Rossby Adjustment Research Articles

Fjord circulation induced by melting icebergs

Abstract. In an iceberg-choked fjord, meltwater can drive circulation. Down-fjord of the ice, buoyancy and rotation lead to an outflowing surface coastal current hugging one side of the fjord with an inflowing counter-current below. To predict the structure and evolution of these currents, we develop an analytical model – complemented by numerical simulations – that involves a rectangular fjord initially at rest. Specifically, we (i) start with the so-called Rossby adjustment problem; (ii) reconfigure it for a closed channel with stratification; and (iii) generalize the conventional “dam-break” scenario to a gradual-release one that mimics the continual, slow injection of meltwater. Implicit in this description is the result that circulation is mediated by internal Kelvin waves. The analytical model shows that if the total meltwater flux increases (e.g., a larger mélange, warmer water, or enhanced ice–ocean turbulence) then circulation strength increases as would be expected. For realistic parameters, a given meltwater flux induces an exchange flow that is ∼50 times larger. This factor decreases with increasing water column stratification and vice versa. Overall, this paper is a step toward making Greenland-wide predictions of fjord inflows and outflows induced by icebergs.

Hysteresis of a periodic or leaking western boundary current flowing by a gap

A 1.5-layer quasi-geostrophic reduced gravity model is used to study the hysteresis of a periodic or leaking western boundary current (WBC) flowing by a gap. When the periods of the WBC variations are much longer than the Rossby adjustment time scales of the circulation in the vicinity of the gap, the Hopf bifurcations during the Reincrease and Re-decrease loops are delayed to produce a new domain of hysteresis of the Reynolds numbers, and the critical Reynolds numbers of the WBC regime transitions change significantly, with the domain of the hysteresis Reynolds number larger for shorter periodic forcing. When the periods of the WBC variations are comparable to those of the Rossby adjustment time scales of the circulation in the vicinity of the gap, the WBC path inside the gap becomes periodic without hysteresis. The intrusion of the WBC into the western basin generally gets smaller as the period decreases. In addition, the partial leakage of the WBC transport through the gap into the western basin is found to have significant impact on the hysteresis loop of the WBC path when the leaked transport is larger than 1/2 of the WBC. Both the intrusion extent and the critical Reynolds numbers of the WBC regime transition are changed, and the larger the throughflow transport, the larger the change.

Intrathermocline Eddies Embedded Within an Anticyclonic Vortex Ring

AbstractHigh‐resolution hydrographic measurements reveal the presence of three intrathermocline eddies (ITEs) embedded within a loop current eddy. ITEs are lenticular bodies of nearly homogeneous water, which contrasts with the well‐stratified surrounding water. Their radii and thickness ranged between 19–32 km and 150–250 m. Negative relative vorticity within their cores (down to −0.85 times the Coriolis frequency), along with a large negative stratification anomaly, results in low Ertel potential vorticity and intense negative Ertel potential vorticity anomalies. Vortex stretching and relative vorticity have comparable contributions to potential vorticity anomaly, resulting in Burger numbers of order unity. The similarity of thermohaline properties within the ITE's cores and the surrounding loop current eddy water suggests that these ITEs likely form by intense mixing events followed by Rossby adjustment.

Dynamics of an idealized fluid model for investigating convective-scale data assimilation

An idealized fluid model of convective-scale numerical weather prediction, intended for use in inexpensive data assimilation experiments, is described here and its distinctive dynamics are investigated. The model modifies the rotating shallow water equations to include some simplified dynamics of cumulus convection and associated precipitation, extending and improving the model of Würsch and Craig. Changes to this original model are the removal of ad hoc diffusive terms and the addition of Coriolis rotation terms, leading to a so-called 1.5-dimensional model. Despite the non-trivial modifications to the parent equations, it is shown that this shallow water type model remains hyperbolic in character and can be integrated accordingly using a discontinuous Galerkin finite element method for nonconservative hyperbolic systems of partial differential equations. Combined with methods to ensure well-balancedness and non-negativity, the resulting numerical solver is novel, efficient and robust. Classical numerical experiments in the shallow water theory, such as the Rossby geostrophic adjustment and flow over topography, are reproduced for the standard shallow water model and used to highlight the modified dynamics of the new model. In particular, it exhibits important aspects of convective-scale dynamics relating to the disruption of large-scale balance and is able to simulate other features related to convecting and precipitating weather systems. Our analysis here and preliminary results suggest that the model is well suited for efficiently and robustly investigating data assimilation schemes in an idealized ‘convective-scale’ forecast assimilation framework.

