Ships of Opportunity

The primary objective of the Ship-of-Opportunity Programme (SOOP) is to fulfill the XBT upper ocean data requirements established by the international scientific and operational communities, which can be met at present by measurements from ships of opportunity. The annual assessment of transect sampling is undertaken by the Joint WMO-IOC Technical Commission for Oceanography and Marine Meteorology (JCOMMOPS) on behalf of the Ship Of Opportunity Programme Implementation Panel (SOOPIP). Data management is taken care of through the Global Temperature Salinity Profile Programme (GTSPP) (Goni et al. 2009). The SOOP is directed primarily towards the continued operational maintenance and co-ordination of the XBT ship of opportunity network but other types of measurements are also being made (e.g. TSG, XCTD, CTD, ADCP, pCO2, phytoplankton concentration). This network in itself supports many other operational needs (such as for fisheries, shipping, defense, etc.) through the provision of upper ocean data for data assimilation in models and for various other ocean analysis schemes. One of the continuing challenges is to optimally combine upper ocean thermal data collected by XBTs with data collected from other sources such as mooring arrays, Argo, and satellites (e.g. AVHRR, altimeter, etc.). However, it is considered most important to have the SOOP focused on supporting climate prediction in order to ensure the continued operation of the present network.

XBT (Expendable BathyThermograph) is an expendable temperature and depth profiling system. It is typically comprised of an acquisition system onboard the ship, a launcher, and a expandable temperature probe. The falling probe is linked to the acquisition system through a thin insulated conductive wire which is used to transmit the temperature data back to the acquisition system in real time. Depth is deduced from elapsed time using a well calibrated fall rate equation (about 6.5 m/s). Processed profile data can be transmitted in real-time through satellite. The realtime data is being archived at Coriolis data center, Brest, France and the delayed mode data at GTSPP managed by NOAA/NODC. Profiles as deep as 1,000 m and comprising (T, D) data points every meter can be made although with usual probes depths range from 500 to 800 m. Accuracy is normally better than 5 m for depth, and better than 0.05°C for temperature. The Global XBT network containing OceanObs'99 recommendations and current proposed transects recommended in OceanObs'09 is shown in Fig. 3.2.

The scientific and operational communities deploy approximately 23,000 XBTs every year. In a typical year, 50% are deployed in the Pacific Ocean, 35% in the Atlantic Ocean and 15% in the Indian Ocean. Profiles from about 90% of the XBT deployments are transmitted in real-time, which represent around 25% of the current real-time vertical temperature profile observations (not counting the continuous temperature profiles made by some moorings). XBT operates three modes of deployment: (a) High Density (HD): 4 transects per year, 1 XBT deployment every approximately 25 km (35 XBT deployments per day with a ship speed of 20 kts), (b) Frequently repeated (FR): 12-18 transects per year, 6 XBT deployments per day (every 100-150 km) and (c) Low Density (LD): 12 transects per year, 4 XBT deployments per day.

The HD transects extend from ocean boundary (continental shelf) to ocean boundary, with temperature profiling at spatial separations that vary from 10 to 50 km in order to resolve boundary currents and to estimate basin-scale geostrophic velocity and mass transport integrals. PX06 (Auckland to Fiji), which began in 1986, is the earliest HD transect in the present network with more than 90 realizations. Some transects are being assessed for their contribution in this mode. For example, the CLIVAR IOP noted that further work is required to assess the value of IX10, which transects the openings of the Bay of Bengal and the Arabian Sea. Scientific objectives of HD sampling and examples of research targeting these objectives are outlined in Goni et al. (2009).

The FR transects cross major ocean current systems and thermal structures. In some cases, for currents near a continental boundary an extra profile that crosses the 200 m depth contour is made to mark the inshore edge of the current. The FR transects are selected to observe specific features of thermal structure (e.g. thermocline ridges), where ocean atmosphere interaction is strong. Estimates of geostrophic velocity and mass transport integrals across the currents are made by low pass mapping of temperature and dynamical properties on the section. The proto-types of FR transects are IX01 and PX02, which now have time series extending more than t-1-1-1-1-1-1-1-1-1-!-1-1-1-r t-1-1-1-1-1-1-1-1-1-!-1-1-1-r

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Fig. 3.2 (top) XBT network containing OceanObs'99 recommendations and (bottom) proposed transects in OceanObs'09. XBT observations transmitted in (red) real and (blue) delayed-time in 2008. (Source: Goni et al. 2009)

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Fig. 3.2 (top) XBT network containing OceanObs'99 recommendations and (bottom) proposed transects in OceanObs'09. XBT observations transmitted in (red) real and (blue) delayed-time in 2008. (Source: Goni et al. 2009)

20 years. The earliest transect from Fremantle to Sunda Strait (Indonesia) began in 1983, has been sampled at 18 times per year after 1986. IX01 crosses the currents between Australia and Indonesia, including the Indonesian Throughflow and has been used in many studies of the Throughflow and the Indian Ocean Dipole. The FR sampling produces well resolved monthly time series of thermal structure along transects. Using IX01, Meyers et al. (1995) shown the mean thermal structure generally westward flow in the deeper part of the thermocline and eastward shear in the shallow (<150 m) layer. Also, brought out the strongest variability in temperature is at the northern end of the transect near Indonesia. The temperature sections were used to understand the relationship of interannual variation in transport of Indonesian Throughflow to ENSO (Meyers 1996). Further, the time-variation of temperature at the north end of IX01 clearly shows the strong, subsurface upwell-ing associated with the start of the IOD events of 1994 and 1997, before the start of surface cooling. These and the other FRX time series have been used to understand how subsurface thermal structure varies across the Indian Ocean during IOD events (e.g. Rao et al. 2002; Feng and Meyers 2003). The use of FR lines in the Indonesian region to study the Indonesian Through-flow is discussed in the Indian Ocean white paper (Masumoto et al. 2009).

Low density transects have both operational and scientific objectives, such as investigate intraseasonal to interannual variability in the tropical oceans, measuring temporal variability of boundary currents, and investigating the historical relationship between sea surface height and upper ocean thermal structure. Many illustrative examples of applications of XBT observations, primarily from LD mode, are presented in the XBT white paper (Goni et al. 2009).

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