This cruise is the last of a series of four that took place as part of the North Brazil Current Rings Experiment, 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 funded by the National Science Foundation (NSF) and the National Oceanic and Atmospheric Administration (NOAA).

The main goal of this program is to study the contribution of the North Brazil Current (NBC) rings to inter-hemispheric exchange of heat and salt and to determine its role in climate.

The specific 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.

During the previous cruises (NBC1: November - December 1998; NBC2: February - March 1999; and NBC3: January - February 2000) the following was accomplished:

Deployment of:

• two sound sources (S1 and S2) ,


• one current meter mooring (CMM1),


• one CTD mooring (CMM2),


• an array of 14 inverted echo sounders (IES),


• one pressure gauge (PG)


• forty-three surface drifters, and


• twenty eight subsurface floats (RAFOS and ALFOS).

In addition,

• CTD casts were obtained at the deployment sites for further calibration.


• Intensive hydrographic and current surveys (CTDO/LADCP and XBT), guided by satellite products, were performed to survey the large-scale field and three rings.

The main objective of this cruise, NBC4, was to recover the moored instruments. If time permitted, ring/s would be surveyed.

The cruise was carried out from the R/V Seward Johnson. The ship departed from Fortaleza, Brazil on June 7, 2000 and arrived to Barbados on June 23, 2000.

The main objectives of the NBC4 cruise were:

In addition, XBT surveys were obtained along the cruise track. Four surface drifters were deployed. A CTD/LADCP survey was performed on a retroflection ring located near 7°N 50.5°W.

The locations of the IESs and the pressure gauge (PG) recovered during NBC4 are given in Table 1 and shown in Figure 1. All dates are GMT.

The location of the CMM deployed during NBC1 and recovered during NBC4 are given in Table 2.

4.3 Sound Sources:
Sound source moorings S1 and S2 were deployed during NBC1 at the locations shown in Table 1 (see also Figure 1) and recovered during NBC4.

Satellite tracked surface were launched at the following locations (Table 4):

A total of 14 hydrographic stations (Table 5) were occupied during the cruise. Station 1-13 were occupied crossing the surface expression of a NBC retroflection ring (Figure 2 and 3) Station 14 was taken at the CMM2 site for calibration of the temperature/salinity sensors after recovery. 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-9 plus CTD system. 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).

Temperature profiles were measured at a total of 53 stations using expendable bathythermograph (XBT) probes launched from the stern of the vessel while underway. The locations of the XBT stations are given in Table 6. The launches are listed in sequential order; probes that failed are not listed.

Time series data records from the pressure gauge and IESs deployed in foreign territorial waters will be provided to the respective countries within 2 years after final recovery of the moored instrumentation (nominally July 1, 2002). Surface drifter data is available in real time at http://www.aoml.noaa.gov . Floats trajectories will be provided to clearance countries via the WOCE Subsurface Float data Assembly Center (at http://www.wfdac.whoi.edu) within a year of the final data transmission (approximately 1 October 2000).