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Fiedler, S., K. Schepanski, P. Knippertz, B. Heinold und I. Tegen. „How important are cyclones for emitting mineral dust aerosol in North Africa?“ Atmospheric Chemistry and Physics Discussions 13, Nr. 12 (10.12.2013): 32483–528. http://dx.doi.org/10.5194/acpd-13-32483-2013.
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Abstract. This study presents the first quantitative estimate of the mineral dust emission associated to atmospheric depressions and migrating, long-lived cyclones in North Africa. Results from a tracking algorithm are combined with dust emission flux calculations based on ERA-Interim data from the European Centre for Medium-Range Weather Forecasts for 1989–2008. The results highlight that depressions are abundant and associated with 55% of the dust emission amount annually and spatially averaged over North African dust sources. Even larger contributions to dust emission from depressions are found south of the Atlas Mountains during spring with regionally up to 90%. It is spring when the largest monthly totals of 250–380 g m−2 of dust emission occur in North Africa. The remaining months have a total dust emission smaller than 80 g m−2. In summer, depressions, particularly Saharan heat lows, coincide with up to 90% of the seasonal total dust emission over wide areas of North Africa. In contrast to depressions, migrating cyclones that live for more than two days are rare and are associated to 4% of the annual and spatial dust emission average. Migrating cyclones over North Africa occur primarily in spring north of 20° N with eastwards trajectories and typical life times of three to seven days. Regionally larger seasonal totals of dust emission are associated to cyclones with up to 25% over Libya. In summer, near-surface signatures of African Easterly Waves (AEWs) emit regionally up to 15% of the total emission. The diurnal cycle of dust emission underlines that emission associated to cyclones at mid-day is substantially larger than at night by a factor of three to five. Soil moisture weakens dust emission during cyclone passage by 10%. Despite the overall small contribution of migrating cyclones to dust emission, cyclones coincide with particularly intense dust emission events exceeding the climatological mean flux by a factor of four to eight. This implies, that both depressions and migrating, long-lived cyclones are important for dust emission in North Africa.
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Hopsch, Susanna B., Chris D. Thorncroft, Kevin Hodges und Anantha Aiyyer. „West African Storm Tracks and Their Relationship to Atlantic Tropical Cyclones“. Journal of Climate 20, Nr. 11 (01.06.2007): 2468–83. http://dx.doi.org/10.1175/jcli4139.1.
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Abstract The automatic tracking technique used by Thorncroft and Hodges has been used to identify coherent vorticity structures at 850 hPa over West Africa and the tropical Atlantic in the 40-yr ECMWF Re-Analysis. The presence of two dominant source regions, north and south of 15°N over West Africa, for storm tracks over the Atlantic was confirmed. Results show that the southern storm track provides most of the storms that reach the main development region where most tropical cyclones develop. There exists marked seasonal variability in location and intensity of the storms leaving the West African coast, which may influence the likelihood of downstream intensification and longevity. There exists considerable year-to-year variability in the number of West African storm tracks, both in numbers over the land and continuing out over the tropical Atlantic Ocean. While the low-frequency variability is well correlated with Atlantic tropical cyclone activity, West African rainfall, and SSTs, the interannual variability is found to be uncorrelated with these. In contrast, variance of the 2–6-day-filtered meridional wind, which provides a synoptic-scale measure of African easterly wave activity, shows a significant, positive correlation with tropical cyclone activity at interannual time scales.
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Chen, Tsing-Chang, Shih-Yu Wang und Adam J. Clark. „North Atlantic Hurricanes Contributed by African Easterly Waves North and South of the African Easterly Jet“. Journal of Climate 21, Nr. 24 (15.12.2008): 6767–76. http://dx.doi.org/10.1175/2008jcli2523.1.
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Abstract A majority of tropical cyclones in the North Atlantic develop from African easterly waves (AEWs), which originate along both the southern and northern flanks of the midtropospheric African easterly jet (AEWS and AEWn, respectively). The purpose of this note is to identify the contribution of AEWSs and AEWns to North Atlantic tropical cyclones that develop from AEWs. Applying a manual backtracking approach to identify the genesis locations of AEWS, it was found that the population ratio of tropical cyclones formed from AEWSs to those formed from AEWns is 1:1.2. Because the population ratio of AEWSs to AEWns is 1:2.5, the conversion rate of the former AEWS to tropical cyclones is twice as effective as the latter waves. In addition, it was found that AEWns travel farther and take longer to transform into tropical cyclones than AEWSs, which is likely because the AEWns are drier and shallower than AEWSs. An analysis of various terms in the moisture and vorticity budgets reveals that the monsoon trough over West Africa provides moisture and enhances low-level vorticity for both AEWns and AEWSs as they move off the West African coast. The monsoon trough appears to be of particular importance in supplying AEWns with enough moisture so that they have similar properties to AEWSs after they have traveled a considerable westward distance across the tropical Atlantic.
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Romem, M., B. Ziv und H. Saaroni. „Scenarios in the development of Mediterranean cyclones“. Advances in Geosciences 12 (05.07.2007): 59–65. http://dx.doi.org/10.5194/adgeo-12-59-2007.
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Abstract. The Mediterranean is one of the most cyclogenetic regions in the world. The cyclones are concentrated along its northern coasts and their tracks are oriented more or less west-east, with several secondary tracks connecting them to Europe and to North Africa. The aim of this study is to examine scenarios in the development of Mediterranean cyclones, based on five selected winter seasons (October–March). We detected the cyclones subjectively using 6-hourly Sea-Level Pressure maps, based on the NCAR/NCEP reanalysis archive. HMSO (1962) has shown that most Mediterranean cyclones (58%) enter the Mediterranean from the Atlantic Ocean (through Biscay and Gibraltar), and from the south-west, the Sahara Desert, while the rest are formed in the Mediterranean Basin itself. Our study revealed that only 13% of the cyclones entered the Mediterranean, while 87% were generated in the Mediterranean Basin. The entering cyclones originate in three different regions: the Sahara Desert (6%), the Atlantic Ocean (4%), and Western Europe (3%). The cyclones formed within the Mediterranean Basin were found to generate under the influence of external cyclonic systems, i.e. as "daughter cyclones" to "parent cyclones" or troughs. These parent systems are located in three regions: Europe (61%), North Africa and the Red Sea (34.5%) and the Mediterranean Basin itself (4.5%). The study presents scenarios in the development of Mediterranean cyclones during the winter season, emphasizing the cyclogenesis under the influence of various external forcing. The large difference with respect to the findings of HMSO (1962) is partly explained by the dominance of spring cyclones generating in the Sahara Desert, especially in April and May that were not included in our study period.
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HUDSON, D. A., und B. C. HEWITSON. „MID-LATITUDE CYCLONES SOUTH OF AFRICA IN THE GENESIS GCM“. International Journal of Climatology 17, Nr. 5 (April 1997): 459–73. http://dx.doi.org/10.1002/(sici)1097-0088(199704)17:5<459::aid-joc147>3.0.co;2-r.
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Bond, Patrick. „Blue Economy threats, contradictions and resistances seen from South Africa“. Journal of Political Ecology 26, Nr. 1 (21.07.2019): 341. http://dx.doi.org/10.2458/v26i1.23504.
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<p>South Africa hosts Africa's most advanced form of the new Blue Economy, named 'Operation Phakisa: Oceans.' In 2014, the McKinsey-designed project was formally launched by now-disgraced President Jacob Zuma with vibrant state and corporate fanfare. Financially, its most important elements were anticipated to come from corporations promoting shipping investments and port infrastructure, a new generation of offshore oil and gas extraction projects and seabed mining. However, these already conflict with underlying capitalist crisis tendencies associated with overaccumulation (overcapacity), globalization and financialization, as they played out through uneven development, commodity price volatility and excessive extraction of resources. Together this metabolic intensification of capital-nature relations can be witnessed when South Africa recently faced the Blue Economy's ecological contradictions: celebrating a massive offshore gas discovery at the same time as awareness rises about extreme coastal weather events, ocean warming and acidification (with profound threats to fast-bleaching coral reefs), sea-level rise, debilitating drought in Africa's main seaside tourist city (Cape Town), and plastic infestation of water bodies, the shoreline and vulnerable marine life. Critics of the capitalist ocean have demanded a greater state commitment to Marine Protected Areas, support for sustainable subsistence fishing and eco-tourism. But they are losing, and so more powerful resistance is needed, focusing on shifting towards post-fossil energy and transport infrastructure, agriculture and spatial planning. Given how climate change has become devastating to vulnerable coastlines – such as central Mozambique's, victim of two of the Southern Hemisphere's most intense cyclones in March-April 2019 – it is essential to better link ocean defence mechanisms to climate activism: global youth Climate Strikes and the direct action approach adopted by the likes of Dakota Access Pipe Line resistance in the US, Extinction Rebellion in Britain, and Ende Gelände in Germany. Today, as the limits to capital's crisis-displacement tactics are becoming more evident, it is the interplay of these top-down and bottom-up processes that will shape the future Blue Economy narrative, giving it either renewed legitimacy, or the kind of illegitimacy already experienced in so much South African resource-centric capitalism.</p><p><strong>Keywords: </strong>Blue Economy, capitalist crisis, Oceans Phakisa, resistance, South Africa</p>
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Ellis, Kelsey N., Jill C. Trepanier und Robert E. Hodges. „Using Synthetic Tropical Cyclones to Characterize Extreme Hurricanes Affecting Charleston, South Carolina“. Journal of Applied Meteorology and Climatology 55, Nr. 4 (April 2016): 883–92. http://dx.doi.org/10.1175/jamc-d-15-0215.1.
