Abrupt ice-age shifts in southern westerly winds and Antarctic climate forced from the north

Abstract

The Southern Hemisphere (SH) mid-latitude westerly winds play a central role in the global climate system via Southern Ocean upwelling 1 , carbon exchange with the deep ocean 2 , Agulhas Leakage 3 , and possibly Antarctic ice sheet stability 4. Meridional shifts of the SH westerlies have been hypothesized in response to abrupt North Atlantic Dansgaard-Oeschger (DO) climatic events of the last ice age 5,6 , in parallel with the well-documented shifts of the intertropical convergence zone (ITCZ) 7. Shifting moisture pathways to West Antarctica 8 are consistent with this view, but may represent a Pacific teleconnection pattern 9. The full SH atmospheric-circulation response to the DO cycle and its impact on Antarctic temperature remain unclear 10. Here we use five volcanically-synchronized ice cores to show that the Antarctic temperature response to the DO cycle can be understood as the superposition of two modes: a spatially homogeneous oceanic "bipolar seesaw" mode that lags Northern Hemisphere (NH) climate by about 200 years, and a spatially heterogeneous atmospheric mode that is synchronous with NH abrupt events. Temperature anomalies of the atmospheric mode are similar to those associated with present-day Southern Annular Mode (SAM) variability, rather than the Pacific South America (PSA) pattern. Moreover, deuterium excess records suggest a zonally coherent migration of the SH westerlies over all ocean basins in phase with NH climate. Our work provides a simple conceptual framework for understanding the circum-Antarctic temperature response to abrupt NH climate change. We provide observational evidence for abrupt shifts in the SH westerlies, which has previouslydocumented 1-3 ramifications for global ocean circulation and atmospheric CO 2. These coupled changes highlight the necessity of a global, rather than a purely North Atlantic, perspective on the DO cycle. During the glacial DO cycle, abrupt variations in northward heat transport by the Atlantic Meridional Overturning Circulation (AMOC) affect Greenland and Antarctic temperature oppositely (Fig. 1), via an oceanic teleconnection called the bipolar seesaw 6,11. Antarctica warms during Greenland cold phases (stadials), and cools during Greenland warmth (interstadials), with the gradual nature of Antarctic climate change reflecting buffering by a large heat reservoir 11-likely the global ocean interior 6. The DO cycle affects atmospheric circulation also; the ITCZ shifts southwards during stadials, and northwards during interstadials 7. General Circulation Model (GCM) simulations suggest parallel shifts of the SH westerlies 5,6,12 , but the available observational evidence (a deuterium excess record from West Antarctica 8) cannot distinguish between such shifts and Pacific-only teleconnections 9. Furthermore, the impact of the atmospheric circulation changes on Antarctic climate remains unknown, and models are inconclusive on this question 10,13. We use water stable isotope ratios, a proxy for site temperature 14 , from five Antarctic ice cores: WAIS (West ...

Decadal-scale progression of Dansgaard-Oeschger warming events

“…This is further supported by the observations by Steffensen et al (2008), that show changes in deuterium excess at the onset of GI-1 and GI-8 occur after the onset of the transition in mineral dust aerosol. Antarctic records of deuterium excess that indicate changes of moisture sources also show a rapid shift at the onset of the Greenland interstadials interpreted as a change in the Southern Hemisphere Westerlies explaining part of the signal observed in multiple Antarctic δ 18 O records (Markle et al, 2016;Buizert et al, 2018). This further support Clim.…”

“…Note that this implicitly assumes that the timing differences for all insterstadial onsets in the records investigated here are the result of the same underlying process or in other words are similar between the interstadial onsets. This assumption differs from the assumption used in other studies of relative phasing of southern and northern hemisphere climate and changes in precipitation source regions during the last glacial (Buizert et al, 2015;Markle et al, 2016;Buizert et al, 2018). In these studies, the stacking of the climate events assumes that the complete progression of the climate event is a realization of the same underlying process which is a wider and more restrictive assumption as it encompasses the whole transition and not only its onset.…”

Decadal-scale progression of the onset of Dansgaard–Oeschger warming events

“…Furthermore, records from Eurasia indicate rapid changes in the local ecosystems (Rousseau et al, 2017). In the lower latitudes, speleothem and sediment records from both South America (Wang et al, 2004;Deplazes et al, 2013) and eastern Asia (Wang et al, 2008) indicate a northward displacement of the Intertropical Convergence Zone (ITCZ) at the time of DO warming (Cheng et al, 2012) resulting in rapid changes in tropical hydroclimate and methane emissions from the tropical wetlands (Baumgartner et al, 2014). A recent synchronization of cosmogenic radionuclide records from ice cores and low-latitude speleothems back to 45 kyr ago shows that atmospheric circulation changes in the tropics occurred synchronously with the Greenland warming within the cross-dating uncertainties of around 180 years (Adolphi et al, 2018).…”

Review for „Spiky Fluctuations and Scaling in High-Resolution EPICA Ice Core Dust Fluxes” by Lovejoy and Lambert

“…P14 L14-17: Please discuss this statement in the context of recent proxy and model studies that indicate fast Southern Hemisphere circulation changes during DO events. (Markle et al 2016, Buizert et al 2018, Pedro et al 2018).…”