Ultra-oligotrophic waters expansion in the North Atlantic subtropical gyre revealed by 21 years of satellite observations

The North Atlantic Subtropical Gyre (NASTG) has experienced the fastest expansion of oligotrophic waters worldwide in response to ocean warming. We study the trophic regime changes in the NASTG by using 21 years (1998-2018) of satellite chlorophyll-a (CHL) data, complemented with other variables such as Sea Surface Temperature, optical backscatter coefficients (bbp), Secchi disk (zsd), and Mixed Layer Depth. To this aim, we describe the spatial/temporal variability of waters with the lowest CHL concentrations (CHL <= 0.04 mg m-3) with an inter-annual variability analysis of key environmental variables. Main results demonstrate that these ultra-oligotrophic waters are spatially expanding and increasing in frequency, shifting the NASTG to a dominant quasi-permanent ultra-oligotrophic condition, thus confirming the ongoing ocean desertification.