Thomas N. Lee

Red tides in the Gulf of Mexico: Where, when, and why?

[1] Independent data from the Gulf of Mexico are used to develop and test the hypothesis that the same sequence of physical and ecological events each year allows the toxic dinoflagellate Karenia brevis to become dominant. A phosphorus-rich nutrient supply initiates phytoplankton succession, once deposition events of Saharan iron-rich dust allow Trichodesmium blooms to utilize ubiquitous dissolved nitrogen gas within otherwise nitrogen-poor sea water. They and the co-occurring K. brevis are positioned within the bottom Ekman layers, as a consequence of their similar diel vertical migration patterns on the middle shelf. Upon onshore upwelling of these near-bottom seed populations to CDOM-rich surface waters of coastal regions, light-inhibition of the small red tide of ~1 ug chl l(-1) of ichthytoxic K. brevis is alleviated. Thence, dead fish serve as a supplementary nutrient source, yielding large, self-shaded red tides of ~10 ug chl l(-1). The source of phosphorus is mainly of fossil origin off west Florida, where past nutrient additions from the eutrophied Lake Okeechobee had minimal impact. In contrast, the P-sources are of mainly anthropogenic origin off Texas, since both the nutrient loadings of Mississippi River and the spatial extent of the downstream red tides have increased over the last 100 years. During the past century and particularly within the last decade, previously cryptic Karenia spp. have caused toxic red tides in similar coastal habitats of other western boundary currents off Japan, China, New Zealand, Australia, and South Africa, downstream of the Gobi, Simpson, Great Western, and Kalahari Deserts, in a global response to both desertification and eutrophication.

The fate of river discharge on the continental shelf: 1. Modeling the river plume and the inner shelf coastal current

We study the development and evolution of buoyant river plumes on the continental shelf. Our calculations are based on three-dimensional numerical simulations, where the river runoff is introduced as a volume of zero salinity water in the continuity equation and mixing is provided by the models turbulence closure scheme and wind forcing. In the absence of wind forcing, the modeled river plumes typically consist of an offshore bulge and a coastal current in the direction of Kelvin wave propagation. We propose a plume classification scheme based on a bulk Richardson number, which expresses the relative magnitude of the buoyancy-induced stratification versus the available mixing. When the ratio of the discharge and shear velocities is greater (less) than 1, the plume is categorized as supercritical (subcritical); that is, the width of the bulge is greater (less) than the width of the coastal current. Supercritical plumes are often characterized by a meandering pattern along the coastal current, caused by a baroclinic instability process. For a given discharge, subcritical plumes are produced by large mixing and/or shallow water depths. In the presence of wind forcing, the favorable conditions for offshore removal of coastal low-salinity waters include high river runoff and strong upwelling-favorable wind stress. When the rivers are treated as individual sources of freshwater (“point source” behavior), the wind-driven flow may exhibit substantial spatial variability. Under the above removal conditions, strong offshore transport takes place in “jetlike” flow regions within the river plume, in contrast to the downwind acceleration of adjacent waters. When the rivers are treated as a long “line source” of freshwater, the plume region resembles a coastal low-salinity band, and the above removal conditions trigger offshore transport that is most pronounced at the “head” of the source.

Observations of a Gulf Stream frontal eddy on the Georgia continental shelf, April 1977

Abstract Satellite, hydrographic, and data from moored current meters are used to show the effect of Gulf Stream frontal disturbances on low-frequency current and temperature variability, water exchange, and nutrient flux in the outer region of the Georgia shelf. Perturbations of the Gulf Stream cyclonic front are commonly observed as folded wave patterns in routine satellite-derived analyses of the western boundary of the Gulf Stream between Cape Hatteras and Miami. The disturbances consist of southward-flowing warm filaments or streamers of near-surface Gulf Stream water, 15 to 20 m deep, which can extend 35 to 40 km over the outer shelf around a cold upwelled core. Downstream dimensions of the filaments reach 100 to 200 km in the region from Jupiter, Florida, to Charleston, South Carolina, 10 to 50 km south of Jupiter, and 200 to 300 km between Charleston and Cape Hatteras. The features are defined as cyclonic, cold-core frontal eddies due to their flow and water mass properties. They appear to form from amplified waves in the Gulf Stream cyclonic front on an annual average of one every two weeks but with considerable monthly variability. They can persist up to three weeks and travel to the north with the same phase speed as the waves, approx. 40 cm s−1. The cyclonic circulation in frontal eddies provides a means for rapid shelf-Gulf Stream water exchange. The eddies appear to control the residence time of the outer shelf waters, defined as the mean separation time between eddy events, or approx. two weeks. Upwelling in the cold core extended into the euphotic zone (45 m) and shoreward (35 to 40 km) beneath the southward-flowing warm filament in a bottom intrusion layer 20 m thick. The annual nitrogen input to the shelf waters by this process is estimated as 55,000 tons each year, about twice all other estimated nitrogen sources combined; it can support an annual carbon production by phytoplankton of 32 to 64 g C m−2y−1 with no nitrogen recycling.

