ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY Lecture 17 Learning objectives: understand and appreciate “mixing processes in the ocean”: 1) Static stability: a) Stable and unstable stratification b) double diffusion & salt fingering 2) Dynamical instabilities: a) Barotropic instability b) Baroclinic instability c) Kelvin-Helmholtz instability
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ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY
Lecture 17Learning objectives: understand and appreciate“mixing processes in the ocean”:
1) Static stability: a) Stable and unstable stratification b) double diffusion & salt fingering2) Dynamical instabilities:a) Barotropic instabilityb) Baroclinic instabilityc) Kelvin-Helmholtz instability
1) Static stability
z
x
Stable stratification:
0m
-5000m
Archimedes Principle states that the buoyant force on a submerged object is equal to the weight of the fluid that is displaced by the object.
𝝆𝒂 < 𝝆𝒘 < 𝝆𝒃
• Stable and unstable stratification;
FN: Net force
FgFb
z
x
Unstable stratification:
𝝆𝒂 > 𝝆𝒘 > 𝝆𝒃
Stable:
We often use: buoyancy frequency N to describestratification.
N: Brunt-Vaisala frequnecy
Unstable:
Initially - stable
T diffusion=100 S diffusion
• Double diffusion and salt fingering
After certain time:
Unstable: falling/risingoccurs in thin columnsSalt fingering!
Small scale mixing: a few meters.
Obs: Mediterranean plume
Lab experiment: salt fingering
T! ≈ T" , 𝑆! > 𝑆"
By contrast, if a layer of colder, fresher water is above a layer of warmer, saltier water,the water just above the interface becomes lighter than its above and thus rises, while water belowthe interface gets heavier and thus sinks.
Layering: may result in homogeneous layersSeparated by thinner regions of large gradients of T and S.
The ocean is NOT static; they’re in constant motion!