University:
Widener University
Course:
ESSC 103 | Intriduction To Earth Science
Academic year:

2023
Views:

310

Pages:

2
Author:

Badeanzugfqr

Geostrophic Flow — Gulf Stream data Purpose: Calculate the geostrophic ﬂow from a Gulf Stream section and compare to the measured currents. Are the diﬀerences dynamical or errors? Data sets: Note – you need to do the ˜lab/mocha/install for the following procedure. • Make a subdirectory for the Matlab ﬁles. • Select from the lab homepage the “Gulf Stream geostrophy” and enter the name of your directory on the page . Press the download button, and the subdirectory should have Matlab ﬁles in it. • Start Matlab in this directory. We have a CTD (conductivity ⇒ salinity, temperature, depth [pressure]) section across the Gulf Stream. You can pull this into Matlab with loadctd This gives you the variables pp, tt, ss for pressure (decibars), temperature, and salinity. From this you can compute the speciﬁc volume anomaly (using function svan) ∆(S, T, p) = α(S, T, p) − α(35, 0, p) , α = 1/ρ The dynamic height (calculated by dynhgt) is deﬁned as � pref D= ∆dp p and is in “dynamic meters” when the speciﬁc volume anomaly is in cm3 /gm and the pressure in decibars. The geostrophic velocity (geost) follows from v(p) − v(pref ) = 1 ∂ D f ∂x and is in cm/sec if D is in dyn. m, f in 1/sec and x in km. (Convince yourself this is right.) The values of f and the station distances are loaded along with the data. More detail on the CTD section can be found by looking at the “CTD position data” link. We also have a subset of the current meter data, picked out for just the days when the ctd section was run (880328–880331). You can pull this information in by loadcm You can look at the data and plot the positions vs. the CTD station locations plot(ctdlon,ctdlat,ctdlon,ctdlat,’x’,cmlon,cmlat,’o’) To subset this data, pick a latitude/ longitude band and type something like id=find(cmlat>39&cmlat<40&cmlon>-55&cmlon<-54) Then u(id), v(id) will be the currents at depths depth(id) These are what we’d like to compare to the nearest geostrophic velocity proﬁle. See also the “Current meter data” link for more information. Helpful Matlab statements: contour(flipud(tt.*(pp>0))) contour(flipud(ss.*(pp>0)+34*(1-(pp>0)))) plot(u(id),-depth(id),’x’,vv(:,10)-12,-pp(:,1))