The impact of a subpolar North Atlantic freshwater anomaly on Eurasian winter climate

The potential impact of a subpolar North Atlantic freshwater anomaly on sea surface temperature and the overlying atmosphere on seasonal timescales is investigated. A coupled climate model is perturbed with a major freshwater anomaly (FWA) in the subpolar gyre similar in structure to the 1970s Great Salinity Anomaly (GSA). A twenty-member ensemble simulation is run from October to March and compared with a parallel twenty-member control simulation. There are robust responses to the FWA in both the ocean and the atmosphere. In the ocean, the FWA is accompanied by an abrupt shoaling of the mixed layer and a decrease in the surface temperature anomalies that stay in place for several months. In the atmosphere, anomalously low pressure develops over the eastern North Atlantic and East Asia. Increased storminess and precipitation develop over the Atlantic (40–5 °W, 30–55 °N), and in the vicinity of Kazakhstan and Central Asia (40–80 °E, 35–50 °N). The changes in storminess are consistent with increased horizontal temperature gradients and a stronger and more zonally-orientated upper-level jet stream. The ability of the freshwater anomaly to sustain an otherwise short-lived SST anomaly highlights the importance of salinity as an initial condition in coupled seasonal forecasts.