Mixing in the North Atlantic Tracer Release Experiment : observations and numerical simulations of Lagrangian particles and passive tracer

Abstract

Mixing and stirring of passive tracer and Lagrangian particles in the open ocean was studied through comparison of observations from the North Atlantic Tracer Release Experiment, a numerical model, and existing theory. Based on the observed distribution of tracer during the first six months of the NATRE field experiment, Ledwell et al. (1993) estimated that on scales of 1 to 10 km small-scale diffusivity κs ≈ 3 m2s-1 and rms strain rate γ ≈ 3 X 10-7 s-1 . From the observed tracer distribution after one year, Ledwell (personal communication) further estimated that on scales greater than the mesoscale the effective eddy diffusivity κe ≈ 1 x 103 m2s-1. In the present study, statistics of the NATRE float data and numerical simulations of Lagrangian particles and passive tracer were used to determine the biases and uncertainties associated with these estimates. The numerical model was calibrated so that the statistics of model floats agreed as closely as possible with the NATRE floats. It is found that observations of a passive tracer such as were made during the NATRE experiment may be used to determine the rms streak width, δs, and the rms strain rate and hence to estimate the effective small-scale diffusivity. However, caution must be exercised when estimating κs from the theoretical balance, δs = square root κs/γ, as this may introduce a bias which would lead to the over-estimation of κs. Of particular relevance to NATRE is that observations of δs may be biased toward larger estimated rms streak width due to the inability of the observer to distinguish individual streaks from those which have resulted from a recent merger of streaks. Numerical experiments show that such a bias could lead to the over-estimation of κs by up to a factor of 2 to 4, suggesting that the estimate of κs made by Ledwell et al., (1993) from the NATRE tracer observations has an associated uncertainty of similar magnitude. Analysis of NATRE float data indicates that the estimate κe ≈ 1 x 103 m2s-1 inferred from the tracer distribution in Spring, 1993 and Fall, 1994 is accurate to within a factor of 2.