Browsing by Author "Hench, JL"

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    Coupled physical, chemical, and microbiological measurements suggest a connection between internal waves and surf zone water quality in the Southern California Bight
    (Continental Shelf Research, 2012-02-15) Wong, SHC; Santoro, AE; Nidzieko, NJ; Hench, JL; Boehm, AB
    Internal waves have been implicated in the cross-shore transport of scalars such as larvae, nutrients, and pollutants at locations around the world. The present study combines physical measurements with a comprehensive set of surf zone water quality measurements to evaluate the possible impact of cross-shore internal wave transport on surf zone water quality during two study periods. An array of oceanographic moorings was deployed in the summer of 2005 and 2006 at 10-20. m depth offshore of the beach to observe internal waves. Concurrently, surf zone water quality was assessed twice daily at night at an adjacent station (Huntington State Beach) by measuring concentration of phosphate, dissolved inorganic nitrogen (DIN), silicate, chlorophyll a, fecal indicator bacteria (FIB), and the human-specific fecal DNA marker in Bacteroidales. The baroclinic component accounted for about 30% of the total variance in water column velocity, indicating the importance of density-driven flow during the summer when the water column was stratified. Arrival of cold subthermocline water in the very nearshore (within 1. km of the surf zone) was characterized by strong baroclinic onshore flow near the bottom of the water column. The near bottom, baroclinic, cross-shore current was significantly lag-correlated with the near bottom temperature data along a cross-shore transect towards shore, demonstrating shoreward transport of cold subthermocline water. Wavelet analysis of temperature data showed that non-stationary temperature fluctuations were correlated with buoyancy frequency and the near bottom cross-shore baroclinic current. During periods of large temperature fluctuations, the majority of the variance was within the semi-diurnal band; however, the diurnal and high frequency bands also contained a substantial fraction of total variance. The bottom cross-shore baroclinic current was proposed as a proxy for shoreward transport potential by internal waves and was positively correlated with phosphate concentration in both years, silicate in 2005, and fecal indicator bacteria measurements in 2006. The results suggest internal waves are an important transport mechanism of nutrient-rich subthermocline water to the very nearshore in the Southern California Bight, and may facilitate the transport of FIB into the surf zone or enhance persistence of land-derived FIB. © 2011 Elsevier Ltd.
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    Heat balances and thermally driven lagoon-ocean exchangeson a tropical coral reef system (Moorea, French Polynesia)
    (Journal of Geophysical Research C: Oceans, 2015-02-25) Herdman, LMM; Hench, JL; Monismith, SG
    © 2015. American Geophysical Union. All Rights Reserved.The role of surface and advective heat fluxes on buoyancy-driven circulation was examined within a tropical coral reef system. Measurements of local meteorological conditions as well as water temperature and velocity were made at six lagoon locations for 2 months during the austral summer. We found that temperature rather than salinity dominated buoyancy in this system. The data were used to calculate diurnally phase-averaged thermal balances. A one-dimensional momentum balance developed for a portion of the lagoon indicates that the diurnal heating pattern and consistent spatial gradients in surface heat fluxes create a baroclinic pressure gradient that is dynamically important in driving the observed circulation. The baroclinic and barotropic pressure gradients make up 90% of the momentum budget in part of the system; thus, when the baroclinic pressure gradient decreases 20% during the day due to changes in temperature gradient, this substantially changes the circulation, with different flow patterns occurring during night and day. Thermal balances computed across the entire lagoon show that the spatial heating patterns and resulting buoyancy-driven circulation are important in maintaining a persistent advective export of heat from the lagoon and for enhancing ocean-lagoon exchange.
