ARTICLE IN PRESS Deep-Sea Research II 52 (2005) 429–463 www.elsevier.com/locate/dsr2 Caribbean Current and eddies as observed bysurface drifters P.L. Richardsonà Department of Physical Oceanography, MS 29, Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543, USA Received 19 December 2003; accepted 29 November 2004 Abstract Recent satellite-tracked surface drifter trajectories were analyzed to describe the mean currents and eddies in the 1 Caribbean Sea. The structure of the Caribbean Current and its variabilitywere determined from high-resolution 2- degree maps of the mean velocityand eddykinetic energy.Looping drifter trajectories were used to identifydiscrete cyclones and anticyclones, and their characteristics were described and related to the structure of the mean flow. The translation rate of eddies in different areas was found to be similar to the mean velocityof the local background flow fields, suggesting that the eddies were largelyadvected bythe background flow. Ten energetic anticyclonestranslated westward at 13 cm/s in the Venezuela and Colombia Basins. These anticyclones tended to lie in two bands, centered near 151N and 171N, coinciding with two jets of the Caribbean Current. The northern weaker jet contains water primarilyfrom the North Atlantic; the southern stronger jet contains water from the tropical and South Atlantic. The anticyclones are thought to have formed in the eastern Caribbean from the anticyclonic vorticity derived from North Brazil Current rings. The ring vorticityenters the eastern Caribbean through island passages and is probablyamplified by the anticyclonic shear on the northern side of the jets. Southwest of Cuba a cyclone–anticyclone pair was observed to translate slowly( 2 cm/s) westward into the Yucatan Current. The cyclone was tracked for 10.5 months with four drifters, making it the longest tracked of the Caribbean eddies. r 2005 Elsevier Ltd. All rights reserved. 1. Introduction (Wilson and Leaman, 2000)(Fig. 2). Other drifters were launched east of the Caribbean in other Recently, a large number of satellite-tracked, experiments including the North Brazil Current drogued, drifting buoys measured trajectories in (NBC) rings experiment (Fratantoni and Richard- the Caribbean Sea (Fig. 1). Manydrifters were son, 2004); these drifters passed through the launched in the Caribbean during 1998–2000 as Antilles Islands passages adding trajectories in part of the ‘‘Year of the Ocean-1999’’ study the Caribbean. Taken together, these trajectories provide a unique data set that reveals previously ÃTel.: +1 508 289 2546; fax: +1 508 457 2163. unknown details of the circulation and eddies in E-mail address: [email protected]. the different parts of the Caribbean. The intent of 0967-0645/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2004.11.001 430 Caribbean Sea Isle of Pines 21° Cuba Yucatan G Yucatan R Peninsula Basin E A P.L. Richardson / Deep-Sea Research II 52 (2005) 429–463 T E Haiti Dominican R Republic 18° Cayman Basin Jamaica P.R. A N T I L L E L S ee 2000 m LE w S a ARTICLE IN PRESS S r E d 2000 m I s R l 200 m a n Venezuela A d Honduras s ° N 15 Basin 2000 s T d 2000 m I n L a l m L s I Columbia E Nicaragua d r Basin S a w d n 12° i W 200 m Costa Columbia Rica Venezuela Panama 9°N 87°W84° 81° 78° 75° 72° 69° 66° 63° Fig. 1. Location diagram showing the major Caribbean basins, the surrounding countries and islands, and the 200 m (dashed) and 2000 m (dotted) depth contours. The relativelyshallow Jamaica Ridge extends from the Honduras shelf to Hispaniola and separates the Yucatan and Cayman Basins of the western Caribbean f rom the Venezuela and Colombia Basins of the eastern Caribbean. Starting Points 21°N P.L. Richardson / Deep-Sea Research II 52 (2005) 429–463 18°N ARTICLE IN PRESS 15°N 12°N 9°N 87°W 84°W 81°W 78°W 75°W 72°W 69°W 66°W 63°W Fig. 2. Launch or start locations of 212 drifters used in this study. One hundred and eighteen drifters were launched in the Caribbean including 40 in the western Caribbean, 24 in the southern Colombia Basin, and 54 in the eastern Caribbean. Ninety-four others were launched outside the Caribbean including 66 that entered the box from the east across 611W and 13 that entered from the north across 221N. 431 ARTICLE IN PRESS 432 P.L. Richardson / Deep-Sea Research II 52 (2005) 429–463 this paper is to analyze these data with the goal of awayfrom the coast into deeper regions and vice learning more about the characteristics of Car- versa. This has important implications for the ibbean eddies in general and specificallyabout the dispersal of pollutants and fish larvae, e.g. Third, energetic anticyclones in the eastern Caribbean eddies appear to have some preferred pathways that might have been generated byNBC rings. through the Caribbean that cause bands of higher Satellite altimetrystudies suggest that some NBC and lower (rectified) mean westward velocity rings can pass coherentlythrough the Antilles coinciding with westward jets in the Caribbean passages into the Caribbean (Goni and Johns, Current and which suggests that eddy-mean flow 2003). The motivation for the present studyis to interactions could be important. Fourth, eddy see whether the drifter data could provide evidence forcing of the circulation of deeper layers in the that Caribbean anticyclones are generated by NBC Caribbean could be significant if the energetic rings. eddies extend deeplyinto the water column like Although the mean velocityfield and velocity some NBC rings. Fifth, Caribbean eddies can be variance have been mapped recentlyfor the advected into the Yucatan Current and influence Caribbean using surface drifters (Wilson and the path of the Loop Current in the Gulf of Leaman, 2000; Fratantoni, 2001; Centurioni and Mexico and the formation of Loop Current rings Niiler, 2003), and trajectories of drifters looping in there. eddies have been shown and brieflydescribed (Centurioni and Niiler, 2003), these data have not yet been used to thoroughly describe discrete 2. Background eddies in the Caribbean. The new drifter trajec- tories provide an important new Lagrangian The Caribbean Current is the major route by picture of Caribbean eddies, but the eddies were which South Atlantic water flows into the Florida discussed onlybrieflyin the short paper by Current and Gulf Stream and is therefore an Centurioni and Niiler. Theyused a high-resolution important conduit of the upper part of the 1 ( 2 degree) mapping grid to show detailed views northward-flowing meridional overturning circu- of velocityvectors in subregions of the Caribbean, lation (MOC) (Schmitz and Richardson, 1991; but no mean velocitymap of the whole Caribbean Schmitz and McCartney, 1993). The Caribbean was included. This present paper builds on these has two nearlyequal sources of inflow water (see earlier studies byfirst using drifter trajectories to Johns et al., 2002). North of (roughly) Martinique map the circulation and eddykinetic energy(EKE) near 151N Caribbean inflow is primarilyGulf 1 of the whole Caribbean at high resolution (2 Stream water returning southwestward in the degree) and then to systematically identify and North Equatorial Current. This water passes describe several different kinds of discrete eddies in through the Leeward Islands of the Lesser order to show where theyoccur in relationship to Antilles, through the Windward Passage between the mean current field and to estimate their Cuba and Hispaniola, and through the Mona translation rates, swirl velocities, and overall sizes. Passage between Hispaniola and Puerto Rico. Eddies in the Caribbean are important for South of 151N Caribbean inflow is primarilyof several reasons. First, the various source waters tropical and South Atlantic origin. South Atlantic of the Caribbean Current coming from the North water crosses the equator in the NBC and flows Atlantic, South Atlantic, and nearbyAmazon and northwestward along the continental margin of Orinoco Rivers are advected and stirred bythe South America in the form of a coastal current eddies’ swirl velocity, which can be much larger (Candela et al., 1992), in NBC rings (Johns et al., than the velocityof the mean field. Numerous 1990; Didden and Schott, 1993; Richardson et al., large and energetic eddies translating westward 1994; Fratantoni et al., 1995), plus some in Ekman through the Caribbean blend and mix the source transport in the ocean interior (Mayer and waters together. Second, eddies interact with the Weisberg, 1993). The NBC retroflects near 61N boundaries and often sweep near-shore water and feeds into the eastward-flowing North ARTICLE IN PRESS P.L. Richardson / Deep-Sea Research II 52 (2005) 429–463 433 Equatorial Countercurrent. Periodically, roughly Caribbean to the Gulf Stream are a consequence 8–9 times per year, large 400-km diameter NBC of the superposition of a realistic MOC and a rings pinch off from the retroflection and translate realistic wind-driven circulation. Without the northwestward toward the Caribbean at around MOC there were no NBC rings and much lower 15 cm/s (Johns et al., 2003; Garzoli et al., 2003; EKE. Thus, model simulations provide evidence of Goni and Johns, 2003; Fratantoni and Richard- a link between the MOC, the formation of NBC son, 2004). Most rings translate northward just rings, the variabilityof flow into the Caribbean, east of the Antilles to around 14–181N where the and the eddyfield there ( Fratantoni et al., 2000; rings stall and decay( Fratantoni and Richardson, Johns et al., 2002; Barnier et al., 2001). The eddy 2004; Goni and Johns, 2003). Some rings dis- field in the Caribbean thus could be caused by appear after theycollide with the continental rings (or their remnants) translating through the margin and islands south of 141N.