Posted by Steve Rintoul on May 13, 2001 at 05:30:52:
I enjoyed this poster very much. ARIANE seems to
be a wonderful tool for analysing model output and teaching us
something about how the ocean works (although I have a
few questions, see below). Here the authors use the
particle-tracking approach to quantify the contribution of
flow through three passages to the northward flow of the upper
limb of the Atlantic overturning circulation. They find that
the well-known "warm" and "cold" routes both contribute, but
westward flow south of Tasmania is also significant. (They
call this path the "tepid" path; since this word also means
"half-hearted" or "lacking in enthusiasm," I hope we can
find another word for it.)
The observational evidence for westward flow south of
Tasmania is strong. In addition to the papers referred to in
the text, evidence for westward flow south of Tasmania is also
discussed in Rintoul and Sokolov (2001, JGR, 106, 2795-2814).
Based on six repeat CTD sections, we find the (top-to-bottom)
westward flow south of Tasmania to vary from close to 0 Sv to
more than 20 Sv, with a mean value of 8 Sv. However, with
observations alone it can be difficult to follow long
trajectories. The model trajectories are a fascinating,
complementary way to look at interbasin exchanges. Having seen
these trajectories, you inevitably would like to see more of
them. For example, what paths connect the NADW exported from
the Atlantic to the intermediate waters passing through the three
passages discussed here?
I'd be interested in further discussion in this forum on
the interpretation of the trajectories, especially from those
with more experience in these techniques than I. The finding,
for example, that the westward flow south of Tasmania supplies
most of the water flowing north in the intermediate water
density range is intriguing, but I'm not quite sure how to
intepret it. The trajectory technique seems to imply a
long-term "coherence" to water parcels that seems hard to
imagine in an eddy-rich (and at least in the model, diffusive)
ocean. The intermediate water flowing west south of Tasmania
is already "old" (relatively low in oxygen), and the path from
Tasmania to the equatorial Atlantic is very long. Are the
water properties of the equatorial Atlantic IW consistent with
a substantial contribution of this old, oxygen-poor and
nutrient-rich IW?
With regard to the Lagrangian particle tracking, I have a
few questions. Are diffusion or sub-grid-scale motions
accounted for in some way? I gather the Gent and McWilliams
eddy parameterisation was not used in this model: if it was,
could the eddy-induced transport velocity be included in this
method, and would it make much difference? Another important
aspect of the large-scale interbasin exchange discussed here
is the transformation of water masses by air-sea fluxes where
layers outcrop. Is this process accounted for in the
trajectory calculation? How about the interior restoring
terms: the fictitious interior sources/sinks of heat and
freshwater drive diapycnal fluxes - are these included in the
trajectories?