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Week 04 Location

R/V Roger Revelle location at Week 04 in the Indian Ocean.

CLIVAR I5 Weekly Science Report 04

12 April 2009
R/V Roger Revelle
from
Jim Swift, SIO, Chief Scientist and
Greg Johnson, NOAA/PMEL, Co-Chief Scientist

34°S, 61°E; 1600 Z (2000 local);18.5°C (65°F); Winds 24 knots from W

The Revelle's officers, crew, and science team are now resting or working quietly after the fine Easter meals provided by Jay and Mark. The ship has been holding into the wind for nearly 24 hours, waiting for seas to calm sufficiently to resume CTD/rosette operations after the yesterday's passage of Tropical Cyclone Jade, which hit us directly. We were able to carry out CTD operations Saturday as the storm built because the wind and swell were from the same direction, making it possible for the mates to hold the ship remarkably steady during casts. On station 077, yesterday afternoon, we didn't count, however, on winds rising as quickly as they did: When we put the CTD into the water for the 5525-meter deep cast, winds were a more or less manageable and fairly steady 35 knots. Four hours later, when we brought the CTD back on deck, average winds were 52 knots, and while the recovery went just fine, it was no easy job. (Hats off to the Captain, mates, winch operator, and deck crew!)
Flow below 2500m

Flow below 2500 m across all Revelle stations along the SW Indian Ocean Ridge. Strong northward intrusion occurs through the Atlantis II Fracture Zone. Weaker intrusions occur in the next three fracture zones to the east. Interestingly, the northward intrusion in the fracture zone around 57.5E is blocked further north by the topography and should feed its neighboring fracture zones, in particular the Atlantis II one. (Francois Ascani)

While we sampled the rosette, winds continued rising into the upper 50s, roaring most impressively over a stormy ocean just feet away from the open hangar doors of the sampling room. During the night the wind shifted direction (and decreased to 40 knots or so), meaning that now swell was going to build from other directions. As these new swells joined in, the high seas around us became ever more confused. It meant a rough ride, and a tough night for sleep for all but a lucky few. Winds only recently dropped to the 25-knot range, and so the mixed-up seas are taking time to smooth to the point where we resume CTD operations, possibly in a few hours.

Meanwhile, since the last report, we enjoyed a good week of stations, with all science systems and personnel working very well. We not only made steady progress eastward with our basic I5 station plan, but were able to take time to occupy four CTD/LADCP stations just north of the I5 section, roughly along the axis of the Atlantis II Fracture Zone, a major conduit for deep and bottom water flow northward from the Crozet Basin through the Southwest Indian Ridge into the Madagascar Basin. Data from the four stations plus one along the I5 section itself support an earlier investigation with further evidence of heightened mixing along the passage. The LADCP data clearly capture the strong northward deep flow along the passage.

AII FZ Flow Summary

Vertical profile of meridional velocity in the Atlantic II Fracture Zone, with the transport below 2500 m annotated in white text at bottom (Figure by Francois Ascani)

In addition to the LADCP data, the transmissometer on the CTD reveals an increase in particulates, presumably resuspended by this strong flow, starting around 3500 db and increasing towards the bottom. In addition, the deep salinity maximum and CFC-12 minimum are both eroded in the station occupied furthest to the north in the fracture zone. The downstream erosion of these extrema could be a result the vertical mixing from above and below. In fact, the CFC-12 data strongly support this assertion, since CFC-12 levels would be expected to decrease northward with decreasing salinity if the latter were simply a signature of increasing influence of older, fresher, and more CFC-poor North Indian Deep Water relative to the more recently ventilated, saltier, and more CFC-rich North Atlantic Deep Water. Instead the CFC-12 concentration at the minimum increases, quite probably a result of mixing with more CFC-12 rich waters above and below the minimum.

All is well.