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A Simple Model of the Indonesian Throughflow and Its Composition

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  • 1 The Institute for Low Temperature Science, Hokkaido University, Sapporo, Japan
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Abstract

A simple analytical model of the closure of the wind-driven gyres in the tropical west Pacific is developed to illustrate the role of Halmahera and variations in the Pacific wind stress on the water mass composition of the flow from the Pacific to the Indian Ocean through the Indonesian archipelago—that is, the Indonesian Throughflow. The model is expressed in terms of the streamfunction, ψ, for the depth-integrated velocity field. There are three principal dynamical states, and therefore composition regimes. In the absence of Halmahera, the throughflow is composed of water wholly originating from the South Pacific (SP) if the streamfunction at the interior edge of the western boundary layer at the latitude of the northern tip of Papua New Guinea (PNG), ψN, is greater than that on the Asian continent, taken as zero, which is typically the situation in the Northern Hemisphere late summer. The throughflow is composed of water of wholly North Pacific origin (NP) if ψN ≤ ψA, the value of the streamfunction on Australia–PNG, which corresponds to winter conditions. The throughflow is fed by both the fresh Mindanao Current and the saltier South Equatorial Current (SEC) otherwise. If Halmahera is taken into account, the criteria determining the composition regimes are modified. Its presence results in a fresher throughflow, 30% NP–70% SP compared with 100% SP, as ψ typically decreases northwards in the equatorial west Pacific. The result of averaging over perturbations in ψN, ψA is also considered.

Estimates of the throughflow composition from observed hydrography imply a predominantly NP source. A possible explanation is nonlinear retroflection of the SEC into the North Equatorial Countercurrent (NECC), which would lead to the throughflow being fed by the Mindanao Current for much of the year. However, the simple model predicts that a large amount of water of NP origin enters the Indonesian basins before exiting to feed the NECC, even when the throughflow is predicted to be wholly SP. Mixing within the basins would modify the composition of the NECC and throughflow. Different west Pacific states and their effect on the composition are investigated further with a flat-bottomed, homogeneous numerical general circulation model with a simplified geometry and climatological monthly mean forcing.

Abstract

A simple analytical model of the closure of the wind-driven gyres in the tropical west Pacific is developed to illustrate the role of Halmahera and variations in the Pacific wind stress on the water mass composition of the flow from the Pacific to the Indian Ocean through the Indonesian archipelago—that is, the Indonesian Throughflow. The model is expressed in terms of the streamfunction, ψ, for the depth-integrated velocity field. There are three principal dynamical states, and therefore composition regimes. In the absence of Halmahera, the throughflow is composed of water wholly originating from the South Pacific (SP) if the streamfunction at the interior edge of the western boundary layer at the latitude of the northern tip of Papua New Guinea (PNG), ψN, is greater than that on the Asian continent, taken as zero, which is typically the situation in the Northern Hemisphere late summer. The throughflow is composed of water of wholly North Pacific origin (NP) if ψN ≤ ψA, the value of the streamfunction on Australia–PNG, which corresponds to winter conditions. The throughflow is fed by both the fresh Mindanao Current and the saltier South Equatorial Current (SEC) otherwise. If Halmahera is taken into account, the criteria determining the composition regimes are modified. Its presence results in a fresher throughflow, 30% NP–70% SP compared with 100% SP, as ψ typically decreases northwards in the equatorial west Pacific. The result of averaging over perturbations in ψN, ψA is also considered.

Estimates of the throughflow composition from observed hydrography imply a predominantly NP source. A possible explanation is nonlinear retroflection of the SEC into the North Equatorial Countercurrent (NECC), which would lead to the throughflow being fed by the Mindanao Current for much of the year. However, the simple model predicts that a large amount of water of NP origin enters the Indonesian basins before exiting to feed the NECC, even when the throughflow is predicted to be wholly SP. Mixing within the basins would modify the composition of the NECC and throughflow. Different west Pacific states and their effect on the composition are investigated further with a flat-bottomed, homogeneous numerical general circulation model with a simplified geometry and climatological monthly mean forcing.

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