality controls on the estimated radial currents that are independent of the estimated velocity. Additionally, the effect of weighting spatial averages of radia...

Time-Varying Across-Shelf Ekman Transport and Vertical Eddy Viscosity on the Inner Shelf

Abstract The event-scale variability of across-shelf transport was investigated using observations made in 15 m of water on the central Oregon inner shelf. In a study area with intermittently upwelling-favorable winds and significant density stratification, hydrographic and velocity observations show rapid across-shelf movement of water masses over event time scales of 2–7 days. To understand the time variability of across-shelf exchange, an inverse calculation was used to estimate eddy viscosity and the vertical turbulent diffusion of momentum from velocity profiles and wind forcing. Depth-averaged eddy viscosity varied over a large dynamic range, but averaged 1.3 × 10−3 m2 s−1 during upwelling winds and 2.1 × 10−3 m2 s−1 during downwelling winds. The fraction of full Ekman transport present in the surface layer, a measure of the efficiency of across-shelf exchange at this water depth, was a strong function of eddy viscosity and wind forcing, but not stratification. Transport fractions ranged from 60%, d...

The Spatial Structure of Tidal and Mean Circulation over the Inner Shelf South of Martha's Vineyard, Massachusetts

AbstractThe spatial structure of the tidal and background circulation over the inner shelf south of Marthas Vineyard, Massachusetts, was investigated using observations from a high-resolution, high-frequency coastal radar system, paired with satellite SSTs and in situ ADCP velocities. Maximum tidal velocities for the dominant semidiurnal constituent increased from 5 to 35 cm s−1 over the 20-km-wide domain with phase variations up to 60°. A northeastward jet along the eastern edge and a recirculation region inshore dominated the annually averaged surface currents, along with a separate along-shelf jet offshore. Owing in part to this variable circulation, the spatial structure of seasonal SST anomalies had implications for the local heat balance. Cooling owing to the advective heat flux divergence was large enough to offset more than half of the seasonal heat gain owing to surface heat flux. Tidal stresses were the largest terms in the mean along- and across-shelf momentum equations in the area of the reci...