Several studies beginning in the early 1990s (Johns et al., 1990; Didden and
Schott, 1993; Richardson et al., 1994) have shown that the North Brazil Current
(NBC) pinches off large anticyclonic eddies in the western tropical Atlantic
at about 8° N during the time of year (summer to winter) when the NBC
retroflects into the ocean interior to feed the North Equatorial Counter
Current (NECC). These NBC rings play a role in the Atlantic meridional
overturning cell (MOC) as they contribute to the transport of South Atlantic
upper ocean water into the North Atlantic.
Two separate studies to quantify
that portion of the MOC transport carried northward by NBC rings reached very
different conclusions. This is a result of their disparate estimates of the
number of anticyclones
transporting South Atlantic water from the equatorial gyre to the North Atlantic
subtropical gyre along the northeast coast of South America. Fratantoni
et al.
(1995, 2000) used a free-running, but realistic 1/4° Atlantic Ocean
model forced by monthly mean wind stresses (Hellerman and Rosenstein, 1983) and
an imposed Atlantic MOC with an amplitude consistent with Schmitz and
McCartney (1993). They concluded that two to four rings are shed each
year from the NBC and the intergyre transport by these rings is 20-25% of the
annual mean northward Atlantic MOC transport. Goni and Johns (2001) examined
about six years of TOPEX/POSEIDON satellite altimeter data in the NBC region
to arrive at a warm ring estimate of more than five per year possibly
carrying greater than 1/3 of the northward Atlantic MOC transport. This
includes eddies they identified in the region that may or may not have been
shed by the NBC retroflection.
The study presented here is an attempt to reconcile this significant
difference in estimated MOC transport by NBC rings. The methodology is a combination of the
previous two approaches in that it utilizes a realistic numerical ocean model
that assimilates satellite altimeter data. The model is nearly global in
coverage and even without data assimilation it simulates realistic currents,
including the currents of the equatorial Atlantic as well as NBC rings.
Using a data assimilating realistic ocean model may yield a better estimate
of the number of NBC rings shed each year because the model acts as a
dynamical interpolator of the assimilated data which facilitates determination
of the origin of the warm rings observed in the NBC region.
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