A two‐dimensional mixed finite‐element pair on rectangles

AbstractMixed finite‐element discretizations provide a promising method for extending highly desirable C‐grid properties to high order on quasi‐uniform horizontal grids. For the degree 1 Raviart–Thomas–quadrilateral (RT1–Q1) finite‐element discretization of the one‐dimensional linear gravity‐wave equations, it has recently been shown that the group velocity goes to zero for waves with wavelength close to or equal to twice the element width, but that this problem can be addressed by partially lumping mass matrices. As an essential stepping stone to generalization to the two‐dimensional linear shallow‐water equations, recent analysis is extended herein to the one‐dimensional linear shallow‐water equations. It is found that the previous two‐parameter partial mass lumping no longer, in general, addresses the dispersion problem; in particular, and surprisingly, there is an unanticipated spectral gap for the asymptotic limit of pure inertial oscillations. Fortunately, the ensuing dispersion problems can be addressed by deducing an additional constraint on parameter values. This is most easily obtained by ensuring removal of the spectral gap for pure inertial oscillations and leads to the original (generally) asymmetric two‐parameter partial mass lumping reducing to a symmetric single‐parameter one.The resulting modified RT1–Q1 discretization is then extended to two dimensions by taking a tensor product on rectangles. Using numerical simulations of the linear Rossby adjustment problem, it is confirmed that problematic group velocity modes also exist for the unmodified scheme in two dimensions but that they can again be removed by symmetric single‐parameter partial mass lumping. Crucially, not only does this modification lead to accurate Rossby adjustment but it also preserves the desirable properties of the unmodified RT1–Q1 scheme.

Gravity wave radiation from unsteady rotational flow in an f-plane shallow water system

Spontaneous gravity wave radiation from an unsteady rotational flow is investigated numerically in an f-plane shallow water system. Unlike the classical Rossby adjustment problem, where free development of an initially unbalanced state is investigated, we consider development of a barotropically unstable zonal flow which is initially balanced but maintained by zonal mean forcing. Gravity waves are continuously radiated from a nearly balanced rotational flow region even when the Froude number is so small that balance dynamics is thought to be a good approximation for the full system. The source of gravity waves is discussed by analogy with the theory of aero-acoustic sound wave radiation (the Lighthill theory). It is shown that the source regions correspond to regions of strong rotational flow. The gradual change of rotational flow causes gravity wave radiation. We propose an approximation for these strong sources on the assumption that the dominant flow in the jet region is non-divergent rotational flow. In addition, we calculate the zonally symmetric component of gravity waves far from the source regions, solving the Lighthill equation. Using scaling analyses for perturbations, these gravity waves can be calculated with only one approximated source term that is related to the latitudinal gradient of the fluid depth and the latitudinal mass flux. In spite of its simplicity, this approximation not only explains the physical cause of gravity wave radiation, but gives an amount of source close to that obtained by classical approximation derived from vortical motion.

Seasonality of Decadal Sea Surface Temperature Anomalies in the Northwestern Pacific

