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A 1000-Year Carbon Isotope Rainfall Proxy Record from South African Baobab Trees (Adansonia digitata L.).

Woodborne S, Hall G, Robertson I, Patrut A, Rouault M, Loader NJ, Hofmeyr M - PLoS ONE (2015)

Bottom Line: Periods of higher rainfall are significantly associated with lower sea-surface temperatures in the Agulhas Current core region and a negative Dipole Moment Index in the Indian Ocean.The correlation between rainfall and the El Niño/Southern Oscillation Index is non-static.The effect of both proximal and distal oceanic influences are insufficient to explain the rainfall regime shift between the Medieval Warm Period and the Little Ice Age, and the evidence suggests that this was the result of a northward shift of the subtropical westerlies rather than a southward shift of the Intertropical Convergence Zone.

View Article: PubMed Central - PubMed

Affiliation: iThemba LABS, Private Bag 11, Wits 2050, South Africa; Mammal Research Institute, University of Pretoria, Private Bag X20, Hatfield 0028, South Africa.

ABSTRACT
A proxy rainfall record for northeastern South Africa based on carbon isotope analysis of four baobab (Adansonia digitata L.) trees shows centennial and decadal scale variability over the last 1,000 years. The record is in good agreement with a 200-year tree ring record from Zimbabwe, and it indicates the existence of a rainfall dipole between the summer and winter rainfall areas of South Africa. The wettest period was c. AD 1075 in the Medieval Warm Period, and the driest periods were c. AD 1635, c. AD 1695 and c. AD1805 during the Little Ice Age. Decadal-scale variability suggests that the rainfall forcing mechanisms are a complex interaction between proximal and distal factors. Periods of higher rainfall are significantly associated with lower sea-surface temperatures in the Agulhas Current core region and a negative Dipole Moment Index in the Indian Ocean. The correlation between rainfall and the El Niño/Southern Oscillation Index is non-static. Wetter conditions are associated with predominantly El Niño conditions over most of the record, but since about AD 1970 this relationship inverted and wet conditions are currently associated with la Nina conditions. The effect of both proximal and distal oceanic influences are insufficient to explain the rainfall regime shift between the Medieval Warm Period and the Little Ice Age, and the evidence suggests that this was the result of a northward shift of the subtropical westerlies rather than a southward shift of the Intertropical Convergence Zone.

No MeSH data available.


Related in: MedlinePlus

Sample sites and austral summer rainfall density over southern Africa.The highest rainfall density is associated with the southernmost position of the intertropical convergence zone (ITCZ) (solid line) after reference [2]. The approximate position of the TTT system is within the dotted line after reference [4]. Sampling sites and reference data sites include: the baobab trees (triangles), mukwa trees from western Zimbabwe (circle), the cedar chronology (diamond), and the sea-surface temperature for the Mozambique Channel is derived from the coral isotope record from Ifaty (star).
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pone.0124202.g001: Sample sites and austral summer rainfall density over southern Africa.The highest rainfall density is associated with the southernmost position of the intertropical convergence zone (ITCZ) (solid line) after reference [2]. The approximate position of the TTT system is within the dotted line after reference [4]. Sampling sites and reference data sites include: the baobab trees (triangles), mukwa trees from western Zimbabwe (circle), the cedar chronology (diamond), and the sea-surface temperature for the Mozambique Channel is derived from the coral isotope record from Ifaty (star).

Mentions: Much of the Southern African rainfall is of convective origin forced by large-scale dynamics during the Austral summer [1]. The summer rainfall region experiences dramatic inter-annual changes leading to severe droughts or floods that affect agricultural productivity, particularly subsistence farming, as well as water reserves. The region lies to the south of the intertropical convergence zone (ITCZ) [2] and modal rainfall is derived from temperate-tropical troughs (TTT) [3]. East/west displacement of the TTT system plays an important role in modulating rainfall amount [1, 4] (Fig 1). Studies of the limited instrumental records from the region, and modeled data, suggest that the seasonal position of this climate system is influenced by remote ocean atmosphere interactions such as the El Niño/Southern Oscillation (ENSO). The relationship between ENSO and rainfall is non-linear and it has become stronger since the 1970s but prior to this correlations are poor [5, 6]. Most severe droughts occur during the mature phase of El Niño [6, 7]. The Indian Ocean Dipole Moment Index (IOD) [8] or (DMI) [9] also has an influence on rainfall. Droughts are associated with a positive phase of the IOD [10]. In addition the Southwestern Indian Ocean (Mozambique Channel and Agulhas Current system) is the main source of moisture for precipitation in southern Africa [11] and local sea surface temperatures (SST) also affects rainfall [6]. In contrast to East Africa where rainfall is strongly linked to SST in the Indian Ocean [12, 13], the correlation between rainfall in southern Africa and Southwestern Indian Ocean SST is reported to be weaker than it is with ENSO or the IOD [6].


