A Consideration on the Mechanism of Generation, Stagnation and Disappearance of the Kuroshio Meander

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  • 黒潮大だ行の発生，停滞，消滅の機構に関する考察
  • クロシオ ダイダコウ ノ ハッセイ テイタイ ショウメツ ノ キコウ ニ カン

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A mechanism of the Kuroshio Meander is discussed by comparing some observedcharacteristics of the Kuroshio path with short-and long-term variations of the wind fieldover the North Pacific. It is suggested that the meander is caused by the blocking of theKuroshio current by the Izu-Ogasawara Ridge. The blocking occurs when the depth of themain current increases or when the vertical shear becomes weak. These structural variationsare closely related to the supposed baroclinic response of the North Pacific Subtropical Gyreto long-term variations of the wind field with a period of about 5-6 years. The KuroshioMeander is initiated by a trigger meander at the offing of Shikoku Island. The triggermeander is closely related to the supposed barotropic response of the gyre to short-termvariations of the wind field with a period of about 3-4 months.<BR>The barotropic response of the North Pacific Subtropical Gyre to the short-term variationof the wind field yields the rapid change of the vertical structure of the Kuroshio current.This change generates the trigger meander in combination with the complicated pattern ofthe continental slope at the offing of Shikoku Island. The trigger meander is carried awaytoward the Izu-Ogasawara Ridge by the Kuroshio current. When the baroclinic response ofthe gyre is favourable for the blocking of the main current, the trigger meander and thecold eddy grow fed by the upwelling of the deep water of the Kuroshio which is blocked atthe west of the ridge. The growing stops when the scale of the trigger meander reaches to thesize of the steady Rossby wave which corresponds to the over-all mean velocity of the Kuroshioat that time, because the meander exceeding the size of the steady Rossby wave moves westwardand separates from the ridge. Then the deep water of the Kuroshio at the west of theridge which has been under the hard constraint of the cyclonic circulation in the form ofthe cold eddy becomes possible to flow arround the ridge. The upwelling stops and thereremains only the general dissipation process of the available potential energy in the cold eddy.Then the meander gradually decreases its size and returns to the ridge when the meanderbecomes smaller than the steady Rossby wave at that time. It is blocked and begins togrow there again. In this way, the Kuroshio Meander behaves as a quasi-steady Rossbywave and stagnates at the west of the ridge until the baroclinic response of the gyre becomesunfavourable for the blocking of the Kuroshio current by the ridge.

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