Record-breaking warming in the Kamchatka Current halocline

Ocean Dynamics - We used historical CTD observations and Argo data to obtain the characteristics of the Kamchatka Current halocline and its eddies. Kamchatka Current eddies have a cold low salinity...     

Vertical copepod aggregations and interactions with chlorophyll and production on the Peru shelf

Data consisting of high resolution profiles of in situ chlorophyll fluorescence and copepods have been measured with a towed Batfish on the Peru shelf during November 1977. The day to day vertical variability of chlorophyll fluorescence was highly complex; however, there were indications of common features consisting of a surface chlorophyll layer about 10 to 15 m thick and a subsurface chlorophyll maximum at about 25-m depth. Of the dominant copepods measured, Eucalanus inermis, Calanus chilensis, and Centropages brachiatus, the latter two were aggregated in layers at about 15 to 20 m directly within a depression located between the surface and subsurface chlorophyll layer. Primary production profiles estimated from light and chlorophyll data indicated that carbon production was generally restricted to the surface layer (≈15m). Estimates of integrated carbon production (m^?2) within the surface layer ranged from 1.3 to 1.5 g C m^?2 d^?3. Copepod grazing estimates indicated that only 10% of daily carbon production was utilized.     

Dynamic model of mesoscale eddies. Eddy parameterization for coarse resolution ocean circulation models

In the framework of the eddy dynamic model developed in two previous papers (Dubovikov, M.S., Dynamical model of mesoscale eddies, Geophys. Astophys. Fluid Dyn., 2003, 97, 311–358; Canuto, V.M. and Dubovikov, M.S., Modeling mesoscale eddies, Ocean Modelling, 2004, 8, 1–30 referred as I–II), we compute the contribution of unresolved mesoscale eddies to the large-scale dynamic equations of the ocean. In isopycnal coordinates, in addition to the bolus velocity discussed in I–II, the mesoscale contribution to the large scale momentum equation is derived. Its form is quite different from the traditional down-gradient parameterization. The model solutions in isopycnal coordinates are transformed to level coordinates to parameterize the eddy contributions to the corresponding large scale density and momentum equations. In the former, the contributions due to the eddy induced velocity and to the residual density flux across mean isopycnals (so called Σ-term) are derived, both contributions being shown to be of the same order. As for the large scale momentum equation, as well as in isopycnal coordinates, the eddy contribution has a form which is quite different from the down-gradient expression.     

Three-dimensional structure of mesoscale eddies in the western tropical Pacific as revealed by a high-resolution ocean simulation

The three-dimensional structure of mesoscale eddies in the western tropical Pacific(6°S–20°N, 120°E–150°E)is investigated using a high-resolution ocean model simulation. Eddy detection and eddy tracking algorithms are applied to simulated horizontal velocity vectors, and the anticyclonic and cyclonic eddies identified are composited to obtain their three-dimensional structures. The mean lifetime of all long-lived eddies is about 52 days, and their mean diameter is 147 km. Two typical characteristics of mesoscale eddies are revealed and possible dynamic explanations are analyzed. One typical characteristic is that surface eddies are generally separated from subthermocline eddies along the bifurcation latitude(~13°N) of the North Equatorial Current in the western tropical Pacific, which may be associated with different eddy energy sources and vertical eddy energy fluxes in subtropical and tropical gyres. Surface eddies have maximum swirl velocities of 8–9 cm s~(-1) and can extend to about 1500 m depth. Subthermocline eddies occur below 200 m, with their cores at about 400–600 m depth, and their maximum swirl velocities can reach 10 cm s~(-1). The other typical characteristic is that the meridional velocity component of the eddy is much larger than the zonal component. This characteristic might be due to more zonal eddy pairs(two eddies at the same latitude),which is also supported by the zonal wavelength(about 200 km) in the high-frequency meridional velocity component of the horizontal velocity.     

