Marine heatwaves in coastal environment: detection in Patagonian Fjords, Case study of the semi-enclosed inner seas of North Chilean Patagonia

During the last decade, marine heatwaves (MHWs) attracted scientists’ attention since their intensity and frequency have increased in connection with human-induced global warming.
In spite of the fact that MHWs in the open ocean have been quite well studied over the last years, the comprehension of their formation and of their impacts on coastal environment remains understudied. Here, we focused our study on the detection of MHWs in the Inner Sea of Chiloé, a semi-enclosed fjord-like environment in the Chilean Patagonia. The detection of MHWs over large scales is based on the comparison between a long-term climatology, generally built using satellite data, and local temperature. However, the Inner Sea of Chiloé is a narrow coastal environment, composed of various fjords and channels and has a rainy and very cloudy weather. Those geographical and meteorological conditions are not favourable for MHW analysis using satellite data, since these are hindered by the presence of clouds and their spatial resolution is not optimal. The Inner Sea of Chiloé has been well sampled during the last 30 years, totalling more than 3 million samples (at both surface and depth); since we could not use satellite data to calculate the climatology, we decided to use in situ data. To interpolate scattered data into a continuous field, we used the tool “Data-Interpolating Variational Analysis in n dimensions” (DIVAnd) enabling to interpolate scattered data onto a regular grid in an optimal way, taking advantage of the information in the 4 dimensions. Hence, we had a 4-dimensional climatology with a spatial resolution of ~900m and a non-regular vertical resolution at 32 different depths, from surface to 400m depth.
Once the climatology was established, we were able to perform the detection of MHWs at different places of the sea. Here we show the example of the Reloncaví Sound, which experienced very intense marine heatwaves during summers, especially in summer 2021 where two successive and particularly intense MHWs occurred with a temperature of 3°C above the mean.