Potential Effect of Bio-Surfactants on Sea Spray Generation in Tropical Cyclone Conditions

Despite significant improvement in computational and observational capabilities, predicting intensity and intensification of major tropical cyclones remains a challenge. In 2017 Hurricane Maria intensified to a Category 5 storm within 24 h, devastating Puerto Rico. In 2019 Hurricane Dorian, predicted to remain tropical storm, unexpectedly intensified into a Category 5 storm and destroyed the Bahamas. The official forecast and computer models were unable to predict rapid intensification of these storms. One possible reason for this is that key physics, including microscale processes at the air-sea interface, are poorly understood and parameterized in existing forecast models. Here we show that surfactants significantly affect the generation of sea spray, which provides some of the fuel for tropical cyclones and their intensification, but also provides some of the drag that limits intensity and intensification. Using a numerical model verified with a laboratory experiment, which predicts spray radii distribution starting from a 100 μm radius, we show that surfactants increase spray generation by 20–34%. We anticipate that bio-surfactants affect heat, energy, and momentum exchange through altered size distribution and concentration of sea spray, with consequences for tropical cyclone intensification or decline, particularly in areas of algal blooms and near coral reefs, as well as in areas affected by oil spills and dispersants

Relative Abundance of Bacillus spp., Surfactant-Associated Bacterium Present in a Natural Sea Slick Observed by Satellite SAR Imagery over the Gulf of Mexico

The damping of short gravity-capillary waves (Bragg waves) due to surfactant accumulation under low wind speed conditions results in the formation of natural sea slicks. These slicks are detectable visually and in synthetic aperture radar satellite imagery. Surfactants are produced by natural life processes of many marine organisms, including bacteria, phytoplankton, seaweed, and zooplankton. In this work, samples were collected in the Gulf of Mexico during a research cruise on the R/V F.G. Walton Smith to evaluate the relative abundance of Bacillus spp., surfactant-associated bacteria, in the sea surface microlayer compared to the subsurface water at 0.2 m depth. A method to reduce potential contamination of microlayer samples during their collection on polycarbonate filters was implemented and advanced, including increasing the number of successive samples per location and changing sample storage procedures. By using DNA analysis (real-time polymerase chain reaction) to target Bacillus spp., we found that in the slick areas, these surfactant-associated bacteria tended to reside mostly in subsurface waters, lending support to the concept that the surfactants they may produce move to the surface where they accumulate under calm conditions and enrich the sea surface microlayer

Relative Abundance of Bacillus spp., Surfactant-Associated Bacterium Present in a Natural Sea Slick Observed by Satellite SAR Imagery over the Gulf of Mexico

The damping of short gravity-capillary waves (Bragg waves) due to surfactant accumulation under low wind speed conditions results in the formation of natural sea slicks. These slicks are detectable visually and in synthetic aperture radar satellite imagery. Surfactants are produced by natural life processes of many marine organisms, including bacteria, phytoplankton, seaweed, and zooplankton. In this work, samples were collected in the Gulf of Mexico during a research cruise on the R/V F.G. Walton Smith to evaluate the relative abundance of Bacillus spp., surfactant-associated bacteria, in the sea surface microlayer compared to the subsurface water at 0.2 m depth. A method to reduce potential contamination of microlayer samples during their collection on polycarbonate filters was implemented and advanced, including increasing the number of successive samples per location and changing sample storage procedures. By using DNA analysis (real-time polymerase chain reaction) to target Bacillus spp., we found that in the slick areas, these surfactant-associated bacteria tended to reside mostly in subsurface waters, lending support to the concept that the surfactants they may produce move to the surface where they accumulate under calm conditions and enrich the sea surface microlayer

Analysis of surfactant-associated bacteria in the sea surface microlayer using deoxyribonucleic acid sequencing and synthetic aperture radar

The sea surface microlayer (SML) is the upper 1 mm of the ocean, where Earth’s biogeochemical processes occur between the ocean and atmosphere. It is physicochemically distinct from the water below and highly variable in space and time due to changing physical conditions. Some microorganisms influence the composition of the SML by producing surfactants for biological functions that accumulate on the surface, decrease surface tension, and create slicks. Slicks can be visible to the eye and in synthetic aperture radar (SAR) satellite imagery. This study focuses on surfactant-associated bacteria in the near-surface layer and their role in slick formation where oil is present

Freshwater Lenses in the Near-Surface Layer of the Ocean Laterally Spreading as Gravity Currents