Impact of eddies on ocean diapycnal mixing in Gulf Stream region

Based on high-resolution,Array for Real-time Geostrophic Oceanography(Argo)profiles and Sea Level Anomaly(SLA)data,this study statistically analyzes and compares turbulent diapycnal mixing profiles inside and outside mesoscale eddies in the Gulf Stream region.The result indicates that average diapycnal diffusivity at 300–540 m depths in anticyclonic eddies reaches4.0×10-5 m2 s-1.This is significantly higher than the 1.6×10-5 m2 s-1 outside eddies and 0.8×10-5 m2 s-1 in cyclonic eddies.Probabilities of diapycnal diffusivity greater than 10-4 m2 s-1 within anticyclonic and cyclonic eddies and outside eddies are29%,5%and 12%,respectively.However,magnitudes of average diapycnal diffusivity at 540–900 m depths in these three cases are of the same order,10-5 m2 s-1.Twenty-four of a total 38 anticyclonic eddies had enhanced mixing in the ocean interior,and 22 were observed during or shortly after strong winds.The coincidence between enhanced mixing and strong wind stress indicates that more wind-induced,near-inertial wave energy propagates downward in anticyclonic eddies.The deeper part of 12 profiles(below 540 m)in anticyclonic eddies had vertical overturns with Thorpe scale exceeding 5 m,among which three profiles had overturns reaching 20 m.Enhanced mixing may have occurred in deep layers of some profiles,although it was not evident in average conditions.     

Flow convergence over a broad,flat continental shelf

The broad (～500 km) southeastern Bering Sea continental shelf contains three fronts; outer (shelf break, ～170-m depth), middle (～100-m depth), and inner (～50-m depth). The shelf break and inner fronts appear to be analogous to similar fronts reported from other mid-latitude continental shelves; extensively studied examples are from the mid-Atlantic bight, off Nova Scotia, and around the British Isles. The middle front may have counterparts on the broad North Sea and East China Sea shelves.One-month current and temperature records from either side of the middle front, ～150 km landward from the shelf break, showed convergence in the layers deeper than 30 m in both the cross-shelf flow field and heat flux. The convergence was ～3 cm s^?1, so an average upwelling at ～1 × 10^?3cm s^?1 and divergence in the surface layer were required to maintain continuity. Variations in the degree of convergence arose primarily from 1 to 5-day fluctuations in sub-tidal flow across the outer shelf domain seaward of the front.Diffusive landward heat flux was dominated by tidal scales. Horizontal eddy conductivities describing the flux were ～1 ? 10⁶ on the landward side and ～5 × 10⁶cm² s^?1 on the seaward side, and were less in the layers above the bottom layer. Advective flux by the mean flow was the same order as diffusive flux, but landward in the bottom layer and seaward in the mid-water column layers, in agreement with deductions from water mass analyses. Frontal effects reduced the net cross-shelf heat flux beneath ～30 m by about 50%. The observation of a flow convergence in the middle of a broad, flat continental shelf poses an important question of dynamics.     

Modification by lateral mixing of the Warm Deep Water entering the Weddell Sea in the Maud Rise region

Deep water originating in the North Atlantic is transported across the Antarctic Circumpolar Current by eddies and, after circumnavigating of the Antarctic, enters the Weddell Gyre south of Africa. As it does so, it rises up from mid-depth towards the surface. The separate temperature and salinity maxima, the Upper and Lower Circumpolar Deep Waters, converge to form the Warm Deep Water. Cores of this water mass on the southern flank of the eastern Weddell Gyre show a change in characteristic as they flow westward in the Lazarev Sea. Observations have been made along four meridional sections at 3° E, 0°, 3° W and 6° W between 60 and 70° S during the Polarstern Cruise ANTXXIII/2 in 2005/2006. These show that a heterogeneous series of warm and salty cores entering the region from the east both north and south of Maud Rise (65° S, 3° W) gradually merge and become more homogeneous towards the west. The gradual reduction in the variance of potential temperature on isopycnals is indicative of isopycnic mixing processes. A multiple regression technique allows diagnosis of the eddy diffusivities and, thus, the relative importance of isopycnic and diapycnic mixing. The method shows that the isopycnic diffusivity lies in the range 70–140 m² s⁻¹ and the diapycnic diffusivity reaches about 3 × 10⁻⁶ m² s⁻¹. Scale analysis suggests that isopycnic diffusion dominates over diapycnic diffusion in the erosion of the Warm Deep Water cores.     

