Surface and subsurface marine heatwaves in a semi-enclosed basin: Modeling based investigation of the Marmara Sea

Marine heatwaves (MHWs) are prolonged periods of anomalously high sea temperatures that disrupt marine ecosystems. While surface MHWs have been widely investigated, their subsurface counterparts remain less understood due to limited long-term observations. This study presents the first basin wide, three-dimensional assessment of surface and subsurface MHWs in the Marmara Sea for 1987–2021 period, utilizing high-resolution hydrodynamic model outputs validated against satellite, in-situ, and ship-based temperature data. The Marmara Sea’s two-layer circulation, constrained by a persistent pycnocline, offers a natural laboratory to examine depth dependent MHW behavior in strongly stratified systems. Results reveal a profound vertical bifurcation in MHW behavior across the permanent pycnocline, governed by distinct physical drivers. Surface layers exhibit more frequent and intense but short-lived (averaging ∼14 days) MHWs, whereas subsurface layers are characterized by lower frequency and intensity but significantly higher persistence (up to 70 days). While surface MHWs are predominantly forced by atmospheric fluxes, subsurface extremes are identified as episodic advective phenomena linked to Mediterranean water inflows through the Dardanelles. We identify a striking inverse relationship between Eddy Kinetic Energy (EKE) and MHW duration; high EKE in the upper layers facilitates rapid heat dissipation, whereas the stagnant subsurface environment acts as a thermal trap, allowing anomalies to persist once established. Furthermore, the Bosphorus jet provides a localized cooling effect in summer, creating surface layer (0–10 m) thermal refugia in the eastern basin. These findings demonstrate that vertical stratification and water exchanges govern the distinct evolution of surface and subsurface MHWs, offering new insights into heatwave dynamics within semi-enclosed, stratified marine environments.