Estimating the maritime component of aerosol optical depth and its dependency on surface wind speed using satellite data

Six years (2003-2008) of satellite measurements of aerosol parameters from the Moderate Resolution Imaging Spectroradiometer (MODIS) and surface wind speeds from Quick Scatterometer (QuikSCAT), the Advanced Microwave Scanning Radiometer (AMSR-E), and the Special Sensor Microwave Imager (SSM/I), are used to provide a comprehensive perspective on the link between surface wind speed and marine aerosol optical depth over tropical and subtropical oceanic regions. A systematic comparison between the satellite derived fields in these regions allows to: (i) separate the relative contribution of wind-induced marine aerosol to the aerosol optical depth; (ii) extract an empirical linear equation linking coarse marine aerosol optical depth and wind intensity; and (iii) identify a time scale for correlating marine aerosol optical depth and surface wind speed. The contribution of wind induced marine aerosol to aerosol optical depth is found to be dominated by the coarse mode elements. When wind intensity exceeds 4 m/s, coarse marine aerosol optical depth is linearly correlated with the surface wind speed, with a remarkably consistent slope of 0.009±0.002 s/m. A detailed time scale analysis shows that the linear correlation between the fields is well kept within a 12 h time frame, while sharply decreasing when the time lag between measurements is longer. The background aerosol optical depth, associated with aerosols that are not produced in-situ through wind driven processes, can be used for estimating the contributions of terrestrial and biogenic marine aerosol to over-ocean satellite retrievals of aerosol optical depth

Discernible rhythm in the spatio/temporal distributions of transatlantic dust

The differences in North African dust emission regions and transport routes, between the boreal winter and summer, are thoroughly documented. Here we re-examine the spatial and temporal characteristics of dust transport over the tropical and subtropical North Atlantic Ocean, using 10 yr of satellite data, in order to better characterize the different dust transport periods. We see a robust annual triplet: a discernible rhythm of transatlantic dust weather . The proposed annual partition is composed of two heavy loading periods, associated here with a northern-route period and southern-route period, and one light-loading period, accompanied by unusually low average optical depth of dust. The two dusty periods are quite different in character: their duration, transport routes, characteristic aerosol loading and frequency of pronounced dust episodes. The southern-route period lasts ~4 months. It is characterized by a relatively steady southern positioning, low frequency of dust events, low background values and high variance in dust loading. The northern-route period lasts ~6.5 months and is associated with a steady drift northward of ~0.1 latitude day−1, reaching ~1500 km north of the southern-route. The northern period is characterized by higher frequency of dust events, higher (and variable) background and smaller variance in dust loading. It is less episodic than the southern period. Transitions between the periods are brief. Separation between the southern and northern periods is marked by northward latitudinal shift in dust transport and by moderate reduction in the overall dust loading. The second transition, between the northern and southern periods, commences with an abrupt reduction in dust loading and rapid shift southward of ~0.2 latitude day−1, and ~1300 km in total. Based on cross-correlation analyses, we attribute the observed rhythm to the contrast between the northwestern and southern Saharan dust source spatial distributions. Despite the vast difference in areas, the Bodélé Depression, located in Chad, appears to modulate transatlantic dust patterns about half the time

Testing the climate intervention potential of ocean afforestation using the Great Atlantic Sargassum Belt

Ensuring that global warming remains 2 emissions reduction. Additionally, 100–900 gigatons CO2 must be removed from the atmosphere by 2100 using a portfolio of CO2 removal (CDR) methods. Ocean afforestation, CDR through basin-scale seaweed farming in the open ocean, is seen as a key component of the marine portfolio. Here, we analyse the CDR potential of recent re-occurring trans-basin belts of the floating seaweed Sargassum in the (sub)tropical North Atlantic as a natural analogue for ocean afforestation. We show that two biogeochemical feedbacks, nutrient reallocation and calcification by encrusting marine life, reduce the CDR efficacy of Sargassum by 20–100%. Atmospheric CO2 influx into the surface seawater, after CO2-fixation by Sargassum, takes 2.5–18 times longer than the CO2-deficient seawater remains in contact with the atmosphere, potentially hindering CDR verification. Furthermore, we estimate that increased ocean albedo, due to floating Sargassum, could influence climate radiative forcing more than Sargassum-CDR. Our analysis shows that multifaceted Earth-system feedbacks determine the efficacy of ocean afforestation

Nitric oxide production and antioxidant function during viral infection of the coccolithophore Emiliania huxleyi

Emiliania huxleyi is a globally important marine phytoplankton that is routinely infected by viruses. Understanding the controls on the growth and demise of E. huxleyi blooms is essential for predicting the biogeochemical fate of their organic carbon and nutrients. In this study, we show that the production of nitric oxide (NO), a gaseous, membrane-permeable free radical, is a hallmark of early-stage lytic infection in E. huxleyi by Coccolithoviruses, both in culture and in natural populations in the North Atlantic. Enhanced NO production was detected both intra- and extra-cellularly in laboratory cultures, and treatment of cells with an NO scavenger significantly reduced viral production. Pre-treatment of exponentially growing E. huxleyi cultures with the NO donor S-nitroso-N-acetylpenicillamine (SNAP) prior to challenge with hydrogen peroxide (H2O2) led to greater cell survival, suggesting that NO may have a cellular antioxidant function. Indeed, cell lysates generated from cultures treated with SNAP and undergoing infection displayed enhanced ability to detoxify H2O2. Lastly, we show that fluorescent indicators of cellular ROS, NO, and death, in combination with classic DNA- and lipid-based biomarkers of infection, can function as real-time diagnostic tools to identify and contextualize viral infection in natural E. huxleyi blooms

