scholarly journals Harmattan, Saharan heat low and West African Monsoon circulation: Modulations on the Saharan dust outflow towards the north Atlantic

2017 ◽  
Author(s):  
Kerstin Schepanski ◽  
Bernd Heinold ◽  
Ina Tegen
Keyword(s):  
North Atlantic ◽  
Dust Emission ◽  
Temporal Distribution ◽  
Saharan Dust ◽  
Monsoon Circulation ◽  
North African ◽  
Dust Source ◽  
The North ◽  
The North Atlantic ◽  
Spatio Temporal

Abstract. The outflow of dust from the North African continent towards the north Atlantic is stimulated by the atmospheric circulation over North Africa, which modulates the spatio-temporal distribution of dust source activation and consequently the entrainment of mineral dust into the boundary layer, as well as the transport of dust out of the source regions. The atmospheric circulation over the North African dust source regions, predominantly the Sahara and the Sahel, is characterised by three major circulation regimes: (1) the Harmattan (trade winds), (2) the Saharan heat low (SHL), and (3) the West African Monsoon circulation. The strength of the individual regimes controls the Saharan dust outflow by affecting the spatio-temporal distribution of dust emission, transport pathways, and deposition fluxes. This study aims at investigating the atmospheric circulation pattern over North Africa with regard to its role favouring dust emission and dust export towards the tropical North Atlantic. The focus of the study is on summer 2013 (June to August), during which also the SALTRACE (Saharan Aerosol Long-range TRansport and Aerosol-Cloud interaction Experiment) field campaign took place. It involves satellite observations by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) flying on-board the geostationary Meteosat Second Generation (MSG) satellite, which are analysed and used to infer a data set of active dust sources. The spatio-temporal distribution of dust source activation frequencies (DSAF) allows for linking the diurnal cycle of dust source activations to dominant meteorological controls on dust emission. In summer, Saharan dust source activations clearly differ from dust source activations over the Sahel regarding the time-of-day when dust emission begins. The Sahara is dominated by morning dust source activations predominantly driven by the break-down of the nocturnal low-level jet. In contrast, dust source activations in the Sahel are predominantly activated during the second half of the day when down-drafts associated with deep moist convection are the major atmospheric driver. Complementary to the satellite-based analysis on dust source activations and implications from their diurnal cycle, simulations on atmosphere and dust life-cycle were performed using the meso-scale atmosphere-dust model system COSMO-MUSCAT (COSMO: COnsortium for Small-scale MOdelling; MUSCAT: MUltiScale Chemistry Aerosol Transport Model). Fields from this simulation were analysed regarding the variability of the Harmattan, the Saharan heat low, and the Monsoon circulation as well as their impact on the variability of the Saharan dust outflow towards the north Atlantic. This study illustrates the complexity of the interaction among the three major circulation regimes and their modulation of the North African dust outflow. Enhanced westward dust fluxes frequently appear following a phase characterised by a deep SHL. Ultimately, findings from this study contribute to the quantification of the interannual variability of the atmospheric dust burden.

scholarly journals Harmattan, Saharan heat low, and West African monsoon circulation: modulations on the Saharan dust outflow towards the North Atlantic

2017 ◽  
Vol 17 (17) ◽  
pp. 10223-10243 ◽  
Author(s):  
Kerstin Schepanski ◽  
Bernd Heinold ◽  
Ina Tegen
Keyword(s):  
North Atlantic ◽  
Dust Emission ◽  
Temporal Distribution ◽  
Saharan Dust ◽  
Monsoon Circulation ◽  
Dust Source ◽  
The North ◽  
The North Atlantic ◽  
Spatio Temporal ◽  
Heat Low

Abstract. The outflow of dust from the northern African continent towards the North Atlantic is stimulated by the atmospheric circulation over North Africa, which modulates the spatio-temporal distribution of dust source activation and consequently the entrainment of mineral dust into the boundary layer, as well as the transport of dust out of the source regions. The atmospheric circulation over the North African dust source regions, predominantly the Sahara and the Sahel, is characterized by three major circulation regimes: (1) the harmattan (trade winds), (2) the Saharan heat low (SHL), and (3) the West African monsoon circulation. The strength of the individual regimes controls the Saharan dust outflow by affecting the spatio-temporal distribution of dust emission, transport pathways, and deposition fluxes.This study aims at investigating the atmospheric circulation pattern over North Africa with regard to its role favouring dust emission and dust export towards the tropical North Atlantic. The focus of the study is on summer 2013 (June to August), during which the SALTRACE (Saharan Aerosol Long-range TRansport and Aerosol-Cloud interaction Experiment) field campaign also took place. It involves satellite observations by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) flying on board the geostationary Meteosat Second Generation (MSG) satellite, which are analysed and used to infer a data set of active dust sources. The spatio-temporal distribution of dust source activation frequencies (DSAFs) allows for linking the diurnal cycle of dust source activations to dominant meteorological controls on dust emission. In summer, Saharan dust source activations clearly differ from dust source activations over the Sahel regarding the time of day when dust emission begins. The Sahara is dominated by morning dust source activations predominantly driven by the breakdown of the nocturnal low-level jet. In contrast, dust source activations in the Sahel are predominantly activated during the second half of the day, when downdrafts associated with deep moist convection are the major atmospheric driver. Complementary to the satellite-based analysis on dust source activations and implications from their diurnal cycle, simulations on atmosphere and dust life cycle were performed using the mesoscale atmosphere–dust model system COSMO-MUSCAT (COSMO: COnsortium for Small-scale MOdelling; MUSCAT: MUltiScale Chemistry Aerosol Transport Model). Fields from this simulation were analysed regarding the variability of the harmattan, the Saharan heat low, and the monsoon circulation as well as their impact on the variability of the Saharan dust outflow towards the North Atlantic. This study illustrates the complexity of the interaction among the three major circulation regimes and their modulation of the North African dust outflow. Enhanced westward dust fluxes frequently appear following a phase characterized by a deep SHL. Ultimately, findings from this study contribute to the quantification of the interannual variability of the atmospheric dust burden.

