Observational bounds on atmospheric heating by aerosol absorption: Radiative signature of transatlantic dust

[1] Aerosols absorb solar radiation thus changing the atmospheric temperature profile but the overall magnitude of this effect is not known. To that end, Saharan dust emissions over the Atlantic Ocean provide an opportunity to examine aerosol‐related heating via satellite imaging. A major difficulty, however, is disentangling a straightforward heating signal caused by the absorbing dust from a meteorological signal, which originates from correlation between dust concentration and air temperature. To tackle the problem, we combine temperature (T) soundings, from the atmospheric infrared sounder (AIRS), with aerosol optical depth (τ) measurements, from the moderate resolution imaging spectroradiometer (MODIS), and data assimilation results from the global data assimilation system (GDAS). We introduce the quantity β(P) ≡ ∂TP/∂τ, the subscript indicating temperature at a given pressure, and study the observed (AIRS) vs. modeled (GDAS) vertical profiles of β(P). Using the vertical as well as horizontal patterns of β(P) and Δβ(P) ≡ βobs. − βmodl., we avoid instrumental and geographic artifacts and extract a remarkably robust radiative heating signal of about 2–4 K within the dust layer. The extracted signal peaks over the mid‐Atlantic Ocean, as a result of competing trends: “memory” of the dust source in the east, and mixing with transparent aerosol in the west

Effect of coarse marine aerosols on stratocumulus clouds

In contrast to fine anthropogenic aerosols (radii ∼μm), large aerosol particles are thought to enhance cloud droplet growth, promote precipitation formation and reduce cloud albedo. While shown in cloud simulation models, the impact of coarse aerosols on marine stratocumulus clouds lacks observational evidence. Here, by combining data from AMSR‐E and MODIS, both aboard NASA\u27s satellite Aqua, we link the amount of coarse marine aerosols emitted to the atmosphere through wind‐driven processes with the size of cloud droplets, at the world\u27s largest deck of marine stratocumulus clouds over the southeastern Pacific. For constrained meteorological conditions, approximately 1/2 of the change in droplet effective radius (reff) is attributed to increase in coarse marine aerosol optical depth (τcm), as surface winds intensify. Accordingly, a twofold increase in τcm is associated with a 1.4 μm +/−0.11 increase in reff. Our results suggest that climatic changes in surface winds may play an important role not only over land for wind‐power estimation but also over the oceans by changing clouds reflectance and lifetime

Multi scale phytoplankton variability in the northeast Atlantic ocean (a satellite view)

L'objectif de cette thèse est de caractériser les modes de variabilité du phytoplancton et d'identifier les mécanismes à l'origine de cette variabilité dans une région de subduction des eaux modales située au Nord Est de l'océan Atlantique. Ceci est rendu possible par l'exploitation des données de couleur de l'océan, issues des données SeaWiFS. Ces données sont alors confrontées d'une part aux champs dynamiques issus de modèles numériques et d'autre part aux observations satellitales. Dans un premier temps nous décrivons les mécanismes à l'origine de la variabilité grande échelle du système de production. Nous montrons ainsi que cette variabilité est quasi-méridienne et résulte principalement des variations de la profondeur de la couche de mélange. Par ailleurs, nos résultats mettent en évidence le transfert de variabilité des grandes échelles vers les petites échelles à travers des processus de cascade directe horizontale associés au mélange géostrophique.PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Long-Term Loss of Coral Reef in the Gulf of Aqaba Estimated from Historical Aerial Images

Located at the northern tip of the Red Sea, the Gulf of Aqaba coral reef is highly susceptible to anthropogenic pressure associated with the massive development of the two neighboring cities of Eilat and Aqaba. Over the years, the reef has been extensively studied in a number of research and monitoring programs, which provide detailed information on a variety of ecological, physiological, biogeochemical and physical variables. Due to the local nature of these surveys, although the state of the reef at specific times and locations is very well characterized, long-term changes in the spatial characteristics of the reef as a whole have not yet been quantified. Here, through analysis of historical aerial images, we address one of the most important aspects of this knowledge gap and quantify the decrease in coral reef coverage, with respect to a baseline level prior to the establishment of the City of Eilat in 1949. Our results show that along a 8460 m segment of the shoreline at the northwestern edge of the GOA (29°29′–30°N/34°54′–35°E), infrastructure construction has led to the loss of more than 4810 m2 of coral reef. This loss has been caused either directly by the construction (most importantly the Eilat Port) or indirectly by changing environmental conditions, which in turn damage the reef. Our results show that historical aerial images can provide a unique source of information on the spatial characteristics of marine and coastal systems prior to the era of Earth observation satellites

