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Effects of the Indonesian Throughflow on the Pacific and Indian Oceans

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  • 1 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California
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Abstract

Effects of the Indonesian Throughflow (ITF) on the circulation and thermal structure of the Pacific and Indian Oceans are studied by comparing solutions of a near-global ocean general circulation model with open and closed Indonesian passages from 1981 to 1997.

The ITF contributes to the maintenance of the model circulation system around eastern Australia and the southern Indian Ocean. Blockage of the ITF weakens the Indian Ocean South Equatorial Current and Agulhas Current and strengthens the East Australian Current. The ITF does not affect the Mindanao Current, but drains waters carried by this current into the Indian Ocean and thus reduces tropical–subtropical exchange in the North Pacific. Meanwhile, it helps maintain a stronger New Guinea Coastal Undercurrent and thus enhances tropical–subtropical exchange in the south. Water parcels traveling along the western boundary of the South Pacific cross the equator in the presence of the ITF but are confined to the Southern Hemisphere without the ITF. The southern “exchange window” in which subducted waters can reach the Tropics is wider with than without the ITF. Some parcels from the southern subtropics can reach the Tropics with open ITF, but recirculate back to the subtropical gyre when the ITF is closed. These “excess” recirculated parcels reach several degrees farther south and outcrop north of Tasmania. The consequence on sea surface temperature (SST) there may have an important implication to the local climate. Blockage of the ITF depresses the mean thermocline of the tropical Pacific, increases SST in the central to eastern equatorial Pacific, and thus reduces the SST difference between the warm pool and cold tongue. It also raises the mean thermocline of the Indian Ocean (especially the southern Indian Ocean) and decreases SST in the southern Indian Ocean.

Blockage of the ITF reduces seasonal-to-interannual thermocline fluctuations in the central to eastern equatorial Pacific because the resulting deeper thermocline attenuates fluctuations in response to local Ekman pumping. The opposite is true in the southern Indian Ocean for the interannual timescale. However, seasonal thermocline fluctuation in that area is weakened when the ITF is blocked (despite a shallower thermocline). It indicates that local Ekman pumping is not the dominant mechanism controlling seasonal thermocline variability there as previously suggested. Radiation of planetary waves from the ITF area and advection by the ITF-dependent South Equatorial Current could also be important. Blockage of the ITF also reduces interannual variability of SST in the eastern equatorial Pacific and enhances those in the tropical southern Indian Ocean and south of Tasmania.

The results indicate that the ITF may affect El Niño–Southern Oscillation by modifying tropical–subtropical exchanges, mean tropical thermocline structure, mean SST difference between the warm pool and cold tongue, and seasonal-to-interannual variabilities of thermocline depth and SST.

Corresponding author address: Tong Lee, Jet Propulsion Laboratory, MS 300-323, 4800 Oak Grove Drive, Pasadena, CA 91109. Email: tlee@pacific.jpl.nasa.gov.

Abstract

Effects of the Indonesian Throughflow (ITF) on the circulation and thermal structure of the Pacific and Indian Oceans are studied by comparing solutions of a near-global ocean general circulation model with open and closed Indonesian passages from 1981 to 1997.

The ITF contributes to the maintenance of the model circulation system around eastern Australia and the southern Indian Ocean. Blockage of the ITF weakens the Indian Ocean South Equatorial Current and Agulhas Current and strengthens the East Australian Current. The ITF does not affect the Mindanao Current, but drains waters carried by this current into the Indian Ocean and thus reduces tropical–subtropical exchange in the North Pacific. Meanwhile, it helps maintain a stronger New Guinea Coastal Undercurrent and thus enhances tropical–subtropical exchange in the south. Water parcels traveling along the western boundary of the South Pacific cross the equator in the presence of the ITF but are confined to the Southern Hemisphere without the ITF. The southern “exchange window” in which subducted waters can reach the Tropics is wider with than without the ITF. Some parcels from the southern subtropics can reach the Tropics with open ITF, but recirculate back to the subtropical gyre when the ITF is closed. These “excess” recirculated parcels reach several degrees farther south and outcrop north of Tasmania. The consequence on sea surface temperature (SST) there may have an important implication to the local climate. Blockage of the ITF depresses the mean thermocline of the tropical Pacific, increases SST in the central to eastern equatorial Pacific, and thus reduces the SST difference between the warm pool and cold tongue. It also raises the mean thermocline of the Indian Ocean (especially the southern Indian Ocean) and decreases SST in the southern Indian Ocean.

Blockage of the ITF reduces seasonal-to-interannual thermocline fluctuations in the central to eastern equatorial Pacific because the resulting deeper thermocline attenuates fluctuations in response to local Ekman pumping. The opposite is true in the southern Indian Ocean for the interannual timescale. However, seasonal thermocline fluctuation in that area is weakened when the ITF is blocked (despite a shallower thermocline). It indicates that local Ekman pumping is not the dominant mechanism controlling seasonal thermocline variability there as previously suggested. Radiation of planetary waves from the ITF area and advection by the ITF-dependent South Equatorial Current could also be important. Blockage of the ITF also reduces interannual variability of SST in the eastern equatorial Pacific and enhances those in the tropical southern Indian Ocean and south of Tasmania.

The results indicate that the ITF may affect El Niño–Southern Oscillation by modifying tropical–subtropical exchanges, mean tropical thermocline structure, mean SST difference between the warm pool and cold tongue, and seasonal-to-interannual variabilities of thermocline depth and SST.

Corresponding author address: Tong Lee, Jet Propulsion Laboratory, MS 300-323, 4800 Oak Grove Drive, Pasadena, CA 91109. Email: tlee@pacific.jpl.nasa.gov.

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