How to better characterise marine heatwaves?

Marine heatwaves (MHWs) are prolonged periods of abnormally warm water temperatures in the marine environment. They can occur in various oceanic regions and affect both coastal and open ocean areas. MHWs are characterised by their intensity, duration, and extent. They occur when the ocean temperature exceeds the average seasonal or regional temperature by a significant margin and can last for weeks to months, impacting large areas of the ocean.

MHWs have significant ecological consequences, including changes in the distribution and abundance of marine species, coral bleaching events, disruptions to food chains, and negative impacts on ecosystems and fisheries. Climate change is believed to contribute to the frequency and intensity of MHWs.

The ongoing project "Detection and Threats of Marine Heatwaves - CAREHeat," funded by the European Space Agency (ESA) as part of the Ocean Health initiative, is dedicated to enhancing the current methodologies for detecting and characterising MHWs. CAREHeat aims to analyse the vertical propagation of MHWs through the development of 4D temperature fields using machine learning approaches. It also seeks to provide a global atlas of MHWs at the sea surface, advance the understanding of the physical processes involved in MHWs development, and assess the impact of MHWs on marine ecosystems and biogeochemistry.

CAREHeat has revisited the commonly used MHWs detection method known as the Hobday approach. The project conducts sensitivity studies on various parameters, including percentile thresholds and minimum event durations, to refine the detection methodology. Additionally, CAREHeat investigates the influence of sea surface temperature (SST) trends and atmosphere-ocean climate modes, such as El Niño Southern Oscillation (ENSO), to differentiate between slow-varying SST components and the abrupt changes characteristic of extreme MHW events.

The project provides multiple metrics alongside the Global MHWs Atlas data product to aid in the characterisation of MHWs. Furthermore, a machine learning approach based on observations has been employed to reconstruct a 4D temperature field from the sea surface to a depth of 300 metres, enabling the estimation of subsurface MHWs. This approach helps analyse the vertical propagation and potential phase shifts with surface events, offering insights into the impacts on ecosystems.

CAREHeat focuses on three regions of interest: the tropical Pacific, the western Mediterranean, and the Madeira Island region. The project has already yielded significant outcomes in terms of 2D and 4D analyses for these regions:

Marine heatwaves detection thresholds by 2x2° boxes in the Pacific Ocean in depths down to 300 m by machine learning approach. MHWs propagate in depth and can persist longer than visible at the surface, with impacts on ecosystems. (Credit: CLS)

CAREHeat is therefore contributing to our understanding and ability to predict MHWs by advancing our monitoring techniques, improving data analysis, and refining computer models. This knowledge is crucial for assessing the impacts of MHWs on marine ecosystems, human activities, and coastal communities. By better understanding and predicting these events, we can develop strategies to mitigate their effects and protect vulnerable ecosystems and communities.

For further information and to stay updated on the research and results of the CAREHeat project, you can visit their website at www.careheat.org and follow them on Twitter (@careheat_).