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AbstractThe characteristics and conditions favoring extreme hurricanes remain largely unknown because of their small number in the observational record. Synthetic tracks are capable of providing a large, representative sample of these events, which provides an opportunity to further understanding of extreme characteristics as compared with those of more common tropical cyclones. The authors compare 300 synthetic extreme (100-yr event, ≥48.9 m s−1) and 300 common (5-yr event, ≤33.6 m s−1) tropical cyclones for Charleston, South Carolina, for differences in spatial, temporal, and other characteristics. Results suggest that extreme hurricanes have a more-defined spatial and temporal behavior, generally forming off the coast of Africa and making a direct landfall at Charleston. Common tropical cyclones sometimes make prior landfalls, may approach from either the Gulf of Mexico or the Atlantic Ocean, and often decay well before reaching Charleston. They are likely to occur through much of the hurricane season, whereas extreme events are most common during a short period toward the end of August. There is no significant difference between common and extreme translational velocity at landfall. This study demonstrates the opportunity that synthetic tracks provide for understanding the rarest hurricanes and provides initial insight into those affecting Charleston.
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Majodina, M., und M. R. Jury. „Composite winter cyclones south of Africa: evolution during eastward transit over the Agulhas warm pool“. South African Journal of Marine Science 17, Nr. 1 (Juni 1996): 241–52. http://dx.doi.org/10.2989/025776196784158518.
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Pérez-Alarcón, Albenis, Rogert Sorí, José Carlos Fernández-Alvarez, Raquel Nieto und Luis Gimeno. „Moisture Sources for Tropical Cyclones Genesis in the Coast of West Africa through a Lagrangian Approach“. Environmental Sciences Proceedings 4, Nr. 1 (13.11.2020): 3. http://dx.doi.org/10.3390/ecas2020-08126.
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Atmospheric moisture transport plays an important role in the genesis of tropical cyclones (TCs). In this study, the moisture sources associated with the genesis of TCs in the tropical Atlantic Ocean near West Africa, from June to November in the period 1980–2018, were identified. To detect the location of the TCs geneses, the HURDAT2 database from the National Hurricane Center was used. Additionally, global outputs of the Lagrangian FLEXPART model were used to determine the moisture sources that provided water vapor for the genesis of TCs. This model permitted us to track backward in time the air masses from the genesis region of the TCs and identify regions where air masses uptake moisture before reach the target regions. The results reveal that 18.1% (108 TC) of the total number of TCs that formed in the North Atlantic basin were originated in the region of study. The largest frequency for the TCs geneses was observed in August and September, with each one representing approximately 45% of the total. The transport of moisture associated with the genesis of TCs mainly comes from the east of the North and South Atlantic Ocean, as well as from West Africa and the Sahel region. The patterns of moisture uptake confirmed an interhemispheric moisture transport. Finally, during the El Niño, the moisture uptake is more intense over the Atlantic Ocean close to West Africa around 15 °N of latitude, while during La Niña, the pattern is slightly weaker but covers a wider area over the Atlantic Ocean and the north of Africa.
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Zhang, L., Q. B. Li, L. T. Murray, M. Luo, H. Liu, J. H. Jiang, Y. Mao, D. Chen, M. Gao und N. Livesey. „A Tropospheric ozone maximum over the equatorial southern Indian Ocean“. Atmospheric Chemistry and Physics Discussions 12, Nr. 1 (20.01.2012): 1979–2024. http://dx.doi.org/10.5194/acpd-12-1979-2012.
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Abstract. We examine the distribution of tropical tropospheric ozone (O3) from the Microwave Limb Sounder (MLS) and the Tropospheric Emission Spectrometer (TES) by using a global three-dimensional model of tropospheric chemistry (GEOS-Chem). MLS and TES observations of tropospheric O3 during 2005 to 2009 reveal a distinct, persistent O3 maximum, both in mixing ratio and tropospheric column, in May over the Equatorial Southern Indian Ocean (ESIO). The maximum is most pronounced in 2006 and 2008 and less evident in the other three years. This feature is also consistent with the total column O3 observations from the Ozone Mapping Instrument (OMI) and the Atmospheric Infrared Sounder (AIRS). Model results reproduce the observed May O3 maximum and the associated interannual variability. The origin of the maximum reflects a complex interplay of chemical and dynamic factors. The O3 maximum is dominated by the O3 production driven by lightning nitrogen oxides (NOx) emissions, which accounts for 62% of the tropospheric column O3 in May 2006. We find the contribution from biomass burning, soil, anthropogenic and biogenic sources to the O3 maximum are rather small. The O3 productions in the lightning outflow from Central Africa and South America both peak in May and are directly responsible for the O3 maximum over the western ESIO. The lightning outflow from Equatorial Asia dominates over the eastern ESIO. The interannual variability of the O3 maximum is driven largely by the anomalous anti-cyclones over the southern Indian Ocean in May of 2006 and 2008. The lightning outflow from Central Africa and South America is effectively entrained by the anti-cyclones followed by northward transport to the ESIO.
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Zhang, L., Q. B. Li, L. T. Murray, M. Luo, H. Liu, J. H. Jiang, Y. Mao, D. Chen, M. Gao und N. Livesey. „A tropospheric ozone maximum over the equatorial Southern Indian Ocean“. Atmospheric Chemistry and Physics 12, Nr. 9 (14.05.2012): 4279–96. http://dx.doi.org/10.5194/acp-12-4279-2012.
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Abstract. We examine the distribution of tropical tropospheric ozone (O3) from the Microwave Limb Sounder (MLS) and the Tropospheric Emission Spectrometer (TES) by using a global three-dimensional model of tropospheric chemistry (GEOS-Chem). MLS and TES observations of tropospheric O3 during 2005 to 2009 reveal a distinct, persistent O3 maximum, both in mixing ratio and tropospheric column, in May over the Equatorial Southern Indian Ocean (ESIO). The maximum is most pronounced in 2006 and 2008 and less evident in the other three years. This feature is also consistent with the total column O3 observations from the Ozone Mapping Instrument (OMI) and the Atmospheric Infrared Sounder (AIRS). Model results reproduce the observed May O3 maximum and the associated interannual variability. The origin of the maximum reflects a complex interplay of chemical and dynamic factors. The O3 maximum is dominated by the O3 production driven by lightning nitrogen oxides (NOx) emissions, which accounts for 62% of the tropospheric column O3 in May 2006. We find the contribution from biomass burning, soil, anthropogenic and biogenic sources to the O3 maximum are rather small. The O3 productions in the lightning outflow from Central Africa and South America both peak in May and are directly responsible for the O3 maximum over the western ESIO. The lightning outflow from Equatorial Asia dominates over the eastern ESIO. The interannual variability of the O3 maximum is driven largely by the anomalous anti-cyclones over the southern Indian Ocean in May 2006 and 2008. The lightning outflow from Central Africa and South America is effectively entrained by the anti-cyclones followed by northward transport to the ESIO.
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Young, Ian R., Ebru Kirezci und Agustinus Ribal. „The Global Wind Resource Observed by Scatterometer“. Remote Sensing 12, Nr. 18 (09.09.2020): 2920. http://dx.doi.org/10.3390/rs12182920.
