Accelerated northward shift of the North Pacific transition zone chlorophyll front under greenhouse warming

Abstract The transition zone chlorophyll front (TZCF) has significant implications for marine ecosystems, fisheries and ocean carbon cycling in the North Pacific. Recent satellite observations show TZCF shifting notably northward at a rate of 1.40 degrees per decade from 1998 to 2022, accompanied by a reduction of the chlorophyll concentration in the TZ. Using CMIP6 models with longer data, we find that this northward shift of TZCF is robust and accelerates with increasing warming. The weakening of zonal wind stress under global warming leads to a shallower vertical mixed layer and reduces southward transport via horizontal Ekman. These changes result in decreased nutrients in the upper ocean, ultimately causing a reduction in phytoplankton biomass in the North Pacific TZ and a northward shift of the TZCF. Our findings reveal a significant intervention of anthropogenic warming on marine ecosystems and provides a framework for understanding the linkage between anthropogenic warming and phytoplankton dynamics in the open ocean.