Assessment of Oceanographic Conditions during the North Atlantic EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) Field Campaign

Abstract

This manuscript presents an overview of NASA’s EXport Processes in the Ocean from Remote Sensing 2021 Field Campaign in the North Atlantic (EXPORTS NA) and provides quantitative and dynamical descriptions of the physical processes modulating water transformations during the study. A major programmatic goal was to conduct the sampling in a Lagrangian mode so that ocean ecological and biogeochemical changes can be observed independent from physical advective processes. To accomplish this goal, EXPORTS NA conducted a multi-ship, multi-asset field sampling program within a retentive, anticyclonic mode water eddy. Beneath depths of ~100 m, Lagrangian sampling assets remained within the eddy core waters (ECWs) throughout the experiment, demonstrating that the ECWs within the mode water eddy were retentive. However, strong westerly winds from four storm events deepened the mixed layer (ML) of the surface core waters (SCWs) above the eddy’s mode water core by 25-40 m and exchanged some of the SCWs with surface waters outside of the eddy via Ekman transport. Estimates of flushing times ranged from 5-8 days, with surface exchange fractions ranging from 20-75% and were consistent with particle tracking advected by combined geostrophic and Ekman velocities. The relative contributions of horizontal and vertical advection on changes in SCW tracers depend on the horizontal and vertical gradients of that tracer. For example, in the surface waters, horizontal advection played a large role in salinity fluxes, yet vertical entrainment played a larger role in the fluxes of nutrients into the ML. Each storm injected nutrients and low oxygen waters into the ML, after which the surface ocean ecosystem responded by reducing nutrient concentrations and increasing %O2 saturation levels. Overall, SCW values of chlorophyll and POC were the largest at the onset of the field program and decreased throughout the campaign. The analysis presented provides a physical oceanographic context for the many measurements made during the EXPORTS NA field campaign. Illustrated are the many challenges of conducting a production-flux experiment even in a Lagrangian frame and the inherent uncertainties of interpreting biological carbon pump observations that were collected in a Eulerian frame of reference.

Eddy Tracking From In Situ and Satellite Observations

“…While not directly measured here, this effect could be important for experiments such as EXPORTS‐NA that seek to understand connections between the surface and interior ocean. For this eddy, wind stress from periodic storm events played a major role in setting the density structure and horizontal velocity field of the eddy near the surface, and Lagrangian particles advected along streamlines set by geostrophic and Ekman effects trace significantly different paths than those advected only geostrophically around the eddy (Johnson et al., 2023).…”

“…This second possibility is supported by steric height data calculated from in situ profiles (not shown here), which also suggest a region to the west of the eddy with high SLA. This area to the west of the eddy core has also been shown to have anomalous surface water properties, possibly indicating injection of different waters into the eddy (Johnson et al., 2023). In contrast, the incorrect predictions on the northeast side of the eddy from the Γ 2 method were due to the apparent merging (only shown in this metric) of this feature with another eddy to the north.…”

“…Since the EXPORTS‐NA deployment was primarily concerned with the biological pump, many of the ship‐based profiles did not extend to this depth, and were therefore not used here (Table 1). Part of the challenge is that surface water characteristics were modified during the course of the deployment by several large storms that entered the region (see Johnson et al., 2023), meaning profiles only near the surface, or underway data from the ships, were less useful in determining the eddy extent.…”

“…The goal of EXPORTS‐NA was to survey biological properties during the demise of the North Atlantic spring bloom and to assess the major export flux pathways connecting the upper layers with the ocean interior (Siegel et al., 2016). The PAP region has high horizontal variability, with significant variation in water properties on small (<10 km) spatial scales (Damerell et al., 2016; Erickson et al., 2020; Johnson et al., 2023). Tracking of sinking particulates in a model suggests that particles at 3,000 m depth are sourced from up to 140 km away, with high interannual variability driven by changing currents and mesoscale variability (Frigstad et al., 2015).…”

“…Field observations of biological processes over time benefit from the knowledge of which measurements are associated with waters retained within the core of the eddy, as opposed to those in an environment exposed to injection of other water masses and, potentially, outside fluxes of nutrients or biomass (Johnson et al., 2023). This paper introduces different metrics for in situ data to estimate whether a given measurement is taken from within or outside of the physically retentive core of the eddy.…”