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.
Armstrong, B. 2006. Models for the relationship between ambient temperature and daily mortality. Epidemiology, 17, 624–631.
Astok, V. & Mardiste, H. 1995. Contemporary sea. In Eesti. Loodus [Estonia. Nature] (Raukas, A., ed), pp. 228–237. Valgus, Tallinn [in Estonian].
Barriopedro, D., Fischer, E. M., Luterbacher, J., Trigo, R. M. & Garcia-Herrera, R. 2011. The hot summer of 2010: redrawing the temperature record map of Europe. Science, 332, 220–224.
Beniston, M., Stephenson, D. B., Christensen, O. B., Ferro, C. A. T., Frei, C., Goyette, S., Halsnaes, K., Holt, T., Jylhä, K., Koffi, B., Palutikof, J., Schöll, R., Semmler, T. & Woth, K. 2007. Future extreme events in European climate: an exploration of regional climate model projections. Climatic Change, 81, 71–95.
Cheng, L., Abraham, J., Zhu, J., Trenberth, K. E., Fasullo, J., Boyer, T., Locarnini, R., Zhang, B., Yu, F., Wan, L., Chen, X., Song, X., Liu, Y. & Mann, M. E. 2020. Record-setting ocean warmth continued in 2019. Advances in Atmospheric Sciences, 37, 137–142.
CMEMS. 2021a. Baltic Sea Anomaly Time Series of Sea Surface Temperature. https://marine.copernicus.eu/access-data/ocean-monitoring-indicators/baltic-sea-anomaly-time-series-sea-surface-temperature [accessed 18 October 2021].
CMEMS. 2021b. Copernicus: Warmest Summer for Europe by Small Margin; August Globally Joint Third Warmest on Record. Press Release, 7th September 2021.
https://climate.copernicus.eu/copernicus-warmest-summer-europe-small-margin-august-globally-joint-third-warmest-record [accessed 18 October 2021].
Easterling, D. R., Meehl, G. A., Parmesan, C., Changnon, S. A., Karl, T. R. & Mearns, L. O. 2000. Climate extremes: observations, modeling, and impacts. Science, 289, 2068–2074.
EWS. 2021a. Riigi Ilmateenistus. Suvi 2021 [Estonian Weather Service. Summer 2021].
https://storymaps.arcgis.com/stories/8bc2e0cbf0944eccb5802deefd4d782f [in Estonian, accessed 18 October 2021].
EWS. 2021b. Estonian Weather Service. Coastline stations.
http://www.ilmateenistus.ee/meri/vaatlusandmed/kogu-rannik/kaart/?lang=en [accessed 18 September 2021].
EWS. 2021c. Riigi Ilmateenistus. Ajaloolised ilmaandmed [Estonian Weather Service. Historical data].
http://www.ilmateenistus.ee/kliima/ajaloolised-ilmaandmed/ [in Estonian, accessed 18 October 2021].
FMI. 2021. Finnish Meteorological Institute. Download observations.
https://en.ilmatieteenlaitos.fi/download-observations [accessed 18 October 2021].
Haapala, J. 1994. Upwelling and its influence on nutrient concentration in the coastal area of the Hanko Peninsula, entrance of the Gulf of Finland. Estuarine, Coastal and Shelf Science, 38, 507–521.
Hobday, A. J., Oliver, E. C. J., Sen Gupta, A., Benthhuysen, J. A., Burrow, M. T., Donat, M. G., Holbrook, N. J., Moore, P. J., Thomsen, M. S., Wernberg, T. & Smale, D. A. 2018. Categorizing and naming marine heatwaves. Oceanography, 31(2), 162–173.
Høyer, J. & Karagali, I. 2016. Sea surface temperature climate data record for the North Sea and Baltic Sea. Journal of Climate, 29, 2529–2541.
IPCC. 2014. IPCC Fifth Assessment Report (AR5).
Keevallik, S. & Vint, K. 2012. Influence of changes in the station location and measurement routine on the homogeneity of the temperature, wind speed and precipitation time series. Estonian Journal of Engineering, 18, 302–313.
