Marine Heat Waves long-term trend assessment in the Northeast Atlantic region.

Marine heat waves (MHWs) may be understood as prolonged periods of anomalously high sea surface temperatures (SST). These events are associated to impacts on marine ecosystems such as coral bleaching, mass mortality of marine invertebrates due to heat stress, rapid species migrations, fishery closures or quota changes, among others. The Iberia-Biscay-Ireland (IBI) region, covering from the Canary basin to the Celtic Sea, demonstrated for the year 2022 peak anomaly values of 15 MHWs events, 128 days of mean durations, and 261 total days of MHW according to a reference period from 1982 to 2022 (Castrillo-Acua et al. 2024). The result of almost 300 days of MHW in some areas for the 2022 suggest that the current methodology may not be complete, as MHW are expected to be an extreme phenomenon. Global assessments such us Oliver et al. (2018) and Schlegel et al. (2019) had demonstrated the correlation between long term mean SST trends and some MHWs properties, but may this influence be strong enough to invalidate the results? In this study we present a sensitive experiment of the affection of long term mean trends of SST and MHW detection by using different detrending methods. Also considering different refence periods. It is performed in the IBI domain which covers upwellings, straits, bays, continental shelfs, open waters, etc. The study aims to investigate how the presence of medium to long-term trends may condition the MHW properties in different key oceanographic areas. In this way, we can differentiate regions where the variability of MHW is not conditioned by SST trends from those where it is and its magnitude. Castrillo-Acua, L., Alonso-Valle, A., de Pascual-Collar, A.: Characterization of Marine Heat Waves in the IBI Region in 2022. Manuscript submitted to the 8th edition of the Copernicus Ocean State Report (OSR8), Copernicus Publications, State Planet, 2024. Oliver, E. C. J., Donat, M. G., Burrows, M. T., Moore, P. J., Smale, D. A., Alexander, L. V., Benthuysen, A., Feng, M., Sen Gupta, A., Hobday, A. J., Holbrook, N. J., Perkins-Kirkpatrick, S. E., Scannell, H. A., Straub, S. C., and Wernberg, T.: Longer and more frequent marine heatwaves over the past century. Nature Communications, 9(1), Article 1. https://doi.org/10.1038/s41467-018-03732-9, 2018. Schlegel, R. W., Oliver, E. C. J., Hobday, A. J., Smit, A. J. : Detecting Marine Heatwaves With Sub Optimal Data. Frontiers in Marine Science, 6. https://www.frontiersin.org/articles/10.3389/fmars.2019.00737, 2019.