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FAU Institutional Repository FAU Institutional Repository http://purl.fcla.edu/fau/fauir This paper was submitted by the faculty of FAU’s Harbor Branch Oceanographic Institute Notice: ©2002 Elsevier Science Ltd. This is the author’s version of a work accepted for publication by Elsevier. Changes resulting from the publishing process, including peer review, editing, corrections, structural formatting and other quality control mechanisms, may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. The definitive version has been published at http://www.elsevier.com/locate/csr and may be cited as Lee, Thomas N., Ned Smith (2002) Volume transport variability through the Florida Keys tidal Channels, Continental Shelf Research 22(9):1361–1377 doi:10.1016/S0278‐4343(02)00003‐1 Continental Shelf Research 22 (2002) 1361–1377 Volume transport variability through the Florida Keys tidal channels Thomas N. Leea,*, Ned Smithb a Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA b Harbor Branch Oceanographic Institution, 5600 US Highway 1, North, Ft. Pierce, FL 34946, USA Received 28 February 2001; received in revised form 13 July 2001; accepted 18 September 2001 Abstract Shipboard measurements of volume transports through the passages of the middle Florida Keys are used together with time series of moored transports, cross-Key sea level slopes and local wind records to investigate the mechanisms controllingtransport variability. Predicted tidal transport amplitudes rangedfrom 76000 m3/s in LongKey Channel to 71500 m3/s in Channel 2. Subtidal transport variations are primarily due to local wind driven cross-Key sea level slopes. Subtidal transports through Long Key Channel ranged from +1000 m3/s inflow to Florida Bay to À2500 m3/s outflow to the reef tract. Wind directions oriented toward 190–3151 will cause a positive cross-Key sea level slope and inflow to Florida Bay. All other wind directions will tend to cause a negative cross-Key slope of sea level and Gulf to Atlantic outflow toward the reef tract. Seasonal variation in local wind forcingresults in maximum outflows from Florida Bay in the winter when increased winds toward the SE and S occur followingcold front passages.Minimum outflow occurs in fall when winds toward the SW and W are more frequent and inflows to Florida Bay can persist for several days. The long-term mean flow is toward the southeast and the reef tract and is estimated at –740 m3/s for the combined flows of the major channels in the middle Keys, with 7-Mile Bridge Channel accounting for about 50% of this flow, LongKey Channel carries about 35% of the flow and Channels 5 and 2 account for about 7% each. The mean Gulf to Atlantic flow is supported by sea level standinghigherin western Florida Bay and the eastern Gulf than in the Keys Atlantic coastal zone. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Florida Keys; Volume transports; Sea level slopes 1. Introduction water bodies as an integral part of the North Atlantic subtropical gyre (Chew et al., 1982). The The Gulf of Mexico is connected to the Atlantic Gulf and Atlantic are also connected at the open through the Straits of Florida where a mean passages between the Florida Keys (Fig. 1). Cross- downstream slope of sea level helps to maintain Key sea level slopes from tidal and non-tidal the strongwestern boundary current system, the processes will drive flows through these passages Loop Current/Florida Current that joins the two that can result in water exchange between the southwest Florida shelf, includingwestern Florida *Correspondingauthor. Bay, and the Keys Atlantic coastal zone that E-mail address: [email protected] (T.N. Lee). encompasses the Florida Keys reef tract. Smith 0278-4343/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0278-4343(02)00003-1 1362 T.N. Lee, N. Smith / Continental Shelf Research 22 (2002) 1361–1377 83W 82W 81W 80W 26N 26N Marco Is. Loop Cape Current Romano 40m 20m 10m 5m 2m Key able Largo SW Florida Shelf ape S C s Florida Bay S 25N r. 25N WFB pper Key 5 U ile B LK 2 Dry 7 M Tenn Tortugas Key Marquesas West -20 M s Middle Key t 100 s Western Keys Lower Key 200 m Florida Curren 24N 24N 83W 82W 81W 80W Fig.1. Study area showingLongKey Channel (LK), Channel 5 (5), Channel 2 (2), 7-Mile BridgeChannel and bottom pressure station locations (solid dots) in west Florida Bay (WFB) and Tennessee Reef (Tenn.). CMAN wind stations at Molasses and Sombrero Reefs are shown by solid squares. Locations of the Loop Current and Florida Current are shown schematically by arrows. (1994, 1998) has documented a long-term mean up of sea level in western Florida Bay than on the Gulf to Atlantic flow through the passages in the Atlantic side of the Keys. Over the longterm the middle Keys that shows a seasonal dependence, as higher mean water level of the eastern Gulf well as low-frequency flow reversals that appear to and variations of the Loop Current may have be related to local wind forcing. Previously, it was important