LETTERS PUBLISHED ONLINE: 21 SEPTEMBER 2014 | DOI: 10.1038/NGEO2249 Uplift of Africa as a potential cause for Neogene intensification of the Benguela upwelling system Gerlinde Jung*, Matthias Prange and Michael Schulz The Benguela Current, located o the west coast of southern negligible Neogene uplift of the South African Plateau18. Recently, Africa, is tied to a highly productive upwelling system1. Over for East Africa, evidence emerged for a rather simultaneous the past 12 million years, the current has cooled, and upwelling beginning of uplift of the eastern and western branches around has intensified2–4. These changes have been variously linked 25 million years ago19 (Ma), in contrast to a later uplift of the to atmospheric and oceanic changes associated with the western part around 5 Ma as previously suggested20. Palaeoelevation glaciation of Antarctica and global cooling5, the closure of change estimates, for example for the Bié Plateau, during the the Central American Seaway1,6 or the further restriction of past 10 Myr range from ∼150 m (ref. 16) to 1,000 m (ref.7 ). the Indonesian Seaway3. The upwelling intensification also These discrepancies depend strongly on the methods used for occurred during a period of substantial uplift of the African the estimation of uplift, some giving more reliable estimates of continent7,8. Here we use a coupled ocean–atmosphere general the timing of uplift than of uplift rates16, whereas others are circulation model to test the eect of African uplift on Benguela better suited for estimating palaeoelevations but less accurate upwelling. In our simulations, uplift in the East African Rift in the timing7. system and in southern and southwestern Africa induces Despite these uncertainties, there is evidence for uplift processes an intensification of coastal low-level winds, which leads that coincide with the two-step development documented for to increased oceanic upwelling of cool subsurface waters. Benguela upwelling intensification at around 12–10 Ma and after We compare the eect of African uplift with the simulated 5 Ma (ref.2 ). Studies exist that indicate an intensified uplift of the impact of the Central American Seaway closure9, Indonesian South African dome in the past 10 Myr, of the Bié Plateau from 5 to Throughflow restriction10 and Antarctic glaciation11, and find 2 Ma (refs7 ,21), increasing uplift rates during the past 10 Myr for that African uplift has at least an equally strong influence as the Namibia dome7, as well as intensified rifting in the western part each of the three other factors. We therefore conclude that of the East African Rift during the late Miocene and at about 4–2 Ma African uplift was an important factor in driving the cooling and (ref. 22). strengthening of the Benguela Current and coastal upwelling A clear intercomparison of uplift history and timing of upwelling during the late Miocene and Pliocene epochs. intensification is not possible at present, owing to the controversy Today, the atmospheric Benguela jet is found at the eastern flank in timing of uplift and palaeoelevations, but also owing to the of the subtropical South Atlantic anticyclone just off the coast of much lower temporal resolution of uplift history dating compared southern Africa. This low-level wind maximum is closely connected with oceanic palaeotemperature and productivity proxy time series. to the area of maximum oceanic upwelling12 and is influenced However, considering the possible co-occurrence of uplift and by the land–sea thermal contrast and the presence of a coastal coastal upwelling intensification, we reason that uplift of Africa mountain chain. As the thermal land–sea contrast increases with was a likely factor in driving long-term changes in the Benguela strengthened upwelling, the intensity of upwelling and that of the upwelling system. Benguela jet are connected through a positive feedback12. Model- We test this hypothesis by means of a coupled general based sensitivity studies for the California jet, which shares common circulation model. Although it has been suggested that uplift altered features with the Benguela jet12, revealed a substantial alteration of atmosphere and ocean dynamics as well as coastal upwelling in other jet strength and shape due to coastal topography13. For the Benguela regions23, no modelling study has investigated the impact of uplift on region it has also been discussed that the steep slope along the the Benguela Current and upwelling system so far. coast leads not only to strengthening, but also a reduced seasonal Our simulations were performed with the Community Climate variability of the wind. This stabilizes the flow and hence favours System Model version 3 (CCSM3; ref. 24). The model simulations do oceanic upwelling1. not represent a specific time in Earth history. Instead we carried out Southern and eastern African plateaux are considered to form a series of