Interdecadal variation of the transition zone chlorophyll front: A physical-biological model simulation between 1960 and 1990

The interdecadal climate variability affects marine ecosystems in both the subtropical and subarctic gyres, consequently the position of the Transition Zone Chlorophyll Front (TZCF). A three-dimensional physical-biological model has been used to study interdecadal variation of the TZCF using a retrospective analysis of a 30-year (1960-1990) model simulation. The physical-biological model is forced with the monthly mean heat flux and surface wind stress from the COADS. The modeled winter mixed layer depth (MLD) shows the largest increase between 30degreesN and 40degreesN in the central North Pacific, with a value of 40-60% higher during 1979-90 relative to 1964-75 values. The winter Ekman pumping velocity difference between 1979-90 and 196475 shows the largest increase located between 30degreesN and 45degreesN in the central and eastern North Pacific. The modeled winter surface nitrate difference between 1979-90 and 1964-75 shows increase in the latitudinal band between 30degreesN and 45degreesN from the west to the east (135degreesE-135degreesW), the modeled nitrate concentration is about 10 to 50% higher during the period of 1979-90 relative to 1964-75 values depending upon locations. The increase in the winter surface nitrate concentration during 1979-90 is caused by a combination of the winter MLD increase and the winter Ekman pumping enhancement. The modeled nitrate concentration increase after 1976-77 enhances primary productivity in the central North Pacific. Enhanced primary productivity after the 1976-77 climatic shift contributes higher phytoplankton biomass and therefore elevates chlorophyll level in the central North Pacific. Increase in the modeled chlorophyll expand the chlorophyll transitional zone and push the TZCF equatorward.