139 research outputs found
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 aerosolârelated 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 (Ď) 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) ⥠âTP/âĎ, 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 Îβ(P) ⥠βobs. â βmodl., we avoid instrumental and geographic artifacts and extract a remarkably robust radiative heating signal of about 2â4 K within the dust layer. The extracted signal peaks over the midâAtlantic Ocean, as a result of competing trends: âmemoryâ 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 âźÎźm), 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 AMSRâE and MODIS, both aboard NASA\u27s satellite Aqua, we link the amount of coarse marine aerosols emitted to the atmosphere through windâdriven 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 (Ďcm), as surface winds intensify. Accordingly, a twofold increase in Ďcm is associated with a 1.4 Îźm +/â0.11 increase in reff. Our results suggest that climatic changes in surface winds may play an important role not only over land for windâpower estimation but also over the oceans by changing clouds reflectance and lifetime
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 0.009Âą0.002 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 dayâ1, 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 dayâ1, 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 BodĂŠlĂŠ Depression, located in Chad, appears to modulate transatlantic dust patterns about half the time
Lagrangian transport through an ocean front in the North-Western Mediterranean Sea
We analyze with the tools of lobe dynamics the velocity field from a
numerical simulation of the surface circulation in the Northwestern
Mediterranean Sea. We identify relevant hyperbolic trajectories and their
manifolds, and show that the transport mechanism known as the `turnstile',
previously identified in abstract dynamical systems and simplified model flows,
is also at work in this complex and rather realistic ocean flow. In addition
nonlinear dynamics techniques are shown to be powerful enough to identify the
key geometric structures in this part of the Mediterranean. In particular the
North Balearic Front, the westernmost part of the transition zone between
saltier and fresher waters in the Western Mediterranean is interpreted in terms
of the presence of a semipermanent ``Lagrangian barrier'' across which little
transport occurs. Our construction also reveals the routes along which this
transport happens. Topological changes in that picture, associated with the
crossing by eddies and that may be interpreted as the breakdown of the front,
are also observed during the simulation.Comment: 34 pages, 6 (multiple) figures. Version with higher quality figures
available from
http://www.imedea.uib.es/physdept/publications/showpaper_en.php?indice=1764 .
Problems with paper size fixe
Oceanic three-dimensional Lagrangian Coherent Structures: A study of a mesoscale eddy in the Benguela ocean region
We study three dimensional oceanic Lagrangian Coherent Structures (LCSs) in
the Benguela region, as obtained from an output of the ROMS model. To do that
we first compute Finite-Size Lyapunov exponent (FSLE) fields in the region
volume, characterizing mesoscale stirring and mixing. Average FSLE values show
a general decreasing trend with depth, but there is a local maximum at about
100 m depth. LCSs are extracted as ridges of the calculated FSLE fields. They
present a "curtain-like" geometry in which the strongest attracting and
repelling structures appear as quasivertical surfaces. LCSs around a particular
cyclonic eddy, pinched off from the upwelling front are also calculated. The
LCSs are confirmed to provide pathways and barriers to transport in and out of
the eddy
Constraining the aerosol influence on cloud fraction
Aerosol-cloud interactions have the potential to modify many different cloud properties. There is significant uncertainty in the strength of these aerosol-cloud interactions in analyses of observational data, partly due to the difficulty in separating aerosol effects on clouds from correlations generated by local meteorology. The relationship between aerosol and cloud fraction (CF) is particularly important to determine, due to the strong correlation of CF to other cloud properties and its large impact on radiation. It has also been one of the hardest to quantify from satellites due to the strong meteorological covariations involved. This work presents a new method to analyze the relationship between aerosol optical depth (AOD) and CF. By including information about the cloud droplet number concentration (CDNC), the impact of the meteorological covariations is significantly reduced. This method shows that much of the AOD-CF correlation is explained by relationships other than that mediated by CDNC. By accounting for these, the strength of the global mean AOD-CF relationship is reduced by around 80%. This suggests that the majority of the AOD-CF relationship is due to meteorological covariations, especially in the shallow cumulus regime. Requiring CDNC to mediate the AOD-CF relationship implies an effective anthropogenic radiative forcing from an aerosol influence on liquid CF of â0.48 W mâ2 (â0.1 to â0.64 W mâ2), although some uncertainty remains due to possible biases in the CDNC retrievals in broken cloud scenes
Testing the climate intervention potential of ocean afforestation using the Great Atlantic Sargassum Belt
Ensuring that global warming remains 2 emissions reduction. Additionally, 100â900 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 20â100%. Atmospheric CO2 influx into the surface seawater, after CO2-fixation by Sargassum, takes 2.5â18 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
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