Article

# Arctic-Antarctic coupling

Arctic-Antarctic coupling refers to the climate coupling between the Arctic and Antarctic. Research is concerned with the question of how past climate fluctuations in the two hemispheres can be reconciled.

## The bipolar temperature rocker model

The concept of opposing mean temperatures on the two hemispheres can be vividly represented as a seesaw

A simple model of interhemispheric coupling is that of the so-called bipolar seesaw, which postulates an exactly opposite temperature gradient between the north and south polar regions. This assumption is based on the thermohaline circulation of the Atlantic: As the water in the North Atlantic sinks due to its low temperature and high salinity, surface water flows in from the south. This current extends to the southern hemisphere and is so strong that the southern Atlantic as a whole transports heat from south to north (i.e. in the direction of higher radiation). If the thermohaline circulation is now weakened or even reversed, e.g. by large inputs of fresh water in the north, this results in a cooling of the North Atlantic and thus of Greenland, as large quantities of warm water are no longer brought in from the south. At the same time, however, less heat is extracted from the southern hemisphere, so that the temperatures of the southern ocean rise. The same relationship leads to a cooling of the south as soon as the ocean circulation strengthens again, bringing an end to the cold period in the north.

## Measurements from ice cores

Of particular interest in this context is the study of strong and abrupt climate changes such as the Dansgaard-Oeschger events. In the last 110 thousand years, 24 such rapid warmings have been identified by examining ice cores obtained in Greenland for their isotope ratios, which provide information about temperatures of earlier ages. The measurements suggested drastic temperature changes of between 9 and 16 degrees Celsius. However, these values do not represent global warming, as ice cores obtained in Antarctica do not show such sharp temperature jumps. However, it can be assumed that the drastic events in the Northern Hemisphere have also influenced the Antarctic climate.
The analysis of possible coupling mechanisms between the northern and southern hemispheres is difficult for several reasons: On the one hand, the Antarctic cores are not well resolved in time due to the lower precipitation rate, and on the other hand, many signals are simply too weak to allow a clear interpretation.
In order to test the temperature swing theory using data from ice cores, the cores obtained in the Arctic and Antarctic must be dated so that it can be ensured which locations in the cores correspond in time. This synchronization is done by measuring air components trapped in the ice, such as methane, which are uniformly distributed globally. The temperature that prevailed when the ice formed is determined from the ratio of the oxygen isotopes

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O

18

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and

${displaystyle O^{16}}$

O

16

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determined.
With the exception of the last ice age, the measurements from Greenland and Antarctica do not show a contrary behaviour of the temperature, but a clear phase shift between the measurement series, which furthermore show only little commonality even when correcting for the shift. Moreover, even modern climate models provide contradictory results on this issue.

## Models and theories

A simple model can be used to demonstrate why the classical idea of a bipolar temperature swing is not so directly observable: the signal transmission from the northern to the southern hemisphere does not occur immediately, but the large, thermally inert ocean masses need a long time to adjust their temperature to the changed flow conditions. The propagation of a temperature signal generally occurs via wave phenomena such as Kelvin and Rossby waves. Kelvin waves are sea level anomalies that can only propagate along coasts or the equator. In particular, coastal Kelvin waves are critical to the timing of signal transmission between the northern and southern hemispheres. Since the Southern Ocean has almost no coasts, the propagation of temperature signals is strongly inhibited there due to the thermal inertia of the ocean, and a climate change in the Northern Hemisphere is only transmitted by the ocean in a strongly delayed and damped manner. Based on this model assumption, it can be estimated how large the delay caused by the southern ocean must be in order to achieve the best possible agreement with the measurements from the ice cores. The highest correlation is in the range of about 1000 years. This result roughly agrees with calculations by climate models, but only for the last 25-23 thousand years. Prior to this time, the coupling period appears to have been much greater, suggesting a change in the physical conditions in the Southern Ocean. Indeed, there is evidence that the stratification of the ocean was stronger at that time, which would imply an increased circulation duration. However, this model is too simplistic to prove that the Southern Ocean is really the key factor in delaying climate signals. It is conceivable, for example, that the inland ice, from where the measurements ultimately originate, is also involved. The clarification of these questions is far from complete and is therefore the subject of current research.