Observations of submesoscale eddies using high-frequency radar-derived kinematic and dynamic quantities

The spatio-temporal variability of submesoscale eddies off southern San Diego is investigated with two-year observations of subinertial surface currents [O(1) m depth] derived from shore-based high-frequency radars. The kinematic and dynamic quantities — velocity potential, stream function, divergence, vorticity, and deformation rates — are directly estimated from radial velocity maps using optimal interpolation. For eddy detection, the winding-angle approach based on flow geometry is applied to the calculated stream function. A cluster of nearly enclosed streamlines with persistent vorticity in time is identified as an eddy. About 700 eddies were detected for each rotation (clockwise and counter-clockwise). The two rotations show similar statistics with diameters in the range of 5–25 km and Rossby number of 0.2–2. They persist for 1–7 days with weak seasonality and migrate with a translation speed of 4–15 cm s⁻¹ advected by background currents. The horizontal structure of eddies exhibits nearly symmetric tangential velocity with a maximum at the defined radius of the eddy, non-zero radial velocity due to background flows, and Gaussian vorticity with the highest value at the center. In contrast divergence has no consistent spatial shape. Two episodic events are presented with other in situ data (subsurface current and temperature profiles, and local winds) as an example of frontal-scale secondary circulation associated with drifting submesoscale eddies.     

Effects of cyclonic mesoscale eddies on the marine ecosystem in the Kuroshio Extension region using an eddy-resolving coupled physical-biological model

Effects of mesoscale eddies on the marine ecosystem in the Kuroshio Extension (KE) region are investigated using an eddy-resolving coupled physical-biological model. The model captures the seasonal and intra-seasonal variability of chlorophyll distribution associated with the mesoscale eddies, front variability, Kuroshio meanders, and upwelling. The model also reproduces the observed interannual variability of sea surface height anomaly (SSHA) in the KE region along a zonal band of 32–34°N from 2002 to 2006. The distribution of high surface chlorophyll corresponds to low SSHA. Cyclonic eddies are found to detach from the KE jet near 150°E and 158°E and propagate westward. The westward propagating cyclonic eddies lift the nutrient-rich thermocline into the euphotic zone and maintain high levels of chlorophyll in summer. In the subsurface layer, the pattern in chlorophyll is influenced by both lateral and vertical advection. In winter, convection inside the eddy entrains high levels of nutrients into the mixed layer, increasing production, and resulting in high chlorophyll concentration throughout the surface mixed layer. There is significant interannual variability in both the cyclonic eddy activity and the surface phytoplankton bloom south of the KE jet, although whether or not there is a causal link is unclear.     

Eddy properties in the Southern California Current System

The California Current System (CCS) is an eastern boundary upwelling system characterized by strong eddies that are often generated at the coast. These eddies contribute to intense, long-distance cross-shelf transport of upwelled water with enhanced biological activity. However, the mechanisms of formation of such coastal eddies, and more importantly their capacity to trap and transport tracers, are poorly understood. Their unpredictability and strong dynamics leave us with an incomplete picture of the physical and biological processes at work, their effects on coastal export, lateral water exchange among eddies and their surrounding waters, and how long and how far these eddies remain coherent structures. Focusing our analysis on the southern part of the CCS, we find a predominance of cyclonic eddies, with a 25-km radius and a SSH amplitude of 6 cm. They are formed near shore and travel slightly northwest offshore for ~?190 days at ~?2 km day^?1. We then study one particular, representative cyclonic eddy using a combined Lagrangian and Eulerian numerical approach to characterize its kinematics. Formed near shore, this eddy trapped a core made up of ~?67% California Current waters and ~?33% California Undercurrent waters. This core was surrounded by other waters while the eddy detached from the coast, leaving the oldest waters at the eddy’s core and the younger waters toward the edge. The eddy traveled several months as a coherent structure, with only limited lateral exchange within the eddy.     

