geostrophic currents from sha

[deba]

hi all
i need to compute geostrophic currents from sha (SSH anomly) as available from TOPEX/Poseidon. can anyone help me out with the necessary fortran code or at least the required equations

[vinu]

Hi Deba

Geostrophic current from SSH anomaly (TOPEX or whatever) is more straight forward
(assuming the SSH is the resultant contributions from all the vertical modes)

The geostrophic balance is
f.v = g.dh/dx
f.u = -g.dh/dy ; where h is the SSH anomaly
f=2.omega.sin(y) ; where 'omega' is angular velocity of earth rotation and 'y' is
the latitude and 'g' is the gravity.

If the softwere FERRET is used,

use ssh_data.nc ! (assuming that the variable is ssh)
let g=9.8
let pi=3.141
let omega = 0.729e-4
let yy = y[g=giw1] !( please note that this 'giw1' is coming from ferret
grid, and can be get by using the command 'show grid ssh' after
loading the data into ferret, It will be different for different data sets)
let f=2.0*omega*sin(yy*pi/180.0)

let u = (-1.0*g/f)*ssh[y=@ddc] !(centered difference is used)
let v = (g/f)*ssh[x=@ddc]

set reg/x=100e:80w/y=10n:40n
vec/l=1 u,v

In FORTRAN, the spatial gradient of SSH can be found in a centered difference
grid. For example , u = -(g/f)*(h(i,j+1) - h(i,j-1))/2dy and v = (g/f)*(h(i+1,j) - h(i-1),j)/2dx;
where 2dx and 2dy are the twice the grid size and h(i,j) is the two-D array of SSH.

There must be a buffer zone at the equator where the 'f' tends to zero
and geostrophy breaks down. a +/- 5 degree on either side of the
equator would be fine.


Thanks