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A 27-year-long calibrated multi-mission scatterometer data set is used to determine the global basin-scale and near-coastal wind resource. In addition to mean and percentile values, the analysis also determines the global values of both 50- and 100-year return period wind speeds. The analysis clearly shows the seasonal variability of wind speeds and the differing response of the two hemispheres. The maximum wind speeds in each hemisphere are comparable but there is a much larger seasonal cycle in the northern hemisphere. As a result, the southern hemisphere has a more consistent year-round wind climate. Hence, coastal regions of southern Africa, southern Australia, New Zealand and southern South America appear particularly suited to coastal and offshore wind energy projects. The extreme value analysis shows that the highest extreme wind speeds occur in the North Atlantic Ocean with extreme wind regions concentrated along the western boundaries of the North Atlantic and North Pacific Oceans and the Indian Ocean sector of the Southern Ocean. The signature of tropical cyclones is clearly observed in each of the well-known tropical cyclone basins.
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Zuluaga, Manuel D., und Robert A. Houze. „Extreme Convection of the Near-Equatorial Americas, Africa, and Adjoining Oceans as seen by TRMM“. Monthly Weather Review 143, Nr. 1 (01.01.2015): 298–316. http://dx.doi.org/10.1175/mwr-d-14-00109.1.
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Abstract This study documents the preferred location and diurnal cycle of extreme convective storms that occur in the tropical band containing the east Pacific Ocean, Central and South America, the Atlantic Ocean, and northern Africa. Data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar show three types of convective-stratiform structures that constitute extreme convective events: deep convective cores (DCCs), wide convective cores (WCCs), and broad stratiform regions (BSRs). Interim ECMWF Re-Analysis (ERA-Interim) data show the associated synoptic environmental factors associated with the occurrence of extreme convective features. The DCC, WCC, and BSR echoes are associated with early, middle, and late stages of convective system development, respectively, and the statistics and timing of their occurrence are related to topography and life cycle behavior of the convection. Storms containing DCC occur primarily over the Sudanian savannas of Africa and near the mountains in northern South America, being diurnally controlled. Storms with WCC manifest over land, in the same regions as the DCC, but also over oceanic regions. They appear around the clock but with maximum frequency at night. They are favored in regions of midlevel synoptic-scale low pressure systems, which over the sub-Sahara are the troughs of easterly waves. Storms containing BSR maximize over oceanic regions west of Africa and South America, where they exhibit a weak diurnal cycle with a slight midmorning maximum. Off the west coast of South America, the storms with WCC and BSR have longer lifetimes enhanced by orographic lifting over the Andes. The storms with BSR in the east Pacific Ocean often develop into tropical cyclones.
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LaCasce, J. H., und P. E. Isachsen. „On Sverdrup Discontinuities and Vortices in the Southwest Indian Ocean“. Journal of Physical Oceanography 37, Nr. 12 (01.12.2007): 2940–50. http://dx.doi.org/10.1175/2007jpo3652.1.
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Abstract The southwest Indian Ocean is distinguished by discontinuities in the wind-driven Sverdrup circulation. These connect the northern and southern tips of Madagascar with Africa and the southern tip of Africa with South America. In an analytical barotropic model with a flat bottom, the discontinuities produce intense westward jets. Those off the northern tip of Madagascar and the southern tip of Africa are always present, while the strength of that off southern Madagascar depends on the position of the zero curl line in the Indian Ocean (the jet is strong if the line intersects Madagascar but weak if the line is north of the island). All three jets are barotropically unstable by the Rayleigh–Kuo criterion. The authors studied the development of the instability using a primitive equation model, with a flat bottom and realistic coastlines. The model produced westward jets at the three sites and these became unstable after several weeks, generating 200–300-km scale eddies. The eddies generated west of Madagascar are in accord with observations and with previous numerical studies. The model’s Agulhas eddies are similar in size to the observed eddies, both the anticyclonic rings and the cyclones that form to the west of the tip of South Africa. However, the model’s Agulhas does not retroflect, most likely because of its lack of stratification and topography, and so cannot capture pinching-off events. It is noteworthy nevertheless that a retroflection is not required to produce eddies here.
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Núñez Ocasio, Kelly M., Jenni L. Evans und George S. Young. „A Wave-Relative Framework Analysis of AEW–MCS Interactions Leading to Tropical Cyclogenesis“. Monthly Weather Review 148, Nr. 11 (November 2020): 4657–71. http://dx.doi.org/10.1175/mwr-d-20-0152.1.
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AbstractAn African easterly wave (AEW) and associated mesoscale convective systems (MCSs) dataset has been created and used to evaluate the propagation of MCSs, AEWs, and, especially, the propagation of MCSs relative to the AEW with which they are associated (i.e., wave-relative framework). The thermodynamic characteristics of AEW–MCS systems are also analyzed. The analysis is done for both AEW–MCS systems that develop into tropical cyclones and those that do not to quantify significant differences. It is shown that developing AEWs over West Africa are associated with a larger number of convective cloud clusters (CCCs; squall-line-type systems) than nondeveloping AEWs. The MCSs of developing AEWs propagate at the same speed of the AEW trough in addition to being in phase with the trough, whereas convection associated with nondeveloping AEWs over West Africa moves faster than the trough and is positioned south of it. These differences become important for the intensification of the AEW vortex as this slower-moving convection (i.e., moving at the same speed of the AEW trough) spends more time supplying moisture and latent heat to the AEW vortex, supporting its further intensification. An analysis of the rainfall rate (MCS intensity), MCS area, and latent heating rate contribution reveals that there are statistically significant differences between developing AEWs and nondeveloping AEWs, especially over West Africa where the fraction of extremely large MCS areas associated with developing AEWs is larger than for nondeveloping AEWs.
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Wiston, Modise, und Kgakgamatso Marvel Mphale. „Mesoscale Convective Systems: A Case Scenario of the ‘Heavy Rainfall’ Event of 15–20 January 2013 over Southern Africa“. Climate 7, Nr. 6 (28.05.2019): 73. http://dx.doi.org/10.3390/cli7060073.
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Southern east Africa is prone to some extreme weather events and interannual variability of the hydrological cycle, including tropical cyclones and heavy rainfall events. Most of these events occur during austral summer and are linked to shifts in the intertropical convergence zone, changes in El Niño Southern Oscillation signatures, sea surface temperature and sea level pressure. A typical example include mesoscale convective systems (MCSs) that occur between October and March along the eastern part, adjacent to the warm waters of Mozambique Channel and Agulhas Current. In this study we discuss a heavy rainfall event over southern Africa, focusing particularly on the period 15–20 January 2013, the period during which MCSs were significant over the subcontinent. This event recorded one of the historic rainfalls due to extreme flooding and overflows, loss of lives and destruction of economic and social infrastructure. An active South Indian Convergence Zone was associated with the rainfall event sustained by a low-level trough linked to a Southern Hemisphere planetary wave pattern and an upper-level ridge over land. In addition, also noteworthy is a seemingly strong connection to the strength of the African Easterly Jet stream. Using rainfall data, satellite imagery and re-analysis (model processed data combined with observations) data, our analysis indicates that there was a substantial relation between rainfall totals recorded/observed and the presence of MCSs. The low-level trough and upper-level ridge contributed to moisture convergence, particularly from tropical South East Atlantic Ocean, which in turn contributed to the prolonged life span of the rainfall event. Positive temperature anomalies favored the substantial contribution of moisture fluxes from the Atlantic Ocean. This study provides a contextual assessment of rainfall processes and insight into the physical control mechanisms and feedback of large-scale convective interactions over tropical southern Africa.
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Bovalo, C., C. Barthe und N. Bègue. „A lightning climatology of the South-West Indian Ocean“. Natural Hazards and Earth System Sciences 12, Nr. 8 (22.08.2012): 2659–70. http://dx.doi.org/10.5194/nhess-12-2659-2012.
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Abstract. The World Wide Lightning Location Network (WWLLN) data have been used to perform a lightning climatology in the South-West Indian Ocean (SWIO) region from 2005 to 2011. Maxima of lightning activity were found in the Maritime Continent and southwest of Sri Lanka (>50 fl km−2 yr−1) but also over Madagascar and above the Great Lakes of East Africa (>10–20 fl km−2 yr−1). Lightning flashes within tropical storms and tropical cyclones represent 50 % to 100 % of the total lightning activity in some oceanic areas of the SWIO (between 10° S and 20° S). The SWIO is characterized by a wet season (November to April) and a dry season (May to October). As one could expect, lightning activity is more intense during the wet season as the Inter Tropical Convergence Zone (ITCZ) is present over all the basin. Flash density is higher over land in November–December–January with values reaching 3–4 fl km−2 yr−1 over Madagascar. During the dry season, lightning activity is quite rare between 10° S and 25° S. The Mascarene anticyclone has more influence on the SWIO resulting in shallower convection. Lightning activity is concentrated over ocean, east of South Africa and Madagascar. A statistical analysis has shown that El Niño–Southern Oscillation mainly modulates the lightning activity up to 56.8% in the SWIO. The Indian Ocean Dipole has a significant contribution since ~49% of the variability is explained by this forcing in some regions. The Madden–Julian Oscillation did not show significative impact on the lightning activity in our study.
