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Samoan Passage |
| Daniel L. Rudnick |
The simplest notion of the ocean's circulation involves sinking at high latitudes and equatorward flow at depth. As a result of the earth's rotation, the equatorward flow is mostly confined to the western boundary of the ocean. The global meridional circulation of the ocean is an important component of the earth's climate system.
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| Figure 1. Bathymetry of the region surrounding the Samoan Passage. The locations of the six moorings in the passage are indicated as black circles. |
The Samoan Passage (10°S, 170°W) is a major conduit through which abyssal water moves northward in the Pacific Ocean. The deep western boundary current of the South Pacific originates near Antarctica, flows northward past New Zealand, and reaches a barrier north of Samoa formed by the Robbie Ridge to the west and the Manihiki Plateau to the east. The only appreciable pathway deeper than 4000 m is the Samoan Passage (Figure 1). The coldest and densest flow northward into the Central Pacific Basin must take place through the passage. This fortuitous arrangement of the bottom topography means that an array of current-meter moorings can determine the coldest transport in the Pacific's deep western boundary current at this latitude. The current meter array, designated PCM11 by the World Ocean Circulation Experiment, was sponsored by the National Science Foundation.
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| Figure 2. Mean along-passage current. The perspective is looking northward, and northward velocity is positive. Note the regions of high northward flow on the west side of the passage and the slightly southward flow on the east. The mean transport is 6.0 Sv. Squares indicate the locations of current meters. |
The central goal of the moored array was to quantify the transport beneath 4000 m. The six moorings, in the water from September 1992 through February 1994 were instrumented densely from 4000 m to the bottom with current meters measuring horizontal velocity and temperature (Figure 2). The 17-month mean along-passage velocity is strongly northward on the west with vanishing flow on the east side of the passage. Integrating the velocity over the area of the passage deeper than 4000 m yields a mean transport of 6.0 Sv (1 Sv = 1,000,000 m³/s). This mean transport is an important constraint on the general circulation of the ocean.
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| Figure 3. The transport deeper than 4000 m. The mean is 6.0 Sv, the standard deviation is 1.5 Sv, and variability at a period of 30 days is evident. The transport varies by an order of magnitude but is never southward. |
The temporal variability of the flow through the Samoan Passage is of interest. Because the ocean is large and difficult to observe, oceanographers have generally made estimates of circulation from one-time surveys of a certain region. The current meters return a time series of transport so that the one-time estimates may be interpreted in terms of natural variability. The Samoan Passage transport is quite variable but is never southward (Figure 3). The minimum and maximum transports are 1.1 and 10.7 Sv, easily bracketing all previously reported estimates. The standard deviation is 1.5 Sv and is due primarily to a roughly 30-day oscillation. This oscillation is speculated to be due to a resonance in the northward flow similar to that occurring in musical wind instruments. The observations suggest a permanent, but variable, northward flow through the Samoan Passage.
Acknowledgment. Dale Pillsbury and the OSU Buoy Group were primarily responsible for the excellent data return from the moorings. Dale also heads the WOCE Current Meter Data Assembly Center where a wealth of current meter data is available.
Johnson, G. C., D. L. Rudnick, and B. A. Taft, 1994: Bottom water variability in the Samoa Passage. J. Mar. Res., 98, 6883-6893.
Roemmich, D., S. Hautala, and D. Rudnick, 1996: Northward abyssal transport through the Samoan Passage and adjacent regions. J. Geophys. Res., 101, 14,039-14055.
Rudnick, D. L., 1997: Direct velocity measurements in the Samoan Passage. J. Geophys. Res., 102, 3293-3302.
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