Surface currents
Below find some discussion of offshore currents and winds, ocean models, and links to data/animations. A previous post (http://openphi.net/tenacious/?p=7) provided links to HF radar measurements of currents in the general region in which Jim was sailing and some specific information.  However, the present search area is much larger and far beyond the range of HF radar.Â
 First, Francisco Chavez, provided the following comments:
“Here are some thoughts regarding the general circulation during this time of the year.
The winter is commonly dominated by the appearance/intensification of the Davidson current which flows poleward along the coast. In an average year a particle, drifter or sailboat would be advected northward and eventually onto shore. Flow north of the olden Gate is often northward.
Having said that this has been an unusual year in terms of wind patterns with stronger than average nothwesterly wind, upwelling and perhaps California Current. The combined effect of the ocean and wind on a sailboat would act to move it offshore almost perpendicular to the coast (with a bit of a southward component) until it reached between 100 and 200 km when flow would become southerly.
So two very different options but it does cut down a few search quadrants.If he went north he is likely to be close to the shore or on the beach, if he went south then offshore and close to the time when it might turn southerly.Hope this helpsâ€
Now, moving to some model output. Winds are not a bad proxy for surface currents. There is a wind product produced by the Naval Research Laboratory and served by JPL below:
Ocean models are not as far advanced with respect to capabilities as are atmospheric models (it is a harder problem). The ocean is much more variable than the atmosphere, and harder to observe. In the offshore region, where there are effectively very few measurements of ocean conditions, the situation is too challenging to expect ocean models to provide reliable output with respect to details of circulation. However, having said that, there are surface current available from various ocean models. One such model is operated at JPL.
An animation of the output of the JPL model run has been put together by Mike Godin, and can be obtained at:Â
http://aosn.mbari.org/tenacious/
Finally, the HF radar links (described in more detail in the post identified at the start of this note) are::
http://cencalcurrents.org/Â Â Surface current for SF.
http://www.cencalcurrents.org/Plots/SFB1/trj/2007_01/animation_2007_01_29_1000.gif   Animation for surface currents showing trajectories of water parcels on July 28 (the day Jim disappeared)
http://www.cencalcurrents.org/Plots/SFB1/trj/2007_01/Â Â Animation files.
Our experience with the HF radar in Monterey Bay is that the HF radar trajectories do a reasonable job of matching the paths of surface drifters.
-Jim BellinghamÂ
February 4th, 2007 at 12:32 pm
I worked on the Farallones for 24 years and was involved in many searches and a couple of rescues. We have also tracked dead whales and transmitters as they have left the island. Most items seem to drift to the west, defying our predictions based on wind and current models. In October 1999 a satellite transmitter drifted 166 nautical miles WNW of the island within four days. A dead humpack whale in November 1995 also drifted over 100 miles to the WSW within a couple of days. In November 2003 the sailboat, “Just Imagine” broke from its mooring (long story). Intensive searches in the week following its escape were concentrated within 100 miles of the coast and turned up nothing. The boat wound up off the Channel Isalnds a month later, and we concluded that it must have gone west to begin with, outside of the search area, and then tracked to the SE to rech this destination. I recall another derelict boat ending up about 300 miles off shore in the late 1980s or early 1990s. Hope this helps and good luck.
February 4th, 2007 at 1:02 pm
THank you! That is great information which I pass along to the folks working on trajectory generation.
February 4th, 2007 at 1:33 pm
Currents should be factored into the trajectory, but keep in mind that a sailboat, even without its sails up, is influenced significantly by wind and swell direction. Much more so than drifting current sensors or tagged whale carcasses. Even sailess, the topsides of C&C 40 will present enough surface to a wind of sufficient velocity to drive it along. The swell direction and state will also have a large influence on the orientation of the hull. Obviously the effect of the wind on trajectory will be increase by a large magnitude if the sails are up, but do not overlook the importance of swell state and direction in calculating an unattended sailboat’s trajectory.