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Decadal-Scale Variability in the Closure of the Wind-driven Gyres in the Western Equatorial Pacific and Implications for Heat and Salt Transport into the Indian Ocean

 

Roxana C. Wajsowicz

Dept. of Meteorology / JCESS, University of Maryland

 

Assuming the existence of a 2-D streamfunction, P to describe the depth-integrated flow over the thermocline, a general analytical method for determining the path of each streamline through a series of channels, which may be western boundary layers or straits, is presented. The method is applied to the western equatorial Pacific, where the equatorward western boundary currents, which close the northern and southern wind-driven tropical gyres, meet. Parameters determining whether streamlines of the cold, fresh Mindanao Current and warm, salty South Equatorial Current (SEC) turn east and enter the North Equatorial Countercurrent (NECC), or west and enter the Indonesian archipelago, are identified. Further, their contributions to flow in Makassar Strait, the Maluku passages and the Halmahera Sea are determined as analytic functions of the values of the streamfunction on Sulawesi and Halmahera, and at the interior edge of the west Pacific boundary layer at latitudes Yn, Yp. The latitude Yn depends on the degree of nonlinearity assumed, and is defined in general terms as the northernmost latitude at which ocean interior streamlines are deflected into the path of the SEC. The latitude Yp is that of the northern tip of Irian Jaya. The model shows that flow in Makassar Strait will always be from the North Pacific and that in the Halmahera Sea always from the South Pacific unless really perverse conditions exist in the western equatorial Pacific; only the transport will vary in time. In contrast, both water masses may pass through the Maluku passages, and variations in both composition and transport expected. These results are in broad agreement with the hydrographic data collected during the ARLlNDO cruises. Taking the single channel representation of the Indonesian archipelago, so that the fractional contributions are a function of P at Yn only, the Sverdrup streamfunction for the Pacific is used to represent variations in P (Yn) on interannual to interdecadal timescales. For Florida State University wind stresses from 1961 to 1998, there is substantial variability in P (Yn), which implies significant composition variations on all timescales considered irrespective of the choice of throughflow magnitude. As expected, the archipelago inflow composition is dominated by the North Pacific contribution, the more northerly the choice of Yn. In contrast, wind stresses derived from European Center for Medium-range Weather Forecasting 10m winds for the period 1987-95 yield a Sverdrup streamfunction, which varies little over possible latitudes of Yn, and importantly P is mainly positive over these latitudes giving a wholly SEC-fed archipelago inflow, and so a relatively fresh NECC. A further contrast is provided by wind stresses derived from SSM / I data for the period 1988-96. These yield a Sverdrup streamfunction, which varies little with time over equatorial latitudes, but takes on substantial negative values, so that the archipelago inflow is almost wholly fed by the Mindanao Current, and so a relatively salty NECC. Insufficient salinity data exists to confirm or refute the estimated FSU-based composition variations on interannual and intradecadal timescales. However, the interdecadal signal of a tendency towards a greater SEC contribution to the archipelago inflow over the last decade can be substantiated. Salinity profile data from the National Oceanographic Data Center, Washington D. C., shows a fresher western equatorial Pacific in the region of the SEC and its retroflection into the NECC in the epoch 1981-95 compared with the epoch 1966-80. The Pacific entrance to the Indonesian archipelago is saltier in the later epoch. There is a hint that this salty signal is carried through into the Indian Ocean over the latitudes of the throughflow outflow. However, on the northwest Australian shelf, the ocean is fresher.

 

 

 

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