A SIMPLE-MODEL OF THE MASS-EXCHANGE BETWEEN THE SUBTROPICAL AND TROPICAL OCEAN

A simple ventilated thermocline model is used to study the subtropical-tropical mass exchange. It is found that the water subducted in the western subtropical gyre (recirculating window) tends to recirculate within the subtropical gyre, while the water subducted in the eastern part (exchange window) tends to penetrate equatorward. The exchange window expands with an increased easterly wind or basin width on the southern boundary of the subtropical gyre, but shrinks with an increased wind curl within the subtropical gyre. Furthermore, the total exchange transport increases with the easterly wind or the width of the basin on the southern boundary of the subtropical gyre, but it is independent of subtropical wind. The ventilation mechanism is important in supporting the exchange transport. For wind with realistic strength at the southern boundary, the reduction of the exchange transport is about 15%-30% of the Ekman transport. Finally, relative to the exchange transport in the interior of the ocean, the exchange transport through the low-latitude western boundary current decreases with increased total exchange transport.