A prior second-moment closure (SMC) model of Langmuir turbulence in the upper ocean is modified by introduction of inhomogeneous pressure-strain rate and pressure-scalar gradient closures that are ...similar to the high Reynolds number, near-wall treatments for solid wall boundaries. This repairs several near-surface defects in the algebraic Reynolds stress model (ARSM) of the prior SMC by redirecting Craik-Leibovich (CL) vortex force production of turbulent kinetic energy out of the surface-normal vertical component and into a horizontal one, with an associated reduction in near-surface CL production of vertical momentum flux. A surface-proximity function introduces a new closure parameter that is tuned to previous results from large-eddy simulations (LES), and a numerical SMC model based on stability functions from the new ARSM produces improved comparisons with mean profiles of momentum and TKE components from steady-state LES results forced by aligned wind and waves. An examination of higher-order quasi-homogeneous closures and a numerical simulation of Langmuir turbulence away from the boundaries both show the near-surface inhomogeneous closure to be both necessary for consistency and preferable for simplicity.
Abstract
The Reynolds stress equation is modified to include the Craik–Leibovich vortex force, arising from the interaction of the phase-averaged surface wave Stokes drift with upper-ocean ...turbulence. An algebraic second-moment closure of the Reynolds stress equation yields an algebraic Reynolds stress model (ARSM) that requires a component of the vertical momentum flux to be directed down the gradient of the Stokes drift, in addition to the conventional component down the gradient of the ensemble-averaged Eulerian velocity. For vertical and horizontal component fluctuations, the momentum flux must be closed using the form , where the coefficient is generally distinct from the eddy viscosity or eddy diffusivity . Rational expressions for the stability functions , , and are derived for use in second-moment closure models where the turbulent velocity and length scales are dynamically modeled by prognostic equations for and . The resulting second-moment closure (SMC) includes the significant effects of the vortex force in the stability functions, in addition to source terms contributing to the and equations. Additional changes are made to the way in which is limited by proximity to boundaries or by stratification. The new SMC model is tuned to, and compared with, a suite of steady-state large-eddy simulation (LES) solutions representing a wide range of oceanic wind and wave forcing conditions. Comparisons with LES show the modified SMC captures important processes of Langmuir turbulence, but not without notable defects that may limit model generality.
Background
The long‐term effect of implantable vagus nerve stimulators (VNS) on seizures has not been evaluated in epileptic dogs.
Objectives
Report seizure frequency in medication‐resistant ...epileptic dogs before and after VNS implantation.
Animals
Twelve client‐owned dogs with idiopathic epilepsy and >1 seizure day per 3 weeks despite 3 months of appropriate use of 2 antiseizure medications and seizure diaries maintained 6 months before and >12 months after VNS implantation.
Methods
Uncontrolled, open‐label, before and after study. Mean monthly seizures and inter‐seizure periods obtained from contemporaneous seizure diaries in the 6 months before implantation were compared with 0 to 6 months, 7 to 12 months, and subsequent 12‐month periods after implantation. The number of dogs with >50% decrease in seizure frequency, >3 times increase in inter‐ictal period interval, and seizure freedom for >3 months at the time of death or last follow‐up were recorded.
Results
Five of 12 dogs were euthanized <12 months after implantation. All 7 remaining dogs showed >50% decrease in seizure frequency until last follow‐up, starting at a median of 37 to 48 months after implantation (range, 0‐6 to 61‐72 months) and a >3‐fold increase in mean inter‐seizure interval starting a median of 25 to 36 months after implantation (range, 0‐6 months to 49‐60 months), 3/7 dogs were seizure‐free at death or last follow‐up.
Conclusions and Clinical Importance
Monthly seizure frequencies decreased and inter‐seizure intervals increased in all dogs 2 to 3 years after VNS implantation, but a high proportion were euthanized before this time point. Prospective clinical trials are required to establish causality and the magnitude of this association.
Background
Implantable vagus nerve stimulation (VNS) devices can be used to treat epilepsy in dogs. Adverse effects and short‐term complications associated with delivering suggested therapeutic ...electrical stimulation (>1.5 mA) are not well‐described.
Objectives
To compare complications and adverse effects observed with standard and rapid protocols of current increase.
Animals
Sixteen client‐owned dogs with idiopathic epilepsy.
Methods
Nonrandomized, nonblinded prospective cohort study. Surgical complications, stimulation‐related adverse effects, modifications to stimulator settings, number of hospital visits, and time to reach 1.5 mA stimulation current without intolerable adverse effects were described in dogs receiving current increases every 1 to 3 weeks (slow ramping) and dogs receiving current increases every 8 to 12 hours (fast ramping).
