Subject: Marine Geospatial Ecology Tools (MGET) help
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| From: | "Jason Roberts" <> |
|---|---|
| To: | "'Robert Rankin'" <> |
| Cc: | <> |
| Subject: | RE: [mget-help] Eddies and southern hemisphere - cyclones versus anticyclones |
| Date: | Sun, 28 Oct 2012 15:31:02 -0400 |
Robert,
I'm glad you like MGET. Thanks for your inquiry.
I believe the tool documentation is correct, that "anticylonic is concave
down, cyclonic is concave up", regardless of which hemisphere you are in. My
understanding of eddy terminology is that regardless of hemisphere,
anticyclonic eddies are concave down and therefore downwell at the center,
while cyclonic eddies are concave up and therefore upwell at the center. The
only thing that differs depending on hemisphere is the direction of
rotation: in the northern hemisphere, anticyclonic rotate clockwise and
cyclonic rotate anticlockwise; in the southern hemisphere, the directions
are reversed.
For a succinct explanation of that, please see Introduction to Geophysical
Fluid Dynamics: Physical and Numeric Aspects, by Benoit Cushman-Roisin and
Jean-Marie Beckers, p. 249, the bottom two paragraphs. Hopefully this Google
Books link will take you to that (I don't know if it will work outside the
United States):
http://books.google.com/books?id=4g-6iX6EQ8YC&pg=PA249&lpg=PA249&dq=anticycl
onic+downwelling+southern+hemisphere&source=bl&ots=xLT1cKYZ-P&sig=z_UUt4ZYXP
HF58-AmUhTb97LB-M&hl=en&sa=X&ei=H4SNUMDhIarg0gH01IDwCA&ved=0CHMQ6AEwDg
If my understanding is wrong, my apologies and please don't hesitate to
correct me!
Assuming all of that is correct, then no, I do not believe you need to alter
your interpretation of the tool's output. I believe you can interpret -1 to
mean cyclonic and 1 to mean anticyclonic, regardless of hemisphere.
Best regards,
Jason
-----Original Message-----
From: Robert Rankin
[mailto:]
Sent: Sunday, October 28, 2012 12:21 PM
To:
Subject: [mget-help] Eddies and southern hemisphere - cyclones versus
anticyclones
Hello,
Thank you for this great tool! I thoroughly enjoy MGET.
I have a question about the polarity of eddies identified in the
"Okubo-Weiss Eddies in Aviso SSH Product" tool, using MGET 0.8a30.
Does the algorithm correct for polarities for the Southern Hemisphere versus
the Northern Hemisphere. I.e., reading the tool help very naively, it states
that "anticyclonic is concave down, cyclonic is concave up". Shouldn't this
be switched for the Southern Hemisphere?
Does the tool automatically flip the polarities or do I have to manually
alter my interpretation of -1,1 from the polarity outputs?
Thanks again,
Robert Rankin
--
"You could give Aristotle a tutorial. And you could thrill him to the core
of his being ... Such is the privilege of living after Newton, Darwin,
Einstein, Planck, Watson, Crick and their colleagues."
-- Richard Dawkins
I'm glad you like MGET. Thanks for your inquiry.
I believe the tool documentation is correct, that "anticylonic is concave
down, cyclonic is concave up", regardless of which hemisphere you are in. My
understanding of eddy terminology is that regardless of hemisphere,
anticyclonic eddies are concave down and therefore downwell at the center,
while cyclonic eddies are concave up and therefore upwell at the center. The
only thing that differs depending on hemisphere is the direction of
rotation: in the northern hemisphere, anticyclonic rotate clockwise and
cyclonic rotate anticlockwise; in the southern hemisphere, the directions
are reversed.
For a succinct explanation of that, please see Introduction to Geophysical
Fluid Dynamics: Physical and Numeric Aspects, by Benoit Cushman-Roisin and
Jean-Marie Beckers, p. 249, the bottom two paragraphs. Hopefully this Google
Books link will take you to that (I don't know if it will work outside the
United States):
http://books.google.com/books?id=4g-6iX6EQ8YC&pg=PA249&lpg=PA249&dq=anticycl
onic+downwelling+southern+hemisphere&source=bl&ots=xLT1cKYZ-P&sig=z_UUt4ZYXP
HF58-AmUhTb97LB-M&hl=en&sa=X&ei=H4SNUMDhIarg0gH01IDwCA&ved=0CHMQ6AEwDg
If my understanding is wrong, my apologies and please don't hesitate to
correct me!
Assuming all of that is correct, then no, I do not believe you need to alter
your interpretation of the tool's output. I believe you can interpret -1 to
mean cyclonic and 1 to mean anticyclonic, regardless of hemisphere.
Best regards,
Jason
-----Original Message-----
From: Robert Rankin
[mailto:]
Sent: Sunday, October 28, 2012 12:21 PM
To:
Subject: [mget-help] Eddies and southern hemisphere - cyclones versus
anticyclones
Hello,
Thank you for this great tool! I thoroughly enjoy MGET.
I have a question about the polarity of eddies identified in the
"Okubo-Weiss Eddies in Aviso SSH Product" tool, using MGET 0.8a30.
Does the algorithm correct for polarities for the Southern Hemisphere versus
the Northern Hemisphere. I.e., reading the tool help very naively, it states
that "anticyclonic is concave down, cyclonic is concave up". Shouldn't this
be switched for the Southern Hemisphere?
Does the tool automatically flip the polarities or do I have to manually
alter my interpretation of -1,1 from the polarity outputs?
Thanks again,
Robert Rankin
--
"You could give Aristotle a tutorial. And you could thrill him to the core
of his being ... Such is the privilege of living after Newton, Darwin,
Einstein, Planck, Watson, Crick and their colleagues."
-- Richard Dawkins
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