Combined impact of summer heat waves and coastal upwelling in the Baltic Sea

Under warming climates, heat waves (HWs) have occurred in increasing intensity in Europe. Also, public interest towards HWs has considerably increased over the last decades. The paper discusses the manifestations of the summer 2014 HW and simultaneously occurring coastal upwelling (CU) events in the Gulf of Finland. Caused by an anticyclonic weather pattern and persisting easterly winds, CUs evolved along the southern coast of the Gulf in four episodes from June to August. Based on data from coastal weather stations, 115 days-long measurements with a Recording Doppler Current Profiler (RDCP) oceanographic complex and sea surface temperature (SST) satellite images, the partly opposing impacts of these events are analysed. Occurring on the background of a marine HW (up to 26°C), the CU-forced SST variations reached about 20 degrees. At the 10 m deep RDCP mooring location, a drop from 21.5 to 2.9°C occurred within 60 hours. Salinity varied between 3.6 and 6.2 and an alongshore coastal jet was observed; the statistically preferred westerly current frequently flowed against the wind. Locally, the cooling effect of the CUs occasionally mitigated the overheating effects by the HWs both in the sea and on the marine-land boundary. However, in the elongated channel-like Gulf of Finland, upwelling at one coast is usually paired with downwelling at the opposite coast, and simultaneously or subsequently occurring HWs and CUs effectively contribute to heat transfer from the atmosphere to the water mass. Rising extremes of HWs and rapid variations by CUs may put the ecosystems under increasing stress.