Streamtube models of gravity currents in the ocean

International audienceResults from numerical simulations of idealised, 2.5-dimensional Boussinesq, gravity currents on an inclined plane in a rotating frame are used to determine the qualitative and quantitative characteristics of such currents. The current is initially geostrophically adjusted. The Richardson number is varied between different experiments. The results demonstrate that the gravity current has a two-part structure consisting of: (1) the vein, the thick part that is governed by geostrophic dynamics with an Ekman layer at its bottom, and (2) a thin friction layer at the downslope side of the vein, the thin part of the gravity current. Water from the vein detrains into the friction layer via the bottom Ekman layer. A self consistent picture of the dynamics of a gravity current is obtained and some of the large-scale characteristics of a gravity current can be analytically calculated, for small Reynolds number flow, using linear Ekman layer theory. The evolution of the gravity current is shown to be governed by bottom friction. A minimal model for the vein dynamics, based on the heat equation, is derived and compares very well to the solutions of the 2.5-dimensional Boussinesq simulations. The heat equation is linear for a linear (Rayleigh) friction law and non-linear for a quadratic drag law. I demonstrate that the thickness of a gravity current cannot be modelled by a local parameterisation when bottom friction is relevant. The difference between the vein and the gravity current is of paramount importance as simplified (streamtube) models should model the dynamics of the vein rather than the dynamics of the total gravity current. In basin-wide numerical models of the ocean dynamics the friction layer has to be resolved to correctly represent gravity currents and, thus, the ocean dynamics

“…The influence of a second scalar, salinity and a nonlinear equation of state, leading to, e.g. the thermobaric effect (see Emms 1998), is not considered here.…”

Section: The Physical Problem Consideredmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

On the basic structure of oceanic gravity currents

Wirth

2009

“…However, the current is enhanced at some regions such as the western slope of Jabuka Pit, where estimated the velocity to be about 10 cm/s, by using streamtube analytical model on a slope (e.g. Emms 1997). The same model emphasized the role of bottom friction responsible for the deepening of the dense currents, estimated to be about 1.1 m/km on the Jabuka Pit western and southern slope.…”

Section: The Spreadingmentioning

confidence: 99%

Dense water generation on a shelf: the case of the Adriatic Sea

Vilibić

Supić²

2005

129

The paper overviews recent and past studies of preconditioning, generation and spreading of North Adriatic Dense Water (NAdDW), by analysing both transient episodes and climatological data. The importance of wind stress, heat and water fluxes, and particularly river discharges during the preconditioning and generative period is emphasized, as well as the advection of saline levantine intermediate water from the southeast. After the generation, NAdDW affects deep and bottom layers of the middle and south Adriatic Sea, flowing as a dense current and mixing with the adjacent waters; it can be traced even in the Otranto Strait, contributing to the formation of deep water in the Eastern Mediterranean. Objective shortcomings and the projections in NAdDW investigations are also discussed, due to their high importance in the circulation of the Adriatic Sea, which may relate other similar basins around the world.

“…In this way, stream-tube models have contributed substantially to understanding the physics of oceanic gravity currents. This adjustment procedure, however, does not always lead to a deeper understanding of the physical processes involved (see Emms 1998). The implementation of stream-tube models or their physics in today's large-scale OGCMs is difficult.…”

mentioning

confidence: 99%

Estimation of friction parameters and laws in 1.5D shallow-water gravity currents on the f-plane, by data assimilation

Wirth¹,

Verron²

2008

International audienceA 1.5-dimensional, 1.5-layer shallow water model and an ensemble Kalman filter are used to evaluate the feasibility of estimating friction parameters and determining friction laws of oceanic gravity currents. The two friction laws implemented are a linear Rayleigh friction and a quadratic drag law. We demonstrate that the assimilation procedure rapidly estimates the total frictional force, whereas the distinction between the two laws is evolving on a slower time scale. We also demonstrate that parameter estimation can, in this way, choose between different parametrisations and help to discriminate between physical laws of nature by estimating the coefficients presented in such parametrisations