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Intraseasonal Variability in the Indo–Pacific Throughflow and the Regions Surrounding the Indonesian Seas

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  • 1 Department of Oceanography, University of Hawaii at Manoa, Honolulu, Hawaii
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Abstract

Intraseasonal oscillations in sea level, sea surface temperature, and upper ocean flow field have recently been observed in various locations surrounding the Indonesian seas. While the observed oscillations at these locations have similar wave periods ranging from 30 to 85 days, their forcing mechanisms can involve different dynamic processes. In order to clarify the dynamic processes essential for the observed intraseasonal signals, the authors adopted in this study a fine-resolution 1½-layer reduced-gravity model and carried out a series of model experiments by varying coastline geometry and surface wind forcing. From these carefully designed experiments, they show that the intense 50-day oscillations observed at the entrance of Celebes Sea are a result of Rossby wave resonance in which the frequency of cyclonic eddy shedding by the retroflecting Mindanao Current matches that of the gravest Rossby mode of the semi-enclosed Celebes Sea basin. These 50-day oscillations of the Celebes Sea have a large impact on the throughflow in the Makassar Strait and Banda Sea, but do not contribute significantly to the throughflow changes in the Lombok, Ombai, and Timor Straits.

The observed intraseasonal signals along the Sumatra/Java coasts in the eastern Indian Ocean are dominated by oscillations with wave periods of 50 and 85 days. The origin of these wave signals can be traced back to the central equatorial Indian Ocean where the surface zonal wind fluctuates intraseasonally with the same wave periods. The remotely induced coastal Kelvin waves along the Sumatra/Java coasts strongly influence the Lombok Strait, but exert no direct influence upon the Ombai and Timor Straits farther to the east; the gap of the Lombok Strait is found to conduit the Kelvin wave signals effectively into the Makassar Strait and the Banda Sea. The intraseasonal signals observed in the Timor Passage have a dominant wave period of 30–35 days. The model simulation further suggests the presence of a second spectral peak around 85 days. Both of these intraseasonal signals are attributable to the local, along-archipelago wind near the Timor Passage.

* Current affiliation: Japan Marine Science and Technology Center, Yokosuka, Japan.

Corresponding author address: Dr. Bo Qiu, Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Road, Honolulu, HI 96822.

Email: bo@lunarmax.soest.hawaii.edu

Abstract

Intraseasonal oscillations in sea level, sea surface temperature, and upper ocean flow field have recently been observed in various locations surrounding the Indonesian seas. While the observed oscillations at these locations have similar wave periods ranging from 30 to 85 days, their forcing mechanisms can involve different dynamic processes. In order to clarify the dynamic processes essential for the observed intraseasonal signals, the authors adopted in this study a fine-resolution 1½-layer reduced-gravity model and carried out a series of model experiments by varying coastline geometry and surface wind forcing. From these carefully designed experiments, they show that the intense 50-day oscillations observed at the entrance of Celebes Sea are a result of Rossby wave resonance in which the frequency of cyclonic eddy shedding by the retroflecting Mindanao Current matches that of the gravest Rossby mode of the semi-enclosed Celebes Sea basin. These 50-day oscillations of the Celebes Sea have a large impact on the throughflow in the Makassar Strait and Banda Sea, but do not contribute significantly to the throughflow changes in the Lombok, Ombai, and Timor Straits.

The observed intraseasonal signals along the Sumatra/Java coasts in the eastern Indian Ocean are dominated by oscillations with wave periods of 50 and 85 days. The origin of these wave signals can be traced back to the central equatorial Indian Ocean where the surface zonal wind fluctuates intraseasonally with the same wave periods. The remotely induced coastal Kelvin waves along the Sumatra/Java coasts strongly influence the Lombok Strait, but exert no direct influence upon the Ombai and Timor Straits farther to the east; the gap of the Lombok Strait is found to conduit the Kelvin wave signals effectively into the Makassar Strait and the Banda Sea. The intraseasonal signals observed in the Timor Passage have a dominant wave period of 30–35 days. The model simulation further suggests the presence of a second spectral peak around 85 days. Both of these intraseasonal signals are attributable to the local, along-archipelago wind near the Timor Passage.

* Current affiliation: Japan Marine Science and Technology Center, Yokosuka, Japan.

Corresponding author address: Dr. Bo Qiu, Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Road, Honolulu, HI 96822.

Email: bo@lunarmax.soest.hawaii.edu

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