sensitivity experiments to give an insight into processes part of the so-called African superswell14, which was emplaced in that are important for coastal upwelling intensification due to uplift. the early Miocene15 and successively lifted during the Neogene16,17 Four experiments were performed, a control run with present-day with phases of major uplift since the late Miocene7,8. The exact topography (HIGH), a simulation identical to that, except for a timing of uplift as well as regional uplift rates and estimates of lowered eastern and southern African topography to half of the palaeoelevations, however, remain controversial. For the Namibian present-day level (LOW), and two additional simulations lowering dome, a continuous uplift with linearly increasing uplift rates East (EALOW) and southern African (SALOW) topographies since the late Miocene was estimated whereas the Bié Plateau and separately (Supplementary Methods). the South African dome show maxima in uplift rates in the late According to our model simulations, the uplift of southern and Miocene/early Pliocene period7. In contrast, another study suggests eastern Africa leads to a strengthening of the horizontal wind at MARUM - Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen, 28334 Bremen, Germany. *e-mail: [email protected] NATURE GEOSCIENCE | VOL 7 | OCTOBER 2014 | www.nature.com/naturegeoscience 741 © 2014 Macmillan Publishers Limited. All rights reserved LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO2249 abLOW HIGH 500 500 600 600 700 700 800 800 Pressure (hPa) Pressure (hPa) Pressure 900 900 1,000 1,000 010203040 010203040 Longitude (° E) Longitude (° E) 0246810 12 0246810 12 Wind speed (m s−1) Wind speed (m s−1) cdLOW HIGH 500 500 600 600 700 700 800 800 Pressure (hPa) Pressure (hPa) Pressure 900 900 1,000 1,000 0 10 20 30 40 0 10 20 30 40 Longitude (° E) Longitude (° E) −6 −2 2 61014182226 −6 −2 2 61014182226 Temperature (°C) Temperature (°C) Figure 1 | Vertical cross-sections at 25◦ S of annual mean horizontal wind speed (representing the low-level Benguela jet) and air temperature. a, Horizontal wind speed for low African topography (LOW). b, Horizontal wind speed for present-day topography (HIGH). c, Temperature for low African topography (LOW). d, Temperature for present-day topography (HIGH). the eastern limb of the subtropical anticyclone with the strongest intensification and pattern change is the elevated heating source intensification occurring in the vicinity of the southwest African over land in the present-day altitude simulation (HIGH). We find coast in the region of the Benguela jet. The maximum intensi- that zonal temperature gradients near the coast increase with uplift fication is seasonally varying in magnitude (around 2.5 m s−1 to (Fig.1 c,d). Consequently, a deeper layer of baroclinity develops, more than 4 m s−1) and in location, leading to a stabilization of which strengthens the along-coast winds, similar to the mechanism the flow (Supplementary Discussion). On average it is up to 3 m s−1 discussed for the California coastal jet involving thermal wind and found at approximately 25◦ S (Fig.1 a,b). Uplift also leads to considerations13,25. Another cause for increased coastal baroclinity a slight change in wind direction with a weakening of the east- with mountain uplift is the barrier effect that leads to a mechanical erly component and hence the wind becoming more parallel to blocking of onshore intrusion of higher momentum air, as was the coast (Fig.2 ). One factor contributing to this regional wind also demonstrated for the California coastal jet25. An important 742 NATURE GEOSCIENCE | VOL 7 | OCTOBER 2014 | www.nature.com/naturegeoscience © 2014 Macmillan Publishers Limited. All rights reserved NATURE GEOSCIENCE DOI: 10.1038/NGEO2249 LETTERS abLOW HIGH−LOW 10 10 10 2 1081 20 1081 20 1084 1084 Latitude (° S) Latitude (° S) 1085 30 30 1085 1087 1087 40 40 01020 30 0102030 Longitude (° E) Longitude (° E) −1 −0.5 0 0.5 1 −0.3−0.2 −0.1 0 0.1 0.2 0.3 Ekman pumping velocity (m d−1) Δ(Ekman pumping velocity) (m d−1) cdHIGH−SALOW HIGH−EALOW 10 10 2 2 1081 20 20 1081 1084 1084 Latitude (° S) Latitude (° S) 1085 30 1085 30 1087 1087 40 40 0102030 0102030 Longitude (° E) Longitude (° E) −0.3−0.2 −0.1 0 0.1 0.2 0.3 −0.3−0.2 −0.1 0 0.1 0.2 0.3 Δ(Ekman pumping velocity) (m d−1) Δ(Ekman pumping velocity) (m d−1) 1 Figure 2 | Annual mean wind vectors at 1,000 hPa (m s− ) and Ekman pumping velocity. a, For lowered African topography (LOW). b, Eect of eastern and southern African uplift (HIGH–LOW). c, Eect of southern African uplift (HIGH–SALOW). d, Eect of eastern African uplift (HIGH–EALOW); positive values indicate a strengthening of upwelling. Note the dierent scaling of the panels. The black circles and the associated numerical identifiers indicate the positions of Ocean Drilling Program sites that show Benguela upwelling intensification during the late Neogene.