Application of Suction-cup-attached VHF Transmitters to the Study of Beluga, Delphinapterus leucas, Surfacing Behavior in Cook Inlet, Alaska

Suction-cup-attached VHF radio transmittes were deployed on belugas, Delphinapterus leucas, in Cook Inlet, Alaska, in 1994 and 1995 to characterize the whales' surfacing behavior. Data from video recordings were also used to characterize behavior of undisturbed whales and whales actively pursued for tagging. Statistics for dive intervals (time between the midpoints of contiguous surfacings) and surfacing intevals (time at the surface per surfacing) were estimated. Operations took place on the tidal delta of the Susitna and Little Susitna Rivers. During the 2-yr study, eight whales were successfully tagged, five tags remained attached for >60 min, and data from these were used in the analyses. Mean dive interval was 24.1 sec (interwhale SD=6.4 sec, n=5). The mean surfacing interval, as determined from the duration of signals received from the radio transmitters, was 1.8 sec (SD=0.3 sec, n=125) for one of the whales. Videotaped behaviors were categorized as "head-lifts" or "slow-rolls." Belugas were more likely to head-lift than to slow-roll during vessel approaches and tagging attempts when compared to undisturbed whales. In undisturbed groups, surfacing intervals determined from video records were significantly different between head-lifting (average = 1.02 sect, SD=0.38 sed, n=28) and slow-rolling whales (average = 2.45 sec, SD=0.37 sec, n=106). Undisturbed juveniles exhibited shorter slow-roll surfacing intervals (average = 2.25 sec, SD=0.32 sec, n=36) than adults (average = 2.55 sec, SD=0.36 sec, n=70). We did not observe strong reactions by the belugas to the suction-cup tags. This tagging method shows promise for obtaining surfacing data for durations of several days

The evolution of a buoyant river plume in response to a pulse of high discharge from a small midlatitude river

Author Posting. © American Meteorological Society, 2020. 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 50(7),(2020): 1915-1935, https://doi.org/10.1175/JPO-D-19-0127.1.A unique feature of small mountainous rivers is that discharge can be elevated by an order of magnitude during a large rain event. The impact of time-varying discharge on freshwater transport pathways and alongshore propagation rates in the coastal ocean is not well understood. A suite of simulations in an idealized coastal ocean domain using the Regional Ocean Modeling System (ROMS) with varying steady background discharge conditions (25–100 m3 s−1), pulse amplitude (200–800 m3 s−1), pulse duration (1–6 days), and steady downwelling-favorable winds (0–4 m s−1) are compared to investigate the downstream freshwater transport along the coast (in the direction of Kelvin wave propagation) following a discharge pulse from the river. The nose of the pulse propagates rapidly alongshore at 0.04–0.32 m s−1 (faster propagation corresponds with larger pulse volume and faster winds) transporting 13%–66% of the discharge. The remainder of the discharge volume initially accumulates in the bulge near the river mouth, with lower retention for longer pulse duration and stronger winds. Following the pulse, the bulge eddy disconnects from the river mouth and is advected downstream at 0–0.1 m s−1, equal to the depth-averaged wind-driven ambient water velocity. As it transits alongshore, it sheds freshwater volume farther downstream and the alongshore freshwater transport stays elevated between the nose and the transient bulge eddy. The evolution of freshwater transport at a plume cross section can be described by the background discharge, the passage of the pulse nose, and a slow exponential return to background conditions.Support for this research was provided by National Science Foundation Grants OCE1131238, OCE1260394, and OCE1829979