influences on mean transports through uncertain what physical mechanisms supported the the Keys passages. Southeastward flow through mean Gulf to Atlantic flow against the prevailing the Keys provides a mechanism for transport of westward winds, likewise the source of the subtidal waters from western Florida Bay that undergo flow variations was not resolved. large changes in water quality related to sea grass In this paper we present recent results from the die-offs (Robblee et al., 1991; Thayer et al., 1994; South Florida Ecosystem Restoration Prediction Boyer et al., 1999), algal blooms (Butler et al., and Modeling(SFERPM) Program.Shipboard 1995), hypersalinity (Fourqurean et al., 1993) and measurements of volume transports through the winter cooling(Roberts et al., 1982; Lee, 1986) middle Keys passages are used to calibrate moored toward the coral reef tract of the Florida Keys time series of volume transports and compare to National Marine Sanctuary (FKNMS). cross-Keys sea level slopes from bottom pressure measurements and to local wind forcing. We show that local wind forcingis the primary cause of the 2. Background low-frequency transport variability through the generation of cross-Key sea level slopes that arise The Florida Keys island chain separates two due to differences in the physical characteristics very different continental shelves that respond (shape, orientation, and depth) of the shelf regimes differently to the same wind forcing: the southwest on either side of the Keys. Surprisingly, winds Florida shelf to the west and north of the Keys, from the southeast can cause a flow against the and the Keys Atlantic coastal zone lyingto the east wind in these shallow waters due to a greater set- and south (Fig. 1). These two shelf regions interact T.N. Lee, N. Smith / Continental Shelf Research 22 (2002) 1361–1377 1363 with each other through the tidal channels between inner shelf to about 10 m in the Hawk Channel the Keys and with strongoffshore boundary region of mid shelf, then decrease again to 0–5 m in currents at their outer edges; the Loop Current the complex bathymetry of the reef tract, before in the Gulf and Florida Current on the Atlantic the sudden steepeningof the slope seaward of the side of the Keys. Loop Current eddies propagate shelf break at about 30 m. The curvingshoreline southward alongthe outer edgeof the wide west causes regional differences in response to prevail- Florida shelf and develop into persistent eddy ingwestward winds and Florida Current influ- structures south of the Dry Tortugas that are ences. The lower Keys are normally aligned with forced downstream alongthe outer shelf and slope the wind and form a downwellingcoast with of the Keys (Paluszkiewicz et al., 1983; Lee et al., onshore flow in the upper layer and offshore in the 1995; Fratantoni et al., 1998). The southwest lower layer, and a westward coastal current that Florida shelf is the southern extension of the wide, intensifies toward the west due to greater persis- shallow west Florida shelf with its smoothly tence of Florida Current eddies off the Tortugas varyingtopographyalignedin a northwest-south- (Lee and Williams, 1999). The prevailingwestward east direction. Just north of the Keys the shallow wind is onshore in the upper Keys, with little inner shelf topography (depths o20 m) turns influence on alongshore flow, whereas the Florida abruptly to the west to wrap around the western Current converges nearer the outer reefs in this Keys, thus forminga shallow cul-de-sac in western region causing strong downstream flows with an Florida Bay. The west Florida shelf displays a onshore component in the upper layer and off- strongcoherent response to synoptic-scale along- shore in the lower layer. The outer shelf region of shore wind forcing(Mitchum and Sturges, 1982; the Keys is dominated by meanders of the Florida Mitchum and Clarke, 1986; Li and Weisberg, Current and downstream propagation of frontal 1999; Weisberget al., 2000) that is also observed eddies that can cause energetic current reversals in the inner shelf of southwest Florida (Lee et al., on time scales of days to weeks and significant 2000). This response has all the characteristics of cross-shelf transports (Lee et al., 1992, 1995; the arrested topographic wave response observed Fratantoni et al., 1998; Lee and Williams, 1999). in the Middle Atlantic Bight (Csanady, 1982) and The eddies decrease in size from 100 to 200 km South Atlantic Bight (Lee et al., 1989). Synoptic- off the Tortugas to tens of km in the middle and scale upwelling (downwelling) alongshore winds, upper Keys, and increase in forward speed from together with the coastal constraint results in 5–15 km/day off the western Keys to 15–30 km/ barotropic alongshore currents over the shelf day off the upper Keys, due to the narrowingof aligned with the wind direction and balanced by the Florida Straits channel as it curves to the north a set-down (set-up) of coastal sea level.
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