Marine Heatwaves and Stratification Extremes in the Gulf of Lion

In the Gulf of Lion (GOL), in the northwestern Mediterranean Sea, a phenomenon known as deep convection often occurs in the late winter / early spring. It occurs when the vertical stability, or stratification, of the ocean is eroded until becoming neutral, allowing the column to overturn. Following these overturning events, there are large phytoplankton blooms, as the mixing evenly distributes oxygen and nutrients throughout the column. Western Mediterranean dense water is also formed by this overturning and aids in the overall Mediterranean Sea thermohaline circulation.

Deep convection is primarily driven by air-sea fluxes on the yearly timescale, forming an annual cycle. When these fluxes turn significantly negative in the fall and winter, cooling the ocean surface, they drive the aforementioned erosion of stratification, leading to deep convection. However, if these fluxes do not provide sufficient cooling, the column retains some stratification and overturning won't occur. Similarly, if enough stratification accumulates during the summer when the air-sea fluxes are positive, typical fall/winter fluxes can be prevented from completely removing it.

Marine heatwaves (MHWs) are anomalously warm sea surface temperature (SST) events, typically identified as extreme SST events above the 90th percentile SST. They are driven either by anomalously warm air-sea fluxes and/or warm advected water masses. These events can significantly affect the local air-sea fluxes and marine biology. One particular impact they can have is increasing the local stratification, by increasing the vertical temperature gradient at/near the surface. For areas with an annual stratification cycle that can result in deep convection, e.g. the GOL, this is a concerning potential consequence of MHWs. For example, in the GOL, 2022 and 2023 saw a significant number of MHWs: 130 and 90 days per year of extreme SSTs, respectively.

In this work, we analyse Infrared Atmospheric Sounding Interferometer (IASI) SST data with in-situ Argo float measurements to observe the co-occurrence of MHWs and extreme stratification events from 2007 to 2024, to evaluate the impact of MHWs on the deep convection cycle in the GOL. During this period, 731 days in the GOL were marked as MHWs, occurring roughly 11.7% of the time. However, of this 11.7%, MHWs only co-occurred roughly half (~45.1%) of the time with an extreme stratification event somewhere in the vertical column. When examining the co-occurrence with extreme stratification per vertical ocean layer, this percentage drops to around 10% for most of the layers.

This means, despite MHWs becoming more frequent with climate change (8 event days in 2010 in the GOL versus 90 in 2023), in the GOL only half of these events co-occur with extreme stratification. While this seems large, to further quantify the impact, the anomalous extreme stratification are small enough in magnitude to be potentially removed by ~2.5 weeks of probable surface cooling (~-100 MJ of integrated, anomalous heat fluxes required, achievable with ~18 days of ~-75 W/m² anomalous heat flux). Therefore, marine heatwaves do not appear to significantly impact the deep convection cycle of the Gulf of Lion, even as they become more frequent.