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Tamay M. Özgökmen
and
Eric P. Chassignet

Abstract

The emergence of Fofonoff-like flows over a wide range of dissipative parameter regimes is explored in a wind-driven two-layer quasigeostrophic model. Two regimes are found in which Fofonoff-like circulations emerge as a direct consequence of the baroclinic nature of the system, since the wind forcing used in these experiments has been shown to inhibit the formation of Fofonoff flows in the barotropic case. The first regime is one in which the magnitudes of the frictional coefficients (viscosity and bottom dissipation) are extremely small. The experiments clearly illustrate the transition of the numerical solution from a conventional wind-driven circulation to an inertial Fofonoff-like regime. The latter circulation first appears in the lower layer and then spreads throughout the water column via barotropization. The second regime, surprisingly, is obtained with very high bottom friction. This result indicates that entropy can be maximized independently in each layer, depending on the distribution of forcing and dissipation. This sheds a new perspective on the common assumption that forcing and dissipation are disruptive effects that prevent the system from displaying a Fofonoff state.

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Tamay M. Özgökmen
and
Eric P. Chassignet

Abstract

In light of previous numerical studies demonstrating a strong sensitivity of the strength of thermohaline circulation to the representation of overflows in ocean general circulation models, the dynamics of bottom gravity currents are investigated using a two-dimensional, nonhydrostatic numerical model. The model explicitly resolves the Kelvin–Helmholtz instability, the main mechanism of mixing in nonrotating bottom gravity currents.

A series of experiments were conducted to explore the impact of density difference and slope angle on the dynamics of bottom gravity currents in a nonrotating and homogeneous environment. The features of the simulated currents; that is, a characteristic head at the leading edge and lumped vortices in the trailing fluid, agree qualitatively well with those observed in laboratory experiments. Quantitative comparisons of speed of descent indicate that laboratory results remain valid at geophysical scales.

Two distinct regimes of entrainment of ambient fluid into bottom gravity currents are identified: (i) the laminar entrainment regime is associated with the initial growth of the characteristic head due to the drag exerted by the fresh fluid in front and (ii) the turbulent entrainment is associated with the Kelvin–Helmholtz instabilities. The turbulent entrainment is found to be much stronger than the laminar entrainment, and entrainment in the turbulent regime is less sensitive to the slope angle than that in the laminar regime. The entrainment is quantified as a function of basic parameters of the system, the buoyancy flux and the slope angle, for the purpose of parameterizing the mixing induced by bottom gravity currents.

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Tamay M. Özgökmen
and
Fulvio Crisciani

Abstract

The dynamics of zonal recirculating flows (β plumes) driven by a source of potential vorticity, or a sink of mass, located at the eastern boundary of an ocean basin are investigated using analytical solutions of the barotropic, linear, steady quasigeostrophic equation with bottom and lateral friction.

By scaling the ratio of the strength of the recirculation to that of the sink by that from the inviscid solution, the regimes are identified in which friction becomes a dominant factor. The primary new finding of this study is that the recirculating flow component disappears due to frictional effects when the meridional extent of the sink becomes small. Unlike for the zonal extent of the sink, which affects the recirculating component only if it is on the order of the frictional boundary layer scale, the deviation of the recirculating flow strength from that given by the inviscid solution is apparent for meridional sink scales, which are much larger than the frictional boundary layer scale. Also, location of the maximum recirculation and westward penetration distance of the plumes are quantified. Finally, a stability analysis is conducted to determine the parameter regime in which the β plumes are candidates for instability.

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Anne Molcard
,
Annalisa Griffa
, and
Tamay M. Özgökmen

Abstract

Because of the increases in the realism of OGCMs and in the coverage of Lagrangian datasets in most of the world's oceans, assimilation of Lagrangian data in OGCMs emerges as a natural avenue to improve ocean state forecast with many potential practical applications, such as environmental pollutant transport, biological, and naval-related problems.

