A thesis submitted March 14, 2012 for the degree of Doctor of Philosophy and defended May 2, 2012.
The PhD School of Science
Centre for Ice and Climate
Niels Bohr Institute
Peter L. Langen
Abrupt climate change and high to low latitude teleconnections as simulated in climate models
High to low latitude atmospheric teleconnections have been a topic of increasing scientific interest since it was shown that high latitude extratropical forcing can induce tropical precipitation shifts through atmosphere-surface ocean interactions. In this thesis, several aspects of high to low latitude atmospheric teleconnections have been considered.
The atmospheric energy transport response during Northern Hemisphere cooling and warming from present day and Last Glacial Maximum (LGM) conditions is investigated using sea surface temperature (SST) anomalies derived from a freshwater hosing experiment. The results showed the enhanced sensitivity of the present day atmospheric mid- latitude energy transport compared to that of the LGM, suggesting its ability to reorganize more easily and thereby dampen high latitude temperature anomalies that could arise from changes in the oceanic transport.
The following question was if the atmosphere-surface ocean interactions would, if enabled, affect these findings. This lead to a further re-examination of the role of tropical SSTs in the tropical precipitation shifts. Using the set of idealized simulations with the fixed tropical sea surface temperatures, it was shown that the ITCZ shifts are not possible without the tropical SST changes. The results also revealed a partial
(local) energy compensation in the high latitudes, while the main energy compensation was shown to come from the southern tropics, with the energy gain originating from the cloud radiative feedbacks, temperature and longwave water vapor feedbacks.
The work was further extended by considering all other scenarios that can induce the southward ITCZ shifts, namely the Northern Hemisphere cooling, Southern Hemisphere warming and a bipolar seesaw like forcing that encompasses both. These simulations have provided an insight into two key areas over which the convection strongly reorganizes in order to provide the energy flux changes required by the system. The two areas are the East Tropical Pacific and the Northern Tropical Atlantic with positive and negative top-of-the-atmosphere energy flux anomalies, respectively.
Furthermore, Southern Hemisphere warming simulations have not only caused the ITCZ shift, but have also affected the high northern latitudes, as seen from the surface temperature and wind strength changes. These results may have important paleo implications to the Dansgaard-Oeschger type of events.