Master Thesis Defense by Julia González San Juan

Title: Storminess in the North Atlantic Ocean. Climate variability and the role of teleconnection patterns on wind speed

Abstract:
Large-scale atmospheric circulation significantly influences variations in weather patterns. Climate variability is highly complex but can be simplified into more manageable patterns known as modes of variability. Understanding the relationship between these modes and other climate variables provides valuable insights into past climate patterns and facilitates predictions for future scenarios. In this thesis, we use Principal Component Analysis to identify the modes of variability in sea level pressure in the North Atlantic Ocean since 1959. This technique manages to capture successfully the three main modes of variability in the area. We also analyze nearsurface wind speeds and investigate how their strength and spatial distribution are influenced by these modes.

Furthermore, four smaller basins have been selected to examine the relationship between the first

mode of variability, the North Atlantic Oscillation (NAO), and wind speeds. In the Atlantic Ocean, the North Sea, and the Norwegian Sea, wind speed and the NAO exhibit a robust positive correlation. In the Mediterranean Sea, the correlation is negative and not as robust. Additionally, we parameterize decadal wind variation using theWeibull distribution. An increase in wind speed occurred between the 1960s and the following decades, but since then, no notable differences are visible.

Lastly, we employ four models from the CMIP6 project (EC-Earth3, CESM2, CNRM-ESM2-1, and HadGEM3-GC31-LL) to identify potential climate trends at the end of the century and assess the impact of global warming in the North Atlantic Ocean. We calculate changes in mean sea level pressure and near-surface wind speeds between a historical period (1990-2014) and three distinct time periods within the SSP5-8.5 future scenario, spanning from 2041 to 2100. A decrease of 0.14 m/s or 2.1% is expected across the entire study region by the end of the century, with larger declines at a regional level, such as a 0.41 m/s or 7.1% decrease in the Mediterranean Sea. We also study potential trends in the first three modes of variability. The Azores high is expected to shift northeastwards, and both centers of action of the NAO are expected to strengthen, thereby increasing the pressure gradient between both dipoles. Regarding the following modes, the Scandinavian pattern remains in the same location, while the East Atlantic pattern is expected to undergo a significant northeastward shift as well as a decrease in its primary pole.

Supervisor: Jens Hesselbjerg Christensen
Censor: Peter Aakjær