Melanie R. Fewings

Observations and a Model of Undertow over the Inner Continental Shelf

Abstract Onshore volume transport (Stokes drift) due to surface gravity waves propagating toward the beach can result in a compensating Eulerian offshore flow in the surf zone referred to as undertow. Observed offshore flows indicate that wave-driven undertow extends well offshore of the surf zone, over the inner shelves of Martha’s Vineyard, Massachusetts, and North Carolina. Theoretical estimates of the wave-driven offshore transport from linear wave theory and observed wave characteristics account for 50% or more of the observed offshore transport variance in water depths between 5 and 12 m, and reproduce the observed dependence on wave height and water depth. During weak winds, wave-driven cross-shelf velocity profiles over the inner shelf have maximum offshore flow (1–6 cm s−1) and vertical shear near the surface and weak flow and shear in the lower half of the water column. The observed offshore flow profiles do not resemble the parabolic profiles with maximum flow at middepth observed within the su...

Observations of Cross-Shelf Flow Driven by Cross-Shelf Winds on the Inner Continental Shelf

Six-yr-long time series of winds, waves, and water velocity from a cabled coastal observatory in 12 m of water reveal the separate dependence of the cross-shelf velocity profile on cross-shelf and along-shelf winds, waves, and tides. During small waves, cross-shelf wind is the dominant mechanism driving the cross-shelf circulation after tides and tidal residual motions are removed. The along-shelf wind does not drive a substantial cross-shelf circulation. During offshore winds, the cross-shelf circulation is offshore in the upper water column and onshore in the lower water column, with roughly equal and opposite volume transports in the surface and bottom layers. During onshore winds, the circulation is nearly the reverse. The observed profiles and cross-shelf transport in the surface layer during winter agree with a simple two-dimensional unstratified model of cross-shelf wind stress forcing. The cross-shelf velocity profile is more vertically sheared and the surface layer transport is stronger in summer than in winter for a given offshore wind stress. During large waves, the cross-shelf circulation is no longer roughly symmetric in the wind direction. For onshore winds, the cross-shelf velocity profile is nearly vertically uniform, because the wind- and wavedriven shears cancel; for offshore winds, the profile is strongly vertically sheared because the wind- and wave-driven shears have the same sign. The Lagrangian velocity profile in winter is similar to the part of the Eulerian velocity profile due to cross-shelf wind stress alone, because the contribution of Stokes drift to the Lagrangian velocity approximately cancels the contribution of waves to the Eulerian velocity.

The propagating response of coastal circulation due to wind relaxations along the central California coast

[1] Following relaxations of prevailing upwelling‐favorable winds, warm waters from the Santa Barbara Channel propagate poleward around Point Conception and along the south central California coast. We examined characteristics of these relaxation flows, including frontal propagation speed and temperature changes during the warm water arrivals, by using multiyear time series of currents and temperatures from four moorings along the ∼15 m isobath, surface current observations from high‐frequency radars, and satellite sea surface temperature images. Propagation speeds of the warm fronts relative to ambient waters ranged from 0.04 to 0.46 m s −1 . As the fronts arrived at the moorings, temperature increases ranged from 0.7°C to 4.2°C. In ensemble averages over many frontal arrivals, alongshore flow speeds increased by 0.1–0.2 m s −1 over the water column during arrivals. Cross‐shore flows were onshore near the surface and offshore near the bottom with speeds of 0.02–0.05 m s −1 . This cross‐shore flow structure persisted as temperature increased during arrivals and ceased when temperatures stopped increasing. Frontal propagation speeds were correlated with temperature increases at the moorings, consistent with forcing by baroclinic pressure gradients. Compared to other buoyant flows such as from the Chesapeake Bay where density contrasts with ambient waters are 2– 3k g m −3 , these relaxation flows are less buoyant with density contrasts of 0.1–0.9 kg m −3 . Consequently, the propagation of these flows is more affected by bottom friction and the speeds are closer to the “slope‐controlled” or “bottom‐advected” limit described in theoretical and laboratory work but not well studied in the ocean.

Satellite sea surface temperatures along the West Coast of the United States during the 2014–2016 northeast Pacific marine heat wave

From January 2014 to August 2016, sea-surface temperatures (SSTs) along the Washington, Oregon, and California coasts were significantly warmer than usual, reaching a maximum SST anomaly of 6.2 °C off southern California. This marine heat wave occurred alongside the Gulf of Alaska marine heat wave, and resulted in major disturbances in the California Current ecosystem and massive economic impacts. Here, we use satellite and blended reanalysis products to report the magnitude, extent, duration, and evolution of SSTs and wind stress anomalies along the west coast of the continental United States during this event. Nearshore SST anomalies along the entire coast were persistent during the marine heat wave, and only abated seasonally, during spring upwelling-favorable wind stress. The coastal marine heat wave weakened in July 2016 and disappeared by September 2016.

