Overcoming Challenges in Coastal Marine Heatwave Detection: Integrating In Situ and Satellite Data in Complex Coastal Environment

Abstract. Marine Heatwaves (MHWs) and Marine Cold Spells (MCSs) are extreme sea temperature events characterised respectively by unusually warm and cold conditions, that can persist from days to months. Both events occur in every ocean, but MHWs are becoming more frequent and intense due to global warming, to the detriment of MCSs. They are generally studied using satellite data with long temporal resolution to build a long-term climatology of the sea temperature. While MHWs and MCSs occurring in the open ocean have been well studied and documented, their occurrence and dynamics in coastal environment remain poorly understood, mainly due to the lack of data available. Indeed, coastal regions exhibit complex characteristics due to intricate coastal dynamics, the presence geographical features such as of gulfs, islands, channels or fjords, posing significant challenges for satellite observation.

In this study, we investigate the development and characteristics of MHWs and MCSs in Chilean Northern Patagonia over the period 2003–2023, as well as their seasonality and trends. Chilean Northern Patagonia is characterised by its complex geography, with the presence of thousands of islands, fjords, channels and gulfs. We present here a new methodology for MHWs and MCSs detection, using a combination of in situ and satellite data. Since the 1990s, Northern Patagonia has been well sampled, resulting in approximatively 3 million of samples across different depths. We interpolated these data using the DIVAnd algorithm to obtain a daily climatology of the sea temperature at 32 different depths (from the surface to 400 m) with a spatial resolution of 900 m that does resolve all the channels and fjords of the study area. Satellite data were used for threshold determination and to compare daily temperature to the climatology. The combination of the in situ-based dataset and the satellite-based do enable the detection of MHWs at very high resolution.

Our findings reveal that MHWs tend to be more frequent across most of the study area, whereas MCSs are becoming less common. MHWs and MCSs intensity is generally much higher in the most enclosed basins of the study area, with an average intensity of 2.5 °C for MHWs and -1.5 °C for MCSs in those places. However, MCSs intensity tends to increase over time whereas MHWs intensity tends to decrease in most basins. Some case studies are also described, including a succession of MCSs and MHWs in 2007–2008, and prolonged MHWs conditions during 2016–2017.