Kevin D. Leaman

Hydrographic Structure of the Convection Regime in the Gulf of Lions: Winter 1987

Abstract Deep winter convection in the northwestern Mediterranean (Gulf of Lions) and the subsequent formation of Mediterranean Deep Water were observed using advanced oceanographic instrumentation during a six-week long experiment in 1987. The severe 1987 European winter forced an intense outbreak of the Mistral, a cold, dry wind blowing down the Rhone valley, in mid-January. Surface cooling and evaporation were of sufficient intensity to cause an initial episode of deep convection shortly before the experiment described here began. However, several more Mistral events took place during the experiment. During several cruises into the area, CTD and absolute horizontal velocity profiles were measured in the mixed area over the Rhone fan as well as across the southern front of this region; in addition, continuous records of shipboard meteorological and oceanographic parameters (air temperature, surface salinity, etc.) were made. An early-February Mistral apparently did not produce enough surface cooling to ...

Structure and Variability of the Florida Current at 27°N: April 1982–July 1984

Abstract Results of a two-year field experiment as part of the SubTropical Atlantic Climate Studies (STACS) program in the Straits of Florida are presented. Temperature and absolute ocean current observations were obtained by PEGASUS acoustic current profilers over 16 cruises during which repeated cross sections of the Florida Current were made at 27°N. Results are shown for the mean velocity and temperature fields, the perturbation horizontal kinetic energy and potential energy fields and for those energy conversion terms that could be computed directly from the data. The barotropic and baroclinic energy conversion terms, although small, indicate that the flow is stable for both types of perturbations. A 1arge part of the variability is contributed by short time scales (one week or less). The average and standard deviation of northward volume transport by the Florida Current during these cruises was (31.7 ± 3.0) × 106 m3 s−1. Barotropic and baroclinic contributions to the total heat flux across the North...

Observations with Moored Acoustic Doppler Current Profilers in the Convection Regime in the Golfe du Lion

Abstract In the Golfe du Lion, south of France, favorable conditions for deep winter convection exist and were documented by the MEDOC experiments during 1969–75. A renewed investigation of that regime with upward-looking moored acoustic Doppler current profilers (ADCPs) was carried out during 24 January–5 March 1987, to record the three-dimensional currents associated with the deep mixing. While in the earlier studies initial deep convection did not begin until fairly late in the winter season, a very strong Mistral around 10 January 1987 had already generated a 1arge deep-mixed patch, homogeneous down to around 2000 m at deployment time. Three ADCPs, two working at 150 kHz and one at 75 kHz, were moored in a triangle of 15 km sidelength at 550–780 m depth. Full records at 1-hour ensemble time intervals, 400 pings per ensemble, 8 m bin lengths were obtained by the 75 kHz and one of the 150 kHz ADCPs. In mid-February, a second Mistral hit the region. With the onset of strong winds and surface cooling the ...

Florida current meanders and gyre formation in the southern Straits of Florida

Moored current measurements, shipboard hydrography, and a time sequence of satellite-derived surface thermal images are used to show the formation and evolution of cold, cyclonic gyres coupled to large offshore meanders of the Florida Current in the southern Straits of Florida (SSF). Gyre formation is dependent upon the orientation of the Loop Current as it enters the SSF. With a well-developed Loop Current the flow overshoots the entry to the SSF causing the formation of a cold recirculation off the Dry Tortugas, approximately 200 km in size, that persists over timescales of about 100 days. The demise of the gyre occurs as it moves to the east at about 5 km d−1, reducing to half its original size off Big Pine and Marathon Keys and unobservable off the northern Keys. Previously observed local gyres between Key West and Islamorada, Florida (Lee et al., 1992), are identified as a latter stage in the downstream evolution of the gyres formed off the Tortugas. When the Loop Current is not developed, flow from the Yucatan Channel turns anticyclonically into the SSF, causing strong eastward flow over the slope off the Dry Tortugas and lower Florida Keys, and gyre formation does not occur.

Transport, potential vorticity, and current/temperature structure across Northwest Providence and Santaren Channels and the Florida Current off Cay Sal Bank

Currents and temperatures were measured using Pegasus current profilers across Northwest Providence and Santaren Channels and across the Florida Current off Cay Sal Bank during four cruises from November 1990 to September 1991. On average, Northwest Providence (1.2 Sv) and Santaren (1.8 Sv) contribute about 3 Sv to the total Florida Current transport farther north (e.g., 27°N). Partitioning of transport into temperature layers shows that about one-half of this transport is of “18°C” water (17°C–19.5°C); this can account for all of the “excess” 18°C water observed in previous experiments. This excess is thought to be injected into the 18°C layer in its region of formation in the northwestern North Atlantic Ocean. Due to its large thickness, potential vorticities in this layer in its area of formation are very low. In our data, lowest potential vorticities in this layer are found on the northern end of Northwest Providence Channel and are comparable to those observed on the eastern side of the Florida Current at 27°N. On average a low-potential-vorticity 18°C layer was not found in the Florida Current off Cay Sal Bank.