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Liang, Jianyu, Yongsheng Chen, Avelino F. Arellano und Abdulla Al Mamun. „Model Sensitivity Study of the Direct Radiative Impact of Saharan Dust on the Early Stage of Hurricane Earl“. Atmosphere 12, Nr. 9 (13.09.2021): 1181. http://dx.doi.org/10.3390/atmos12091181.
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Current studies report inconsistent results about the impacts of Saharan dust on the development of African Easterly Waves (AEWs), the African Easterly Jet (AEJ), and tropical cyclones (TCs). We present a modeling case study to further elucidate the direct radiative impacts of dust on the early development stage of a TC. We conducted experiments using the Weather Research and Forecasting model coupled with chemistry (WRF-Chem-V3.9.1) to simulate Hurricane Earl (2010) which was influenced by the dusty Saharan Air Layer (SAL). We used the aerosol product from ECMWF MACC-II as the initial and boundary conditions to represent aerosol distribution, along with typical model treatment of its radiative and microphysical effects in WRF. Our simulations at 36-km resolution show that, within the first 36 h, the presence of dust weakens the low-pressure system over North Africa by less than 1 hPa and reduces its mean temperature by 0.03 K. Dust weakens and intensifies the AEJ at its core and periphery, respectively, with magnitudes less than 0.2 m/s. Dust slightly shifts the position of 600 hPa AEW to the south and reduces its intensity prior to impacting the TC. Finally, TC with dust remains weaker.
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Almar, Rafael, Elodie Kestenare und Julien Boucharel. „On the key influence of remote climate variability from Tropical Cyclones, North and South Atlantic mid-latitude storms on the Senegalese coast (West Africa)“. Environmental Research Communications 1, Nr. 7 (12.07.2019): 071001. http://dx.doi.org/10.1088/2515-7620/ab2ec6.
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Kobayashi, Mutsuo, Osamu Komagata und Naoko Nihei. „Global Warming and Vector-borne Infectious Diseases“. Journal of Disaster Research 3, Nr. 2 (01.04.2008): 105–12. http://dx.doi.org/10.20965/jdr.2008.p0105.
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Vector-borne diseases result from infections transmitted to humans by blood-feeding arthropods such as mosquitoes, ticks, and fleas. Such cold-blooded animals are influenced by environmental change. A recent IPCC report clearly showed that the emission of greenhouse gases has already changed world climates. Heat waves in Europe, rises in global mean sea level, summer droughts and wild fires, more intense precipitation, and increasing numbers of large cyclones and hurricanes may be typical example of extreme climate phenomena related to global warming. High temperatures may increase survival among arthropods, depending on their vector, behavior, ecology, and valuable factors, and temperate zone warming may accelerate the spread of mosquitoes such asAedes albopictus. The MIROK (K1) Model clearly shows a northern limit forAe.albopictus, particularly in northern Honshu in 2035 and southern and middle Hokkaido Island in 2100 in Japan. The spread of the mosquito vector through global used-tire trading in recent decades to Africa, the Mideast, Europe, and North and South America caused an outbreak of Chikungunya fever in north Italy in 2007. Global warming, extreme climate change, changing physical distribution, and an increase in oversea travel are also expected to influence the epidemiology of vector-borne infectious diseases.
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Li, Xiaohui, Dongkai Yang, Jingsong Yang, Guoqi Han, Gang Zheng und Weiqiang Li. „Validation of NOAA CyGNSS Wind Speed Product with the CCMP Data“. Remote Sensing 13, Nr. 9 (07.05.2021): 1832. http://dx.doi.org/10.3390/rs13091832.
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The National Aeronautics and Space Administration (NASA) Cyclone Global Navigation Satellite System (CyGNSS) mission was launched in December 2016, which can remotely sense sea surface wind with a relatively high spatio-temporal resolution for tracking tropical cyclones. In recent years, with the gradual development of the geophysical model function (GMF) for CyGNSS wind retrieval, different versions of CyGNSS Level 2 products have been released and their performance has gradually improved. This paper presents a comprehensive evaluation of CyGNSS wind product v1.1 produced by the National Oceanic and Atmospheric Administration (NOAA). The Cross-Calibrated Multi-Platform (CCMP) analysis wind (v02.0 and v02.1 near real time) products produced by Remote Sensing Systems (RSS) were used as the reference. Data pairs between the NOAA CyGNSS and RSS CCMP products were processed and evaluated by the bias and standard deviation SD. The CyGNSS dataset covers the period between May 2017 and December 2020. The statistical comparisons show that the bias and SD of CyGNSS relative to CCMP-nonzero collocations when the flag of CCMP winds is nonzero are –0.05 m/s and 1.19 m/s, respectively. The probability density function (PDF) of the CyGNSS winds coincides with that of CCMP-nonzero. Furthermore, the average monthly bias and SD show that CyGNSS wind is consistent and reliable generally. We found that negative deviation mainly appears at high latitudes in both hemispheres. Positive deviation appears in the China Sea, the Arabian Sea, and the west of Africa and South America. Spatial–temporal analysis demonstrates the geographical anomalies in the bias and SD of the CyGNSS winds, confirming that the wind speed bias shows a temporal dependency. The verification and comparison show that the remotely sensed wind speed measurements from NOAA CyGNSS wind product v1.1 are in good agreement with CCMP winds.
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Raveh-Rubin, Shira. „Dry Intrusions: Lagrangian Climatology and Dynamical Impact on the Planetary Boundary Layer“. Journal of Climate 30, Nr. 17 (September 2017): 6661–82. http://dx.doi.org/10.1175/jcli-d-16-0782.1.
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Dry-air intrusions (DIs) are dry, deeply descending airstreams from the upper troposphere toward the planetary boundary layer (PBL). The significance of DIs spans a variety of aspects, including the interaction with convection, extratropical cyclones and fronts, the PBL, and extreme surface weather. Here, a Lagrangian definition for DI trajectories is used and applied to ECMWF interim reanalysis (ERA-Interim) data. Based on the criterion of a minimum descent of 400 hPa during 48 h, a first global Lagrangian climatology of DI trajectories is compiled for the years 1979–2014, allowing quantitative understanding of the occurrence and variability of DIs, as well as the dynamical and thermodynamical interactions that determine their impact. DIs occur mainly in winter. While traveling equatorward from 40°–50° latitude, DIs typically reach the lower troposphere (with maximum frequencies of ~10% in winter) in the storm-track regions, as well as over the Mediterranean Sea, Arabian Sea, and eastern North Pacific, off the western coast of South America, South Africa, and Australia, and across the Antarctic coast. The DI descent is nearly adiabatic, with a mean potential temperature decrease of 3 K in two days. Relative humidity drops strongly during the first descent day and increases in the second day, because of mixing into the moist PBL. Significant destabilization of the lower levels occurs beneath DIs, accompanied by increased 10-m wind gusts, intense surface heat and moisture fluxes, and elevated PBL heights. Interestingly, only 1.2% of all DIs are found to originate from the stratosphere.
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Ibebuchi, Chibuike Chiedozie. „On the Relationship between Circulation Patterns, the Southern Annular Mode, and Rainfall Variability in Western Cape“. Atmosphere 12, Nr. 6 (10.06.2021): 753. http://dx.doi.org/10.3390/atmos12060753.