Abstract The seasonality of the decadal sea surface temperature (SST) anomalies and the related physical processes in the northwestern Pacific were investigated using a three-dimensional bulk mixed layer model. In the Kuroshio–Oyashio Extension (KOE) region, the strongest decadal SST anomaly was observed during December–February, while that of the central North Pacific occurred during February–April. From an examination of the seasonal heat budget of the ocean mixed layer, it was revealed that the seasonal-scale enhancement of the decadal SST anomaly in the KOE region was controlled by horizontal Ekman temperature transport in early winter and by vertical entrainment in autumn. The temperature transport by the geostrophic current made only a slight contribution to the seasonal variation of the decadal SST anomaly, despite controlling the upper-ocean thermal conditions on decadal time scales through the slow Rossby wave adjustment to the wind stress curl. When averaging over the entire KOE region, the contribution from the net sea surface heat flux was also no longer significantly detected. By examining the horizontal distributions of the local thermal damping rate, however, it was concluded that the wintertime decadal SST anomaly in the eastern KOE region was rather damped by the net sea surface heat flux. It was due to the fact that the anomalous local thermal damping of the SST anomaly resulting from the vertical entrainment in autumn was considerably strong enough to suppress the anomalous local atmospheric thermal forcing that acted to enhance the decadal SST anomaly.

On the use of the super compact scheme for spatial differencing in numerical models of the atmosphere

AbstractThe ‘Super Compact Finite‐Difference Method’ (SCFDM) is applied to spatial differencing of some prototype linear and nonlinear geophysical fluid dynamics problems. An alternative form of the SCFDM relations for spatial derivatives is derived. The sixth‐order SCFDM is compared in detail with the conventional fourth‐order compact and the second‐order centred differencing. For the frequency of linear inertia‐gravity waves on different numerical grids (Arakawa's A–E and Randall's Z) related to the Rossby adjustment process, the sixth‐order SCFDM shows a substantial improvement on the conventional methods. For the Jacobians involved in vorticity advection by non‐divergent flow and in the Bolin–Charney balance equation, a general framework, valid for every finite‐difference method, is derived to present the discrete forms of the Jacobians. It is found that the sixth‐order SCFDM provides a noticeably more accurate representation of the wave‐number distribution of the Jacobians, when compared with the conventional methods. The problem of reconstructing the stream‐function field from the vorticity field on a sphere is also considered. For the Rossby–Haurwitz wave, the computation of a normalized global error at different horizontal resolutions in longitude and latitude directions shows that the sixth‐order SCFDM can markedly improve on the fourth‐order compact. The sixth‐order SCFDM is thus proposed as a viable method to improve the accuracy of finite‐difference models of the atmosphere. Copyright © 2005 Royal Meteorological Society.

Nonlinear adjustment of density fronts. Part 1. The Rossby scenario and the experimental reality

This study deals with the nonlinear cyclo-geostrophic adjustment of a circular density front in a two-layer fluid. Laboratory experiments have been performed to investigate the dynamical evolution of a fixed volume of buoyant water, initially confined within a bottomless cylinder, which is quickly released in a dense rotating fluid. This configuration corresponds to a rapid input of potential energy in a geostrophic fluid layer and reproduces some dynamical processes which occur during oceanic upwelling or stratospheric warming events. We focus our efforts on the visualization techniques in order to have simultaneous and independent measurements of both the horizontal velocity field and the vertical density field. We thus obtained, for the first time, quantitative measurements of the potential vorticity and the flow balance after a geostrophic adjustment process. The density profile of the mean adjusted state observed in the experiment is in good agreement with the prediction of the standard adjustment theory based on Lagrangian conservation of potential vorticity except in the frontal region. There, strong three-dimensional motions (plume structures, shocks and rapid transient instabilities) take place during the early stage of adjustment. These transient three-dimensional motions could dissipate up to 50% of the initial energy of the system, especially when the size of the initial density anomaly is close to or larger than the deformation radius. Therefore, it significantly changes the velocity and the energy budget predicted by the standard Rossby adjustment. Both the kinetic energy of the mean adjusted state and the energy transferred to inertia-gravity wave modes are reduced by these transient dissipative processes.

