Acoustically and visually tracked drogue measurements of nearsurface water velocities in Lake Huron, plus observations of a coastal upwelling

During July and August of 1980 our research group measured nearsurface water velocities near the eastern coast of Lake Huron by tracking drogues using acoustic travel time and compass sighting techniques. The velocity fields appeared to consist of two components. These have been termed: a sub-current, which varied slowly with depth (compared to the deepest drogue depth of 5.2 m) and, in most cases, was apparently in geostrophic balance with the cross shore pressure gradient; and, a surface layer-current (defined by the relative velocity from deeper to shallower drogues) which decayed rapidly with depth and was directed nearly parallel with the wind and waves. There was no discernable relationship between wind speed and relative velocity. There was, however, a direct dependence of relative velocity with estimated surface roughness, suggesting that Stokes drift may have been primarily responsible for the shear. The magnitudes of the observed relative velocities were approximately equal to Stokes drift magnitudes calculated from representative wave energy spectra. Also reported are measurements of current and temperature structure made prior to and following a coastal upwelling.Prepared for the Department of Energy under Contract DE-AC02-79EV10005 and the National Oceanic and Atmospheric Administration under Contract 03-5-022-26

Shelf water and chlorophyll export from the Hatteras slope and outer shelf

© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 4291–4304, doi:10.1002/2014JC009809.Using high-resolution data acquired from a shipboard ADCP and a towed Scanfish equipped with a CTD and fluorometer, we examine the properties and transport of Middle Atlantic Bight (MAB) shelf water over a region of the Hatteras outer shelf and slope where MAB shelf water is commonly deflected offshore and entrained into the Gulf Stream. The data are from a period in early August 2004 when the seasonal pycnocline of the MAB is well developed and situated over a weakly stratified, near-bottom shelf water mass commonly referred to as the cold pool. Our data show chlorophyll-rich cold pool water carried rapidly southward over the slope and outer shelf, at a rate of up to 60 cm s−1, as part of the shelf-edge frontal jet. This southward transport of chlorophyll-rich cold pool water is shunted eastward and entrained into the Gulf Stream. However, the latitude band over which this export occurs varies significantly over the 7 day course of our study, a variation which is linked to an order 50 km shift in the latitude at which the Gulf Stream separates from the continental margin. The coupled rapid translation of the Gulf Stream frontal separation and the cold pool export zone is likely to have a significant impact on the movement and accumulation of biogenic material over the Hatteras slope and rise.This work was supported by the U.S. National Science Foundation through grants OCE-03–27249 and OCE-0926999

Shelfbreak frontal eddies over the continental slope north of Cape Hatteras

Author Posting. © American Geophysical Union, 2009. 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 114 (2009): C02017, doi:10.1029/2007JC004642.Shelfbreak and slope eddies have been implicated as important agents in the exchange of water between the shelf and slope domains of the Middle Atlantic Bight (MAB). Here we present temperature, salinity, and velocity data from a series of shipboard transects that intercepted a rich eddy field over the slope of the southern MAB. Attention is focused on a well-sampled cyclonic eddy, of roughly 60-km diameter and 300-m depth, that translated southward at 0.1 m s−1. The eddy was composed of a mix of water masses including MAB shelf and slope water, Gulf Stream water, and water from the MAB shelfbreak front. Gradient Richardson numbers suggest that these water masses were subject to vigorous turbulent vertical mixing. The transport of shelfbreak frontal water contained within the eddy was substantial. In the upper 100 m, shelfbreak frontal water comprised ∼75% of the eddy's volume. This frontal water fraction moved southward with a transport of ∼0.4 Sv, comparable with the volume transport within the shelfbreak frontal jet. A number of factors indicate that this highly energetic eddy, with maximum azimuthal velocity of 0.7 m s−1, was generated through instability of the shelfbreak frontal jet. The eddy had apparently developed rapidly (in <3 days), consistent with models of eddy generation through baroclinic instability of the shelfbreak frontal jet. The eddy's potential temperature/salinity (θ/S) properties and energy density closely matched the θ/S properties and energy density found in the frontal jet to the north of the eddy.This work was supported by the U.S. National Science Foundation through grant OCE-03-27249

