Smith Keston W.

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Smith
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Keston W.
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  • Article
    Expectation-maximization analysis of spatial time series
    (Copernicus Publications on behalf of the European Geosciences Union and the American Geophysical Union, 2007-02-01) Smith, Keston W. ; Aretxabaleta, Alfredo L.
    Expectation maximization (EM) is used to estimate the parameters of a Gaussian Mixture Model for spatial time series data. The method is presented as an alternative and complement to Empirical Orthogonal Function (EOF) analysis. The resulting weights, associating time points with component distributions, are used to distinguish physical regimes. The method is applied to equatorial Pacific sea surface temperature data from the TAO/TRITON mooring time series. Effectively, the EM algorithm partitions the time series into El Nino, La Nina and normal conditions. The EM method leads to a clearer interpretation of the variability associated with each regime than the basic EOF analysis.
  • Article
    Model simulations of the Bay of Fundy Gyre : 1. Climatological results
    (American Geophysical Union, 2008-10-29) Aretxabaleta, Alfredo L. ; McGillicuddy, Dennis J. ; Smith, Keston W. ; Lynch, Daniel R.
    The characteristics of a persistent gyre in the mouth of the Bay of Fundy are studied using model simulations. A set of climatological runs are conducted to evaluate the relative importance of the different forcing mechanisms affecting the gyre. The main mechanisms are tidal rectification and density-driven circulation. Stronger circulation of the gyre occurs during the later part of the stratified season (July–August and September–October). The density-driven flow around the gyre is set up by weak tidal mixing in the deep basin in the central Bay of Fundy and strong tidal mixing on the shallow flanks around Grand Manan Island and western Nova Scotia. Spring river discharge has an important influence on near-surface circulation but only a small effect when averaged over the entire water column. Retention of particles in the gyre is controlled by the residual tidal circulation, increased frontal retention during stratified periods, wind stress, and interactions with the adjacent circulation of the Gulf of Maine. Residence times longer than 30 days are predicted for particles released in the proximity of the gyre.
  • Article
    Wind-induced, cross-frontal exchange on Georges Bank : a mechanism for early summer on-bank biological particle transport
    (American Geophysical Union, 2003-11-29) Chen, Changsheng ; Schlitz, Ronald J. ; Lough, R. Gregory ; Smith, Keston W. ; Beardsley, Robert C. ; Manning, James P.
    Water exchange across the tidal-mixing front on the southern flank of Georges Bank (GB) is examined using a two-dimensional (2D) primitive equation ocean model. The model domain features a cross-frontal transect including a June 1999 hydrographic (CTD)/ADCP study made as part of the U.S. GLOBEC Northwest Atlantic/Georges Bank program. The model was initialized with temperature and salinity fields taken on the 15 June 1999 CTD section and run prognostically with tidal forcing, measured winds, and representative surface heat flux. The results show that fluctuations of wind plus tidal mixing can play the following essential role in the short-term transport of water and particles from the stratified region to the mixed region on GB in early summer, when stratification is just developing with a weak thermocline at a depth of about 10 m. First, a passing weather front drives a wind-induced on-bank Ekman transport of the upper part of the water column at the tidal-mixing front and associated particles in the surface mixed layer. Then, when the wind relaxes or changes direction, the water in the on-bank extension of the front (above the thermocline) mixes quickly through enhanced tidal motion in shallower depths of water. As a result, particles that are advected along the extended front stay in the previously well-mixed region of the bank. Surface heating tends to increase the strength of the thermocline and reduce the thickness of the surface mixed layer. This in turn accelerates the on-bank movement of the front under an easterly wind favorable for Ekman transport and thus enhances the on-bank, cross-frontal transport of particles. Since the wind-induced, cross-frontal on-bank transport of water can occur episodically during passages of meteorological fronts, these could produce a larger net cross-frontal flux than that produced by just tidal forcing on equivalent timescales. Therefore wind-induced processes can be important in the on-bank cross-frontal flux of copepods and other zooplankton species that exhibit shallow maxima in their vertical distributions over the southern flank of GB in early summer.
