Subduction Of The Low Potential Vorticity Water In The Central North Pacific And Transportation To The East Of Taiwan Island

The low potential vorticity (PV) water subductes into the pycnocline from the late winter mixed layer in the central North Pacific, and is transported to the east of Taiwan Island by the mean pycnocline flow. Based on previous researches, the climatological subduction rate in the North Pacific has been provided by using the assimalated ocean model data (SODA) and the general global ocean model (IAP-LASG-LICOM1.0). Three maximum subduction rate regions have been found in the subtropical North Pacific, which are considered as the formation areas of three subtropical mode waters (the low PV water in the pycnocline). The formation of the subtropical mode waters are influenced by variations of the sea surface wind sress and the net heat flux via the annaul subduction rate. Obvious interannaul-decadal variations of subduction rate have also been found in the three mode water formation areas, resluting from different mechanisms. Then the subsurface pathway has been quantitatively determined of low PV water from central North Pacific to the east of Taiwan Island. As a signal of subduction, the subsurface high salinity water have been studied, in the aspect of its formation and distribution in the north Pacific. Finally, effect by the transport of the low PV water has been discussed to the decadal variation in the North Pacific. The main innovations of this research are as follows:1. Character of the interannual and decadal variations of subduction rate have been given in the three subtropical mode water formation areas. Interannal variation of subduction rate in the western mode water formation area was stronger before 1985 and then trailed off with weaker amplitudes. In the central mode water formation area, the amplitudes of the annual variation were greater between 1976 to 1993. An abrupt change of subduction rate happened around 1993 in the eastern mode water formation area.2. Mechanisms have been given of interannual and decadal variations of the subduction rate in three mode water formation area. The net heat flux had dominant effect on the variation of the subduction rate in the western formation area. Variations of the sea surface zonal wind were the main reason of the subduction rate variation in the central area, and the surface wind curl played a critical role in the subduction rate in eastern formation area.3. It has been found that 30o-35oN,150o-170oE was one of the obvious interannual variation areas of the subduction rate in the North Pacific. The low PV water subducted in the 30o-35oN,150o-170oE area with potential vorticity 25.4σθ-25.8σθwas transported southwest to the east of Taiwan Island in the subsurface pathway and spent approximately 12 years in arriving to the east of Taiwan island after subduction. The decadal variation in the North Pacific ocean-atmospgere coupled system can modify the subduction rate of low PV water by changing the sea surface wind, and then affect the pycnocline thickness to the east of Taiwan island. Using the Argo float data, the existence of the subsurface pathway has been further comfirmed from the analysis the subsurface high salinity water in the subtropical North Pacific.4. A possible negative feedback of the oscillation has been found in the North Pacific with a period about 25 years. Stronger sea surface winds in the central North Pacific produce bigger subduction rate and more low PV water in the pycnocline from the winter mixed layer, and increase the thickness of pycnocline there. Subsequently, the low PV water is subducted in the 30o-35oN,150o-170oE area with potential vorticity 25.4σθ-25.8σθand then transfered southwest to the east of Taiwan Island in the subsurface pathway, with a period of about 12 years to arrive at the east of Taiwan island after subduction. Then the arrival of the low PV water enhances the local pycnocline thickness to the east of Taiwan island and strengthens the recalculating currents in the recirculation region; Finally, as a result of the stronger current, the heat content of upper ocean in the KOE becomes bigger, with more heat loss from sea surface to the atmosphere, leading to the weaker phase of the Aleutian Low and weaker sea surface wind.