Who We Are
What We Do
The Western Boundary Time Series (WBTS) project carries out a sustained monitoring effort of the volume transport and seawater properties of the western boundary currents in the subtropical North Atlantic: the northward flowing Florida Current, which is part of the Gulf Stream as it passes through the Straits of Florida from the southernmost Florida Keys to the northernmost Bahamas Islands; and the near-surface northward Antilles Current and the southward Deep Western Boundary Current that are both found to the east of the Bahamas.
The WBTS project consists of two components:
1. Measurements of the Florida Current volume transport at about 27ºN with a submarine telephone cable supplemented with regular (14-16 times per year) calibration cruises that also observe changes in seawater properties;
2. Measurements of seawater properties and meridional flows across the 26.5ºN transect to the east of the Bahamas Islands with dedicated regular (approximately every 9 months) hydrographic surveys and an array of moorings.
Objectives of the Western Boundary Time Series project:
- Monitor the volume transport of the Florida Current.
- Observe changes in water mass properties in the Straits of Florida.
- Monitor the near-surface Antilles Current and the Deep Western Boundary Current, and associated water mass properties.
- Serve as a western boundary endpoint of a subtropical meridional overturning circulation/heat transport monitoring system at about 26.5ºN established in 2004 in partnership with the University of Miami and the National Oceanographic Centre (United Kingdom).
Field Operations and Engineering Support
AOML Researchers Monitor Important Boundary Currents in the North Atlantic Ocean Through Direct Measurements at Sea
July 6, 2021
Researchers from the Physical Oceanography Division of AOML conduct regular hydrographic surveys to monitor the western boundary current system in the subtropical North Atlantic Ocean. These cruises are a part of the laboratory’s long-running Western Boundary Time Series (WBTS) project and are designed to monitor both the Florida Current, east of Florida in the Florida Straits, and the North Atlantic Deep Western Boundary Current east of the Bahamas in the North Atlantic Ocean. These western boundary currents are important parts of the Atlantic Meridional Overturning Circulation (AMOC).
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Research Impacts & Key Findings
The Western Boundary Time Series project has the longest continuous observations of a western boundary current in existence. Daily monitoring of the Florida Current volume transport with a submarine cable has been maintained nearly continuously since 1982.
Due to the Earth’s rotation, the direction of major oceanic currents is parallel to the lines of constant sea level, with higher sea level to the right of the current in the Northern Hemisphere. For the Florida Current, this translates into sea level near the Bahamas being about 1-meter higher than sea level along Florida’s east coast. Variations in the Florida Current volume transport are linked to sea level changes on both sides of the Current.
The monitoring of the Deep Western Boundary Current (DWBC) in the subtropical Atlantic Ocean works toward validation of ocean models and forecast systems. For example, the Florida Current transport data are a core component in the verification efforts on the operational Global Real-Time Ocean Forecast System.
The Florida Current accounts for the bulk of both the upper limb of the Meridional Overturning Circulation and the western boundary component of the North Atlantic subtropical gyre circulation. Climate models have shown that variations in the Atlantic MOC, in part driven by variations in the Florida Current, have significant impacts on the climate at both the regional and global level. Observed changes in the Florida Current and the Atlantic MOC transports are essential metrics for climate studies and documented yearly in State of the Climate reports.
The Western Boundary Time Series project addresses NOAA’s mission to observe and detect long-term climate changes by continuing time series measurements of thermohaline circulation through full water column hydrographic observations and moored time series. Measurements of the Florida Current volume transport are a key component of the Atlantic Meridional Overturning Circulation Observing Array at 26.5ºN.
AOML has led more than 90 WBTS hydrographic cruises, measuring water mass properties and transport, across the project focus area, which includes the Florida Straits, Northwest Providence Channel, and along 26.5N east of Abaco Island. The hydrographic measurements east of Abaco reveal decadal changes in seawater properties within the Deep Western Boundary Current.
In the Florida Straits at 27N, AOML has conducted more than 14,000 daily estimates of the Florida Current transport since 1982, has led 252+ dropsonde cruises since 1991, 110+ XBT cruises since 1998, and 77+ CTD/ADCP cruises on board the R/V Walton Smith since 2001. With an array of six Pressure Inverted Echo Sounders (PIES) to the east of Abaco Island, AOML has collected about 6,000 daily values of bottom pressure and travel time since 2004.
The Meridional Overturning Circulation
Climate models have shown that variations of the transport of the Meridional Overturning Circulation (MOC) in the Atlantic Ocean have significant impacts on the climate at both the regional and global level. In the subtropical North Atlantic, the MOC consists primarily of two western boundary components: the northward flowing Gulf Stream/ Florida Current and the southward flowing Deep Western Boundary Current.
