scholarly journals Holocene climate variability in the winter rainfall zone of South Africa

2013 ◽  
Vol 9 (5) ◽  
pp. 2347-2364 ◽  
Author(s):  
S. Weldeab ◽  
J.-B. W. Stuut ◽  
R. R. Schneider ◽  
W. Siebel
Keyword(s):  
South Africa ◽  
Surface Water ◽  
Thermal State ◽  
Causal Link ◽  
South Atlantic ◽  
Ice Age ◽  
Time Interval ◽  
Dust Deposition ◽  
The South ◽  
Winter Rainfall

Abstract. We established a multi-proxy time series comprising analyses of major elements in bulk sediments, Sr and Nd isotopes, grain size of terrigenous fraction, and δ18O and δ13C in tests of Neogloboquadrina pachyderma (sinistral) from a marine sediment sequence recovered off the Orange River. The records reveal coherent patterns of variability that reflect changes in wind strength, precipitation over the river catchments, and upwelling of cold and nutrient-rich coastal waters off western South Africa. The wettest episode of the Holocene in the winter rainfall zone (WRZ) of South Africa occurred during the "Little Ice Age" (700–100 cal years BP) most likely in response to a northward shift of the austral westerlies. Wet phases and strengthened coastal water upwellings are companied by a decrease of Agulhas water leakage into the South Atlantic and a reduced dust incursion over Antarctica, as indicated in previous studies. A continuous aridification trend in the WRZ and a weakening of the southern Benguela Upwelling System (BUS) between 9000 and 5500 cal years BP parallel with increase of dust deposition over Antarctica and an enhanced leakage of warm Agulhas water into the eastern South Atlantic. The temporal relationship between precipitation changes in the WRZ, the thermal state of the coastal surface water, and leakage of warm water in the South Atlantic, and variation of dust incursion over Antarctica suggests a causal link that most likely was related to latitudinal shifts of the Southern Hemisphere westerlies. Our results of the mid-Holocene time interval may serve as an analogue to a possible long-term consequence of the current and future southward shift of the westerlies. Furthermore, warming of the coastal surface water as a result of warm Agulhas water incursion into the southern BUS may affect coastal fog formation.

scholarly journals Holocene climate variability in the Winter Rainfall Zone of South Africa

2013 ◽  
Vol 9 (3) ◽  
pp. 2309-2356 ◽  
Author(s):  
S. Weldeab ◽  
J.-B. W. Stuut ◽  
R. R. Schneider ◽  
W. Siebel
Keyword(s):  
South Africa ◽  
Surface Water ◽  
Thermal State ◽  
Causal Link ◽  
Major Elements ◽  
Ice Age ◽  
Time Interval ◽  
Dust Deposition ◽  
Winter Rainfall ◽  
Phytoplankton Productivity

Abstract. We established a multi-proxy time series comprising analyses of major elements in bulk sediments, Sr and Nd isotopes and grain size of terrigenous fraction, and δ18O and δ13C in tests of Neogloboquadrina pachyderma (sinistral) from a marine sediment sequence recovered off the Orange River. The records reveal coherent patterns of variability that reflect changes in wind strength, precipitation over the river catchments, and upwelling of cold and nutrient-rich coastal waters off western South Africa. The wettest episode of the Holocene in the Winter Rainfall Zone (WRZ) of South Africa occurred during the "Little Ice Age" (700–100 yr BP). Wet phases were accompanied by strengthened coastal water upwellings, a decrease of Agulhas water leakage into the southern Atlantic, and a reduced dust incursion over Antarctica. A continuous aridification trend in the WRZ and a weakening of the southern Benguela Upwelling System (BUS) between 9000 and 5500 yr BP parallel with increase of dust deposition over Antarctica and an enhanced leakage of warm Agulhas water into the southeastern Atlantic. The temporal relationship between precipitation changes in the WRZ, the thermal state of the coastal surface water, and leakage of warm water in southern Atlantic, and variation of dust incursion over Antarctica suggests a causal link that most likely was related to latitudinal shifts of the Southern Hemisphere westerlies. Our results of the mid-Holocene time interval may serve as an analogue to a possible long-term consequence of the current and future southward shift of the westerlies that may result in a decline of rainfall over southwest Africa and a weakened upwelling with implication for phytoplankton productivity and fish stocks. Furthermore, warming of the coastal surface water as a result of warm Agulhas water incursion into the southern BUS may affect coastal fog formation that is critical as moisture source for the endemic flora of the Namaqualand.