Anomalies in the carbonate system of Red Sea coastal habitats

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Baldry, K., Saderne, V., McCorkle, D. C., Churchill, J. H., Agust, S., & Duarte, C. M. Anomalies in the carbonate system of Red Sea coastal habitats. Biogeosciences, 17(2), (2020): 423-439, doi:10.5194/bg-17-423-2020.We use observations of dissolved inorganic carbon (DIC) and total alkalinity (TA) to assess the impact of ecosystem metabolic processes on coastal waters of the eastern Red Sea. A simple, single-end-member mixing model is used to account for the influence of mixing with offshore waters and evaporation–precipitation and to model ecosystem-driven perturbations on the carbonate system chemistry of coral reefs, seagrass meadows and mangrove forests. We find that (1) along-shelf changes in TA and DIC exhibit strong linear relationships that are consistent with basin-scale net calcium carbonate precipitation; (2) ecosystem-driven changes in TA and DIC are larger than offshore variations in >70 % of sampled seagrass meadows and mangrove forests, changes which are influenced by a combination of longer water residence times and community metabolic rates; and (3) the sampled mangrove forests show strong and consistent contributions from both organic respiration and other sedimentary processes (carbonate dissolution and secondary redox processes), while seagrass meadows display more variability in the relative contributions of photosynthesis and other sedimentary processes (carbonate precipitation and oxidative processes). The results of this study highlight the importance of resolving the influences of water residence times, mixing and upstream habitats on mediating the carbonate system and coastal air–sea carbon dioxide fluxes over coastal habitats in the Red Sea.This research has been supported by the King Abdullah University of Science and Technology (KAUST) (grant nos. BAS/1/1071-01-01 and BAS/1/1072-01-01) and the Investment in Science fund at WHOI

Tracking near surface drogues using an acoustic travel time technique in shallow, highly stratified water : problems and observations

During July and August of 1980 near surface water velocities of Lake Huron were measured by tracking drogues, equipped with sonobuoys, using an acoustic travel time technique. Prior to these experiments difficulties associated with acoustic ray bending in the shallow, highly stratified environment were anticipated. Simple models were developed to predict the errors in drogue position and velocity determination resulting from ray bending. During the experiments round trip travel times of acoustic pulses transmitted between three bottom transponders and a transducer (lowered from a ship) were recorded. These combined with ray diagrams strongly suggested that, for a separation between the transducer and a bottom transponder of about 1.2 km, pulses which were detected first traveled by two paths, that of an inflected ray and that of a ray trapped beneath the thermocline. The error in position and velocity determination associated with these paths was 1 to 2%. Evidence also indicated that increased thermocline depth resulted in decreased tracking range.Prepared for the Department of Energy under Contract DE-AC02-79EV10005 and for NOAA under Contract 03-5-022-26

Coral reef drag coefficients—surface gravity wave enhancement

Author Posting. © American Meteorological Society, 2018. 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 48 (2018): 1555-1566, doi:10.1175/JPO-D-17-0231.1.A primary challenge in modeling flow over shallow coral reefs is accurately characterizing the bottom drag. Previous studies over continental shelves and sandy beaches suggest surface gravity waves should enhance the drag on the circulation over coral reefs. The influence of surface gravity waves on drag over four platform reefs in the Red Sea is examined using observations from 6-month deployments of current and pressure sensors burst sampling at 1Hz for 4–5min. Depth-average current fluctuations U0 within each burst are dominated by wave orbital velocities uw that account for 80%–90%of the burst variance and have a magnitude of order 10 cm s21, similar to the lower-frequency depth-average current Uavg. Previous studies have shown that the cross-reef bottom stress balances the pressure gradient over these reefs. A bottom stress estimate that neglects the waves (rCdaUavgjUavgj, where r is water density and Cda is a drag coefficient) balances the observed pressure gradient when uw is smaller than Uavg but underestimates the pressure gradient when uw is larger than Uavg (by a factor of 3–5 when uw 5 2Uavg), indicating the neglected waves enhance the bottom stress. In contrast, a bottom stress estimate that includes the waves [rCda(Uavg 1 U0)jUavg 1 U0j)] balances the observed pressure gradient independent of the relative size of uw and Uavg, indicating that this estimate accounts for the wave enhancement of the bottom stress. A parameterization proposed by Wright and Thompson provides a reasonable estimate of the total bottom stress (including the waves) given the burst-averaged current and the wave orbital velocity.The Red Sea field program was supported by Awards USA 00002 and KSA 00011 made by KAUST. S. Lentz was supported for the analysis by NSF Award OCE-1558343.2019-01-1