Results
Self‐resolving surgery site seromas formed in 6 dogs. No other surgical complications were observed. Fourteen dogs reached 1.5 mA. Coughing (11/14 dogs; 5 slow, 6 fast ramping) was the most common adverse effect. Intolerable coughing that limited current increases despite changing other stimulus parameters occurred in 6/7 of the fast‐ramping group and in none of the slow‐ramping group. Median time to 1.5 mA was 72 days (range, 28‐98) in the slow‐ramping group and 77 days (range, 3‐152) in the fast‐ramping group. Median number of clinic visits was 6 for the slow‐ramping group (range, 5‐6) and 3 for the fast‐ramping group (range, 1‐7).
Conclusions and Clinical Importance
Coughing is a common adverse effect of VNS in dogs and generally is well tolerated, particularly if current is increased slowly and other stimulation parameters are adapted for effect.
A fourth production region for the globally important Antarctic bottom water has been attributed to dense shelf water formation in the Cape Darnley Polynya, adjoining Prydz Bay in East Antarctica. ...Here we show new observations from CTD-instrumented elephant seals in 2011-2013 that provide the first complete assessment of dense shelf water formation in Prydz Bay. After a complex evolution involving opposing contributions from three polynyas (positive) and two ice shelves (negative), dense shelf water (salinity 34.65-34.7) is exported through Prydz Channel. This provides a distinct, relatively fresh contribution to Cape Darnley bottom water. Elsewhere, dense water formation is hindered by the freshwater input from the Amery and West Ice Shelves into the Prydz Bay Gyre. This study highlights the susceptibility of Antarctic bottom water to increased freshwater input from the enhanced melting of ice shelves, and ultimately the potential collapse of Antarctic bottom water formation in a warming climate.
Nearly all operational ocean models use air‐sea fluxes and the ocean shear and stratification to estimate upper ocean boundary layer mixing rates. This approach implicitly parameterizes surface wave ...effects in terms of these inputs. Here we test this assumption using parallel experiments in a lake with small waves and in the open ocean with much bigger waves. Under the same wind stress and adjusting for buoyancy flux, we find the mixed layer average turbulent vertical kinetic energy in the open ocean typically twice that in the lake. The increase is consistent with models of Langmuir turbulence, in which the wave Stokes drift, and not wave breaking, is the dominant mechanism by which waves energize turbulence in the mixed layer. Applying these same theories globally, we find enhanced mixing and deeper mixed layers resulting from the inclusion of Langmuir turbulence in the boundary layer parameterization, especially in the Southern Ocean.
Key Points
Mixed layer turbulence is stronger in ocean than in lake for same air‐sea flux
The extra turbulence is due to larger surface waves in the ocean
Langmuir circulation theories explain most of the observed difference
Abstract
The scaling of turbulent kinetic energy (TKE) and its vertical component (VKE) in the upper ocean boundary layer, forced by realistic wind stress and surface waves including the effects of ...Langmuir circulations, is investigated using large-eddy simulations (LESs). The interaction of waves and turbulence is modeled by the Craik–Leibovich vortex force. Horizontally uniform surface stress τ0 and Stokes drift profiles uS(z) are specified from the 10-m wind speed U10 and the surface wave age CP/U10, where CP is the spectral peak phase speed, using an empirical surface wave spectra and an associated wave age–dependent neutral drag coefficient CD. Wave-breaking effects are not otherwise included. Mixed layer depths HML vary from 30 to 120 m, with 0.6 ≤ CP/U10 ≤ 1.2 and 8 m s−1 < U10 < 70 m s−1, thereby addressing most possible oceanic conditions where TKE production is dominated by wind and wave forcing.
The mixed layer–averaged “bulk” VKE 〈w2〉/u*2 is equally sensitive to the nondimensional Stokes e-folding depth D*S/HML and to the turbulent Langmuir number Lat = u*/US, where u* = |τ0|/ρw in water density ρw and US = |uS|z=0. Use of a D*S scale-equivalent monochromatic wave does not accurately reproduce the results using a full-surface wave spectrum with the same e-folding depth. The bulk VKE for both monochromatic and broadband spectra is accurately predicted using a surface layer (SL) Langmuir number LaSL = u*/〈uS〉SL, where 〈uS〉SL is the average Stokes drift in a surface layer 0 > z > − 0.2HML relative to that near the bottom of the mixed layer. In the wave-dominated limit LaSL → 0, turbulent vertical velocity scales as wrms ∼ u*La−2/3SL. The mean profile (z) of VKE is characterized by a subsurface peak, the depth of which increases with D*S/HML to a maximum near 0.22HML as its relative magnitude /〈w2〉 decreases. Modestly accurate scalings for these variations are presented. The magnitude of the crosswind velocity convergence scales differently from VKE. These results predict that for pure wind seas and HML ≅ 50 m, 〈w2〉/u*2 varies from less than 1 for young waves at U10 = 10 m s−1 to about 2 for mature seas at winds greater than U10 = 30 m s−1. Preliminary comparisons with Lagrangian float data account for invariance in 〈w2〉/u*2 measurements as resulting from an inverse relationship between U10 and CP/U10 in observed regimes.
