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Project OC3 – Holocene changes in the position and intensity of the southern westerly wind belt
Frank Lamy, Rolf Kilian, Helge W. Arz, Jean-Pierre Francois, Jérôme Kaiser, Matthias Prange, and Tatjana Steinke
The position and intensity of the southern westerly wind belt (SWW) varies seasonally as a consequence of changes in sea surface temperature. During the austral winter, the belt expands northward and the wind intensity in the core decreases. Conversely, during the summer, the belt contracts, and the intensity within the core is strengthened. Reconstructions of the westerly winds since the last glacial maximum, however, have suggested that changes at a single site reflected shifts throughout the entire southern wind belt. Here we use sedimentological and pollen records to reconstruct precipitation patterns over the past 12,500 yr from sites along the windward side of the Andes. Precipitation at the sites, located in the present core and northern margin of the westerlies, is driven almost entirely by the wind belt, and can be used to reconstruct its intensity. Rather than varying coherently throughout the Holocene epoch, we find a distinct anti-phasing of wind strength between the core and northern margin over multi-millennial timescales. During the early Holocene, the core westerlies were strong whereas the northern margin westerlies were weak. We observe the opposite pattern in the late Holocene. As this variation resembles modern seasonal variability, we suggest that our observed changes in westerly wind strength can best be explained by variations in sea surface temperature in the eastern South Pacific Ocean.
Figure 1: Proxy records for precipitation and wind strength in the core SWW (~53°S) and at the northern SWW margin (~34°S). a) Rainfall reconstruction from Lake Aculeo. b) Clay/silt ratios from core Sk1. c) Biogenic carbonate AR from core Palm2 (reversed y-scale). d) AR of siliciclastic material at site TM1 and, e) terrestrial organic carbon (Corg_terr.) AR from core TM1. f) Corg_terr.AR from core TML1. Vertical bars indicate the occurrence and thickness (13-34 cm) of terrestrial mass flow layers. g) Relative pollen percentages of Misodendron (core TML1). h) Pollen percentage of hygrophytic taxa (core GC2). Triangles indicate radiocarbon dates and stars ash-layers. Coloured bars distinguish the early (red), middle (yellow), and late Holocene (blue) multi-millennial phases in the evolution of the SWW.
Figure 2: Compilation of paleoclimatic records to explain Holocene SWW changes. a, Summer insolation (June, 21) at 65°N. b, 231Pa/230Th record from a subtropical North Atlantic sediment core taken as a proxy for the strength of the Atlantic meridional overturning circulation. c, Composite alkenone SST index from the eastern Pacific equatorial cold tongue. d, Alkenone SST record from the Chilean margin at ~30°S. e, Alkenone SST record from the Chilean margin at ~41°S. f, Diatom assemblage-based summer SST record from the Atlantic sector of the Southern Ocean at ~53°S. g, CO2 record from the EDC ice-core. h, Summer insolation (December, 21) at 65°S. Coloured bars as in Figure 1.
Reference
Lamy, F., R. Kilian, H. W. Arz, J.-P. Francois, J. Kaiser, M. Prange, and T. Steinke (2010), Nature Geoscience 3, 695–699, doi:10.1038/ngeo959