The summer 2021 heat waves (HWs) and marine heat waves (MHWs) were analysed on the basis of air and water temperatures measured both in the open part of the Gulf of Finland at buoys and lighthouses and some land-based stations. The summer of 2021 (June, July, August) was the hottest on record in Estonia with distinct HW episodes around 18–23 June, 4–10 July and 13–18 July. Water temperatures reached up to 26.5 °C at the Kunda coastal station on 16 July and up to 28.1 °C at the Narva-Jõesuu coastal station on 15–16 July. At the Gulf of Finland wave buoys, high water temperatures were measured on 14 July: 26.6 °C (24-h average 25.5 °C) in the central part of the gulf and 27.9 °C (24-h average 25.8 °C) at the Kotka buoy. Analysis of satellite sea surface temperature (SST) data showed that the gulf-wide average SST was 26.3 °C on 15 July. The MHW events were interrupted by several coastal upwelling (CU) episodes, occasionally causing water temperatures lower than 5 °C. The CU events occurred either on the gulf’s northern side, with prevailing westerlies, or its southern side, with easterlies. A comparison of summers suggested the following ranking of the HWs, valid for the region of the Gulf of Finland in 2010–2021: 2021 and 2010 (more or less a tie), 2018, 2014, 2011 and 2019. In the ranking of the MHWs, the summer of 2021 clearly was the first, followed (tied) by those of 2014, 2010 and 2018. Because of the remarkable warming trend in the Baltic Sea area over the last half-century, it is unlikely that any other summer from earlier periods could have had more influential HWs and MHWs than the summers of 2021 and 2010 had.
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