JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. Cll, PAGES 25,805-25,820, NOVEMBER 15, 1999 Direct observations of velocity and transport in the passages between the Intra-Americas Sea and the Atlantic Ocean, 1984 -1996 Elizabeth Johns,W. DouglasWilson, and Robert L. Molinari Atlantic Oceanographicand MeteorologicalLaboratory, National Oceanicand AtmosphericAdministration, Miami, Florida Abstract. Shipboardacoustic Doppler currentprofiler observationsof the velocityin the upper 200 rn of the water columncollected during 1984-1996 usingthe National Oceanic and AtmosphericAdministration R/V MalcolmBaldrige are usedto examinethe velocity structureand transportin the passagesbetween the Atlantic Ocean and the Intra- AmericasSea (IAS). Data were collectedduring 23 cruisesalong the followingsections: acrossthe Straitsof Florida, in the NorthwestProvidence Channel (NWPC), acrossthe northernpassages into the CaribbeanSea (Windward,Mona, and Anegada),across the easternCaribbean along 63030 ' W, therebyforming a closedquadrangle, and in the GrenadaPassage. The Florida Current, the easternCaribbean, and the Grenada Passage sharea similarmean velocitystructure characterized by high-velocity,surface-intensified flowswith strongvertical and horizontalshears. The northern Caribbeanpassages (NWPC, Windward,Mona, and Anegada)share a differentcommon mean velocity structure,with subsurfacevelocity maxima directed into the IAS, and surface-intensified counterflowsalong one side of each passage.On average,there is a transportbalance in the upper200 rn betweenwaters entering and exitingthe IAS, with the 16.5 _+2.4 Sv (1 Sv = 106m3/s) transport of theFlorida Current at 27øNcomprised of 0.4 _+0.8 Sv from the NWPC, 2.2 _ 1.5 Sv from the Windward Passage,2.8 _ 2.1 and 2.4 _ 2.8 Sv from the Mona and Anegadapassages, respectively, and 9.5 +_4.7 Sv acrossthe easternCaribbean, for a total of 17.3 Sv. The four passagesnorth of 17øN(from NWPC to AnegadaPassage) have a combinedtransport of 7.8 Sv, nearlyhalf of the transportof the Florida Current in the upper200 m. Of the 9.5 Sv flowingthrough the easternCaribbean between lløN and 17øN,4.9 _+2.6 Sv, or more than half, come from the Grenada Passage.This is significant to the subjectof cross-equatorialexchange of mass,heat, and salt, as the GrenadaPassage is where the highesttransport of watersoriginating in the southernhemisphere is thought to enter the Caribbean. 1. Introduction Caribbean and into the southeastern corner of the Gulf of Mexico and eventuallyexit via the Florida Current. The wind- The Intra-AmericasSea (IAS) is the semienclosedbody of driven circulationof the Caribbeanis dominatedby the Trade water comprisedof the CaribbeanSea, the Gulf of Mexico,and Winds, which have a strong seasonalcomponent and a wind the Straits of Florida, connected to the North Atlantic Ocean stresscurl field that respondsto the seasonalnorthward and by a numberof passagesbetween the islandsof the Bahamas, southwardmigration of the Intertropical ConvergenceZone Greater Antilles, and LesserAntilles (Figure 1). The circula- [Mayerand Weisburg,1993]. tion of the IAS is dominatedby the Caribbeanand Florida In addition to its strictly physicaloceanographic interest, currents.These currents form a major componentof the North attention has recentlybeen givento the interconnectednature Atlantic subtropicalgyre, leading directly into the Gulf Stream, of the marine and atmosphericenvironments of the IAS in and as suchare an importantconduit for mass,heat, and salt terms of pollutant dispersal,larval transports,the health of fluxes in the Atlantic. coral reefs,and commercialand recreationalfisheries [Roberts, The IAS is a fascinatingregion from a physicaloceano- 1997; Ogden,1997; Mooersand Maul, 1998]. It is becoming graphicviewpoint, with variabilityon all timescalesfrom syn- apparentthat only throughthe useof focusednumerical mod- opticto seasonal,interannual, and decadal,driven by wind and els of the circulationwhich can properly simulatethe atmo- thermohalineforcing. The deep circulationis dominatedby sphericand oceanicforcing and the ocean'sresponse on all sporadicinflow eventsover the sillsof the major Caribbean relevantspace and timescaleswill the modelsbe able to move passages[Sturges, 1970], while the upper layer circulationis a toward the nowcast/forecastcapability that could be used to throughflowsystem in whichinflows from the passages,includ- address the environmental and societal concerns critical to the ing recirculatingNorth Atlantic subtropicalgyre waters as well IAS. as watersoriginating in the SouthAtlantic, passthrough the However,the developmentof effectivenumerical circulation Copyright1999 by the AmericanGeophysical Union. modelsrequires high-quality and abundantobservational data Paper