scholarly journals Holocene climate variability in the winter rainfall zone of South Africa

2012 ◽  
Vol 8 (3) ◽  
pp. 2281-2320 ◽  
Author(s):  
S. Weldeab ◽  
J.-B. W. Stuut ◽  
R. R. Schneider ◽  
W. Siebel
Keyword(s):  
South Africa ◽  
Surface Water ◽  
Thermal State ◽  
Causal Link ◽  
Major Elements ◽  
Ice Age ◽  
Time Interval ◽  
Water Leakage ◽  
Winter Rainfall ◽  
Phytoplankton Productivity

Abstract. We established a multi-proxy time series comprising analyses of major elements in bulk sediments, Sr and Nd isotopes, grain size of terrigenous fraction, and δ18O and δ13C in tests of Neogloboquadrina pachyderma (sinistral) from a marine sediment sequence recovered off the Orange River. The records reveal coherent patterns of variability that reflect changes in wind strength, precipitation over the river catchments, and upwelling of cold and nutrient-rich coastal waters off Western South Africa. The wettest episode of the Holocene in the Winter Rainfall Zone (WRZ) of South Africa occurred during the "Little Ice Age" (700–100 yr BP). Wet phases were accompanied by strengthened coastal water upwellings, a decrease of Agulhas water leakage into the Southern Atlantic, and a reduced dust incursion over Antarctica. A continuous aridification trend in the WRZ and a weakening of the Southern Benguela Upwelling System (BUS) between 9000 and 5500 yr BP parallel with evidence of a poleward shift of the austral mid-latitude westerlies and an enhanced leakage of warm Agulhas water into the Southeastern Atlantic. The temporal relationship between precipitation changes in the WRZ, the thermal state of the coastal surface water, and variation of dust incursion over Antarctica suggests a causal link that most likely was related to latitudinal shifts of the Southern Hemisphere westerlies and changes in the amount of Agulhas water leakage into the Southern BUS. Our results of the mid-Holocene time interval may serve as an analogue to a possible long-term consequence of the current and future southward shift of the westerlies that may result in a decline of rainfall over Southwest Africa and a weakened upwelling with implication for phytoplankton productivity and fish stocks. Furthermore, warming of the coastal surface water as a result of warm Agulhas water incursion into the Southern BUS may affect coastal fog formation that is critical as moisture source for the endemic flora of the Namaqualand.

A Lagrangian view of the transfer of Southern waters to the South Atlantic Ocean

2021 ◽  
Author(s):  
Anna Olivé Abelló ◽  
Josep L. Pelegrí ◽  
Ignasi Vallès-Casanova
Keyword(s):  
South Africa ◽  
Surface Water ◽  
Drake Passage ◽  
South Atlantic ◽  
Subtropical Gyre ◽  
The South ◽  
The North ◽  
Brazil Current ◽  
North Brazil ◽  
North Brazil Current

<p>The Atlantic Meridional Overturning Circulation (AMOC), a key component of the Earth's climate system, is sustained through the northward transport of Southern Ocean waters to high latitudes. This returning limb of the AMOC consists largely of relatively cold waters entering from the Pacific Ocean through the Drake Passage, what is commonly referred to as cold-water route. Here, we explore the pathways and transit times of these Antarctic waters that are incorporated to the South Atlantic, with special attention to their recirculation in the subtropical gyre and their escape northward through the North Brazil Current. For this purpose, we use daily values of the climatological GLORYS12v1 velocity field, as obtained using data for 2002-2018 and track the trajectories with the help of the OceanParcels software. We trace the particles transiting through four sections in the Southern and South Atlantic Oceans: 64°W and 27°E, crossing entire Antarctic Circumpolar Current (ACC) through the Drake Passage and off South Africa, respectively; 32°S, from the African coast out to 5°S, sampling the eastern boundary current system; and 21°S, from the American coast out to 30°W, sampling the North Brazil Current.</p><p>Particles are released daily in the Drake Passage down to 1800 m during one full year, its spatial distribution and number being proportional to the transport crossing each vertical portion of the section. This represents an annual-mean of 116.3 Sv entering the Atlantic sector through the Drake Passage, split into 13.3 Sv for surface (Subantarctic Surface Water, SASW, and Subantarctic Mode Water, SAMW), 40.2 Sv for intermediate (Antarctic Surface Water, AASW, and Antarctic Intermediate Water, AAIW) and 62.8 Sv for deep (Upper Circumpolar Deep Water, UCDW) water masses. The particles are then tracked forward, with a one-day resolution, during 20 years. The simulation shows that about 83% of the SASW/SAMW transport follow the ACC past South Africa while the remaining 17% are incorporated to the subtropical gyre. Among the latter, only 13% veer northward and cross the 21°S section. Regarding the intermediate waters, AASW/AAIW, 93% of transport follows the ACC, and 7% join the subtropical gyre. Finally, for the UCDW transport, which remains part of ACC, about 97% follow eastward as the ACC and only 3% drift cross the 32°S section, and only 4% of the latter reach through the 21°S section. The median times for the Drake Passage water particles to get to the 27°E, 32°S and 21°S sections are: 1.7, 2.1 and 5.7 yr for the SASW/SAMW; 2.3, 5.3 and 6.5 yr for the AASW/AAIW; and 3.3, 6.0 and 11.7 yr for the UCDW, respectively. Long tails in the age distributions reflect a high degree of recirculation, being remarkable the high presence of mesoscale eddies around 32°S over Cape Basin.</p>

