Have the Southern Westerlies changed in a zonally symmetric ...

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M.-S. Fletcher, P.I. Moreno / Quaternary International 253 (2012) 32e46 43evident in the multi-millennial trend toward lower moisture innorthwest Patagonia (41 S), in opposition to an increase in moisturein central Chile (33 S) and Southwest Patagonia and Tierra delFuego (52 S) after 5 ka. These trends were coeval with low lakelevelsin southeast Australia (28 S) and southwest Victoria (35 S)and decreasing moisture in western Tasmania (41 S), a patterninginconsistent with the relationship between the SWW and precipitationin the modern climate of southern Australia (Figs. 1b). Southof the modern core of the SWW, a multi-millennial increase inmoisture at Lago Guanaco and Laguna Potrok Aike impliesincreasing westerly influence in that region through the lateHolocene that is consistent with arguments for increased westerlyflow after 5 ka over New Zealand (McGlone et al., 1993;Shulmeister, 1999; Shulmeister et al., 2004), yet is at odds withthe decrease in aquatic pollen content at Okarito Bog (Figs. 6) andwhile it is possible that the Okarito Bog aquatic pollen contentreflects authigenic processes on the mire surface at this time, ratherthan regional climate forcing, the complex and regionally heterogeneoustrends in moisture across the Southern Hemisphere afterw5 ka signals a departure from the marked zonal symmetry of theproceeding period (14e5 ka), implying that multi-millennialmoisture balances were modulated by factors other than zonallysymmetric westerly changes.6.2. Possible mechanisms6.2.1. Insolation-driven westerly change?Precessional-scale changes in solar insolation are the mostcommonly invoked driver of multi-millennial scale changes inatmospheric circulation, via their effect on meridional temperaturegradients (e.g. Harrison and Dodson, 1993; Dodson, 1998; Mageeet al., 2004; Cruz et al., 2005; Gilli et al., 2005a, 2005b; Donderset al., 2007), although which component of insolation drivesglobal circulation is far from clear. Palaeoenvironmental data havebeen used to support hypotheses of both inter- and intra-seasonalinsolation gradients to explain changes in the SWW over and sincethe LGM (Harrison and Dodson, 1993; Dodson et al., 1998;Shulmeister et al., 2004). Our results tentatively support thenotion of an inter-seasonal driver of multi-millennial scale changesin the position and/or intensity of the SWW over the last 14,000years, with a close relationship apparent between Decemberinsolation at 60 S (a proxy for the inter-seasonal insolationgradient; Shulmeister et al., 2004) and trends in westerly winddrivenmoisture balances between 14 and 5 ka (Fig. 6a.). Theapparent breakdown in the relationship between insolation andmoisture balances after w5 ka may reflect the dominance ofclimate modes not directly related to precessional-scale insolation,such as ENSO, the Southern Annular Mode, or the Pacific DecadalOscillation of the Pacific-South American Mode.6.2.2. A mid to late Holocene ENSO signal?Two possible explanations are proposed for the development ofregionally asymmetric trends in moisture balances of the southernlandmasses after 5 ka: (1) a breakdown in the zonal symmetry ofthe SWW, or (2) the establishment of high-frequency precipitationvariability resulting from more frequent ENSO oscillations sincew5ka (Figs. 6b; Moy et al. 2002). The geographic patterning ofmulti-millennial moisture trends after w5ka (Figs. 6 and 7) isremarkably consistent with the modern expression of ENSO in theclimate of the Southern Hemisphere (Power et al., 1999; Garreaudet al., 2009; Hill et al., 2009): the warm phase of ENSO (El Niño)results in the development of a strong high pressure cell below thesouthwest tip of southern South America that deflects the zonalwesterlies north, resulting in anomalously dry conditions innorthwest Patagonia and anomalously wet conditions in centralChile (Garreaud et al., 2009); while an anomalously high pressuresystem develops over southern Australia resulting in dry conditionsalong the eastern seaboard of Australia and all of Tasmania (Poweret al., 1999; Hill et al., 2009). Evidence for the onset of ENSO in thewestern tropical Pacific after w5ka (Shulmeister and Lees, 1995;Wanner et al., 2008) and in the eastern tropical Pacific afterw6ka(Moy et al., 2002) supports this notion of ENSO driving thebreakdown in zonal symmetry after w6e5 ka in the extra-tropicalregions analysed here.The paucity of data from southern Africa in this analysisprecludes discussion of multi-millennial drivers of climate changethrough this period in that region, although evidence indicatesenhanced climate variability in the late Holocene (Tyson et al., 2000;Lee Thorp et al., 2001). Jerardino (1995) discusses the similaritiesbetween palaeoclimate records in southern South America andsouthern Africa over the last w5000 and these findings, coupledwith evidence of an ENSO influence over modern South Africanclimate (Mason, 2001) and evidence for ENSO-like millennial-scaleclimate variability in southern Africa (Holmgren et al., 2003), altogether,suggest that ENSO may have played a part in this region also.6.3. The Southern Westerly Winds and atmospheric CO 2Toggweiler et al. (2006) proposed that the