Papers by Sebastian Engelstaedter
Geophysical Research Letters, 2007
1] Topographic depressions serve as the key spatial control on large global dust sources. In cont... more 1] Topographic depressions serve as the key spatial control on large global dust sources. In contrast, the temporal control on these hotspots has remained elusive. We provide the first global observational evidence that the annual cycle of emissions from dust hotspots is determined by an erosivity feature in the form of wind gustiness. We use TOMS AI and an aridity index to define 131 global dust hotspots. The correlation between the annual cycle of hotspot dust and the annual cycle of gustiness is 70% stronger than the corresponding correlation with wind. The mean significant correlation with wind (n = 106 hotspots) is 0.37 (s = 0.24) and the mean of significant correlation (n = 118 hotspots) with gustiness is 0.63 (s = 0.12). Whereas most model simulations of dust have relied on the broadscale wind, gustiness holds overwhelmingly more power in explaining the annual cycle of dust emissions from global dust hotspots.
Geophysical Research Letters, 2007
1] Topographic depressions serve as the key spatial control on large global dust sources. In cont... more 1] Topographic depressions serve as the key spatial control on large global dust sources. In contrast, the temporal control on these hotspots has remained elusive. We provide the first global observational evidence that the annual cycle of emissions from dust hotspots is determined by an erosivity feature in the form of wind gustiness. We use TOMS AI and an aridity index to define 131 global dust hotspots. The correlation between the annual cycle of hotspot dust and the annual cycle of gustiness is 70% stronger than the corresponding correlation with wind. The mean significant correlation with wind (n = 106 hotspots) is 0.37 (s = 0.24) and the mean of significant correlation (n = 118 hotspots) with gustiness is 0.63 (s = 0.12). Whereas most model simulations of dust have relied on the broadscale wind, gustiness holds overwhelmingly more power in explaining the annual cycle of dust emissions from global dust hotspots.
Quaternary Science Reviews, 2011
We present a new method of analyzing model results to help identify the sensitivity of the locati... more We present a new method of analyzing model results to help identify the sensitivity of the location of different paleodust records to estimate local to regional scale variability of dust and climate variables. We use model simulations of global dust distribution from the last glacial maximum, preindustrial, current, and predicted future. The dust model has been previously shown to match available observational data for the current and last glacial maximum climate. Here, the model is compared to available source provenance data and is shown to agree with these limited observations. Using correlations and slopes across different time periods, the modeled relationships between deposition at specific observational sites and regional deposition and dustiness are shown. In addition, we evaluate the modeled relative slope of these cores to determine the location of paleodust sites that are especially easy to interpret as regional indicators of dustiness. Model predictions suggest that deposition in Antarctic ice cores is usually better than dust concentration to capture regional deposition and dustiness variability over glacialeinterglacial time periods, in agreement with ice core interpretations. For Greenland, the model predicts a possible shift from dominantly wet deposition under modern conditions to dominantly dry deposition during glacial climate conditions indicating that deposition may be better suited to capture dustiness variability under LGM conditions in Greenland. The model also identifies specific regions that are not well covered by observations for glacial/interglacial or anthropocene dust variability. In addition, we evaluate the modeled relative slope of the location of these cores to determine regions that would provide ideal localities for pursuing records that would provide easily interpretable paleo-proxy records of regional dustiness.
Journal of Geophysical Research, 2009
1] Atmospheric dust concentrations have been measured continuously since 1965 at Barbados providi... more 1] Atmospheric dust concentrations have been measured continuously since 1965 at Barbados providing one of the longest records of modern atmospheric dust downwind the dust transport route from North Africa toward the Caribbean. The Barbados dust concentration (BDC) record shows a high variability on daily to decadal timescales. This study aims to assess the importance of four processes that have the potential to contribute to the observed day-to-day variability: (1) changes in the transport pathways, (2) changes in atmospheric transport time, (3) changes in the atmospheric dust loadings in the source regions in North Africa, and (4) changes in rainfall. Back-trajectories are calculated for low and high dust days in June (peak month in annual cycle) and analyzed with respect to these four processes. The results show that statistically significant differences exist between low and high BDC days for all four processes. For high BDC days we observe (1) an increase in the number of trajectories (especially at 100 and 150 hPa above the surface) which originated over the North African continent due to differences in the transport pathways, (2) a faster trans-Atlantic transport at the height of the SAL by up to 0.87 days which would mean less dust being removed as a result of dry deposition and less diffusion, (3) increased dust loadings over West Africa and the tropical North Atlantic, and (4) less rainfall in ITCZ region which may have a minor impact of dust loadings due to wet deposition. Our results support the findings of previous studies that changes in dust emission and rainfall impact dust variability over the Atlantic Ocean and the Caribbean. In addition to these two processes, our findings suggest that also changes in the synoptic meteorology over the North Atlantic and varying transport speed can affect BDC variability by modulating dust transport pattern.
