Fewings Melanie R.

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Fewings
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Melanie R.
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  • Article
    Impact of recently upwelled water on productivity investigated using in situ and incubation-based methods in Monterey Bay
    (John Wiley & Sons, 2017-03-11) Manning, Cara C. ; Stanley, Rachel H. R. ; Nicholson, David P. ; Smith, Jason M. ; Pennington, Timothy ; Fewings, Melanie R. ; Squibb, Michael E. ; Chavez, Francisco P.
    Photosynthetic conversion of inline image to organic carbon and the transport of this carbon from the surface to the deep ocean is an important regulator of atmospheric inline image. 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 inline image/Ar and inline image triple isotopes ( inline image), respectively. We simultaneously conducted incubations measuring the uptake of 14C, inline image, and inline image, 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 inline image/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 inline image. Later in the cruise (Phase 2), recently upwelled water with higher nutrient concentrations entered the study area, causing 14C and inline image uptake to increase substantially. Continuous inline image/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 inline image/Ar and inline image incubation-based NCP estimates can give equivalent results in an N-limited, coastal system, when the nonsteady state inline image fluxes are negligible or can be quantified.
  • Article
    Momentum balances on the inner continental shelf at Martha's Vineyard Coastal Observatory
    (American Geophysical Union, 2010-12-09) Fewings, Melanie R. ; Lentz, Steven J.
    The subtidal, depth-average momentum balances in 12 m and 27 m water depth are investigated using observations from 2001 to 2007 of water velocity, temperature, and density; bottom pressure; surface gravity waves; and wind stress. In the fluctuating across-shelf momentum budget, the dominant terms are surface wind stress, pressure gradient, and Coriolis acceleration. The balance is a combination of (1) the geostrophic balance expected at midshelf sites and (2) the coastal setup and setdown balance driven by the across-shelf wind stress expected where surface and bottom boundary layers overlap. At the 12 m site, the estimated wave radiation stress gradient due to surface gravity wave shoaling is also large but is uncorrelated with the observed pressure gradient. A simple model suggests the wave radiation stress gradient is balanced by an across-shelf pressure gradient with a spatial scale too small to resolve with this mooring array. In the fluctuating along-shelf momentum balance, the dominant terms are surface wind stress, pressure gradient, and bottom stress at the shallower site, but the other estimated terms are not negligible. Our results support the Grant and Madsen (1986) formulation for wave-induced bottom stress. The fluctuating along-shelf pressure gradient is mainly a local sea level response to wind forcing, not a remotely generated pressure gradient. A strong correlation between along-shelf velocity and along-shelf wind stress at the shallower site is captured by a simple steady model of imbalance between wind stress and pressure gradient balanced by linear bottom drag.
  • Article
    Summertime cooling of the shallow continental shelf
    (American Geophysical Union, 2011-07-19) Fewings, Melanie R. ; Lentz, Steven J.
    In summer on the shallow New England continental shelf, near the coast the water temperature is much cooler than the observed surface heat flux suggests. Using depth-integrated heat budgets in 12 and 27 m water depth calculated from observed surface heat flux, water temperature, and velocity, we demonstrate that on time scales of weeks to months the water is persistently cooled due to a mean upwelling circulation. Because the mean wind is weak, that mean circulation is likely not wind driven; it is partly a tidal residual circulation. A feedback exists between the cross-shelf and surface heat fluxes: the two fluxes remain nearly in balance for months, so the water temperature is nearly constant in spite of strong surface heating (the heat budget is two-dimensional). A conceptual model explains the feedback mechanism: the short flushing time of the shallow shelf produces a near steady state heat balance, regardless of the exact form of the circulation, and the feedback is via the influence of surface heating on temperature stratification. Along-shelf heat flux divergence is apparently small compared to the surface and cross-shelf heat flux divergences on time scales of weeks to months. Heat transport due to Stokes drift from surface gravity waves is substantial, warms the shallow shelf in summer, and was previously ignored. In winter, the surface heat flux dominates and the observed water temperature is close to the temperature predicted from surface cooling (the heat budget is one-dimensional); weak winter stratification makes the cross-shelf heat flux small even during strong cross-shelf circulation.
  • Article
    Observations of cross-shelf flow driven by cross-shelf winds on the inner continental shelf
    (American Meteorological Society, 2008-11) Fewings, Melanie R. ; Lentz, Steven J. ; Fredericks, Janet J.
    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 wave-driven 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.
  • Technical Report
    Long-term evolution and coupling of the boundary layers in the Stratus Deck Regions of the eastern Pacific (STRATUS)
    (Woods Hole Oceanographic Institution, 2001-06) Lucas, Lisanne E. ; Way, Bryan S. ; Weller, Robert A. ; Bouchard, Paul R. ; Fischer, Albert S. ; Moffat, Carlos F. ; Schneider, Wolfgang ; Fewings, Melanie R.
