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

摘要

The North Brazil Current (NBC) forms an inter-hemispheric connection for South Atlantic tropical water into theNorthern Hemisphere and is the dominant circulation feature in the western tropical Atlantic. The NBC separatesfrom 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 ananticyclonic eddy, known as an NBC ring, five to eight times annually. The NBCR undergoes a seasonal increasein transport during boreal summer and fall, advancing well to the northwest (up to 10°N) with a correspondingshift in the position of ring generation. The strong currents of the NBCR and its rings adversely impact deepwateroil exploration lease blocks throughout the region. Deepwater exploration is expanding throughout the region, butlimited metocean observational data poses a major challenge for predicting the impact of the circulation ondrilling 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 driftingbuoys. Ring radii range from 80 to 320 km (40 - 170 n.mi.) and exhibit azimuthal surface currents from 70 to200 cm s~(-1) (1.3-3.8 knots). We describe the local impact of complete NBCR ring-shedding cycles. A state-of-theart 1/24~(th) 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 theglobal HYCOM model. A hindcast scenario of the observations in 2011-2012 is presented with emphasis on thepredictability horizon of the NBC and associated circulation.