The location of the Antarctic Polar Front (PF) is mapped in the Southern Indian Ocean by decomposing the shape of temperature and salinity profiles into vertical modes using a functional Principal ...Component Analysis. We define the PF as the northernmost minimum of temperature at the subsurface and represent it as a linear combination of the first three modes. This method is applied on an ocean reanalysis data set and on in situ observations, revealing a seasonal variability of the PF latitudinal position that is most pronounced between the Conrad Rise and the Kerguelen Plateau. This shift coincides with variations in the transport across the Northern Kerguelen Plateau. We suggest that seasonal changes of the upper stratification may drive the observed variability of the PF, with potentially large implications for the pathways and residence time of water masses over the plateau and the phytoplankton bloom extending southeast of the Kerguelen Islands.
Plain Language Summary
The Antarctic Polar Front (PF) is a water mass boundary that flows around Antarctica between approximately 48°S and 56°S in the Southern Indian Ocean. The position of the PF in space and time is important to understand the oceanic circulation, the heat and salt exchanges, and also marine ecosystems. In the Indian sector the PF has to cross the Kerguelen Plateau, a major bottom topography feature. The present study develops and then applies a novel method for mapping the PF taking into account the whole hydrographic structure in the upper 300 m of the ocean. We are able to map the PF position and find that it presents large seasonal variations that are more intense just west of the Kerguelen Plateau. Between the Conrad Rise and the Kerguelen Plateau, the PF is essentially zonally orientated in September and found farther south by up to 4° latitude in March. Shifts in the PF position are shown to correlate with a seasonal variation in volume transport between Kerguelen and Heard Islands. We discuss how these seasonal variations in circulation pathways could have an impact on the local marine ecosystems.
Key Points
A novel method classifying TS profiles reveals the spatiotemporal variability of the Polar Front (PF) in the Southern Indian Ocean
West of the Kerguelen Plateau, the Polar Front follows the 49 degrees south latitude in the Austral spring, while in the autumn it is located at 53 degrees south
This seasonal meandering covaries with the transport over the Plateau, suggesting important variations in circulation pathways
Human–shark encounters garner a disproportionate amount of public attention. Long-term datasets from shark mitigation programs can help determine the environmental conditions that influence abundance ...of potentially dangerous sharks. We used 25 yr (1992–2016) of shark catches from the New South Wales (NSW) Shark Meshing Program (SMP) to model the abundance of all potentially dangerous shark species (tiger Galeocerdo cuvier, white Carcharodon carcharias and whaler sharks genus Carcharhinus) and individual species/genus to determine: (1) the temporal/spatial variability in catches and (2) the oceanographic and physical variables that could influence abundance. Too few tiger sharks were caught to individually model their abundance. Generalised additive mixed models revealed seasonal and inter-annual abundance trends that differ between white and whaler sharks. Overall, sea surface temperatures (SSTs), years with SSTs colder or warmer than the long-term average, El Niño events, moon illumination, and beach length influenced the abundance of shark groups tested. White shark abundance was highest during water temperatures of ~17–18°C and declined when SST increased above 19°C. Whaler abundance increased with higher SSTs. Shark abundance was higher during El Niño events than during La Niña, although the number of whalers caught was highest during neutral phases. All groups showed a decrease in the number of catches with increasing moon illumination and higher abundance on longer beaches. These results may aid public safety methods aimed at reducing human–shark encounters by highlighting when higher numbers of sharks may occur.
Movement responses to environment Jonsen, I. D.; McMahon, C. R.; Patterson, T. A. ...
Ecology (Durham),
01/2019, Letnik:
100, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Like many species, movement patterns of southern elephant seals (Mirounga leonina) are being influenced by long-term environmental change. These seals migrate up to 4,000 km from their breeding ...colonies, foraging for months in a variety of Southern Ocean habitats. Understanding how movement patterns vary with environmental features and how these relationships differ among individuals employing different foraging strategies can provide insight into foraging performance at a population level. We apply new fast-estimation tools to fit mixed effects within a random walk movement model, rapidly inferring among-individual variability in southern elephant seal environment–movement relationships. We found that seals making foraging trips to the sea ice on or near the Antarctic continental shelf consistently reduced speed and directionality (move persistence) with increasing sea-ice coverage but had variable responses to chlorophyll a concentration, whereas seals foraging in the open ocean reduced move persistence in regions where circumpolar deep water shoaled. Given future climate scenarios, open-ocean foragers may encounter more productive habitat but sea-ice foragers may see reduced habitat availability. Our approach is scalable to large telemetry data sets and allows flexible combinations of mixed effects to be evaluated via model selection, thereby illuminating the ecological context of animal movements that underlie habitat usage.
PDF
