[Atmosdyn] weekly summary: ep fluxes continued

[Andres, Heather]

Hi everyone.

Ryan and Taimaz and I discussed Iakov's comments about EP fluxes.  I would
summarize them as follows:

1.  Don't worry about analysing what any particular eddy would do - it's
all turbulence and is best treated on average.

We found this a little dissatisfying.  However, through discussion at the
meeting today, we think we may have resolved the problem we discussed last
time of why the enstrophy decreases as eddies head toward the jet.  The
reason is that this result is not saying that there are fewer eddies at the
jet. Rather, the wind shear creates a background PV gradient that causes
small perturbations to be amplified as they propagate away from the jet in
either direction.  For eddies propagating toward the jet, the reverse is
true, and their enstrophy decreases.  It's a directional phenomenon.

2.  Given a dataset of winds, you remove the zonal average and then
evaluate the zonal (and time) means of u'v' and where they converge will
tell you where the eddies are accelerating the flow - i.e. the eddy-driven
jet.

The last element of this chain fitted a lot of pieces together for me that
I wasn't making sense of before.  EP flux convergence means that more
eddies/waves are entering a region than leaving it.  The net effect is the
transport of zonal momentum into that region. Now, it's not clear to me how
a circular eddy, like i"m used to thinking of, would transport a net
momentum unless the eddy as a whole were being carried in a zonal
direction.  For waves, apparently the wave breaks and deposits its momentum
at that site, creating an eddy-driven jet.  Once the jet is there, it
creates a wind shear that spins off eddies as discussed in point 1.

3.  Dissipation seems to be very important, but it's not clear to me what
would cause latitudinal variations in this term. Iakov mentioned that wind
shear can stretch eddies out into filaments and effectively dissipate
them.  Otherwise, the main mode of dissipation is molecular, which only
affects eddies on the smallest scales.  However, via an energy cascade from
larger to smaller eddies, the effect of this dissipation can be
parametrized as a diffusive process (i.e. eddy diffusivity or eddy
viscosity).

Plan going ahead:  I'm going to be moving back to Winnipeg next week, and
it's getting close to Christmas and holidays.  We are proposing to break
until January.  We decided to shift from Vallis to some papers that apply
the methods we've learned so far to particular physical phenomena.  For
example, I'd like to get back to the papers from the 1980s that discuss
Rossby wave formation due to topographic or thermal anomalies.  If there
are other papers of interest to you, suggest them. Next week, I'll look
through what I've got lined up and send some papers out.

Enjoy your holidays!
Heather

-- 
********************************************************
Dr. Heather Andres
Post-doctoral Research Fellow
Physics and Physical Oceanography
Memorial University of Newfoundland
St. John's, CANADA
(709)864-8846
*********************************************************
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