scholarly journals Southern Hemisphere anticyclonic circulation drives oceanic and climatic conditions in late Holocene southernmost Africa

2017 ◽  
Vol 13 (6) ◽  
pp. 649-665 ◽  
Author(s):  
Annette Hahn ◽  
Enno Schefuß ◽  
Sergio Andò ◽  
Hayley C. Cawthra ◽  
Peter Frenzel ◽  
...  
Keyword(s):  
South Africa ◽  
Climatic Conditions ◽  
Sea Surface ◽  
South Coast ◽  
Ice Age ◽  
Sea Surface Temperatures ◽  
The South ◽  
Anticyclonic Circulation ◽  
Surface Temperatures ◽  
Precipitation Regimes

Abstract. Due to the high sensitivity of southern Africa to climate change, a reliable understanding of its hydrological system is crucial. Recent studies of the regional climatic system have revealed a highly complex interplay of forcing factors on precipitation regimes. This includes the influence of the tropical easterlies, the strength of the southern hemispheric westerlies as well as sea surface temperatures along the coast of the subcontinent. However, very few marine records have been available in order to study the coupling of marine and atmospheric circulation systems. Here we present results from a marine sediment core, recovered in shallow waters off the Gouritz River mouth on the south coast of South Africa. Core GeoB18308-1 allows a closer view of the last  ∼  4 kyr. Climate sensitive organic proxies, like the distribution and isotopic composition of plant-wax lipids as well as indicators for sea surface temperatures and soil input, give information on oceanographic and hydrologic changes during the recorded time period. Moreover, the micropaleontology, mineralogical and elemental composition of the sediments reflect the variability of the terrigenous input to the core site. The combination of down-core sediment signatures and a catchment-wide provenance study indicate that the Little Ice Age ( ∼  300–650 cal yr BP) was characterized by climatic conditions favorable to torrential flood events. The Medieval Climate Anomaly ( ∼  950–650 cal yr BP) is expressed by lower sea surface temperatures in the Mossel Bay area and humid conditions in the Gouritz River catchment. These new results suggest that the coincidence of humid conditions and cooler sea surface temperatures along the south coast of South Africa resulted from a strengthened and more southerly anticyclonic circulation. Most probably, the transport of moisture from the Indian Ocean by strong subtropical easterlies was coupled with Agulhas Bank upwelling pulses, which were initiated by an increase in Agulhas Current strength.

Norwegian-British-Swedish Antarctic Expedition, 1949–52

Polar Record ◽  
1953 ◽  
Vol 6 (45) ◽  
pp. 608-616 ◽  
Author(s):  
G. De Q. Robin
Keyword(s):  
South Africa ◽  
Cape Town ◽  
South Atlantic ◽  
The South ◽  
Scotia Sea ◽  
Weather Bureau ◽  
Heavy Equipment ◽  
Antarctic Expedition ◽  
Australian Department

