VENTILATION OF THE NORTH-ATLANTIC AND NORTH-PACIFIC - SUBDUCTION VERSUS OBDUCTION

Ventilation in the North Atlantic and North Pacific is examined by ana lyzing the Levitus climatological data and the Hellerman and Rosenstei n wind stress data. Ventilation between the permanent pycnocline and t he overlying seasonal pycnocline and mixed layer consists of two physi cal processes: subduction and obduction. Subduction takes place mainly in the subtropical basin where surface water is irreversibly transfer red into the permanent pycnocline below. Obduction takes place in the subpolar basin where water from the permanent pycnocline is irreversib ly transferred into the mixed layer above. Ventilation in the North At lantic and North Pacific can be classified into four physically differ ent regions: the subductive region, the obductive legion, the ambiduct ive region where both subduction and obduction take place, and the ins ulated region where neither subduction nor obduction occurs. Although the total subduction rates in these two oceans are comparable, the tot al 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 shea renewal time of the s ubpolar water masses in the Atlantic basin Obduction is weak in the No rth Pacific (7.8 Sv); this is consistent with the sluggish thermohalin e circulation and the slower renewal process of the subpolar water mas ses there. Accordingly, the water mass renewal time based on the subdu ction/obduction rate is calculated and compared with previous estimati ons.