Thursday, March 23, 2017

The weather, and other considerations

Doing research from a ship poses a number of logistical challenges that must be taken into consideration along with the science goals of the mission. We are out here to look for and study a particular phenomenon, but this means not only finding it in the first place (as discussed in earlier posts), but also having acceptable conditions in which to operate. In the case of this project, since we are hoping to look at restratification processes (several days of de-mixing) following a strong mixing event (i.e. a storm) at a front (a fairly narrow physical feature), we need to be in the right place at the right time without putting the ship in danger. Ideally, we’d like to time it so that the equipment is already in the water when the storm hits, because if things get too exciting, we either can’t deploy our instruments or we need to vacate the region, neither of which is good for the science. Fortunately, we have a very experienced ship crew on the Sikuliaq that is both very helpful in making sure the science happens but also conscientious about making sure that no unnecessary risks are taken.

Consulting about the optimal measurement spot on the bridge. From left: Chief Scientist James Girton, Captain of the Sikuliaq Diego Mello, Co-PI Eric Kunze, Co-PI John Mickett, and Second Mate John Hamill. (Photo credit: RE)
We are currently surveying the region of our final deployment with SWIMS to determine where exactly we want to place the buoy and the floats for the next round of observations. The first step in selecting this location was looking at the most recent data sent back from Argo[1] floats in the region to see if we could find places with a deeper mixed layer that would also better coincide with the storm track in the northern Pacific Ocean. We also looked at satellite altimetry (ocean height) and sea-surface temperature measurements to confirm that the deeper mixed layers could be found near a front. This allowed us to define a general box of interest, with the intent of doing on-site verification of the required features as we approached.

Ryan Newell, Eric Kunze, and Chris Siani work on reconnecting the winch cable to SWIMS after repairing the electrical connection, which is essential for transmitting data back to the ship as the instrument profiles. (Photo credit: RE)
Because the ship has to go so much slower when we are towing SWIMS, once we arrived at the beginning of our large spatial box, we used a different instrument to get a rough picture of the mixed layer, known as an “expendable bathythermograph” (XBT). This instrument is standard fare in the world of oceanography, and is frequently used on ships of opportunity because they are so simple (and inexpensive) to use. The XBT is a small torpedo shaped instrument with a lead weight at one end and a thermistor that measures temperature. Depth is calculated from a simple speed-of-fall estimation, and the data is transmitted to the ship in real time as a thin copper wire unspools rapidly. All three deployments, spaced roughly one degree of latitude apart, showed a mixed layer closer to 70 or 80 meters, rather than the 30 meters we observed at our last site, which is great news.

Marine Tech Bern explains how the XBT works to volunteer Ethan Brush. The tip of the actual instrument is just visible inside the black plastic cylinder it is housed in until deployment. (Photo credit: RE)
As mentioned earlier, however, picking a site is not as simple as finding a place in the ocean that has the right properties and going to work. Since we are now closer to the storm track, it is also important that we are not putting the ship at risk when doing the science. The box we had initially outlined for this portion of the cruise included some regions that were considerably farther north. After consultation between the chief scientists, the captain, and the mates, it was determined that if we could achieve the same scientific goals without entering that region of the ocean, both the ship and our instruments would probably be better off. As a result, we are about to embark on round three of buoy and float deployment for the home stretch of the project.

[1] The Argo program is a worldwide consortium of researchers using floats very similar to our EM-APEX floats that do a 2000 meter temperature-salinity-depth profile once every 10 days, often for 5 or more years. (Some floats are now equipped with additional sensors, such as dissolved oxygen and nitrogen sensors). One huge asset to this program is that the floats are distributed throughout the world ocean and profile year-round, often in places that are not readily accessible to ships all the time. You can find out more about the program and the data at

No comments:

Post a Comment