Simulation of Distribution of Tritium in the Global Ocean General Circulation Model

Tritium (³H) is often used as an important passive tracer to study physical processes in the ocean and to assess the performance of the ocean general circulation model (OGCM). A global OGCM (LICOM) is employed to study the distribution, storage, and transport of tritium in the ocean. Simulated results for the global tritium flux show that before 1975, tritium entered the ocean mainly from the atmosphere via vapor exchange, especially in 1963, when the tritium input by vapor exchange was 2.5 times larger than that by precipitation. However, after 1975 these two types of inputs decreased quickly and became quite close to each other. Compared to the observations obtained during GEOSECS (Geochemical Ocean Sections Study, 1972-1978) and WOCE (World Ocean Circulation Experiment, 1989-1995), our model reproduces well the observed features of the sea surface distribution, water column inventory, meridional gradient, and local maximum values of tritium. The main weakness of our model is that the simulated penetration depth of tritium is too small, especially in the two subtropical regions. Uncertainties are believed to exist in the development of the tritium input function, and the description of the physical processes, which probably influences the simulated results, needs to be improved in the OGCM. The total tritium inventory estimated by our model is close to the data-based estimate. For example, our model estimates that the North Pacific contained 20.4 kg and 20.7 kg of tritium during 1973-1974 and 1989-1995, respectively, which is in good agreement with the data-based estimates of 21.1±4.7 kg and 23.4±2.0 kg. The lateral ventilation of tritium from the subarctic to tropical regions is obvious along isopycnal surfaces. Our model successfully simulates the entry of tritium into the ocean from the high-latitude sea surface and its transport along isopycnals to the subsurface layer in the low-latitude region and then to the Southern Hemisphere and high-latitude regions by circulation and diffusion.