Lynch Daniel R.

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Lynch
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Daniel R.
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  • Preprint
    Modeling turbulent dispersion on the North Flank of Georges Bank using Lagrangian Particle Methods
    ( 2004-09-29) Proehl, Jeffrey A. ; Lynch, Daniel R. ; McGillicuddy, Dennis J. ; Ledwell, James R.
    Circulation and transport at the North Flank of Georges Bank are studied using a data-assimilative 3-D model of frontal dynamics under stratified, tidally energetic conditions over steep topography. The circulation model was used in real-time during a cross-frontal transport study. Skill is evaluated retrospectively, relative to CTD, ADCP, drifter, and fluorescent dye observations. Hydrographic skill is shown to be retained for periods of weeks, requiring only initialization from routine surveys and proper atmospheric heating subsequently. Transport skill was limited by the wind stress input; real-time forecast winds taken from an operational meteorological model produced cross-isobath Ekman transport which was not observed locally. Retrospective use of observed local wind stress removed this cross-frontal bias. The contribution of tidal-time motion to the dispersion of a passive tracer is assessed using an ensemble of passive particles. The particle release simulates an at-sea dye injection in the pycnocline, which is followed for four days. Non-advective vertical tracer transport is represented as a random walk process sensitive to the local eddy diffusivity and its gradient, as computed from the turbulence closure. Non-advective horizontal tracer transport is zero for these ensembles. Computations of ensemble variance growth support estimates of (Lagrangian) horizontal dispersion. Off-bank, ensembles are essentially non-diffusive. As an ensemble engages the mixing front, its vertical diffusivity rises by 3 orders of magnitude, and horizontal spreading occurs in the complex front. The resultant horizontal dispersion is estimated from the ensemble variance growth, in along-bank and cross-bank directions. It is partitioned, roughly, between that contributed by 3-D advection alone, and that initiated by vertical diffusion. Engagement in the mixing front occurred in the forecast ensemble as a result of Ekman drift produced by an erroneous wind prediction. In the hindcast, observed wind left the ensemble non-diffusive and compact, advecting parallel to the mixing front and experiencing some advective shear dispersion. Lagrangian dispersion is event-specific and both simulations here represent credible events with dramatically different ecological outcomes. The skill metrics used are less sensitive, indicating that metrics tailored to surface-layer phenomena would be more appropriate in a data-assimilative context. The hindcast is closer to truth, based on first principles (better information). The level 2.5 closure used is realistic in the ocean interior; the near-surface processes need further refinement, especially as both surface- and bottom-generated turbulence affect these events strongly.
  • Preprint
    Mechanisms regulating large-scale seasonal fluctuations in Alexandrium fundyense populations in the Gulf of Maine : results from a physical–biological model
    ( 2005-04-27) McGillicuddy, Dennis J. ; Anderson, Donald M. ; Lynch, Daniel R. ; Townsend, David W.
    Observations of Alexandrium fundyense in the Gulf of Maine indicate several salient characteristics of the vegetative cell distributions: patterns of abundance are gulf-wide in geographic scope; their main features occur in association with the Maine Coastal Current; and the center of mass of the distribution shifts upstream from west to east during the growing season from April to August. The mechanisms underlying these aspects are investigated using coupled physical-biological simulations that represent the population dynamics of A. fundyense within the seasonal mean flow. A model that includes germination, growth, mortality, and nutrient limitation is qualitatively consistent with the observations. Germination from resting cysts appears to be a key aspect of the population dynamics that confines the cell distribution near the coastal margin, as simulations based on a uniform initial inoculum of vegetative cells across the Gulf of Maine produces blooms that are broader in geographic extent than is observed. In general, cells germinated from the major cyst beds (in the Bay of Fundy and near Penobscot and Casco Bays) are advected in the alongshore direction from east to west in the coastal current. Growth of the vegetative cells is limited primarily by temperature from April through June throughout the gulf, whereas nutrient limitation occurs in July and August in the western gulf. Thus the seasonal shift in the center of mass of cells from west to east can be explained by changing growth conditions: growth is more rapid in the western gulf early in the season due to warmer temperatures, whereas growth is more rapid in the eastern gulf later in the season due to severe nutrient limitation in the western gulf during that time period. A simple model of encystment based on nutrient limitation predicts deposition of new cysts in the vicinity of the observed cyst bed offshore of Casco and Penobscot Bays, suggesting a pathway of re-seeding the bed from cells advected downstream in the coastal current. A retentive gyre at the mouth of the Bay of Fundy tends to favor re-seeding that cyst bed from local populations.