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1 Interannual-to-decadal Variation of Tropical-subtropical
2 Exchange in the Pacific Ocean: Boundary Versus Interior
3 Pathways
4
5 Tong Lee, Ichiro Fukumori, Dimitris Menemenlis, Lee-Lueng Fu
6 Jet Propulsion Laboratory, California Institute of Technology
7
8 Tropical-subtropical exchange of water masses is considered
9 to be important to interannual-to-decadal variability in the
10 tropical Pacific. On average, the exchange is accomplished
11 by a shallow (< 400 m) meridional circulation which connects
12 the equatorial upwelling, poleward transport of surface
13 Ekman flow, and equatorward transport of pycnocline waters
14 originating from the subtropics. Pycnocline waters arrive
15 at the tropics via western-boundary and interior pathways.
16 It is well-established that, on average, the two pathways
17 re-enforces the net exchange. However, less is known about
18 their relative role on interannual-to-decadal time scales
19 and the underlying physical processes. In this study, we
20 address these issues using sea level measurements obtained
21 by the TOPEX/Poseidon satellite altimter and circulation
22 estimated by the Consortium for Estimating the Circulation
23 and Climate of the Ocean (ECCO; http://www.ecco-group.org).
24 Variation of pycnocline transport via the boundary is found
25 to be (1) anti-correlated to and (2) smaller in magnitude
26 than that of the interior. These features are attributed
27 to the combined effect of two different forcing mechanisms:
28 (1) the variation of local wind stress curl changes the
29 strength of horizontal circulation and results in variation
30 of boundary flow that is opposite in direction and
31 comparable in magnitude to that of interior flow; (2) the
32 variation of equatorial zonal wind stress which affects
33 the strength of meridional circulation with net pycnocline
34 flow opposing the surface Ekman flow. Due to the partial
35 compensation by boundary flow, the convergence of
36 pycnocline waters into the tropics is about 50% of that
37 inferred from interior pycnocline flow alone. The net
38 pycnocline transport reflects ENSO forcing on interannual
39 time scales. There is less equatorward intrusion of
40 pycnocline water in the 1990s than in the 1980s,
41 consistent with recent observation. In the North Pacific,
42 variability of interior pycnocline transport is larger
43 than that through the boundary. This is different from the
44 time mean where the interior transport is substantially
45 smaller than that through the boundary.
46

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