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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

Oceanic forcing of Southern African palaeorainfall.(A) Periods characterised by a low Indian Ocean Dipole Moment Index (DMI) (red, inverted central axes) correlate with periods of high rainfall in the baobab record from South Africa (blue), and (B) the mukwa tree ring record from Zimbabwe. (C) Low sea-surface temperatures in the Agulhas Current core region (purple, inverted central axes) are associated with high rainfall in the baobab record and (D) in the mukwa record. (E) High rainfall in the Pafuri area is associated with El Niño conditions over most of the ENSO record [54] (grey, left axis), but specifically in the periods AD 1301-AD 1550 and AD 1710 – AD 1970 (blue, right axis). (F) ENSO also has a significant correlation with the mukwa record from Zimbabwe (green, right axis).
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pone.0124202.g005: Oceanic forcing of Southern African palaeorainfall.(A) Periods characterised by a low Indian Ocean Dipole Moment Index (DMI) (red, inverted central axes) correlate with periods of high rainfall in the baobab record from South Africa (blue), and (B) the mukwa tree ring record from Zimbabwe. (C) Low sea-surface temperatures in the Agulhas Current core region (purple, inverted central axes) are associated with high rainfall in the baobab record and (D) in the mukwa record. (E) High rainfall in the Pafuri area is associated with El Niño conditions over most of the ENSO record [54] (grey, left axis), but specifically in the periods AD 1301-AD 1550 and AD 1710 – AD 1970 (blue, right axis). (F) ENSO also has a significant correlation with the mukwa record from Zimbabwe (green, right axis).

Mentions: The baobab δ13C record is characterized by decadal as well as centennial variability. It indicates a shift from wetter to dryer conditions around AD 1600. Maximum rainfall occurred c. AD 1075 with minima in c. AD 1635, c. AD 1695 and c. AD1805 (Fig 3). In order to elucidate the underlying cause for the decadal variability we compare the baobab biweight mean δ13C and Zimbabwe mukwa records with palaeo-proxies of three oceanic mechanisms that have been linked to recent rainfall variability. These are the ENSO phenomenon which proxies SST dynamics in the Niño3,4 region in the Pacific Ocean, the DMI phenomenon in the Indian Ocean [8] as well as the SST in the Agulhas Current core area (Fig 5A–5F). There are several palaeo-reconstructions for the ENSO phenomenon: the 1100-year high-resolution reconstruction from North American droughts [50], two tree ring reconstruction of 700 years [51] and 273 years [52], and the 458 year multi-proxy record of Braganza et al. [53]. Caution must be exercised when using these ENSO proxies as there are inconsistencies in the way in which they are calculated. The indices of Stahle and Braganza are inverted relative to the instrumental Niño 3.4 record but when this is taken into account the records all provide a reassuringly coherent record of past ENSO variability (S3 Fig). The updated record of Li et al. [54] covers most of the duration of the baobab record, and we make use of this in assessing the influence of ENSO on southern African precipitation. We make use of the DMI reconstructed from coral records for the period AD 1846–1995 [9] and the SST reconstruction from Ifaty in the Mozambique Channel [55] that is argued to represent the SST in the Agulhas Current core region [56]. Correlation statistics between the biweight mean record of baobab and these parameters are summarized in S1 Table.


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)

Oceanic forcing of Southern African palaeorainfall.(A) Periods characterised by a low Indian Ocean Dipole Moment Index (DMI) (red, inverted central axes) correlate with periods of high rainfall in the baobab record from South Africa (blue), and (B) the mukwa tree ring record from Zimbabwe. (C) Low sea-surface temperatures in the Agulhas Current core region (purple, inverted central axes) are associated with high rainfall in the baobab record and (D) in the mukwa record. (E) High rainfall in the Pafuri area is associated with El Niño conditions over most of the ENSO record [54] (grey, left axis), but specifically in the periods AD 1301-AD 1550 and AD 1710 – AD 1970 (blue, right axis). (F) ENSO also has a significant correlation with the mukwa record from Zimbabwe (green, right axis).
© Copyright Policy
Related In: Results  -  Collection

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

pone.0124202.g005: Oceanic forcing of Southern African palaeorainfall.(A) Periods characterised by a low Indian Ocean Dipole Moment Index (DMI) (red, inverted central axes) correlate with periods of high rainfall in the baobab record from South Africa (blue), and (B) the mukwa tree ring record from Zimbabwe. (C) Low sea-surface temperatures in the Agulhas Current core region (purple, inverted central axes) are associated with high rainfall in the baobab record and (D) in the mukwa record. (E) High rainfall in the Pafuri area is associated with El Niño conditions over most of the ENSO record [54] (grey, left axis), but specifically in the periods AD 1301-AD 1550 and AD 1710 – AD 1970 (blue, right axis). (F) ENSO also has a significant correlation with the mukwa record from Zimbabwe (green, right axis).
Mentions: The baobab δ13C record is characterized by decadal as well as centennial variability. It indicates a shift from wetter to dryer conditions around AD 1600. Maximum rainfall occurred c. AD 1075 with minima in c. AD 1635, c. AD 1695 and c. AD1805 (Fig 3). In order to elucidate the underlying cause for the decadal variability we compare the baobab biweight mean δ13C and Zimbabwe mukwa records with palaeo-proxies of three oceanic mechanisms that have been linked to recent rainfall variability. These are the ENSO phenomenon which proxies SST dynamics in the Niño3,4 region in the Pacific Ocean, the DMI phenomenon in the Indian Ocean [8] as well as the SST in the Agulhas Current core area (Fig 5A–5F). There are several palaeo-reconstructions for the ENSO phenomenon: the 1100-year high-resolution reconstruction from North American droughts [50], two tree ring reconstruction of 700 years [51] and 273 years [52], and the 458 year multi-proxy record of Braganza et al. [53]. Caution must be exercised when using these ENSO proxies as there are inconsistencies in the way in which they are calculated. The indices of Stahle and Braganza are inverted relative to the instrumental Niño 3.4 record but when this is taken into account the records all provide a reassuringly coherent record of past ENSO variability (S3 Fig). The updated record of Li et al. [54] covers most of the duration of the baobab record, and we make use of this in assessing the influence of ENSO on southern African precipitation. We make use of the DMI reconstructed from coral records for the period AD 1846–1995 [9] and the SST reconstruction from Ifaty in the Mozambique Channel [55] that is argued to represent the SST in the Agulhas Current core region [56]. Correlation statistics between the biweight mean record of baobab and these parameters are summarized in S1 Table.

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