The eXpendable BathyThermograph (XBT) has changed the face of oceanography. For three decades, the instruments that resemble small torpedoes have been launched by scientists riding on repeating container ship transects to measure the temperature of the upper ocean, opening an entirely new world of data to scientists about long-term changes in the ocean.
It all started when two oceanographers, Dean Roemmich and Bruce Cornuelle, ushered a transformative piece of technology into existence that ultimately set the bar for the future of XBTs. The two researchers from Scripps Institution of Oceanography at the University of California San Diego persevered through sleepless nights and grueling shifts of field deployments to build an unprecedented ocean observing program known as the High Resolution XBT (HR-XBT) Network.
HR-XBT measurements along key ocean transects have allowed researchers to calculate the seasonal and interannual variation in the transport of heat, mass and fresh water, and to determine trends in temperature, velocity, and circulation in the ocean. The transport of heat and fresh water have major implications for surrounding ecosystems, especially in a changing climate. Not only does the HR-XBT Network allow scientists to observe these trends in different locations and at various depths, but it allows them to observe trends over time.
“Thirty-year datasets are rare gems in oceanography. The longer these transects can be maintained, the more scientifically valuable they become,” said Roemmich.
A central goal of the HR-XBT program is to allow for deeper understanding of the world oceans and ocean-atmosphere interactions by enhancing data resolution. XBTs are dropped frequently along the ocean transects, providing observations that are closer together in order to more accurately capture the structure of narrower ocean features. Additionally, ships repeat the same route in order to note changes in observations made at the same location.
Thanks to HR-XBT ocean temperature measurements, the world has gained insight on global ocean warming and other ocean and climate related variability in heat content. HR-XBT data have also been used to calculate the ocean circulation across sections of the ocean. Researchers have been able to use HR-XBT data along with an ocean model to account for the heat stored in the north Pacific Ocean, and to investigate how heat moves and how heat content changes from year to year.
Roemmich and Cornuelle’s XBT adventures began in March 1986 when they initiated the first mission of the High Resolution XBT Network with funding from the National Science Foundation (NSF). Over three days, they traversed the Pacific Ocean between Auckland, New Zealand and Suva, Fiji, crossing through the South Pacific subtropical gyre. The oceanographers rode aboard a container ship, dropping manufactured XBTs along the route using their hand-built automated XBT launcher.
Cornuelle and Roemmich were no strangers to exploring the world oceans. Both obtained their doctorates in the early 1980s under the same advisor at the Massachusetts Institute of Technology–Woods Hole Oceanographic Institution Joint Program in Oceanography and then performed field work including at-sea observations.
The pair of researchers wanted their observational and data collection techniques to contribute to a major survey of oceans known as the World Ocean Circulation Experiment, or WOCE, that was planned to start four years after their first XBT expedition. By proposing easily deployable XBTs and repeated transects to the WOCE planning committee, Roemmich and Cornuelle intended to demonstrate that XBTs paired with their automated XBT launcher would work well, and that the XBTs would add an important component to the ocean survey by allowing for higher resolution temperature-depth profiles.
In 1990, the NSF decided to make measurements with XBT instruments a component of WOCE. The HR-XBT Network later evolved to include U.S. and international partners which have included NOAA-AOML (Atlantic Oceanographic & Meteorological Laboratory), NOAA, CSIRO (Commonwealth Scientific and Industrial Research Organisation) in Australia, and Tohoku University in Japan.
In previous missions using XBTs, researchers routinely launched one XBT every hour, and as often as every 15 minutes. In order to do so, a researcher had to stand next to the ship’s rail with a handheld XBT launcher connected to a computer until the XBT sank to a maximum depth. Roemmich and Cornuelle aimed to make consecutive measurements across transects that could require three days of ship travel. There was, however, only one of them on the ship at a time, so they needed to improve XBT launching methods.
Rather than stay awake for three days in a row, with less than an hour for sleeping in between XBT launches, Cornuelle and Roemmich built an automated launcher that could hold three manufactured XBTs at a time. This would allow the researchers a couple hours of sleep between launches.
Cornuelle and Roemmich worked day and night onboard the cargo ships. The two researchers often worked on a minimal amount of sleep, as they routinely reloaded the XBT launcher every three hours for days at a time. Roemmich and Cornuelle had to remain alert and ready to reload the autolauncher and make necessary repairs at all times.
"You’re pretty tired, and there was an alarm to wake you up to launch another probe, or to tell you if something went wrong while collecting data. I’d jolt out of bed and bump my head on my way out,” said Cornuelle. "I’ve taken out a fair number of light fixtures in my time."
The initial version of the XBT autolauncher was entirely homemade, built by the pair of researchers using aluminum angle brackets that they secured to the ship’s rail. They used computer-activated electric motors that tugged on a string connected to the pin securing the instrument. Once the pin was released, the XBT could drop into the ocean to begin taking measurements along the transect.
