Master Thesis defense by Anna Ida Katharina Kirchner

Title:  North Atlantic Circulation Variability and Teleconnections to Surface Climate Conditions in CMIP6 Models

Weather conditions and extreme events in Europe are strongly influenced by the state of the atmospheric circulation over the North Atlantic. Typical modes of circulation variability can be identified, with the North Atlantic Oscillation (NAO) being the dominant mode, driving surface conditions in the region and further away through teleconnections connected to changes between its positive and negative phase. There is a high interest in the scientific community in improving predictability of the NAO and other modes of circulation variability, especially in the context of anthropogenic climate change. To this end, the mechanisms driving circulation variability can be better understood by studying them under different climate conditions with the help of climate models or proxy-based reconstructions, that rely on the observed relationship between circulation variability and surface conditions. This work addresses the need to verify the correct representation of modes of variability in climate models and the robustness of their teleconnection patterns in space and time. It aims to investigate how state-of-the-art Global Circulation Models models represent different modes of North Atlantic circulation variability and their relationship with surface climate conditions under current climate conditions, and how these modes and teleconnections can be captured and compared. We use an Empirical Orthogonal Functions analysis to identify different modes of circulation variability in the winter geopotential height field over the North Atlantic and determine their surface temperature and precipitation response patterns in the domain. We investigate the spatial patterns of the modes and their teleconnections in a subset of different CMIP6 models historical runs and explore approaches to evaluate them against reanalysis data. The models show a high ability to represent three distinct modes of variability and realistic corresponding temperature and precipitation responses. Notable differences between the models provide insights into the performance of different models, potential drivers of these differences, and the role of natural spatial variability of the modes and its impacts on teleconnections. A robust evaluation of the models is prevented by limitations of the used methods and the lacking consideration of uncertainties connected to decadal variability.

Supervisor: Jens Hesselbjerg Christensen