Gulf Stream frontal eddy influence on productivity of the southeast U.S. continental shelf

Weekly period meanders and eddies are persistent features of Gulf Stream frontal dynamics from Miami, Florida, to Cape Hatteras, North Carolina. Satellite imagery and moored current and temperature records reveal a spatial pattern of preferred regions for growth and decay of frontal disturbances. Growth regions occur off Miami, Cape Canaveral, and Cape Fear due to baroclinic instability, and decay occurs in the confines of the Straits of Florida between Miami and Palm Beach, between 30° and 32°N where the stream approaches the topographic feature known as the Charleston bump and between 33°N and Cape Hatteras. Eddy decay regions are associated with elongation of frontal features, offshore transport of momentum and heat, and onshore transport of nutrients. Onshore transport of new nitrogen from the nutrient-bearing strata beneath the Gulf Stream indicates that frontal eddies serve as a “nutrient pump” for the shelf. New nitrogen flux to the shelf due to Gulf Stream input could support new production of 7.4×1012 g C yr−1 or about 8 million tons carbon per year if all nitrate were utilized. Calculations indicate that approximately 70% of this potential new production is realized, yielding an annual new production for the outer shelf of 4.3×1012 g C.

Annual Cycle and Variability of the North Brazil Current

Abstract Current meter observations from an array of three subsurface moorings located on the Brazil continental slope near 4°N are used to describe the annual cycle and low-frequency variability of the North Brazil Current (NBC). The moored array was deployed from September 1989 to January 1991, with further extension of the shallowest mooring, located over the 500-m isobath near the axis of the NBC, through September 1991. Moored current measurements were also obtained over the adjacent shelf for a limited time between February and May 1990. The NBC has a large annual cycle at this latitude, ranging from a maximum transport of 36 Sv (Sv ≡ 106 m3 s−1) in July–August to a minimum of 13 Sv in April–May, with an annual mean transport of approximately 26 Sv. The mean transport is dominated by flow in the upper 150 m, and the seasonal cycle is contained almost entirely in the top 300 m. Transport over the continental shelf is 3–5 Sv and appears to be fairly constant throughout the year, based on the available...

The Kuroshio East of Taiwan: Moored Transport Observations from the WOCE PCM-1 Array

Observations from the WOCE PCM-1 moored current meter array east of Taiwan for the period September 1994 to May 1996 are used to derive estimates of the Kuroshio transport at the entrance to the East China Sea. Three different methods of calculating the Kuroshio transport are employed and compared. These methods include 1) a “direct” method that uses conventional interpolation of the measured currents and extrapolation to the surface and bottom to estimate the current structure, 2) a “dynamic height” method in which moored temperature measurements from moorings on opposite sides of the channel are used to estimate dynamic height differences across the current and spatially averaged baroclinic transport profiles, and 3) an “adjusted geostrophic” method in which all moored temperature measurements within the array are used to estimate a relative geostrophic velocity field that is referenced and adjusted by the available direct current measurements. The first two methods are largely independent and are shown to produce very similar transport results. The latter two methods are particularly useful in situations where direct current measurements may have marginal resolution for accurate transport estimates. These methods should be generally applicable in other settings and illustrate the benefits of including a dynamic height measuring capability as a backup for conventional direct transport calculations. The mean transport of the Kuroshio over the 20-month duration of the experiment ranges from 20.7 to 22.1 Sv (1 Sv ≡ 106 m3 s−1) for the three methods, or within 1.3 Sv of each other. The overall mean transport for the Kuroshio is estimated to be 21.5 Sv with an uncertainty of 2.5 Sv. All methods show a similar range of variability of ±10 Sv with dominant timescales of several months. Fluctuations in the transport are shown to have a robust vertical structure, with over 90% of the transport variance explained by a single vertical mode. The moored transports are used to determine the relationship between Kuroshio transport and sea-level difference between Taiwan and the southern Ryukyu Islands, allowing for long-term monitoring of the Kuroshio inflow to the East China Sea.

