Late Holocene intensification of the westerly winds at the subantarctic Auckland Islands (51° S), New Zealand

Imogen M Browne,Christopher M Moy,Christina R Riesselman,Lorelei G Curtin,Gary S Wilson,Andrew R Gorman,Helen L Neil

doi:10.5194/cp-13-1301-2017

Abstract

Abstract. The Southern Hemisphere westerly winds (SHWWs) play a major role in controlling wind-driven upwelling of Circumpolar Deep Water (CDW) and outgassing of CO2 in the Southern Ocean, on interannual to glacial–interglacial timescales. Despite their significance in the global carbon cycle, our understanding of millennial- and centennial-scale changes in the strength and latitudinal position of the westerlies during the Holocene (especially since 5000 yr BP) is limited by a scarcity of palaeoclimate records from comparable latitudes. Here, we reconstruct middle to late Holocene SHWW variability using a fjord sediment core collected from the subantarctic Auckland Islands (51° S, 166° E), located in the modern centre of the westerly wind belt. Changes in drainage basin response to variability in the strength of the SHWW at this latitude are interpreted from downcore variations in magnetic susceptibility (MS) and bulk organic δ13C and atomic C ∕ N, which monitor influxes of lithogenous and terrestrial vs. marine organic matter, respectively. The fjord water column response to SHWW variability is evaluated using benthic foraminifer δ18O and δ13C, both of which are influenced by the isotopic composition of shelf water masses entering the fjord. Using these data, we provide marine and terrestrial-based evidence for increased wind strength from ∼ 1600 to 900 yr BP at subantarctic latitudes that is broadly consistent with previous studies of climate-driven vegetation change at the Auckland Islands. Comparison with a SHWW reconstruction using similar proxies from Fiordland suggests a northward migration of the SHWW over New Zealand during the first half of the last millennium. Comparison with palaeoclimate and palaeoceanographic records from southern South America and West Antarctica indicates a late Holocene strengthening of the SHWW after ∼ 1600 yr BP that appears to be broadly symmetrical across the Pacific Basin. Contemporaneous increases in SHWW at localities on either side of the Pacific in the late Holocene are likely controlled atmospheric teleconnections between the low and high latitudes, and by variability in the Southern Annular Mode and El Niño–Southern Oscillation.

Highlights

The Southern Hemisphere westerly winds (SHWWs) influence mid-latitude climate and global carbon cycling on a variety of timescales
Particulate δ13C and δ15N are somewhat more positive (+ ∼ 1 ‰) during rainy and high-wind synoptic conditions (Fig. 4b) compared to times when dry/low wind velocities persist (Fig. 4a), there is no significant difference in the vertical isotopic gradient in either regime
We reconstructed past changes in the strength of the SHWW using a sediment core recovered from a silled inlet at the Auckland Islands, located in the Subantarctic Zone (SAZ) south of mainland New Zealand, within the modern SHWW core

Summary

Introduction

The Southern Hemisphere westerly winds (SHWWs) influence mid-latitude climate and global carbon cycling on a variety of timescales. The position, strength, and symmetry of the SHWW determines precipitation patterns over mid-latitude southern landmasses, and modifies upwelling of nutrient- and carbon-rich water along oceanic fronts associated with the Antarctic Circumpolar Current (ACC; Carter et al, 2009; Dinniman et al, 2012; Lovenduski and Gruber, 2005; Menviel et al, 2008). Southern Ocean upwelling releases CO2 into the atmosphere and provides nutrients to lower latitudes via northward advection of Subantarctic Mode Water (SAMW), which forms at the Subantarctic. Browne et al.: Late Holocene intensification of the westerly winds

Methods

Results

Discussion

Conclusion