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