Master´s thesis defense by Jonathan Rheinlænder – Niels Bohr Institute - University of Copenhagen

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Master´s thesis defense by Jonathan Rheinlænder

Interhemispheric climate variability in a pre-industrial control
simulation of CCSM4

Climate reconstructions from ice core records in Greenland and Antarctica have revealed a series of abrupt climate transitions, showing a distinct relationship between northern and southern hemisphere climate during the last glacial period. It is generally believed that changes in the AMOC are responsible for this interhemispheric connection. This so-called bipolar seesaw mechanism, links the two hemispheres on centennial timescales and provides a framework for understanding the timing of the abrupt climate transitions during glacial times. The recent ice core records from West Antarctica (WAIS) points towards an atmospheric teleconnection as a possible trigger for the interhemispheric climate variability. This recent progress in paleoclimate observations, together with recent modelling efforts, calls for the classic bipolar seesaw theory to be revised.

In this modelling study, dynamics of abrupt climate transitions in an unforced control simulation of CCSM4 are explored. The model exhibits a series of abrupt changes in Greenland surface temperature triggered by internal climate variability, which closely resemble a Dansgaard-Oeschger event. Similar, but weaker changes are observed in the Southern Hemisphere synchronous with the abrupt changes in the Northern Hemisphere. We argue that both north and south high latitude climate variability is triggered by stochastic precipitation anomalies in the western tropical Pacific. The atmospheric wave guide then provides a fast communication pathway connecting the deep tropics and the polar regions. In the Southern Hemisphere this is manifested as a distinct pressure pattern over West Antarctica that strongly influences deep ocean convection in the Weddell Sea.

The ocean circulation response to the abrupt climate transitions show a characteristic asymmetric pattern in the subsurface temperature of the Atlantic, resembling a bipolar see-saw response. After an initial surface cooling in the subpolar gyre, the subsequent ocean adjustment is dominated by heat convergence at the subpolar-subtropical gyre boundary. This warming anomaly, located at mid-depth, spreads into the South Atlantic along the deep western boundary current, on time scales associated with slow advective processes. The anomaly is significantly reduced, as the signal reaches the
South Atlantic midlatitudes, where further southward propagation is inhibited by the presence of the Antarctic Circumpolar Current (ACC). This suggest an essential role of the ACC in the setting the magnitude and time scale of the hemispheric response.

In line with a number of recent studies, the modelling evidence presented here shows that internal climate variability is a potential trigger for the abrupt climate transitions observed during the last glacial through the existence of a fast atmospheric teleconnection mechanism. Furthermore, our results show that propagation of an ocean temperature signal from the North Atlantic to Antarctica is hindered by the ACC, which presents an essential issue in the traditional bipolar see-saw mechanism.