Dispersion of passive and active tracers in the upper ocean

This dissertation consists of two separate investigations of tracer dispersion in the ocean. In the first part, I study the horizontal processes that redistribute heat and salt in the mixed layer. In the second part, I examine the effect of jets and vortices on the dispersion of passive tracers in geophysical flows.;Simple mixed layer models suggest that horizontal fluxes of heat and salt depend nonlinearly on density gradients, e.g. diffusivities are proportional to some power of the density gradient. In chapter 2, I examine the consequences of these nonlinear-diffusion parameterizations by solving an initial value problem in which the t = 0 thermohaline fields are prepared as random and uncorrelated distributions of temperature and salinity. Solutions of the nonlinear diffusion equation as a 'rundown' problem show that correlations develop between the horizontal temperature and salinity gradients. These correlations are such that the evolving thermohaline gradients tend to be strongly compensating in their joint effect on density. In chapter 3, I use high-resolution measurements, collected in the Subtropical North Pacific during the Spice experiment, to show that mixed-layer temperature and salinity gradients tend to compensate on horizontal scales of 20 meters to 10 kilometers. In the thermocline, temperature and salinity gradients tend to counteract in their joint effect on density but they do not compensate.;Dispersion of passive tracers in geophysical flows characterized by long lived coherent structures is the topic of chapter 4. I show that the growth rate of various moments of particle displacements is a useful diagnostic to characterize dispersion in complex flows. An obvious application of this result is to study dispersion of neutrally buoyant floats in the ocean.