The North Brazil Current
is a western boundary current in the tropical Atlantic that transports
upper ocean waters across the equator. Particularly during summer
and fall, the NBC retroflects from the coast at 6° to 7°N
and feeds the North Equatorial Countercurrent and North Equatorial
Undercurrent. During this retroflection phase large anticyclonic rings
are shed. These features then move northwestward toward the Caribbean
Sea, roughly paralleling the South American coastline. As part of
the NBC Ring study, an analysis of altimetric data was made (Goni
and Johns, 2001). Using a two-layer reduced gravity model, sea height
anomaly was converted into upper layer thickness. The thickness maps
are used to infer the NBC rings formation and propagation. Analysis
of the historical altimetric record indicates that ring shedding is
nearly a factor of two greater than previously estimated even though
the altimeter does not track all the rings formed at the retroflection,
(Garzoli et al., 2002).
North Brazil
Current rings characteristics
-
Transport
resulting from mean annual ring shedding:
The estimated yearly mass transported by rings is 9 Sv.
-
Interannual variability:
The available time series of ring shedding
derived from the altimeter is shown in Figure 3.
-
Annual cycle:
There are insufficient data to determine if
there is an annual signal in ring generations. However, the analysis
of the IES data obtained during the North Brazil Current ring experiment
(Garzoli et al., 2002) indicates that there is no seasonality.
Figure 3. Time
series of the latitude of penetration of the NBC retroflection estimated
from synoptic dynamic height maps. It is measured as the distance
(in km) between the northern most point of the retroflection and an
arbitrary point in space (0°N 40°W). Diamonds indicate the
time of a ring shedding. The northward motion can be considered as
the motion of the northward penetration (30km/day). The southward
motion is only a resetting of the index. As the ring separates, the
retroflection reforms further south.
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