I have carefully reviewed the manuscript and found no points that requires revision.

The manuscript “Establishing a Monitoring Approach for Marine Heatwaves in the Black and Mediterranean Seas” provides a comprehensive analysis of marine heatwaves (MHWs) in two climatically important semi-enclosed basins. Overall, this is a strong contribution that combines well-established surface temperature metrics with a novel subsurface perspective. The study addresses an important gap – previous MHW research in the Mediterranean Sea is extensive, but the Black Sea has seen very limited attention (indeed only one prior dedicated study). By tackling both regions together and integrating subsurface indicators, the authors advance our understanding of MHW characteristics in these basins. The work is timely, given the pronounced warming trends observed in the Mediterranean and Black Seas over recent decades, and the need for improved monitoring of extreme thermal events in the upper ocean.

My overall impression is very positive. To further improve the manuscript, I suggest the following minor refinements:

• Emphasize key findings in plain language: After sections of heavy analysis, add one or two sentences that plainly state what the numbers mean. These statements help ensure the reader takes away the main messages.
• SST vs OHC definition clarity: Clarify the parallel between the SST-based and OHC-based MHW definitions. The paper could explicitly say, “We apply the same 90th-percentile threshold approach to daily OHC anomalies (0–40 m) to define subsurface MHW events,” just to eliminate any doubt. This would reinforce that the two approaches are methodologically consistent.
• Figure presentation: Ensure all multi-panel figures are reader-friendly. Large composite figures might be split across pages or be hard to read. The authors might consider splitting a figure into two if it’s overly crowded (e.g. Figure 1), or at least use panel labels (a, b, c, …) and refer to them in the caption and text. For instance, instead of “rows 6 and 7 in Figure 1,” using “Figure 1f–g” (if panels are labelled) could be clearer.
• Discussion of implications: The conclusion could go a step further in discussing what the findings imply for ecosystem management or forecasting. The introduction references mass mortality events and bloom disruptions from MHWs. Perhaps the authors can speculate briefly in the conclusion about how having a subsurface MHW indicator might aid early warning systems for marine parks or fisheries in these seas. This would enhance the impact of the work for a broad audience.

Please consider my referee report attached

Please, find attached the comments.