Estimation of the particle flux from the convective mixed layer by large eddy simulation

With an aim to estimate how much phytoplankton population remains within the mixed layer during winter and acts as seeds for blooming in the following spring, the downward particle flux from the convective mixed layer is investigated by analyzing the motion of a large number of Lagrangian particles in the ocean mixed layer simulated by large eddy simulation. It is found that W/w_s decreases gradually from 1 to 0 with wd/w_s, where W is the sedimentation rate, defined as the ratio of downward particle flux to the mean particle concentration within the mixed layer; w_s is the terminal velocity of a particle settling in still fluid; and wd is the deepening velocity of the mixed layer depth (MLD). It is in contrast with the previous hypothesis that W = w_s for wd < w_s and W = 0 otherwise. The new parameterization W/ws = exp[-1.4(wd/ws)] leads to much higher particle concentration after winter than the previous estimation, when wd > w _s. The present result suggests that the escape of particles from the mixed layer occurs stochastically, and its probability is determined by turbulence at the MLD and by the mixed layer deepening, both of which are represented by wd/w_s.