Etudes et évaluation de processus océaniques par des hiérarchies ...

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62 CHAPTER 10. PENETRATION OF SURFACE FLUXES
10.6 Gravity Currents
tel-00545911, version 1 - 13 Dec 2010
Gravity currents are currents that evolve due to their different density with respect to the
surrounding water masses. We can thus distinguish buoyant gravity currents and dense gravity
currents.
Buoyant gravity currents are lighter than the surrounding and are thus confined to the
surface, an example is fresh river water that enters the ocean. Dense gravity currents on
the contrary are composed of water heavier than the surrounding and they thus flow along
the topography. Important examples are dense currents that pass through straits (Gibraltar,
Denmark, ...) and flow down the continental slopes. We will here consider only the case of
dense gravity currents.
When a dense gravity current leaves a strait it is deviated to the right by the Coriolis force
and flows along the slope of the topography. When we neglect friction, mixing and entrainment
(see section 10.5), the parameters determining the dynamics of the gravity current are the
reduced gravity g ′ = g∆ρ/ρ the slope α and the Coriolis parameter f.
α
⊗
F g
′ ✾
F c
′ ✿
✲
F g
F c
Figure ❄10.3: Force balance in gravity currents
F g = mg ′ , F ′ g = mg ′ sin α, (10.26)
F c = mfu, F ′ c = mfu cos α. (10.27)
If we suppose that the gravity current is in a stationary state, the buoyancy force and the
Coriolis force projected on the slope have to balance, that is F ′ g ′ = F ′ c (see Fig. 10.6) and thus,
u Nof = g′
tanα, (10.28)
f
which is called the Nof-speed.
Exercise 61: What happens when we include bottom friction in the force balance?
Exercise 62: What happens when we include entrainment in the dynamics (see section 10.5)?