  • Preprint
    Model initialization in a tidally energetic regime : a dynamically adjusted objective analysis
    ( 2011-01-24) Aretxabaleta, Alfredo L. ; Smith, Keston W. ; McGillicuddy, Dennis J. ; Ballabrera-Poy, Joaquim
    A simple improvement to objective analysis of hydrographic data is proposed to eliminate spatial aliasing e ects in tidally energetic regions. The proposed method consists of the evaluation of anomalies from observations with respect to circulation model elds. The procedure is run iteratively to achieve convergence. The method is applied in the Bay of Fundy and compared with traditional objective analysis procedures and dynamically adjusted climatological elds. The hydrographic skill (di erence between observed and model temperature and salinity) of the dynamically adjusted objective analysis is signi cantly improved by reducing bias and correcting the vertical structure. Representation of the observed velocities is also improved. The resulting ow is consistent with the known circulation in the Bay.
  • Article
    Model simulations of the Bay of Fundy Gyre : 2. Hindcasts for 2005–2007 reveal interannual variability in retentiveness
    (American Geophysical Union, 2009-09-03) Aretxabaleta, Alfredo L. ; McGillicuddy, Dennis J. ; Smith, Keston W. ; Manning, James P. ; Lynch, Daniel R.
    A persistent gyre at the mouth of the Bay of Fundy results from a combination of tidal rectification and buoyancy forcing. Here we assess recent interannual variability in the strength of the gyre using data assimilative model simulations. Realistic hindcast representations of the gyre are considered during cruises in 2005, 2006, and 2007. Assimilation of shipboard and moored acoustic Doppler current profiler velocities is used to improve the skill of the simulations, as quantified by comparison with nonassimilated drifter trajectories. Our hindcasts suggest a weakening of the gyre system during May 2005. Retention of simulated passive particles in the gyre during that period was highly reduced. A recovery of the dense water pool in the deep part of the basin by June 2006 resulted in a return to particle retention characteristics similar to climatology. Retention estimates reached a maximum during May 2007 (subsurface) and June–July 2007 (near surface). Interannual variability in the strength of the gyre was primarily modulated by the stratification of the dense water pool inside the Grand Manan Basin. These changes in stratification were associated with mixing conditions the preceding fall–winter and/or advectively driven modification of water mass properties.
  • Article
    Data assimilative hindcast of the Gulf of Maine coastal circulation
    (American Geophysical Union, 2005-10-12) He, Ruoying ; McGillicuddy, Dennis J. ; Lynch, Daniel R. ; Smith, Keston W. ; Stock, Charles A. ; Manning, James P.
    A data assimilative model hindcast of the Gulf of Maine (GOM) coastal circulation during an 11 day field survey in early summer 2003 is presented. In situ observations include surface winds, coastal sea levels, and shelf hydrography as well as moored and shipboard acoustic Doppler D current profiler (ADCP) currents. The hindcast system consists of both forward and inverse models. The forward model is a three-dimensional, nonlinear finite element ocean circulation model, and the inverse models are its linearized frequency domain and time domain counterparts. The model hindcast assimilates both coastal sea levels and ADCP current measurements via the inversion for the unknown sea level open boundary conditions. Model skill is evaluated by the divergence of the observed and modeled drifter trajectories. A mean drifter divergence rate (1.78 km d−1) is found, demonstrating the utility of the inverse data assimilation modeling system in the coastal ocean setting. Model hindcast also reveals complicated hydrodynamic structures and synoptic variability in the GOM coastal circulation and their influences on coastal water material property transport. The complex bottom bathymetric setting offshore of Penobscot and Casco bays is shown to be able to generate local upwelling and downwelling that may be important in local plankton dynamics.
  • Preprint
    Analyzing state-dependent model–data comparison in multi-regime systems
    ( 2011-01-07) Aretxabaleta, Alfredo L. ; Smith, Keston W.
    An approach to analyze regime change in spatial time series data sets is followed and extended to jointly analyze a dynamical model depicting regime shift and observational data informing the same process. We analyze changes in the joint model-data regime and covariability within each regime. The method is applied to two observational data sets of equatorial sea surface temperature (TAO/TRITON array and satellite) and compared with the predicted data by the ECCO-JPL modeling system.