Arctic-Antarctic coupling

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With Arctic Antarctic coupling the air are couplings between Arctic and Antarctic , respectively. The research deals with the question of how past climate fluctuations in the two hemispheres can be reconciled.

The model of the bipolar temperature rocker

The concept of opposing mean temperatures on the two hemispheres can be clearly illustrated as a seesaw

A simple model of the interhemispheric coupling is that of the so-called bipolar seesaw, which postulates an exactly opposite temperature curve 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 now the thermohaline circulation z. B. weakened by large inputs of fresh water in the north or even vice versa, this results in a cooling of the North Atlantic and thus Greenland, as not so large amounts of warm water are brought in from the south. At the same time, less heat is extracted from the southern hemisphere, so that the temperatures of the southern ocean rise. The same connection leads to a cooling of the south as soon as the ocean circulation increases again and brings the north to an end of the cold period.

Measurements from ice cores

In this context, the investigation of strong and abrupt climate changes such as the Dansgaard-Oeschger events is of particular interest . In the last 110 thousand years, 24 such rapid warmings have been identified by examining ice cores extracted in Greenland for their isotope ratios , which provide information about temperatures of earlier ages. The measurements indicated drastic temperature changes between 9 and 16 degrees Celsius. However, these values ​​do not represent global warming, since ice cores that were obtained in the Antarctic do not show such sharp temperature jumps. However, it can be assumed that the drastic events in the northern hemisphere also influenced the Antarctic climate. The analysis of possible coupling mechanisms between the northern and southern hemisphere is difficult for various reasons: On the one hand, the drill cores of the Antarctic are not well timed due to the lower precipitation rate, on the other hand, many signals are simply too weak to allow a clear interpretation. In order to test the theory of the temperature swing on the basis of data from ice cores, the drill cores obtained in the Arctic and Antarctic must be dated so that it can be ensured which locations in the drill cores correspond in time. This synchronization takes place via the measurement of air components trapped in the ice, such as B. methane , which are evenly distributed globally. The temperature that prevailed when the ice formed is determined from the ratio of the oxygen isotopes and . With the exception of the last Ice Age, the measurements from Greenland and the Antarctic do not show any opposing temperature behavior, but rather a clear phase shift between the series of measurements, which, even if the shift is corrected, show little in common. In addition, even modern climate models provide contradicting results on this matter.

Models and theories

A simple model can be used to demonstrate why the classic idea of ​​a bipolar temperature swing cannot be directly observed: The signal transmission from the northern to the southern hemisphere does not take place immediately, but the large, thermally inert ocean masses need a long time to change their temperature to the changed one Adjust flow conditions. A temperature signal is generally propagated via wave phenomena such as Kelvin and Rossby waves . Kelvin waves are anomalies in sea level that can only propagate along coastlines or the equator. In particular, coastal Kelvin waves are decisive for the duration of the signal transmission between the northern and southern hemisphere. Since the Southern Ocean has practically no coasts, the spread 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 passed on by the ocean with great delay and attenuation. On the basis of this model assumption, it is possible to estimate how great the delay through the southern ocean must be in order to achieve the best possible agreement with the measurements from the ice cores. The highest correlation is around 1000 years. This result roughly coincides with calculations by climate models, but only for the last 25-23 thousand years. Before this time, the coupling duration seems to have been significantly longer, which suggests a change in the physical conditions in the Southern Ocean. In fact, there is evidence that the ocean was stratified back then, which would mean increased overturning times. However, this model is too simple to prove that the southern ocean is really the deciding factor in delaying climate signals. It would be conceivable, for example, that the ice sheet, 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.

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