Ventilation of the North Atlantic and North Pacific: Subduction Versus Obduction

Abstract

Ventilation in the North Atlantic and North Pacific is examined by analyzing the Levitus climatological data and the Hellerman and Rosenstein wind stress data. Ventilation between the permanent pycnocline and the overlying seasonal pycnocline and mixed layer consists of two physical processes: subduction and obduction. Subduction takes place mainly in the subtropical basin where surface water is irreversibly transferred into the permanent pycnocline below. Obduction takes place in the subpolar basin where water from the permanent pycnocline is irreversibly transferred into the mixed layer above. Veatilation in the North Atlantic and North Pacific can be clarified into four physically different regions: the subductive region, the obductive region, the ambiductive region where both subduction and obduction take place, and the insulated region where neither subduction nor obduction occurs. Although the total subduction rates in these two oceans are comparable, the total obduction rates are considerably different. In the North Atlantic, obduction is strong (23.5 Sv), consistent with the notion of the fast thermohaline circulation and the relatively short renewal time of the subpolar water masses in the Atlantic basin. Obduction is weak in the North Pacific (7.8 Sv), this is consistent with the sluggish thermohaline circulation and the slower renewal process of the subpolar water masses there. Accordingly, the water mass renewal time based on the subduction/obduction rate is calculated and compared with previous estimations.