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This study investigates circulation types (CTs) in Africa, south of the equator, that are related to wet and dry conditions in the Western Cape, the statistical relationship between the selected CTs and the Southern Annular Mode (SAM), and changes in the frequency of occurrence of the CTs related to the SAM under the ssp585 scenario. Obliquely rotated principal component analysis applied to sea level pressure (SLP) was used to classify CTs in Africa, south of the equator. Three CTs were found to have a high probability of being associated with wet days in the Western Cape, and four CTs were equally found to have a high probability of being associated with dry days in the Western Cape. Generally, the dry/wet CTs feature the southward/northward track of the mid-latitude cyclone, adjacent to South Africa; anti-cyclonic/cyclonic relative vorticity, and poleward/equatorward track of westerlies, south of South Africa. One of the selected wet CTs was significantly related to variations of the SAM. Years with an above-average SAM index correlated with the below-average frequency of occurrences of the wet CT. The results suggest that through the dynamics of the CT, the SAM might control the rainfall variability of the Western Cape. Under the ssp585 scenario, the analyzed climate models indicated a possible decrease in the frequency of occurrence of the aforementioned wet CT associated with cyclonic activity in the mid-latitudes, and an increase in the frequency of the occurrence of CT associated with enhanced SLP at mid-latitudes.
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Tsvieli, Y., und A. Zangvil. „Synoptic climatological analysis of Red Sea Trough and non-Red Sea Trough rain situations over Israel“. Advances in Geosciences 12 (15.10.2007): 137–43. http://dx.doi.org/10.5194/adgeo-12-137-2007.
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Abstract. Winter (October to April) rainfall in Israel is caused mostly by migrating Mediterranean cyclones but certain rain situations are accompanied by a low pressure trough extending northward from the southern Red Sea towards the Eastern Mediterranean (EM) and the Levant. This system, the Red Sea Trough (RST) is one of the most frequent surface atmospheric circulation patterns over the southeastern Mediterranean, but most of the RSTs are not accompanied by rain. This paper presents a synoptic climatological comparative analysis between rain situations associated with RSTs, and those not connected with RSTs (non-RST rain, NRR). The RST situations were identified objectively with the aid of a new algorithm applied to the GEOS-1 reanalysis gridded data set from NASA, for the period of March 1985–November 1995. Results show that RST rain is accompanied by a relatively narrow 500-hPa trough, located west of the Israeli coast-line and characterized by a deep southward penetration, while Non-RST-associated rain (NRR) is accompanied by a wider upper trough, located over the Israeli coast-line with a shallower southward penetration. We found a south-southwesterly wind vector anomaly at 200 hPa over Israel during RST rains, while during NRRs a similar wind vector anomaly pattern is observed east of Israel. There is a divergence center over, or a few degrees east of Israel during RST rains, while NRR is associated with a divergence value of nearly zero over Israel and a maximum divergence center located east of Israel. The moisture flux during NRR at 700 and 900 hPa is from the Mediterranean, while during RST rain there is a south-westerly moisture flux at 700 hPa from equatorial Africa to Israel and vicinity. A steeper temperature lapse-rate between 950–500 hPa was found during RST rain compared with NRR, resulting from a combination of cooling aloft together with heating near the surface.
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Ionita, Monica, Viorica Nagavciuc und Bin Guan. „Rivers in the sky, flooding on the ground: the role of atmospheric rivers in inland flooding in central Europe“. Hydrology and Earth System Sciences 24, Nr. 11 (05.11.2020): 5125–47. http://dx.doi.org/10.5194/hess-24-5125-2020.
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Abstract. The role of large-scale atmospheric circulation and atmospheric rivers (ARs) in producing extreme flooding and heavy rainfall events in the lower part of the Rhine catchment area is examined in this study. Analysis of the largest 10 floods in the lower Rhine, between 1817 and 2015, shows that all these extreme flood peaks have been preceded up to 7 d in advance by intense moisture transport from the tropical North Atlantic basin in the form of narrow bands also known as atmospheric rivers. Most of the ARs associated with these flood events are embedded in the trailing fronts of the extratropical cyclones. The typical large-scale atmospheric circulation leading to heavy rainfall and flooding in the lower Rhine is characterized by a low pressure center south of Greenland, which migrates toward Europe, and a stable high pressure center over the northern part of Africa and the southern part of Europe and projects on the positive phase of the North Atlantic Oscillation. On the days preceding the flood peaks, lower (upper) level convergence (divergence) is observed over the analyzed region, which indicates strong vertical motions and heavy rainfall. Vertically integrated water vapor transport (IVT) exceeds 600 kg m−1 s−1 for the largest floods, marking these as very strong ARs. The results presented in this study offer new insights regarding the importance of moisture transport as a driver of extreme flooding in the lower part of the Rhine catchment area, and we show, for the first time, that ARs are a useful tool for the identification of potentially damaging floods in inland Europe.
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Wei, Ho-Hsuan, und Simona Bordoni. „On the Role of the African Topography in the South Asian Monsoon“. Journal of the Atmospheric Sciences 73, Nr. 8 (25.07.2016): 3197–212. http://dx.doi.org/10.1175/jas-d-15-0182.1.
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Abstract The Somali jet, a strong low-level cross-equatorial flow concentrated in a narrow longitudinal band near the coast of Somalia, is a key feature of the South Asian monsoon (SAM) circulation. Previous work has emphasized the role of the East African highlands in strengthening and concentrating the jet. However, the fundamental dynamics of the jet remains debated, as does its relation to the SAM precipitation. In this study, numerical experiments with modified topography over Africa are conducted with the GFDL atmospheric model, version 2.1 (AM2.1), general circulation model (GCM) to examine the influence of topography on the Somali jet and the SAM precipitation. It is found that when the African topography is removed, the SAM precipitation moderately increases in spite of a weakening of the cross-equatorial Somali jet. The counterintuitive precipitation increase is related to lower-level cyclonic wind anomalies, and associated meridional moisture convergence, which develop over the Arabian Sea in the absence of the African topography. Potential vorticity (PV) budget analyses along particle trajectories show that this cyclonic anomaly primarily arises because, in the absence of the blocking effect by the African topography and with weaker cross-equatorial flow, air particles originate from higher latitudes with larger background planetary vorticity and thus larger PV.
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Schutte, Quintin, Leon Vivier und Digby P. Cyrus. „Changes in the fish community of the St Lucia estuarine system (South Africa) following Cyclone Gamede, an episodic cyclonic event“. Estuarine, Coastal and Shelf Science 243 (September 2020): 106855. http://dx.doi.org/10.1016/j.ecss.2020.106855.
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Wood, Alan R., und J. Ginns. „A new dieback disease of Acacia cyclops in South Africa caused by Pseudolagarobasidium acaciicola sp.nov.“ Canadian Journal of Botany 84, Nr. 5 (Mai 2006): 750–58. http://dx.doi.org/10.1139/b06-032.
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Acacia cyclops A. Cunn. ex G. Don (Fabaceae, Mimosoideae), originating from Western Australia, is a serious environmental weed in South Africa. A dieback disease of A. cyclops occurring in South Africa is described, and a fungus herein named Psuedolagarobasidium acaciicola Ginns sp.nov. (Basidiomycetes, Polyporales, Hyphodermataceae) was consistently isolated from diseased roots of A. cyclops trees showing early dieback symptoms. Isolates of P. acaciicola caused 100% mortality in pathogenicity screening tests using seedlings of A. cyclops. Saplings of A. cyclops were inoculated with one isolate of P. acaciicola, and all plants were killed within 2–3 months. No control plants died. Psuedolagarobasidium acaciicola was reisolated from all inoculated plants that died. Trees growing in the field were inoculated at two sites on two occasions. Most inoculated trees died within 2 years, whereas no control plants died. Eventually P. acaciicola fruited in vitro and in vivo allowing the basidiomes and cultures to be described. This fungus has potential to be developed as a bioherbicide to aid in the control of this serious environmental weed.
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Braun, Scott A. „Reevaluating the Role of the Saharan Air Layer in Atlantic Tropical Cyclogenesis and Evolution“. Monthly Weather Review 138, Nr. 6 (01.06.2010): 2007–37. http://dx.doi.org/10.1175/2009mwr3135.1.
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Abstract 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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Scheinert, Josh. „Refusal to Save Lives: A Perspective from International Criminal Law“. International Criminal Law Review 13, Nr. 3 (2013): 627–63. http://dx.doi.org/10.1163/15718123-01303003.
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This article examines whether or not the refusal to accept life-saving humanitarian aid can qualify as a crime against humanity in international criminal law. By looking at the refusal to accept a certain HIV/AIDS drug in South Africa, and humanitarian aid in the wake of Cyclone Nargis in Burma, this article seeks to test the limits of the current understanding and conception of what a crime against humanity is. After a thorough review of the jurisprudence the article turns to apply the law to what transpired in South Africa and to Burma, and concludes that those situations do comport with the crime against humanity of ‘other inhumane acts’.
