Ocean tracer distributions: Part I. A preliminary numerical experiment

Abstract

A numerical experiment has been carried out to determine the three-dimensional distributions of dissolved oxygen and radiocarbon as well as the fields of temperature, salinity, and velocity in an ocean driven by wind and thermohaline processes. The aim in this preliminary calculation was to test the usefulness of a tracer model for studying the origin of water masses in the sea and to provide a framework for looking further into the nature of the sources, age, and circulation of the abyssal ocean. Much of our knowledge about circulation patterns and mixing in the sea has come from examining the distribution of properties in simplified one and two dimensional models. In this investigation tracer studies are extended to three dimensions using a numerical model of the ocean circulation of the kind described by Bryan (1969). With a given set of parameters and boundary conditions governing the model ocean, a calculation is made to predict the distributions of four “tracers”, temperature, salinity, dissolved oxygen, and radiocarbon, as well as the structure of the circulation. Based upon this first experiment some important conclusions can be made regarding those features necessary in any model to satisfactorily account for observed distributions. Part I describes the physical model and its numerical analogue as well as the numerical techniques used in finding a steady state. The results of an experiment with tracer eddy diffusivities K = 1 cm²/sec (vertical) and A_H = 5times10⁷ cm²/sec (horizontal) are examined in some detail to understand the time scales involved for the deep sea as well as the shallower regions. The patterns of flow and the distributions of the four tracers are related to each other by determining the roles of diffusion, advection, and decay processes in the model. Comparisons are made between observed and predicted surface fluxes of heat and evaporation minus precipitation in order to ascertain the effect of the boundary conditions on the nature of the distributions. These results suggest the direction further studies should take in constructing realistic models of the ocean. Calculations with smaller diffusivities and other boundary conditions are underway and will be reported in Part II. DOI: 10.1111/j.2153-3490.1971.tb00583.x