After loading stores at Göteborg and Oslo, the expedition sailed from London on 23 November 1949 in the chartered Norwegian sealing vessel Norsel, G. Jakobsen, master. After calling at Cape Town, where P. G. Law of the Australian Department of External Affairs and J. A. King of the Union Weather Bureau of South Africa joined the vessel as observers, the Norsel headed south on 27 December to meet the Norwegian whaling factory Thorshovdi, which was carrying an advanced party of five men, with sixty dogs and some heavy equipment. An unexpectedly wide detour had to be made across the South Atlantic as the Thorshevdi was at that time in the Scotia Sea.

scholarly journals South Atlantic Surface Boundary Current System during the Last Millennium in the CESM-LME: The Medieval Climate Anomaly and Little Ice Age

Geosciences ◽  
2019 ◽  
Vol 9 (7) ◽  
pp. 299
Author(s):  
Fernanda Marcello ◽  
Ilana Wainer ◽  
Peter R. Gent ◽  
Bette L. Otto-Bliesner ◽  
Esther C. Brady
Keyword(s):  
Little Ice Age ◽  
South Atlantic ◽  
Subtropical Gyre ◽  
Ice Age ◽  
Surface Boundary ◽  
South American ◽  
Last Millennium ◽  
Medieval Climate Anomaly ◽  
The South ◽  
Gyre Circulation

Interocean waters that are carried northward through South Atlantic surface boundary currents get meridionally split between two large-scale systems when meeting the South American coast at the western subtropical portion of the basin. This distribution of the zonal flow along the coast is investigated during the Last Millennium, when natural forcing was key to establish climate variability. Of particular interest are the changes between the contrasting periods of the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). The investigation is conducted with the simulation results from the Community Earth System Model Last Millennium Ensemble (CESM-LME). It is found that the subtropical South Atlantic circulation pattern differs substantially between these natural climatic extremes, especially at the northern boundary of the subtropical gyre, where the westward-flowing southern branch of the South Equatorial Current (sSEC) bifurcates off the South American coast, originating the equatorward-flowing North Brazil Undercurrent (NBUC) and the poleward Brazil Current (BC). It is shown that during the MCA, a weaker anti-cyclonic subtropical gyre circulation took place (inferred from decreased southern sSEC and BC transports), while the equatorward transport of the Meridional Overturning Circulation return flow was increased (intensified northern sSEC and NBUC). The opposite scenario occurs during the LIA: a more vigorous subtropical gyre circulation with decreased northward transport.

scholarly journals Latitudinal distribution of <i>Trichodesmium</i> spp. and N<sub>2</sub> fixation in the Atlantic Ocean

2010 ◽  
Vol 7 (2) ◽  
pp. 2195-2225 ◽  
Author(s):  
A. Fernández ◽  
B. Mouriño-Carballido ◽  
A. Bode ◽  
M. Varela ◽  
E. Marañón
Keyword(s):  
Atlantic Ocean ◽  
N2 Fixation ◽  
Large Scale ◽  
Strong Association ◽  
Absolute Magnitude ◽  
South Atlantic ◽  
Equatorial Region ◽  
Latitudinal Distribution ◽  
Dust Deposition ◽  
The South

Abstract. We have determined the latitudinal distribution of Trichodesmium spp. abundance and community N2 fixation in the Atlantic Ocean along a meridional transect from ca. 30° N to 30° S in November–December 2007 and April–May 2008. The observations from both cruises were highly consistent in terms of absolute magnitude and latitudinal distribution, showing a strong association between Trichodesmium abundance and community N2 fixation. The highest Trichodesmium abundances (mean = 220 trichomes L−1) and community N2 fixation rates (mean = 60 μmol m−2 d−1) occurred in the Equatorial region between 5° S–15° N. In the South Atlantic gyre, Trichodesmium abundance was very low (ca. 1 trichome L−1) but N2 fixation was always measurable, averaging 3 and 10 μmol m2 d−1 in 2007 and 2008, respectively. We suggest that N2 fixation in the South Atlantic was sustained by other, presumably unicellular, diazotrophs. Comparing these distributions with the geographical pattern in atmospheric dust deposition points to iron supply as the main factor determining the large scale latitudinal variability of Trichodesmium spp. abundance and N2 fixation in the Atlantic Ocean. We observed a marked South to North decrease in surface phosphate concentration, which argues against a role for phosphorus availability in controlling the large scale distribution of N2 fixation. Scaling up from all our measurements (42 stations) results in conservative estimates for total N2 fixation of ~6 TgN yr−1 in the North Atlantic (0–40° N) and 1.2 TgN yr−1 in the South Atlantic (0–40° S).