The Importance of the Western Boundary Currents
Western boundary currents are some of the strongest ocean currents in the world, carrying with them large amounts of heat, fresh water, nutrients and other chemicals that alter the global and regional weather, climate, sea level, and marine ecosystem. The WBTS project addresses NOAA’s mission to observe, understand, and predict changes in climate, weather, oceans, and coasts by carrying out sustained time series measurements of the western boundary currents in the subtropical North Atlantic.
What is the Western Boundary Time Series Project?
The WBTS project continuously monitors two important components of the thermohaline circulation in the subtropical North Atlantic with the ultimate goal of determining the state of the overturning circulation and providing a monitoring system for rapid climate change. The components are the northward flowing Florida Current and the southward flowing Deep Western Boundary Current: both key components of the meridional overturning circulation (MOC) in the subtropical North Atlantic. Over the past 30+ years the program has worked to measure both the warm upper and cold lower limbs of the MOC near the western boundary of the Atlantic Ocean at 27°N and is the longest time series of the flow that makes up the MOC available globally.
History of the WBTS project
The earliest observations of the total Florida Current transport in the northern Straits of Florida were made with dropsonde measurements between 1964 and 1974. Submarine telephone cable use for the estimates of the Florida Current transport dates back to 1975-1979.
In the modern era (2000-present) the cable measurements and hydrographic surveys in the Straits of Florida and east of Abaco are now part of the NOAA Western Boundary Time Series program.
Check out the Western Boundary Time Series project timeline.
The Western Boundary Time Series Project
Scientists at AOML use a decommissioned submarine telephone cable, along with calibration cruises, full water column hydrographic surveys, and moored observations to monitor the Atlantic’s western boundary currents and seawater properties.
Hydrographic cruises are conducted approximately every 9 months across the Deep Western Boundary Current (DWBC) area east of Abaco Island to measure water mass properties and transports. Tracer sampling, such for chlorofluorocarbon (CFC) and carbon dioxide (CO2), are also present in many sections.
The repeated hydrographic and tracer sampling at Abaco has established a high-resolution record of water mass properties in the Deep Western Boundary Current at 26N. This data set is unique because it is a transport section of a single time series site, of which very few are available in the ocean that approach a decade in length.
The Florida Current
Transport Time Series and Cruises
The Florida Current is a strong oceanic current flowing northward along the eastern coast of Florida carrying warm tropical waters that eventually feed the Gulf Stream. The Florida Current transports are collected two ways: measuring the voltage difference across the Straits of Florida using submerged submarine telephone cables; and via small-boat dropsonde/XBT cruises and research vessel LADCP/CTD cruises. The cable measurements provide daily transports – which have been measured nearly continuously since 1982.
The sections provide both calibration information for the transport estimates from the submarine cable and observations of changes in water properties within the Florida Current over time. Accounting for the platform-specific limitation, the Florida Current volume transport can also be inferred from cross-stream sea level differences measured by bottom pressure recorders deployed near Florida and Grand Bahama Island and by altimetry satellites that cross the Straits of Florida approximately every 10 days.
The daily Florida Current volume transport has been measured with a submarine telephone cable between Florida and Grand Bahama Island since 1982. These measurements are calibrated/validated with regular direct dropsonde and LADCP measurements at nine stations along the 27N transect during calibration/validation cruises. The full-depth temperature and salinity profiles are also obtained with CTD and XBT casts. Two bottom pressure recorders are deployed near the Florida coast and near Grand Bahama Island. Altimetry satellites, from Topex/Poseidon to Jason series, have measured sea surface height along the descending track 178 and the ascending track 243 across the Florida Current since the end of 1992.
(Upper) Vertical section of meridional velocity obtained by merging underway ship ADCP (SADCP) and lowered ADCP (LADCP) measurements at the nine stations (shown by triangles) and averaged for 25 cruises between 2012 and 2018.
(Lower) The Florida Current volume transport: (black) daily transport estimates from the cable measurements, (green dots) transport measured with “Pegasus” profilers, (red dots) transports measured with dropsonde floats, and (blue dots) transports measured with LADCP.
Deep Western Boundary Current
Transport Time Series
A quasi-continuous monitoring of the Deep Western Boundary Current using six pressure inverted echo sounders (PIES) that yield hourly measurements of bottom pressure and acoustic travel time between the moored instrument and the sea surface. Data from these instruments are acoustically downloaded by a passing research vessel about every 9 months without actually recovering the instruments. The data are then brought to the lab for processing. It has been shown that combining PIES measurements with historic hydrography data can reproduce the transport of a more traditional picket-fence of current meter moorings to within the accuracy of the current meter array (Meinen et al., 2004). The image to the right shows the location of the inverted echo sounders.