A Satellite-Based Lagrangian View on Phytoplankton Dynamics

The well-lit upper layer of the open ocean is a dynamical environment that hosts approximately half of global primary production. In the remote parts of this environment, distant from the coast and from the seabed, there is no obvious spatially fixed reference frame for describing the dynamics of the microscopic drifting organisms responsible for this immense production of organic matter—the phytoplankton. Thus, a natural perspective for studying phytoplankton dynamics is to follow the trajectories of water parcels in which the organisms are embedded. With the advent of satellite oceanography, this Lagrangian perspective has provided valuable information on different aspects of phytoplankton dynamics, including bloom initiation and termination, spatial distribution patterns, biodiversity, export of carbon to the deep ocean, and, more recently, bottom-up mechanisms that affect the distribution and behavior of higher-trophic-level organisms. Upcoming submesoscale-resolving satellite observations and swarms of autonomous platforms open the way to the integration of vertical dynamics into the Lagrangian view of phytoplankton dynamics

Shallow Convective Cloud Field Lifetime as a Key Factor for Evaluating Aerosol Effects

Summary: Clouds control much of the Earth's energy and water budgets. Aerosols, suspended in the atmosphere, interact with clouds and affect their properties. Recent studies have suggested that the aerosol effect on warm convective cloud systems evolve in time and eventually approach a steady state for which the overall effects of aerosols can be considered negligible. Using numerical simulations, it was estimated that the time needed for such cloud fields to approach this state is >24 hr. These results suggest that the typical cloud field lifetime is an important parameter in determining the total aerosol effect. Here, analyzing satellite observations and reanalysis data (with the aid of numerical simulations), we show that the characteristic timescale of warm convective cloud fields is less than 12 hr. Such a timescale implies that these clouds should be regarded as transient-state phenomena and therefore can be highly susceptible to changes in aerosol properties. : Earth Sciences; Atmospheric Science; Atmosphere Modelling Subject Areas: Earth Sciences, Atmospheric Science, Atmosphere Modellin

Long range transport of a quasi isolated chlorophyll patch by an agulhas ring

International audienceUsing satellite retrievals of sea surface chlorophyll and geostrophic currents we study the evolution of a distinct chlorophyll patch transported by an Agulhas ring along a ~1,500 km track. Throughout an ~11 months period of the total 2 years eddy lifetime, the shape of the chlorophyll patch is consistently delimited by the horizontal transport barriers associated with the eddy. Analysis of Lagrangian time series of sea surface variables in and around the eddy suggests that the evolution of the chlorophyll patch is driven by two processes (i) slow lateral mixing with ambient waters mediated by horizontal stirring in filaments, and (ii) rapid events of wind induced vertical mixing. These results support the idea that mesoscale eddies shape biological production through the combination of horizontal and vertical dynamical processes, and emphasize the important role of horizontal eddy transport in sustaining biological production over the otherwise nutrient-depleted subtropical gyres

Long range transport of a quasi isolated chlorophyll patch by an Agulhas ring

International audienceUsing satellite retrievals of sea surface chlorophyll and geostrophic currents we study the evolution of a distinct chlorophyll patch transported by an Agulhas ring along a ˜1,500 km track. Throughout an ˜11 months period of the total 2 years eddy lifetime, the shape of the chlorophyll patch is consistently delimited by the horizontal transport barriers associated with the eddy. Analysis of Lagrangian time series of sea surface variables in and around the eddy suggests that the evolution of the chlorophyll patch is driven by two processes (i) slow lateral mixing with ambient waters mediated by horizontal stirring in filaments, and (ii) rapid events of wind induced vertical mixing. These results support the idea that mesoscale eddies shape biological production through the combination of horizontal and vertical dynamical processes, and emphasize the important role of horizontal eddy transport in sustaining biological production over the otherwise nutrient-depleted subtropical gyres

Dispersion/dilution enhances phytoplankton blooms in low-nutrient waters

International audienceSpatial characteristics of phytoplankton blooms often reflect the horizontal transport properties of the oceanic turbulent flow in which they are embedded. Classically, bloom response to horizontal stirring is regarded in terms of generation of patchiness following large-scale bloom initiation. Here, using satellite observations from the North Pacific Subtropical Gyre and a simple ecosystem model, we show that the opposite scenario of turbulence dispersing and diluting fine-scale (∼1–100 km) nutrient-enriched water patches has the critical effect of regulating the dynamics of nutrients–phytoplankton–zooplankton ecosystems and enhancing accumulation of photosynthetic biomass in low-nutrient oceanic environments. A key factor in determining ecological and biogeochemical consequences of turbulent stirring is the horizontal dilution rate, which depends on the effective eddy diffusivity and surface area of the enriched patches. Implementation of the notion of horizontal dilution rate explains quantitatively plankton response to turbulence and improves our ability to represent ecological and biogeochemical processes in oligotrophic oceans