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Ocean-atmosphere interactions : the case of Marine Heatwaves in the North Atlantic
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Resumo(s)
Excessive greenhouse gas emissions are placing severe pressure on the oceans, which play a very important role in climate regulation. Consequently, the oceans are experiencing an exacerbated warming, leading to an increase in the occurrence of extreme events, such as Marine Heatwaves (MHWs). MHWs are characterized as prolonged periods of anomalously high sea surface temperatures (SSTs). Quantitatively, they are identified when SST anomalies exceed the 90th percentile of a reference climatology for at least five consecutive days. Understanding the occurrence of MHWs is challenging due to their rarity (less than 10 per cent of the surface temperature values, as the 90th percentile indicates) and the limited availability of consistent long-term observational data (last four decades). However, it is now known that they can be modulated by oceanic or atmospheric factors or a combination of both, affecting the ocean's ability to absorb incident solar radiation and dissipate it through currents and the mixing layer, leading to its warming. This study focused on large-scale atmospheric factors driving the MHW occurrence and characteristics in the North Atlantic (an area relatively understudied, especially when compared with the Mediterranean or with the Tasman Seas) from 1982 to 2022, with the objectives of (i) identifying spatial-temporal trends of MHWs, (ii) determining the atmospheric factors contributing to their occurrence, and (iii) exploring their relationship with prevalent climate variability modes. MHWs were identified using ESA Copernicus Climate Change Initiative Sea Surface Temperature data and Hobday et al. (2016)'s method. An event dataset generated using a new event detection algorithm developed by the team at +ATLANTIC CoLAB was used, with the events ranked by severity. Atmospheric data were obtained from ERA 5 reanalysis, including sea-level pressure, geopotential height, temperature, heat fluxes, solar radiation, and wind components. The study also analysed the North Atlantic Oscillation (NAO) index to examine its influence on MHWs. The results show positive trends in MHW frequency, duration, and intensity, especially since 1995, with regional variability. High-pressure systems with weak pressure gradients and associated reduced wind speeds, and increased solar radiation seem to be crucial for the formation of the analysed events. The annual NAO appears to modulate the spatial distribution of MHWs, with its positive phase favouring MHWs in mid-latitude regions, while the negative phase impacts subpolar and tropical regions.
Descrição
Tese de Mestrado, Ciências Geofísicas, 2025, Universidade de Lisboa, Faculdade de Ciências
Palavras-chave
Marine heat extremes Heat extreme trends Atmospheric Drivers Ocean-atmosphere interactions North Atlantic Oscillation