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It is commonly believed that the thermohaline circulation in the North Pacific is much weaker than that in the North Atlantic, because there is no deep water formation in the North Pacific. However, it is not yet clear to what extent we can disregard the thermohaline circulation in the North Pacific. We cannot exclude possibility that the upper-layer circulation of the North Pacific is regulated by the thermohaline effect. There is some evidence which indicates its importance. For example, the distribution of salinity shows that a part of the salinity minima forms in the subpolar gyre and extends into the subtropical gyre on the eastern side of the North Pacific, and that the North Pacific Intermediate Water (NPIW) extends into the tropical gyre on the western side. These features are believed to be due to cross-gyre flows, and not to effects of meso-scale eddies, although the latter effects are considered to be important in the Kuroshio-Oyashio confluence zone to transfer low salinity water to the subtropical gyre, forming NPIW. We consider that these cross-gyre flows are thermohaline.
In the present study, we intend to reproduce these cross-gyre flows in a coarse resolution, non-eddy-resolving, OGCM. First, we make process studies using a simple square basin model with ideal forcings. Three forcings are considered: side-wall cooling along the southern boundary, sea-surface differential heating, and wind-stresses. Case-studies are made with different combinations of these forcings. Next, we perform a simulation experiment using a model with realistic to topography and observed sea-surface temperature and salinity. It is found that at the upper thermocline depths in the subtropical and the tropical gyre, the flows in the poleward and westward region are wind-driven but those in the equatorward and eastward region are thermohaline. Our results suggest that the Okhotsk Sea is very important to induce the thermohaline circulation in the North Pacific, though its effect is not properly reproduced in the model. Incorporation of enhanced sub-grid phenomena caused by tidal effects along Kuril Islands into OGCM may be indispensable. The behavior of the Antarctic Intermediate Water (AAIW) may be accounted for in the same way as that of NPIW. Also, as is observed, AAIW can easily cross the equator as the thermohaline circulation.
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