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Baker-Yeboah, S., G. R. Flierl, G. G. Sutyrin und Y. Zhang. „Transformation of an Agulhas eddy near the continental slope“. Ocean Science Discussions 6, Nr. 2 (19.08.2009): 1819–56. http://dx.doi.org/10.5194/osd-6-1819-2009.
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Abstract. The transformation of Agulhas eddies near the continental slope of South Africa and their subsequent self-propagation are analyzed in both observational data and numerical simulations. Self-propagation results from a net dipole moment of a generalized heton structure consisting of a surface-intensified anticyclonic eddy and deep cyclonic pattern. Such Agulhas vortical structures can form near the retroflection region and further north along the western coast of South Africa. We analyze nonlinear topographic wave generation, vortex deformations, and filament production as an important part in water mass exchange. Self-propagating structures provide a conduit for exchange between the deep ocean and shelf regions in the Benguela upwelling system.
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Jury, Mark R. „Environmental controls on marine productivity near Cape St. Francis, South Africa“. Ocean Science 15, Nr. 6 (02.12.2019): 1579–92. http://dx.doi.org/10.5194/os-15-1579-2019.
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Abstract. This study considers ocean–atmosphere influences on marine productivity over the shelf near Cape St. Francis, South Africa. Multiday estimates of chlorophyll fluorescence in the period 2006–2017 with an area outlined by 34.5–33.75∘ S and 24–26.5∘ E provide the basis for evaluation using data from high-resolution reanalyses. Correlations with the mean annual cycle of chlorophyll fluorescence were significant for salinity, linking marine productivity and the coastal hydrology. A strengthened Agulhas Current induces cyclonic shear that lifts water at the shelf edge. Composite high-chlorophyll-fluorescence events were dominated by a large-scale midlatitude atmospheric ridge of high pressure. The resultant easterly winds caused offshore transport and the upwelling of cool nutrient-rich water in multiday events at the beginning and end of austral summer. Environmental controls on interannual fluctuations of the commercial fishery were also explored. Southwestward currents and diminished heat fluxes favored the squid catch, while anchovy and sardine catches were linked with upper northerly wind, consistent with large-scale weather patterns that underpin coastal upwelling and river discharge. Productivity lags a few days behind cyclonic wind and current shear and the upstream coastal hydrology, which shares a common atmospheric driver.
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Ibebuchi, Chibuike Chiedozie. „Circulation pattern controls of wet days and dry days in Free State, South Africa“. Meteorology and Atmospheric Physics 133, Nr. 5 (24.07.2021): 1469–80. http://dx.doi.org/10.1007/s00703-021-00822-0.
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AbstractAtmospheric circulation is a vital process in the transport of heat, moisture, and pollutants around the globe. The variability of rainfall depends to some extent on the atmospheric circulation. This paper investigates synoptic situations in southern Africa that can be associated with wet days and dry days in Free State, South Africa, in addition to the underlying dynamics. Principal component analysis was applied to the T-mode matrix (variable is time series and observation is grid points at which the field was observed) of daily mean sea level pressure field from 1979 to 2018 in classifying the circulation patterns in southern Africa. 18 circulation types (CTs) were classified in the study region. From the linkage of the CTs to the observed rainfall data, from 11 stations in Free State, it was found that dominant austral winter and late austral autumn CTs have a higher probability of being associated with dry days in Free State. Dominant austral summer and late austral spring CTs were found to have a higher probability of being associated with wet days in Free State. Cyclonic/anti-cyclonic activity over the southwest Indian Ocean, explained to a good extent, the inter-seasonal variability of rainfall in Free State. The synoptic state associated with a stronger anti-cyclonic circulation at the western branch of the South Indian Ocean high-pressure, during austral summer, leading to enhanced low-level moisture transport by southeast winds was found to have the highest probability of being associated with above-average rainfall in most regions in Free State. On the other hand, the synoptic state associated with enhanced transport of cold dry air, by the extratropical westerlies, was found to have the highest probability of being associated with (winter) dryness in Free State.
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Francis, Diana, Clare Eayrs, Jean-Pierre Chaboureau, Thomas Mote und David M. Holland. „A meandering polar jet caused the development of a Saharan cyclone and the transport of dust toward Greenland“. Advances in Science and Research 16 (16.04.2019): 49–56. http://dx.doi.org/10.5194/asr-16-49-2019.
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Abstract. In this study, we identify a new mechanism by which dust aerosols travel over long distances across the eastern side of the North Atlantic Ocean toward the Arctic. The meandering polar jet was at the origin of both dust emission through cyclogenesis over Northwest Africa and poleward transport of the uplifted dust towards the Arctic, through cut-off circulation. The dust emission was associated with an intense Saharan cyclone that formed over Northwest Africa in early April 2011. The formation of the cyclone was caused by the intrusion into subtropics, of a high-latitude-upper-level trough, linked to the meandering polar jet. The trough initiated cyclogenesis over Northwest Africa after orographic blocking by the Anti-Atlas Mountains. The still meandering polar jet led to the formation of a cut-off low further south with which the Saharan dust-cyclone merged 2 d later and moved northward with the main stream. Beside satellite observations, a simulation at high resolution was performed using the prognostic-dust permitting model MesoNH. The total dust load carried during this event to areas located north of 40∘ N was estimated by the model to be 38 Tg and dust deposition was estimated to be 1.3 Tg. The Saharan dust reaching Greenland was accompanied by warm and moist air masses that caused a rise in surface temperature of about 10 ∘C for more than 3 consecutive days over the southeastern Greenland. Ice melt over this area of Greenland was detected in the brightness temperature observations.
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Chen, Tsing-Chang. „Characteristics of African Easterly Waves Depicted by ECMWF Reanalyses for 1991–2000“. Monthly Weather Review 134, Nr. 12 (01.12.2006): 3539–66. http://dx.doi.org/10.1175/mwr3259.1.
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Abstract Several interesting characteristics of African easterly waves (AEWs) were observed and investigated by previous studies: two separate propagation paths, genesis mechanisms, restriction of vertical development, and the interaction with the African easterly jet (AEJ). However, some aspects of these characteristics have been neglected: the contrast of the AEW population along the two propagation paths, the AEW genesis mechanism over the Saharan thermal low and the role played by the low-level North African circulation in this mechanism, the dynamical mechanism restricting the vertical development of AEWs, and the synoptic relationship and interaction between the AEJ and the AEWs along the two propagation paths. The ECMWF reanalyses for the 1991–2000 period supplemented with those of 1979 were analyzed to explore these AEW features. Major findings of this effort are the following: The population of AEWs along the propagation path north of the AEJ (AEWn) is approximately 2.5 times of that along the propagation path south of the AEJ (AEWs). The AEWn geneses primarily occur over the three convergent centers and the southwestward extension of the Saharan thermal low. Underneath the midtropospheric Saharan high, the baroclinic instability of a shallow, low static stability environment, which may be triggered by the intrusion of dry northerlies over central North Africa, leads to the AEW genesis. Continental-scale upward motion along the Saharan thermal low and the cyclonic-shear side of the AEJ maintains positive vortex stretching below the Saharan high and the western part of the Asian monsoon high. These two regions thus form a favorable environment for the development of AEWs within the near-surface troposphere along the Saharan thermal low and the midtroposphere south of the AEJ. The passage of AEWn (AEWs) across the coastal zone of West Africa is accompanied by a weak (strong) AEJ and weak (strong) Saharan high. The westward propagation and development/maintenance of the two types of AEWs are achieved through vorticity advection by the AEJ, which is the major AEW–AEJ interaction. These findings will facilitate the search for AEW dynamics and aid in assessing the impact of AEW activity on North African climate change.
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Henderson, L. „Invasive, naturalized and casual alien plants in southern Africa: a summary based on the Southern African Plant Invaders Atlas (SAPIA)“. Bothalia 37, Nr. 2 (18.08.2007): 215–48. http://dx.doi.org/10.4102/abc.v37i2.322.
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The primary objective of this publication is to provide an overview of the species identity, invasion status, geographical extent, and abundance of alien plants in South Africa, Swaziland and Lesotho, based on field records from 1979 to the end of 2000. The dataset is all the species records for the study area in the Southern African Plant Invaders Atlas (SAPIA) database during this time period. A total of 548 naturalized and casual alien plant species were catalogued and invasion was recorded almost throughout the study area. Most invasion, in terms of both species numbers and total species abundance, was recorded along the southern, southwestern and eastern coastal belts and in the adjacent interior. This area includes the whole of the Fynbos and Forest Biomes, and the moister eastern parts of the Grassland and Savanna Biomes. This study reinforces previous studies that the Fynbos Biome is the most extensively invaded vegetation type in South Africa but it also shows that parts of Savanna and Grassland are as heavily invaded as parts of the Fynbos. The Fabaceae is prominent in all biomes and Acacia with 17 listed species, accounts for a very large proportion of all invasion. Acacia mearmii was by far the most prominent invasive species in the study area, followed by A. saligna, Lantana camara, A. cyclops, Opuntia ficus-indica. Solarium mauritianum, Populus alba/xcanescens, Melia azedarach, A. dealbata and species of Prosopis.
