Anslope - iceberg C19 commentary

AnSlope Ross Sea ice/berg info NBP03-02 C-19 tracking

The Tao of C-19 - commentary by Doug MacAyeal

Just a few comments about iceberg drift (that oceanographers should know,
but often haven't yet realized):

1. An iceberg this size reacts to the surrounding "ocean surface
topography" created by the wind, not to the wind itself. The slope of
the sea-surface produces a "body force" (i.e., a component of gravity
in the "horizontal" plane of the iceberg...), whereas the wind itself
produces only a force that is proportional to area.

Think of the iceberg as "feeling" the same force as sea ice, but which
has 300 times the inertia...

2. These icebergs steer the ocean currents. I've spent the last year
watching little tiny icebergs (say less than a kilometer size up to
several kilometers) do actual (really!) *circumnavigations" around
B15a... (which incidentally is **not** grounded! It moves around with
the tide every day... only C16 is "hard aground")

3. B15a "bucks the wind" (e.g., the famous freak storm of December 2001
that saved McMurdo from the fast ice that is still there this year) by
"sliding down" the inclined plane created by upwelling along the edge
of Ross Island.

4. When a "giant" iceberg is adrift, then it's Rossby number is so low
that it quite literally is in geostrophic balance with all forces that
are acting on it... If wind is the main force, then the response of the
iceberg is at a right angle to the left... given that it's flow is so
slow as to not generate significant skin friction at the ice/water
interface (you can rule out edge friction as too small scale for such a
large iceberg).

5. C19 is probably in a static equilibrium (we can't tell if it's
aground because, quite literally, the weather hasn't allowed us to
geolocate it on the meteorological imagery that the AMRC maintains...
unlike B15a which is easy to geolocate because of its proximity to Ross
Island) rather than aground. The wind tries to push it, but it only
ends up bumping into a "road block" then converting it's motion into
"inertial oscillations" of some sort... (B15a sometimes has magnificent
inertial oscillations after big storms, but rarely moves in any way
other than to simply gyrate with the tide...)

Finally, a few speculations:

My hunch is that C19 and the "25-50 year iceberg" getting stuck out
there just off the slope is going to be the main player in determining
"off slope" hydrographic fluxes (e.g., what you guys are studying) over
the next year. This might suggest that the "static, stationary state"
mind-set for doing physical oceanography along the slope of the Ross
Sea is inappropriate, ... that a notion of "extreme, but relatively
rare" events might some how lend themselves to producing the "steady
conditions" that are witnessed elsewhere in the ocean, e.g., the
"Anslope" is simply a physical "integrator" of extreme, ephemeral
events in the "chaos zone" around the coast of Antarctica...