Nonorographic generation of Arctic polar stratospheric clouds during December 1999

During December 1999, polar stratospheric clouds (PSCs) were observed in the absence of conditions conducive to generation by topographic gravity waves. The possibility is explored that PSCs can be generated by inertia gravity waves (IGW) radiating from breaking synoptic‐scale Rossby waves on the polar front jet. The aerosol features on 7 and 12 December are selected for comparison with theory and with simulations using the University of Wisconsin Nonhydrostatic Modeling System (UWNMS). Consistent with Rossby adjustment theory, a common feature in the UWNMS simulations is radiation of IGW from the tropopause polar front jet, especially from sectors which are evolving rapidly in the Rossby wave breaking process. Packets of gravity wave energy radiate upward and poleward into the cold pool, while individual wave crests propagate poleward and downward, causing mesoscale variations in vertical motion and temperature. On 12 December the eastbound DC‐8 lidar observations exhibited a fairly uniform field of six waves in aerosol enhancement in the 14–20 km layer, consistent with vertical displacement by a field of IGW propagating antiparallel to the flow, with characteristic horizontal and vertical wavelengths of ∼300 and ∼10 km. UWNMS simulations show emanation of a field of IGW upward and southwestward from a northward incursion of the polar front jet. The orientation and evolution of the aerosol features on 7 December are consistent with a single PSC induced by an IGW packet propagating from a breaking Rossby wave over western Russia toward the northeast into the coldest part of the base of the polar vortex, with characteristic period ∼9 hours, vertical wavelength ∼12 km, and horizontal wavelength ∼1000 km. Linear theory shows that for both of these cases, IGW energy propagates upward at ∼1 km/hour and horizontally at ∼100 km/hour, with characteristic trace speed ∼30 m/s. The spatial orientation of the PSC along IGW phase lines is contrasted with the nearly horizontal filamentary structures in the PSC, which are indicative of flow streamlines. It is suggested that vertical displacement is a crucial factor in determining whether a PSC will form and that most PSCs are relatable to specific synoptic and mesoscale motions.

Tropical Cyclogenesis via Convectively Forced Vortex Rossby Waves in a Shallow Water Primitive Equation Model

This work examines further the problem of tropical cyclogenesis by convective generation of vertical vorticity within a preexisting cyclonic circulation whose initial maximum tangential wind is approximately 5 m s 21. This paper validates and extends recent work examining the suggested upscale cascade mechanism in a three-dimensional quasigeostrophic framework using a simple shallow water primitive equation (SWPE) numerical model and helps clarify certain aspects of the Rossby adjustment problem on a nonresting basic state for finite-amplitude nonaxisymmetric disturbances. The SWPE approach serves as a meaningful intermediate step between the quasigeostrophic and full-physics frameworks and allows a simple investigation of the effects of unbalanced dynamics (contributions of gravity waves) and Rossby numbers of order unity. The authors compare quantitative results of the two models on the storm spinup time and magnitude. For asymmetric initial conditions whose mass and wind field are out of balance, robust spinup is still obtained provided the initial asymmetries possess a significant vortical component. Episodic convective forcing parameterized via unbalanced vorticity anomalies is shown to lead to spinup of a tropical storm strength vortex on a timescale of approximately 40 h. When the convective vorticity anomaly has a large amplitude compared to the initial 5 m s21 basic-state vortex, the convective anomaly becomes the dominant or ‘‘master vortex,’’ remaining essentially intact and shearing the basic-state vortex. This behavior is understood heuristically in terms of a ‘‘vortex beta Rossby number,’’ which provides a local measure of the strength of the nonlinear terms in the vorticity equation compared to the corresponding linear vortex Rossby wave restoring term. Additional experiments show that, if the convection in a single pulse mode occurs in multiple patches (or ‘‘subclusters’’) rather than in a single cluster with equal cyclonic circulation, a reduced spinup is obtained. This effect is captured in simulations with a nonlinear nondivergent semispectral model, establishing that gravity wave dynamics are not responsible for the reduction of spinup in the multiple-cluster case. A wave-mean-flow approximation with the nondivergent model also reproduces the effect of a reduced spinup with multiple-cluster convection. The applicability of the wave-mean-flow approximation at these finite amplitudes is explained by the fact that the vortex beta Rossby number of these configurations is not large. A case study using satellite observations shows that, although the observations are for a tropical storm rather than for genesis, an intensification mechanism similar to that discussed here is suggested. Further tests of the theory are proposed.

