This program is designed to measure the upper ocean thermal structure in key regions of the Atlantic Ocean. eXpendable BathyThermographs (XBTs) in High Density mode are deployed approximately every three months and are deployed approximately 25 km apart in order to measure the mesoscale structure of the ocean to diagnose the ocean circulation responsible for redistributing heat and other water properties globally. This transects are carried globally, with NOAA/AOML taking the lead in the operations in the Atlantic Ocean.
Data from these transects have been used extensively (Meyers et al, 1991; Taft and Kessler, 1991; Goni and Baringer, 2002). For example, the scales of mode water and the distribution and circulation of associated water properties can be readily captured by LD/FRX sampling (Hanawa and Yoritaka, 1999). XBT data are also used in ocean analysis and in climate model initialization. For instance, for El Nino prediction XBT data complement that from the TAO array and from satellite-derived sea surface temperature and sea height observations. The use of XBT data serves to measure the seasonal and interannual fluctuations in the upper layer heat storage, now being complemented by profiling float measurements. Heat transport and geostrophic ocean circulation can be measured using the high-density XBT data that measures the meso-scale field.
Within this context, eleven XBT transects have been chosen to monitor properties in the upper layers of the Atlantic Ocean in HD mode. The continuation of AX07 and AX10 and the implementation of AX18 were recommended by the Upper Ocean Thermal Review Panel in St. Raphael in 1999. The location of the transects recommended at the St. Raphael meeting and the GCOS Implementation Plan (GCOS-92) are based on specific advantages of each lines location. HD transects AX07 and AX10 have been maintained since 1994 and 1996, respectively, providing a homogeneous data set for more than a decade. Sustained observations from these and the other three HD transects are required to have observations with adequate spatial and temporal resolution for climate studies. High-density observations in AX08, AX18, AX25, and AX97 provide observations in poorly surveyed regions. A summary of the justification for each of the HD transect is provided below.
- XBT transects AX01, AX02, and AX20 have been implemented in collaboration with the French Institut de Recherche pour le Developpement (IRD) and the University of Paris. AX01 runs between Greenland, Iceland, and Denmark, AX02 from Iceland to Canada, and AX20 from Spain to French Guyana. For these new transects, NOAA/AOML provides equipments and XBT probes, as well as data management, quality control and distribution, while the French collaborators provide ship logistics and ship riders. Transects AX01 and AX02 are located across the northward upper transport to the Nordic Seas and Arctic Ocean and provides the data necessary to monitor the subpolar gyre. The data is also used to study ocean current transport on seasonal time scales. Data have been also used to perform comparisons between an intake temperature sensor (TSG, installed in the ships used for these transects) and the XBTs, in order to assess possible temperature biases. Additionally data from AX01, AX02 and AX20 data helps to document the ocean heat storage and global transport of heat and fresh water, which is crucial to improving climate prediction models that are initialized with temperature profiles. The seasonal to interannual variability in upper ocean heat content and transport is monitored to understand how the ocean responds to changes in atmospheric and oceanic conditions and how the ocean response may feedback to the important climate fluctuations such as the North Atlantic Oscillation (NAO).
- The HD XBT transect AX07 is located nominally along 30oN extending from the Straits of Gibraltar in the eastern Atlantic to the east coast of the United States at Miami, Florida. This latitude is ideal for monitoring heat flux variability in the Atlantic because it lies near the center of the subtropical gyre, which has been shown to be the latitude of the maximum poleward heat flux in the Atlantic Ocean.
- The HD XBT transect AX10 is located between New York City and Puerto Rico. This line closes off the United States eastern seaboard, where subtropical temperature anomalies could have the greatest interaction with the atmosphere. This transect was chosen to monitor the location of the Gulf Stream and its link to the NAO.
