Presenter: Darren McKee (University of Virginia)

Description:
While Eulerian chlorophyll time series alias a variance component from advective and mixing divergence, in principle, a Lagrangian framework isolates net biological accumulation from physical transport. Because Bio-Argo floats primarily drift with currents at depth, they are neither Lagrangian nor Eulerian with respect to the surface flow. Here we study how Bio-Argo floats sample evolving mesoscale chlorophyll fields in a region of the North Atlantic ocean by comparing time series of float velocity and mixed-layer-mean chlorophyll (from fluorescence) to Eulerian (satellite altimetry and ocean color) and Lagrangian (surface drifter velocity and ocean color projected onto drifter trajectory) time series. We compute decorrelation times from space-composited autocorrelation functions and quantify horizontal float motion as the mean distance between float and surface geostrophic trajectories over the time spanned by three consecutive profiles (Quasi-Planktonic Index; QPI). Ratios of Lagrangian to Eulerian velocity timescales are quantitatively consistent with drifter motion in the "frozen field" regime of turbulence (advective term dominates material derivative) and float motion in the “fixed-float” regime (tendency term dominates). However, when QPI is small (i.e., float and surface geostrophic trajectories are similar), float behavior approaches drifter behavior as velocity variance increases and Lagrangian timescales decrease. Chlorophyll anomalies about a seasonal cycle have Eulerian scales similar to those of velocity, suggesting stirring helps set geostatistics of biological properties. As such, ratios of Lagrangian to Eulerian chlorophyll timescales depend on observer speed relative to an evolution speed of the chlorophyll fields. By lagging surface chlorophyll patches, floats underestimate the Lagrangian tendency and advective terms, and the Eulerian tendency primarily sets timescales. As QPI increases nonlinearly with profiling interval, rapid profiling can generate time series of biological accumulation less contaminated by transport.