Global patterns of ocean transparency: Implications for the new production of the open ocean

Climatological mean fields of ocean transparency derived from Secchi depth measurements held by the National Oceanographic Data Center are presented. Clearly evident are the major coastal upwelling regions off Peru, northwest and south Africa as well as the coastally enriched areas globally. The central gyre regions in both North Atlantic and North Pacific waters are separated by a sharp discontinuity in water clarity from the more northern waters; the change in Secchi depth across this subtropical front is equivalent to a difference of ∼1 mg chlorophyll a m −3 over meridional distances of 1000 km. Sufficient data exist in the western Pacific to demonstrate that the front migrates approximately 1000 km seasonally from a northern extreme at ∼28° N in September–November, to a southern extreme at 18° N in March–May. A simple explanation for this feature, which is also clearly evident in recent satellite ocean color images of the the northern ocean basins, is offered in terms of the effective vertical ‘upwelling’ and resulting new production associated with seasonal variations in deep convective mixing.