Convective rains and/or river runoff produce localized freshwater lenses in the near-surface layer of the ocean that have significant density anomalies and thus horizontal pressure gradients. As a result, these lenses can spread and propagate as gravity currents, interacting with wind stress. We have modeled freshwater lens dynamics in the near-surface layer of the ocean using computational fluid dynamics (CFD) tools. We are able to reproduce generic features of freshwater lens spreading and interaction with wind using a 3D CFD model developed with ANSYS Fluent software. The model set up included an initial 0.5 psu salinity and 0.8° C temperature anomaly with a 50 m radius. Wind stress corresponding to U10 = 8 m/s was applied to the water surface. The freshwater lens spread laterally as a gravity current, producing a typical gravity current head with some asymmetry of the lens edges due to the effect of the wind. Interestingly, coherent structures developed at the frontal edge of the spreading freshwater lens, apparently intensifying mixing. These structures resemble a complex pattern of three-dimensional water flow motions in the leading edge of the gravity current and trailing fluid, as previously reported by Özgökmen et al. (2004) and Soloviev et al. (2015). The results of the CFD model have been compared with measurements conducted as part of the Gulf of Mexico Research Initiative Consortium for Advanced Research on Transport of Hydrocarbons in the Environment. These results have a number of practical applications including pollution propagation in coastal waters (e.g., oil spills), open ocean dynamics (e.g., Madden-Julian Oscillation), and interpretation of Aquarius and SMOS sea surface salinity satellite measurements

The Air-Sea Interface as a Factor in Rapid Intensification of Tropical Cyclones

Observations suggest that under extreme wind speed conditions there is a widespread disruption of the air-sea interface. The mechanisms that control disruption of the air-sea interface in tropical cyclones are somewhat analogous to the process of atomization that is well studied in such engineering applications as fuel injection in combustion and cryogenic rocket engines, food processing, and inkjet printing. The related instabilities may include the well-known interfacial mode (Kelvin-Helmholtz instability) and the “liquid” mode (which has some resemblance to the Holmboe instability). In this work, computational fluid dynamics experiments have been performed using a multi-phase volume of fluid large eddy simulation model (ANSYS Fluent) to reproduce properties of the air-sea interface under tropical cyclone conditions. A very fine resolution mesh 0.75 mm x 0.75 mm x 0.75 mm and a realistic surface tension coefficient (0.072 N/m) were set at the air-water interface. The model was forced with hurricane force wind stress at the top of the air layer. The periodic boundary condition along the wind direction was equivalent to an infinite fetch. The model reveals a noticeable asymmetry between the air and water sides of the interface (most of the action is on the air side), which has previously been observed in laboratory experiments. Such asymmetry is typical for the Kelvin-Helmholtz instability at a gas-liquid interface with a significant density difference. Computational and laboratory experiments have resulted in the development of a non-monotonous parameterization of the air-sea drag coefficient dependence on wind speed that can contain the aerodynamic drag well near 60 m/s wind and can explain the rapid intensification and rapid decline of tropical cyclones (Soloviev et al., JGR-Oceans, 2017). One serious complication is that the enthalpy exchange coefficient is still a poorly known parameter in tropical cyclones. A volume of fluid to discrete phase model is under development for a more realistic enthalpy exchange parameterization. We are considering other related factors involved in the tropical cyclone intensification and decline including vapor advection in the cyclone, coupled air-sea system effects, and atmospheric conditions

Analysis of the Magnetic Signature of Surface Waves Measured in a Laboratory Experiment

A magnetic signature is created by secondary magnetic field fluctuations caused by the phenomenon of seawater moving in Earth’s magnetic field. A laboratory experiment was conducted at the SUrge STructure Atmosphere INteraction (SUSTAIN) facility to measure the magnetic signature of surface waves using a differential method: a pair of magnetometers, separated horizontally by one-half wavelength, were placed at several locations on the outer tank walls. This technique significantly reduced the extraneous magnetic distortions that were detected simultaneously by both sensors and additionally doubled the magnetic signal of surface waves. Accelerometer measurements and local gradients were used to identify magnetic noise produced from tank vibrations. Wave parameters of 4 m long waves with a 0.56 Hz frequency and a 0.1-m amplitude were used in this experiment. Freshwater and saltwater experiments were completed to determine the magnetic difference generated by the difference in conductivity. Tests with an empty tank were conducted to identify the noise of the facility. When the magnetic signal was put through spectral analysis, it showed the primary peak at the wave frequency (0.56 Hz) and less pronounced higher frequency harmonics, which are caused by the non-linearity of shallow water surface waves. The magnetic noise induced by the wavemaker and related vibrations peaked around 0.3 Hz, which was removed using filtering techniques. These results indicate that the magnetic signature produced by surface waves was an order of magnitude larger than in traditional model predictions. The discrepancy may be due to the magnetic permeability difference between water and air that is not considered in the traditional model

In Situ and Satellite Observations of an Oil Seep in the Gulf of Mexico

No abstract provided

Measurements of Surface Wave Patterns at the Sharp Front Produced by the Mississippi River Runoff during SAR Satellite Overpasses

Abstract is not available