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1 "An Open-Circuit and a Short-Circuit in the Pacific Ocean
2 Subtropical-Tropical Exchange"
3
4 Ichiro Fukumori, Tong Lee, Benny Cheng, and Dimitris Menemenlis
5
6 Water mass exchange between subtropical and tropical regions of the
7 Pacific Ocean has been hypothesized as being part of a mechanism
8 controlling inter-decadal changes in the nature of ENSO. The pathway
9 of this exchange is analyzed using model estimates from 1980 to 2001
10 of the Consortium for Estimating the Circulation and Climate of the
11 Ocean (ECCO; http://www.ecco-group.org).
12
13 A passive tracer and its adjoint are employed, the evolution of which
14 describe, respectively, where the tagged water mass goes to and where
15 it comes from. Over ten years, on average, water mass of the Nino3
16 region can be traced back to eastern subtropical thermocline waters of
17 the northern (27%) and southern hemispheres (39%). The Nino3 water
18 subsequently returns to these subtropical latitudes in the upper
19 ocean. But in contrast to the hypothesized "Subtropical Cell" (STC),
20 this circulation is an open-circuit with water returning to the
21 western regions of the two hemispheres (subtropical gyres) and to the
22 Indian Ocean.
23
24 Temporal variability causes the tropical circulation inferred from a
25 time-mean state to differ significantly from the average circulation.
26 In particular, non-seasonal, intra-annual variability significantly
27 enhances the magnitude of the so-called interior pathway relative to
28 that of the circuitous western boundary pathway. Such short-circuit
29 in the subtropical-tropical exchange may help better explain observed
30 tracer distributions. Significant differences in circulation pathways
31 are also identified that are associated with El Nino and La Nina
32 events. The strength of the subtropical-tropical water mass exchange
33 is found to have weakened during the 1990s.
34

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