Date: Sun, 15 Sep 1996 00:33:55 +0000 (GMT)

Dear Colleague,

Despite heavy ice cover at 64 deg S, we managed to achieve our
objective of blanket coverage of a 20 mile X 20 mile area within which
a sediment trap mooring will be deployed.

Presently, we are nearing the completion of a much larger survey
between 60 S and 62 S.  Bottom topography within this region proved to
be more rugged than anticipated, requiring us to incorporate additional
lines into the survey to provide nearly complete coverage of the sea
floor.  Although this region lies farther from the spreading axis of
the mid-ocean ridge, bottom topography here has far more relief than at
64 S.

For example, two striking fault scarps, with relief in excess of 1500m,
run diagonally across the survey area following a NW - SE trend.

The area between 60 S and 62 S was selected for intensive study by
JGOFS because the Antarctic Polar Front (APF) is typically located
within this region.  Indeed, the underway thermosalinograph record
shows an abrupt temperature gradient from about +1 deg C to - 1 deg C
as we cross the front during each transect of the survey.  In addition,
we regularly observe mesoscale features in the SST record, presumably
associated with eddies, on both sides of the principal thermal

Mesoscale variability is manifest in the hydrocast profiles, too, and
not only a the APF.  During the course of surveying the AESOPS site at
64 S, which was covered by pack ice, we completed a deep (64 07'S, 169
45'W) and a shallow (63 54'S, 169 22'W) hydrocast.  The position of the
deep hydrocast was 30 km SW of the shallow cast.  The mixed layer was
nearly twice as deep at the northern location of the shallow cast,
while the mixed layer temperature at the location of the deep cast was
clearly above the freezing point.  Dissolved oxygen concentrations in
the deep mixed layer were 0.1 ml/liter lower than in the shallow mixed
layer.  Dissolved Si concentrations in the deep mixed layer were 8 uM
higher than in the shallow mixed layer.  At 61S, a deep cast (61 00'S,
170 00 W) was taken only about 7 miles SW of the shallow cast (60 55'S,
169 51'W), yet the two profiles show strongly-dissimilar structure in
the upper water column.  This station is on the southern fringe of the
APF, and shows no significant true mixed layer, but shows evidence of
interleaving water masses. 

AESOPS process study cruises in the APF zone will need to incorporate
an assessment of mesoscale features in order to define the context in
which fluxes, rate measurements, etc. are being evaluated.

Bob Anderson