Journal of Geophysical Research, 2009
1] Atmospheric dust concentrations have been measured continuously since 1965 at Barbados providi... more 1] Atmospheric dust concentrations have been measured continuously since 1965 at Barbados providing one of the longest records of modern atmospheric dust downwind the dust transport route from North Africa toward the Caribbean. The Barbados dust concentration (BDC) record shows a high variability on daily to decadal timescales. This study aims to assess the importance of four processes that have the potential to contribute to the observed day-to-day variability: (1) changes in the transport pathways, (2) changes in atmospheric transport time, (3) changes in the atmospheric dust loadings in the source regions in North Africa, and (4) changes in rainfall. Back-trajectories are calculated for low and high dust days in June (peak month in annual cycle) and analyzed with respect to these four processes. The results show that statistically significant differences exist between low and high BDC days for all four processes. For high BDC days we observe (1) an increase in the number of trajectories (especially at 100 and 150 hPa above the surface) which originated over the North African continent due to differences in the transport pathways, (2) a faster trans-Atlantic transport at the height of the SAL by up to 0.87 days which would mean less dust being removed as a result of dry deposition and less diffusion, (3) increased dust loadings over West Africa and the tropical North Atlantic, and (4) less rainfall in ITCZ region which may have a minor impact of dust loadings due to wet deposition. Our results support the findings of previous studies that changes in dust emission and rainfall impact dust variability over the Atlantic Ocean and the Caribbean. In addition to these two processes, our findings suggest that also changes in the synoptic meteorology over the North Atlantic and varying transport speed can affect BDC variability by modulating dust transport pattern.
Geophysical Research Letters, 2003
1] The degree to which dust emissions are controlled by vegetation cover and geomorphic setting (... more 1] The degree to which dust emissions are controlled by vegetation cover and geomorphic setting (specifically closed topographic depressions) was investigated using dust storm frequency (DSF) data based on visibility measurements from >2400 meteorological stations worldwide. Comparisons with distributions of vegetation types suggest that DSF is highest in desert/bare ground (median: 60-80 d/yr) and shrubland (median: 20-30 d/yr) regions, and comparatively low in grassland regions (median: 2 -4 d/yr). Average DSF is inversely correlated with leaf area index (an index of vegetation density) and net primary productivity. In nonforested regions, DSF increases as the fraction of closed topographic depressions increases, likely due to the accumulation of fine sediments in these areas. These findings support the importance of incorporating vegetation and geomorphic setting as explicit controls on emissions in global dust cycle models. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0315 Atmospheric Composition and Structure: Biosphere/ atmosphere interactions. Citation: Engelstaedter, S., K. E. Kohfeld, I. Tegen, and S. P. Harrison, Controls of dust emissions by vegetation and topographic depressions: An evaluation using dust storm frequency data, Geophys.
Geophysical Research Letters, 2003
1] The degree to which dust emissions are controlled by vegetation cover and geomorphic setting (... more 1] The degree to which dust emissions are controlled by vegetation cover and geomorphic setting (specifically closed topographic depressions) was investigated using dust storm frequency (DSF) data based on visibility measurements from >2400 meteorological stations worldwide. Comparisons with distributions of vegetation types suggest that DSF is highest in desert/bare ground (median: 60-80 d/yr) and shrubland (median: 20-30 d/yr) regions, and comparatively low in grassland regions (median: 2 -4 d/yr). Average DSF is inversely correlated with leaf area index (an index of vegetation density) and net primary productivity. In nonforested regions, DSF increases as the fraction of closed topographic depressions increases, likely due to the accumulation of fine sediments in these areas. These findings support the importance of incorporating vegetation and geomorphic setting as explicit controls on emissions in global dust cycle models. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0315 Atmospheric Composition and Structure: Biosphere/ atmosphere interactions. Citation: Engelstaedter, S., K. E. Kohfeld, I. Tegen, and S. P. Harrison, Controls of dust emissions by vegetation and topographic depressions: An evaluation using dust storm frequency data, Geophys.
