JOURNAL OF GEOPHYSICAL RESEARCH,VOL 104,NO. Cl, PAGES 1431-1453,JANUARY 15, 1999 The connectivity of eddy variability in the Caribbean Sea, the Gulf of Mexico, and the Atlantic Ocean SylviaJ. Murphy and Harley E. Hurlburt Naval Research Laboratory, Stennis Space Center, Mississippi James J. O'Brien Centerfor OceanAtmospheric Pr~diction Studies, Florida StateUniversity, Tallahassee Abstract. A set of numericalsimulations is usedto investigatethe connectivityof mesoscalevariability in the Atlantic Ocean,the Caribbean,and the Gulf of Mexico. The primitive equationmodels used for thesesimulations have a free surfaceand realistic coastlinegeometry including a detailedrepresentation of the LesserAntilles island arc. Two simulationshave 1/4° resolution and includea 5.5-layerreduced gravity and a 6-layer model with realisticbottom topography.Both are wind forced and include the global thermohalinecirculation. The third simulationis from a 1(2°linear wind-drivenmodel. In the two nonlinearnumerical simulations, potential vorticity from decayingrings shedby the North Brazil Current retroflection can be advectedthrough the LesserAntilles. This potentialvorticity actsas a finite amplitudeperturbation for mixedbarotropic and internal modebaroclinic instabilities, which amplify mesoscalefeatures in the Caribbean.The eddiesassociated with the CaribbeanCurrent are primarily anticyclonicand transit a narrowcorridor acrossthe Caribbeanbasin along an axis at 14°to 15°Nwith an average speedof 0.15m/s. It takesthem an averageof 10 monthsto transit from the Lesser Antilles to the YucatanChannel. Along the way,many of the eddiesintensify greatly.The amountof intensificationdepends substantially on the strengthof the CaribbeanCurrent and is greatestduring a multiyear period when the current is anomalouslystrong owing to interannualvariation in the wind forcing. SomeCaribbean eddies squeeze through the YucatanChannel into the Gulf of Mexico,where they can influencethe timing of Loop Current eddy-sheddingevents. There is a significantcorrelation of 0.45between the Loop Current eddyshedding and the eddiesnear the LesserAntilles with a time lag of 11 months.However, Canobean eddies show no statisticallysignificant net influenceon the meaneddy-shedding period nor on the sizeand strengthof shededdies in the Gulf of Mexico.Additionally, no significantcorrelation is found betweeneddy shedding in the Gulf of Mexicoand transpott variationsin the Florida Straits,although transport fluctuationsin the Florida Straits at 27°Nand the YucatanChannel and showeda correlationof about 0.7 with a lag of 15 days.The linear solution exhibiteda multiyear anomalyin the strengthof the Canobeancirculation that wasconcentrated in the central and easternCaribbean due to a multiyearanomaly in the wind field over the basin. In the nonlinearsimulation this anomalyextended into the westernCanobean and acrossthe entire Gulf of Mexico.This westwardextension resulted from the nonlinearity and instabilityof the CaribbeanCurrent, the westwardpropagation of the eddies,and the passageof Canobeaneddies through the YucatanChannel into the Gulf of Mexico. 1. Introduction of inflow through the various Caribbean passages.The Carib- bean Current exits the basin to the northwest through the The Caribbean is a semienclosed sea bounded on the east Yucatan Channel and flows into the Gulf of Mexico to form and the north by a chain of closely spaced islands that act as a the Loop Current (Figure 3). Both the Caribbean Current and sieve for inflow from the Atlantic Ocean. The islands from the Loop Current are essential components of the Gulf Stream Guadaloupe south to Grenada (Figure 1) comprise the Lesser system,which is partly wind driven but which is also augmented Antilles. while the larger islands to the north (Cuba, Hispani- by a contnoution from the global thermohaline circulation ola, and Puerto Rico) are referred to as the Greater Antilles. [Schmitz and Richardson, 1991; Schmitz, 1995]. Westward flow between the islands of the Lesser Antilles en- Many aspectsof Canobean circulation, including mesoscale ters the Caribbean and forms th~ westward flowing Caribbean variability, are summarized by Kinderet al. [1985). Maul [1993) Current (Figure 2), the principal current within the region. A also presents an overview including the environmental and great deal of uncertainty exists, however, about the distnoution socioeconomic implications of global climate change in the Copyright1999 by the AmericanG.:ophysical Union. Caribbean. In this paper we investigate the connectivity of eddy vari- Paper number 1998JC9(XX)10. 0 148-{)227/99/1998Jcxxx) 10$09.