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7.7.5 Notes on the flux form of the PV equation and vertical PV fluxes

7.7.5.1 Flux form of the PV equation

The conservative flux form of the potential vorticity equation is:
$\displaystyle \frac{\partial \rho Q}{\partial t} + \nabla \cdot \vec{J}$ $\displaystyle =$ 0 (7.11)

where the potential vorticity $ Q$ is given by the Eq.7.3.

The generalized flux vector of potential vorticity is:

$\displaystyle \vec{J}$ $\displaystyle =$ $\displaystyle \rho Q \vec{u} + \vec{N_Q}$ (7.12)

which allows to rewrite Eq.7.11 as:
$\displaystyle \frac{DQ}{dt}$ $\displaystyle =$ $\displaystyle -\frac{1}{\rho}\nabla\cdot\vec{N_Q}$ (7.13)

where the nonadvective PV flux $ \vec{N_Q}$ is given by:
$\displaystyle \vec{N_Q}$ $\displaystyle =$ $\displaystyle -\frac{\rho_0}{g}B\vec{\omega_a} + \vec{F}\times\nabla\sigma_\theta$ (7.14)

Its first component is linked to the buoyancy forcing7.1:

$\displaystyle B$ $\displaystyle =$ $\displaystyle -\frac{g}{\rho_o}\frac{D \sigma_\theta}{dt}$ (7.16)

and the second one to the nonconservative body forces per unit mass:
$\displaystyle \vec{F}$ $\displaystyle =$ $\displaystyle \frac{D \vec{u}}{dt} + 2\Omega\times\vec{u} + \nabla p$ (7.17)

7.7.5.2 Determining the PV flux at the ocean's surface

In the context of mode water study, we're particularly interested in how the PV may be reduced by surface PV fluxes because a mode water is characterised by a low PV level. Considering the volume limited by two $ iso-\sigma_\theta$ , PV flux is limited to surface processes and then vertical component of $ \vec{N_Q}$ . It is supposed that $ B$ and $ \vec{F}$ will only be nonzero in the mixed layer (of depth $ h$ and variable density $ \sigma_m$ ) exposed to mechanical forcing by the wind and buoyancy fluxes through the ocean's surface.

Given the assumption of a mechanical forcing confined to a thin surface Ekman layer (of depth $ \delta_e$ , eventually computed by the package) and of hydrostatic and geostrophic balances, we can write:

$\displaystyle \vec{u_g}$ $\displaystyle =$ $\displaystyle \frac{1}{\rho f} \vec{k}\times\nabla p$ (7.18)
$\displaystyle \frac{\partial p_m}{\partial z}$ $\displaystyle =$ $\displaystyle -\sigma_m g$ (7.19)
$\displaystyle \frac{\partial \sigma_m}{\partial t} + \vec{u}_m\cdot\nabla\sigma_m$ $\displaystyle =$ $\displaystyle -\frac{\rho_0}{g}B$ (7.20)

where:
$\displaystyle \vec{u}_m$ $\displaystyle =$ $\displaystyle \vec{u}_g + \vec{u}_{Ek} + o(R_o)$ (7.21)

is the full velocity field composed by the geostrophic current $ \vec{u}_g$ and the Ekman drift:
$\displaystyle \vec{u}_{Ek}$ $\displaystyle =$ $\displaystyle -\frac{1}{\rho f}\vec{k}\times\frac{\partial \tau}{\partial z}$ (7.22)

(where $ \tau$ is the wind stress) and last by other ageostrophic components of $ o(R_o)$ which are neglected.

Partitioning the buoyancy forcing as:

$\displaystyle B$ $\displaystyle =$ $\displaystyle B_g + B_{Ek}$ (7.23)

and using Eq.7.21 and Eq.7.22, the Eq.7.20 becomes:
$\displaystyle \frac{\partial \sigma_m}{\partial t} + \vec{u}_g\cdot\nabla\sigma_m$ $\displaystyle =$ $\displaystyle -\frac{\rho_0}{g} B_g$ (7.24)

revealing the "wind-driven buoyancy forcing":
$\displaystyle B_{Ek}$ $\displaystyle =$ $\displaystyle \frac{g}{\rho_0}\frac{1}{\rho f}\left(\vec{k}\times\frac{\partial \tau}{\partial z}\right)\cdot\nabla\sigma_m$ (7.25)

Note that since:
$\displaystyle \frac{\partial B_g}{\partial z}$ $\displaystyle =$ $\displaystyle \frac{\partial}{\partial z}\left(-\frac{g}{\rho_0}\vec{u_g}\cdot\...
...-\frac{g}{\rho_0}\frac{\partial \vec{u_g}}{\partial z}\cdot\nabla\sigma_m
= 0$ (7.26)