Proceedings of The National Academy of Sciences, 2009
We discuss the existence of cryospheric "tipping points" in the Earth's climate system. Such crit... more We discuss the existence of cryospheric "tipping points" in the Earth's climate system. Such critical thresholds have been suggested to exist for the disappearance of Arctic sea ice and the retreat of ice sheets: Once these ice masses have shrunk below an anticipated critical extent, the ice-albedo feedback might lead to the irreversible and unstoppable loss of the remaining ice. We here give an overview of our current understanding of such threshold behavior. By using conceptual arguments, we review the recent findings that such a tipping point probably does not exist for the loss of Arctic summer sea ice. Hence, in a cooler climate, sea ice could recover rapidly from the loss it has experienced in recent years. In addition, we discuss why this recent rapid retreat of Arctic summer sea ice might largely be a consequence of a slow shift in ice-thickness distribution, which will lead to strongly increased year-to-year variability of the Arctic summer sea-ice extent. This variability will render seasonal forecasts of the Arctic summer seaice extent increasingly difficult. We also discuss why, in contrast to Arctic summer sea ice, a tipping point is more likely to exist for the loss of the Greenland ice sheet and the West Antarctic ice sheet.
Journal of Geophysical Research, 2006
1] Dust plays an important role in climate, recognition of which has led to a concentrated resear... more 1] Dust plays an important role in climate, recognition of which has led to a concentrated research effort in field campaigns, development and analysis of remotely sensed data, and modeling to better understand dust. There have, however, been very few direct surface-based field measurements from key dust source regions. The Bodélé, Chad, has been shown to be one of the premier sources of dust in the world. This paper reports on the Bodélé Field Experiment (BoDEx 2005) which took place during February and March 2005 and presents the first surface-based measurements of the circulation over the Bodélé. On the basis of Pilot Balloon and AWS data, we confirm the existence of the Bodélé Low Level Jet (LLJ) and show that winds undergo a strong diurnal cycle such that strongest surface winds typically occur in the midmorning when momentum is mixed downward in turbulence induced by radiative heating. In contrast, the core of the LLJ, near 500 m, peaks during the evening and is weakest during the day. The LLJ was present on all days during BoDEx 2005, but winds at the surface reached speeds necessary for large-scale dust entrainment on only a few days. The winds strength during the main dust plume event of BoDEx (10-12 March 2005) was in the bottom third of March plume events of the last 4 years. Pathways of dust transport from the Bodélé using a trajectory model show potential advection of dust over the west African coastline within 5 days.
Geophysical Research Letters, 2003
The degree to which dust emissions are controlled by vegetation cover and geomorphic setting (spe... more The degree to which dust emissions are controlled by vegetation cover and geomorphic setting (specifically closed topographic depressions) was investigated using dust storm frequency (DSF) data based on visibility measurements from >2400 meteorological stations worldwide. Comparisons with distributions of vegetation types suggest that DSF is highest in desert/bare ground (median: 60-80 d/yr) and shrubland (median: 20-30 d/yr) regions, and comparatively low in grassland regions (median: 2-4 d/yr). Average DSF is inversely correlated with leaf area index (an index of vegetation density) and net primary productivity. In non-forested regions, DSF increases as the fraction of closed topographic depressions increases, likely due to the accumulation of fine sediments in these areas. These findings support the importance of incorporating vegetation and geomorphic setting as explicit controls on emissions in global dust cycle models.
Journal of Geophysical Research, 2006
1] Dust plays an important role in climate, recognition of which has led to a concentrated resear... more 1] Dust plays an important role in climate, recognition of which has led to a concentrated research effort in field campaigns, development and analysis of remotely sensed data, and modeling to better understand dust. There have, however, been very few direct surface-based field measurements from key dust source regions. The Bodélé, Chad, has been shown to be one of the premier sources of dust in the world. This paper reports on the Bodélé Field Experiment (BoDEx 2005) which took place during February and March 2005 and presents the first surface-based measurements of the circulation over the Bodélé. On the basis of Pilot Balloon and AWS data, we confirm the existence of the Bodélé Low Level Jet (LLJ) and show that winds undergo a strong diurnal cycle such that strongest surface winds typically occur in the midmorning when momentum is mixed downward in turbulence induced by radiative heating. In contrast, the core of the LLJ, near 500 m, peaks during the evening and is weakest during the day. The LLJ was present on all days during BoDEx 2005, but winds at the surface reached speeds necessary for large-scale dust entrainment on only a few days. The winds strength during the main dust plume event of BoDEx (10-12 March 2005) was in the bottom third of March plume events of the last 4 years. Pathways of dust transport from the Bodélé using a trajectory model show potential advection of dust over the west African coastline within 5 days.