(X) ability in the Caribbean Sea, the Gulf of Mexico, and the 1431 1432 MURPHY ET AL: CONNECrlVITY IN INTRA-AMERICA SEAS , MURPHY ET AI..: CONNECfIVITY IN INTRA-AMERICA SEAS 1433 1966;Kinder, 1983;Lemming, 1971; Molinari et ai., 1981;Nys- tuen andAndrade, 1993},but these studies have been limited in spatial and temporal resolution and coverage.The utilization of a global numerical model aUowsfor research at time and space scalescurrently unfeasible observationaIly. Three versions of the Naval Research Laboratory (NRL) Layered Ocean Model (NLOM) [WaI1craft,1991} are used in this study. All are global in domain and contain detailed Ca- ribbean geometry. The simulations include a reduced gravity, thermodynamic version (simulation 1, Table 1) with 1/40reso- lution and a finite depth, hydrodynamic version (simulation 2, Table 1) also with 1/40 resolution. Finally, a linear, reduced gravity version (simulation 3, Table 1) with 1/20resolution is utilized. After spin-up to statistical equilibriUin, all of the sim- ulations were forced by daily averaged winds from 1981 to 1994, so that interannual variability is also present. The for- mulation for the NLOM is presented in section 2 with detailed equations for the thermodynamic version. The modifications necessaryto create a hydrodynamic version are also presented 64W 60W 56W as well as a more detailed discussionof the design of the three simulations specifically used in this study. These simulations contain different physics,which are used to include or exclude dynamical processesthat may be relevant to the simulation of mean currents and variability in the Ca- ribbean. For instance, simulation 2 contains realistic bottom topography. Simulation 1 is reduced gravity and therefore ex- cludes the barotropic mode and the bottom topography. Since eddies are observed in both simulations, these differences pro- vide a means to investigate (1) the influence bottom topogra- phy and baroclinic instability involving the barotropic mode on the simulated eddies in the Caribbean and (2) topographic effects on eddy formation and pathways of mean currents. Simulation 1 is also thermodynamic and therefore contains more accurate stratification that can influence eddy generation and propagation speeds.This simulation also has a more ac- curate representation of the return flow of the global thermo- haline circulation, which affects the mean transports through the Lesser Antilles. The linear simulation excludesflow insta- bility dynamics, providing a linear, deterministic ocean model 50 66 82 98 114 130 146 responseto the atmospheric forcing. It is used to demonstrate inm the impact of nonlinearity on a multiyear anomaly seen in the interannual simulations and to isolate the causeof this anomaly. 0.28..vs This investigation can be viewed as an attempt to use the Plate 1. Snapshotsof upper layer thickness (meters) and ve- model simulations to extend our understanding of a region locity vectors (meters per second) from simulation 1 for (a) with limited observations after comparing the simulations with September 2, 1992, where a ring from the North Brazil ret- the observational data currently available. However, additional roflection is impacting the Lesser Antilles, and (b) September observations are required to adequately assessthe model re- 8, 1992, where potential vorticity from the ring is advected through the Lesser Antilles. sults. Hence some of the results must be viewed as model predictions subject to future verification. The connectivity of eddy variability in the Caribbean, Gulf of Mexico, and Atlantic Ocean is discussedin section 3. Topics include (1) the influence of the Atlantic Ocean on the forma- Atlantic Ocean. This includes (1) the influence of eddies that tion of Caribbean eddies; (2) a description of the eddies, their impinge on the eastern side of the Lesser Antilles, principally rings shed from the North Brazil Current retroflection [Johns mean pathways, and velocities; (3) a comparison between Ca- et al., 1990;FraJan/oni et al., 1995]; (2) the role of mesoscale ribbean eddies observed by Geosat altimetry and model ed- flow instabilities in the Caribbean Current; and (3) the influ- dies; -(4) the relationship between Caribbean eddies and the ence of the Caribbean eddies on eddy shedding by the Loop eddy shedding from the Loop Current in the Gulf of Mexico; Current in the Gulf of Mexico, including eddies from the (5) discussionof an intradecadal cycle simulated in Caribbean easternmost Caribbean. variability and its potential origins; and, finally (6) a discussion Caribbean eddies have been discussedin the literature [Fu of variability connectivity within the Gulf Stream system. Sec- and Holt, 1983; Hebum et al., 1982; Ingham and Mahnken, tion 4 contains the summary and conclusions. , 1434 MURPHY ET AL.: CONNEcrrvITY IN INtRA-AMERICA SEAS 0.75m/s 50 66 82 98 114 130 146 ~ inm Plate 2. Snapshotsof upper layer