Origins of the Indonesian Throughflow controled by the Mindanao Current strength: Simple Model Study
Toru Miyama (International Pacific Research Center, Frontier Research Program for Global Change, SOEST, the Univ. of Hawaii, 1000 Pope Road, Honolulu, HI,96822,USA)
e-mail: tmiyama@iniki.soest.hawaii.edu
Bo Qiu (Department of Oceanography, the Univ. of Hawaii)
Toshiyuki Awaji (Department of Geophysics, Graduate School of Science, Kyoto Univ., Kyoto 606-8502, JAPAN)
Abstract
Dependence of the Indonesian Throughflow (IT) upon the strength of the Mindanao Current (MC) is investigated using a reduced-gravity model. The horizontal resolution of the model is 1/4°in longitude and latitude. The strength of the MC is controlled by the various steady wind stress curl in the North Pacific Ocean in this study. The average total transport of the IT is found to be hardly affected by the wind strength in the northern hemisphere, which supports Godfrey (1989) island rule. However, the transport through each strait depends on the wind stress curl in the northern hemisphere. As the MC strengthens, the water from the MC increases in the IT passing through the Makassar Strait. The IT oscillates with a period about 40 days in some parameter range. This oscillation is due to the eddy detachment from the Mindanao Eddy. Analysis using constant absolute vorticity trajectories shows that this oscillation tends to happen with 70cm/s ＜υc ＜ 160cm/s where υc is the velocity of the MC at the southeastern coast of the Mindanao island.
1. Introduction
It is well known there is a powerful theoretical framework, so called Godfrey (1989) Island Rule, which estimates total Indonesian Throughflow (IT) transport. According to Godfrey (1989)'s Island. Rule, the transport of the IT is determined by the southern hemisphere wind.
Besides, many studies showed that the Mindanao Current (MC) is the main origin of the water of the Indonesian Throughflow [Miyama et al. (1995). Ffield and Gordon (1992), Bingham and Lukas (1994)]
One of the distinguished features of the MC that it strongly fluctuates in various time scales. Those fluctuations are related with wind stress variations of the Asian Monsoon (Masumoto and Yamagata 1991). Lukas (1988) found prominent quasi biennial variation in the MC transport estimated form tide gauge data. This feature was well reproduced in the model by Qiu and Lukas (1996). Some studies identified variations which had higher frequency than the seasonal variation. Direct velocity observation by Kashino et al. (1998) revealed oscillation with period of two-months (50-60 days). Recently, Qiu et al. (1998) thoroughly investigated the high frequency variation in MC and the Indonesian Seas.
Figure 1 (a) shows the interannual variation of the MC transport estimation using Sverdrup relation calculated with the FSU wind stress. In fact, the Sverdrup transport strongly fluctuates from nearly 0 to about 60 Sv. Figure 1 (b) and (c) correspond to quasi-biennial-timescale variation and ENSO-timescale variation respectively. Both signals have similar amplitude. There is even longer cycle or trend in the Sverdrup transport.
Godfrey (1989)'s Island Rule tells the total transport Is affected only by the wind stress in the southern hemisphere. This does not tell how the wind stress in the northern hemisphere, which is the main origin of the IT, affects the IT. In this study, we address this question in this study. We show the wind stress does affect the origin of the IT.
