What are marine heatwaves?
Marine heat waves (MHW) are prolonged periods of high sea temperature at an extensive ocean. MHWs can affect both the surface and deeper oceanic levels and are present worldwide. In the last decades and associated with sea surface temperature (SST) warming, their magnitude and frequency have increased, with harmful impacts on ecosystems, marine industries and human activities.
The Mediterranean Sea is one of the most affected areas by MHWs, as it has experienced significant sea warming over the last decades. Both, the SST increase due to long-term anthropogenic change and the consequent increased MHW frequency have been having devastating effects on local ecosystems.
MHW 2024 daily evolution for the Mediterranean Sea
MHWs can be classified in categories depending on their intensity. In Hobday et al. (2018) a naming convention for MHWs was proposed stating four categories based on the maximum observed intensity.
| Category | Description |
|---|---|
| I Moderate | MHWs with a maximum intensity that does not double the distance between the seasonal climatology and the threshold detection value. |
| II Strong | MHWs with a maximum intensity doubling the distance from the seasonal climatology to the threshold, but does not triple it. |
| III Severe | MHWs with a maximum intensity that triples the distance between the seasonal climatology and the threshold detection value. |
| IV Extreme | MHWs with a maximum intensity that is four times or greater the distance between the seasonal climatology and the threshold detection value. These events are currently rare and very localised. |