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Leah Johnson

Senior Oceanographer





Department Affiliation

Ocean Physics


2000-present and while at APL-UW

Assessment of oceanographic conditions during the North Atlantic EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign

Johnson, L. and 17 others including C.M. Lee, "Assessment of oceanographic conditions during the North Atlantic EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign," Prog. Oceanogr., 220, doi:10.1016/j.pocean.2023.103170, 2024

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1 Jan 2024

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 to 8 days, with surface exchange fractions ranging from 20 to 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 ECW tracers depended on the horizontal and vertical gradients of that tracer. For example, horizontal advection played a large role in ECW salinity fluxes, while vertical entrainment played a larger role in the fluxes of nutrients into SCW 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, ECW 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 while illustrating 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.

A finite-time ensemble method for mixed layer model comparison

Johnson, L., B. Fox-Kemper, Q. Li, H.T. Pham, and S. Sarkar, "A finite-time ensemble method for mixed layer model comparison," J. Phys. Oceanogr., 53, 2211-2230, doi:10.1175/JPO-D-22-0107.1, 2023.

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1 Sep 2023

This work evaluates the fidelity of various upper-ocean turbulence parameterizations subject to realistic monsoon forcing and presents a finite-time ensemble vector (EV) method to better manage the design and numerical principles of these parameterizations. The EV method emphasizes the dynamics of a turbulence closure multimodel ensemble and is applied to evaluate 10 different ocean surface boundary layer (OSBL) parameterizations within a single-column (SC) model against two boundary layer large-eddy simulations (LES). Both LES include realistic surface forcing, but one includes wind-driven shear turbulence only, while the other includes additional Stokes forcing through the wave-average equations that generate Langmuir turbulence. The finite-time EV framework focuses on what constitutes the local behavior of the mixed layer dynamical system and isolates the forcing and ocean state conditions where turbulence parameterizations most disagree. Identifying disagreement provides the potential to evaluate SC models comparatively against the LES. Observations collected during the 2018 monsoon onset in the Bay of Bengal provide a case study to evaluate models under realistic and variable forcing conditions. The case study results highlight two regimes where models disagree 1) during wind-driven deepening of the mixed layer and 2) under strong diurnal forcing.

Eddy tracking from in situ and satellite observations

Erickson, Z.K., E. Fields, L. Johnson, A.F. Thompson, L.A. Dove, E. D'Asaro, and D.A. Siegel, "Eddy tracking from in situ and satellite observations," J. Geophys. Res., 128, doi:10.1029/2023JC019701, 2023.

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1 Aug 2023

Mesoscale eddies are a dominant source of spatial variability in the surface ocean and play a major role in the biological marine carbon cycle. Satellite altimetry is often used to locate and track eddies, but this approach is rarely validated against in situ observations. Here we compare measurements of a small (under 25 km radius) mode water anticyclonic eddy over the Porcupine Abyssal Plain in the northeastern Atlantic Ocean using CTD and Acoustic Doppler Current Profiler (ADCP) measurements from three ships, two gliders, two profiling floats, and one Lagrangian float with those derived from sea level anomaly (SLA). In situ estimates of the eddy center were estimated from maps of the thickness of its central isopycnal layer, from ADCP velocities at a reference depth, and from the trajectory of the Lagrangian float. These were compared to three methods using altimetric SLA: one based on maximizing geostrophic rotation, one based on a constant SLA contour, and one which maximizes geostrophic velocity speed along the eddy boundary. All algorithms were used to select CTD profiles that were within the eddy. The in-situ metrics agreed to 97%. The altimetry metrics showed only a small loss of accuracy, giving >90% agreement with the in situ results. This suggests that current satellite altimetry is adequate for understanding the spatial representation of even relatively small mesoscale eddies.

More Publications

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center