Variations in gyre closure at the water mass crossroads of the western equatorial Pacific Ocean

Assuming the flow over the depth of interest may be described by a 2D strea mfunction, psi, a general analytical method for determining the path of eac h streamline through a series of channels, which may be western boundary la yers or straits: is presented. At each intersection, a channel operator, C- a(b) = mid(psi(a), psi(b), psi(C)), is defined, where psi(a), psi(C) are th e values of psi on either side of the channel, and the operator is applied to the bounding streamlines of the how psi(b). By assigning water mass prop erties to each streamline entering the system, the composition of the flow in each channel is determined. The method is applied to the western equatorial Pacific, where the equatorw ard western boundary currents, which close the northern and southern wind-d riven tropical gyres, meet. Parameters determining whether streamlines of t he cold. Fresh Mindanao Current and warm, salty South Equatorial Current (S EC) turn east and enter the North Equatorial Countercurrent (NECC), or west and enter the Indonesian archipelago, are identified. Further, their contr ibutions to how in Makassar Strait. the Maluku passages, and the Halmahera Sea are determined as analytic functions of the values of the streamfunctio n on Sulawesi and Halmahera, and psi(F)(y), the value at the interior edge of the west Pacific boundary layer. at latitudes y(N), y(P). The latitude y (N) depends on the degree of nonlinearity assumed, and is defined in genera l terms as the northernmost of either the latitude of the northern tip of H almahera or the northernmost latitude at which ocean interior streamlines o f the NECC originate from the SEC. The latitude y(P) is that of the norther n tip of Irian Jaya. In general, outside parameter ranges where the archipelago inflow is from a single source, the model gives that the fractional contributions to the in flow, and so NECC, are less sensitive to variations in the west Pacific, th e larger the throughflow. Also, the larger the throughflow, the more South Pacific component is present in the archipelago inflow, and so the fresher the NECC, for given conditions in the western equatorial Pacific. Interesti ngly, the model's Indonesian channels act as a dynamic filter on the inflow with that from the North (South) Pacific entering through the western (eas tern) channels: which is consistent with observations. Taking the single channel representation of the Indonesian archipelago so t hat the fractional contributions are a function of psi(N) = psi(F)(y(N)) on ly, the Sverdrup streamfunction for the Pacific is used to represent variat ions in psi(N) on interannual to interdecadal timescales. For Florida State University (FSU) wind stresses from 1961 to 1998, there is substantial var iability in psi(N) that implies significant composition Variations on all t imescales considered irrespective of the choice of throughflow magnitude. A s expected, the archipelago in-flow composition is dominated by the North P acific contribution, the more northerly the choice of y(N) In contrast, win d stresses derived from European Centre for Medium-Range Weather Forecasts 10-m winds for the period 1987-95 yield a Sverdrup streamfunction that vari es little over possible latitudes of y(N), and importantly psi is mainly po sitive over these latitudes giving a wholly SEC-fed archipelago inflow. and so a relatively fresh NECC. A further contrast is provided by wind stresse s derived from SSM/I data for the period 1988-96. These yield a Sverdrup st reamfunction, which varies little with time over equatorial latitudes, but takes an substantial negative values, so that the archipelago inflow is alm ost wholly fed by the Mindanao Current, and the NECC is relatively salty. Insufficient salinity data exist to confirm or refute the estimated FSU-bas ed composition variations on interannual and intradecadal timescales. Howev er. the interdecadal signal of a tendency toward a greater SEC contribution over the last 15 years can be substantiated. Salinity profile data from th e National Oceanographic Data Center show a fresher western equatorial Paci fic in the region of the SEC and its retroflection into the NECC in the epo ch 1981-95 compared with the epoch 1966-80. The Pacific entrance to the Ind onesian archipelago is saltier in the later epoch.