There is an electrical component inside the XBT probe, called a thermistor, that allows the computer to read temperature measurements from the probe. As the XBT drops, two copper wires unravel from spools along with it; one wire unravels with the XBT and the other unravels from a spool attached to the ship.
The copper wire links the probe to the ship down to a maximum depth of around 800 meters (2,625 feet). After the wire fully unravels, it breaks, allowing the XBT probe to sink freely to the ocean floor.
Depth measurements are made indirectly. They are calculated using what are known as fall-rate equations. In order to solve these equations, scientists must consider the known rate at which the probes fall through seawater, and the time after the probe entered the water.
The oceanographers used a programming language to automate commands and make their own data logging system for the computer. This helps reduce data collection errors, and gives scientists more control over commands given to the device.
Along the ocean transects, the two scientists used their hand-built autolauncher to drop XBT probes to measure changes in ocean temperature and other variability in heat content in the upper ocean. Probes were dropped frequently, as close as every ten kilometers across narrow boundary currents. They use measurements from their voyages to build a database outlining how heat is transferred through the oceans, and to look at basin-scale phenomena including boundary currents and fronts. The new component of repeated high-resolution ocean observations using XBTs had never been carried out before.
Not only did the oceanographers gain expertise on data collection using their hand-built launcher for the ocean probes, but they also gained insight on life as a member of a multinational shipping crew. They secured container ships by identifying shipping routes matching their research interests, obtaining permission, and installing their equipment onboard. The scientists made consecutive measurements while the shipping crew members performed their own mission and duties.
“Most container ships have spare cabins, and we provide a form of interest or entertainment for the crew by talking about the program. It’s really very interesting to chat with officers and crew on the bridge and in the mess and to learn something of the shipping business,” said Roemmich.
Fifteen years after its launch, the HR-XBT Network became a part of NOAA’s sustained observing system, funded by NOAA-CIMEC (the Cooperative Institute for Marine Ecosystems and Climate). As the program evolved, the two-scientist team grew to include more professionals and numerous part-time riders, many of whom are still involved with the program today. Scripps marine engineer Glenn Pezzoli joined the team in 1997 and, alongside Roemmich, designed the modern version of the XBT autolauncher that allows for higher efficiency and fewer sleepless nights.
Pezzoli and Scripps marine technician Justine Parks now head the at-sea data collection program. Lisa Lehmann leads alongside as computer programmer and data manager. Scripps physical oceanographer Janet Sprintall is now the principal investigator of the program. Today the group manages about 30 cruises per year.
As a team, the researchers have re-designed the XBT autolauncher to hold six XBTs, identified ever-changing shipping routes, and modified data collection techniques to become more efficient as the network expanded.
The HR-XBT Network allows for higher spatial resolution of eddies and significant ocean boundary currents such as the Kuroshio current off Japan, California Current, the East Australian Current and the Gulf Stream. The program also reveals information on frontal systems, and observes climate variability in ocean temperature and circulation. The temporal resolution of ocean phenomena is also enhanced through the HR-XBT program since, by repeating transects, it provides a dataset of changes over time along particular ocean sections.
“You have to be able to see it [the ocean] and understand what’s going on to have the vision for what’s going to happen; and to have it be the right vision,” said Cornuelle.
Some of the greatest mysteries the XBT measurements have helped scientists solve include identifying the heat transport involved with permanent ocean boundary currents and fronts, and investigating the significance of eddies in transporting heat. No program before the HR-XBT Network has made measurements closely enough or made repeated transects that resolve these phenomena to the same degree.
Measurements from HR-XBT have revealed new information on a decadal shift in the East Australian Current System that is due to stronger winds brought on by climate change. In studies of the southwest Pacific’s western boundary current using HR-XBT data and satellite altimetry, researchers also gained new information on long-term heat and freshwater transport in the Tasman Sea.
Because HR-XBT measurements provide researchers with a better understanding of how the ocean affects Earth’s climate system, they are also key in helping improve operational ocean computer models and future predictions of the ocean in a changing climate.
“We study the ocean in an effort to understand and predict climate,” said Pezzoli. “The ocean currents are the air conditioning system of the earth; moving heat around. If those currents stop or change, then the equator would be unbearably warm and the poles would be unbearably cold, so the zone of habitability would shrink.”
The HR-XBT Network served as a precursor to a groundbreaking ocean-observing program called Argo that was led by Roemmich and established in the early 2000s. Since then, Argo has deployed over 3,000 free-drifting profiling floats across world oceans. The floats provide data on subsurface ocean temperature, salinity, and currents.
While the HR-XBT Network measures the heat and mass entering and leaving a section of the ocean, Argo floats drift with the current to measure the heat remaining in the ocean section. Together Argo and XBT measurements provide information on the heat flux from the ocean to the atmosphere. Argo measurements, therefore, make XBT measurements more valuable and vice versa.
“Transect-based sampling (XBT or repeat hydrography) and area sampling (Argo) are very complementary modes in the kind of information they provide,” said Roemmich.
Tashiana Osborne is a second-year student in the laboratory of climate researcher F. Martin Ralph.