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John M. Klinck

Abstract

A one-layer numerical model was developed to analyze the vorticity dynamics of the seasonal variations of currents in the Southern Ocean. The model includes the continental geometry and bathymetry of the Southern Ocean and is forced by monthly climatological wind stress. Five cases are considered that compare (i) circulation over a flat bottom to that with bathymetry, (ii) effects of zonally averaged wind stress forcing versus the climatological forcing and (iii) anomaly wind stress (winds with the annual mean removed) versus the fun stress. The individual terms in the vorticity conservation equation are calculated from the model solution along two latitude lines; 57.5°S, which passes through Drake Passage, and 43.5°S, which is in the subtropical gyre. In the zonal part of the flat bottom simulation, the curl of the surface stress balances bottom stress curl. However, in Drake Passage, beta (advection of planetary vorticity) balances bottom loss—the western boundary balance. Such vorticity interactions depend on the partial barrier of South America and, thus, do not occur in zonal channel models. The removal of vorticity occurs throughout the Southern Ocean for the seasonally varying winds but the mean circulation is balanced mainly by losses near Drake Passage. The location of the Antarctic Circumpolar Current (ACC) is controlled by the lip of South America rather than the structure of the wind. The seasonal changes in the model surface elevation in Drake Passage occur largely in the southern part of the passage, in agreement with pressure observations. The calculated ACC transport is similar for climatological and zonally averaged winds but the structure of the forced circulation is rather different for the two cases. Bottom topography changes the vorticity interactions so that the largest effect occur where the flow is forced over bathymetry creating relative vorticity by stretching, which is then removed by bottom friction. The major loss in the model occurs near Drake Passage, although there are smaller losses at other locations along 57.5°S. Bathymetry provides a strong counterforce to the wind stress and the transport is reduced by a factor of ten compared to the comparable uniform depth simulation. Friction plays a secondary role by determining the width of the currents and the spinup time but has only a weak effect on the total transport.

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John M. Klinck

Abstract

A T–S volumetric census, with a resolution of 0.2°C and 0.1 psu, for years 20-25 of the World Ocean Circulation Experiment Community Modeling Effort eddy-resolving simulation of the equatorial and North Atlantic Ocean, reveals how the thermohaline character of the model has changed from the initial conditions, which were taken from the Levitus climatology. Any changes in the thermohaline structure, other than stirring, mixing, or geostrophic adjustment of smoothed climatology, must be due to the boundary conditions, which are imposed at the surface and at four sponge layers (northern boundary, southern boundary, Labrador Sea and Mediterranean Sea), where water temperature and salinity are nudged toward climatological conditions.

Several unrealistic thermohaline features appear in the solution, which can be traced to these surface and lateral sponge boundary conditions. 1) Water masses from the Arctic Ocean are overrepresented in the model. The volume transport across the northern sponge is twice the value estimated from observations. The heat flux is approximately correct, while the salt flux is large by a factor of 4.2) Water masses from the South Atlantic are underrepresented. The transport of water across the southern sponge is about two-thirds of the observed value, but the salt flux is comparable with estimates. However, the heat flux is only 10% of measured values due to a missing equatorward motion of warm surface waters. 3) Water masses from the Labrador Sea and Baffin Bay are overrepresented. The volume flux is twice that observed, while the heat flux from the sponge is realistic. The salt flux is about 20% of the observed value. 4) Finally, Mediterranean Water is underrepresented. Even though the volume transport across the sponge is eight times the observed value, the net salt flux is small by a factor of 400, leading to an insufficient production of salt.

All of these difficulties with the model T-S structure are traced to three general problems. First, the flow at the outer edge of the sponges is strongly barotropic in spite of the fact that the temperature and salinity fields are from climatology. Part of the problem with the sponges may be the smoothed nature of the climatology, which has the effect of reducing density gradients, thereby reducing geostrophic shears In all case, except the southern sponge, the volume transport across the sponge is two to eight times larger than the value expected from other analyses or observations. Since the vertical structure of the Bow is set by the climatology, the only way to create this additional transport is through barotropic flow. The reason for the additional transport is not entirely clear, but it may be due to the excessive vertical velocities that are demanded by the conversion process in the sponges. These vertical motions create bound vortices in the sponge layers that drive recirculation in the vicinity of the sponges, increasing the transport without changing the heat or salt flux. The second problem is due to geometric effects within the sponges. One such problem is that Iceland blocks the exchange along the northern sponge. Another problem is that the ocean bathymetry is specified in the sponge layer. For example, the inner Mediterranean sponge is so shallow (around 100 m) that there is very little area in which to modify the water. Similar conditions occur in the Labrador sponge where the water is also 100 m deep. The third general problem is the use of relaxation to climatology to represent surface freshwater Fluxes, which leads to unrealistic surface forcing it the currents are displaced from climatological locations. The combination of a displaced Gulf Stream and the relaxation of surface salinity to climatology produces mode waters that are unrealistically cool and fresh.

