Mason Evan (Mediterranean Institute for Advanced Studies, IMEDEA (CSIC-UIB), Spain); Capet Arthur (CNR, OGS, Trieste, Italy); Pascual Ananda (Mediterranean Institute for Advanced Studies, IMEDEA (CSIC-UIB), Spain)

An innovative approach is used to analyse the impact of vertical velocities associated with quasi-geostrophic (QG) dynamics on the redistribution and uptake of nitrate in the southeast Pacific (SEP). Twelve years of vertical and horizontal currents are derived from an observation-based estimate of the ocean state. Horizontal velocities are obtained through application of thermal wind balance to weekly temperature and salinity fields. Vertical velocities are estimated by integration of the QG Omega equation. Seasonal variability of the synthetic vertical velocity and kinetic energy associated with the horizontal currents is coincident, with peaks in austral summer (November-December) in accord with published observations. The impact of vertical velocity on SEP nitrate uptake rates is assessed by using two Lagrangian particle tracking experiments that differ according to vertical forcing (w=wqg versus w=0). From identical initial distributions of nitrate-tagged particles, the Lagrangian results show that vertical motions are responsible for local increases in nitrate uptake of up to 30%. Despite being weaker than horizontal currents by a factor of up to 10^-4, vertical velocity is demonstrated to make an important contribution to nutrient distributions in low uptake regions with high mesoscale activity.