Lagrangian Observations of Inner-Shelf Motions in Southern California: Can Surface Waves Decelerate Shoreward-Moving Drifters Just outside the Surf Zone?

AbstractThis study explores Eulerian and Lagrangian circulation during weak winds at two inner-shelf locations off the Southern California coast where the shoreline, shelf, wind, and wave characteristics differ from those in previous studies. In agreement with recent observational studies, wave-driven Eulerian offshore flow just outside the surf zone, referred to as undertow, is a substantial component of the net cross-shore circulation during periods of weak winds. Drifter observations show onshore surface flow, likely due to light onshore winds, and a consistent decrease in onshore velocity of roughly 4 cm s−1 within a few hundred meters of the surf zone. Undertow is examined as a possible explanation for the observed Lagrangian decelerations. Model results suggest that, even when waves are small, undertow can decrease the velocity of shoreward-moving drifters by >2 cm s−1, roughly half the observed deceleration. The coastal boundary condition also has the potential to contribute to the observed deceler...

Synoptic forcing of wind relaxations at Pt. Conception, California

Over the California Current upwelling system in summer, the prevailing upwelling-favorable winds episodically weaken (relax) or reverse direction for a few days. Near Pt. Conception, California, the wind usually does not reverse, but wind relaxation allows poleward oceanic coastal flow with ecological consequences. To determine the offshore extent and synoptic forcing of these wind relaxations, we formed composite averages of wind stress from the QuikSCAT satellite and atmospheric pressure from the North American Regional Reanalysis (NARR) using 67 wind relaxations during summer 2000–2009. Wind relaxations at Pt. Conception are the third stage of an event sequence that repeatedly affects the west coast of North America in summer. First, 5–7 days before the wind weakens near Pt. Conception, the wind weakens or reverses off Oregon and northern California. Second, the upwelling-favorable wind intensifies along central California. Third, the wind relaxes at Pt. Conception, and the area of weakened winds extends poleward to northern California over 3–5 days. The NARR underestimates the wind stress within 200 km of coastal capes by a factor of 2. Wind relaxations at Pt. Conception are caused by offshore extension of the desert heat low. This synoptic forcing is related to event cycles that cause wind reversal as in Halliwell and Allen (1987) and Mass and Bond (1996), but includes weaker events. The wind relaxations extend 600 km offshore, similarly to the California-scale hydraulic expansion fan shaping the prevailing winds, and 1000 km alongshore, limited by an opposing pressure gradient force at Cape Mendocino.

Wind relaxation and a coastal buoyant plume north of Pt. Conception, CA: Observations, simulations, and scalings

In upwelling regions, wind relaxations lead to poleward propagating warm water plumes that are important to coastal ecosystems. The coastal ocean response to wind relaxation around Pt. Conception, CA is simulated with a Regional Ocean Model (ROMS) forced by realistic surface and lateral boundary conditions including tidal processes. The model reproduces well the statistics of observed subtidal water column temperature and velocity at both outer and inner-shelf mooring locations throughout the study. A poleward-propagating plume of Southern California Bight water that increases shelf water temperatures by ≈ 5°C is also reproduced. Modeled plume propagation speed, spatial scales, and flow structure are consistent with a theoretical scaling for coastal buoyant plumes with both surface-trapped and slope-controlled dynamics. Plume momentum balances are distinct between the offshore (>30 m depth) region where the plume is surface-trapped, and onshore of the 30 m isobath (within 5 km from shore) where the plume water mass extends to the bottom and is slope controlled. In the onshore region, bottom stress is important in the alongshore momentum equation and generates vertical vorticity that is an order of magnitude larger than the vorticity in the plume core. Numerical experiments without tidal forcing show that modeled surface temperatures are biased 0.5°C high, potentially affecting plume propagation distance and persistence.

