Interannual-to-decadal Variation of Tropical-subtropical
Exchange in the Pacific Ocean: Boundary Versus Interior 
Pathways 

Tong Lee, Ichiro Fukumori, Dimitris Menemenlis, Lee-Lueng Fu
Jet Propulsion Laboratory, California Institute of Technology

Tropical-subtropical exchange of water masses is considered
to be important to interannual-to-decadal variability in the
tropical Pacific.  On average, the exchange is accomplished 
by a shallow (< 400 m) meridional circulation which connects
the equatorial upwelling, poleward transport of surface
Ekman flow, and equatorward transport of pycnocline waters
originating from the subtropics.  Pycnocline waters arrive
at the tropics via western-boundary and interior pathways.
It is well-established that, on average, the two pathways 
re-enforces the net exchange. However, less is known about
their relative role on interannual-to-decadal time scales 
and the underlying physical processes.  In this study, we 
address these issues using sea level measurements obtained
by the TOPEX/Poseidon satellite altimter and circulation  
estimated by the Consortium for Estimating the Circulation  
and Climate of the Ocean (ECCO; http://www.ecco-group.org). 
Variation of pycnocline transport via the boundary is found 
to be (1) anti-correlated to and (2) smaller in magnitude   
than that of the interior.  These features are attributed   
to the combined effect of two different forcing mechanisms: 
(1) the variation of local wind stress curl changes the
strength of horizontal circulation and results in variation
of boundary flow that is opposite in direction and
comparable in magnitude to that of interior flow; (2) the
variation of equatorial zonal wind stress which affects  
the strength of meridional circulation with net pycnocline
flow opposing the surface Ekman flow.  Due to the partial 
compensation by boundary flow, the convergence of 
pycnocline waters into the tropics is about 50% of that
inferred from interior pycnocline flow alone.  The net 
pycnocline transport reflects ENSO forcing on interannual
time scales.  There is less equatorward intrusion of
pycnocline water in the 1990s than in the 1980s,
consistent with recent observation.  In the North Pacific,
variability of interior pycnocline transport is larger
than that through the boundary.  This is different from the
time mean where the interior transport is substantially 
smaller than that through the boundary.