SWW play a centralrole in governing global atmospheric CO 2 content via wind-drivenupwelling and ventilation of CO 2 -rich deep waters in theSouthern Ocean, a notion that has received considerable attention(e.g. Tschumi et al., 2008; Anderson et al., 2009; Sijp and England,2009; Toggweiler, 2009; Moreno et al., 2010; Sijp et al., 2010;Fletcher and Moreno, 2011). In a recent synthesis of a number ofselected Southern Hemisphere palaeoenvironmental records,Fletcher and Moreno (2011) argue for a southward displacement ofthe SWW at w12.5 ka that initiated a phase of enhanced winddrivenoceanic upwelling at circumpolar sites located >52 S(Anderson et al., 2009), driving an increase in atmospheric CO 2 viaenhanced ventilation of the Southern Ocean (Toggweiler et al.,2006; Toggweiler, 2009). This was followed by a weakening ofwesterly flow across the entire zone of westerly influence, asindicated by a cessation of westerly driven oceanic upwelling in thesub-polar Southern Ocean (Anderson et al., 2009), a reversal of theatmospheric CO 2 trend and negative westerly derived moisturenorth (Lago Condorito) and south (Lago Guanaco) of the modernwesterly core, a notion supported by the ample evidence presentedin the present paper and by palaeoenvironmental reconstructionsfrom sub-Antarctic sites (>52 S) in New Zealand (McGlone et al.,2000, 2010), Southwest Patagonia (Fesq-Martin et al., 2004; Lamyet al. 2010), Tierra del Fuego (Heusser, 2003) and Isla de los Estados(Ponce et al., 2011) for decreased westerly flow between 11 and8 ka, and by substantial evidence that changes in SWW intensityare at least as important as latitudinal displacements for drivingCO 2 fluxes out of the deep ocean at high southern latitudes(Tschumi et al., 2008; Sijp and England, 2009; Sijp et al., 2010). Theevidence for a multi-millennial increase in SWW flow after w8kaisconsistent with the multi-millennial increase in atmospheric CO 2content (EPICA, 2004) (see Moreno et al., 2010; Fletcher andMoreno, 2011) and which appears to be unaffected by the breakdownin zonal symmetry evident after w5 ka at sites located above50 S, suggesting that the onset of ENSO variability in the tropicaland extra-tropical Pacific did not affect the SWW-Southern Oceancoupled system (sensu Fletcher and Moreno, 2011).7. ConclusionThis synthesis and analysis represents the first attempt atreconstructing changes in the globally important Southern
44M.-S. Fletcher, P.I. Moreno / Quaternary International 253 (2012) 32e46Westerly Winds (SWW) since 14 ka based on palaeoenvironmentaldata from all Southern Hemisphere landmasses. This intervalshows a clear multi-millennial zonal symmetry of SWW, which isgreatest between 14 and 5 ka, with a striking degree of synchronyand co-variability that clearly mirrors the modern relationshipbetween zonal westerly wind speed and precipitation across theSouthern Hemisphere. A breakdown in zonal symmetry coincidedwith the onset of ENSO variability after w6-5 ka, possibly reflectingthe influence of insolation-driven changes on the pole to equatorthermal gradient on atmospheric circulation. A steepened gradientat this time would have driven stronger westerly winds anda general increase in the intensity of atmospheric circulation which,in turn, may have driven changes in oceanic circulation that alteredoceanic SST and SLP gradients, triggering intensified Walker Cellcirculation and the onset of an ENSO-dominated climate. Thisinterpretation of zonally symmetric changes in the westerliesthrough much of the last 14,000 years lends significant support tonotions of a SWW driver of global atmospheric CO 2 content viawind-driven overturning and ventilation of the Southern Ocean.The data analysed here only allows hemisphere-wide comparisonat multi-millennial timescales and focus now needs to be directedat developing high-resolution and precisely dated palaeoenvironmentalrecords targeted in areas within the entire zone ofwesterly influence, north and south of w50 S, that specificallyaccounts for and addresses the heterogeneous manner in whichwesterlies impact upon the climate of southern landmasses andwhich will allow comparisons of millennial and centennial scaletrends.AcknowledgementsM.-S.F. is funded by the Institute of Ecology and Biodiversity,Chile, and Fondecyt 3110180. P.I.M. is funded by Fondecyt 1080485,1110612 and Iniciativa Científica Milenio P05-002, contract PFB-23.We would like to thank René Garreaud for discussions on themechanics of Southern Hemisphere climate. Comments on anearlier draft of the manuscript by Peter Kershaw and two anonymousreferees were much appreciated.ReferencesAbarzúa, A.M., Moreno, P.I., 2008. Changing fire regimes in the temperate rainforestregion of southern Chile over the last 16,000 yr. Quaternary Research 69 (1),62e71.Abarzúa, A.M., Villagran, C., Moreno, P.I., 2004. Deglacial and postglacial climatehistory in east-central Isla Grande de Chiloe, southern Chile (43 S). QuaternaryResearch 62, 49e59.Anderson, B., Mackintosh, A., 2006. Temperature change is the major driver of late-Glacial and Holocene glacier fluctuations in New Zealand. Geology 34 (2), 121.Anderson, R.F., Ali, S., Bratmiller, L.I., Nielsen, S.H.H., Fleisher, M.Q., Anderson, B.E.,Burkle, L.H., 2009. 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