Cold anticyclonic eddies formed from cold pool water in the southern Middle Atlantic Bight

AVHRR satellite imagery of the southern Mid-Atlantic Bight during May 1993 revealed a large area of cold water over the shelf break and slope that appeared to spin up into a series of southward propagating anticyclonic eddies. The eddies had diameters of 35–45 km at the surface and moved southward at about 20 cm/sec. A radial TOYO CTD (to 50m) and ADCP velocity (to 400m) transect was conducted across the southern-most of these eddies. The upper 50 meters had minimum temperatures of less than 7°C and salinities of about 33 pss, characteristics similar to cold pool waters usually found over the continental shelf. ADCP velocity data from one of the eddies revealed anticyclonic flow extending to a depth of about 250m. The transport of cold pool water by the eddies was estimated to be 0.1 to 0.2 Sv which is of the same order as the annual mean alongshore transport of shelf water in this region. The origin of the deeper water within the eddy is unlikely to be the continental shelf because the shelf break is less than 100 m. The depth and velocity profiles along the TOYO transect were consistent with the constant potential vorticity eddy model of Flierl (1979) although the source of the eddy kinetic energy is uncertain. The cause for the exodus of cold pool water from the shelf, which extended northward to at least 38°N, is unclear but must involve the establishment of an alongshore baroclinic pressure gradient against the usual southwestward shelf flow. It is possible that the intrusion of Gulf Stream waters onto the shelf near Cape Hatteras was a precursor of this off shelf transport. The southern-most eddy was marked by high biological productivity and very high oxygen supersaturation. The phytoplankton bloom detected within the exported cold pool water, located over the continental slope, suggests a mechanism whereby production fueled by nutrients derived from the shelf can be locally exported into deep water.     

Benchmarking the mesoscale variability in global ocean eddy-permitting numerical systems

The role of data assimilation procedures on representing ocean mesoscale variability is assessed by applying eddy statistics to a state-of-the-art global ocean reanalysis (C-GLORS), a free global ocean simulation (performed with the NEMO system) and an observation-based dataset (ARMOR3D) used as an independent benchmark. Numerical results are computed on a 1/4 ^° horizontal grid (ORCA025) and share the same resolution with ARMOR3D dataset. This “eddy-permitting” resolution is sufficient to allow ocean eddies to form. Further to assessing the eddy statistics from three different datasets, a global three-dimensional eddy detection system is implemented in order to bypass the need of regional-dependent definition of thresholds, typical of commonly adopted eddy detection algorithms. It thus provides full three-dimensional eddy statistics segmenting vertical profiles from local rotational velocities. This criterion is crucial for discerning real eddies from transient surface noise that inevitably affects any two-dimensional algorithm. Data assimilation enhances and corrects mesoscale variability on a wide range of features that cannot be well reproduced otherwise. The free simulation fairly reproduces eddies emerging from western boundary currents and deep baroclinic instabilities, while underestimates shallower vortexes that populate the full basin. The ocean reanalysis recovers most of the missing turbulence, shown by satellite products , that is not generated by the model itself and consistently projects surface variability deep into the water column. The comparison with the statistically reconstructed vertical profiles from ARMOR3D show that ocean data assimilation is able to embed variability into the model dynamics, constraining eddies with in situ and altimetry observation and generating them consistently with local environment.     

Detailed structure of the Kuroshio frontal eddy along the shelf edge of the East China Sea

The detailed three-dimensional structure of the Kuroshio frontal eddy along the shelf edge of the East China Sea is revealed by the CTD, ADCP, and satellite-tracked drifter observations. The length and width of the cold core of the Kuroshio frontal eddy are about 60 and 40 km, respectively, and its phase speed is about 30 cm s^-1. The calculated buoy tracks with the use of the observed ADCP data well reproduce the observed tracks of satellite-tracked drifters around the frontal eddy. The observed maximum horizontal velocity around this frontal eddy are 40 cm s^-1 and the center of this eddy shifts offshore in the deep layer. Nutrient is advected onshore across the shelf edge by passing of this frontal eddy while it is advected offshore without the frontal eddy at the shelf edge.     

