The Southward Intrusion of North Pacific Intermediate Water along the Mindanao Coast

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  • 1 Joint Institute for Marine and Atmospheric Research, University of Hawaii at Manoa, Honolulu, Hawaii
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Abstract

A tongue of low salinity intermediate water was observed along the coast of Mindanao during the WEPOCS III experiment in June and July 1988. The tongue, delineated by a discontinuity in θ–S relations, is a southward intrusion of water at 26–27 σθ. It is the Northern Hemisphere counterpart of the northward flow of Antarctic Intermediate Water in the New Guinea Coastal Undercurrent. In the WEPOCS III data, it is seen entering the Celebes Sea near 5°N at the southern tip of Mindanao. Once it passes the southern tip of Mindanao, it circulates within the Celebes Sea. Relatively fresh water at 26.55 σθ is seen continuing to flow toward the Makassar Strait and into the Indonesian Throughflow, although some fraction mixes with intermediate water of equatorial Pacific and South Pacific origin and flows eastward in the northern subsurface countercurrent. The tongue is present in a number of sections along 8°N in the Mindanao Current, but it is often wider than is found in the WEPOCS III sections as measured by the distance from the Mindanao coast to the high horizontal salinity gradient edge of the tongue. The tongue plays a part in the exchange of water at intermediate density between the tropical and subtropical gyres. It is estimated that 6% of the northward salt transport in the Pacific at 8°N is accomplished by the southward flow of fresh intermediate water in the Mindanao Current. This transport is sensitive to fluctuations in the basin-scale wind forcing and is highly variable.

Abstract

A tongue of low salinity intermediate water was observed along the coast of Mindanao during the WEPOCS III experiment in June and July 1988. The tongue, delineated by a discontinuity in θ–S relations, is a southward intrusion of water at 26–27 σθ. It is the Northern Hemisphere counterpart of the northward flow of Antarctic Intermediate Water in the New Guinea Coastal Undercurrent. In the WEPOCS III data, it is seen entering the Celebes Sea near 5°N at the southern tip of Mindanao. Once it passes the southern tip of Mindanao, it circulates within the Celebes Sea. Relatively fresh water at 26.55 σθ is seen continuing to flow toward the Makassar Strait and into the Indonesian Throughflow, although some fraction mixes with intermediate water of equatorial Pacific and South Pacific origin and flows eastward in the northern subsurface countercurrent. The tongue is present in a number of sections along 8°N in the Mindanao Current, but it is often wider than is found in the WEPOCS III sections as measured by the distance from the Mindanao coast to the high horizontal salinity gradient edge of the tongue. The tongue plays a part in the exchange of water at intermediate density between the tropical and subtropical gyres. It is estimated that 6% of the northward salt transport in the Pacific at 8°N is accomplished by the southward flow of fresh intermediate water in the Mindanao Current. This transport is sensitive to fluctuations in the basin-scale wind forcing and is highly variable.

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