Modeling the Lateral Circulation in Straight, Stratified Estuaries*

The dynamics of lateral circulation in an idealized, straight estuary under varying stratification conditions is investigated using a three-dimensional, hydrostatic, primitive equation model in order to determine the importance of lateral circulation to the momentum budget within the estuary. For all model runs, lateral circulation is about 4 times as strong during flood tides as during ebbs. This flood–ebb asymmetry is due to a feedback between the lateral circulation and the along-channel tidal currents, as well as to time-varying stratification over a tidal cycle. As the stratification is increased, the lateral circulation is significantly reduced because of the adverse pressure gradient set up by isopycnals being tilted by the lateral flow itself. When rotation is included, a timedependent, cross-channel Ekman circulation is driven, and the tidally averaged, bottom lateral circulation is enhanced toward the right bank (when looking toward the ocean in the Northern Hemisphere). This asymmetry in the tidally averaged bottom lateral circulation may lead to asymmetric sediment transport, leading to asymmetric channel profiles in straight estuaries. For the weakly stratified model run, advection due to lateral currents is a dominant term in both the along-channel and cross-channel momentum equations over a tidal cycle and for the tidally averaged momentum equations. In the tidally averaged, along-channel momentum equation, lateral advection acts as a driving term for the estuarine exchange flow and can be larger than the along-channel pressure gradient. Therefore, it should not be ignored when estimating momentum budgets in estuaries

Estuarine exchange flow quantified with isohaline coordinates : contrasting long and short estuaries

Author Posting. © American Meteorological Society, 2012. 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 42 (2012): 748–763, doi:10.1175/JPO-D-11-086.1.Isohaline coordinate analysis is used to compare the exchange flow in two contrasting estuaries, the long (with respect to tidal excursion) Hudson River and the short Merrimack River, using validated numerical models. The isohaline analysis averages fluxes in salinity space rather than in physical space, yielding the isohaline exchange flow that incorporates both subtidal and tidal fluxes and precisely satisfies the Knudsen relation. The isohaline analysis can be consistently applied to both subtidally and tidally dominated estuaries. In the Hudson, the isohaline exchange flow is similar to results from the Eulerian analysis, and the conventional estuarine theory can be used to quantify the salt transport based on scaling with the baroclinic pressure gradient. In the Merrimack, the isohaline exchange flow is much larger than the Eulerian quantity, indicating the dominance of tidal salt flux. The exchange flow does not scale with the baroclinic pressure gradient but rather with tidal volume flux. This tidal exchange is driven by tidal pumping due to the jet–sink flow at the mouth constriction, leading to a linear dependence of exchange flow on tidal volume flux. Finally, a tidal conversion parameter Qin/Qprism, measuring the fraction of tidal inflow Qprism that is converted into net exchange Qin, is proposed to characterize the exchange processes among different systems. It is found that the length scale ratio between tidal excursion and salinity intrusion provides a characteristic to distinguish estuarine regimes.SNC is supported by a WHOI postdoctoral scholarship, a NSF Grant OCE-0926427, and a Taiwan National Science Council Grant NSC 100- 2199-M-002-028.WRGis supported byNSFGrantOCE- 0926427. JAL is supported by NSF Grant OCE-0452054.2012-11-0

A Comparison of Bulk Estuarine Turnover Timescales to Particle Tracking Timescales Using a Model of the Yaquina Bay Estuary

The ability to determine a bulk estuarine turnover timescale that is well defined under realistic conditions is in high demand for estuarine research and management. We compare how turnover timescales vary with tidal and river forcing from idealized forcing scenarios using a three-dimensional circulation model of the Yaquina Bay estuary in order to understand the limitations and benefits of different timescale methods for future application. Using model results, we compare bulk formula approaches—the tidal prism method, freshwater fraction method, and a relatively new estuarine timescale calculation method based on the total exchange flow (TEF)—to directly calculated timescales from particle tracking in order to assess the utility of the bulk formula timescales. All of the timescales calculated had similar magnitudes during high river discharge but varied significantly at low discharge and had different dependences on tidal amplitude. Even in the application of a single estuary-averaged timescale, we did not find that any of the bulk timescales described the estuary over a realistic range of tidal and river discharge forcing. During high discharge, the Yaquina Bay timescale is on the order of 2–5 tidal cycles based on the particle tracking analysis, but during low discharge, the turnover time varies across methods and spatial considerations appear to be more importantKeywords: Turnover time, Particle tracking, Residence time, Total exchange flowKeywords: Turnover time, Particle tracking, Residence time, Total exchange flo

