A pseudo-Lagrangian method for remapping ocean biogeochemical tracer data: Calculation of net Chl-a growth rates

Abstract

A key goal in understanding the ocean's biogeochemical state is estimation of rates of change of critical tracers, particularly components of the planktonic ecosystem. Unfortunately, because ship survey data are not synoptic, it is difficult to obtain spatially resolved estimates of the rates of change of tracers sampled in a moving fluid. Here we present a pseudo‐Lagrangian transformation to remap data from underway surveys to a pseudo‐synoptic view. The method utilizes geostrophic velocities to back advect and relocate sampling positions, removing advection aliasing. This algorithm produces a map of true relative sampling locations, and allows for determination of the relative locations of observations acquired along streamlines, as well as a corrected view of the tracer's spatial gradients. We then use a forward advection scheme to estimate the tracer's relative change along streamlines, and use these to calculate spatially resolved, net specific rates of change. Application of this technique to Chlorophyll‐a (Chl‐a) fluorescence data around an ocean front is presented. We obtain 156 individual estimates of Chl‐a fluorescence net specific rate of change, covering ∼1200 km2. After incorporating a diffusion‐like model to estimate error, the method shows that the majority of observations (64%) were significantly negative. This pseudo‐Lagrangian approach generates more accurate spatial maps than raw survey data, and allows spatially resolved estimates of net rates of tracer change. Such estimates can be used as a rate budget constraint that, in conjunction with standard rate measurements, will better determine biogeochemical fluxes.

The role of submesoscale currents in structuring marine ecosystems

“…It follows that it is difficult to determine whether a submesoscale phytoplankton patch formed as an active response to submesoscale flow or is simply the result of stirring of a larger patch that formed in response to larger-scale processes. An improved understanding of submesoscale processes will thus rely on our ability to reconstruct the Lagrangian histories of submesoscale patches from observations that are often Eulerian 62 , 119 , 121 – 124 . Real-time estimates of stirring from satellite altimetry can be useful for guiding adaptive sampling strategies during ship-based field campaigns to observe the evolution of tracer fields 125 .…”

Investigating the Nutrient Landscape in a Coastal Upwelling Region and Its Relationship to the Biological Carbon Pump

“…On the P1408 (August 2014) and P1604 (April 2016) cruises, our goal was to investigate system responses to anomalous warming associated with the 2014-2015 North Pacific warm anomaly and ensuing El Niño using similar sampling patterns (Kelly et al, 2018;Morrow et al, 2018;Nickels & Ohman, 2018). On the P1106 (June 2011) and P1208 (August 2012) cruises, we sought to determine ecosystem responses to mesoscale fronts and hence conducted cycles within and to either side of strong frontal gradients (de Verneil & Franks, 2015;Krause et al, 2015;Stukel et al, 2017). On the P1706 (June 2017) cruise, we investigated cross-shore transport mediated by a coastal filament, and hence cycles from this cruise represent a progression from coastal upwelling conditions to bloom decay as water is advected offshore.…”

Phytoplankton Patches at Oceanic Fronts Are Linked to Coastal Upwelling Pulses: Observations and Implications in the California Current System

“…Chronologically, over the month-long study, sampling included SeaSoar Survey 1, a Moving Vessel Profiler survey, two cross-frontal transects separated by five Lagrangian "cycles", and SeaSoar Survey 2. Other studies of this frontal system interpreted the decreasing phytoplankton biomass along the front as driven by an in situ biological sink (de Verneil & Franks, 2015) or vertical export along the front (Stukel et al, 2017). These studies assumed that the productive waters at the front either had a "similar source" (de Verneil or were geographically stationary and at steady state between the various sampling campaigns conducted (Stukel et al, 2017).…”