In this study, a Lagrangian data assimilation method, which was introduced in prior studies in the context of single-layer quasigeostrophic and primitive equation models, is extended for use in multilayer OGCMs using statistical correlation coefficients between velocity fields in order to project the information from the data-containing layer to the other model layers. The efficiency of the assimilation scheme is tested using a set of twin experiments with a three-layer model, as a function of the layer in which the floats are launched and of the assimilation sampling period normalized by the Lagrangian time scale of motion.

It is found that the assimilation scheme is effective provided that the correlation coefficient between the layer that contains the data and the others is high, and the data sampling period Δt is smaller than the Lagrangian time scale TL . When the assimilated data are taken in the first layer, which is the most energetic and is characterized by the fastest time scale, the assimilation is very efficient and gives relatively low errors also in the other layers (≈ 40% in the first 120 days) provided that Δt is small enough, Δt << TL . The assimilation is also efficient for data released in the third layer (errors < 60%), while the dependence on Δt is distinctively less marked for the same range of values, since the time scales of the deeper layer are significantly longer. Results for the intermediate layer show a similar insensitivity to Δt, but the errors are higher (exceeding 70%), because of the lower correlation with the other layers. These results suggest that the assimilation of deep-layer data with low energetics can be very effective, but it is strongly dependent on layer correlation. The methodology also remains quite robust to large deviations from geostrophy.

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Toshio M. Chin
,
Tamay M. Özgökmen
, and
Arthur J. Mariano

Abstract

A recipe for a cubic B-spline-based solution for multivariate variational formulation of a data analysis and assimilation problem is provided. To represent a signal whose smallest wavelength is L, the spline scale must be at most L/2, or approximately the Nyquist wavelength. This spline scale defines the computational grid, which tends to be coarser than the typical grid required for finite-difference discretization and hence offers a significant advantage in computational efficiency. The geostrophy–thin-plate model is introduced and applied to a set of analysis problems to demonstrate the effectiveness of the solution technique.

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Tamay M. Özgökmen
,
Eric P. Chassignet
, and
Afonso M. Paiva

Abstract

The persistence of unrealistic Gulf Stream separation in numerical models of the ocean has prompted many theories about possible mechanisms that influence the separation of a western boundary current from the coast. In this paper, the joint effects of (a) coastline orientation, (b) bottom topography, and (c) inertia on the midlatitude jet separation are explored in a wind-driven two-layer quasigeostrophic model. It is shown that topographic effects are of importance in high eddy activity regions and that eddy–topography interactions strongly influence the separation process.

In order for the western boundary current to separate from the coastline and cross the f/h contours associated with the continental rise, eddy fluctuations need to be weak at the separation point. This can be achieved either by introducing a positive wind stress curl in the northern part of the domain or by increasing the inertia of the western boundary current. In both cases, the separation is facilitated by low eddy activity, resulting in a decoupling of the upper layer from the lower layer when the current crosses the f/h contours.

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Mine Çağlar
,
Tamay M. Özgökmen
, and
Leonid I. Piterbarg

Abstract

In light of the recent high-resolution radar data of surface velocity, which have revealed submesoscale eddies between the Florida Current and the coast, an objective method of detecting eddies and estimating their parameters such as center coordinates, size, and intensity is suggested. The obtained statistics are used to parametrically represent the birth–death process of eddies filling up the observation area via a model stochastic velocity field known as Çinlar flow. It appears that the suggested approach leads to a reasonable parameterization of this process for potential future use in OGCMs or coastal models.

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Arthur J. Mariano
,
Annalisa Griffa
,
Tamay M. Özgökmen
, and
Enrico Zambianchi

Abstract

The first Lagrangian Analysis and Predictability of Coastal and Ocean Dynamics (LAPCOD) meeting took place in Ischia, Italy, 2–6 October 2000. The material presented at LAPCOD 2000 indicated both a maturing of Lagrangian-based observing systems and the development of new analysis and assimilation techniques for Lagrangian data. This summary presents a review of the state-of-the-art technology in Lagrangian exploration of oceanic and coastal waters that was presented at the meeting.