Earth-science Reviews, 2006
The need for a better understanding of the role of atmospheric dust in the climate system and its... more The need for a better understanding of the role of atmospheric dust in the climate system and its impact on the environment has led to research of the underlying causes of dust variability in space and time in recent decades. North Africa is one of the largest dust producing regions in the world with dust emissions being highly variable on time scales ranging from diurnal to multiannual. Changes in the dust loading are expected to have an impact on regional and global climate, the biogeochemical cycle, and human environments. The development of satellite derived products of global dust distributions has improved our understanding of dust source regions and transport pathways in the recent years. Dust models are now capable of reproducing more realistic patterns of dust distributions due to an improved parameterization of land surface conditions. A recent field campaign has improved our understanding of the natural environment and emission processes of the most intense and persistent dust sources in the world, the Bodélé Depression in Chad. In situ measurements of dust properties during air craft observations in and down wind of source regions have led to new estimates of the radiative forcing effects which are crucial in predicting future climate change. With a focus on the North African desert regions, this paper provides a review of the understanding of dust source regions, the variability of dust emissions, climatic controls of dust entrainment and transport, the role of human impact on dust emission, and recent developments of global and regional dust models.
Geomorphology, 2007
The Bodélé in northern Chad is the dustiest place on earth. It is dusty because of the co-locatio... more The Bodélé in northern Chad is the dustiest place on earth. It is dusty because of the co-location of strong near-surface winds and a large exposure of low-density, friable diatomite. Data are reported from three kinds of observation on dust generation in the Bodélé: (a) two automatic weather stations and pilot balloon tracking, complemented with reanalysis data, measuring and estimating wind velocities; (b) sun-photometers, measuring atmospheric dust concentrations; (c) a vacuum-pump dust sampler and an array of MWAC sediment samplers, measuring in-situ sediment transport. With these data, we develop four hypotheses about dust generation: (1) the pulsing of dust output from the Bodélé depends primarily on the varying strength of a Low Level Jet (LLJ); i.e., at this scale, dust production is transport-limited; (2) the most prolific mechanism of dust production is the breakdown of saltating diatomite flakes as they collide with each other and with the surface; (3) disturbance of flow around the large barchans, particularly turbulence and convergence on their "horns", is a major contributor to dust production, i.e., the dunes are "dust-mills"; (4) in consequence, dust production is localised within the main exposure of diatomite to places where there are the most dunes, as on the eastern and northern fringes of the exposure.
1] Dust plays an important role in climate, recognition of which has led to a concentrated resear... more 1] Dust plays an important role in climate, recognition of which has led to a concentrated research effort in field campaigns, development and analysis of remotely sensed data, and modeling to better understand dust. There have, however, been very few direct surface-based field measurements from key dust source regions. The Bodélé, Chad, has been shown to be one of the premier sources of dust in the world. This paper reports on the Bodélé Field Experiment (BoDEx 2005) which took place during February and March 2005 and presents the first surface-based measurements of the circulation over the Bodélé. On the basis of Pilot Balloon and AWS data, we confirm the existence of the Bodélé Low Level Jet (LLJ) and show that winds undergo a strong diurnal cycle such that strongest surface winds typically occur in the midmorning when momentum is mixed downward in turbulence induced by radiative heating. In contrast, the core of the LLJ, near 500 m, peaks during the evening and is weakest during the day. The LLJ was present on all days during BoDEx 2005, but winds at the surface reached speeds necessary for large-scale dust entrainment on only a few days. The winds strength during the main dust plume event of BoDEx (10-12 March 2005) was in the bottom third of March plume events of the last 4 years. Pathways of dust transport from the Bodélé using a trajectory model show potential advection of dust over the west African coastline within 5 days.