Nutrient dynamics across the Israeli coastal shelf: An unusual oligotrophic coastal system

The nutrient dynamics and biogeochemical properties of the Pelagic Eastern Mediterranean (EMS), an inland sea with many of the characteristics of a mid-ocean gyre, has been well studied. By contrast there are few studies designed to understand these processes on the coastal shelves of the EMS. In this study the nutrient dynamics were determined using ultra-sensitive analytical procedures together with Primary Productivity (PP) and Chlorophyll across the Israeli Coastal shelf (ICS). This includes 12 monthly cruises over a seasonal cycle at an outer shelf station and several transects from the coast to 100 m depth. Sample preservation tests were carried out for nutrients at the ultra-low concentrations found on the ICS, which found that filtered fresh samples were optimal while confirming that using unfiltered frozen samples gave results for Dissolved Inorganic Phosphorus (DIP) that were too high and NO3− and NO2− (N&N) were too low. The extra DIP released from frozen unfiltered samples may be due to DIP stored in the periplasm of cyano- and other bacteria (Kamennaya et al., 2020). Nutrient data for the ICS showed moderate concentrations of N&N (∼300–500 nM) throughout the water column in winter, which decreased to less than 50 nM in mid-late summer. DIP was depleted (<10 nM) throughout the year. Net PP at the outer shelf station (120 m) was 30 gC m2 y−1. The seasonality and concentration of nutrient concentrations and Net PP as well as the observed seasonal signal of Chlorophyll from remote sensing were similar to that found in the pelagic EMS. The ICS is ultra-oligotrophic and is very different from the eutrophied coastal shelves found in many locations globally because of the unusual anti-estuarine circulation

Lyapunov exponents and oceanic fronts

International audienceLyapunov exponents and Lyapunov vectors are precious tools to study dynamical systems: they provide a mathematical framework characterizing sensitive dependence on initial conditions, as well as the stretching and the contraction occurring along a trajectory. Their extension to finite size and finite time calculation has been shown to lead to the location of Coherent Lagrangian Structures, which correspond in geophysical flows to frontal regions. In this case, the Lyapunov exponent and the Lyapunov vector provide respectively the cross front gradient amplification and the front orientation. Here we present global maps of Lyapunov exponents/vectors computed from satellite-derived surface currents of the oceans and we quantify their capability of predicting fronts by comparing with Sea Surface Temperature images. We find that in high energetic regions like boundary currents, large relative separations are achieved in short times (few days) and Lyapunov vector mostly align with the direction of jets; in contrast, in lower energetic regions (like the boundaries of subtropical gyres) the Lyapunov calculation allows to predict tracer lobes and filaments generated by the chaotic advection occurring here. These results may be useful for a global calibration and validation of the Lagrangian technique for multidisciplinary oceanographic applications like co-localization of marine animal behaviours to frontal systems and adaptive strategies for biogeochemical field studies. The ocean is a turbulent system where its physical and biogeochemical trac-ers (like heat, salinity, phytoplankton) present strong inhomogeneities that are structured over a large range of spatiotemporal scales by features like vortices (eddies) and fronts. Several methods have been proposed to analyze the surface currents and track the physical features that constrain tracer distributions through the horizontal transport. In particular, Lagrangian methods allow to mimic the transport dynamics by creating synthetic particle trajectories which are obtained by integrating the velocity field and then analyzed. One powerful diagnostic which has been used to identify frontal structures, i.e. lines where discontinuities or strong gradients are expected to occur in the ocean, is the calculation of the local Lyapunov exponent. In general the Lyapunov exponents are used in a dynamical system approach in order to detect chaotic behaviour for an invariant system by measuring the growth of the perturbations occurring along particle trajectories. For geophysical systems, the calculation of the Lyapunov exponent is usually performed at finite time and finite space

Intrusion of coastal waters into the pelagic eastern Mediterranean: in situ and satellite-based characterization

A combined dataset of near-real-time multi-satellite observations and in situ measurements from a high-resolution survey is used for characterizing physical-biogeochemical properties of a patch stretching from the coast to the open sea in the Levantine Basin (LB) of the eastern Mediterranean (EM). Spatial analysis of the combined dataset indicates that the patch is a semi-enclosed system, bounded within the mixed layer and separated from ambient waters by transport barriers induced by horizontal stirring. As such, the patch is characterized by physical-biogeochemical properties that significantly differ from those of the waters surrounding it, with lower salinity and higher temperatures, concentrations of silicic acid and chlorophyll <i>a</i>, and abundance of <i>Synechococcus</i> and picoeukaryote cells. Based on estimates of patch dimensions (&sim;40 km width and &sim;25 m depth) and propagation speed (&sim;0.09 m s⁻¹), the volume flux associated with the patch is found to be on the order of 0.1 Sv. Our observations suggest that horizontal stirring by surface currents is likely to have an important impact on the ultra-oligotrophic Levantine Basin ecosystem, through (1) transport of nutrients and coastally derived material, and (2) formation of local, dynamically isolated niches. In addition, this work provides a satellite-based framework for planning and executing high-resolution sampling strategies in the interface between the coast and the open sea