Upper Ocean Thermal Data Sampling Requirements
TOGA and WOCE demands for UOT data employ several strategies. As the objectives for these programs are somewhat different, the sampling requirements also vary. TOGA requirements are primarily based on a series of statistical studies in which the dominant time and space scales of upper ocean thermal variability were defined (White and Bernstein (1979), White et al. (1982), Sprintall and Meyers (1991), Festa and Molinari (1992), for example). To meet WOCE requirements, two types of sampling strategies have been developed. Low-density sampling is used to map upper ocean heat storage fields. Along a few of the transects shown in Fig. 2, high-density sampling is used to estimate oceanic heat flux and geostrophic currents.
The low-density sampling requirement is based on the statistical studies. A UOT profile is required every two months on a 1.5° of latitude by 7° of longitude grid. WOCE spatial requirements outside the tropics are similar, i.e., 4 probes per day on a 15 knot ship gives 1° of latitude resolution. However, WOCE is attempting to achieve monthly resolution of the upper ocean heat content distribution. TOGA has also established "frequently repeated transequatorial sections", transects that are occupied approximately 18 times per year at the 1° spatial resolution (Meyers et al., 1994). Finally, WOCE includes high density sampling on selected lines. Seasonal sampling is recommended with 50 km spacing, higher along boundaries and in high gradient regions, to resolve changes in the large-scale circulation and transport (Roemmich and Cornuelle, 1990). Until the network evaluations described above are completed we will adopt the recommendation of the Ocean Observing System Development Panel (OOSDP, 1994) and "follow TOGA for the tropical region and WOCE for the subtropics and mid-latitudes" in continuing sampling along established ONB transects.
Figure 9 displays the level of NOAA's contribution to the international effort. Of the total 37,440 XBT profiles received in 1992, NOAA VOS XBT Program contributed 15,288 upper-ocean temperature profiles and 5,912 probes to other countries in support for the TOGA XBT Network. This represents 57% of all XBT observations reported in 1992. NOS and AOML have coordinated NOAA's XBT contribution to this international effort through the TOGA/WOCE XBT/xCTD Programme Planning Committee (TWXXPPC). UOTC plans to continue this cooperation through this committee or its successor.
The existing NOAA VOS network currently supported is identified in Table 1. The corresponding track lines are shown in Fig. 2. Continued support for the NOAA XBT network is proposed. However, it is recognized that sampling will be modified as new sampling requirements are identified.
ONB and AOML personnel have worked closely in the past to provide the ship-greeting and other operational functions needed to insure a healthy network. Early on in the program it became obvious that neither group alone possessed adequate resources to sustain a global network. Only by working together could the necessary tasks be accomplished. This coordination will be enhanced within the UOTC.
WOCE has specified AX-7 in the subtropical North Atlantic as an important candidate for a high density line. AOML will implement this line. Unfortunately, because of the loss of one ship by sinking and a change in trackline of another, the first occupation of this line did not occur until January, 1995. The line will be occupied at two times per year (half the WOCE requirements) to estimate changes in the large-scale structure of the subtropical gyre and meridional heat fluxes.