With global warming and the importance of the seas to our planet’s ability to sustain life, monitoring of oceans for changes is becoming vital work for scientists.  By understanding the operation of one type of monitoring device, such as the Spray underwater glider described in this article, scientists can improve the way they use their own devices in their fields of study and possibly come up with new ideas for data collection methods.

Spray underwater gliders are small devices that have been in use for about a dozen years to help scientists collect data on the oceans.  These gliders, which can be launched and recovered from small boats near land, move horizontally and vertically and have been used extensively to monitor boundary currents carrying warm water.  

Spray underwater glider being deployed in the ocean

The gliders travel in a U-shaped path, going from the surface of the ocean, where they communicate data to shore and collect navigational data by satellite, to depths of between 500 and 1,000 meters.  The gliders “carry small, low-power sensors to measure such quantities as temperature, salinity, pressure, velocity, chlorophyll fluorescence, dissolved oxygen, nitrate, acoustic backscatter, and many other variables for long durations and distances.”

 

The developers and builders of the spray glider used descriptive statistics to analyze the effectiveness of the gliders and to help quantify the value of the missions of the gliders.

Data Collection

Sample. Since 2004, Spray gliders have completed nearly 200,000 dives covering over 564,000 kilometers horizontally through the oceans and almost 210,000 kilometers vertically.  The sample for this study was 297 missions that lasted at least 5 days each.  Missions of under 5 days were not included because most of these represented test missions (usually after technological changes to the glider) or missions cut short because of some sign of a problem (in which cases the gliders were recovered, repaired, and re-deployed as quickly as possible).  A total of 66 short missions were excluded from the study.

Descriptive Statistics.  The researchers calculated descriptive statistics for several variables.  They first looked at length of deployment for their gliders.  The mode fell in the range of 95-125 days; 54% of the missions fell in this range. They found other peaks at 10 days, which they attribute to test missions and aborted missions, and at 60 days, which represented missions that were divided into two parts because of the need to repair the glider.  The median duration of 100 days was greater than that for a similar type of glider.

There were also several long missions (of over 180 days), including one of 375 days.  The researchers attributed most these long missions to the inability of damaged gliders to be recovered.  These gliders would collect and broadcast data only intermittently, so they were not as effective as gliders that worked properly, but they did have value.

The distance covered over ground is another important, but somewhat “ambiguous” measure of a glider’s mission.  The mode for distance covered was in the 1,600 to 2,800 kilometer range, with the lower end of this range coming from missions off of California, where the currents are relatively slow and the gliders made numerous back-and-forth runs during their missions.  The longest distances were over 3,500 km; these were in the faster currents in the Gulf of Mexico and the Gulf Stream.

Spray glider’s path.

The measurements of horizontal distance through water had a mode between 2,000 and 3,000 kilometers, with 56% of missions covering distances in this range.  Because of the more-consistent measurements of horizontal distance as compared to the other measurements, the researchers believe it is the best measure of value on its own.

Of the sample missions, only 28% of the glider missions were ended due to complications.

Of the 297 missions in the study, 84 (28%) had problems that ended the mission, with 49 (16% of the total) being critical enough to affect glider flight.  Nine gliders were lost; 6 of these came in the first six years of operation and 3 during the second half, indicating an improvement in loss rate.  Three of the losses were on missions that were using some untested technology or computer code, meaning they were at a higher risk for loss.

Discussion and Conclusion

In this study, the researchers discuss the California Underwater Glider Network (CUGN) as an example of one of the operations that employ Spray gliders to monitor the ocean off the California coast.  The operational goal is to keep one glider on each of the three lines in the network at all times.  On average, Spray gliders made 26 dives per day down to 500 meters and covered 59 kilometers over ground and 65 kilometers through water per day.  In analyzing the descriptive statistics (given above), they conclude that, since 2009, they have met their goal 97% of the time.  Thus, they conclude that their mission has been a success.

Resources:

Rudnick, D. L., Davis, R. E., & Sherman, J. T. (2016). Spray underwater glider operations. Journal of Atmospheric and Oceanic Technology, 33(6), 1113-1122.

Woods Hole Oceanographic Institution.  https://www.whoi.edu/main/spray-glider

Author: Neil Starr