$ B_g$ must be uniform throughout the depth of the mixed layer and then being related to the surface buoyancy flux by integrating Eq.7.23 through the mixed layer:
$\displaystyle \int_{-h}^0B\,dz$ $\displaystyle =\, hB_g + \int_{-h}^0B_{Ek}\,dz \,=$ $\displaystyle \mathcal{B}_{in}$ (7.27)

where $ \mathcal{B}_{in}$ is the vertically integrated surface buoyancy (in)flux:
$\displaystyle \mathcal{B}_{in}$ $\displaystyle =$ $\displaystyle \frac{g}{\rho_o}\left( \frac{\alpha Q_{net}}{C_w} - \rho_0\beta S_{net}\right)$ (7.28)

with $ \alpha\simeq 2.5\times10^{-4}\, K^{-1}$ the thermal expansion coefficient (computed by the package otherwise), $ C_w=4187J.kg^{-1}.K^{-1}$ the specific heat of seawater, $ Q_{net}[W.m^{-2}]$ the net heat surface flux (positive downward, warming the ocean), $ \beta[PSU^{-1}]$ the saline contraction coefficient, and $ S_{net}=S*(E-P)[PSU.m.s^{-1}]$ the net freshwater surface flux with $ S[PSU]$ the surface salinity and $ (E-P)[m.s^{-1}]$ the fresh water flux.

Introducing the body force in the Ekman layer:

$\displaystyle F_z$ $\displaystyle =$ $\displaystyle \frac{1}{\rho}\frac{\partial \tau}{\partial z}$ (7.29)

the vertical component of Eq.7.14 is:
$\displaystyle \vec{N_Q}_z$ $\displaystyle =$ $\displaystyle -\frac{\rho_0}{g}(B_g+B_{Ek})\omega_z
+ \frac{1}{\rho}
\left( \frac{\partial \tau}{\partial z}\times\nabla\sigma_\theta \right)\cdot\vec{k}$  
  $\displaystyle =$ $\displaystyle -\frac{\rho_0}{g}B_g\omega_z
-\frac{\rho_0}{g}
\left(\frac{g}{\...
...( \frac{\partial \tau}{\partial z}\times\nabla\sigma_\theta \right)\cdot\vec{k}$  
  $\displaystyle =$ $\displaystyle -\frac{\rho_0}{g}B_g\omega_z
+ \left(1-\frac{\omega_z}{f}\right)...
...\frac{\partial \tau}{\partial z}
\times\nabla\sigma_\theta \right)\cdot\vec{k}$ (7.30)

and given the assumption that $ \omega_z\simeq f$ , the second term vanishes and we obtain:
$\displaystyle \vec{N_Q}_z$ $\displaystyle =$ $\displaystyle -\frac{\rho_0}{g}f B_g %\label{sec:diag:pv:eq12}
$ (7.31)

Note that the wind-stress forcing does not appear explicitly here but is implicit in $ B_g$ through Eq.7.27: the buoyancy forcing $ B_g$ is determined by the difference between the integrated surface buoyancy flux $ \mathcal{B}_{in}$ and the integrated "wind-driven buoyancy forcing":
$\displaystyle B_g$ $\displaystyle =$ $\displaystyle \frac{1}{h}\left( \mathcal{B}_{in} - \int_{-h}^0B_{Ek}dz \right)$  
  $\displaystyle =$ $\displaystyle \frac{1}{h}\frac{g}{\rho_0}\left( \frac{\alpha Q_{net}}{C_w} - \r...
...1}{\rho f}\vec{k}\times \frac{\partial \tau}{\partial z} \cdot\nabla\sigma_m dz$  
  $\displaystyle =$ $\displaystyle \frac{1}{h}\frac{g}{\rho_0}\left( \frac{\alpha Q_{net}}{C_w} - \r...
...- \frac{g}{\rho_0}\frac{1}{\rho f \delta_e}\vec{k}\times\tau\cdot\nabla\sigma_m$ (7.32)

Finally, from Eq.7.14, the vertical surface flux of PV may be written as:
$\displaystyle \vec{N_Q}_z$ $\displaystyle =$ $\displaystyle J^B_z + J^F_z$ (7.33)
$\displaystyle J^B_z$ $\displaystyle =$ $\displaystyle -\frac{f}{h}\left( \frac{\alpha Q_{net}}{C_w}-\rho_0 \beta S_{net}\right)$ (7.34)
$\displaystyle J^F_z$ $\displaystyle =$ $\displaystyle \frac{1}{\rho\delta_e} \vec{k}\times\tau\cdot\nabla\sigma_m$ (7.35)


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Next: 8. Ocean State Estimation Up: 7.7 Potential vorticity Matlab Previous: 7.7.4 Technical details   Contents
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