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, NOAA's Atlantic Oceanographic and Meteorological Laboratory, the Woods Hole Oceanographic Institution, and Columbia University's Lamont Doherty Earth Observatory. The program is jointly funded by the National Science Foundation (NSF) and the National Oceanic and Atmospheric Administration (NOAA).

The main objectives of the program are:

1. To obtain a thorough description of the temporal evolution of the North Brazil Current retroflection and the shedding of rings from the retroflection.

2. To determine the physical structure of NBC rings after they separate from the retroflection and the volume of South Atlantic water they trap and transport, and

3. To determine the rates of translation, decay, and mixing with resident North Atlantic waters as the rings move northwestward toward the Caribbean Sea.

In order to achieve these objectives, intensive field work will take place during four cruises. Cruise SJ9901 was the second project cruise and had the following main objectives:

1) To conduct a hydrographic and direct current survey of the North Brazil Current Retroflection and any North Brazil Current Rings that could be identified in the study region.

2) To deploy surface drifters and acoustically-tracked subsurface (RAFOS) floats within the retroflection and selected NBC rings found during the survey.

3) To deploy 4 inverted echo sounders (IES) and one shallow-water pressure gauge to complete the array of instrumentation to monitor the NBC retroflection and shedding of rings.

4) To perform CTD/LADCP casts at the deployment sites for calibration of the data collected with the IES and moorings.

2. Cruise Dates and Personnel

Cruise SJ9901 was broken into two consecutive legs. Leg 1 took place from February 6 to February 22, 1999, beginning in Bridgetown, Barbados and ending in Cayenne, French Guiana. Leg 2 took place from February 24 to March 9, 1999, from Cayenne, French Guiana to Natal, Brazil. The purpose of the first leg was to locate and conduct surveys of NBC rings present to the northwest of the retroflection, including rings that had been surveyed previously on the first project cruise in Nov.-Dec., 1998 (SJ9808). In addition these rings would be seeded with more float and drifter launches. The purpose of the second leg was to carry out a survey of the NBC retroflection and to deploy the four IES and one additional pressure gauge mooring that were not successfully deployed on the first project cruise as part of the NBC time series monitoring array.

Cruise Participants:

Leg 1. February 6 to February 22, 1999 - Bridgetown, Barbados to Cayenne, French Guiana:

Leg 2. February 24 - March 9, 1999; Cayenne, French Guiana to Natal, Brazil:

3. Instrument Deployments

3.1. Inverted Echo Sounders

The original plan called for the deployment of 4 inverted echo sounders (IES) during Leg 2. However, due to instrument problems, only one new deployment and one replacement of a previous deployment were carried out successfully. The locations of these IESs are given in Table 1 and shown in Figure 1 (along with the locations of IES deployed in 1998).

Table 1: Location of IES Deployments

3.2. Moored Pressure Gauge

A shallow water pressure gauge was moored on the Brazilian shelf to measure tidal and low-frequency fluctuations of sea level at the following location (also shown in Fig. 1):

Table 2: Location of Moored Pressure Gauge

3.3. Surface Drifters and Subsurface Floats

A total of 24 surface drifters and subsurface floats were launched during both legs of this cruise. Details are given in Table 3:

Table 3: Location of Surface Drifter and Subsurface Float Launches

4. Station Data

4.1. CTDO/LADCP Stations

A total of 68 hydrographic stations were occupied on Legs 1 and 2 (Table 4). At each station, profiles of temperature, salinity (conductivity), and dissolved oxygen concentration (CTDO) were collected to a depth of 2000 m or to within approximately 20 m of the bottom at shallower depths, using a Sea-Bird SBE-9plus CTD system, and water samples for calibration of the salinity and dissolved oxygen profiles were collected. Current profiles were also measured using a lowered 300 kHz R.D. Instruments Acoustic Doppler Current Profiler (LADCP).

4.2. Expendable Bathythermograph Profiles

Temperature profiles were measured at a total of 116 stations using type T-7 expendable bathythermograph (XBT) probes launched from the stern of the vessel while underway. The locations of the XBT stations are given in Table 5. The launches are listed in sequential order; probes that failed are not listed.

Table 5: XBT Drop Locations:

Leg 1:

Leg 2:

5. Underway Measurements

5.1. Thermosalinograph

Values of surface temperature and salinity were continuously monitored and logged on the ships computer using a Sea-Bird temperature-conductivity recorder installed in the ship's 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 m below the vessel, depending on sea state conditions.

6. Preliminary Results

Survey operations during the first cruise leg found two North Brazil Current Rings northwest of the NBC retroflection. (Fig.3). The westernmost of these two rings had been seeded with drifters and floats on the first project cruise shortly before it pinched off from the retroflection in December, 1998, and had been tracked continuously by project scientists with satellite altimetry. The second ring was newly formed in late January, 1999 and the survey thus took place within a few weeks of its generation. Both rings were very large, strong NBC rings with maximum surface swirl velocities of greater than 1 m/s. A third ring which had been surveyed on the first project cruise (SJ9808, Nov.-Dec., 1998) and was anticipated to be in the vicinity of Barbados or Tobago could not be found and was assumed to have disintegrated or passed west of the survey area to interact with the Lesser Antilles.

Results from the second leg showed that the tip of the NBC retroflection reached 7.5°N, nearly touching the southeastward edge of the newly formed ring, and that the retroflection had a well developed internal recirculation centered near 6°N, 48.5°W. (see Fig. 3). The eastward flow leaving the retroflection and flowing into the North Equatorial Countercurrent had a strong northerly component and cyclonic curvature which suggests that the western part of the North Equatorial Countercurrent was locked in a stationary meander pattern that is typical for this time of year. The NBC entering the region along the western boundary near 2°N was found to be surprisingly weak and narrow compared to the strength of the retroflection and the NBC rings, with maximum surface currents less than 1 m/s and a total width of only ~100 km.

7. Release of Project Data

In accordance with the provisions specified in the cruise prospectus and application for foreign clearances, the full data results from this experiment will be provided to all clearance countries according to the following schedule:

Shipboard Measurements

All shipboard measurements, including underway data records, CTDO/LADCP station data, and XBT profile data will be provided within 6 months of the termination of the cruise (September 1, 1999).

Moored Instrumentation and Drifters/Floats

Time series data records from current meters, IESs, surface drifters, and subsurface floats will be provided within 2 years after final recovery of the moored instrumentation (nominally June 1, 2002).

8. Foreign Observers and Participants

A scientific participant from Brazil joined Leg 2 of the cruise (Ms. Denise Velhote from IOUSP), and a Brazilian Naval Officer, Frigate Captain (Capitão-de-Fragata) Helio Zampier, was an observer on both cruise legs.

9. Acknowledgements

The support and able assistance provided by the Captain and crew of the R/V Seward Johnson is gratefully acknowledged. The scientific participation from Brazil was extremely helpful in all aspects of the cruise operations and is much appreciated. Financial support for this research was provided by the U.S. National Science Foundation and the National Oceanic and Atmospheric Administration.

Figure 1: Inverted echo sounders (IES 4 and 7) and pressure gauge (PG) deployed during this cruise. (black closed symbols). Positions of current meter and CTD moorings (open square), and IES (open circles) deployed on previous cruise are shown.

Figure 2. Location of CTD and XBT profiles obtained during Leg 1 (upper panel) and Leg 2 (lower panel).

Figure 3. Shiptrack and surface currents measured during Leg 1 (upper panel) and Leg 2 (lower panel) with the shipboard ADCP.