Remote Sensing of Diatom Bloom Succession

Marine diatoms are major biogeochemical and ecological influencers that contribute to a large fraction of the carbon export and supplying fisheries (Falkowski 2015). The fluxes of carbon transfer to the food web or to the deep ocean vary according to the stage of a diatom bloom (Du Toit 2018). Stages can be determined using inherent optical properties that reflect their physiological state, such as the chlorophyll fluorescence to particulate backscattering ratio (ChlF/b(sub bp), Cetinic et al. 2015). Identifying the bloom stage can potentially improve biogeochemical models of carbon export and fishery management. However, it is not yet possible to adequately determine the stage of phytoplankton blooms using satellites. Satellite-derived remote sensing reflectance R(sub rs)() allow for remote identification of diatom blooms in the open ocean (Sathyendranath et al. 2004), and there are techniques to estimate the fluorescence quantum yield () that, when high, can indicate the nutrient limitation that often takes place when blooms start to senesce (Behrenfeld et al. 2009). The goal of this study is to use the ratio between the normalized fluorescence line height from R(sub rs)() (nFLH) and the particulate backscattering (b(sub bp)(443)) provided by satellites to identify exponentially growing and senescent diatom blooms from space

Ocean convergence and the dispersion of flotsam

Floating oil, plastics, and marine organisms are continually redistributed by ocean surface currents. Prediction of their resulting distribution on the surface is a fundamental, long-standing, and practically important problem. The dominant paradigm is dispersion within the dynamical context of a nondivergent flow: objects initially close together will on average spread apart but the area of surface patches of material does not change. Although this paradigm is likely valid at mesoscales, larger than 100 km in horizontal scale, recent theoretical studies of submesoscales (less than ∼10 km) predict strong surface convergences and downwelling associated with horizontal density fronts and cyclonic vortices. Here we show that such structures can dramatically concentrate floating material. More than half of an array of ∼200 surface drifters covering ∼20 × 20 km2 converged into a 60 × 60 m region within a week, a factor of more than 105 decrease in area, before slowly dispersing. As predicted, the convergence occurred at density fronts and with cyclonic vorticity. A zipperlike structure may play an important role. Cyclonic vorticity and vertical velocity reached 0.001 s−1 and 0.01 ms−1, respectively, which is much larger than usually inferred. This suggests a paradigm in which nearby objects form submesoscale clusters, and these clusters then spread apart. Together, these effects set both the overall extent and the finescale texture of a patch of floating material. Material concentrated at submesoscale convergences can create unique communities of organisms, amplify impacts of toxic material, and create opportunities to more efficiently recover such material

Inertial Oscillations and Frontal Processes in an Alboran Sea Jet: Effects on Divergence and Vertical Transport

Vertical transport pathways in the ocean are still only partially understood despite their importance for biogeochemical, pollutant, and climate applications. Detailed measurements of a submesoscale frontal jet in the Alboran Sea (Mediterranean Sea) during a period of highly variable winds were made using cross-frontal velocity, density sections and dense arrays of surface drifters deployed across the front. The measurements show divergences as large as ±f implying vertical velocities of order 100 m/day for a ≈ 20 m thick surface layer. Over the 20 hr of measurement, the divergences made nearly one complete oscillation, suggesting an important role for near-inertial oscillations. A wind-forced slab model modified by the observed background frontal structure and with initial conditions matched to the data produces divergence oscillations and pattern compatible with that observed. Significant differences, though, are found in terms of mean divergence, with the data showing a prevalence of negative, convergent values. Despite the limitations in data sampling and model uncertainties, this suggests the contribution of other dynamical processes. Turbulent boundary layer processes are discussed, as a contributor to enhance the observed convergent phase. Water mass properties suggest that symmetric instabilities might also be present but do not play a crucial role, while downward stirring along displaced isopycnals is observed.This work has been supported and co-financed by the CALYPSO project, within the Office of Naval Research Departmental Research Initiative, under the following grants: N00014-18-1-2782 and N00014-22-1-2039 (GE,SD,MB,AG), N00014-18-1-2139 (AYS,EAD), N00014-18-1-2138 (TO), N00014-18-1-2418 and N00014-20-1-2754 (PMP), N00014-19-1-2692 and N00014-19-1-2380 (LC and part of the drifter data collection/analysis), N00014-18-1-2431 (JTF), N00014-18-1-2416 (TMSJ), N00014-16-1-3130 (AP,DRT,SR), N00014-21-1-2702 (AM). MF was supported by the Scripps Institutional Postdoctoral Fellowship (MAF). Investigation of front dynamics in the Mediterranean Sea from multiplatform observations is supported as well by the European Union's JERICO-S3 project through Grant 871153. Open Access Funding provided by Consiglio Nazionale delle Ricerche within the CRUI-CARE Agreement.Peer reviewe

Evidence of Langmuir Mixing Effects in the Upper Ocean Layer During Tropical Cyclones Using Observations and a Coupled Wave-Ocean Model

Mixing of the ocean beneath tropical cyclones (TC) cools the surface temperature thereby modifying the storm intensity. Modeling studies predict that surface wave forcing through Langmuir turbulence (LT) increases the mixing and cooling and decreases near-surface vertical velocity shear. However, there are very few quantitative observational validations of these model predictions, and the validation efforts are often limited by uncertainties in the drag coefficient (Cd). We combine EM-APEX and Lagrangian float measurements of temperature, salinity, velocity, and vertical turbulent kinetic energy (VKE) from five TCs with a coupled ocean-wave model (Modular Ocean Model 6—WAVEWATCH III) forced by the drag coefficient Cd directly constrained for these storms. On the right-hand of the storms in the northern hemisphere, where wind and waves are nearly aligned, the measured VKE is consistent with predictions of models including LT and 2–3 times higher than predictions without LT. Similarly, vertical shear in the upper 20 m is small, consistent with predictions of LT models and inconsistent with the large shears predicted by models without LT. On the left-hand of the storms, where wind and waves are misaligned, the observed VKE and cooling are reduced compared to those on the right-hand, consistent with the measured decrease in Cd. These results confirm the importance of surface waves for ocean cooling and thus TC intensity, through both Cd and LT effects. However, the model predictions, even with the LT parameterization, underestimate the upper ocean cooling and mixed layer deepening by 20%–30%, suggesting possible deficiency of the existing LT parameterization

Drag Coefficient and Its Sea State Dependence under Tropical Cyclones

The drag coefficient under tropical cyclones and its dependence on sea states are investigated by combining upper-ocean current observations [using electromagnetic autonomous profiling explorer (EM-APEX) floats deployed under five tropical cyclones] and a coupled ocean–wave (Modular Ocean Model 6–WAVEWATCH III) model. The estimated drag coefficient averaged over all storms is around 2–3 3 1023 forwindspeedsof25–55 m s21.Whilethedragcoefficient weakly depends on wind speed in this wind speed range, it shows stronger dependence on sea states. In particular, it is signif-icantly reduced when the misalignment angle between the dominant wave direction and the wind direction exceeds about 458, a feature that is underestimated by current models of sea state–dependent drag coefficient. Since the misaligned swell is more common in the far front and in the left-front quadrant of the storm (in the Northern Hemisphere), the drag coefficient also tends to be lower in these areas and shows a distinct spatial distribution. Our results therefore support ongoing efforts to develop and implement sea state–dependent parameterizations of the drag coefficient in tropical cyclone conditions