CRUISE REPORT

North Brazil Current Rings Experiment

R/V Seward Johnson Cruise SJ0002
January 29 - February 24, 2000

1. Introduction and Objectives

The North Brazil Current Rings Experiment is a joint effort between the University of Miami's Rosenstiel School of Marine and Atmospheric Science, the Woods Hole Oceanographic Institution, and the U.S. National Oceanic and Atmospheric Administration's (NOAA) Atlantic Oceanographic and Meteorological Laboratory. The main objectives of the program are:

1. To determine the physical structure of North Brazil Current (NBC) rings after they separate from the NBC retroflection.
   
2. To determine the rates of NBC ring translation, decay, and mixing with resident North Atlantic waters as the rings move northwestward towards the Caribbean Sea.
   
3. To describe the temporal evolution of the NBC retroflection and the dynamics responsible for NBC ring generation.

To achieve these scientific objectives, four research cruises will be performed. This cruise is the third in the series. The specific objectives of this cruise are to: (1) Conduct a hydrographic and direct-velocity survey of the NBC retroflection and any NBC rings which might be identified in the region, and (2) To deploy satellite-tracked surface drifters and acoustically-tracked subsurface RAFOS floats to measure the circulation in and around NBC rings as they translate and evolve.

2. Cruise Participants:

Leg 1: Fortaleza, Brazil - Bridgetown, Barbados
January 29 - February 10, 2000

Leg 2: Bridgetown, Barbados - Bridgetown, Barbados
February 12 - February 24, 2000

3. Instrument Deployment

Thirteen satellite-tracked surface drifting buoys were launched during the cruise. One was launched during Leg 1 and the remaining 12 were launched during Leg 2. Two acoustically-tracked RAFOS floats were launched during Leg 2 in the center of the ring at depths of 200 m and 450 m. The RAFOS floats will surface in June 2000. Launch times and positions are shown in Table 1.

4. Station Data

4.1 CTD/LADCP Stations

Seventy-seven hydrographic stations were occupied on Legs 1 and 2. At each station, profiles of temperature, salinity, dissolved oxygen, and vector velocity were collected to a depth of 2000 m or to within 10 m of the bottom, whichever was less. A Sea-Bird SBE-911plus CTD system was used for collecting the hydrographic profiles. Water samples were collected using a 14-bottle rosette sampler and 10-L Niskin bottles. Velocity measurements were made with a lowered 150 kHz R. D. Instruments acoustic Doppler current profiler (ADCP). A summary of station positions is shown in Figure 1 and Table 2.

4.2 XBT Stations

Ten temperature profiles were collected using Sippican type T-4 expendable bathythermograph probes launched from the stern of the ship while underway. The locations of the XBT stations are given in Table 3.

5. Underway Measurements

5.1 Thermosalinograph

Measurements of surface temperature and salinity were continuously monitored and logged on the ship's computer using a Sea-Bird temperature-conductivity recorder installed in the ship's clean seawater intake line.

5.2 Shipboard Acoustic Doppler Current Profiler

Upper-ocean currents were continuously measured with a 150 kHz narrow-band acoustic Doppler current profiler (ADCP) mounted in the ship's transducer well. The depth range of good velocity data typically extended to 250-300 m below the vessel, depending on sea conditions. Maps of surface and 200 m velocity are shown in Figure 2 for Legs 1 and 2.

6. Preliminary Results

Observations made during the first leg of the cruise indicated that one detached NBC ring was present in the study area. A single drifter was launched in the center of this ring towards the end of Leg 1 to facilitate tracking the ring's westward translation. A thorough ring survey, including 4 full sections across the ring, was completed during Leg 2. Shipboard and lowered ADCP measurements indicated ring swirl velocities greater than 100 cm/s near the surface, decreasing to less than 10 cm/s at 250-300 m depth. The ring core was characterized by a reduced salinity maximum at 100-150 m and relatively high oxygen between 150-400 m. Three CTD/LADCP sections across the continental shelfbreak were made on Leg 2 to explore the characteristics of the southeastward Guiana Undercurrent at 150-500 m depth. Southeastward flow was absent at the first of these sections offshore of Guiana due to the presence of the ring, but the undercurrent was clearly visible in the LADCP data at the sections east of Trinidad and north of Tobago. On the final section between Tobago and Barbados we noticed a strong anticyclonic circulation at 200 m (with purely westward flow above). Salinity and oxygen values within this feature are similar to those within an NBC ring.

7. Release of Project Data

In accordance with the provisions specified in the cruise prospectus and applications for foreign research clearances, the complete dataset collected during this cruise will be provided to all clearance countries according to the following schedule:

Shipboard Measurements: All shipboard measurements including underway data records, CTD/LADCP station data and XBT profiles will be provided within 6 months of the termination of the cruise (September 1, 2000).

Float/Drifter Measurements: Time series data from surface drifters and subsurface RAFOS floats will be provided within 2 years after completion of their drifting missions (June 1, 2002).

8. Foreign Observers and Participants

Carlos Fonseca, a Brazilian student from IOUSP, participated in Leg 1 of the cruise. Lieutenant Commander Luis Augusto de Oliveira of the Brazilian Navy's Department of Hydrography and Navigation was an observer on both legs of the cruise.

9. Acknowledgements

The assistance of the Captain and crew of the R/V Seward Johnson is gratefully acknowledged. Financial support for this research program was provided by the U.S. National Science Foundation with additional field support from NOAA/AOML.

Figure 1: Location of occupied CTD/LADCP stations. Large circles indicate locations of IES, sound source, and current meter moorings installed on a previous cruise.

Figure 2a: Surface currents (Leg 1)