Lagrangian Water Age trajectories initiated from the coastal 500m isobath and derived from surface velocities obtained from satellite observations

We conduct a Lagrangian particle trajectory analysis of surface velocities. We
define an “offshore water age” as the time taken by a water parcel to be advected
backward in time from its current position along its trajectory until it crosses the
500 m isobath. The rationale of this diagnostic is to detect filaments of coastal
water advected offshore by horizontal transport and to estimate the time for water
parcels in the filament o leave the coastal area. For example, a value of “20 days”
assigned to a pixel means that the water parcel in that area was in the coastal area
approximately 20 days before, where it was likely enriched in nutrients.