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Tamay M. Özgökmen
,
William E. Johns
,
Hartmut Peters
, and
Silvia Matt

Abstract

Given the motivation that overflow processes, which supply source waters for most of the deep and intermediate water masses in the ocean, pose significant numerical and dynamical challenges for ocean general circulation models, an intercomparison study is conducted between field data collected in the Red Sea overflow and a high-resolution, nonhydrostatic process model. The investigation is focused on the part of the outflow that flows along a long narrow channel, referred to as the “northern channel,” that naturally restricts motion in the lateral direction such that the use of a two-dimensional model provides a reasonable approximation to the dynamics. This channel carries about two-thirds of the total Red Sea overflow transport, after the overflow splits into two branches in the western Gulf of Aden. The evolution of the overflow in the numerical simulations can be characterized in two phases: the first phase is highly time dependent, during which the density front associated with the overflow propagates along the channel. The second phase corresponds to that of a statistically steady state. The primary accomplishment of this study is that the model adequately captures the general characteristics of the system: (i) the gradual thickening of the overflow with downstream distance, (ii) the advection of high salinity and temperature signals at the bottom along the channel with little dilution, and (iii) ambient water masses sandwiched between the overflow and surface mixed layer. To quantify mixing of the overflow with the ambient water masses, an entrainment parameter is determined from the transport increase along the slope and is expressed explicitly as a function of mean slope angle. Bulk Richardson numbers are estimated both from data and model and are related to the entrainment parameter. The range of entrainment parameter and its functional dependence on bulk Richardson number in this study are found to be in reasonable agreement with those reported from various laboratory experiments and that based on measurements of the Mediterranean overflow. The results reveal a complex dynamical interaction between shear-induced mixing and internal waves and illustrate the high computational and modeling requirements for numerical simulation of overflows to capture (at least in part) turbulent transports explicitly.

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Tamay M. Özgökmen
,
William E. Johns
,
Hartmut Peters
, and
Silvia Matt

Abstract

Given the motivation that overflow processes, which supply source waters for most of the deep and intermediate water masses in the ocean, pose significant numerical and dynamical challenges for ocean general circulation models, an intercomparison study is conducted between field data collected in the Red Sea overflow and a high-resolution, nonhydrostatic process model. The investigation is focused on the part of the outflow that flows along a long narrow channel, referred to as the “northern channel,” that naturally restricts motion in the lateral direction such that the use of a two-dimensional model provides a reasonable approximation to the dynamics. This channel carries about two-thirds of the total Red Sea overflow transport, after the overflow splits into two branches in the western Gulf of Aden. The evolution of the overflow in the numerical simulations can be characterized in two phases: the first phase is highly time dependent, during which the density front associated with the overflow propagates along the channel. The second phase corresponds to that of a statistically steady state. The primary accomplishment of this study is that the model adequately captures the general characteristics of the system: (i) the gradual thickening of the overflow with downstream distance, (ii) the advection of high salinity and temperature signals at the bottom along the channel with little dilution, and (iii) ambient water masses sandwiched between the overflow and surface mixed layer. To quantify mixing of the overflow with the ambient water masses, an entrainment parameter is determined from the transport increase along the slope and is expressed explicitly as a function of mean slope angle. Bulk Richardson numbers are estimated both from data and model and are related to the entrainment parameter. The range of entrainment parameter and its functional dependence on bulk Richardson number in this study are found to be in reasonable agreement with those reported from various laboratory experiments and that based on measurements of the Mediterranean overflow. The results reveal a complex dynamical interaction between shear-induced mixing and internal waves and illustrate the high computational and modeling requirements for numerical simulation of overflows to capture (at least in part) turbulent transports explicitly.

Full access