Master Thesis defense by Mia Theil Larsen og Rasmus Lund Andersen

Title: Arctic Freshwater, North Atlantic SST, and the Winter NAO - Assessing SST-Mediated Links Between Greenland Runoff, Fram Strait Sea-Ice Export, and the Winter NAO

Abstract:
This thesis investigated whether freshwater fluxes associated with runoff from Greenland and sea ice transport through the Fram Strait are indirectly linked to winter North Atlantic Oscillation (NAO) variability through North Atlantic sea surface temperature (SST) anomalies. In addition, the influence of sea ice concentration (SIC) in the Barents-Kara (BK) Seas on the NAO was examined. SST and SIC data were obtained from ERA5, runoff data from the HIRAM5 model, Fram Strait sea ice flux from observational and satellite-derived data, and NAO indices from NOAA and NCAR. Spatial correlation analyses were used to identify SST regions associated with freshwater fluxes and subsequent winter NAO phases. These regions were then used in mediation analyses to assess whether SST anomalies may act as a mediator between freshwater fluxes and the NAO. 

The results indicate a delayed oceanic response to freshwater fluxes. Regional SST anomalies and SIC anomalies in the BK Seas were also found to be linked to subsequent winter NAO phases. However, the results suggest a complex and coupled freshwater-ocean-atmosphere system, in which the examined variables interact in multiple directions and the relationships depended on both the spatial region and the period analysed.  

The spatial correlation results showed that both summer runoff from Greenland and winter sea ice fluxes through Fram Strait are associated with regional SST anomalies in the North Atlantic. Positive correlations between runoff and SST anomalies were found south of Iceland in the Nordic Seas, and farther south in the central North Atlantic, while negative correlations were found within the Subpolar Gyre in the year following the runoff. A similar signal was found for the sea ice transport through Fram Strait, with the relationships being more pronounced for volume-based sea ice flux than for area-based flux. SST anomalies in the subpolar North Atlantic, the Nordic Seas and the Gulf Stream region were also found to be associated with subsequent winter NAO. The strongest correlations were found when the SST anomalies led the winter NAO by one year and for the period 1950–2023 using the NOAA-based index, showing negative correlations in the Subpolar Gyre and positive correlations in the Nordic Seas. A statistically significant positive correlation was found between late autumn SIC anomalies in the BK Seas and the following winter NAO for both NAO indices. The mediation analyses provided limited and spatially localized statistically significant evidence for an SST-mediated pathway between freshwater fluxes and the NAO. However, spatial mediation analyses indicated localized indirect effects within sea ice influenced regions. Therefore, further spatial analyses and the inclusion of additional mediating variables are needed to fully evaluate the proposed freshwater-SST-NAO mediation framework. 

Supervisor: Jens-Hesselbjerg Christensen
Censor: Martin Drews, DTU