Contrasting the 2007 and 2005 hurricane seasons: Evidence of possible impacts of Saharan dry air and dust on tropical cyclone activity in the Atlantic basin

Sun, Donglian; Lau, K.-M.; Kafatos, Menas

doi:10.1029/2008gl034529

Cited by 63 publications

(71 citation statements)

References 34 publications

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“…These results have led to assumptions in several studies (Jones et al 2007;Reale et al 2009;Sun et al 2008Sun et al , 2009) that the SAL is very dry throughout its depth. As was shown in sections 3 and 4, the SAL in the eastern Atlantic is often characterized by a midlevel layer of more elevated humidity (Carlson and Prospero 1972;Ismail et al 2010;Messager et al 2009).…”

Section: B the Sal Is Dry Throughout Its Depthmentioning

confidence: 99%

“…While it was not Dunion and Velden's intention to imply that the SAL's impacts were always negative or were the dominant factor affecting hurricane activity (J. Dunion and C. Velden 2009, personal communication), it appears from recent research papers (e.g., Jones et al 2007;Sun et al 2008Sun et al , 2009Reale et al 2009;Shu and Wu 2009) that that view is becoming more widespread.…”

Section: Introductionmentioning

confidence: 99%

“…A frequent assumption made in many recent SAL papers (Jones et al 2007;Dunion and Marron 2008;Reale et al 2009;Shu and Wu 2009;Sun et al 2008;Sun et al 2009) is that very dry low-to midlevel tropical air is necessarily of Saharan origin. Dunion and Marron (2008) separated soundings at four Caribbean sites into SAL and non-SAL categories, with the separation being accomplished based upon Geostationary Operational Environmental Satellite (GOES) multichannel SAL analyses (DV).…”

Section: A Dry Tropical Air Is Sal Airmentioning

confidence: 99%

“…A more recent study by Dunion and Velden (2004, hereafter DV) described several potentially negative influences of the SAL, and Wu et al (2006), Jones et al (2007), and Shu and Wu (2009) assumed the general applicability of the DV results to describe SAL impacts on hurricanes. The reduced Atlantic hurricane activity of 2006 and 2007 compared to 2004 and 2005, particularly as it affected the United States, has led to speculation in some research papers (e.g., Lau and Kim 2007a,b;Sun et al 2008) that dustiness or dry air from increased SAL activity contributed significantly to the decline in hurricane activity in those two years. related recent increased Atlantic hurricane activity to a decrease in SAL activity (dry-air outbreaks and enhanced vertical wind shear).…”

Section: Introductionmentioning

confidence: 99%

“…The link between the SAL's properties and its supposed negative impacts (DV;Shu and Wu 2009;Sun et al 2008) has been built largely on very limited evidence and some false assumptions. Alternative causes of storm weakening or lack of intensification, such as vertical shear or weak convective activity not associated with the SAL, and hurricane-induced ocean cooling, were not examined.…”

Section: Introductionmentioning

confidence: 99%

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Reevaluating the Role of the Saharan Air Layer in Atlantic Tropical Cyclogenesis and Evolution

Braun

2010

Monthly Weather Review

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The existence of the Saharan air layer (SAL), a layer of warm, dry, dusty air frequently present over the tropical Atlantic Ocean, has long been appreciated. The nature of its impacts on hurricanes remains unclear, with some researchers arguing that the SAL amplifies hurricane development and with others arguing that it inhibits it. The potential negative impacts of the SAL include 1) vertical wind shear associated with the African easterly jet; 2) warm air aloft, which increases thermodynamic stability at the base of the SAL; and 3) dry air, which produces cold downdrafts. Multiple NASA satellite datasets and NCEP global analyses are used to characterize the SAL's properties and evolution in relation to tropical cyclones and to evaluate these potential negative influences. The SAL is shown to occur in a large-scale environment that is already characteristically dry as a result of large-scale subsidence. Strong surface heating and deep dry convective mixing enhance the dryness at low levels (primarily below ;700 hPa), but moisten the air at midlevels. Therefore, mid-to-upper-level dryness is not generally a defining characteristic of the SAL, but is instead often a signature of subsidence. The results further show that storms generally form on the southern side of the jet, where the background cyclonic vorticity is high. Based upon its depiction in NCEP Global Forecast System meteorological analyses, the jet often helps to form the northern side of the storms and is present to equal extents for both strengthening and weakening storms, suggesting that jet-induced vertical wind shear may not be a frequent negative influence. Warm SAL air is confined to regions north of the jet and generally does not impact the tropical cyclone precipitation south of the jet.Composite analyses of the early stages of tropical cyclones occurring in association with the SAL support the inferences from the individual cases noted above. Furthermore, separate composites for strongly strengthening and for weakening storms show few substantial differences in the SAL characteristics between these two groups, suggesting that the SAL is not a determinant of whether a storm will intensify or weaken in the days after formation. Key differences between these cases are found mainly at upper levels where the flow over strengthening storms allows for an expansive outflow and produces little vertical shear, while for weakening storms, the shear is stronger and the outflow is significantly constrained.