Prognostic Modeling Studies of the Keweenaw Current in Lake Superior. Part I: Formation and Evolution

Abstract The formation and evolution of the Keweenaw Current in Lake Superior were examined using a nonorthogonal-coordinate primitive equation numerical model. The model was initialized by the monthly averaged temperaturefield observed in June and September 1973 and run prognostically under different forcing conditions with and without winds. As a Rossby adjustment problem, the model predicted the formation of a well-defined coastal current jet within an inertial period of 16.4 h after the current field adjusted to the initial temperature field. The magnitude and direction of this current jet varied with the cross-shelf temperature gradient and wind velocity. It tended to intensify during northeastward (downwelling favorable) winds, and to lessen, or even reverse, during southwestward to northwestward (upwelling favorable) or southeastward (downwelling favorable) winds. In a case with strong stratification and without external atmospheric forcings, a well-defined clockwise warm-core eddy formed near the ...

Nonlinear Rossby adjustment in a channel

The Rossby adjustment problem for a homogeneous fluid in a channel is solved for large values of the initial depth discontinuity. We begin by analysing the classical dam break problem in which the depth on one side of the discontinuity is zero. An approximate solution for this case can be constructed by assuming semigeostrophic dynamics and using the method of characteristics. This theory is supplemented by numerical solutions to the full shallow water equations. The development of the flow and the final, equilibrium volume transport are governed by the ratio of the Rossby radius of deformation to the channel width, the only non-dimensional parameter. After the dam is destroyed the rotating fluid spills down the dry section of the channel forming a rarefying intrusion which, for northern hemisphere rotation, is banked against the right-hand wall (facing downstream). As the channel width is increased the speed of the leading edge (along the right-hand wall) exceeds the intrusion speed for the non-rotating case, reaching the limiting value of 3.80 times the linear Kelvin wave speed in the upstream basin. On the left side of the channel fluid separates from the sidewall at a point whose speed decreases to zero as the channel width approaches infinity. Numerical computations of the evolving flow show good agreement with the semigeostrophic theory for widths less than about a deformation radius. For larger widths cross-channel accelerations, absent in the semigeostrophic approximation, reduce the agreement. The final equilibrium transport down the channel is determined from the semigeostrophic theory and found to depart from the non-rotating result for channels widths greater than about one deformation radius. Rotation limits the transport to a constant maximum value for channel widths greater than about four deformation radii.The case in which the initial fluid depth downstream of the dam is non-zero is then examined numerically. The leading rarefying intrusion is now replaced by a Kelvin shock, or bore, whose speed is substantially less than the zero-depth intrusion speed. The shock is either straight across the channel or attached only to the right-hand wall depending on the channel width and the additional parameter, the initial depth difference. The shock speeds and amplitudes on the right-hand wall, for fixed downstream depth, increase above the non-rotating values with increasing channel width. However, rotation reduces the speed of a shock of given amplitude below the non-rotating case. We also find evidence of resonant generation of Poincaré waves by the bore. Shock characteristics are compared to theories of rotating shocks and qualitative agreement is found except for the change in potential vorticity across the shock, which is very sensitive to the model dissipation. Behind the leading shock the flow evolves in much the same way as described by linear theory except for the generation of strongly nonlinear transverse oscillations and rapid advection down the right-hand channel wall of fluid originally upstream of the dam. Final steady-state transports decrease from the zero upstream depth case as the initial depth difference is decreased.

Observations of coastal filaments in the Adriatic Sea

At the end of August 1990, 1991 and 1992, temperature readings from satellite AVHRR images provided a description of cold filaments departing from the Croatian coast directed offshore in the Adriatic Sea. The main filament was observed in the middle Adriatic extending from the Croatian to the Italian side along a topographic escarpment, giving rise to a double recirculation cell. Surface velocities associated to the offshore flow, estimated with the maximum cross-correlation technique, exhibited values on the order of 10–20 cm/s. ECMWF wind data indicate the Adriatic was characterized by winds with intensity of 5–7 m/s (breeze superposed to the climatological signal) excluding the eventuality of a wind driven filament. The filament formation process was also analyzed in terms of residual currents associated to tidal rectification processes. Comparisons between velocity values predicted by this theory and observations indicate the dynamics associated to the observed structures cannot be explained in terms of the theory of tidal residual flows. Although much more experimental evidence is required to finally prove the following hypothesis, it is shown that the baroclinic extension of the theory of the Rossby adjustment over a bottom escarpment can ascribe the filament formation in the middle Adriatic to interaction of steady coastal currents with bathymetry.

