Trends of satellite-derived thermal fronts and frontal chlorophyll concentration in marine hotspots: between 2003 and 2020

Since the mid-20th century, humans have had an unprecedented impact on Earth’s climate. Carbon dioxide emissions and other greenhouse gas have driven global warming, leading to rising ocean temperatures. Oceanic frontal activity is expected to have changed significantly due to climate change and warming oceans. Such changes could strongly impact local ocean biogeochemistry and marine ecosystems in marine hotspots, regions with the most rapid warming, causing changes and redistribution of biomass and impacting higher trophic levels. A better understanding of these changing fronts today will help us predict and manage future responses of such regions to ocean warming. We use MODIS Aqua satellite-derived products: sea surface temperature (SST) and chlorophyll-a concentration; and a histogram-based frontal detection algorithm to derive 18-year time series (2003-2020) of the spatial density, annual probability, mean strength (SST gradient), and mean chlorophyll concentration of oceanic fronts. This analysis is applied in nine global marine hotspots: South East Australia, South West Australia, South California-Mexico, South-Brazil Uruguay, South East Canada-a/b, Galapagos, South Africa, and South Indian Ocean regions. Our results show frontal density has decreased significantly (fewer fronts) in six of the nine hotspots from 2003 to 2020. Frontal probability shows a decreasing likelihood for fronts to occur in five in nine hotspots. However, the mean frontal strength has been increasing in three of the nine hotspot regions, and frontal chlorophyll has increased in five hotspot regions. Based on our current results, the preliminary conclusions are that fronts are becoming fewer but stronger and with higher chlorophyll concentrations in marine hotspots. We are still working on covering more hotspots to check how widespread this is.