Since 1984, the repeated hydrographic and tracer sampling at Abaco has established a high-resolution record of water mass properties in the Deep Western Boundary Current at 26N. Approximately one hydrographic cruise has been conducted east of Abaco Island every nine months. Events such as the intense convection period in the Labrador Sea and the renewal of classical Labrador Sea Water in the 1980’s are clearly reflected in the cooling and freshening of the Deep Western Boundary Current waters off Abaco, and the arrival of a strong chlorofluorocarbon pulse approximately 10 years later. This data set is unique in that it is not just a single time series site but a transport section, of which very few are available in the ocean that approach a decade in length. The image to the left shows stations frequently visited during the Deep Western Boundary Current Hydrography cruise.
Three Main Purposes for Climate Variability Studies
These continued time series observations at Abaco serve three main purposes for climate variability studies:
- Monitoring the DWBC for water mass and transport signatures related to changes in the strengths and regions of high latitude water mass formation in the North Atlantic for the ultimate purpose of assessing rapid climate change.
- Serving as a western boundary endpoint of a subtropical meridional overturning circulation (MOC)/heat flux monitoring system designed to measure the interior dynamic height difference across the entire Atlantic basin and its associated baroclinic heat transport.
- Monitoring the intensity of the Antilles Current as an index (together with the Florida Current) of interannual variability in the strength of the subtropical gyre.
Building a Weather-Ready Nation.
Ocean Observing Technologies.
Scientists use different observing technologies and instruments to measure and observe the global oceans. AOML works with partners around the world to develop and maintain key components of these systems of observing technologies, known collectively as the Global Ocean Observing System.
AOML scientists design and operate new observing technologies and systems, deploy them in the ocean around the world, coordinate deployments with partners to maximize efficiency, and manage several critical aspects of data flow to insure that the data are available and of the highest quality. Visit the Global Ocean Observing System (GOOS) page to learn more.
Inferring Florida Current Volume Transport From Satellite Altimetry
Volkov, D. L., Domingues, R., Meinen, C. S., Garcia, R., Baringer, M., Goni, G., & Smith, R. H. (2020). Inferring Florida Current volume transport from satellite altimetry. Journal of Geophysical Research: Oceans, e2020JC016763.
Plain Language Summary: Florida Current (FC) is one of the major conduits of heat, salt, carbon, nutrients and other properties in the subtropical North Atlantic, with profound influences on regional weather, climate, sea‐level, and ecosystems. Daily monitoring of the FC volume transport with a submarine cable has been maintained nearly continuously since 1982. Because of the extremely high value of these measurements for Earth system studies, efforts are underway to find a suitable backup observing system for the inevitable future when the cable fails. Satellites have been providing accurate measurements of sea level for nearly 3 decades. Due to the Earth’s rotation, the direction of major oceanic currents is parallel to the lines of constant sea level, which for the FC translates into sea level near the Bahamas being about 1‐m higher than sea level along Florida east coast. Variations in the FC…
Driving Innovative Science with Data.
Our Contribution to Global Ocean Observing.
The following is a list of scientific publications that reference Western Boundary Time Series (WBTS) data.
Please contact publication authors to obtain an electronic or printed version of the articles. Alternatively, you may conduct a Google search of the publication title, which may direct you to an electronic version of the article.
View WBTS Peer Reviewed Publications by year.
View WBTS Data Reports by year.
Sharing Resources Delivers Results.
Expanding Reach Through Partnerships.
Hydrographic and mooring cruises have been conducted with our national and international partners.
Years of the Atlantic MOC monitoring at 26.5N.
The Rapid Climate Change-Meridional Overturning Circulation and Heatflux Array (RAPID or MOCHA) program is a collaborative research project between the National Oceanography Centre (Southampton, U.K.), the University of Miami’s Rosenstiel School of Marine and Atmospheric Science (RSMAS), and AOML that measures the meridional overturning circulation (MOC) and ocean heat transport in the North Atlantic Ocean.
Monitoring of the Florida Current volume transport with along-track satellite altimetry, providing an alternative observing platform that is not influenced by severe weather conditions and can fill out the existing gaps in the cable data back to 1993.
Bottom pressure measurements at 27N began in an attempt to establish a backup observing system for the cable measurements.
The AMOC observing array at 26.5N was established, greatly facilitating climate-related studies.
First Inverted Echo Sounders were deployed at 26.5N east of Abaco Island, allowing quasi-continuous estimates of the meridional transports and thermohaline structure.
CTD/LADCP cruises at 27N on board the University of Miami RSMAS R/V “Walton Smith” started, providing direct measurements of the Florida Current for cable calibration.
The start of the NOAA’s Western Boundary Time Series program.
XBT measurements at 27N started.
The advent of high-accuracy satellite altimetry, providing nearly global measurements of sea level anomalies.
Dropsonde measurements at 27N started as a replacement for Pegasus measurements, serving for the calibration and validation of cable measurements.
The Abaco time series program began when the STACS extended its Straits of Florida program to include measurements of western boundary current transports and water mass properties east of Abaco Island, Grand Bahamas.
The NOAA Subtropical Atlantic Climate Studies (STACS) program began, which included quasi-continuous measurements of the Florida Current volume transport with a submarine cable.