W. D. Wilson & R. L. Molinari, AOML;
R. A. Fine, RSMAS, University of
Miami.
The Deep Western Boundary Current (DWBC) of the North Atlantic Ocean is a principal component of the global meridional overturning circulation (MOC), transporting cold water from high latitude source regions southward across the equator. Recent ACCP studies have shown decadal time scale variability in temperature and salinity characteristics at the location of the source waters, correlated with atmospheric indices such as the North Atlantic Oscillation (NAO) (Dickson, et al., 1996), as well as lower latitude ocean SST and heat storage fields (Joyce and Robbins, 1996; Hansen and Bezdek, 1996; Curry and McCartney, 1996). Temperature and salinity at the DWBC sources controls deep convection, and presumably affects subsequent DWBC intensity and MOC heat transport.
The water mass and velocity characteristics of the DWBC at 26.5 N, east of Abaco Island, the Bahamas, have been monitored by AOML and RSMAS/University of Miami researchers since 1984, with support from STACS and ACCP. The section has been occupied from one to four times per year, with observations of temperature, salinity, and dissolved oxygen on all sections, and direct velocity measurements and chlorofluorocarbon (CFC) data on many of the sections. Prior to 1996, CFC cores were found primarily at the levels of southern Labrador Sea Water (1200-1600m) and lower North Atlantic Deep Water (LNADW, 3200-4500m). While the on/offshore location of the velocity core has varied in this record, there has been very little change in the structure of the salinity, temperature and tracer fields. At the level of "classical" Labrador Sea Water, there was a velocity core but little evidence of recent ventilation. Data collected on a cruise in July, 1996, however, clearly shows the presence of colder, fresher waters (by at least 2 standard deviations relative to the previous record) with CFC values higher by a factor of two (0.5 pmol/kg to 1.0 pmol/kg), in the Labrador Sea Water density range. These characteristics were confined very near the coast (50 km) and coincident with the upper DWBC velocity core.
Earlier studies of the DWBC at 26.5 N using CFC observations suggested advective time scales of about 20 years for southern Labrador Sea Water, however little was known about the pathways and recirculations enroute. The T-S time series at Abaco appears to lag similar series in the Labrador Sea convection region by about 10 years, suggesting more rapid advective velocities or a more direct pathway. Characteristics of the LNADW core also changed noticeably in 1996. NOAA's commitment to maintaining this long time series at an important location has provided new insight into the propagation of decadal scale climate anomalies in the deep ocean, including their effects on basin scale heat transports and methods of feedback to the upper ocean and atmosphere.
References
Dickson, R.R., J. Lazier, J. Meincke, P. Rhines, J Swift, 1996. Long-term
coordinated changes in the convective activity of the North Atlantic. submitted to Progress in
Oceanography.
Joyce, T. M. , and P. Robbins, 1996. The long-term hydrographic record at Bermuda, J.
Climate, in press.
Hansen, D. V. and H.F. Bezdek, 1996. On the nature of decadal anomalies in the North Atlantic sea
surface, J. Geophys. Res., 101:8749-8758.
Curry, R.G, and M.S. McCartney, 1996. Links between sub-tropical mid-depth warming/cooling
patterns and variations in convection intensity in the subpolar Labrador Sea.
Proceedings from the Principal Investigators Meeting of the Atlantic Climate
Change Program, May 14-16, 1996.