Comment on egusphere-2022-827

Abstract

We examine 20-years of monthly global ocean color data and modelling outputs of nutrients using self-organizing map analysis (SOM) to identify characteristic spatial and temporal patterns of High Nutrient Low Chlorophyll (HNLC) regions and their association with different climate modes. Analyzing the properties of the probability distribution function of the global nitrate to chlorophyll ratio (NO3:Chl), we estimate that NO3:Chl>17 (mmol NO3/mg Chl) is a good indicator of the distribution limit of this unproductive biome that extends over ~25 % of the ocean. Trends in satellite-derived surface chlorophyll (0.6±0.4 to 2±0.4 % yr-1) suggest that HNLC regions in polar and subpolar areas have experienced an increase in phytoplankton biomass over the last decades. However, much of this variation is produced by a foremost climate-driven transition occurring after the year 2010, which resulted in a reduction in the extension of polar HNLC regions and an increase in their productivity. Chlorophyll variations at HNLC regions respond to all three major climate variability signals (Sea Surface Temperature, SST; El Niño Southern Oscillation, ENSO; and Meridional Overturning Circulation, MOC) and their annual and semiannual variabilities are coherent with seasonal temperature variations. At larger scales, ENSO driven variability (2–4 yr) and decadal-scale processes of heat uptake and redistribution by ocean circulation influence the HNLC extension. Our results are indicative of the long-term changes in phytoplankton biomass and productivity in the ocean and suggest global coupling in the functioning of distant biogeochemical regions.