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

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2007.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.My last three years of thesis work were supported by National Aeronautics and Space Administration Headquarters under the Earth System Science Fellowship Grant NNG04GQ14H, and by WHOI Academic Programs Fellowship Funds. I also benefited from the freedom of a Clare Boothe Luce Fellowship during my first year in the Joint Program, which allowed me more time than is usual to explore different research topics before choosing an advisor. This research was also funded by the National Aeronautics and Space Administration under grant NNG04GL03G and the Ocean Sciences Division of the National Science Foundation under grants OCE-0241292 and OCE-0548961. The Martha's Vineyard Coastal Observatory is partly funded by the Woods Hole Oceanographic Institution and the Jewett/EDUC/Harrison Foundation. The ADCP deployments at CBLAST site F were funded by National Science Foundation Small Grant for Exploratory Research OCE-0337892. Ship time for deployment and recovery of the F ADCP was provided by Robert Weller through Office of Naval Research contracts N00014-01-1-0029 and N00014-05-10090 for the Low-Wind Component of the Coupled Boundary Layers Air-Sea Transfer Experiment

Observations of cross-shelf flow driven by cross-shelf winds on the inner continental shelf

Author Posting. © American Meteorological Society, 2008. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 38 (2008): 2358-2378, doi:10.1175/2008JPO3990.1.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.This research was funded by the Ocean Sciences Division of the National Science Foundation under Grants OCE-0241292 and OCE-0548961 and by National Aeronautics and Space Administration Headquarters under Grant NNG04GL03G and the Earth System Science Fellowship Grant NNG04GQ14H. MVCO is partly funded by the Woods Hole Oceanographic Institution and the Jewett/EDUC/Harrison Foundation

Observations and a model of undertow over the inner continental shelf

Author Posting. © American Meteorological Society, 2008. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 38 (2008): 2341-2357, doi:10.1175/2008JPO3986.1.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.This research was funded by the Ocean Sciences Division of the National Science Foundation under Grants OCE-0241292 and OCE-0548961

Long-term evolution and coupling of the boundary layers in the Stratus Deck Regions of the eastern Pacific (STRATUS)

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.Funding was provided by the National Oceanic and Atmospheric Administration under grant number NA96GP0429

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

Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 1901–1926, doi:10.1002/2016JC012306.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.Funding for this work was provided by NSF awards OCE-1060840 to R.H.R. Stanley, OCE-1129644 to D.P. Nicholson, OCE-1357042 to F.P. Chavez, NASA award NNX14AI06G to M.R. Fewings, the David and Lucile Packard Foundation through their generous annual donation to the Monterey Bay Aquarium Research Institute, an Ocean Ventures Fund award from the WHOI Academic Programs Office to CC Manning, and graduate scholarships from NSERC and CMOS to CC Manning.2017-09-1

Planktonic foraminiferal assemblages reflect warming during two recent mid-latitude marine heatwaves

Under future climate scenarios, ocean temperatures that are presently extreme and qualify as marine heatwaves (MHW) are forecasted to increase in frequency and intensity, but little is known about the impact of these events on one of the most common paleoproxies, planktonic foraminifera. Planktonic foraminifera are globally ubiquitous, shelled marine protists. Their abundances and geochemistry vary with ocean conditions and fossil specimens are commonly used to reconstruct ancient ocean conditions. Planktonic foraminiferal assemblages are known to vary globally with sea surface temperature, primary productivity, and other hydrographic conditions, but have not been studied in the context of mid-latitude MHWs. For this study, the community composition and abundance of planktonic foraminifera were quantified for 2010-2019 along the Newport Hydrographic Line, a long-term monitoring transect at 44.6°N in the Northern California Current (NCC). Samples were obtained from archived plankton tows spanning 46 to 370 km offshore during annual autumn (August – October) cruises. Two MHWs impacted the region during this timeframe: the first during 2014-2016 and a second, shorter duration MHW in 2019. During the 2014-2016 MHW, warm water subtropical and tropical foraminifera species were more prevalent than the typical polar, subpolar, and transitional species common to this region. Cold water species were abundant again after the first MHW dissipated in late 2016. During the second, shorter-duration MHW in 2019, the assemblage consisted of a warm water assemblage but did not include tropical species. The foraminiferal assemblage variability correlated with changes in temperature and salinity in the upper 100 meters and was not correlated with distance offshore or upwelling. These results suggest that fossil foraminiferal assemblages from deep sea sediment cores may provide insight into the magnitude and frequency of past MHWs

A stitch in time: Combining more than two decades of mooring data from the central Oregon shelf