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Ross, Robert S., und T. N. Krishnamurti. „Low-Level African Easterly Wave Activity and Its Relation to Atlantic Tropical Cyclogenesis in 2001“. Monthly Weather Review 135, Nr. 12 (01.12.2007): 3950–64. http://dx.doi.org/10.1175/2007mwr1996.1.
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Abstract This paper provides new information on the low-level (850 hPa) structure and behavior of African easterly waves (AEWs) and relates this information to previous studies. Individual AEWs that occurred during June–September of 2001 are studied by a synoptic approach that employs Hovmöller diagrams, wave track maps, and case studies. The focus is on two AEW regimes in the lower troposphere over North Africa: a dry regime to the north of the African easterly jet (AEJ) coincident with the surface position of the monsoon trough near 20°N, and a wet regime to the south of the jet coincident with the near-equatorial rainbelt near 10°N. The following issues are addressed: the origin of the waves seen in the two wave regimes, relation of the wave activity to the mean positions of the surface monsoon trough and the 600–700-hPa AEJ, collocation of the tracks of the two wave regimes off the African coast, and diversity in low-level wave behavior that includes merging, splitting, and dissipation of the cyclonic vorticity centers associated with the wave troughs. The relationship between the waves following the two tracks is examined as well as the relationship between the low-level wave activity and Atlantic tropical cyclogenesis in 2001. It is shown that the two wave regimes can interact, and that both regimes were instrumental in Atlantic tropical cyclogenesis in 2001.
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Victor, J. E., D. B. Hoare und R. A. Lubke. „Checklist of plant species of the coastal fynbos and rocky headlands, south of George, South Africa“. Bothalia 30, Nr. 1 (03.02.2000): 97–101. http://dx.doi.org/10.4102/abc.v30i1.544.
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A checklist of vascular plants and cryptograms was compiled for the fynbos and rocky headland communities of the coastal region south of George The area studied is a 12 km stretch of steep sandstone cliffs forming alternating bays and headlands situated between Glentana and Wilderness. The plant communities of the natural vegetation inhabiting the coastline are a mixture of coastal thicket, riparian thicket, fynbos and rocky headland types. The extent of natural vegetation has been reduced by the spread of agricultural land and urban development and is under further threat from the spread of naturalised alien invader species, particularly Acacia cyclops. The checklist records the occurrence of 271 taxa including 16 alien species (6% of taxa). Of the flowering plant species recorded, 6% were regional or local endemics.
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Dieng, Abdou L., Saidou M. Sall, Laurence Eymard, Marion Leduc-Leballeur und Alban Lazar. „Trains of African Easterly Waves and Their Relationship to Tropical Cyclone Genesis in the Eastern Atlantic“. Monthly Weather Review 145, Nr. 2 (Februar 2017): 599–616. http://dx.doi.org/10.1175/mwr-d-15-0277.1.
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In this study, the relationship between trains of African easterly waves (AEWs) and downstream tropical cyclogenesis is studied. Based on 19 summer seasons (July–September from 1990 to 2008) of ERA-Interim reanalysis fields and brightness temperature from the Cloud User Archive, the signature of AEW troughs and embedded convection are tracked from the West African coast to the central Atlantic. The tracked systems are separated into four groups: (i) systems originating from the north zone of the midtropospheric African easterly jet (AEJ), (ii) those coming from the south part of AEJ, (iii) systems that are associated with a downstream trough located around 2000 km westward (termed DUO systems), and (iv) those that are not associated with such a close downstream trough (termed SOLO systems). By monitoring the embedded 700-hPa-filtered relative vorticity and 850-hPa wind convergence anomaly associated with these families along their trajectories, it is shown that the DUO generally have stronger dynamical structure and statistically have a longer lifetime than the SOLO ones. It is suggested that the differences between them may be due to the presence of the previous intense downstream trough in DUO cases, enhancing the low-level convergence behind them. Moreover, a study of the relationship between system trajectories and tropical depressions occurring between the West African coast and 40°W showed that 90% of tropical depressions are identifiable from the West African coast in tracked systems, mostly in the DUO cases originating from the south zone of the AEJ.
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Henderson, L. „Invasive alien woody plants of the southern and southwestern Cape region, South Africa“. Bothalia 28, Nr. 1 (02.10.1998): 91–112. http://dx.doi.org/10.4102/abc.v28i1.624.
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The frequency and abundance of invasive alien plants were recorded along roadsides and at watercourse crossings in 82.9% (145/175) of the quarter degree squares in the study area (3I-35°S, 17-25°E and covering ± 90 000 km:). The survey yielded 102 species of which the most prominent (in order of prominence) in roadside and veld (natural and modified) habitats were: Acacia cyclops, A. saligna and A. mearnsii The most prominent species (in order of prominence) in stream- bank habitats were: A. mearnsii, A. saligna and Populus x canescens.
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Jaars, K., J. P. Beukes, P. G. van Zyl, A. D. Venter, M. Josipovic, J. J. Pienaar, V. Vakkari et al. „Ambient aromatic hydrocarbon measurements at Welgegund, South Africa“. Atmospheric Chemistry and Physics Discussions 14, Nr. 4 (17.02.2014): 4189–227. http://dx.doi.org/10.5194/acpd-14-4189-2014.
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Abstract. Aromatic hydrocarbons are associated with direct adverse human health effects and can have negative impacts on ecosystems due to their toxicity, as well as indirect negative effects through the formation of tropospheric ozone and secondary organic aerosol that affect human health, crop production and regional climate. Measurements were conducted at the Welgegund measurement station (South Africa) that is considered to be a regionally representative background site. However, the site is occasionally impacted by plumes from major anthropogenic source regions in the interior of South Africa, which include the western Bushveld Igneous Complex (e.g. platinum, base metal and ferrochrome smelters), the eastern Bushveld Igneous Complex (platinum and ferrochrome smelters), the Johannesburg–Pretoria metropolitan conurbation (>10 million people), the Vaal Triangle (e.g. petrochemical and pyrometallurgical industries), the Mpumalanga Highveld (e.g. coal-fired power plants and petrochemical industry) and also a region of anti-cyclonic recirculation of air mass over the interior of South Africa. The aromatic hydrocarbon measurements were conducted with an automated sampler on Tenax-TA and Carbopack-B adsorbent tubes with heated inlet for one year. Samples were collected twice a week for two hours during daytime and two hours during night-time. A thermal desorption unit, connected to a gas chromatograph and a mass selective detector was used for sample preparation and analysis. Results indicated that the monthly median total aromatic hydrocarbon concentrations ranged between 0.01 to 3.1 ppb. Benzene levels did not exceed local air quality standards. Toluene was the most abundant species, with an annual median concentration of 0.63 ppb. No statistically significant differences in the concentrations measured during daytime and night-time were found and no distinct seasonal patterns were observed. Air mass back trajectory analysis proved that the lack of seasonal cycles could be attributed to patterns determining the origin of the air masses sampled. Aromatic hydrocarbon concentrations were in general significantly higher in air masses that passed over anthropocentrically impacted regions. Interspecies correlations and ratios gave some indications of the possible sources for the different aromatic hydrocarbons in the source regions defined in the paper. The highest contribution of aromatic hydrocarbon concentrations to ozone formation potential was also observed in plumes passing over anthropocentrically impacted regions.
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Roberts, M. J., T. Zemlak und A. Connell. „Cyclonic eddies reveal Oegopsida squid egg balloon masses in the Agulhas Current, South Africa“. African Journal of Marine Science 33, Nr. 2 (August 2011): 239–46. http://dx.doi.org/10.2989/1814232x.2011.600294.
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Pokam, Wilfried M., Caroline L. Bain, Robin S. Chadwick, Richard Graham, Denis Jean Sonwa und Francois Mkankam Kamga. „Identification of Processes Driving Low-Level Westerlies in West Equatorial Africa“. Journal of Climate 27, Nr. 11 (29.05.2014): 4245–62. http://dx.doi.org/10.1175/jcli-d-13-00490.1.