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Section: B the Sal Is Dry Throughout Its Depthmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: A Dry Tropical Air Is Sal Airmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Reevaluating the Role of the Saharan Air Layer in Atlantic Tropical Cyclogenesis and Evolution

Braun

2010

Monthly Weather Review

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Large‐scale impact of Saharan dust on the North Atlantic Ocean circulation

Serra¹,

Avellaneda

Stammer³

2014

JGR Oceans

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The potential for a dynamical impact of Saharan mineral dust on the North Atlantic Ocean large-scale circulation is investigated. To this end, an ocean general circulation model forced by atmospheric fluxes is perturbed by an idealized, seasonally varying, net shortwave flux anomaly, as it results from remote sensing observations of aerosol optical thickness representing Saharan dust load in the atmosphere. The dust dynamical impact on the circulation is assessed through a comparison between perturbed and an unperturbed run. Results suggest that, following the dust-induced shortwave irradiance anomaly, a buoyancy anomaly is created in the Atlantic offshore the African coast, which over the course of the time propagates westward into the interior Atlantic while progressively subducting. Changes in the large-scale barotropic and overturning circulations are significant after 3 years, which coincides with the elapsed time required by the bulk of the buoyancy perturbation to reach the western boundary of the North Atlantic. Although small in amplitude, the changes in the meridional overturning are of the same order as interannual-to-decadal variability. Variations in the amplitude of the forcing lead to changes in the amplitude of the response, which is almost linear during the first 3 years. In addition, a fast, but dynamically insignificant, response can be observed to propagate poleward along the eastern boundary of the North Atlantic, which contributes to a nonlinear response in the subpolar region north of 40 N.

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Quantifying particle size and turbulent scale dependence of dust flux in the Sahara using aircraft measurements

Rosenberg

Parker

Ryder

et al. 2014

J. Geophys. Res. Atmos.

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The first size-resolved airborne measurements of dust fluxes and the first dust flux measurements from the central Sahara are presented and compared with a parameterization by Kok (2011a). High-frequency measurements of dust size distribution were obtained from 0.16 to 300 μm diameter, and eddy covariance fluxes were derived. This is more than an order of magnitude larger size range than previous flux estimates. Links to surface emission are provided by analysis of particle drift velocities. Number flux is described by a À2 power law between 1 and 144 μm diameter, significantly larger than the 12 μm upper limit suggested by Kok (2011a). For small particles, the deviation from a power law varies with terrain type and the large size cutoff is correlated with atmospheric vertical turbulent kinetic energy, suggesting control by vertical transport rather than emission processes. The measured mass flux mode is in the range 30-100 μm. The turbulent scales important for dust flux are from 0.1 km to 1-10 km. The upper scale increases during the morning as boundary layer depth and eddy size increase. All locations where large dust fluxes were measured had large topographical variations. These features are often linked with highly erodible surface features, such as wadis or dunes. We also hypothesize that upslope flow and flow separation over such features enhance the dust flux by transporting large particles out of the saltation layer. The tendency to locate surface flux measurements in open, flat terrain means these favored dust sources have been neglected in previous studies.

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Contrasting the 2007 and 2005 hurricane seasons: Evidence of possible impacts of Saharan dry air and dust on tropical cyclone activity in the Atlantic basin

Cited by 63 publications

References 34 publications

Reevaluating the Role of the Saharan Air Layer in Atlantic Tropical Cyclogenesis and Evolution

Reevaluating the Role of the Saharan Air Layer in Atlantic Tropical Cyclogenesis and Evolution

Large‐scale impact of Saharan dust on the North Atlantic Ocean circulation

Quantifying particle size and turbulent scale dependence of dust flux in the Sahara using aircraft measurements

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