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Modeling Seasonal Variability in the Wind-driven Upper-Layer Circulation in the Indo–Pacific Region

Masamichi InoueCoastal Studies Institute and Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, Louisiana

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Susan E. WelshDepartment of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana

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Abstract

A basin-scale fine-resolution primitive equation reduced-gravity model forced by the climatological monthly wind of Hellerman and Rosenstein is used to study seasonal variability of the wind-driven upper-layer circulation in the Indo–Pacific region. The model domain is limited to the tropical Pacific Ocean and the eastern Indian Ocean. The use of the open boundary conditions allows the model to account for a net transport of mass from the Pacific to the Indian Ocean. An active eddy field is modeled east of Mindanao in the strong shear zone formed between the North Equatorial Current and the North Equatorial Countercurrent. This eddy field gives rise to the energetic shorter time scale variations in the western Pacific and appears to be responsible for considerable variability in surface currents noted previously. A distinct current bifurcation at the southern coast of New Britain in the Solomon Sea is modeled. The western branch of the resulting flow exits through Vitiaz Strait, while the eastern branch exits through Solomon Strait and St. George's Channel. The modeled annual cycle of the total Indonesian throughflow ranges from 0.5 Sv (Sv ≡ 106 m3 s−1) toward the Pacific in February to 18.1 Sv toward the Indian Ocean in July–October with a mean of 9.8 Sv. Among the three passages carrying the throughflow, Lombok appears to be the most dominant, accounting for nearly 40% of the total. Timor is a close second accounting for 37% of the total, while Savu carries the rest. Seasonal variability in transport through each of the three straits correlates well with pressure gradient between the two basins representing the flow path for each strait. The bulk of the water feeding the throughflow comes through Makassar Strait, with a significant additional contribution coining from the north through the Banda Sea during the peak season. Seasonal variability in the Indonesian throughflow appears to be dominated by the sea level lowering in the eastern Indian Ocean resulting from the seasonal development of coastal upwelling centered around south of Java. This coastal upwelling is in response to the seasonal development of the southeast trade winds during the boreal summer.

Abstract

A basin-scale fine-resolution primitive equation reduced-gravity model forced by the climatological monthly wind of Hellerman and Rosenstein is used to study seasonal variability of the wind-driven upper-layer circulation in the Indo–Pacific region. The model domain is limited to the tropical Pacific Ocean and the eastern Indian Ocean. The use of the open boundary conditions allows the model to account for a net transport of mass from the Pacific to the Indian Ocean. An active eddy field is modeled east of Mindanao in the strong shear zone formed between the North Equatorial Current and the North Equatorial Countercurrent. This eddy field gives rise to the energetic shorter time scale variations in the western Pacific and appears to be responsible for considerable variability in surface currents noted previously. A distinct current bifurcation at the southern coast of New Britain in the Solomon Sea is modeled. The western branch of the resulting flow exits through Vitiaz Strait, while the eastern branch exits through Solomon Strait and St. George's Channel. The modeled annual cycle of the total Indonesian throughflow ranges from 0.5 Sv (Sv ≡ 106 m3 s−1) toward the Pacific in February to 18.1 Sv toward the Indian Ocean in July–October with a mean of 9.8 Sv. Among the three passages carrying the throughflow, Lombok appears to be the most dominant, accounting for nearly 40% of the total. Timor is a close second accounting for 37% of the total, while Savu carries the rest. Seasonal variability in transport through each of the three straits correlates well with pressure gradient between the two basins representing the flow path for each strait. The bulk of the water feeding the throughflow comes through Makassar Strait, with a significant additional contribution coining from the north through the Banda Sea during the peak season. Seasonal variability in the Indonesian throughflow appears to be dominated by the sea level lowering in the eastern Indian Ocean resulting from the seasonal development of coastal upwelling centered around south of Java. This coastal upwelling is in response to the seasonal development of the southeast trade winds during the boreal summer.

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