A 1000-Year Carbon Isotope Rainfall Proxy Record from South African Baobab Trees (Adansonia digitata L.).

Woodborne S, Hall G, Robertson I, Patrut A, Rouault M, Loader NJ, Hofmeyr M - PLoS ONE (2015)

Sample sites and austral summer rainfall density over southern Africa.The highest rainfall density is associated with the southernmost position of the intertropical convergence zone (ITCZ) (solid line) after reference [2]. The approximate position of the TTT system is within the dotted line after reference [4]. Sampling sites and reference data sites include: the baobab trees (triangles), mukwa trees from western Zimbabwe (circle), the cedar chronology (diamond), and the sea-surface temperature for the Mozambique Channel is derived from the coral isotope record from Ifaty (star).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4430471&req=5

pone.0124202.g001: Sample sites and austral summer rainfall density over southern Africa.The highest rainfall density is associated with the southernmost position of the intertropical convergence zone (ITCZ) (solid line) after reference [2]. The approximate position of the TTT system is within the dotted line after reference [4]. Sampling sites and reference data sites include: the baobab trees (triangles), mukwa trees from western Zimbabwe (circle), the cedar chronology (diamond), and the sea-surface temperature for the Mozambique Channel is derived from the coral isotope record from Ifaty (star).
Mentions: Much of the Southern African rainfall is of convective origin forced by large-scale dynamics during the Austral summer [1]. The summer rainfall region experiences dramatic inter-annual changes leading to severe droughts or floods that affect agricultural productivity, particularly subsistence farming, as well as water reserves. The region lies to the south of the intertropical convergence zone (ITCZ) [2] and modal rainfall is derived from temperate-tropical troughs (TTT) [3]. East/west displacement of the TTT system plays an important role in modulating rainfall amount [1, 4] (Fig 1). Studies of the limited instrumental records from the region, and modeled data, suggest that the seasonal position of this climate system is influenced by remote ocean atmosphere interactions such as the El Niño/Southern Oscillation (ENSO). The relationship between ENSO and rainfall is non-linear and it has become stronger since the 1970s but prior to this correlations are poor [5, 6]. Most severe droughts occur during the mature phase of El Niño [6, 7]. The Indian Ocean Dipole Moment Index (IOD) [8] or (DMI) [9] also has an influence on rainfall. Droughts are associated with a positive phase of the IOD [10]. In addition the Southwestern Indian Ocean (Mozambique Channel and Agulhas Current system) is the main source of moisture for precipitation in southern Africa [11] and local sea surface temperatures (SST) also affects rainfall [6]. In contrast to East Africa where rainfall is strongly linked to SST in the Indian Ocean [12, 13], the correlation between rainfall in southern Africa and Southwestern Indian Ocean SST is reported to be weaker than it is with ENSO or the IOD [6].

Bottom Line: Periods of higher rainfall are significantly associated with lower sea-surface temperatures in the Agulhas Current core region and a negative Dipole Moment Index in the Indian Ocean.The correlation between rainfall and the El Niño/Southern Oscillation Index is non-static.The effect of both proximal and distal oceanic influences are insufficient to explain the rainfall regime shift between the Medieval Warm Period and the Little Ice Age, and the evidence suggests that this was the result of a northward shift of the subtropical westerlies rather than a southward shift of the Intertropical Convergence Zone.

View Article: PubMed Central - PubMed

Affiliation: iThemba LABS, Private Bag 11, Wits 2050, South Africa; Mammal Research Institute, University of Pretoria, Private Bag X20, Hatfield 0028, South Africa.

ABSTRACT
A proxy rainfall record for northeastern South Africa based on carbon isotope analysis of four baobab (Adansonia digitata L.) trees shows centennial and decadal scale variability over the last 1,000 years. The record is in good agreement with a 200-year tree ring record from Zimbabwe, and it indicates the existence of a rainfall dipole between the summer and winter rainfall areas of South Africa. The wettest period was c. AD 1075 in the Medieval Warm Period, and the driest periods were c. AD 1635, c. AD 1695 and c. AD1805 during the Little Ice Age. Decadal-scale variability suggests that the rainfall forcing mechanisms are a complex interaction between proximal and distal factors. Periods of higher rainfall are significantly associated with lower sea-surface temperatures in the Agulhas Current core region and a negative Dipole Moment Index in the Indian Ocean. The correlation between rainfall and the El Niño/Southern Oscillation Index is non-static. Wetter conditions are associated with predominantly El Niño conditions over most of the record, but since about AD 1970 this relationship inverted and wet conditions are currently associated with la Nina conditions. The effect of both proximal and distal oceanic influences are insufficient to explain the rainfall regime shift between the Medieval Warm Period and the Little Ice Age, and the evidence suggests that this was the result of a northward shift of the subtropical westerlies rather than a southward shift of the Intertropical Convergence Zone.

No MeSH data available.


Related in: MedlinePlus