- The HD XBT transect AX08, a component of the Tropical Atlantic Observing System, crosses the tropical Atlantic in a NW-SE direction between North America and South Africa. Historical data along AX08 and other historical temperature observations in the tropics exhibit decadal and multi-decadal signals. It has been hypothesized that this large time scale signal may cause atmospheric variability. Given the importance of the tropical Atlantic in climate variability, and the scarcity of observations in this region, data obtained from the measurements along this transect are key to improving our understanding of the ocean and our ability to forecast climate. Temperature profiles obtained from this transect will help to monitor the main zonal currents, countercurrents and undercurrents in the tropical Atlantic and to investigate their spatial and temporal variability.
- The HD XBT transect AX18, which runs between Cape Town and South America (Montevideo, Uruguay, or Buenos Aires, Argentina) is geared towards improving the current climate observing system in the South Atlantic, a region of poor data coverage. Similarly to the AX07 transect in the North Atlantic, the goal of AX18 is to monitor the meridional mass and heat transport in the upper 800 m across 30oS. Given the importance of the South Atlantic and the scarcity of observations in this region, data obtained from the measurements along this transect will be used to investigate the role of the South Atlantic in improving climate forecasts.
- The HD XBT transect AX25 was implemented to monitor the variability in the upper layer interocean exchanges between South Africa and Antarctica on seasonal and interannual time scales. In addition, by exploiting the relationship between upper ocean temperature and dynamic height, XBTs are used to infer velocities and to monitor the various frontal locations in the region.
- Almost all warm North Atlantic water entering the Nordic Seas does so in roughly equal parts between Scotland and the Faroes (the Faroe-Shetland Channel), and between the Faroes and Iceland. These two choke points make it especially easy to measure the transport of water, heat, and freshwater and how these vary with time. In the Nordic Seas this warm water is gradually transformed by atmospheric cooling into dense water that is returned to the deep North Atlantic over the deep sills between Scotland, the Faroes, Iceland and Greenland. Thus measuring currents and water properties here is key to an improved knowledge and understanding of the Nordic Seas' role in the global meridional overturning circulation (MOC). A program to measure currents in this regions has been underway for several years. Starting in summer 2013 temperature profiles will be also measured on a monthly basis. The combination of velocity and temperature sections will provide new and more accurate knowledge of heat transport and how it varies seasonally and eventually on inter-annual timescales. For this purpose XBT transect AX90 has been designed and implemented to monitor the surface-to-bottom temperature of all water between Scotland and Iceland.
- The HD XBT transect AX97 supports the MOVAR (from the Portuguese: Monitoring the upper ocean transport variability in the western South Atlantic) Project. The fluctuations of the zonally integrated baroclinic transport across this transect will be studied and linked to the variability of the Brazil-Malvinas frontal region. This region is critical since Brazil Current rings are the main mechanism to carry subtropical waters to high latitudes.
- XBT transects MX01, MX02 and MX04 have been implemented as a collaboration between NOAA/AOML and Agenzia Nazionale per le Nuove Tecnologie, l'Energia e lo Sviluppo Economico Sostenibile (ENEA), with NOAA/AOML providing XBT probes, data management, quality control and distribution, and the Italian collaborators providing equipment, ship logistics, and ship riders. The monitoring activity on these transects crossing the Mediterranean Sea has a great importance because of several reasons. MX01, MX02 and MX04 transects cross along the warmer and saltier part of the Mediterranean waters, where some permanent and transient structures and gyres can be observed and monitored with the XBT data. Several studies have located in that region unexpected changes and unexplained variability since the 80's (for instance Malanotte-Rizzoli et al.,1999). The intrusion of seawater from eastern basin into western part is almost continuously monitored but not the region where such warm and salt water forms. In the past thirty years the traditional flow of water in the Mediterranean has been modified due to climate change: Mediterranean is still continuing to be warmer and saltier and the rise in sea level is accelerating in recent years. In the Mediterranean, reported temperature anomalies often locally translating into disruption of the wind regime, of the water mass stratification and of the thermocline depth, could in part be correlated with positive NAO.