scholarly journals Modulation of Saharan dust export by the North African dipole

2015 ◽  
Vol 15 (13) ◽  
pp. 7471-7486 ◽  
Author(s):  
S. Rodríguez ◽  
E. Cuevas ◽  
J. M. Prospero ◽  
A. Alastuey ◽  
X. Querol ◽  
...  
Keyword(s):  
Interannual Variability ◽  
North Africa ◽  
North Atlantic ◽  
Large Scale ◽  
Saharan Dust ◽  
North African ◽  
The North ◽  
The North Atlantic ◽  
The Tropics

Abstract. We have studied the relationship between the long-term interannual variability in large-scale meteorology in western North Africa – the largest and most active dust source worldwide – and Saharan dust export in summer, when enhanced dust mobilization in the hyper-arid Sahara results in maximum dust impacts throughout the North Atlantic. We address this issue by analyzing 28 years (1987–2014) of summer averaged dust concentrations at the high-altitude Izaña observatory (~ 2400 m a.s.l.) on Tenerife, and satellite and meteorological reanalysis data. The summer meteorological scenario in North Africa (aloft 850 hPa) is characterized by a high over the the subtropical Sahara and a low over the tropics linked to the monsoon. We measured the variability of this high–low dipole-like pattern in terms of the North African dipole intensity (NAFDI): the difference of geopotential height anomalies averaged over the subtropics (30–32° N, Morocco) and the tropics (10–13° N, Bamako region) close to the Atlantic coast (at 5–8° W). We focused on the 700 hPa standard level due to dust export off the coast of North Africa tending to occur between 1 and 5 km a.s.l. Variability in the NAFDI is associated with displacements of the North African anticyclone over the Sahara and this has implications for wind and dust export. The correlations we found between the 1987–2014 summer mean of NAFDI with dust at Izaña, satellite dust observations and meteorological re-analysis data indicate that increases in the NAFDI (i) result in higher wind speeds at the north of the Inter-Tropical Convergence Zone that are associated with enhanced dust export over the subtropical North Atlantic, (ii) influence the long-term variability of the size distribution of exported dust particles (increasing the load of coarse dust) and (iii) are associated with enhanced rains in the tropical and northern shifts of the tropical rain band that may affect the southern Sahel. Interannual variability in NAFDI is also connected to spatial distribution of dust over the North Atlantic; high NAFDI summers are associated with major dust export (linked to winds) in the subtropics and minor dust loads in the tropics (linked to higher rainfall), and vice versa. The evolution of the summer NAFDI values since 1950 to the present day shows connections to climatic variability (through the Sahelian drought, ENSO (El Niño–Southern Oscillation) and winds) that have implications for dust export paths. Efforts to anticipate how dust export may evolve in future decades will require a better understanding of how the large-scale meteorological systems represented by the NAFD will evolve.

scholarly journals Modeling dust emission response to MIS 3 millennial climate variations from the perspective of East European loess deposits

2013 ◽  
Vol 9 (1) ◽  
pp. 143-185 ◽  
Author(s):  
A. Sima ◽  
M. Kageyama ◽  
D.-D. Rousseau ◽  
G. Ramstein ◽  
Y. Balkanski ◽  
...  
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Reference Site ◽  
Dust Emission ◽  
Dust Storms ◽  
Surface Model ◽  
Climate Signal ◽  
The North ◽  
Loess Deposits ◽  
The North Atlantic