The dynamics of weather-band sea level variations in the Red Sea

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Regional Studies in Marine Science 24 (2018): 336-342, doi:10.1016/j.rsma.2018.09.006.The variations of sea level over the Red Sea may be divided into three broad categories: tidal, seasonal and weather-band. Our study employs a variety of in situ and satellite-derived data in the first comprehensive examination of the Red Sea water level variations in the weather-band (covering periods of 4–30 days). In the central Red Sea, the range of the weather-band sea level signal is of order 0.7 m, which exceeds the tidal and seasonal sea level ranges. From EOF and correlation analysis, we find that a large fraction of the weather-band sea level variation is due to a single mode of motion that extends over the entire Red Sea. In this mode, the water level rises and falls in unison with an amplitude that declines going southward over the southern Red Sea. The temporal signal of this mode is highly correlated with the along-axis surface wind stress over the southern Red Sea, and is closely reproduced by a simple one-dimensional barotropic model with forcing by the along-axis wind stress. Although this model does not account for the full suite of dynamics affecting weather-band sea level variations in the Red Sea, it may serve as a useful predictive tool. Sea level changes associated with the development and movement of sub-mesoscale features (e.g., eddies and boundary currents) are also shown to contribute to weather-band sea level motions in the Red Sea.The pressure sensor and meteorological buoy data were acquired as part of a program supported by Award Nos. USA00001, USA00002 and KSA00011 made by KAUST to WHOI. The data analysis and modeling work of this study were supported General Commission for Survey (GCS), under a project number RGC/3/1612-01-01 made by Office of Sponsored research (ORS)/KAUST, Kingdom of Saudi Arabia

Extracting wind sea and swell from directional wave spectra derived from a bottom-mounted ADCP

Recent advances in processing velocity data from bottom-mounted Acoustic Doppler Current Profilers (ADCPs) offer the capability of partitioning directional wave specctra of surface wave height in order to separate locally generated wind waves from swell. In the study described here, we have partitioned directional wavee spectra, derived from bottom-mounted ADCP measurements at the Martha’s Vineyard Coastal Observatory (MVCO) south of Martha’s Vineyard, MA, into dominant swell and locally generated wind-wave components. The partitioning was carried out following the method of Hanson and Phillips (2001) using an exploratory approach. As part of tthis approach, we assessed the validity of the ADCP-derived wave spectra by comparing them with one-dimensional wavee spectra derived from laser altimeter measurements. This comparison identified a frequency range over which the ADCP-derived wave field may be suspeect. We also carried out a series of sensitivity tests in which we evaluated how the results of wave partitioning according to the Hanson and Phillips (2001) method is influenced by varying the parameters required to implement the method. In this report, we describe and assess the data sources used in our study, outline the methods employed for wave spectra partitioning and describe partitioning results.Funding was provided by the Office of Naval Research under Contract No. N00014-03-1-0681

Properties of Red Sea coastal currents

© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Continental Shelf Research 78 (2014): 51–61, doi:10.1016/j.csr.2014.01.025.Properties of coastal flows of the central Red Sea are examined using 2 years of velocity data acquired off the coast of Saudi Arabia near 22 °N. The tidal flow is found to be very weak. The strongest tidal constituent, the M2 tide, has a magnitude of order 4 cm s−1. Energetic near-inertial and diurnal period motions are observed. These are surface-intensified currents, reaching magnitudes of >10 cm s−1. Although the diurnal currents appear to be principally wind-driven, their relationship with the surface wind stress record is complex. Less than 50% of the diurnal current variance is related to the diurnal wind stress through linear correlation. Correlation analysis reveals a classical upwelling/downwelling response to the alongshore wind stress. However, less than 30% of the overall sub-inertial variance can be accounted for by this response. The action of basin-scale eddies, impinging on the coastal zone, is implicated as a primary mechanism for driving coastal flows.This research is based on work supported by Award nos.USA 00002 and KSA 00011 made by KAUST to WHOI

The transport of nutrient-rich Indian Ocean water through the Red Sea and into coastal reef systems

Driven by upwelling-favorable monsoon winds, nutrient-rich Gulf of Aden Intermediate Water (GAIW) enters the Red Sea from the Indian Ocean each summer. Hydrographic and velocity data acquired in autumn 2011 provide the first indication that GAIW is carried rapidly northward along the eastern Red Sea margin in a well-defined subsurface current with speeds \u3e30 cm s–1. The nutrient-rich (NO2 + NO3 concentrations up to 17 μmol l–1) GAIW overlaps the euphotic zone and appears to fuel enhanced productivity over depths of 35–67 m. GAIW is broadly distributed through the Red Sea, extending northward along the eastern Red Sea boundary to ∼24°N and carried across the Red Sea in the circulation of a basin-scale eddy. Of particular significance is the observed incursion of GAIW into coastal areas with dense coral formations, suggesting that GAIW could be an important source of new nutrients to coral reef ecosystems of the Red Sea