Marine Heatwaves in Fjords: Combining In Situ and Satellite Data for Better Understanding

Marine heatwaves (MHWs) are phenomena characterized by anomalously warm water events that can persist from a few days to several months. These events are increasingly observed worldwide and are typically studied using satellite data with long temporal resolution. While MHWs in the open ocean have been well documented, their occurrence and dynamics in coastal environments remain poorly explored. Coastal areas, with their complex features such as the presence of islands, channels, gulfs or even fjords, pose significant challenges for observation using satellite data alone. Although high-resolution sea surface temperature datasets derived from satellites exist, they are often not able to resolve very complex areas such as fjord systems which feature highly complex bathymetry. In this study, we investigate the development of MHWs in the Chilean Patagonian Inner Sea, which is a fjord ecosystem, using a combination of in situ and satellite data. Since the 1990s, the Patagonian Fjords have been well sampled, resulting in about 3 million of samples across different depths. We used this data to construct a climatology of the sea temperature. To achieve a continuous field with daily temporal resolution and high spatial resolution, we interpolated the in situ data using the DIVAnd algorithm. It resulted in a climatology with a spatial resolution of ~900m that does resolve the fjords and channels of the study area. Detecting MHWs implies defining a temperature threshold above which conditions are considered “extreme”. Typically, this threshold is set as the 90th percentile of the temperature dataset. However, due to temporal and spatial sparsity of the in situ dataset in some areas, defining such a threshold was not feasible. To address this problem, we used sea surface temperature data from MODIS-AQUA (4km resolution) as a baseline to calculate the threshold over the study area, and subsequently interpolating it to match the high resolution of the climatology. Our findings reveal that MHWs intensity is generally higher in enclosed areas such as gulfs and fjords, with an average frequency of 1.5 to 2.5 event per year across most of the study area. An analysis of long-term trends shows that MHWs intensity is decreasing over time in the enclosed areas but increasing in the open ocean. On the other hand, MHWs frequency is mostly increasing except in one of the basins that is characterised by faster ocean dynamics. Over the past two decades, we identified several particularly strong and/or prolonged MHWs. To understand which role played the atmospheric parameters, we used ERA5 reanalysis data to investigate their interactions with MHWs. Our results are showing that the reduction of the cloud coverage (resulting in an increase of the solar radiation), atmospheric heatwaves and reduced winds are often linked to the development of MHWs. Additionally, we investigated the relationship between MHWs and variations in chlorophyll concentration within this ecosystem. To study the changes in chlorophyll concentration during MHW events, we used OLCI datasets with a resolution of 4km and 300m to observe the changes in chlorophyll concentration during MHWs events. This analysis offers insights into how these extreme thermal events impact primary productivity and ecological dynamics in the region.