number 1999JC900235. for modelverification. For someparts of the IAS, suchas the 0148-0227/99/1999JC900235509.00 Florida Current, adequatedata exist for this purpose.The 25,805 25,806 JOHNS ET AL.: VELOCITY AND TRANSPORT IN THE IAS PASSAGES estimateof the transportdistribution based on the limited data availableat that time. More recently,Wilson and Johns[1997] conducteda comprehensivestudy of the inflow to the eastern 2826 _:j•GUlfof Mexico •FC:,•"•-•, Atlantic Ocean Caribbean during the Windward Islands PassagesProgram. Using a cable-lowered acoustic Doppler current profiler uJ 22 (ADCP), they were able to obtain enoughdirect observations • 20 • • MP L.. ..... of transportand velocitystructure in the southernpassages over several years to more closely examine the mean inflow • 16 and its variability. Caribbean Sea 14 The northern passagesto the IAS (Windward, Mona, and Anegada;Figure 1) are the least studied,and few direct mea- 12 surementsof their transporthave been obtained. Thus a key 10 motivationfor the presentstudy was to utilize direct velocity data collectedon a series of NOAA cruisesboth passing 90 88 86 84 82 80 78 76 74 72 70 68 66 64 62 60 58 throughand directed at the region between 1984 and 1996 to LONGITUDE addressthis deficiency.Direct velocity observationswere col- Figure 1. Bottom topography of the Intra-Americas Sea. lectedusing a hull-mountedADCP along a number of sections Transect locations are indicated as follows: FC, Florida Cur- acrossthe passagesof the IAS, in the easternCaribbean Sea, rent; NW, Northwest Providence Channel; WP, Windward and in the Straitsof Florida (Figure 1). Passage;GI, Great Inagua Passage;MP, Mona Passage;AP, Herein, the ADCP velocitydata from thesecruises are used Anegada Passage;CA, eastern Caribbean; and GP, Grenada to examinethe velocitystructure in the upper 200 m of the Passage. OBC indicates the location of the Old Bahama Channel. water column and to estimatetransports through the Carib- bean Sea and the Straitsof Florida. We beginwith a descrip- tion of the available observationsand data reduction, followed by an analysisof the velocityand transportresults for each Florida Current is probably the most studied current in the passage,and a discussionof their significanceboth compared world, from the pioneeringphysical oceanographic study of to earlier transportestimates and in terms of addressingsome Pillsbury[1890] which utilized the first current meter to mea- of the issues mentioned above. sure the transportof the Florida Current, to more sophisti- cated studiessuch as that by Richardsonand Schmitz[1965] and Schmitzand Richardson[1968] using dropsondes.More 2. Data and Methodology recently, NOAA's SubtropicalAtlantic Climate Study [Moli- The eight sectionsconsidered herein are located in the nari et al., 1985]used a combinationof shipboardobservations, Straits of Florida at 27øN and 26øN: the Northwest Providence moored current meters and pressuregauges, and a submarine Channel (NWPC), the Great Inagua Passage(between the cable to examine the velocity structureand variability of the Bahamianislands of Great Inaguaand Hispaniola),the Mona Florida Current at 27øNand to continuouslymonitor the trans- Passage,the AnegadaPassage, the easternCaribbean Sea, and port usingthe cable[Larsen and Sanford,1985]. The Subtrop- the Grenada Passage(Figure 1). Ideally, for estimatingthe ical Atlantic Climate Studywas begun in 1982, and the trans- transportbalance of the IAS, the Great Inagua sectionwould port monitoringcontinues to the presenttime. Resultsfrom insteadhave been located acrossthe Windward Passage,be- this studyinclude determination of a stablemean for the trans- tweenCuba and Haiti. However,this was not possiblebecause port of 32.3 +_3.2 Sv [Larsen,1992] and a better understanding of clearanceconstraints. Thus flow throughthe Great Inagua of its seasonaland interannualvariability [see, e.g., Molinari et sectionis comprisedof flow into or out of the CaribbeanSea al., 1985;Lee et al., 1985;Schott and Zantopp, 1985;Johns and through the Windward Passageand flow into or out of the Schott,1987; Larsen, 1992;Leaman et al., 1987]. Straitsof Florida throughthe Old BahamasChannel and can Suchis not the casefor the other passagesof the IAS and for only be used as a proxy for flow through the Windward Pas- the Caribbean Sea as a whole. Although the Caribbean has sage.The datesof the 23 cruisesused in this analysisand the been the subjectof a number of hydrographicstudies, few sectionsoccupied and transportsobtained during each passage direct observations of the circulation have been made. Earlier transect are listed in Table 1. historicalstudies used hydrographic and tracerdata to describe ShipboardADCP observationsare the primary data source the circulationusing either the