Keevallik, S. & Vint, K. 2015. Temperature extremes and detection of heat and cold waves at three sites in Estonia. Proceedings of the Estonian Academy of Sciences, 64, 473–479.
Konik, M., Kowalewski, M., Bradtke, K. & Darecki, M. 2019. The operational method of filling information gaps in satellite imagery using numerical models. International Journal of Applied Earth Observation and Geoinformation, 5, 68–82.
Kont, A., Endjärv, E., Jaagus, J., Lode, E., Orviku, K., Ratas, U., Rivis, R., Suursaar, Ü. & Tõnisson, H. 2007. Impact of climate change on Estonian coastal and inland wetlands – a summary with new results. Boreal Environment Research, 12, 653–671.
Le Page, M. 2021. Sicily hits 48.8°C, the highest temperature ever recorded in Europe. New Scientist. 12 August 2021.
Lhotka, O., Kyselý, J. & Farda, A. 2018. Climate change scenarios of heat waves in Central Europe and their uncertainties. Theoretical and Applied Climatology, 131, 1043.
Lips, I., Lips, U. & Liblik, T. 2009. Consequences of coastal upwelling events on physical and chemical patterns in the central Gulf of Finland (Baltic Sea). Continental Shelf Research, 29, 1836–1847.
Minnett, P. J., Alvera-Azcárate, A., Chin, T., Corlett, G., Gentemann, C., Karagali, I., Li, X., Marsouin, A., Marullo, S., Maturi, E., Santoleri, R., Saux Picart, S., Steele, M. & Vazquez-Cuervo, J. 2019. Half a century of satellite remote sensing of sea-surface temperature. Remote Sensing of Environment, 233, 111366.
Näyha, S. 2007. Heat mortality in Finland in the 2000s. International Journal of Circumpolar Health, 66, 418–424.
NOAA. 2021. Global Climate Report – Annual 2020.
https://www.ncdc.noaa.gov/sotc/global/202013 [accessed 18 October 2021].
Õispuu, T.-M. 2019. Heat waves in Estonia in 1951–2018. In Yearbook of Estonian Geographical Society, Vol. 44 (Järvet, A., ed), pp. 93–110. Tallinn.
http://egs.ee/wp-content/uploads/2019/12/EGS_Aastaraamat_44kd.pdf [in Estonian, with English summary; accessed 18 October 2021].
Oliver, E. C. J., Burrows, M. T., Donat, M. G., Sen Gupta, A., Alexander, L. V., Perkins-Kirkpatrick, S. E., Benthuysen, J. A., Hobday, A. J., Holbrook, N. J., Moore, P. J., Thomsen, M. S., Wernberg, T. & Smale, D. A. 2019. Projected marine heatwaves in the 21st century and the potential for ecological impact. Frontiers in Marine Science, 6(734), 1–12.
Paalme, T., Torn, K., Martin, G., Kotta, I. & Suursaar, Ü. 2020. Littoral benthic communities under effect of heat wave and upwelling events in NE Baltic Sea. Journal of Coastal Research, Special Issue 95, 133–137.
Perkins, S. E. & Alexander, L. V. 2013. On the measurement of heat waves. Journal of Climatology, 26, 4500–4517.
Peterson, T. C. & Manton, M. J. 2008. Monitoring changes in climate extremes: A tale of international collaboration. Bulletin of the American Meteorological Society, 89, 1266–1271.
Robine, J. M., Cheung, S. L. K., Roy, S., Oyen, H., Griffiths, C., Michel, J. P. & Herrmann, F. R. 2008. Death toll exceeded 70,000 in Europe during the summer of 2003. Comptes Rendus Biologies, 331, 171.
Robinson, P. J. 2001. On the definition of heat wave. Journal of Applied Meteorology, 40, 762–775.
Russo, S., Sillmann, J. & Fischer, E. M. 2015. Top ten heatwaves since 1950 and their occurrence in the coming decades. Environmental Research Letters, 10, 124003.
Rutgersson, A., Jaagus, J., Schenk, F., Stendel, M., Bärring, L., Briede, A., Claremar, B., Hanssen-Bauer, I., Holopainen, J., Moberg, A., Nordli, Ø., Rimkus, E. & Wibig, J. 2015. Recent change – atmosphere. In The Second Assessment of Climate Change for the Baltic Sea Basin (BACC II Author Team, eds), pp. 69–97. Springer, Cham.
