Inter-decadal changes in stratification and double diffusion in a transatlantic section along 7.5°N

Authors

  • Miguel Ángel San Antolín Plaza Departament d’Oceanografia Física, Institut de Ciències del Mar, CSIC - Facultad de Ciencias del Mar, Universidad de Las Palmas
  • Josep L. Pelegrí Departament d’Oceanografia Física, Institut de Ciències del Mar, CSIC - LINCGlobal, CSIC-PUC, Facultad de Ciencias Biológicas, Pontificia Universidad Católica de Chile
  • Francisco José Machín Facultad de Ciencias del Mar, Universidad de Las Palmas
  • Verónica Benítez Barrios Departament d’Oceanografia Física, Institut de Ciències del Mar, CSIC

DOI:

https://doi.org/10.3989/scimar.03616.19G

Keywords:

transatlantic section, stratification, neutral buoyancy frequency, Turner angle, inter-decadal changes, vertical diffusion

Abstract


We use three transatlantic cruises (1957, 1993 and 2010) along 7.5°N to analyse inter-decadal variations of the neutral buoyancy frequency (with neutral density replacing potential density) and Turner angle. We also use Argo data from the 2003-2011 period to show that, within central and intermediate waters, the changes between the three sections are slightly greater than the seasonal and inter-annual variations, so they may be interpreted as actual inter-decadal variations. The results point to a generalized sinking of isoneutrals between 1957 and 2010, with maximum zonally-averaged values of about 100 m in the central and upper deep layers. They also reveal the occurrence of substantial changes in the intermediate and neighbouring water strata, with differential vertical sinking of isoneutrals and the transformation of their thermohaline characteristics. The neutral buoyancy frequency increased in the lower central and upper intermediate layers and decreased in the lower intermediate and upper deep layers. The distributions of Turner angle highlight a predominance of salt-fingering in the North Atlantic, except for a gravitationally doubly-stable layer located immediately below the intermediate water core; this stable layer thinned substantially between 1957 and 2010 because of the sinking of the lower intermediate isoneutrals acting together with actual water transformations from Antarctic Intermediate Waters into North Atlantic Deep Waters. We conclude that a significant portion of the water column underwent both increased vertical stratification and enhanced salt-fingering, two mechanisms with opposite effects on the effective vertical diffusion.

Downloads

Download data is not yet available.

References

Arhan M., Mercier H., Bourles B., Gouriou Y. 1998. Hydrographic sections across the Atlantic at 7°30N and 4°30S. Deep-Sea Res. Part I 45: 829-872. http://dx.doi.org/10.1016/S0967-0637(98)00001-6

Bindoff N.L., McDougall T.J. 1994. Diagnosing climate-change and ocean ventilation using hydrographic data. J. Phys. Oceanogr. 24: 1137-1152. http://dx.doi.org/10.1175/1520-0485(1994)024<1137:DCCAOV>2.0.CO;2

Brandt P., Eden C. 2005. Annual cycle and interannual variability of the mid-depth tropical Atlantic Ocean. Deep-Sea Res. Part I 52: 199-219. http://dx.doi.org/10.1016/j.dsr.2004.03.011

Bryden H.L., Longworth H.R., Cunningham S.A. 2005. Slowing of the Atlantic meridional overturning circulation at 25° N. Nature 435(7068): 655-657. http://dx.doi.org/10.1038/nature04385 PMid:16319889

Cunningham S.A., Kanzow T., Rayner D., Baringer M.O., Johns W.E., Marotzke J., Longworth H.R., Grant E.M., Hirschi J.J.M., Beal L.M., Meinen C.S., Bryden H.L. 2007. Temporal Variability of the Atlantic Meridional Overturning Circulation at 26.5°N. Science 317(5840): 935-938. http://dx.doi.org/10.1126/science.1141304 PMid:17702940

Curry R.G., McCartney M.S. 2001. Ocean gyre circulation changes associated with the North Atlantic Oscillation. J. Phys. Ocea nogr. 12: 3374-3400. http://dx.doi.org/10.1175/1520-0485(2001)031<3374:OGCCAW>2.0.CO;2

Fuglister F.L. 1960. Atlantic Ocean Atlas of Temperature and Salinity Profiles and Data from the International Geophysical Year of 1957-1958. Atlas Series, Woods Hole Oceanographic Institution, Vol. 1. http://dx.doi.org/10.1575/1912/4331