Cross-shelf circulation and momentum and heat balances over the inner continental shelf near Martha's Vineyard, Massachusetts

Abstract : The water circulation and evolution of water temperature over the inner continental shelf are investigated using observations of water velocity, temperature, density, and bottom pressure; surface gravity waves; wind stress; and heat flux between the ocean and atmosphere during 2001-2007. When waves are small, cross-shelf wind stress is the dominant mechanism driving cross-shelf circulation. The along-shelf wind stress does not drive a substantial cross- shelf circulation. The response to a given wind stress is stronger in summer than winter. The cross-shelf transport in the surface layer during winter agrees with a two-dimensional, unstratified model. During large waves and onshore winds the cross-shelf velocity is nearly vertically uniform, because the wind- and wave-driven shears cancel. During large waves and offshore winds the velocity is strongly vertically sheared because the wind- and wave-driven shears have the same sign. The subtidal, depth-average cross-shelf momentum balance is a combination of geostrophic balance and a coastal set-up and set-down balance driven by the cross-shelf wind stress. The estimated wave radiation stress gradient is also large. The dominant along-shelf momentum balance is between the wind stress and pressure gradient, but the bottom stress, acceleration, Coriolis, Hasselmann wave stress, and nonlinear advection are not negligible. The fluctuating along-shelf pressure gradient is a local sea level response to wind forcing, not a remotely generated pressure gradient. In summer, the water is persistently cooled due to a mean upwelling circulation. The cross-shelf heat flux nearly balances the strong surface heating throughout mid-summer, so the water temperature is almost constant. The along-shelf heat flux divergence is apparently small. In winter, the change in water temperature is closer to that expected due to the surface cooling. Heat transport due to surface gravity waves is substantial.

Impact of recently upwelled water on productivity investigated using in situ and incubation-based methods in Monterey Bay

Photosynthetic conversion of CO2 to organic carbon and the transport of this carbon from the surface to the deep ocean is an important regulator of atmospheric CO2. To understand the controls on carbon fluxes in a productive region impacted by upwelling, we measured biological productivity via multiple methods during a cruise in Monterey Bay, California. We quantified net community production and gross primary production from measurements of O2/Ar and O2 triple isotopes (17Δ), respectively. We simultaneously conducted incubations measuring the uptake of 14C, 15NO3- and 15NH4+, and nitrification, and deployed sediment traps. At the start of the cruise (Phase 1) the carbon cycle was at steady state and the estimated net community production was 35(10) and 35(8) mmol C m−2 d−1 from O2/Ar and 15N incubations respectively, a remarkably good agreement. During Phase 1, net primary production was 96(27) mmol C m−2 d−1 from C uptake, and gross primary production was 209(17) mmol C m−2 d−1 from 17Δ. Later in the cruise (Phase 2), recently upwelled water with higher nutrient concentrations entered the study area, causing 14C and 15NO3- uptake to increase substantially. Continuous O2/Ar measurements revealed submesoscale variability in water mass structure and likely productivity in Phase 2 that was not evident from the incubations. These data demonstrate that O2/Ar and 15N incubation-based NCP estimates can give equivalent results in an N-limited, coastal system, when the non-steady state O2 fluxes are negligible or can be quantified. This article is protected by copyright. All rights reserved.

Large‐scale anomalies in sea‐surface temperature and air‐sea fluxes during wind relaxation events off the United States West Coast in summer

In summertime along the U.S. West Coast, the winds exhibit a three-stage cycle spanning ∼12 days. The prevailing upwelling-favorable winds weaken (relax) or reverse off the Pacific Northwest, then reintensify, then weaken off central California. We study the sea-surface temperature (SST) response to these “northern” and “southern” wind relaxations. (1) Satellite data indicate northern wind relaxations result in SST anomalies O(+1°C) extending ∼2000 km offshore. Surface heat flux reanalyses indicate the warm anomaly is mainly from decreased latent cooling. (2) After the winds reintensify, SST becomes anomalously cold along central and southern California. (3) During the southern wind relaxations, the cold SST anomaly persists but the SST warms with time. This warming is not driven by surface heat flux. The latent cooling is reduced, yet unlike during the northern relaxation, this change is canceled by a decrease in solar radiation due to increased cloudiness. In the region south of Point Conception, reduced southward advection of cold water and increased northward advection of warm water by the coastal countercurrent could explain the warming. Reduced Ekman pumping likely contributes to the warming trend during the southern relaxations, and reduced wind-driven entrainment at the base of the mixed layer likely contributes to the warming during both relaxations. Whether the net surface heat flux is the main driver of SST anomalies during wind relaxation depends on the regional response of clouds. Southern wind relaxations follow episodes of enhanced surface cooling, which may contribute to greater cloudiness during southern than northern wind relaxations.