[MITgcm-support] 1. question about Cartesian Grid in Matlab (yang.zhang)

Sun Mar 3 22:39:20 EST 2013

Hi, Yang!

First of all: I don´t have an answer, but I´d be glad to think about that together with you. 

I started working with MITgcm recently, so I´m pretty new to the model too. 
However, I started with the Barotropic Gyre tutorial and have been adapting it to what I want, so I feel I´m in position to join such a discussion. 

I posted some of my run results of this tutorial on https://www.dropbox.com/sh/7e55p0f4g4lz5cv/3vAQaZFL5I?m,so we can think over those plots and try to put some ideas together.
(notice: I won´t keep those files forever due to disk space usage, so if anyone wants to take a look at them in the future, contact me at peixotohn at hotmail.com)

Fact 1: upwelling/downwelling result from cyclonic/anti-cyclonic circulation. Cyclonic circulation = ccw (NH)/cw (SH) = Ekman transport to the right (NH)/left (SH) = divergence at the surface = upwelling through continuity. Thus, we need to know in which hemisphere we are in to know if this CCW circulation at the upper half means cyclonic or anti-cyclonic circulation. 

Fact 2: We are in the NH because f0=1.0E4 by default. I changed it to f0=-1.0E-04 (SH) and got exactly opposite results regarding vertical velocity and horizontal flow. 

Fact 3: The manual says that W.00000nIter - vertical component of velocity field (ocean: m/s and positive upward, atmosphere: Pa/s and positive towards increasing pressure i.e. downward) in in 3.5.1.1 MDSIO output files. Therefore, positive is really upwelling because we are in the ocean given that the default is buoyancyRelation = 'OCEANIC'.

I run the 10-year suggested endTime:- in the first 12 hours, gyres look pretty symmetric;- in the first month, gyres start to skew to the West;- this tendency is confirmed ad time passes toward the 10-year limit; and- results look stable after 9 months (visually, I didn´t use any threshold to assess it).

I attribute the skewing to the West to pilling up of water to the East, so it starts to get harder to overcome the pressure field due to the surface elevation. This pilling up is concentrated meridonally in the N-S center due to the wind stress. There is outflow from this Eastern elevation along the Northern and Southern boundaries.  Here is one piece of the puzzle that could be (really not sure) misleading our physical concepts: there are walls on the East and on the North. The outflow from the Eastern pilling up could be affected by this walls, causing some unknown problems. I´ll run some simulations changing the walls to see if I have any new ideas. One more time, I´d be more than glad of discussing further about this tutorial. I´m planning on creating levels and input temperature and salinity profiles, so it would very useful to get more in deep. Cheers,Daniel Peixoto==========================================> From: mitgcm-support-request at mitgcm.org
> Subject: MITgcm-support Digest, Vol 117, Issue 3
> To: mitgcm-support at mitgcm.org
> Date: Sat, 2 Mar 2013 13:54:20 -0500
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>    1. question about Cartesian Grid in Matlab (yang.zhang)
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> ----------------------------------------------------------------------
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> Message: 1
> Date: Sat, 2 Mar 2013 15:24:02 -0330
> From: "yang.zhang" <yang.zhang at mun.ca>
> To: mitgcm-support at mitgcm.org
> Subject: [MITgcm-support] question about Cartesian Grid in Matlab
> Message-ID:
> 	<CAMJz=_x+utcX6fcC2qw+-8B_UZnfHBsXidm3PLSmdfoM6kiAFA at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
> 
> Hello, Colleagues
> I have a Tutorial_barotropic_gyre result here to show question (I am quite
> new in MITgcm);
> The wind force is from the west to the east with a Sine  variation in
> south-north direction;
> I draw the wind force in 'Barotropic Gyre1 tif' in the attachment.
> 
> So the wind-driven flow is like 'Barotropic Gyre2 tif' in the attachment.
> The contour is Eta(sea surface height);
> They both seems good; But the vertical velocity W in 'Barotropic Gyre3 tif'
> confused me a lot, Because a anti-clockwise gyre in the north should has a
> UPwelling effect (Positive W); and It should be a Downwelling (Negative W)
> in south because of the clockwise gyre in the south. But 'Barotropic
> Gyre3.tif' gives opposite.
> 
> if Xc, Yc are the grid file for U V output:
> These pictures in the attachment is drawn in Matlab like this:
> contour?Xc,Yc,Eta?;
> hold on;
> quiver(Xc,Yc,U,V)
> 
> Barotropic Gyre4 tif is made by this:
> contour(Eta);
> hold on;
> quiver(U,V);
> It gives a reasonable result: the divergence results in Positive W; while
> the convergence gives Negative W.
> 
> I don't know which kind of plot is right? or they are both right?
> Any reply will be appreciated!
> 
> Yang
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