Abstract. European loess sequences of the last glacial period (~ 100–15 kyr BP) show periods of strong dust accumulation alternating with episodes of reduced sedimentation, favoring soil development. In the western part of the loess belt centered around 50° N, these variations appear to have been caused by the North Atlantic rapid climate changes: the Dansgaard-Oeschger (DO) and Heinrich (H) events. It has been recently suggested that the North-Atlantic climate signal can be detected further east, in loess deposits from Stayky (50° 05.65' N, 30° 53.92' E), Ukraine. Here we use climate and dust emission modeling to investigate this data interpretation. We focus on the areas north and northeast of the Carpathians, where loess deposits can be found, and the corresponding main dust sources must have been located as well. The simulations, performed with the LMDZ atmospheric general circulation model and the ORCHIDEE land-surface model, represent a Greenland stadial, a DO interstadial and an H event respectively. Placed in Marine Isotope Stage 3 (~ 60–25 kyr BP) conditions, they only differ by the surface conditions imposed in the North Atlantic between 30° and 63° N. The main source for the loess deposits in the studied area is identified as a dust deflation band, with two very active spots located west–northwest from our reference site. Emissions only occur between February and June. Differences from one deflation spot to another, and from one climate state to another, are explained by analyzing the relevant meteorological and surface variables. Over most of the source region, the annual emission fluxes in the "interstadial" experiment are 30 to 50% lower than the "stadial" values; they would only be about 20% lower if the inhibition of dust uplift by the vegetation were not taken into account. Assuming that lower emissions result in reduced dust deposition leads us to the conclusion that the loess-paleosol stratigraphic succession in the Stayky area reflects indeed North-Atlantic millennial variations. In the main deflation areas of Western Europe, the vegetation effect alone determined most of the ~ 50% stadial-interstadial flux differences. Even if its impact in Eastern Europe is less pronounced, this effect remains a key factor in modulating aeolian emissions at millennial timescale. Conditions favorable to initiating particularly strong dust storms within a few hundred kilometers upwind from our reference site, simulated in the month of April of the "H event" experiment, support the identification of H events as layers of particularly coarse sedimentation in some very detailed profiles.

Infrared radiative forcing of a Saharan dust plume in the North Atlantic

1998 ◽  
Vol 29 ◽  
pp. S1301-S1302 ◽  
Author(s):  
P. Chazette ◽  
J. Pelon ◽  
I. Carrasco ◽  
V. Trouillet ◽  
F. Dulac
Keyword(s):  
North Atlantic ◽  
Radiative Forcing ◽  
Saharan Dust ◽  
The North ◽  
The North Atlantic

scholarly journals The response of Iberian rivers to the North Atlantic Oscillation

2011 ◽  
Vol 8 (3) ◽  
pp. 4459-4493 ◽  
Author(s):  
J. Lorenzo-Lacruz ◽  
S. M. Vicente-Serrano ◽  
J. I. López-Moreno ◽  
J. C. González-Hidalgo ◽  
E. Morán-Tejeda
Keyword(s):  
Water Resources ◽  
North Atlantic Oscillation ◽  
Iberian Peninsula ◽  
North Atlantic ◽  
The North ◽  
Surface Water Resources ◽  
The North Atlantic ◽  
Spatio Temporal ◽  
The North Atlantic Oscillation ◽  
Iberian Rivers

Abstract. In this study we analyzed the influence of the North Atlantic Oscillation (NAO) on the streamflow in 187 sub-basins of the Iberian Peninsula. Monthly and one-month lagged correlations were conducted to assess the spatio-temporal extent of the NAO influence on Iberian river discharges. Analysis of the persistence of the winter NAO throughout the year was also undertaken, together with analysis of streamflow anomalies during positive and negative NAO phases. Moving-window correlation analyses were conducted to assess potential changes in the temporal evolution of the NAO influence on Iberian streamflows. The results show that the NAO has a large impact on surface water resources throughout the Iberian Peninsula during winter, and in the Atlantic watershed during autumn. We showed that water resources management and snowmelt are causing the persistent dependence of streamflows on the previous winter NAO. We found that strongly positive streamflow anomalies occurred during winter, especially in the Atlantic watershed, and provide evidence of non-stationarity and spatial variability in the NAO influence on Iberian water resources.

scholarly journals Earlier colony arrival but no trend in hatching timing in two congeneric seabirds ( Uria spp.) across the North Atlantic

2019 ◽  
Vol 15 (10) ◽  
pp. 20190634 ◽  
Author(s):  
Benjamin Merkel ◽  
Sébastien Descamps ◽  
Nigel G. Yoccoz ◽  
Jóhannis Danielsen ◽  
Francis Daunt ◽  
...  
Keyword(s):  
North Atlantic ◽  
Global Analysis ◽  
Breeding Site ◽  
Egg Laying ◽  
Breeding Phenology ◽  
Arrival Date ◽  
Temperature Changes ◽  
The North ◽  
The North Atlantic ◽  
Spatio Temporal

A global analysis recently showed that seabird breeding phenology (as the timing of egg-laying and hatching) does not, on average, respond to temperature changes or advance with time (Keogan et al. 2018 Nat. Clim. Change 8 , 313–318). This group, the most threatened of all birds, is therefore prone to spatio-temporal mismatches with their food resources. Yet, other aspects of the breeding phenology may also have a marked influence on breeding success, such as the arrival date of adults at the breeding site following winter migration. Here, we used a large tracking dataset of two congeneric seabirds breeding in 14 colonies across 18° latitudes, to show that arrival date at the colony was highly variable between colonies and species (ranging 80 days) and advanced 1.4 days/year while timing of egg-laying remained unchanged, resulting in an increasing pre-laying duration between 2009 and 2018. Thus, we demonstrate that potentially not all components of seabird breeding phenology are insensitive to changing environmental conditions.

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