scholarly journals Linking catchment hydrology and ocean circulation in Late Holocene southernmost Africa

2016 ◽  
Author(s):  
Annette Hahn ◽  
Enno Schefuß ◽  
Sergio Andó ◽  
Hayley C. Cawthraw ◽  
Peter Frenzel ◽  
...  
Keyword(s):  
South Africa ◽  
Ocean Circulation ◽  
Climatic Conditions ◽  
Sea Surface ◽  
Ice Age ◽  
Time Interval ◽  
Sea Surface Temperatures ◽  
Surface Temperatures ◽  
Precipitation Regimes ◽  
Climatic System

Abstract. Recent studies of the South African climatic system revealed a highly complex interplay of forcing factors on precipitation regimes. This includes the influence of the tropical easterlies, the strength of the Southern Hemispheric Westerlies as well as sea surface temperatures along the coast of the subcontinent. This study of a sediment core at the terrestrial-marine interface spanning a time interval of ~ 4 ka provides insights on the highly dynamic climatic system in southernmost South Africa. Several organic proxies sensitive to changes in climatic parameters like the distribution and isotopic composition of plant-wax lipids as well as indicators for sea surface temperatures and soil input give information on climatic changes during the investigated time period. Moreover, the micropaleontology, mineralogical and elemental composition of the sediments reflects the variability of the terrigenous input to the core site. The combination of downcore sediment signatures and a catchment-wide provenance study indicate that the Little Ice Age was characterized by relatively warm sea surface temperatures in Mossel Bay and arid climatic conditions favorable to torrential flood events sourced in the Gouritz headlands. In contrast, the so-called Medieval Climate Anomaly is expressed by humid conditions in the Gouritz River catchment with lower, but highly variable sea surface temperatures in the Mossel Bay area. The coincidence between humid climatic conditions and cooler sea surface temperatures has been attributed to a strengthened and more southerly anticyclonic circulation. In this climatic setting strong tropical easterlies supply Indian Ocean moisture to South Africa and at the same time Agulhas Bank upwelling pulses become more common due to an increase in Agulhas Current transport as well as alongshore southeasterly winds. These processes resemble the modern day oceanography in summer and can be conceptualized in a regional climate model.

A Taxonomic Revision of the Othonna bulbosa Group (Asteraceae: Senecioneae: Othonninae)

2019 ◽  
Vol 104 (4) ◽  
pp. 515-562 ◽  
Author(s):  
Simon L. Magoswana ◽  
James S. Boatwright ◽  
Anthony R. Magee ◽  
John C. Manning
Keyword(s):  
South Africa ◽  
South African ◽  
Geographical Distribution ◽  
Taxonomic Revision ◽  
Cape Floristic Region ◽  
The South ◽  
Winter Rainfall ◽  
African Species ◽  
Taxonomic Treatment ◽  
Rainfall Region

Othonna L. (Asteraceae: Senecioneae: Othonninae) is a genus of some 120 species concentrated in the Greater Cape Floristic Region (GCFR) of South Africa, with a few species extending into southern Namibia, Angola, and Zimbabwe. The South African species of Othonna were last revised more than a century ago, and many species, particularly from the southern African winter rainfall region, remain poorly understood. This study focused on the geophytic species comprising the O. bulbosa group, distinguished by their tuberous rootstock and annual, leafy, aerial stems. A comprehensive taxonomic treatment is presented, including descriptions, complete nomenclature and typification, illustrations, and geographical distribution. Twenty-five species are recognized, of which four are newly described (O. lilacina Magoswana & J. C. Manning, O. nigromontana Magoswana & J. C. Manning, O. revoluta Magoswana & J. C. Manning, and O. sinuata Magoswana & J. C. Manning), and 18 names are reduced to synonymy. The species differ in habit, shape and incision of foliage, capitulum type (radiate vs. disciform), number of involucral bracts, pappus length, and cypselae (myxogenic vs. nonmyxogenic). We place the species into four morphologically diagnosable series (series Heterophyllae Magoswana & J. C. Manning, series Disciformes Magoswana & J. C. Manning, series Perfoliatae Magoswana & J. C. Manning, and series Undulosae Magoswana & J. C. Manning) based on habit and capitulum type.

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