A Geostrophic Adjustment Model of Two Buoyant Fluids

See article for Abstract.Keywords: Fronts, Laboratory/physical models, Boundary currents, Buoyancy, Baroclinic flows, Coastal flow

The influence of lateral advection on the residual estuarine circulation : a numerical modeling study of the Hudson River Estuary

Author Posting. © American Meteorological Society, 2009. 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 39 (2009): 107-124, doi:10.1175/2008JPO3952.1.In most estuarine systems it is assumed that the dominant along-channel momentum balance is between the integrated pressure gradient and bed stress. Scaling the amplitude of the estuarine circulation based on this balance has been shown to have predictive skill. However, a number of authors recently highlighted important nonlinear processes that contribute to the subtidal dynamics at leading order. In this study, a previously validated numerical model of the Hudson River estuary is used to examine the forces driving the residual estuarine circulation and to test the predictive skill of two linear scaling relationships. Results demonstrate that the nonlinear advective acceleration terms contribute to the subtidal along-channel momentum balance at leading order. The contribution of these nonlinear terms is driven largely by secondary lateral flows. Under a range of forcing conditions in the model runs, the advective acceleration terms nearly always act in concert with the baroclinic pressure gradient, reinforcing the residual circulation. Despite the strong contribution of the nonlinear advective terms to the subtidal dynamical balance, a linear scaling accurately predicts the strength of the observed residual circulation in the model. However, this result is largely fortuitous, as this scaling does not account for two processes that are fundamental to the estuarine circulation. The skill of this scaling results because of the compensatory relationship between the contribution of the advective acceleration terms and the suppression of turbulence due to density stratification. Both of these processes, neither of which is accounted for in the linear scaling, increase the residual estuarine circulation but have an opposite dependence on tidal amplitude and, consequently, strength of stratification.This research was supported by the Beacon Institute for Rivers and Estuaries—Woods Hole Oceanographic Institution postdoctoral fellowship program, as well as NSF Grants OCE-0452054 and OCE-0451740

The Temporal Response of the Length of a Partially Stratified Estuary to Changes in River Flow and Tidal Amplitude

The temporal response of the length of a partially mixed estuary to changes in freshwater discharge Q and tidal amplitude Uₜ is studied using a 108-day time series collected along the length of the Hudson River estuary in the spring and summer of 2004 and a long-term (13.4 yr) record of Q, Uₜ, and near-surface salinity. When Q was moderately high, the tidally averaged length of the estuary L₅, here defined as the distance from the mouth to the up-estuary location where the vertically averaged salinity is 5 psu, fluctuated by more than 47 km over the spring–neap cycle, ranging from 28 to .75 km. During low flow periods, L₅ varied very little over the spring–neap cycle and approached a steady length. The response is quantified and compared to predictions of a linearized model derived from the global estuarine salt balance. The model is forced by fluctuations in Q and Uₜ relative to average discharge Qₒ and tidal amplitude Uₜₒ and predicts the linear response time scale τ and the steady-state length Lₒ for average forcing. Two vertical mixing schemes are considered, in which 1) mixing is proportional to Uₜ and 2) dependence of mixing on stratification is also parameterized. Based on least squares fits between L₅ and estuary length predicted by the model, estimated τ varied by an order of magnitude from a period of high average discharge (Qₒ = 750 m³ s⁻¹, τ = 4.2 days) to a period of low discharge (Qₒ = 170 m³ s⁻¹, τ = 40.4 days). Over the range of observed discharge, Lₒ } Qₒ⁻⁰.³⁰±⁰.⁰³, consistent with the theoretical scaling for an estuary whose landward salt flux is driven by vertical estuarine exchange circulation. Estimated t was proportional to the discharge advection time scale (LₒA/Qₒ, where A is the cross-sectional area of the estuary). However, τ was 3–4 times larger than the theoretical prediction. The model with stratification-dependent mixing predicted variations in L₅ with higher skill than the model with mixing proportional to Uₜ. This model provides insight into the time-dependent response of a partially stratified estuary to changes in forcing and explains the strong dependence of the amplitude of the spring–neap response on freshwater discharge. However, the utility of the linear model is limited because it assumes a uniform channel, and because the underlying dynamics are nonlinear, and the forcing Q and Uₜ can undergo large amplitude variations. River discharge, in particular, can vary by over an order of magnitude over time scales comparable to or shorter than the response time scale of the estuary