Geophysical Research Letters, 2006
Journal of Geophysical Research, 2007
1] Dust emitted from desert regions and transported in the atmosphere has been recognized for its... more 1] Dust emitted from desert regions and transported in the atmosphere has been recognized for its potential to alter the Earth's climate and environments. Satellite data show that the largest source regions of dust (i.e., hot spots) are located in dry, nonvegetated areas of West Africa and central Chad. Dust emissions from these sources follow a distinct seasonal cycle. Whereas our understanding of processes controlling the dust cycle of the Chad dust source has been improved through recent studies, our understanding of the West African sources is limited because of the remoteness of the sources and lack of surface observations. Using a satellite-derived dust index and reanalysis atmospheric fields, we show that the annual dust cycle at the West African dust hot spots is not related to changes in mean surface wind strength but is linked to small-scale high-wind events. We find that the annual dust cycle correlates well with changes in near-surface convergence associated with the annual north-south movement of the Inter-Tropical Convergence Zone (ITCZ). Dust emissions in West Africa are highest in June coinciding with the crossing of the convergence zone on its northward bound over the dust hot spots. The increase in convergence leads to enhanced surface gustiness suggesting that dry convection associated with an increase in the occurrence of small-scale high-wind events and vertical velocity are the main processes controlling the annual dust cycle at the West African dust sources.
Journal of Geophysical Research, 2007
1] Mineral dust in the atmosphere is an important component of the climate system but is poorly q... more 1] Mineral dust in the atmosphere is an important component of the climate system but is poorly quantified. The Bodélé Depression of northern Chad stands out as the world's greatest source region of mineral dust into the atmosphere. Frequent dust plumes are a distinguishing feature of the region's climate. There is a need for more detailed information on processes of dust emission/transport and dust optical properties to inform model simulations of this source. During the Bodélé Dust Experiment (BoDEx) in 2005, instrumentation was deployed to measure dust properties and boundary layer meteorology. Observations indicate that dust emission events are triggered when near-surface wind speeds exceed 10 ms À1 , associated with synoptic-scale variability in the large-scale atmospheric circulation. Dust emission pulses in phase with the diurnal cycle of near-surface winds. Analysis of dust samples shows that the dust consists predominantly of fragments of diatomite sediment. The particle size distribution of this diatomite dust estimated from sun photometer data, using a modified Aeronet retrieval algorithm, indicates a dominant coarse mode (radius centered on 1-2 mm) similar to other Saharan dust observations. Single-scattering albedo values are high, broadly in line with other Saharan dust even though the diatomite composition of dust from the Bodélé is likely to be unusual. The radiative impact of high dust loadings results in a reduction in surface daytime maximum temperature of around 7°C in the Bodélé region. Using optical and physical properties of dust obtained in the field, we estimate the total dust flux emitted from the Bodélé to be 1.18 ± 0.45 Tg per day during a substantial dust event. We speculate that the Bodélé Depression ($10,800 km 2 ) may be responsible for between 6-18% of global dust emissions, although the uncertainty in both the Bodélé and global estimates remains high.
Journal of Geophysical Research, 2007
1] Dust emitted from desert regions and transported in the atmosphere has been recognized for its... more 1] Dust emitted from desert regions and transported in the atmosphere has been recognized for its potential to alter the Earth's climate and environments. Satellite data show that the largest source regions of dust (i.e., hot spots) are located in dry, nonvegetated areas of West Africa and central Chad. Dust emissions from these sources follow a distinct seasonal cycle. Whereas our understanding of processes controlling the dust cycle of the Chad dust source has been improved through recent studies, our understanding of the West African sources is limited because of the remoteness of the sources and lack of surface observations. Using a satellite-derived dust index and reanalysis atmospheric fields, we show that the annual dust cycle at the West African dust hot spots is not related to changes in mean surface wind strength but is linked to small-scale high-wind events. We find that the annual dust cycle correlates well with changes in near-surface convergence associated with the annual north-south movement of the Inter-Tropical Convergence Zone (ITCZ). Dust emissions in West Africa are highest in June coinciding with the crossing of the convergence zone on its northward bound over the dust hot spots. The increase in convergence leads to enhanced surface gustiness suggesting that dry convection associated with an increase in the occurrence of small-scale high-wind events and vertical velocity are the main processes controlling the annual dust cycle at the West African dust sources.
Journal of Geophysical Research, 2008
Journal of Geophysical Research, 2008
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Papers by Sebastian Engelstaedter