Jet Streams of the Tropical Atlantic: Observing and Forecasting the North Brazil Current Retroflection

Abstract The North Brazil Current (NBC) forms an inter-hemispheric connection for South Atlantic tropical water into the Northern Hemisphere and is the dominant circulation feature in the western tropical Atlantic. The NBC separates from the South American coastline near 6–8°N and retroflects to feed the North Equatorial Counter Current (NECC). The NBC Retroflection (NBCR) typically pinches off to form a closed circulation and then sheds an anticyclonic eddy, known as an NBC ring, five to eight times annually. The NBCR undergoes a seasonal increase in transport during boreal summer and fall, advancing well to the northwest (up to 10°N) with a corresponding shift in the position of ring generation. The strong currents of the NBCR and its rings adversely impact deepwater oil exploration lease blocks throughout the region. Deepwater exploration is expanding throughout the region, but limited metocean observational data poses a major challenge for predicting the impact of the circulation on drilling operations. We present characteristic patterns of the NBC and the NBCR derived from 11 years (2002–2012) of remotely-sensed synoptic observations (color, temperature and height) and satellite-tracked drifting buoys. Ring radii range from 80 to 320 km (40 – 170 n.mi.) and exhibit azimuthal surface currents from 70 to 200 cm s-1 (1.3–3.8 knots). We describe the local impact of complete NBCR ring-shedding cycles. A state-of-the art 1/24th degree resolution HYbrid Coordinate Ocean Model (HYCOM) was configured and tuned for the region. The domain and resolution were chosen to capture the NBC from its origin in the Atlantic and nested within the global HYCOM model. A hindcast scenario of the observations in 2011–2012 is presented with emphasis on the predictability horizon of the NBC and associated circulation. Introduction The North Brazil Current (NBC) is a warm, northward-flowing western boundary current that dominates the circulation of the western tropical Atlantic. The NBC originates in the southern hemisphere from the westward- flowing South Equatorial Current (SEC). It flows along the South American continental slope to latitudes near 6°–8° where it separates from the coast as the North Brazil Current Retroflection (NBCR) and ultimately joins the eastward-flowing North Equatorial Counter Current (NECC). The NBC varies from a shallow, coastally trapped flow in the spring, with nearly half of its transport confined over the Brazilian shelf, to a broader, deeper-reaching flow in the fall [Johns et al., 1998]. Minimum and maximum transport occurs in boreal spring and late summer/fall, respectively, with most of the seasonal transport variation confined to the upper 300 m [Johns et al., 1998]. During the months of maximum transport, the NBC and NBCR current typically exceed >3.0 knots at the surface with strong flow present to depths of at least 150 m.