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John M. Klinck

Abstract

A rotary Empirical Orthogonal Function analysis was performed on the current meter observations made dining DRAKE 79 to quantify current variability in central Drake Passage in the vicinity of the Polar Front. Two forms of variability are revealed by the analysis: a large scale north-south shift of the Polar Front and meandering of the Polar Front. The frontal shift influences the current at the three nominal observation levels (500 m, 1400 m, 2500 m) over most of the central passage, with a time scale of about three months. Variability associated with meanders (also warm-core and cold-core rings) extends over the whole central passage. Currents as far south as ML-10 are influenced by rings that pass by the northern side of the MS array. These events occur at somewhat regular intervals of one and a half to two months.

This analysis shows that the rugged bottom topography in the central part of Drake Passage plays a dominant role in the variability of the currents. Additionally, the strong current associated with the Polar Front tends to flow around the seamounts located in the central passage. Steering also affects the cold-core rings which travel through the region.

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Julie L. McClean and John M. Klinck

Abstract

The WOCE Community Modeling Effort (CME) general circulation model of the North Atlantic was used to investigate the behavior, nature, and dynamics of 50-day oscillations seen in the meridional component of velocity between 5° and 11°N, 35° and 55°W. Oscillations of the meridional component of velocity, with a period of about 50 days, appear as the seasonal meander pattern of the North Equatorial Countercurrent starts to break down in December. They appear first near 35° and are advected westward. They have a westward phase velocity of about O.1 m s−1, wavelength of about 6000 km, and a very slow eastward group velocity. Their period, phase speed and wavelength agree with recent observations. Calculation of the leading terms from the full vorticity equation following a model decomposition in the propagation region showed that the oscillations were first and second mode baroclinic Rossby waves. Repetition of the vorticity analysis on an undecomposed snapshot during the period of retroflection revealed the NECC meanders also to be baroclinic Rossby waves, the same as the 50-day oscillations. These findings, together with the time evolution of the individual flow components over an annual cycle, suggested that the 50-day oscillations were the westward advected residue of the NECC meander pattern that is released as the NECC slows in December. The retroflecting North Brazil Current produces Rossby waves with very slow eastward group velocity that are advected eastward by the NECC until they reach 35°W, where they dissipate. A standing wave pattern is established for several months, while the NECC is active. Once it slows, the waves am advected westward and disappear totally by May. Neither wind forcing nor barotropic instability were considered to be responsible for the oscillations in the mode1.

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John M. Klinck and Eileen E. Hofmann

Abstract

A rotary empirical orthogonal function analysis of the currents measured in central Drake Passage during DRAKE 79 shows that the deep (2500 m) flow has the same spatial and temporal structure as the flow at 500 m, suggesting that current variability in this region penetrates to the bottom. However, comparison of the time amplitude of the corresponding modes indicates that the variability of the 2500 m flow resulting from north to south shifts in the location of the Polar Front lap that at 500 m by one to three days. This implies that the Polar Front slopes to the east or south (looking up from the bottom). A similar time structure was associated with the flow variations detected at moorings located downstream of a line of seamounts that extend into central Drake Passage. Additionally, the presence of mesoscale features (warm- and cold-core rings and meanders) can block or enhance the deep flow through the narrow channels separating the seamounts in Drake Passage. Such episodic changes in transport through channels has implications for deep water exchange between ocean basins, as determined from short-term current meter observations.

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Michael S. Dinniman and John M. Klinck

Abstract

It is impractical to create gridded numerical models of coastal circulation with sufficient resolution around small topographic features, such as submarine canyons, and still have the alongshore boundaries placed beyond the decay distance of coastal trapped waves. Two solutions to this problem are to make the alongshore boundaries either open or periodic. Numerical simulations were performed with upwelling and downwelling winds to compare the effects of these different choices for boundary conditions.

Several open boundary formulations were tried and three are discussed in detail. The offshore boundary was specified as “no gradient” for all variables with no serious effect. The “modified” Orlanski radiation condition is used for all variables at the alongshore boundaries, except the vertically integrated flow that has the strongest effect on the model solution.

An alongshore pressure gradient, opposing the wind, develops in the model if the modified Orlanski radiation condition is applied to the barotropic flow, causing slower currents near the surface and deep undercurrents away from the shelf. The other cases, which combined either a radiation or a relaxation boundary condition with a local solution of the barotropic equations on the boundary, were at least initially similar to the periodic case but with slower alongshore flow. The initial impact of these differences on the circulation within the canyon was small. The models with the open boundaries were more stable (did not develop strong flow meanders) than the cases with periodic conditions as initial transients are not trapped, and amplified, within the domain. Thus, open cases, especially with the upwelling winds, could run for extended times.