Upper layer currents in the western tropical North Atlantic (1989-1991)

Shipboard acoustic Doppler current profiler (ADCP) measurements and hydrographic observations of temperature, salinity, and dissolved oxygen are used to examine the upper water column flow field in the North Brazil Current (NBC) retroflection region of the western tropical Atlantic Ocean. Observations are presented from six cruises, one conducted in August 1989 and the other five conducted during the Western Tropical Atlantic Experiment between January 1990 and September 1991. The upper water column is divided into two layers, an upper thermocline layer located between the surface and the 24.5 o- 0 isopycnal surface and a lower subthermocline layer located between the 24.5 and 26.75 isopycnals. In the upper layer the NBC retroflects north of the equator to form the eastward flowing North Equatorial Countercurrent (NECC). During the six cruises the retroflection appeared complete. However, data coverage did not extend shoreward of the 200-m isobath, so the possibility of a continuous flow over the shelf still remains. There were also indications of several NBC rings that had apparently separated from the NBC retroflection and drifted to the northwest toward the eastern Caribbean Sea. North of the NBC retroflection and the NECC, the North Equatorial Current (NEC) flows west as the southern limb of the subtropical gyre. Part of the NEC is observed to retroflect cyclonically to join the eastward NECC flow. In the lower layer, beneath the NBC, the North Brazil Undercurrent retroflects to feed the eastward North Equatorial Undercurrent (NEUC). To the north a deeper component of the NEC recurves to also contribute to the NEUC.

Volume and Heat Transports of the Florida Current: April 1982 Through August 1983

Absolute velocity and temperature profiles are used to estimate the volume transport through the Straits of Florida and, in combination with historical midbasin data, to estimate the total meridional heat flux through a section at 27 �N. The mean annual volume transport of the Florida Current from April 1982 through August 1983 is 30.5 (� 1)x 106 cubic meters per second. The net northward heat flux through the 27�N section is 1.2 (� 0.1)x 1015 watts. The volume transport is characterized by high values in the late spring and early summer and low values in the late fall and early winter. There is a similar cycle in total heat flux.

Transports through the Straits of Florida

Abstract Transports were calculated for four sections of the Florida Current from Key West to Jupiter, Florida, using a moored current-meter array and voltages from cross-channel telephone cables at the western and northern ends of the Straits of Florida. In addition, moored arrays were used to estimate transport through the Northwest Providence, Santaren, and Old Bahama Channels that connect the Florida Current to the southwestern part of the North Atlantic Ocean. Transport measurements were obtained for an 11-month period from December 1990 to November 1991. Mean transports of ∼25 Sv (1 Sv ≡ 106 m3 s−1) for the flow across the western ends of the straits, which agree quite well with recent estimates of 23.8 ± 1 Sv entering the Gulf of Mexico through the Yucatan Channel, were obtained from both the Key West to Havana cable and the moored array. This estimate is about 5 Sv less than the generally accepted transport through the northern end of the straits at 27°N. This difference was partially accounted fo...

Current meter observations in the Old Bahama Channel

Current meter observations were made at 50, 250, and 435 m in 495 m of water at the center of the Old Bahama Channel between November 1990 and November 1991. The mean speed at those depths was 2.6, 50, and 26 cm s−1 toward the Straits of Florida. A maximum speed of 193 cm s−1 was found at the 250-m level. There was evidence of a speed maximum between 50 and 250 m. Large internal diurnal tidal currents were observed that produced high shears in the water column under some conditions. Transport calculated from the single mooring using simple assumptions varied from −2.4 to +6.6 Sv with a mean of +1.9 Sv toward the Straits of Florida. These values, while based on limited data, are very significant compared to the mean flow in the Straits of Florida. Thus flow through the Old Bahama Channel may be an important component of heat and salt transport in the straits.

Subtropical Atlantic Climate Studies: Introduction

This report is an introduction to the accompanying collection of reports that present the results of a 2-year period of intensive monitoring of the Florida Current. Both direct observing systems (ship-deployed current profilers and moored current meters) and indirect observing systems (coastal tide gauge stations, bottom pressure gauge arrays, a submarine cable, acoustic arrays, and radar installations) were used to measure temperature and volume transport.