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Photo of XBT deployment from ship

How We Observe the Ocean

Expendable Bathythermographs (XBTs) & Ships of Opportunity

The world-wide network of eXpendable BathyThermographs (XBTs), which are instruments dropped from shipboard to measure the temperature of the upper kilometer of the ocean, is deployed from about 50 "ships of opportunity." XBT measurements, which have been repeated for decades at intervals ranging from hours to months, provide the foundation for understanding long-term changes in marine climate. The ships of opportunity are commercial carriers that transit scientifically important trans-oceanic routes; they volunteer to make ocean measurements using NOAA-supplied instruments, and in some cases host NOAA technicians on board to make the measurements. In addition to observations of upper ocean thermal structure and sea surface temperature, these ships provide highly accurate measurements of surface metereorology as well the air-sea exchange of carbon dioxide, and also serve as platforms for deployment of surface drifting buoys and Argo profiling floats. Over two dozen countries contribute to the operation of this network.

  • XBT Details
  • SOOP Details
  • SOOP/XBT Links

An eXpendable BathyThermograph (XBT) is a temperature probe that is dropped into the ocean from a ship, either by hand or using using an automatic launching system. Temperatures are recorded as the probe drops at a known rate through about the upper kilometer of the ocean. By making measurements at the same location at regular intervals, it is possible to observe the evolution of the thermal structure of the upper ocean.

NOAA employs two sampling modes for deployment of XBT probes, each serving a different scientific purpose: Frequently Repeated (FR) and High Density (HD).

Modes of XBT Deployment
Mode Spacing Frequency
Frequently Repeated (FR) ~ 150 km 18 times per year
High Density (HD) ~ 25 km 4 times per year

Frequently repeated XBT transects, located primarily in tropical regions, are designed to monitor strong seasonal-to-interannual variability in the presence of intra-seasonal oscillations and other small-scale variability. They are intended to capture the large-scale thermal response to changes in equatorial and extra-equatorial winds.

High density transects, used to observe smaller scale ocean features, are designed to resolve fronts and mesoscale eddies that are prevalent in much of the ocean, and boundary currents that transport large quantities of heat in regions where thermal gradients are large (and where sampling intervals ma, therefore, be as fine as 10-15km).

NOAA deployment of XBTs is managed in close cooperation with about two dozen other countries that contribute to this network under the framework of the JCOMM Ship of Opportunity Programme. NOAA scientists at the Atlantic Oceanographic and Meteorological Laboratory (AOML) arrange and conduct XBT deployments in the Atlantic Ocean; scientists at the Scripps Oceanographic Institution arrange and conduct XBT deployments in the Pacific Ocean.

XBT data are made available in near-real time on the Global Telecommunications System for weather forecasting efforts, and are freely available in quality controlled delayed more via the internet from the Global Temperature-Salinity Profile Program (GTSPP) operated by the NOAA National Oceanic Data Center (NODC). Delayed mode data are also archived and distributed by the French Coriolis Data Assembly Center. Final quality controlled XBT data merged with other temperature profiles can be found in the NODC World Ocean Database. In addition, NOAA Pacific XBT datasets are available at the Scripps Institution of Oceanography High Resolution XBT/XCTD Network Site; NOAA Atlantic XBT data sets are available at the NOAA Atlantic Oceanographic and Meteorological Laboratory XBT web site.

Ship of Opportunity Natalie Schulte

Ship of Opportunity Natalie Schulte, operating in 2011 between Long Beach, California and New Zealand

The Ship Of Opportunity Programme (SOOP), operating internationally under the auspices of the Joint WMO-IOC Technical Commission for Oceanography and Marine Meteorology (JCOMM), oversees collection of upper ocean temperature profiles using eXpendable BathyThermographs (XBTs), mostly from volunteer vessels. The XBT deployments are designated by their spatial and temporal sampling goals or modes of deployment (Frequently Repeated, and High Density) and sample along well-observed transects, on either large or small spatial scales or at special locations such as boundary currents and chokepoints, all of which are complementary to the global array of Argo profiling floats. In addition to deploying XBTs, many ships of the Ship of Opportunity Programme serve as platforms for other scientific instruments, including TSGs (ThermoSalinoGraphs), xCTDs (eXpendable Conductivity Temperature and Depth), pCO2 (CO2 partial pressure instruments), ADCPs (Acoustic Doppler Current Profilers), and CRPs (Continuous Plankton Recorders).

The Ship Of Opportunity Program Implementation Panel (SOOPIP) serves to implement the XBT upper ocean data requirements established by the international scientific and operational communities. Annual reporting of transect sampling is undertaken by the JCOMM in situ Observations Program Support Centre (JCOMMOPS). SOOPIP, which deals with ocean observations from ships, is one of the three components of the WMO-IOC Ship of Observations Team, the other two being the Voluntary Observing Ship Programme, which deals with surface meteorological observations, and the Automated Shipboard Aerological Programme, which deals with upper-air profiles.