Australia: The Land Where Time Began

A biography of the Australian continent 

Dansgaard-Oeschger Events – Global Atmospheric Teleconnections During Such Events

The North Atlantic region underwent a series of Dansgaard-Oeschger cycles during which the climate alternated abruptly between warm and cold periods. In Antarctica corresponding variations were out of phase with their northern counterparts. The ocean overturning circulation, which leads to interhemispheric redistribution of heat, is commonly believed to be the cause of the temperature relationship between the hemispheres. Markle et al. suggest changes in the circulation of the atmosphere to satisfy global energy budget constraints should accompany changes in heat transport in the ocean. There is a lack of Evidence for changes that are predicted for atmospheric circulation in the Southern Hemisphere during Dansgaard-Oeschger cycles, though changes in the tropical atmospheric circulation that are linked to abrupt events in the Northern Hemisphere are well documented. In this study Markle et al. show by the use of a deuterium excess record at high resolution from West Antarctica that the latitude of the mean source of moisture for precipitation in Antarctica changed in phase with abrupt shifts in the climate of the Northern Hemisphere, and significantly prior to temperature changes in Antarctica. Direct evidence is provided by this that storm tacks at mid-latitudes in the Southern Hemisphere shifted within decades of abrupt changes in the North Atlantic in parallel with intertropical convergence zone meridional migrations. Markle et al. concluded that the hemispheres are linked during abrupt changes of climate by processes, oceanic and atmospheric, operating on different time scales.

In the Northern Hemisphere (NH) (Dansgaard et al., 1982; Sachs & Lehman, 1999) Dansgaard-Oeschger (DO) events, and in the Southern Hemisphere (SH), the Antarctic isotope maximum (AIM) events are coupled by variations in the meridional oceanic transport of heat. The abrupt changes in the oceanic heat transport are integrated by the large effective heat capacity of the oceans in the Southern Hemisphere (Stocker & Johnsen, 2003), which leads to the muted, out-of-phase character of Antarctic temperature variations (Blunier et al., 1998). It has been found that the temperature response in the Antarctic systematically lags DO transitions by about 200 years, and this time scale is consistent with oceanic processes (WAIS Divide Project Members, 2015).

Anomalies in the interhemispheric heat flux must be accommodated by opposing changes in atmospheric heat transport (Kang et al., 2009) or changes in local radiative processes in order to satisfy the energy budget at the top of the atmosphere. Some responses are robust, such as the migration of the Hadley circulation and intertropical convergence zone (ITCZ) towards the warmer hemisphere (Kang et al., 2009; Broccoli et al., 2006), though atmospheric models do not necessarily agree on the relative roles of these mechanisms. It is suggested by recent studies that migration of the intertropical convergence zone can influence the position of Southern Hemisphere eddy-driven (Ceppi et al., 2013; Chiang et al., 2014) jet and surface westerlies. The mid-latitude westerlies of the Southern Hemisphere are a key component of the global climate, as they transport heat and momentum towards the pole and influence wind-driven upwelling (Marshall & Speer, 2012) and exchange of CO₂ between atmosphere and ocean (Anderson et al., 2009). Evidence of abrupt changes in tropical circulation and precipitation, which are synchronous with Dansgaard-Oeschger events in the Northern Hemisphere has been found in sediment cores (Deplazes et al., 2013; Peterson et al., 2009; Pedro et al., 2016) and speleothem records (Wang et al., 2006). Contrasting with this, evidence of changes on a millennial scale in the dynamics or meridional position of the Southern Hemisphere westerlies is yet to be found (Kohfeld et al., 2013).

Conclusions

The new data from WDC of Markle et al. demonstrate the importance of atmospheric and oceanic teleconnections that link the climate of the Northern and Southern Hemispheres on millennial timescales. Sea Surface Temperatures of the Southern Hemisphere followed the temporal signature of the Antarctic isotopic maximum, which is driven by changes in ocean heat transport. Coupled changes in global atmospheric circulation are reflected in the shift of the Southern Hemisphere winds in phase with the Northern Hemisphere Dansgaard-Oeschger events. These rapid atmospheric teleconnections shifted position of the moisture source locations for WDC (recorded in d_ln) within decades of the Northern Hemisphere Dansgaard-Oeschger events, about 2 centuries prior to significant Antarctic temperature change (recorded in δ¹⁸O). The findings of Markle et al. complement evidence for atmospheric-circulation-imposed variability in Greenland deuterium excess (Mason-Delmotte, 2005). The tropical Hadley circulation is linked by atmospheric dynamics to the storm tracks at mid-latitudes in both hemispheres. According to Markle et al. a global ‘atmospheric seesaw’ is superimposed on the classic oceanic bipolar seesaw, and they suggest it may be important to the dynamics of millennial climate change.

Sources & Further reading

1. Markle, B. R., E. J. Steig, C. Buizert, S. W. Schoenemann, C. M. Bitz, T. J. Fudge, J. B. Pedro, Q. Ding, T. R. Jones, J. W. C. White and T. Sowers (2017). "Global atmospheric teleconnections during Dansgaard-Oeschger events." Nature Geosci 10(1): 36-40.