From: Pat Wheeler
Subject: Summary of COAST Downwelling Cruise - to 29 Jan 2003

Here's Jim's summary of the COAST results from the mixing group.

Summary of COAST Downwelling Cruise - to 29 Jan 2003

Ocean Mixing Group aboard Revelle

We were fortunate to arrive off Cascade Head following an upwelling event. Conditions included a cool fresh surface layer offshore to 30 km and a dense bottom layer all the way inshore across the shelf. Alongshore currents were predominantly southward across the shelf, with a thin ribbon of northward flow inshore and a broader northward flow offshore. The thin and strongly stratified surface interface provided encouragement for the shoreward propagation of large amplitude internal solitary waves of depression, one of which blew the ship backward over our profiler, Chameleon, exposing it to the notorious, non-shrouded Z-drives of the Revelle. We have since adopted an extremely conservative mode of operation in order to avoid further mishaps with Chameleon's sisters.

Downwelling trapped the fresh surface water next to the shore and drew the dense bottom fluid offshore. A well-defined northward flow developed across the shelf, concentrated at both the offshore front and the inshore front. In between (5 ÿ 10 km offshore of CH1; in the 60-100 m depth range) formed a region of weak (possibly 0) stratification and strong mixing throughout the water column. The degree to which this was unstratified will be important to assess as it will determine the extent of top-to-bottom communication of water column properties.

One consequence of the downwelling seems to be the intensification of a cross-shelf density gradient near the bottom as the dense bottom fluid is forced offshore. During weakening of the downwelling, we have observed what appears to be the onshore propagation of a highly turbulent bore at the bottom, presumably forced by the cross-shelf density gradient. At the leading edge appear to be internal solitary waves of elevation which sometimes propagate ahead of the bore. These are intensely turbulent and act to draw fluid adjacent to the bottom up into the field of view of Burke Hale's pumped profiler (4 m above bottom), where apparently unique chemical signatures have been observed.

We have struggled with the Doppler sonar systems on Revelle. Over the past few years, Rob Pinkel has installed a high-res system of 140 kHz and 50 kHz Doppler sonars on Revelle and we had made plans to use these on this trip. Rob and his group provided a lot of support to get us spun up on how to use these. While we did an initial transect of very nice velocity data, the 140 kHz system blew a power supply for which there is no backup aboard and we cannot rely on the 50 Khz system alone in shallow water. Most unfortunately, after testing Revelle's 150 kHz broadband ADCP for 2 days, we finally discovered 1 transducer was fatally blown and have reverted to the much poorer (for our purposes) 150 kHz narrow band ADCP. Having gone through several iterations of revising our procedures for analyzing quite different data streams and figuring out how to navigate these, we are becoming somewhat expert at it (esp. Jody Klymak, who assumed the role of Doppler sonar evaluator).