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

Mon Mar 4 03:48:03 EST 2013

Yang, I run some simulations, and get to these conclusions:- If you use a negative f0 (SH), you get the same horizontal circulation (what sounds strange, but I´ll try to explain it), but opposite vertical motion (upwelling in the northern half);- It led me to the conclusion that the vertical motion in this tutorial is not completely due to Ekman pumping caused by cyclonic/anti-cyclonic circulation because we have opposite vertical motions with the same circulation. Something else is driving the vertical motion; - Indeed, I believe (my opinion) whoever made up this tutorial was not interest in looking at vertical motion, but only horizontal flow and how gyres are represented by the model;- I believe that the pilling up on the walls and the "rip currents" are driving the vertical motion. It is based on the fact that if there were no walls, it would be difficult to generate so well defined gyres in so simple a box;- Here comes the caveat: in figure 4, when I remove all walls, some diffuse vertical motion show up, probably due to the wind stress in the form of a sine;- In order to simulate Ekman pumping, I made a negative cos wind stress that simulates the midlatitudes regular conditions (Westerlies at 45ºN, Easterlies at 15ºN);- Finally, I got downwelling caused by the anti-cyclonic clockwise simulation induced by the negative cosine wind stress (indeed, I got upwelling in the center for the first 11 days, probably due to the covergence, but after day 12 I started getting downwelling that kept increasing downward as time passes). My plots are posted in the same link (https://www.dropbox.com/sh/7e55p0f4g4lz5cv/3vAQaZFL5I?m) I attached some plots in a doc file that can illustrate what I mean. I have a comment for you: maybe your quiver plot could be quiver(U',V') due to the logics gendata.m uses to generate the grid with the matlab function ndgrid. Btw, the space between vectors in your quiver plot are better than in my plot with vectors at all grid points. What was your matlab line for the horizontal flow? You also plotted the wind stress in a manner that I could visualize better than I usually do. Would you mind sharing the matlab code for the wind stress plot as well? Hope it helped you. At least, it helped me a lot to understand what was going on. I am definetely open to hear any more expert coleague that could add to this discussion. Best,Daniel PeixotoFrom: peixotohn at hotmail.com
To: mitgcm-support at mitgcm.org
Subject: 1. question about Cartesian Grid in Matlab (yang.zhang)
Date: Mon, 4 Mar 2013 00:39:20 -0300

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
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> 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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