    A surface mooring was deployed in the eastern tropical Pacific west of northern Chile from the R/V Melville as part of the Eastern Pacific Investigation of Climate (EPIC). EPIC is a CLIVAR study with the goal of investigating links between sea surface temperature variability in the eastern tropical Pacific and climate over the American continents. Important to that goal is an understanding of the role of clouds in the eastern Pacific in modulating atmosphere-ocean coupling. The mooring was deployed near 20°S 85°W, at a location near the western edge of the stratocumulus cloud deck found west of Peru and Chile. This deployment started a three-year occupation of that site by a WHOI surface mooring in order to collect accurate time series of surface forcing and upper ocean variability. The surface mooring was deployed by the Upper Ocean Processes Group of the Woods Hole Oceanographic Institution (WHOI). In collaboration with investigators from the University of Concepcion, Concepcion, Chile, an XBT section was made on the way out to the mooring from Arica, Chile, and an XBT and CTD section was made on the way into Arica. The buoy was equipped with meteorological instrumentation, including two Improved METeorological (IMET) systems. The mooring also carried Vector Measuring Current Meters, single-temperature recorders, and conductivity and temperature recorders located in the upper meters of the mooring line. In addition to the instrumentation noted above, a variety of other instruments, including an acoustic current meter, an acoustic doppler current profiler, a bio-optical instrument package, and an acoustic rain guage, were deployed. This report describes, in a general manner, the work that took place and the data collected during the Cook 2 cruise aboard the R/V Melville. The surface mooring deployed during this cruise will be recovered and re-deployed after approximately 12 months and again after 24 months, with a final recovery planned for 36 months after the first setting. Details of the mooring design and preliminary data from the XBT and CTD sections are included.
  • Article
    Observations and a model of undertow over the inner continental shelf
    (American Meteorological Society, 2008-11) Lentz, Steven J. ; Fewings, Melanie R. ; Howd, Peter A. ; Fredericks, Janet J. ; Hathaway, Kent
    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 surf zone. Instead, the vertical structure is similar to the Stokes drift velocity profile but with the opposite direction. This vertical structure is consistent with a dynamical balance between the Coriolis force associated with the offshore flow and an along-shelf “Hasselmann wave stress” due to the influence of the earth’s rotation on surface gravity waves. The close agreement between the observed and modeled profiles provides compelling evidence for the importance of the Hasselmann wave stress in forcing oceanic flows. Summer profiles are more vertically sheared than either winter profiles or model profiles, for reasons that remain unclear.
  • Thesis
    Cross-shelf circulation and momentum and heat balances over the inner continental shelf near Martha's Vineyard, Massachusetts
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2007-09) Fewings, Melanie R.
    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 midsummer, 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.
  • Article
    Integrated observations of global surface winds, currents, and waves: Requirements and challenges for the next decade
    (Frontiers Media, 2019-07-24) Villas Bôas, Ana B. ; Ardhuin, Fabrice ; Ayet, Alex ; Bourassa, Mark A. ; Brandt, Peter ; Chapron, Bertrand ; Cornuelle, Bruce D. ; Farrar, J. Thomas ; Fewings, Melanie R. ; Fox-Kemper, Baylor ; Gille, Sarah T. ; Gommenginger, Christine ; Heimbach, Patrick ; Hell, Momme C. ; Li, Qing ; Mazloff, Matthew R. ; Merrifield, Sophia T. ; Mouche, Alexis ; Rio, Marie H. ; Rodriguez, Ernesto ; Shutler, Jamie D. ; Subramanian, Aneesh C. ; Terrill, Eric ; Tsamados, Michel ; Ubelmann, Clement ; van Sebille, Erik
    Ocean surface winds, currents, and waves play a crucial role in exchanges of momentum, energy, heat, freshwater, gases, and other tracers between the ocean, atmosphere, and ice. Despite surface waves being strongly coupled to the upper ocean circulation and the overlying atmosphere, efforts to improve ocean, atmospheric, and wave observations and models have evolved somewhat independently. From an observational point of view, community efforts to bridge this gap have led to proposals for satellite Doppler oceanography mission concepts, which could provide unprecedented measurements of absolute surface velocity and directional wave spectrum at global scales. This paper reviews the present state of observations of surface winds, currents, and waves, and it outlines observational gaps that limit our current understanding of coupled processes that happen at the air-sea-ice interface. A significant challenge for the coming decade of wind, current, and wave observations will come in combining and interpreting measurements from (a) wave-buoys and high-frequency radars in coastal regions, (b) surface drifters and wave-enabled drifters in the open-ocean, marginal ice zones, and wave-current interaction “hot-spots,” and (c) simultaneous measurements of absolute surface currents, ocean surface wind vector, and directional wave spectrum from Doppler satellite sensors.