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An Adjoint Data Assimilation Approach to the Estimation of
*Pseudocalanus* Population Dynamics

Dennis McGillicuddy, Daniel Lynch, Peter Wiebe,
Wendy Gentleman and Cabell Davis

Our underlying scientific objective here is to determine the
mechanisms that control variations in the seasonal abundance
patterns of *Pseudocalanus*. We hypothesize that the observed
distributions result from the interaction of the population dynamics
with the physical circulation. This is expressed in the two
dimensional advection-diffusion-reaction equation for a scalar
variable C which represents the concentration of *Pseudocalanus*.
Given initial conditions C0 at time t0, we seek the population
dynamics source term R(x,y) such that integration of the forward
model equation will result in predictions which minimize the sum of
squares of differences between modeled and observed concentrations.
An adjoint data assimilation technique has been designed for these
purposes.
This approach has been used to invert for the population dynamics
between the January-February and March-April climatological distributions of
*Pseudocalanus* derived from the MARMAP data. Climatological flow
fields for this time period are specified. The solution converges
rapidly in the first ten iterations, reaching a minimum in the cost
function by approximately the 25th iteration. The general
characteristics of the source term include strong growth on the
crest of the bank, and high mortality in two areas: (1) a strip
just off the northwest flank of the bank extending from the northern
Great South Channel almost to Georges Basin, and (2) Wilkinson
Basin.