From: Wecoma Chief Scientist
Subject: Wecoma report for 28-30 January 2003

COAST R/V Wecoma report
28-30 January 2003

During Tuesday to Thursday, 28-30 January, we completed the following sampling:

-- two SeaSoar/ADCP/HTI/iron surveys on the SmallBox North grid (5th and 6th repeats)
-- 10 deep CTD stations along the shelfbreak to help delineate the downwelling front
-- three tow-yo CTD sections along the Cascade Head line between the 150 and 100-m isobaths (more details below)

Winds came up on Thursday to peak at 35 knots and we had to call off operations late Thursday night. After a bumpy night winds are back down to 25 knots and we're going to attempt towing SeaSoar.

Results from our survey on Tuesday, after a period of weak winds, showed that the dense, lower layer water was intruding back up the shelf reaching about the 110m isobath. The tongue of high-density water looks like the result of onshore transport in a bottom Ekman layer. Currents were weak to the south inshore and near the bottom on the CH line, consistent with the bottom Ekman layer interpretation. Our survey on Thursday showed that the larger-scale bottom downwelling front had moved farther up the shelf, again as a response to lighter winds.

On Tuesday and Wednesday nights we completed three tow-yo CTD sections along the CH line between the 150 and 100-m isobaths. We cycled the CTD from 75-m depth in the relatively well-mixed upper layer to within 5 m of the bottom and back. We couldn't safely get closer to the bottom as the ship went through 3-m rolls in the waves. While traveling onshore at 1-1.5 knots, the horizontal spacing between up-down traces ranged from a few hundred meters to about 100 m. All the SeaSoar watches did a great job doing the 3-5 hour tow-yo transects. Thanks to the Wecoma ABs (Brian, Harold and Doug) for the attentive, continuous operation of the hydro winch and the captain and mates for maintaining speed and course through the rough weather. Andy Dale took the lead on the tow-yo operation and plotting up the results, with help from Steve Pierce contributing the ADCP plots.

The overall picture of the near-bottom downwelling front showed the thinning of the lower layer as it intersected the shelf topography. In addition to a near-bottom layer of high particle concentration, there was a ribbon of bottom material carried up and offshore along the density front. On the very inshore side of the thin layer, we saw large undulations in the lower layer height. These extended up to 20-30 m above the bottom, were about 200 m wide and contained large amounts of particles. The east-west ADCP velocities showed short-scale divergence and convergence consistent with the interpretation of the large-amplitude deviations as internal waves. There was considerable difference between the tow-yo results from one evening to another, as the strong northward winds shoved the front back down the shelf. We hope to do some more tow-yo CTDs along the CH-line over the next couple of nights.

We are about to deploy SeaSoar/HTI/ironfish for a tow along the CH line and an attempt to complete a SmallBox North grid, weather dependent. We'll also attempt another tow-yo CTD along the CH line. If the weather gets better on the weekend, we're considering a sampling strategy for the BigBox lines 3-8, south over Heceta Bank.

Jack Barth
Chief Scientist, R/V Wecoma
0900, 31 Jan 2003