Influence of the vertical motion field on ozone concentration in the stratosphere

Computations of the mean meridional motion field in the stratosphere are applied to ozone distributions to evaluate the associated ozone concentration changes. These changes are compared with those produced by photochemical and quasi-horizontal eddy processes. For the period January–April 1964 there is a cooperative action between the mean and eddy motions with mean subsidence in middle latitudes supplying ozone to be carried polawards and equatorwards by quasi-horizontal eddy processes. At low latitudes mean horizontal motions offset the eddy transport while at high latitudes mean rising motion is the offsetting term. The mean ozone flux through 50 mb, 3.5×10²⁹ molecules sec^–1, is comparable with the fluxes evaluated by other techniques.The spring maximum is thought to be due to a modulation of the energy supply to the stratospheric eddies which, in turn, force the mean motions. Longer-term changes are to be expected; for example during Ice Ages when increased tropospheric eddy activity is anticipated there should be higher total ozone.     

Two-layer Ekman model for wind-induced shear flow near the sea surface

A simple two-layered model for steady wind-induced shear flow near the sea surface has been formulated. Basic assumptions of Ekman's theory are retained, including horizontal uniformity, infinite depth and constant (but differing) vertical eddy viscosities in the respective layers. Employing information coming from observational data, the parameters of the model (depth of the surface layer and the two eddy viscosities) are evaluated and optimized. The results thereby obtained favour the presence of a high shear layer, about 1 m thick, immediately below the sea surface. The eddy viscosity in that layer increases approximately linearly with wind speed but remains comparatively small, being generally less than 20 cm² s⁻¹ for wind speeds less than 20 m s⁻¹. In contrast, the eddy viscosity directly below the layer is two orders of magnitude larger and increases more steeply with wind speed.     