Ruuhela, R., Votsis, A., Kukkonen, J., Jylhä, K., Kankaanpää, S. & Perrels, A. 2021. Temperature-related mortality in Helsinki compared to its surrounding region over two decades, with special emphasis on intensive heatwaves. Atmosphere, 12, 46.
Saava, A., Rekker, K. & Indermitte, E. 2015. Äärmusliku kuuma ilma (sh kuumalainete) mõju rahvastiku suremusele [Impact of exceptionally hot weather (incl. heat waves) on population mortality]. Eesti Arst, 94(5), 288–293 [in Estonian, with English summary].
Schär, C., Vidale, P. L., Lüthi, D., Frei, C., Häberli, C., Liniger, M. A. & Appenzeller, C. 2004. The role of increasing temperature variability in European summer heatwaves. Nature, 427, 332–336.
She, J., Su, J. & Zinck, A.-S. 2020. Anomalous surface warming in the Baltic Sea in summer 2018 and mechanism analysis. Copernicus Marine Service Ocean State Report, Issue 4, Journal of Operational Oceanography, 13, sup1, s125–s132.
Siegel, H., Gerth, M. & Tschersich, G. 2006. Sea surface temperature development of the Baltic Sea in the period 1990–2004. Oceanologia, 48(S), 119–131.
Soomere, T., Myrberg, K., Leppäranta, M. & Nekrasov, A. 2008. The progress in knowledge of physical oceanography of the Gulf of Finland: a review for 1997–2007. Oceanologia, 50, 287–362.
Suursaar, Ü. 2010. Waves, currents and sea level variations along the Letipea – Sillamäe coastal section of the southern Gulf of Finland. Oceanologia, 52, 391−416.
Suursaar, Ü. 2020. Combined impact of summer heat waves and coastal upwelling in the Baltic Sea. Oceanologia, 62, 511–524.
Suursaar, Ü. 2021. Winter upwelling in the Gulf of Finland, Baltic Sea. Oceanologia, 63, 356–369.
Suursaar, Ü. & Aps, R. 2007. Spatio-temporal variations in hydro-physical and -chemical parameters during a major upwelling event off the southern coast of the Gulf of Finland in summer 2006. Oceanologia, 49, 209–228.
Suursaar, Ü., Jaagus, J. & Tõnisson, H. 2015. How to quantify long-term changes in coastal sea storminess? Estuarine Coastal and Shelf Science, 156, 31–41.
Suursaar, Ü., Elken, J. & Belkin, I. M. 2021. Fronts in the Baltic Sea: A review with a focus on its north-eastern part. In Chemical Oceanography of Frontal Zones (Belkin, I. M., ed). Springer Nature,
SYKE. 2021. TARKKA open web service.
http://syke.fi/tarkka [accessed 18 October 2021].
Takolander, A., Cabeza, M. & Leskinen, E. 2017. Climate change can cause complex responses in Baltic macroalgae: A systematic review. Journal of Sea Research, 123, 16–29.
Uiboupin, R. & Laanemets, J. 2009. Upwelling characteristics derived from satellite sea surface temperature data in the Gulf of Finland, Baltic Sea. Boreal Environment Research, 14, 297–304.
Zhurbas, V., Laanemets, J. & Vahtera, E. 2008. Modeling of the mesoscale structure of coupled upwelling/downwelling events and the related input of nutrients to the upper mixed layer in the Gulf of Finland, Baltic Sea. Journal of Geophysical Research Oceans, 113, 1–8.
Zujev, M. & Elken, J. 2018. Testing marine data assimilation in the northeastern Baltic using satellite SST products from the Copernicus Marine Environment Monitoring Service. Proceedings of the Estonian Academy of Sciences, 67, 217–230.
Zujev, M., Elken, J. & Lagemaa, P. 2021. Data assimilation of sea surface temperature and salinity using basin-scale reconstruction from empirical orthogonal functions: a feasibility study in the northeastern Baltic Sea. Ocean Science, 17, 91–109.