Sub-Milankovitch Variability in the Transequatorial Atlantic Surface Circulation: A Response to Subtropical Trade Wind Forcing?

R R Schneider (Bremen University, 28334 Bremen, Germany; ph. 49-421-218-3579; fax +49-421-218-3116; Internet: rschneid@zfn.uni-bremen.de); H W Arz (Bremen University, 28334 Bremen, Germany; ph. +49-421-218- ; fax +49-421-218-311; Internet: arz@allgeo.uni-bremen.de)

In the eastern subtropical South Atlantic relative abundances of distinct planktonic foraminifera, e.g. left-coiling N. pachyderma and G. bulloides, show rapid fluctuations over the past 80 kyr BP, indicating millenial scale changes in Benguela upwelling intensity (Little et al. 1997). Periodic fluctuations at a similar sub-Milankovitch time scale are recorded also for the sea surface temperatures and the thermocline structure in the western tropical Atlantic north of the Equator, which are paralleled by terrigenous sedimentation pulses on the continental margin of northeast Brasil, indicating humid periods on the continent (Arz et al. 1998). Based on a first correlation of the two radiocarbon dated records from the Benguela and North Brasil Current regimes, periods of intense upwelling in the eastern South Atlantic match those of warm mixed-layer thickening in the western tropical North Atlantic and of increased rainfall and river runoff in northeast Brasil. This implies, in accordance to the modern Atlantic transequatorial SST dipole forced by the opposing trade-wind patterns between hemispheres, that millenial scale changes in southern hemisphere trade wind strength have contributed to changes in heat and salt flux from the South into the North Atlantic. Moreover, the co-occurence of warm and humid pulses in the western tropics with interstadials as recorded in the Greenland ice cores may indicate that sub-Milankovitch climate changes in high-latitudes resulted to some extent from climatic perturbations in low latitudes, e.g. from nonlinear reponse to precessional insolation changes (e.g. McIntyre & Molfino 1996). The link between low and high latitude climate change could have been a variable supply of warm waters to the North Atlantic, originating from periodic extensions of the warm surface water pool in the tropical ocean forced by southern hemisphere trade winds and movements in the position of the intertropical convergence zone.