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John M. Klinck, Leonard J. Pietrafesa, and Gerald S. Janowitz

Abstract

A linear, two-dimensional model of a rotating, stratified fluid is constructed to investigate the circulation induced by a moving, localized line of surface stress. This model is used to analyze the effect of moving cold fronts on continental shelf circulation.

The nature of the induced circulation depends on the relative magnitude of the translation speed of the storm and the natural internal wave speed. If the surface stress moves slower than the internal wave speed. the disturbance is quasi-geostrophic and moves with the storm. If the storm moves faster than the internal wave speed, two sets of internal-inertial waves are produced. One set of waves is forced by the surface forcing and travels at the speed of the storm. Another set of waves is produced by reflection of the directly forced waves from the coastal wall.

We conclude that free surface deflection (slope) is responsible for the low-frequency. quasi-geostrophic currents due to passing cold fronts. The internal response is composed of tree inertia waves which radiate away from the coast, leaving no residual circulation.

Model results are compared to current meter data collected during the passage of a cold front over the South Atlantic Bight on 9 January 1978. The inertia frequency response observed at the mooring is reproduced by the model calculation.

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John M. Klinck, James J. O'Brien, and Harald Svendsen

Abstract

The dynamical interaction of a narrow fjord with a wind-driven coastal regime is investigated using a linear, two-layer numerical model. The Coriolis acceleration is important in the coastal regime but assumed to he unimportant in the fjord dynamical because v = 0. For a wide variety of wind conditions, bottom topography and model parameters, the wind-forced coastal circulation, with its geostrophic alongshore currants, has a strong effect on the circulation within the fjord.

These geostrophic currents control the free surface and pycnocline displacement at the fjord mouth, thereby strongly affecting fjord circulation. This mechanism is an alternative to the classical idea of hydraulic control at the mouth by sills or constrictions. Model simulators also show that the free surface slope is a baroclinic effect and that alongshore and across-shore winds affect the fjord differently. Alongshore winds produce flooding while up- and down-fjord winds simply sat up the surface. We find that offshore winds can produce large velocity shear in the fjord which can have a significant effect on turbulent intensity and diffusion.

Data available from Norwegian lords, the Strait of Juan de Fuca and Alberni Inlet. British Columbia. support this idea of dynamic control of fjord circulation by offshore wind-driven coastal currents.

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Pierre St-Laurent, John M. Klinck, and Michael S. Dinniman

Abstract

Oceanic exchanges across the continental shelves of Antarctica play an important role in biological systems and the mass balance of ice sheets. The focus of this study is on the mechanisms responsible for the circulation of warm Circumpolar Deep Water (CDW) within troughs running perpendicular to the continental shelf. This is examined using process-oriented numerical experiments with an eddy-resolving (1 km) 3D ocean model that includes a static and thermodynamically active ice shelf. Three mechanisms that create a significant onshore flow within the trough are identified: 1) a deep onshore flow driven by the melt of the ice shelf, 2) interaction between the longshore mean flow and the trough, and 3) interaction between a Rossby wave along the shelf break and the trough. In each case the onshore flow is sufficient to maintain the warm temperatures underneath the ice shelf and basal melt rates of O(1 m yr−1). The third mechanism in particular reproduces several features revealed by moorings from Marguerite Trough (Bellingshausen Sea): the temperature maximum at middepth, a stronger intrusion on the downstream edge of the trough, and the appearance of warm anticyclonic anomalies every week. Sensitivity experiments highlight the need to properly resolve the small baroclinic radii of these regions (5 km on the shelf)—simulations at 3-km resolution cannot reproduce mechanism 3 and the associated heat transport.

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Michael S. Dinniman, John M. Klinck, and Eileen E. Hofmann

Abstract

Circumpolar Deep Water (CDW) can be found near the continental shelf break around most of Antarctica. Advection of this relatively warm water (up to 2°C) across the continental shelf to the base of floating ice shelves is thought to be a critical source of heat for basal melting in some locations. A high-resolution (4 km) regional ocean–sea ice–ice shelf model of the west Antarctic Peninsula (WAP) coastal ocean was used to examine the effects of changes in the winds on across-shelf CDW transport and ice shelf basal melt. Increases and decreases in the strength of the wind fields were simulated by scaling the present-day winds by a constant factor. Additional simulations considered effects of increased Antarctic Circumpolar Current (ACC) transport. Increased wind strength and ACC transport increased the amount of CDW transported onto the WAP continental shelf but did not necessarily increase CDW flux underneath the nearby ice shelves. The basal melt underneath some of the deeper ice shelves actually decreased with increased wind strength. Increased mixing over the WAP shelf due to stronger winds removed more heat from the deeper shelf waters than the additional heat gained from increased CDW volume transport. The simulation results suggest that the effect on the WAP ice shelves of the projected strengthening of the polar westerlies is not a simple matter of increased winds causing increased (or decreased) basal melt. A simple budget calculation indicated that iron associated with increased vertical mixing of CDW could significantly affect biological productivity of this region.

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