"An Open-Circuit and a Short-Circuit in the Pacific Ocean
Subtropical-Tropical Exchange"

Ichiro Fukumori, Tong Lee, Benny Cheng, and Dimitris Menemenlis

Water mass exchange between subtropical and tropical regions of the
Pacific Ocean has been hypothesized as being part of a mechanism
controlling inter-decadal changes in the nature of ENSO.  The pathway
of this exchange is analyzed using model estimates from 1980 to 2001 
of the Consortium for Estimating the Circulation and Climate of the  
Ocean (ECCO; http://www.ecco-group.org).

A passive tracer and its adjoint are employed, the evolution of which
describe, respectively, where the tagged water mass goes to and where
it comes from.  Over ten years, on average, water mass of the Nino3  
region can be traced back to eastern subtropical thermocline waters of
the northern (27%) and southern hemispheres (39%).  The Nino3 water 
subsequently returns to these subtropical latitudes in the upper
ocean.  But in contrast to the hypothesized "Subtropical Cell" (STC),
this circulation is an open-circuit with water returning to the
western regions of the two hemispheres (subtropical gyres) and to the
Indian Ocean.

Temporal variability causes the tropical circulation inferred from a
time-mean state to differ significantly from the average circulation.
In particular, non-seasonal, intra-annual variability significantly  
enhances the magnitude of the so-called interior pathway relative to 
that of the circuitous western boundary pathway.  Such short-circuit 
in the subtropical-tropical exchange may help better explain observed
tracer distributions.  Significant differences in circulation pathways
are also identified that are associated with El Nino and La Nina
events.  The strength of the subtropical-tropical water mass exchange
is found to have weakened during the 1990s.