Holocene dynamics of the Southern Hemisphere westerly winds and possible links to CO2 outgassing

Abstract

The Southern Hemisphere westerly winds (SHW) play a significant role in regulating the 15 capacity of the Southern Ocean carbon sink. They modulate upwelling of carbon-rich deep 16 water and, with sea ice, determine the ocean surface area available for air-sea gas exchange. 17 Some models suggest the current strengthening and poleward shift of the SHW will weaken 18 the carbon sink. If correct, centennial-to millennial-scale reconstructions of SHW intensity 19 should be linked with past changes in atmospheric CO 2 , temperature, and sea ice. Here, we 20 present a 12,300-year reconstruction of wind strength based on three independent proxies 21 that track inputs of sea salt aerosols and minerogenic particles accumulating in lake 22 sediments on sub-Antarctic Macquarie Island. Between c. 12.1-11.2 ka BP and since c. 7 ka 23 BP, wind intensities were above their long-term mean, and corresponded with increasing 24 atmospheric CO 2. Conversely, from c. 11.2-7.2 ka BP, wind intensities were below their long-25 term mean and corresponded with decreasing atmospheric CO 2. These observations are 26 consistent with model inferences of enhanced SHW contributing to the long-term outgassing 27 of CO 2 from the Southern Ocean. 28 29 The Southern Ocean currently accounts for 43 ± 3% of the global oceanic anthropogenic CO 2 30 uptake 1 mitigating (perhaps temporarily) the climatic effects of enhanced greenhouse gases in 31 the atmosphere. The capacity of the Southern Ocean to absorb CO 2 at the surface is 32 determined by the balance between processes sequestering carbon (e.g., diffusion and the 33 biological carbon pump) versus processes releasing old carbon from the deep ocean to the 34 atmosphere (e.g., upwelling and outgassing) 2,3. Attempts to model these processes have 35 yielded conflicting results. One model based on instrumental data collected between 1981 and 36 2004 suggested that the Southern Ocean carbon sink has weakened 4. Conversely, analyses of 37 the same data extended to 2011 suggest a reinvigoration of the carbon sink since 2002 5. This 38 latter trend is also seen in measurements of the difference between the partial pressure of CO 2 39 in ocean surface water and the overlying atmosphere (ΔpCO 2) of the Southern Ocean in Drake 40 Passage 6. Spatial extrapolation of the relatively few pCO 2 measurements from the Southern 41 Ocean as a whole suggests a trend towards a weakening sink in the 1990s, but a strengthening 42 one in the 2000s 7. 43 44 3 One of the main drivers of the Southern Ocean CO 2 sink are the Southern Hemisphere westerly 45 winds (SHW), which are strongest from 50-55°S over the Southern Ocean 8 (Fig. 1a). Changes in 46 the SHW are mainly determined by atmospheric temperature gradients, sea surface 47 temperature and regional sea ice 9. In turn they influence ocean circulation 10,11 , regulate sea 48 ice extent 12 and control the upwelling of dissolved inorganic carbon-rich deep water to 49 Antarctic surface waters 2. All are processes that modulate the net uptake of CO 2 by the ocean 50 from t...

Paleoproductivity in the SW Pacific Ocean During the Early Holocene Climatic Optimum

“…The bathymetry has not changed significantly during the Holocene; thus, Prebble et al () suggest that the expanded STFZ may have been the result of a reduction of the SWW. This has recently been supported by direct wind proxy evidence from a small lake on Macquarie Island immediately south of New Zealand, which suggested a reduction in SWW during the early Holocene (~54°S; Saunders et al, ). Alternatively, there may have been a change in the structure of the SWW (Chiang et al, , ).…”

“…Paleoceanography and Paleoclimatology south of New Zealand, which suggested a reduction in SWW during the early Holocene (~54°S; Saunders et al, 2018). Alternatively, there may have been a change in the structure of the SWW (Chiang et al, 2014(Chiang et al, , 2018.…”

Sea surface temperature in the Indian sector of the Southern Ocean over the Late Glacial and Holocene

“…Alternatively, the strength of the SWW may have influenced the SST. Reconstructions support that the SWW were weaker between 11 and 7 ka BP (Fletcher and Moreno, 2011;Saunders et al, 2018; Figure 8C) and that atmospheric circulation was more meridional (Mayewski et al, 2013), which was potentially caused by the reduced latitudinal 300 insolation and temperature gradient at this time (Divine et al, 2010;Saunders et al, 2018). The more meridional circulation may have led to a greater poleward penetration of warm air masses and therefore warming over the Southern Ocean.…”

“…It is considered that this in turn caused poleward-shifted and strengthened SWW (Denton et al, 2010;Pedro et al, 2018). Evidence from the Southern Ocean supports that the SWW were stronger over a longer interval from 14 to 11 ka BP but only shifted southwards to a more poleward position at 12.5 ka BP ( Fig 8C; Fletcher and Moreno, 2011;Saunders et al, 2018).…”

“…Greater upwelling has been shown by higher opal deposition to the south of the Polar Front in the Southern Ocean (Atlantic, Indian and Pacific sectors) through the period 12.7-11.5 ka BP, which has been attributed to increased wind-driven upwelling and a response to the reduction in sea ice extent ( Figure 8D; Dezileau et al, 2003;Anderson et al, 2009;Siani et al, 2013). Finally, the combination of greater upwelling and reduced sea ice allowed more outgassing of CO2 to the atmosphere ( Figure 8G), causing warming globally (Monnin et al, 270 2001;Anderson et al, 2009;Clark et al, 2012;Saunders et al, 2018).…”