Observations and modeling of coastal internal waves driven by a diurnal sea breeze

During the Internal Waves on the Continental Margin (IWAVES) field experiments of 1996 and 1997 off of Mission Beach, California (32.75° N), we observed energetic, dirunal-band motions across the entire study site in water depths ranging from 15 to 500 m and spanning a cross-shore distance of 15 km. The spectral peak of the currents was at the diurnal frequency (σD₁ = 1 cpd) and was sufficiently well resolved to be clearly separated from the slightly higher local inertial frequency (f = 1.08 cpd). These motions were surface enhanced and clockwise circularly polarized and had an upward phase propagation speed of ~68 m d¯¹, suggesting that the motions were driven predominantly by the diurnal sea breeze. However, the downward energy (upward phase) propagation seems irreconcilable with the subinertial diurnal period, and moreover, the intermittent diurnal current events were not obviously associated with the diurnal sea breeze events. We rationalize these features using a flat-bottomed linear modal sum internal wave model that includes advection and refraction due to subtidal alongshore flow, V(x,t). Fluctuations in V at the observing site can change the “effective” local Coriolis parameter f + Vx by as much 50%, thus making the diurnal motions at different times effectively either subinertial or superinertial. The model is integrated numerically for 200 days at a latitude of 32.75°N under different wind and subtidal flow conditions: purely diurnal winds and no V, purely diurnal winds and a time-independent V, narrow-band diurnal winds and no V, and narrow-band diurnal winds and subtidal, time-dependent V. Model diurnal currents forced by narrow-band diurnal winds and subtidal V show complex offshore structure with realistic intermittency and spectral broadening. This study suggests that continental margins in the vicinity of the 30° latitude (where σD₁ = f) are regions that could potentially produce energetic, sea breeze-driven baroclinic motions and that these motions could be regulated by the vorticity of the local subtidal currents

The Influence of Lateral Advection on the Residual Estuarine Circulation: A Numerical Modeling Study of the Hudson River Estuary

In most estuarine systems it is assumed that the dominant along-channel momentum balance is between the integrated pressure gradient and bed stress. Scaling the amplitude of the estuarine circulation based on this balance has been shown to have predictive skill. However, a number of authors recently highlighted important nonlinear processes that contribute to the subtidal dynamics at leading order. In this study, a previously validated numerical model of the Hudson River estuary is used to examine the forces driving the residual estuarine circulation and to test the predictive skill of two linear scaling relationships. Results demonstrate that the nonlinear advective acceleration terms contribute to the subtidal along-channel momentum balance at leading order. The contribution of these nonlinear terms is driven largely by secondary lateral flows. Under a range of forcing conditions in the model runs, the advective acceleration terms nearly always act in concert with the baroclinic pressure gradient, reinforcing the residual circulation. Despite the strong contribution of the nonlinear advective terms to the subtidal dynamical balance, a linear scaling accurately predicts the strength of the observed residual circulation in the model. However, this result is largely fortuitous, as this scaling does not account for two processes that are fundamental to the estuarine circulation. The skill of this scaling results because of the compensatory relationship between the contribution of the advective acceleration terms and the suppression of turbulence due to density stratification. Both of these processes, neither of which is accounted for in the linear scaling, increase the residual estuarine circulation but have an opposite dependence on tidal amplitude and, consequently, strength of stratification.Keywords: Baroclinic flows, Advection, Density currents, Estuarine circulation, Frictio