Averaging over Fast Gravity Waves for Geophysical Flows with Unbalanced Initial Data

Various facets of recent mathematical theories for averaging over fast gravity waves on advective time scales for geophysical flows with unbalanced initial data are presented here including nonlinear Rossby adjustment and simplified reduced dynamics. This work is presented within the context of simplified geophysical models involving the rotating shallow-water equations and the rotating stably stratified Boussinesq equations. Novel mechanisms for enhanced gravity wave dissipation through the catalytic interaction with potential vortical modes are also developed here within the context of the rotating shallow-water equations.

Nonlinear adjustment of a rotating homogeneous atmosphere to zonal momentum forcing

Idealized numerical simulations using a simple shallow water model are performed to study ageneralized Rossby adjustment problem which focuses on the nonlinear response of a rotating, uniform, homogeneous, barotropic zonal flow to meso-α and β scale zonal momentum forcing. The prescribed forcings propagate downstream at a speed (c) which is less than the basic stateflow speed (U), and represent the local effects of momentum deposition/redistribution attributableto a variety of physical processes. For small Rossby number flow and t≤ τ =2a/(U — c), the near-field response to meso-α scale forcing in the moving frame of reference is to producelocalized zonal jets of finite longitudinal and latitudinal extent whose geometries are similar tothe imposed forcing structure. The perturbation mass (height) field adjusts to the wind fieldassociated with these disturbances. Although the free surface vertical motion is dominated bytransient inertia-gravity waves at early times, well-defined localized vertical motions also formin the vicinity of the forcing center. For isolated forcing, ascending and descending verticalmotion occurs south and north of the forcing center, respectively. For dipole forcing, a fourcellpattern of vertical motion characterized by ascent in the southwest and northeast quadrantsand descent in the northwest and southeast quadrants flanks the forcing center where a pairof easterly and westerly jets form. For t >t, the exit region of the localized zonal jet producedby isolated forcing is advected downstream, carrying portions of the meridional perturbationwinds and free surface displacement fields with it. The long term asymptotic response is azonally elongated, synoptic scale jet due to the temporally continuous relative vorticity generatedby the zonal momentum forcing. A divergent cross-stream ageostrophic flow in the jetentrance region produces an isolated region of ascending vertical motion which is compensatedby weaker regions of descent to the east and west of the forcing center. The easterly jet producedby flow deceleration in the exit region of the dipole forcing is advected downstream during thesame time period. A four-cell pattern of vertical motion accompanies this easterly jet. Theresponse in the vicinity of the forcing center is an isolated meso-α scale westerly jet, withmeridionally confluent flow in its entrance region and meridionally diffluent flow in its exitregion. The ageostrophic circulation produces rising motion in the jet entrance region andsinking motion in the jet exit region. For moderately large Rossby number flow and meso-bscale dipole forcing, a mesoscale cyclone forms in response to fluid parcels being displacedsouthward into deeper fluid around a ridge in the height field. The moderately strong meso-bscale zonal wind maximum which is produced has associated vertical motions whose geometryis similar to those produced by larger meso-α scale dipole forcing. Stronger nonlinear advectionallows the meso-β scale jet to form four times sooner than the westerly jet produced by smallerRossby number meso-α scale dipole forcing.

Time-Dependent Fully Nonlinear Geostrophic Adjustment

Shock-capturing numerical methods are employed to integrate the fully nonlinear, rotating 1D shallow-water equations starting from steplike nongeostrophic initial conditions (a Rossby adjustment problem). Such numerical methods allow one to observe the formation of multiple bores during the transient adjustment process as well as their decay due to rotation. It is demonstrated that increasing the rotation and/or the nonlinearity increases the rate of decay. Additionally, the time required for adjustment to be completed and its dependence on nonlinearity is examined; this time is found to be highly measure dependent. Lastly, the final adjusted state of the system is observed through long time integrations. Although the bores that form provide a mechanism for dissipation, their decay results in a final state in very good agreement with the one computed by well-known (dissipationless) conservation methods.

