Centennial-scale variability of the Southern Hemisphere westerly wind belt in the eastern Pacific over the past two millennia

Abstract

Abstract. We present the first high-resolution (sub-annual) dust particle data set from West Antarctica, developed from the West Antarctic Ice Sheet (WAIS) Divide deep ice core (79.468 • S, 112.086 • W), and use it to reconstruct changes in atmospheric circulation over the past 2400 years. We find a background dust flux of ∼ 4 mg m −2 year −1 and a mode particle size of 5-8 µm diameter. Through comparing the WAIS Divide record with other Antarctic ice core particle records, we observe that coastal and lower-elevation sites have higher dust fluxes and coarser particle size distributions (PSDs) than sites on the East Antarctic plateau, suggesting input from local dust sources at these lower-elevation sites. In order to explore the use of the WAIS Divide dust PSD as a proxy for past atmospheric circulation, we make quantitative comparisons between both mid-latitude zonal wind speed and West Antarctic meridional wind speed and the dust size record, finding significant positive interannual relationships. We find that the dust PSD is related to midlatitude zonal wind speed via cyclonic activity in the Amundsen Sea region. Using our PSD record, and through comparison with spatially distributed climate reconstructions from the Southern Hemisphere (SH) middle and high latitudes, we infer that the SH westerlies occupied a more southerly position from circa 1050 to 1400 CE (Common Era), coinciding with the Medieval Climate Anomaly (MCA). Subsequently, at ca. 1430 CE, the wind belt shifted equatorward, where it remained until the mid-to-late twentieth century. We find covariability between reconstructions of El Niño-Southern Oscillation (ENSO) and the mid-latitude westerly winds in the eastern Pacific, suggesting that centennial-scale circulation changes in this region are strongly influenced by the tropical Pacific. Further, we observe increased coarse particle deposition over the past 50 years, consistent with observations that the SH westerlies have been shifting southward and intensifying in recent decades.

Surface formation, preservation, and history of low-porosity crusts at the WAIS Divide site, West Antarctica

“…Many graded beds were also present, likely indicative of changes during a specific storm event or primarily before the next storm. This was later confirmed on site with accumulation stakes and measurements following specific large storm events (see also Koffman et al, 2014;Criscitiello et al, 2014).…”

Particle shape accounts for instrumental discrepancy in ice core dust size distributions

“…Although both instruments are typically calibrated using standard spheres of known diameter, they produce substantially different size distribution results when ice core samples are measured (Ruth et al, 2003;Lambert et al, 2008Lambert et al, , 2012Koffman et al, 2014). It has been proposed that this difference is because ice core dust is generally nonspherical (Lambert et al, 2012;Potenza et al, 2016).…”

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

“…Weakening of westerly circulation over the Auckland Islands between 1000 and 900 yr BP (Turney et al, 2016b and current study) occurred a few centuries prior to weakening over Fiordland (Knudson et al, 2011), suggesting a northward migration of the SHWW at this time. This northward migration coincided with the end of the Northern Hemisphere expression of the Medieval Climate Anomaly (MCA; 1000-700 yr BP) and the beginning of the Little Ice Age (LIA; 500-100 yr BP; Masson-Delmotte et al, 2013), and is likely related to a (Shevenell et al, 2011). shift to more negative phase of the Southern Annular Mode (SAM) (Moreno et al, 2014;Koffman et al, 2014;Abram et al, 2014;Villalba et al, 1997Villalba et al, , 2012Ummenhofer et al, 2009). …”

“…Dust particles in the West Antarctic Ice Sheet (WAIS) Divide ice core (79 • S, 112 • W) record changes in mid-latitude SHWW circulation over the past 2400 years (Koffman et al, 2014). An increase in percentage of coarse particles occurs at 1850 and 1650 yr BP and more recently from 900 to 550 yr BP (similar in timing to the MCA).…”