Moored observations of western boundary current variability and thermohaline circulation at 26.5°N in the subtropical North Atlantic

Abstract A 5.8-year time series of moored current meter observations is used with hydrographic section data, CME model results, and gridded wind fields over the North Atlantic to describe the mean structure and variability of circulation and volume transports east of Abaco, Bahamas, at 26.5°N. A mean Antilles Current, with 5 Sv of northward transport, is confined against the Bahamas boundary in the upper 800 m and combines with approximately 19 Sv of Florida Current transport to balance the Sverdrup interior circulation, and does not contribute to interhemispheric exchange. The mean transport of the deep western boundary current (DWBC) off the Bahamas is approximately 40 Sv, of which 13 Sv compensates the upper branch of the thermohaline circulation, requiring a 27 Sv deep recirculation. Robust annual and semiannual cycles of meridional are found in both moored observations and model results with remarkable agreement in amplitude (±13 Sv) and phase. Maximum northward transports occur in winter and summer,...

Influence of Florida Current, gyres and wind-driven circulation on transport of larvae and recruitment in the Florida Keys coral reefs

Abstract Physical processes with high potential influence on the transport and recruitment of fish, lobster and other larvae in the Florida Keys are discussed using current measurements from standard moored instrumentation, plus bottom mounted Acoustic Doppler Current Profile observations, interdisciplinary surveys of water mass properties, nutrients and planktonic distributions, and satellite derived surface thermal patterns. A cold, cyclonic gyre forms over the Pourtales Terrace seaward of the middle and lower Keys where the Florida Current shifts from eastward to northward flow. Formation of the gyre appears to be related to offshore meander motion of the Florida Current and its cyclonic curvature. Prevailing easterly winds over the gyre circulation cause a convergence of Ekman transports into the coastal zone. The gyre circulation combined with the shoreward convergence of Ekman flow facilitates the transport of pelagic larvae from the Current to the fringing reefs. Duration of the gyre is approximately 1 month, which matches the planktonic stage of fish and slipper lobster larvae, and thus provides a mechanism for larvae retention and local recruitment. Abundant microzooplankton food supply for the larvae is available in the gyre interior due in part to concentration mechanisms and ecosystem response to gyre-induced upwelling of deeper, nutrient enriched waters. Gyre retention times are too short to be a factor in recruitment of locally spawned Florida spiny lobster larvae, which have a planktonic stage that may last up to 12 months. This indicates that Florida spiny lobster should recruit from remote upstream sources in the Caribbean or spend most of their long planktonic stage in Florida Bay of the southwest Florida shelf.

Variability of Structure and Transport of the Florida Current in the Period Range of Days to Seasonal

Abstract Current measurements with five repeated moored arrays were carried out from April 1982 to June 1984 across the Florida Current between West Palm Beach and Grand Bahama Island; a reduced sixth array was continued until June 1985. Transport were calculated directly by vertical integration of the 40-hour low-passed northward current components and extrapolation to the surface using the mean vertical shears ova the extent of the mooring. These transports compared well with 96 transport sections measured by PEGASUS and with transports determined by cable voltage measurements. During 1982–84 the transport variations ranged between 20 and 40 Sv (Sv = 106 m3 s−1) with a mean of 30.5 Sv and standard deviations of the 40-hour low-passed data of ±3 Sv. However, monthly mean deviations from the mean annual cycle were only of order ± 1 Sv, indicating that the climate relevant long-period variations of this current are fairly small. The volume transport shows a continuous spectrum with no particularly energeti...

The influence of Loop Current perturbations on the formation and evolution of Tortugas eddies in the southern Straits of Florida

Large cyclonic eddies on the northern edge of the Florida Current are the dominant mesoscale features within the southern Straits of Florida. The most prominent of these features is a quasi-stationary eddy that forms near the Dry Tortugas. Our observations, compiled from 3 years of advanced very high resolution radiometer measurements in the Straits of Florida and Gulf of Mexico, demonstrate a strong relationship between the generation of anticyclonic rings from the Gulf of Mexico Loop Current and the evolution of Tortugas eddies within the southern Straits of Florida. In six cases, Tortugas eddies evolve from cyclonic frontal eddies which form along the boundary of the Loop Current. The eddies remain stationary near the Dry Tortugas until they are impacted by an approaching Loop Current frontal eddy. The length of time an eddy spends near the Dry Tortugas is increased when the Loop Current sheds an anticyclonic ring. The involvement of a Loop Current frontal eddy in the ring-shedding process results in a delay in its, and hence the Tortugas eddys, downstream propagation. Results suggest that the lifetime of a Tortugas eddy can be as long as 140 days when a ring-shedding event occurs, or as short as 50 days in the absence of any ring-shedding events. Upon entering the Straits of Florida, the Tortugas eddies are deformed by the narrowing topography and shrink to approximately 55% of their original size as they propagated downstream. The shrinking of these eddies is accompanied by an accelerated translation from 5 km/d in the western Straits of Florida to 16 km/d in the east.