Effect of a freshwater pulse on mesoscale circulation and phytoplankton distribution in the lower St. Lawrence Estuary

As part of a multidisciplinary program to study the physical-biological interactions regulating carbon flows in the lower St. Lawrence Estuary (LSLE), three cruises were conducted in June-July 1990 during a neap-spring tidal cycle when biological production was expected to be maximal. Nutrient (nitrates and silicates), phytoplankton biomass (chlorophyll), oxygen, temperature, salinity, and current fields were used to elucidate the effect of a freshwater pulse produced by the discharge of the St. Lawrence and Saguenay rivers on the current fields and the biological variability and productivity of the LSLE. A simple Rossby adjustment model is presented to explain the temporal (3-5 days) and spatial (40-50 km) scales of motion in our study region (impact of the freshwater pulse on the circulation). Prior to the passage of the pulse during the neap tide, the circulation was dominated by a downstream outflow and phytoplankton blooms were limited to areas of weak baroclinic currents downstream and along the south shore. The arrival of the pulse during the tidal transition led to the intensification of a transverse current that most likely reduced flushing and allowed phytoplankton biomass to develop further upstream and toward the north shore. During the spring tide, lower salinity waters and the bloom spread along the north shore as the transverse current weakened. Based on these observations, a new conceptual model of mesoscale physical-biological interactions in the LSLE is presented that emphasizes the importance of transverse motions in regulating mesoscale patterns in phytoplankton blooms.

Initialization, Asymmetry, and Spindown of Arctic Eddies

Abstract Initialization and development of cyclonic and anticyclonic eddies under the influence of a frictional surface, as with Arctic eddies under ice cover, are examined both analytically and numerically using a three-dimensional numerical model. Solutions of the linear Rossby adjustment problem show that energy released from an initial density anomaly in the barotropic mode and the first few baroclinic modes destabilizes the numerical computation for small Arctic eddies. This result suggests the necessity of slow spinup to reduce energy release in these modes and sufficient vertical resolution to resolve higher baroclinic modes. The numerical computation shows that in an initially stratified and motionless ocean, a surface cyclone (an anticyclone) is produced by a localized salinity source (sink). In open waters, the stable flow field consists of a vertically aligned pair of counterrotating eddies. When dampened by surface friction, an eddy pair produced by deep forcing has features dissimilar to the ...

The influence of canyons on shelf currents: A theoretical study

The influence of submarine canyons on shelf currents is studied using the Rossby adjustment method for a homogeneous, inviscid fluid on an f plane. The canyon in the model is assumed to have vertical edges and constant width. The geostrophic flow around a canyon is found to be dependent upon two geometric parameters: the ratio of the depth of the canyon to the depth of the shelf and the ratio of the width of the canyon to the Rossby radius over the canyon. Moreover, a single parameter determines most of the properties of the geostrophic state. This parameter is called the canyon number and is a combination of the two basic geometric parameters. In the geostrophic state an infinitely long flat‐bottom canyon will act as a complete barrier to an approaching shelf flow. The approaching flow is asymmetrically diverted along the canyon, and a net flux is generated to the left of the flow in the northern hemisphere. If the canyon cuts a shelf between the shelf break and the coast and connects to a strait (the geometry of Juan de Fuca Canyon) an in‐canyon (out‐canyon) current will be generated when the shelf break current flows keeping the shelf at its left (right) in the northern hemisphere. If the canyon has a stepped or sloped bottom, the geostrophic flow has a singularity where the step or slope meets the left canyon edge (looking upcanyon) in the northern hemisphere. Flow can cross the canyon edge through the singularity, so the canyon is no longer a complete barrier to the approaching shelf flow. In this case, as above, a net flux is generated to the left of the approaching shelf flow.