Dynamic Model of Mesoscale Eddies

Oceanic mesoscale eddies which are analogs of well known synoptic eddies (cyclones and anticyclones), are studied on the basis of the turbulence model originated by Dubovikov (Dubovikov, M.S., "Dynamical model of turbulent eddies", Int. J. Mod. Phys. B7, 4631-4645 (1993).) and further developed by Canuto and Dubovikov (Canuto, V.M. and Dubovikov, M.S., "A dynamical model for turbulence: I. General formalism", Phys. Fluids 8, 571-586 (1996a) (CD96a); Canuto, V.M. and Dubovikov, M.S., "A dynamical model for turbulence: II. Sheardriven flows", Phys. Fluids 8, 587-598 (1996b) (CD96b); Canuto, V.M., Dubovikov, M.S., Cheng, Y. and Dienstfrey, A., "A dynamical model for turbulence: III. Numerical results", Phys. Fluids 8, 599-613 (1996c)(CD96c); Canuto, V.M., Dubovikov, M.S. and Dienstfrey, A., "A dynamical model for turbulence: IV. Buoyancy-driven flows", Phys. Fluids 9, 2118-2131 (1997a) (CD97a); Canuto, V.M. and Dubovikov, M.S., "A dynamical model for turbulence: V. The effect of rotation", Phys. Fluids 9, 2132-2140 (1997b) (CD97b); Canuto, V.M., Dubovikov, M.S. and Wielaard, D.J., "A dynamical model for turbulence: VI. Two dimensional turbulence", Phys. Fluids 9, 2141-2147 (1997c) (CD97c); Canuto, V.M. and Dubovikov, M.S., "Physical regimes and dimensional structure of rotating turbulence", Phys. Rev. Lett. 78, 666-669 (1997d) (CD97d); Canuto, V.M., Dubovikov, M.S. and Dienstfrey, A., "Turbulent convection in a spectral model", Phys. Rev. Lett. 78, 662-665 (1997e) (CD97e); Canuto, V.M. and Dubovikov, M.S., "A new approach to turbulence", Int. J. Mod. Phys. 12, 3121-3152 (1997f) (CD97f); Canuto, V.M. and Dubovikov, M.S., "Two scaling regimes for rotating Raleigh-Benard convection", Phys. Rev. Letters 78, 281-284, (1998) (CD98); Canuto, V.M. and Dubovikov, M.S., "A dynamical model for turbulence: VII. The five invariants for shear driven flows", Phys. Fluids 11, 659-664 (1999a) (CD99a); Canuto, V.M., Dubovikov, M.S. and Yu, G., "A dynamical model for turbulence: VIII. IR and UV Reynolds stress spectra for shear driven flows", Phys. Fluids 11, 656-677 (1999b) (CD99b); Canuto, V.M., Dubovikov, M.S. and Yu, G., "A dynamical model for turbulence: IX. The Reynolds stress for shear driven flows", Phys. Fluids 11, 678-694 (1999c) (CD99c).). The CD model derives from general principles and does not resort to any free parameters. Yet, it successfully describes a wide variety of quite different turbulent flows. In the present work we apply CD model to the compressible ocean. The model yields mesoscale eddies generated by the baroclinic instability. The latter, in turn, arises from the nonhorizontal orientation of the surfaces of the constant potential density (isopycnals). The obtained dynamic equations for eddy fields reduce to a vertical eigen value problem, an eigen value real part yielding an eddy radius, while an imaginary part - an eddy drift velocity. The size of the eddy is about 3r_d (where r_d is the Rossby deformation radius). The eddy dynamics has the following distinctive features: (1) the large scale potential energy feeds the eddy potential energy (EPE) at scales ～ r_d , (2) from r_d EPE cascades to the smaller scales down to ～ l ₁ determined from the condition that the spectral Rossby number Ro(q) ≡ qU'(q)f^?1 becomes ～ 1 (q is two-dimensional wave number within an isopycnal surface), (3) at scales ～ l ₁ EPE transforms into eddy kinetic energy (EKE) which cascades backwards to the larger scales up to ～ r_d , where it transforms back into EPE, thereby closing the energy flux circulation in a wavenumber space, (4) dissipation of the eddy energy (EE) occurs at scales ～ l ₁ since at those scales the fluctuating component of the vertical shear is maximal and equals to the Brunt-Vaisala frequency. The latter equality is the well known condition for generating the vertical turbulence which dissipates EE. The model enables to determine all turbulence characteristics, including the horizontal (isopycnal) diffusivity κ _h in terms of the large scale mean fields. From the typical values of the latter follow estimates for the parameters of an eddy which agree well with the observational and simulational data: k_h ～ 10³m²s^?1, EKE K ～ 10³m²s^?1, r_d ～ 3 × 10⁴m, l_I ～ 10. In what concerns the bolus velocity, it contains additional terms (as compared to the model of Gent and McWilliams (Gent, P.R. and McWilliams, J.C., "Isopycnal mixing in ocean circulation models", J. Phys. Oceanogr. 20, 150-155 (1990)) which result from the eddy fields advection by a mean velocity ū. Since the latter varies with depth, it is inevitable to differ from the eddy drift velocity that produces a shearing force eroding the eddy coherent structures and, therefore, contributing negatively to EE production. This is in contrast with the positive contribution from the GM term (which is due to the baroclinic instability). In those regions where the disruptive action is stronger, there is no eddy generation.     

Modification of halocline source waters during freezing on the Beaufort Sea shelf: evidence from oxygen isotopes and dissolved nutrients

During some, but not all winters, waters on the Mackenzie shelf of the Beaufort Sea become sufficiently saline to ventilate the halocline of the adjacent Canada Basin. This occurred in March 1988, at which time a survey of the temperature, salinity, dissolved nutrient and ¹⁸O properties of the ventilating waters was completed. The regional hydrography of 1988 was very similar to that of 1981, when ventilation also occurred in this area. The δ¹⁸O-salinity properties of the cold, saline shelf waters revealed that in the winter of 1987–1988, ice was grown from water initially more saline by about 1.5 [psu] than is typical for the area. The higher initial salinity appears to have been a consequence of a two-stage conditioning of shelf waters by storms in the autumn of 1987. Since the amount of ice growth, and consequent salt rejection, over the winter of 1987–1988 was abnormally low, this conditioning played a crucial role in the formation of the ventilating water mass. Nutrient concentrations in ventilating waters were the same as those of waters unaffected by freezing. Thus significant regeneration of nutrients within the cold saline shelf waters did not occur during their 6-month period of formation. In consequence, the nutrient signatures carried into the arctic halocline by winter shelf waters from this area tended to erode, rather than to reinforce the nutrient maxima. For this reason they are not the dominant source of supply to the arctic halocline. Waters in the Chukchi and northern Bering Seas during the same period had δ¹⁸O values intermediate between those on the Mackenzie shelf and those in the arctic halocline. Thus winter shelf waters are supplied to the arctic halocline with a range of nutrient, temperature, salinity and δ¹⁸O properties. On average, the southern Canada Basin is an impressive net producer of sea ice. The net rate of production from waters in the upper 350 m in this area is about 2 m y⁻¹, approximately twice the net rate of production in the central Arctic Ocean.     

