|
||
CLIVAR I5 Weekly Science Report 0229 March 2009R/V Roger Revelle from Jim Swift, SIO, Chief Scientist and Greg Johnson, NOAA/PMEL, Co-Chief Scientist 33°S, 37°E; 1300 Z (1500 local); 25.5°C (78°F); Winds 6 knots from SW We are now about 600 km from South Africa, just east of the Mozambique Ridge, having completed the 26th station of our CTD/hydro/carbon/tracer/LADCP crossing of the south Indian Ocean. Here in the Mozambique Basin, we are beginning some of the deepest casts of the cruise. Deeper casts take longer to complete, plus the stations to come are somewhat farther apart than those completed so far, and that combination is providing some welcome relief to the water sample analysts, who have been working flat-out to keep up with the more closely-spaced, shallower stations we do while crossing bathymetry. Oceanographically, the most interesting aspect of the past week was our crossing of the Agulhas Current, which is the western boundary current of the South Indian Ocean. The Agulhas is a strong current, which has been of practical as well as scientific interest to participants on this expedition. During our sampling of the Agulhas during the CLIVAR/CO2 2009 reoccupation of WOCE Section I05, the near-surface currents measured by the shipboard ADCP reached speeds of over 2.0 m/s (about 3.7 kts) near the coast. This high velocity made stations challenging, but the officers and crew of the R/V Revelle overcame the impediment competently. On the continental slope, we were aiming to occupy a CTD station at every 500 m increase in bottom depth from the continental shelf break all the way down to the base of the continental rise. Since the current runs mostly parallel to isobaths, we would find our target depth, and then steam a few nm along that isobath upstream (roughly northeast) from the nominal station track. The officer on watch would then orient the ship properly with respect to winds, waves, and currents, after which we would begin our station, with the ship drifting southwest with the current during the station to minimize wire angle. By the time the CTD reached the bottom, the ship was usually pretty close to our target position on the section, and then moved past it as the CTD was brought back to the surface. During these cruises we generally try to sample to within 10 m of the bottom. However, when drifting along at over 3 kt, with far more wire paid out than there was depth below the ship, and uncertain bathymetry ahead, we sometimes settled for a 20-m gap. One oceanographic consequence of this strong velocity is that, with the admittedly very crude (and likely erroneous) assumption of zero velocity at the deepest common level of each station pair, the preliminary data yield and estimated volume transport of the Agulhas across the section was roughly 85 Sverdrups to the southwest (typical of other similarly derived estimates). Another interesting feature associated with the Agulhas is the northeastward flowing Agulhas Undercurrent, a reversal in flow that is usually found deeper than 800 m, adjacent to the continental slope, below the core of the Agulhas. Since we have Eric Firing's trusty (except for a sticky mercury switch used for sensing vertical orientation) old 150-KHz broadband lowered ADCP on the CTD frame, we were able to measure the expression of this current in March 2009 as we crossed the Agulhas starting from near Durban. On this cruise the Agulhas Undercurrent was remarkable for its absence. While our data are very closely spaced (every 500 m of bottom change, and from 2 to 25 km in distance), we have not find any velocity structure that would merit the designation of Undercurrent in the preliminary LADCP data. Our water sample data appear to be of outstanding quality, all systems are working well, the weather has been very good, and the officers and crew continue to provide outstanding support. We had to re-terminate the CTD cable one time, but lost no data and little time in the process. The only unwelcome news to report is that a highly unpleasant GI virus is making the rounds of the officers, crew, and science team. But at some point soon it will run out of new victims. Other than that, all is well. |
||
