Investigating the role of extratropical cyclones in North Atlantic deep water formation

The Atlantic Meridional Overturning Circulation (AMOC) is a vital mechanism of heat transport in the climate system, but it has been suggested that its strength will change in the coming decades. This strength depends in part on water mass transformations in the North Atlantic, and understanding the factors that contribute to this variability is crucial to predicting the future behaviour of the AMOC. This research aims to deepen our understanding of one such factor: the role that cyclonic storms play in priming and initiating deep water formation in the Labrador Sea and Nordic Seas. We study cyclone statistics for both regions, but primarily study the ocean’s response in the Labrador Sea. To do this, we used pressure and wind fields from two atmospheric datasets (ERA5 and the Canadian Meteorological Centre’s Global Deterministic Prediction System Reforecasts, or CGRF) to identify and track cyclones. We then look at output from a very-high resolution NEMO (Nucleus for European Modelling of the Ocean) model configuration, run over 2002-2019 and forced with the above atmospheric datasets to evaluate what effects passing cyclones exert on upper ocean properties. 

Preliminary results indicate that the passage of individual cyclones noticeably cools and deepens the mixed layer, likely via an associated increase in surface heat loss. The other key points we still aim to investigate are the linkage between deep convection and cyclones, and how the presence of this cyclone forcing affects the properties of the resulting deep water masses. We also aim to quantify the contribution of cyclones to deep convection over the study period relative to the amount contributed by the background environmental conditions. We explore whether the cyclones have a positive contribution to deep water formation, particularly after multiple cyclones transit in relatively short succession.