Tracing ventilation source of tropical pacific oxygen minimum zones with an adjoint global ocean transport model

Abstract

To better understand the changes in the ventilation of Oxygen Minimum Zones (OMZs) in the Eastern Tropical North Pacific (ETNP) and South Pacific (ETSP), we use an adjoint global transport model to trace the surface locations of their source waters. The transit times for surface waters to reach the OMZs are obtained for steady-state transport. For the ETNP OMZ, 26% of the water volume makes last contact with the atmosphere in the Equatorial North Pacific with mean transit time of 109 years. Mid-latitude North and South Pacific make comparable volumetric contributions (17% and 18% respectively), but have mean transit times that differ by a factor of 2 (186 and 382 years, respectively). The combined contribution of the Southern Ocean and North Atlantic accounts for 16% of the ETNP OMZ volume. The mean transit time from these remote regions is in excess of 2500 years. For the ETSP OMZ, 29% of water volume originates from the surface of the equatorial South Pacific with a mean transit time of 65 years while 36% originates from the mid-latitude South Pacific with a mean transit-time of 157 years. The relative contribution of the two regions indicates that the meridional ventilation pathway from the mid- latitudes is of equal importance as the ventilation via eastward equatorial current system. The mean transit-times computed here help clarify the timescales with which advective- diffusive processes link changes in surface ocean processes to the OMZs of the Eastern Tropical Pacific.