Hidden Heat: The case of 2023 Gulf of Trieste Bottom Marine Heatwave

Abstract. In summer and autumn 2023, unusually high bottom temperatures were recorded at the Vida buoy (LTER site, 22 m depth) in the Gulf of Trieste (northern Adriatic, Mediterranean Sea). This long-term dataset enabled rare in situ observation of a bottom marine heatwave (BMHW), which began in early August and lasted over three months. At its peak, bottom temperature reached 4.3^°C above the 20-year average and 1.2^°C above the previous maximum recorded since 2002. Observations and modelling indicate that the BMHW was preconditioned by a prolonged drought starting in 2022 and continuing into 2023. This resulted in elevated surface salinity and weakened stratification, allowing unusually deep mixing that extended to the seafloor. Although 2023 was warm, heat was distributed over a thicker water column than usual, limiting sea surface warming but causing extreme temperatures at depth. The event persisted into autumn due to freshwater inflow, which re-established stratification and trapped the heat below. As surface temperatures remained moderate, the BMHW was undetectable by satellites or surface measurements. Only bottom sensors and models revealed the stress in deeper layers. Such events may therefore remain unnoticed while exerting extreme stress on the marine ecosystem. Our findings highlight reduced precipitation and deep mixing as overlooked drivers of subsurface marine heatwaves, which may become more frequent in similar shallow and stratified areas subject to increasingly frequent droughts driven by climate change.