The highly biologically productive northern California Current, which includes the Oregon continental shelf, is an archetypal eastern boundary region with summertime upwelling driven by prevailing equatorward winds and wintertime downwelling driven by prevailing poleward winds. Between 1960 and 1990, monitoring programs and process studies conducted off the central Oregon coast advanced the understanding of many oceanographic processes, including coastal trapped waves, seasonal upwelling and downwelling in eastern boundary upwelling systems, and seasonal variability of coastal currents. Starting in 1997, the U.S. Global Ocean Ecosystems Dynamics – Long Term Observational Program (GLOBEC-LTOP) continued those monitoring and process study efforts by conducting routine CTD (Conductivity, Temperature, and Depth) and biological sampling survey cruises along the Newport Hydrographic Line (NHL; 44.652°N, 124.1 – 124.65°W), located west of Newport, Oregon. Additionally, GLOBEC-LTOP maintained a mooring slightly south of the NHL, nominally at 44.64°N, 124.30°W, on the 81-meter isobath. This location is referred to as NH-10, as it is located 10 nautical miles or 18.5 km west of Newport. A mooring was first deployed at NH-10 in August 1997. This subsurface mooring collected water column velocity data using an upward-looking acoustic Doppler current profiler. A second mooring with a surface expression was deployed at NH-10 starting in April 1999. This mooring included velocity, temperature and conductivity measurements throughout the water column as well as meteorological measurements. GLOBEC-LTOP and the Oregon State University (OSU) National Oceanographic Partnership Program (NOPP) provided funding for the NH-10 moorings from August 1997 to December 2004. Since June 2006, the NH-10 site has been occupied by a series of moorings operated and maintained by OSU with funding from the Oregon Coastal Ocean Observing System (OrCOOS), the Northwest Association of Networked Ocean Observing Systems (NANOOS), the Center for Coastal Margin Observation & Prediction (CMOP), and most recently the Ocean Observatories Initiative (OOI). While the objectives of these programs differed, each program contributed to long-term observing efforts with moorings routinely measuring meteorological and physical oceanographic variables. This article provides a brief description of each of the six programs, their associated moorings at NH-10, and our efforts to combine over twenty years of temperature, practical salinity, and velocity data into one coherent, hourly averaged, quality-controlled data set. Additionally, the data set includes best-fit seasonal cycles calculated at a daily temporal resolution for each variable using harmonic analysis with a three-harmonic fit to the observations. The stitched together, hourly NH-10 time series and seasonal cycles are available via Zenodo at https://doi.org/10.5281/zenodo.7582475

Momentum balances on the inner continental shelf at Martha's Vineyard Coastal Observatory

Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 115 (2010): C12023, doi:10.1029/2009JC005578.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.This research was funded by the Ocean Sciences Division of the National Science Foundation under grants OCE‐ 0241292 and OCE‐0548961 and by the National Aeronautics and Space Administration Headquarters under grant NNG04GL03G and the Earth System Science Fellowship grant NNG04GQ14H. MVCO is partly funded by the Woods Hole Oceanographic Institution and the Jewett/EDUC/ Harrison Foundation. CBLAST 2003 was funded by the Office of Naval Research contracts N00014‐01‐1‐0029 and N00014‐05‐10090 for the Low‐Wind Component of the Coupled Boundary Layers Air‐Sea Transfer Experiment. The F ADCP, T1 and T2 moorings, and temperature measurements near the Node in 2003 were funded by the National Science Foundation Small Grant for Exploratory Research OCE‐0337892 (“Observational Mesoscale Context for Oceanic Turbulence Measurements Obtained during CBLAST‐Low”)

Biological and Physical Interactions on a Tropical Island Coral Reef: Transport and Retention Processes on Moorea, French Polynesia

The Moorea Coral Reef Long Term Ecological Research project funded by the US National Science Foundation includes multidisciplinary studies of physical processes driving ecological dynamics across the fringing reef, back reef, and fore reef habitats of Moorea, French Polynesia. A network of oceanographic moorings and a variety of other approaches have been used to investigate the biological and biogeochemical aspects of water transport and retention processes in this system. There is evidence to support the hypothesis that a low-frequency counterclockwise flow around the island is superimposed on the relatively strong alongshore currents on each side of the island. Despite the rapid flow and flushing of the back reef, waters over the reef display chemical and biological characteristics distinct from those offshore. The patterns include higher nutrient and lower dissolved organic carbon concentrations, distinct microbial community compositions among habitats, and reef assemblages of zooplankton that exhibit migration behavior, suggesting multigenerational residence on the reef. Zooplankton consumption by planktivorous fish on the reef reflects both retention of reef-associated taxa and capture by the reef community of resources originating offshore. Coral recruitment and population genetics of reef fishes point to retention of larvae within the system and high recruitment levels from local adult populations. The combined results suggest that a broad suite of physical and biological processes contribute to high retention of externally derived and locally produced organic materials within this island coral reef system