the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 May 2026
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.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Ocean Science.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-2567', Anonymous Referee #1, 29 Jun 2026
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RC2: 'Comment on egusphere-2026-2567', Anonymous Referee #2, 08 Jul 2026
This manuscript investigates a rare bottom marine heatwave observed in situ at a mooring buoy in 22 m of water in the Gulf of Trieste in 2023. The authors show that the event lasted for more than three months, with bottom temperatures reaching 4.3°C above the 20-year average, and that it was not detectable from satellite or surface observations.By estimating the mixed layer depth and analysing in situ salinity, precipitation, and runoff, the authors link the bottom marine heatwaves (MHW) to prolonged drought, elevated salinity, weakened stratification, and unusually deep mixing. They also use a model to present spatial maps of the bottom heat anomaly and its spatial extent, and to rule out ocean advection as the primary cause of the bottom MHW.This study is one of the first to show the role of salinity in stratification and the trapping of bottom heat, and is therefore very valuable. The availability of this long-term dataset is invaluable, and the mechanism identified here is likely important in many coastal areas. The authors provide an interesting estimate of the mixed layer depth from atmospheric forcing and background stratification, which appears to work well despite the limitations, which are clearly stated. The modelling study used to further investigate the generation of the event is also well motivated.However, the manuscript remains very qualitative and descriptive, and misses opportunities to synthesise and complete the story in a more robust way. I therefore recommend major revisions.My main concerns are as follows:
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The relative roles of the atmospheric forcing terms should be quantified more clearly. The manuscript argues that drought, evaporation, reduced freshwater input, elevated salinity, and weakened stratification were central to the development of the bottom MHW. However, the relative contributions of these processes remain mostly qualitative. The authors should quantify, at least approximately, the influence of net heat flux, evaporation, river run-off and precipitation on the mixed layer depth evolution. This should be possible from Equation 7 and would make the proposed mechanism much more convincing. The mixed layer depth estimate is one of the strengths of the manuscript, and it performs surprisingly well when compared with CTD estimates in Figure 7, so I would really like to see it used for a decomposition of terms, which would substantially strengthen the paper. Moreover, you seem to have discarded the potential effect of wind-driven downwelling, can you explain why?
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The model validation should be quantified. The model is used to support key conclusions about the spatial extent of the bottom heat anomaly and the processes responsible for its generation. However, the validation currently appears too descriptive. The authors should provide quantitative skill metrics comparing the model with available observations, such as temperature, salinity, and mixed layer depth. Even simple metrics such as bias, root-mean-square error, correlation, or variance explained would help assess how much confidence can be placed in the model results. This is particularly important because the model is used not only for illustration, but also to support mechanistic interpretation.
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A heat budget is ideal to assess the relative roles of advection and air-sea heat fluxes. The authors argue that ocean advection was not the primary cause of the bottom MHW, but this conclusion is not yet demonstrated robustly. The current descriptive figures are useful but insufficient to separate the effects of local air-sea forcing, vertical mixing, lateral advection, and storage. One of the main advantage of the model in the marine heatwave space is that ability to perform a heat budget. It would allow the authors to properly quantify the mechanisms responsible for the onset, persistence, and decay of the event. Without such analysis, the added value of the model remains limited and the conclusion regarding the role of advection is not fully supported.
- The presentation of the figures should be streamlined. The manuscript contains too many simple figures, with 14 figures in total. The authors should reconsider how the results are presented and merge several figures where possible. For example, Figure 11, showing the modelled MLD, could be overlaid on Figures 9 and 10. Figure 14 presents the same line in four panels, and Figures 3 and 7 are essentially the same. These examples are not exhaustive. Reducing repetition and combining related diagnostics would make the paper clearer and would help the reader focus on the main mechanism. Also, please revise the captions which need to contain all information (rather than in the text).
Citation: https://doi.org/10.5194/egusphere-2026-2567-RC2 -
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Fabio Giordano
Matjaž Ličer
Stefano Querin
Stefano Salon
In summer 2023, the Gulf of Trieste (Adriatic, Mediterranean Sea) experienced extreme bottom temperatures recorded by a thermometer at 20 m depth. However, the three-month bottom marine heatwave was not detectable from the surface. The event followed a prolonged drought: high salinity and density of surface waters caused unusually deep mixing, allowing warm surface waters to reach the seafloor and imposing extreme stress on bottom-dwelling biota.
In summer 2023, the Gulf of Trieste (Adriatic, Mediterranean Sea) experienced extreme bottom...
This paper presents in situ observations of a bottom marine heatwave (BMHW), based on a 22-year long temperature, salinity and wind record from the Vida buoy in the Gulf of Trieste, complemented by nested high-resolution MITgcm simulations of the 2023 event. The authors show that an extreme summer 2023 BMHW (up to 4.3°C above the long-term average, lasting over three months) developed concomitantly to only moderate surface warming, and attribute this to a salinity increase that weakened the stratification enough to let mixing by wind reach the seafloor. The combination of a rare two-decade in situ bottom record with a high-resolution, non-assimilative regional model, strengthen the interpretation and provides a convincing full picture. The resulting dataset is a valuable contribution and show how such in situ records are crucial. The manuscript is well written, the proposed mechanism is convincing, and the results are presented in a notably didactic and honest way, with the authors transparently flagging the approximations in their own analysis.
I only have three main suggestions:
A list of more specific minor comments is provided below.
- The manuscript would benefit from some consolidation of the figures, of which there are currently fourteen, several of which somehow overlap. Figure 7 (bottom panel) repeats Figure 3 almost exactly and could be removed (the MLD-event ellipses could go on figure 3). In Figure 14, the left-hand size time series panels are duplicates of one another with only the highlighted date changing and could be combined into a single panel, freeing up space to enlarge the corresponding maps. None of this is essential, but given the density of material already in the paper, a more compact set of main-text figures would improve readability. Regarding the figures, captions are not always self-contained, I would suggest the authors to revise captions so that each figure can be understood without reference to the main text.
- 21–22 and reference list: Gómez-Gras et al. 2021 is cited twice here but appear to only refer to one reference.
- 142: “Ponetalgoscuro” likely a typo for “Pontelagoscuro”.
- Fig. 4 caption: “Blue w lines” appears to be a typographical error.