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    Hydrodynamics influence coral performance through simultaneous direct and indirect effects
    (ECOLOGY, 2015-06) Lenihan, HS; Hench, JL; Holbrook, SJ; Schmitt, RJ; Potoski, M
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    Observations of spatial flow patterns at the coral colony scale on a shallow reef flat
    (Journal of Geophysical Research: Oceans, 2013-01-01) Hench, JL; Rosman, JH
    Although small-scale spatial flow variability can affect both larger-scale circulation patterns and biological processes on coral reefs, there are few direct measurements of spatial flow patterns across horizontal scales <100 m. Here flow patterns on a shallow reef flat were measured at scales from a single colony to several adjacent colonies using an array of acoustic Doppler velocimeters on a diver-operated traverse. We observed recirculation zones immediately behind colonies, reduced currents and elevated dissipation rates in turbulent wakes up to 2 colony diameters downstream and enhanced Reynolds stresses in shear layers around wake peripheries. Flow acceleration zones were observed above and between colonies. Coherent flow structures varied with incident flow speeds; recirculation zones were stronger and wakes were more turbulent in faster flows. Low-frequency (<0.03 Hz) flow variations, for which water excursions were large compared with the colony diameters (Keulegan-Carpenter number, KC >1), had similarspatial patterns to wakes, while higher-frequency variations (0.05-0.1 Hz, KC<1) had no observable spatial structure. On the reef flat, both drag and inertial forces exerted by coral colonies could have significant effects on flow, but within different frequency ranges; drag dominates for low-frequency flow variations and inertial forces dominate for higher frequency variations, including the wave band. Our scaling analyses suggest that spatial flow patterns at colony and patch scales could have important implications or both physical and biological processes at larger reef scales through their effects on forces exerted on the flow, turbulent mixing, and dispersion. © 2013. American Geophysical Union. All Rights Reserved.
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    The island-scale internal wave climate of Moorea, French Polynesia
    (Journal of Geophysical Research: Oceans, 2012-01-01) Leichter, JJ; Stokes, MD; Hench, JL; Witting, J; Washburn, L
    Analysis of five-year records of temperatures and currents collected at Moorea reveal strong internal wave activity at predominantly semi-diurnal frequencies impacting reef slopes at depths 30m around the entire island. Temperature changes of 1.5C to 3C are accompanied by surges of upward and onshore flow and vertical shear in onshore currents. Superimposed on annual temperature changes of approximately 3C, internal wave activity is high from Oct-May and markedly lower from Jun-Sep. The offshore pycnocline is broadly distributed with continuous stratification to at least 500m depth, and a subsurface fluorescence maximum above the strong nutricline at approximately 200m. Minimum buoyancy periods range from 4.8 to 6min, with the maximum density gradient occurring at 50 to 60m depth in summer and deepening to approximately 150 to 200m in winter. The bottom slope angle around all of Moorea is super-critical relative to the vertical stratification angle suggesting that energy propagating into shallow water is only a portion of total incident internal wave energy. Vertical gradient Richardson numbers indicate dominance by density stability relative to current shear with relatively limited diapycnal mixing. Coherence and lagged cross-correlation of semi-diurnal temperature variation indicate complex patterns of inter-site arrival of internal waves and no clear coherence or lagged correlation relationships among island sides. Semi-diurnal and high frequency internal wave packets likely arrive on Moorea from a combination of local and distant sources and may have important impacts for nutrient and particle fluxes in deep reef environments. © 2012 American Geophysical Union. All Rights Reserved.
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    Wave transformation and wave-driven flow across a steep coral reef
    (Journal of Physical Oceanography, 2013-08-14) Monismith, SG; Herdman, LMM; Ahmerkamp, S; Hench, JL
    Observations of waves, setup, and wave-driven mean flows were made on a steep coral forereef and its associated lagoonal system on the north shore of Moorea, French Polynesia. Despite the steep and complex geometry of the forereef, and wave amplitudes that are nearly equal to the mean water depth, linear wave theory showed very good agreement with data. Measurements across the reef illustrate the importance of including both wave transport (owing to Stokes drift), as well as the Eulerian mean transport when computing the fluxes over the reef. Finally, the observed setup closely follows the theoretical relationship derived from classic radiation stress theory, although the two parameters that appear in the model-one reflecting wave breaking, the other the effective depth over the reef crest-must be chosen to match theory to data. © 2013 American Meteorological Society.