Simulation of airflow around rain gauges: Comparison of LES with RANS models

Wind is responsible for systematic errors that affect rain gauge measurements. The authors investigate the use of computational fluid dynamics (CFD) to calculate airflow around rain gauges by applying a high-resolution large eddy simulation (LES) model to determine the flow fields around a measuring system of two rain gauges. The simulated air flow field is characterized by the presence of massive separation which induces the formation and shedding of highly unsteady eddies in the detached shear layers and wakes. Parts of these detached structures occur over the orifice of the rain gauges and may substantially affect the dynamics of the raindrops in this critical region. Non-dissipative LES methods used with fine enough meshes can successfully predict these eddies and their associated fluctuations. The authors compare statistics from LES with steady-state Reynolds averaged Navier–Stokes (RANS) simulations using the k–ε and shear stress transport k–ω turbulence models. They find that both RANS and LES models predict similar mean velocity distributions around the rain gauges. However, they determine the distribution of the resolved turbulent kinetic energy (TKE) to be strongly dependent on the RANS model used. Neither RANS model predictions of TKE are close to those of LES. The authors conclude that the failure of RANS to predict TKE is an important limitation, as TKE is needed to scale the local velocity fluctuations in stochastic models used to calculate the motion of raindrops in the flow field.     

Water masses in the Monterey Bay during the summer of 2000

Water masses in Monterey Bay are determined from the CTD casts of the Monterey Ocean Observing System (MOOS) Upper-water-column Science Experiment (MUSE) August 2000 dataset. It is shown through cluster analysis that the MUSE 2000 CTD dataset contains 5 water masses. These five water masses are: bay surface water (BSW), bay warm water (BWW), bay intermediate water (BIW), sub arctic upper water (SUW) and North Pacific deep water (NPDW). The BWW is a new water mass that exists in one area and is attributed to the effects of solar heating. The volumes occupied by each of the water masses are obtained. The BIW water is the most dominant water mass and occupies 68.8% of the volume. The statistical means and standard deviations for each water parameter, including spiciness and oxygen concentration, are calculated during separate upwelling and relaxed periods.The water mass content and structure are analyzed and studied during upwelling and a relaxed period. During upwelling, along a CTD track off Pt. Ano Nuevo, the water mass T, S distribution tended to be organized along three branches. Off Pt. Ano Nuevo the innovative coastal observation network (ICON) model showed the formation of a cyclonic eddy during the analyzed upwelling period. In time the eddy moved southwest and became absorbed into the southerly flow during the initial phases of the following wind-relaxed period.     

A comparison of numerical simulations of eddy generation performed with a two- and a three-layer quasi-geostrophic model of oceanic mesoscale eddies

Abstract

The generation of eddies by a large-scale flow over mesoscale topography is studied with the help of two- and three-layer nonlinear quasi-geostrophic models of the open ocean. The equations are integrated forward in time with no eddies present initially. For a given time, the displacement of the interface between layers two and three (ζ) tends to a well-defined limit (function of the horizontal spatial coordinates) as ρ ₃ - ρ ₂ → 0 (ρ_r is the density of layer r). Even for values of α[= (ρ ₃ - ρ ₂)/(ρ ₂ - ρ ₁)] ^as small as 0.01 the potential energy due to ζ is not negligible and it can reach, in some cases, a considerable fraction of the total eddy energy.