Feature ArticleAnnual CTD Profiling On Transit to Canadian Arctic
By Dr. Svein Vagle • Adrian McDonald • Dr. Jochen Klinke
The Icebreaker CGGS Sir Wilfrid Laurier.
The ocean plays a critical role in the Earth’s climate and climate change. Because of the ocean’s vastness, an adequate monitoring system is complex and expensive. Satellite-based sensors are now highly successful in mapping and monitoring surface processes economically over large geographical scales and with reasonable temporal coverage. However, critical observations of the ocean’s interior still rely on making in-situ observations using profiling instrumentation.
The most fundamental parameters required to understand and monitor the ocean are temperature, salinity and density as functions of depth, time and location. From CTD profiles, salinity, density and sound speed can all be calculated using known equations relating to the thermodynamic properties of seawater.
Spatial and vertical density variations are some of the primary drivers of ocean currents, mixing and heat transport. Sound-speed profiles are used extensively in numerical modeling of sound propagation in the upper ocean to study, for example, marine mammal movements and anthropogenic and natural noise levels.
Every summer, icebreakers leave southern ports on the west and east coast of Canada and travel north to the Canadian Arctic where they perform icebreaking duties, are on standby for search and rescue, service navigational aids, supply remote communities and conduct science. While in transit, these ships cover vast distances all around Canada, and this represents a unique opportunity to collect oceanographic data every year, as well as to establish baselines and study effects of climate change. This is the objective of Canada’s Three Ocean’s (C3O) project, which has been making these types of measurements from icebreakers for many years. The Underway CTD (UCTD) system designed and built by The Oceanscience Group (San Diego, California) has significantly increased the amount of data collected along the ship tracks, without slowing down the ships, and the data collected are of significantly better quality than earlier data.
Alternative Techniques for Water-Mass Characterization
The most mature and accurate technique to characterize water masses at a given location at a given time is to lower carefully calibrated sensors from oceanographic research ships stopped on station and implement heave compensation to minimize the effects of a rolling ship on the movement of the sensors. These sensors are calibrated in situ by collecting and analyzing in the lab water collected at selected depths during a given profile. This is expensive and time consuming, normally requiring ships assigned specifically for this task. The number of profiles collected in a given geographical location will be limited due to limited resources and available ships.
These limitations have partially been overcome by the deployment of thousands of Argo floats around the world. These floats profile the water column from a typical depth of 1,000 meters and surface every 10 days for up to 5 years. They give a synoptic picture of the world’s ocean but are not capable of fine-scale (space or time) studies of particular oceanic processes.
Ocean gliders are now becoming common tools to gather ocean data over large geographical areas for extended periods of time. However, these are complicated vehicles that move relatively slowly through the water and require significant technical support to operate. Also, in shallower waters, where the gliders need to reballast frequently, the operational period is relatively short.
Making profiling observations from moving vessels has the advantages of saving time on ships that are on a tight schedule and the opportunity to collect profiles over a larger area at smaller temporal scales. The Moving Vessel Profiler (MVP) built by ODIM Brooke Ocean (Dartmouth, Canada) is a multipurpose instrument designed for this task. This system can be controlled by a computer inside the ship and can therefore be operated autonomously, without personnel on deck. However, this instrumentation is large, with big winches and cabled tow fish. And even though these systems deliver superior data quality in real time, the deck-space requirements are significant, including special large-attachment mounts, and these systems are technically complex and expensive.
At the other end of the complexity and cost scales are expendable bathythermographs (XBT). Even though these probes were originally developed for use on navy ships at high speeds, they have also been in wide use on oceanographic research ships, survey vessels and ships of opportunity for gathering temperature profiles while underway. However, limited data quality, lack of salinity (conductivity) measurements and a residual environmental footprint meant that, while convenient, XBTs are not ideal. More recent expendable CTDs (XCTD) overcome the lack of salinity measurements but are expensive and leave an environmental footprint.
The Underway CTD System
Not satisfied with the available options, researchers at Scripps Institution of Oceanography developed an Underway CTD profiling system to provide oceanographers access to the high-quality data available from complex and expensive profilers, with the convenience of expendable probes. The new system also had to be compact enough to fit on a range of vessel sizes and easily transferable from one vessel to another. Subsequently commercialized and developed by The Oceanscience Group, and using a special free-falling CTD probe developed by Sea-Bird Electronics Inc. (Bellevue, Washington), the Underway CTD has been in use by oceanographic research groups, fisheries scientists and hydrographic surveyors since its introduction in 2008.
The UCTD consists of four main components. The electric winch, mounted close to the aft rail, has a low-friction, free-spooling payout action to deploy the probe. The CTD probe is a customized low-drag design, developed by Sea-Bird Electronics, based on an earlier prototype Scripps design. Sampling at 16 hertz allows approximately 25-centimeter vertical resolution in typical operation. The rewinder serves to load line from the main winch onto the CTD probe assembly before each cast. A power supply supplies 24 volts to the system components. The principle behind the system is to load tether line onto the probe assembly, which is spooled off during the descent of the probe to its desired target depth while the winch on the ship pays out line along the ocean surface at the speed of the ship. This technique allows the deployed probe to quickly reach a constant, relatively high fall speed. To continue this article please click here.
Dr. Svein Vagle received a Ph.D. in physics from the University of Victoria in 1989. In 1996, he joined the Ocean Science Division at Institute of Ocean Sciences Fisheries and Oceans Canada. He studies upper-ocean processes and how they affect biological distributions using standard and new techniques and instrumentation.
Adrian McDonald is a senior sales executive at The Oceanscience Group. He received a Ph.D. in chemical oceanography from University of Southampton. He had 10 years of sales and marketing experience in the oil and gas industry prior to joining Oceanscience.
Dr. Jochen Klinke is currently senior scientist at The Oceanscience Group, responsible for the company’s underway profiling systems. After post-doctoral research at Scripps Institution of Oceanography, Dr. Klinke joined Oceanscience to help commercialize the Underway CTD.