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Abstract This paper investigates and characterizes the control mechanisms of the low-level circulation over west equatorial Africa (WEA) using four reanalysis datasets. Emphasis is placed on the contribution of the divergent and rotational circulation to the total flow. Additional focus is made on analyzing the zonal wind component, in order to gain insight into the processes that control the variability of the low-level westerlies (LLW) in the region. The results suggest that the control mechanisms differ north and south of 6°N. In the north, the LLW are primarily a rotational flow forming part of the cyclonic circulation driven primarily by the heat low of the West African monsoon system. This northern branch of the LLW is well developed from June to August and disappears in December–February. South of 6°N, the seasonal variability of the LLW is controlled by the heating contrast between cooling associated with subsidence over the ocean and heating over land regions largely south of the equator, where ascent prevails. The heating contrasts lead to a Walker-type circulation with development of LLW as its lower branch. Thus, evidence is presented that the LLW are driven by differential heating. This contrasts with the traditional conceptual view that the Saint Helena high is the primary driver of low-level circulation off the Atlantic Ocean to WEA. Forest cover in WEA may modulate the latent heating that helps to drive the differential heating and maintain the LLW, and this interaction should be the focus of further study.
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Fitchett, J., und S. Grab. „Changes in south-east African tropical cyclone landfall frequency over the past 161 years“. Quaternary International 404 (Juni 2016): 192. http://dx.doi.org/10.1016/j.quaint.2015.08.152.
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Magnusdottir, Gudrun, und Chia-Chi Wang. „Intertropical Convergence Zones during the Active Season in Daily Data“. Journal of the Atmospheric Sciences 65, Nr. 7 (01.07.2008): 2425–36. http://dx.doi.org/10.1175/2007jas2518.1.
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Abstract Synoptic-scale variability of vorticity structures in the lower troposphere of the tropics is analyzed in 23 yr of daily averaged high-resolution reanalysis data. The vorticity structures can be divided into zonally elongated vorticity strips, classified as intertropical convergence zones (ITCZs), and more localized maxima, termed westward-propagating disturbances. A composite of such variability is presented for the east to central Pacific and for the east Atlantic/Africa region, both in summer. The composite in the east Pacific is zonally elongated and ITCZ-like, propagating westward over a number of days before dissipating. The spatial structure of the vorticity strip shows the characteristic cyclonic tilt into the latitudinal direction with time that is also seen in modeling experiments. The composite over the Atlantic/Africa region shows two active regions that are correlated on synoptic time scales. The disturbances in the southern region are better developed and longer lasting, even though the time and space scales are smaller than over the east Pacific. Overall, variability over the Atlantic is consistent with variability due to African easterly waves. The double ITCZ in spring in the east Pacific is different from the few earlier studies available. It is stronger south of the equator and located at 10°S, which is farther poleward than earlier studies have indicated. The northern branch that is weak in comparison is located at 5°N. The two branches of the double ITCZ tend to appear in tandem on the 2-week time scale.
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Fitchett, Jennifer M., und Stefan W. Grab. „A 66-year tropical cyclone record for south-east Africa: temporal trends in a global context“. International Journal of Climatology 34, Nr. 13 (12.02.2014): 3604–15. http://dx.doi.org/10.1002/joc.3932.
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Reason, C. J. C. „Tropical cyclone Dera, the unusual 2000/01 tropical cyclone season in the South West Indian Ocean and associated rainfall anomalies over Southern Africa“. Meteorology and Atmospheric Physics 97, Nr. 1-4 (14.03.2007): 181–88. http://dx.doi.org/10.1007/s00703-006-0251-2.
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Barimalala, Rondrotiana, Ross C. Blamey, Fabien Desbiolles und Chris J. C. Reason. „Variability in the Mozambique Channel Trough and Impacts on Southeast African Rainfall“. Journal of Climate 33, Nr. 2 (15.01.2020): 749–65. http://dx.doi.org/10.1175/jcli-d-19-0267.1.
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AbstractThe Mozambique Channel trough (MCT) is a cyclonic region prominent in austral summer in the central and southern Mozambique Channel. It first becomes evident in December with a peak in strength in February when the Mozambique Channel is warmest and the Mascarene high (MH) is located farthest southeast in the Indian Ocean basin. The strength and the timing of the mean MCT are linked to that of the cross-equatorial northeasterly monsoon in the tropical western Indian Ocean, which curves as northwesterlies toward northern Madagascar. The interannual variability in the MCT is associated with moist convection over the Mozambique Channel and is modulated by the location of the warm sea surface temperatures in the south Indian Ocean. Variability of the MCT shows a strong relationship with the equatorial westerlies north of Madagascar and the latitudinal extension of the MH. Summers with strong MCT activity are characterized by a prominent cyclonic circulation over the Mozambique Channel, extending to the midlatitudes. These are favorable for the development of tropical–extratropical cloud bands over the southwestern Indian Ocean and trigger an increase in rainfall over the ocean but a decrease over the southern African mainland. Most years with a weak MCT are associated with strong positive south Indian Ocean subtropical dipole events, during which the subcontinent tends to receive more rainfall whereas Madagascar and northern Mozambique are anomalously dry.
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Ndarana, Thando, Mary-Jane Bopape, Darryn Waugh und Liesl Dyson. „The Influence of the Lower Stratosphere on Ridging Atlantic Ocean Anticyclones over South Africa“. Journal of Climate 31, Nr. 15 (August 2018): 6175–87. http://dx.doi.org/10.1175/jcli-d-17-0832.1.
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The link between Rossby wave breaking and ridging Atlantic Ocean anticyclones in the South African domain is examined using NCEP–DOE AMIP-II reanalysis data. A simple composite analysis, which used the duration of ridging events as a basis of averaging, reveals that ridging anticyclones are coupled with Rossby wave breaking at levels higher than the dynamical tropopause. Lower-stratospheric PV anomalies extend to the surface, thus coupling the ridging highs with the lower stratosphere. The anomaly extending from the 70-hPa level to the surface contributes to a southward extension of the surface negative anomaly over the Namibian coast, which induces a cyclonic flow, causing the ridging anticyclone to take a bean-like shape. The surface positive anomaly induces the internal anticyclonic flow within the large-scale pressure system, causing the ridging end to break off and amalgamate with the Indian Ocean high pressure system. Lower-stratospheric Rossby wave breaking lasts for as long as the ridging process, suggesting that the former is critical to the longevity of the latter by maintaining and keeping the vertical coupling intact.
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Pretorius, I., und H. Rautenbach. „A Long-term synoptic-scale climate study over Mariepskop, Mpumalanga, South Africa“. Clean Air Journal 22, Nr. 2 (03.12.2012): 2–6. http://dx.doi.org/10.17159/caj/2012/22/2.7076.
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Mariepskop forms part of the northernmost edge of the Drakensberg Mountain range and is known for its complex topography associated with meso-scale atmospheric circulation, and therefore its numerous climatic zones. The mountain hosts a high biodiversity. The peak of Mariepskop lies at approximately 1900m Above Mean Sea Level (AMSL), which is higher than the surrounding escarpment to the east. Its foothills also extend well into the Lowveld at about 700 m AMSL. Mariepskop is therefore ideal for studying airflow exchange between the industrial Highveld and the Lowveld with its diversity of natural resources. It is also ideal for detecting global warming signals on altitudinal gradients extending from the Lowveld to altitudes above the Highveld escarpment. In this study long-term National Centre for Atmospheric Research / National Centre for Environmental Prediction (NCAR/NCEP) wind data at two atmospheric pressure levels (850 and 700 hPa), as well as near-surface temperature data, were obtained for the Mariepskop region for the summer (December-January-February: DJF (1981-2011 )) and winter (June-July-August: JJA (1980-2012)) seasons. The data was used to study synoptic wind flow across the mountain in the upper (700 hPa) and lower (800 hPa) atmosphere, as well as near-surface temperature gradients. During the summer season, east-south-easterly and south-easterly winds were found to be the most prominent. These winds are commonly associated with both continental and ridging anticyclonic conditions. During winter, the predominant wind direction at 850 hPa is south-easterly, which is also due to the influence of ridging anticyclones, while at 700 hPa the dominant winter wind direction becomes west-south-west, which is due to the more frequent eastward passing of cyclonic frontal systems across the Highveld towards the Lowveld. Long-term near-surface temperatures exhibit a weak increasing linear temperature trend for both seasons, which might be due to global warming.