Search

Recent observations collected as part of the North Brazil Current Rings Experiment are used to assess the role played by NBC rings in tropical to subtropical cross-gyre transport in the Atlantic Ocean. During the course of the 20 month experiment, four different NBC Rings were surveyed by ships and twelve additional rings were identified by moored current meters and temperature/salinity recorders. Of the total of 16 rings observed, four were subsurface-intensified rings with little or no surface signal. Except for these subsurface rings, generally good agreement was found in the identification of NBC rings during the experiment by various techniques including satellite altimetry, ocean color, and inverted echo sounders. The observations of water properties in the ring cores provided by the in-situ temperature and salinity measurements are used to estimate the trapped core volumes of South Atlantic water in the rings. Based on these new measurements we estimate a ring formation rate of 8-9 rings per year, with no apparent seasonal variation in the formation rate. However, the surface rings show a seasonal cycle in their vertical penetration and associated trapped core volumes. Deeper rings tend to occur in fall and early winter, while shallower rings occur in spring and summer. The subsurface rings are usually smaller in diameter than the surface rings (average radius of maximum velocity 100 km versus 130 km), but have a thicker layer of trapped South Atlantic water and consequently a larger transport per ring. The average ring-induced transport including all ring types is about 1.1 Sv per ring, leading to an estimate of 9.3 Sv for the total annualized ring transport. This value is nearly twice that of most previous estimates, and suggests that NBC rings could account for more than half of the northward transport in the warm limb of the Atlantic meridional overturning cell.

Ten years of altimeter data are used in conjunction with temperature and salinity data within a two-layer reduced gravity approximation to investigate the shedding and translation of North Brazil Current rings. Space-time diagrams of sea height anomalies and residues along the altimeter groundtracks show large seasonal and interannual variability. Results presented here confirm previous estimates that indicate a shedding rate of 3 to 7 rings per year with no marked seasonal variability but with very strong year-to-year variability. Additionally, eddies not shed by the retroflection travel through the region as well. Most of the rings pass very near of Barbados, affecting the environment in the region, of which seven rings during the study period are seen to enter into the Caribbean Sea. A link is found in this study between long-term surface temperature changes in the tropical Atlantic and the number of rings shed at the NBC retroflection, where periods of time with warmer surface temperatures are associated to a higher number of rings shed.

Output from a very high resolution (1/12 deg.) North Atlantic simulation with the Miami Isopycnic Coordinate Ocean Model (MICOM) is analyzed in a region of the Tropical Atlantic characterized by the presence of the North Brazil Current (NBC) retroflection and North Brazil Current rings. The model mean and seasonal circulations present a good qualitative agreement with observations. Quantitatively, the modeled NBC in summer and fall does not completely retroflect into the North Equatorial Counter Current, and the model upper 100 m NBC is more intense than the observed values by 3-4 Sv. The modeled NBC generates a variety of rings, which we classify as ‘shallow’, ‘intermediate’, ‘deep’, and ‘subsurface’. An average of 8.3 rings of all types are generated per year, of which 6 are surface intensified, in good agreement with altimetry (5.7 rings per year, Goni and Johns, 2001). The transport of southern origin water by the the rings was estimated using two methods. First, the transport was computed kinematically from the rings' volume, resulting in an average transport of 6.6 Sv. Second, an estimation of southern water transport based on an explicit calculation of water mass content was done, resulting in an average transport of 7.5 Sv. The rings' contribution represents 0% of the total meridional transport from the surface to the intermediate water layers. Possible mechanisms operating in the model ring generation are briefly discussed.

The Intra-Americas Sea (IAS) (Caribbean Sea, Gulf of Mexico, Straits of Florida, and adjacent waters) is a semi-enclosed sea dominated by the throughflow of the Gulf Stream System that serves to link the physics, chemistry, ecology, and fisheries of the region. A regional GOOS, IAS-GOOS, has been conceived that exploits the geography and dynamics of the region. Euro-GOOS could play a role in the development of IAS-GOOS as well as with IOCARIBE-GOOS and individual national Coastal GOOS activities in the IAS.