You’ve heard of glacial and interglacial periods, but have you heard of Dansgaard-Oeschger (DO) events? Unless you’ve studied climate change and ocean surface processes, perhaps not. Named after researchers who discovered their existence by studying ancient Greenland ice cores, DO events are smaller climate fluctuations that occur within larger glacial/interglacial cycles. In the Northern Hemisphere, they take the form of rapid warming episodes of a few decades that are typically followed by gradual cooling over a longer period.
The processes behind the timing and magnitude of these events (recorded in ice cores) are still unclear. Some evidence points to Heinrich events, during which large flotillas of icebergs break off from northern glaciers and traverse the North Atlantic. The resulting influx of freshwater affects the circulation of the North Atlantic, and could be linked to DO events; additionally, it appears that Heinrich events only occur in the cold spells immediately preceding DO warmings.
However, in the Southern Hemisphere, the pattern is different. The temperature relationship between the hemispheres is commonly attributed to a redistribution of heat by the global ocean’s circulation. The atmosphere also plays a part: changes in ocean heat transport should be accompanied by changes in atmospheric circulation to balance global energy budget constraints.
Researchers can study past sea surface temperatures (SSTs) by studying oxygen isotopes (also called deuterium, or d-O18) in ice cores. By studying about 25 ancient Antarctic Isotope Maxima (AIM), researchers have found that the warming in the Southern Hemisphere during DO events occurs much slower and with much smaller temperature fluctuations, though they appear to reflect changes in the North Atlantic ocean circulation. The ice core evidence from Antarctica suggests that the DO events are related to the Antarctic Isotope Maxima by a coupling of the climate of the two hemispheres, a.k.a. the Bi-polar Seesaw (the theory that a change in one hemisphere may take a long time to effect change in the other).
Although changes in atmospheric circulation linked to DO events in the Northern Hemisphere are well studied, evidence for corresponding changes in the Southern Hemisphere’s atmospheric circulation during DO events is lacking, and it appears the changes are out of phase (lagging by centuries to millennia). But just how out of phase are they, and what processes may link Northern Hemisphere DO events and AIM?
Investigating the Past with Oxygen Isotopes
Researchers at the University of Washington and other institutes set out to try and shed more light on the relationship. The researchers used a high-resolution oxygen isotope record from West Antarctica to show past SST during known DO events revealed by Greenland ice cores. These records also are a proxy for relative moisture at the site of the record, and using this proxy, they can investigate storm tracks and atmospheric circulation.
In past studies, interpretation of Antarctic isotope records has proven difficult due to Antarctic ice cores not being well correlated. Here, the researchers found that the lack of correlation is largely an side product of the method used to analyze them. So, they used a new method using logarithmically-defined records (the math is, frankly, above this oceanbites author), and found more coherence between the records. They could begin their study in earnest to delve into SST and moisture, which would also translate to precipitation and storms.
Their results showed that the latitude of the moisture source for Antarctic precipitation changed soon after the abrupt DO shifts in Northern Hemisphere climate. This provides evidence that Southern Hemisphere mid-latitude storm tracks shifted within decades of abrupt changes in the North Atlantic. However, these atmospheric shifts occurred significantly before changes Antarctic SST of the ocean.
Why might this be? The large effective heat capacity (ability to “soak up” heat energy) of the Southern Hemisphere ocean integrates the abrupt changes in oceanic heat transport, leading to the less intense, out-of-phase nautre of Antarctic SST variation. This could explain why the Antarctic SST response – and the many AIMs – lags behind DO events by about 200 years, the timing of which is consistent with ocean circulation processes.
The researchers concluded that both oceanic and atmospheric processes are important and link the hemispheres during abrupt climate change events like DO events, though they appear to be operating on different timescales. Southern Hemisphere SSTs followed the signature of AIM events and were driven by oceanic heat transport changes. Southern Hemisphere winds shifted soon after Northern Hemisphere DO events, connecting global atmospheric circulation, and shifting the position of the moisture source for Antarctica within decades of DO events and about 200 years before significant Antarctic temperature change. Thus, the researchers can add a global ‘atmospheric seesaw’ to the theory of the oceanic Bipolar seesaw. Such a component may be important to the dynamics of millennial climate change.