Warm European summers initiated by freshwater events in the North Atlantic

Abstract:

Recent decades have been characterised by record Arctic sea ice losses and an increased occurrence of weather extremes at mid-latitudes. While earlier studies have noticed statistical linkages between the two, the underlying dynamical connections remain elusive. Here, we focus on the mechanisms linking Arctic freshwater discharges with warm summers in Europe. Combining remote sensing with atmospheric reanalysis data, we show that large freshwater events trigger intense cold anomalies in the North Atlantic in winter, resulting in sharp temperature gradients and amplified baroclinic instability. The initiated, atmospheric response forces a northward shift in the North Atlantic Current and in turn the jet stream in the following summer, leading to increased air temperatures over Europe. The magnitude of the freshwater event still controls the ocean and atmospheric evolution including the location and strength of the heat anomalies two years after the event with the correlations exceeding 0.9. Considering these high sensitivities and the growing Arctic freshwater discharges, we conclude that Europe is facing hot summers.