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  1. Free, publicly-accessible full text available October 1, 2024
  2. Exchange of material across the nearshore region, extending from the shoreline to a few kilometers offshore, determines the concentrations of pathogens and nutrients near the coast and the transport of larvae, whose cross-shore positions influence dispersal and recruitment. Here, we describe a framework for estimating the relative importance of cross-shore exchange mechanisms, including winds, Stokes drift, rip currents, internal waves, and diurnal heating and cooling. For each mechanism, we define an exchange velocity as a function of environmental conditions. The exchange velocity applies for organisms that keep a particular depth due to swimming or buoyancy. A related exchange diffusivity quantifies horizontal spreading of particles without enough vertical swimming speed or buoyancy to counteract turbulent velocities. This framework provides a way to determinewhich processes are important for cross-shore exchange for a particular study site, time period, and particle behavior. 
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  3. Abstract

    An along‐isobath current in stratified waters leads to a bottom boundary layer. In models with no alongshore variation, cross‐isobath density transport in this bottom boundary layer reduce the velocity in the bottom boundary layer via thermal wind, and thus the bottom friction experienced by the current above the boundary layer—this is bottom‐boundary‐layer arrest. If, however, alongshore variation of the flow is allowed, the bottom boundary layer is baroclinically unstable. We show with high resolution numerical models that these instabilities reduce this arrest and allow bottom friction to decelerate the flow above the bottom boundary layer when the flow is in the Kelvin wave direction (so that the bottom Ekman transport is downwelling). Both the arrest of the bottom boundary layer and the release from this arrest are asymmetric; the friction experienced by flows in the direction of Kelvin‐wave propagation (downwave) is much greater than flows in the opposite direction. The strength of the near bottom currents, and thus the magnitude of bottom friction, is found to be governed by the destruction of potential vorticity near the bottom balanced by the offshore along‐isopycnal transport of this anomalous potential vorticity. A simple model of this process is created and used to quantify the magnitude of this effect and the resulting reduction of arrest of the bottom boundary layer.

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  4. Abstract

    Glacial troughs are flat‐bottomed, steep‐sided submarine valleys that incise the shelf and significantly alter coastal circulation. We examine how these features drive exchange between the shelf and the slope in the barotropic and linear limits. When the alongshore flow is in the Kelvin‐wave (downwave/downwelling favorable) direction, the troughs move transport from the shelf upwave of the trough to the slope downwave of the trough, diminishing wind‐driven alongshore transport on the shelf downwave of the trough. Conversely, when the alongshore flow is against the Kelvin wave direction (upwave/upwelling favorable), the troughs move transport from the slope downwave of the trough to the shelf upwave of the trough. These cross‐shelf flows are driven by the acceleration and curvature of the flows induced by the narrowing and turning isobaths around the trough, and the bottom friction experienced by these accelerated flows. These dynamics are quantified by examining the along‐isobath evolution of potential vorticity in the model's limits.

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  5. Abstract

    Dispersal and adaptation are the two primary mechanisms that set the range distributions for a population or species. As such, understanding how these mechanisms interact in marine organisms in particular – with capacity for long‐range dispersal and a poor understanding of what selective environments species are responding to – can provide useful insights for the exploration of biogeographic patterns. Previously, the barnacleNotochthamalus scabrosushas revealed two evolutionarily distinct lineages with a joint distribution that suggests an association with one of the two major biogeographic boundaries (~30°S) along the coast of Chile. However, spatial and genomic sampling of this system has been limited until now. We hypothesized that given the strong oceanographic and environmental shifts associated with the other major biogeographic boundary (~42°S) for Chilean coastal invertebrates, the southern mitochondrial lineage would dominate or go to fixation in locations further to the south. We also evaluated nuclear polymorphism data from 130 single nucleotide polymorphisms to evaluate the concordance of the signal from the nuclear genome with that of the mitochondrial sample. Through the application of standard population genetic approaches along with a Lagrangian ocean connectivity model, we describe the codistribution of these lineages through a simultaneous evaluation of coastal lineage frequencies, an approximation of larval behavior, and current‐driven dispersal. Our results show that this pattern could not persist without the two lineages having distinct environmental optima. We suggest that a more thorough integration of larval dynamics, explicit dispersal models, and near‐shore environmental analysis can explain much of the coastal biogeography of Chile.

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