Each spring, the North Atlantic experiences one of the largest open-ocean phytoplankton blooms in the global ocean. Diatoms often dominate the initial phase of the bloom with succession driven by exhaustion of silicic acid. The North Atlantic was sampled over 3.5 weeks in spring 2021 following the demise of the main diatom bloom, allowing mechanisms that sustain continued diatom contributions to be examined. Diatom biomass was initially relatively high with biogenic silica concentrations up to 2.25 μmol Si L^-1. A low initial silicic acid concentration of 0.1–0.3 μM imposed severe Si limitation of silica production and likely limited the diatom growth rate. Four storms over the next 3.5 weeks entrained silicic acid into the mixed layer, relieving growth limitation, but uptake limitation persisted. Silica production was modest and dominated by the >5.0 μm size fraction although specific rates were highest in the 0.6–5.0 μm size fraction over most of the cruise. Silica dissolution averaged 68% of silica production. The resupply of silicic acid via storm entrainment and silica dissolution supported a cumulative post-bloom silica production that was 32% of that estimated during the main bloom event. Diatoms contributed significantly to new and to primary production after the initial bloom, possibly dominating both. Diatom contribution to organic-carbon export was also significant at 40%μ70%. Thus, diatoms can significantly contribute to regional biogeochemistry following initial silicic acid depletion, but that contribution relies on physical processes that resupply the nutrient to surface waters.

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.

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.

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.

Sexual harassment is a pervasive problem on oceanographic research vessels and while conducting fieldwork in general. A variety of factors contribute to inadequate protection against sexual harassment, such as poor training in prevention, support, and response; remoteness of field sites; academic hierarchies that reinforce uneven power dynamics that extend to fieldwork; and multi-institutional teams with distinct policies or reporting structures that can lead to confusion in reporting and responding to incidents in the field. In compromising individuals' physical and mental health, sexual harassment can negatively affect research expeditions. For example, harassed individuals may decide to refrain from working on complicated team-based tasks, which can be a safety issue. A broader concern is that sexual harassment deters talented people from pursing or maintaining employment in ocean science. Harassment must be treated with the same gravity as research misconduct and safety policy infringements. When planning a research expedition, science team leaders are responsible for the safety of their team and other colleagues aboard and would benefit from resources aimed at helping team leadership create a plan to ensure safety and inclusivity. To address this resource gap, 18 participants in the Workshop to Promote Field Safety in Ocean Sciences, convened by the Consortium for Ocean Leadership and held May 17–18, 2022, in Washington, DC, developed a checklist for use by scientific leaders and others to assist in planning for participant safety and to prevent harassment the field. The checklist specifies the timing of, and who is responsible for, specific actions that should be taken to improve safety while conducting fieldwork, whether on a research vessel or on land. It also provides additional resources and suggestions for leaders on how to amend the checklist to address their specific fieldwork situations.

Current submesoscale restratification parameterizations, which help set mixed layer depth in global climate models, depend on a simplistic scaling of frontal width shown to be unreliable in several circumstances. Observations and theory indicate that frontogenesis is common, but stable frontal widths arise in the presence of turbulence and instabilities that participate in keeping fronts at the scale observed, the arrested scale. Here we propose a new scaling law for arrested frontal width as a function of turbulent fluxes via the turbulent thermal wind (TTW) balance. A variety of large-eddy simulations (LES) of strain-induced fronts and TTW-induced filaments are used to evaluate this scaling. Frontal width given by boundary layer parameters drawn from observations in the General Ocean Turbulence Model (GOTM) are found qualitatively consistent with the observed range in regions of active submesoscales. The new arrested front scaling is used to modify the mixed layer eddy restratification parameterization commonly used in coarse-resolution climate models. Results in CESM-POP2 reveal the climate model's sensitivity to the parameterization update and changes in model biases. A comprehensive multimodel study is in planning for further testing.

The NORSE DRI focuses on characterizing the key physical parameters and processes that govern the predictability of upper-ocean rapid evolution events occurring in the ice-free high latitudes. The goal is to identify which observable parameters are most influential in improving model predictability through inclusion by assimilation, and to field an autonomous observing network that optimizes sampling of high-priority fields. The overall goal is to demonstrate improvements in the predictability of the upper ocean physical fields associated with acoustic propagation over the course of the study. This Science Plan describes the specific objectives and implementation plan.

Submesoscale frontal dynamics are thought to be of leading-order importance for stratifying the upper ocean by slumping horizontal density gradients to produce vertical stratification. Presented here is an investigation of submesoscale instabilities in the mixed layer—mixed layer eddies (MLEs)—as a potential mechanism of frontal slumping that stratifies the upper ocean during the transition from winter to spring, when wintertime forcings weaken but prior to the onset of net solar warming. Observations from the global Argo float program are compared to predictions from a one-dimensional mixed layer model to assess where in the world’s oceans lateral processes influence mixed layer evolution. The model underestimates spring stratification for ~75% ± 25% of the world’s oceans. Relationships between vertical and horizontal temperature and salinity gradients are used to suggest that in 30% ± 20% of the oceans this excess stratification can be attributed to the slumping of horizontal density fronts. Finally, 60% ± 10% of the frontal enhanced stratification is consistent with MLE theory, suggesting that MLEs may be responsible for enhanced stratification in 25% ± 15% of the world’s oceans. Enhanced stratification from frontal tilting occurs in regions of strong horizontal density gradients (e.g., midlatitude subtropical gyres), with a small fraction occurring in regions of deep mixed layers (e.g., high latitudes). Stratification driven by MLEs appears to constrain the coexistence of sharp lateral gradients and deep wintertime mixed layers, limiting mixed layer depths in regions of large lateral density gradients, with an estimated wintertime restratification flux of order 100 W m^−2.