Ganachaud A. 2003. Large-scale mass transports, water mass formation, and diffusivities estimated from World Ocean Circulation Experiment (WOCE) hydrographic data. J. Geophys. Res. 108: 3213. http://dx.doi.org/10.1029/2002JC001565

Jackett T.R., McDougall T.J. 1997. A neutral density variable for the world's ocean. J. Phys. Oceanogr. 27: 237-263. http://dx.doi.org/10.1175/1520-0485(1997)027<0237:ANDVFT>2.0.CO;2

Kanzow T., Cunningham S.A., Rayner R., Hirschi J.J.M., Johns W.E., Baringer M.O., Bryden H.L., Beal L.M., Meinen C.S., Marotzke J. 2011. Observed flow compensation associated with the MOC at 26.5°N in the Atlantic. Science 317(5840): 938-941. http://dx.doi.org/10.1126/science.1141293 PMid:17702941

Lappo S.S., Lozovatsky I.D., Morozov E.G., Sokov A.V., Shapovalov S.M. 2001. Variability of water structure in the Equatorial Atlantic. Dokl.-Earth Sci. Sect. 379: 739-743.

Lozier M.S., Owens W.B., Curry R. 1995. The climatology of the North Atlantic. Prog. Oceanogr. 36: 1-44. http://dx.doi.org/10.1016/0079-6611(95)00013-5

Lux M., Mercier H., Arhan M. 2001. Interhemispheric exchanges of mass and heat in the Atlantic Ocean in January-March 1993. Deep-Sea Res. Part I 48: 605-638. http://dx.doi.org/10.1016/S0967-0637(00)00033-9

Marin F., Gouriou Y. 2000. Heat fluxes across 7°30'N and 4°30'S in the Atlantic Ocean. Deep-Sea Res. Part I 47: 2111-2139. http://dx.doi.org/10.1016/S0967-0637(00)00022-4

Marsh R., de Cuevas B.A., Coward A.C., Bryden H.L., Alvarez M. 2005. Thermohaline circulation at three key sections in the North Atlantic over 1985-2002. Geophys. Res. Lett. 32: L10604. http://dx.doi.org/10.1029/2004GL022281

McDougall T.J. 1987. Neutral surfaces. J. Phys, Oceanogr. 17: 1950-1964. http://dx.doi.org/10.1175/1520-0485(1987)017<1950:NS>2.0.CO;2

Oudot C., Morin P., Baurand F., Wafar M., Le Corre P. 1998. Northern and southern water masses in the equatorial Atlantic: distribution of nutrients on the WOCE A6 and A7 lines. Deep-Sea Res. Part I 45: 873-902. http://dx.doi.org/10.1016/S0967-0637(98)00002-8

Ruddick B. 1983. A practical indicator of the stability of the water column to double-diffusive activity. Deep-Sea Res. 30: 1105-1107. http://dx.doi.org/10.1016/0198-0149(83)90063-8

Ruddick B., Gargett A.E. 2003. Oceanic double-diffusion: introduction. Prog. Oceanogr. 56: 381-393. http://dx.doi.org/10.1016/S0079-6611(03)00024-7

Schmitt R.W., Perkins H., Boyd J.D., Stalcup M.C. 1987. C-Salt: An investigation of the thermohaline staircase in the western tropical North Atlantic. Deep-Sea Res. Part A 34: 1655-1665. http://dx.doi.org/10.1016/0198-0149(87)90014-8

Schmitt R.W., Ledwell J.R., Montgomery E.T., Polzin K.L., Toole J.M. 2005. Enhanced diapycnal mixing by salt fingers in the thermocline of the Tropical Atlantic. Science 308: 685-688. http://dx.doi.org/10.1126/science.1108678 PMid:15860625

Schott F.A., McCreary J.P., Johnson G.C. 2004. Shallow overturning circulations of the tropical-subtropical oceans. In: Wang C., Xie S.-P., Carton J.A. (eds.), Earth Climate: The Ocean-Atmosphere Interaction. AGU Geophys. Monogr. Ser. 147: 261-304.

Steinacher M., Joos F., Frölicher T.L., Boop L., Cadule P., Cocco V., Doney S.C., Gehlen M., Lindsay K., Moore J.K., Schneider B., Segschneider J. 2010. Projected 21st century decrease in marine productivity: a multi-model analysis. Biogeosciences 7: 979-1005. http://dx.doi.org/10.5194/bg-7-979-2010

Stramma L., Schott F. 1999. The mean flow field of the tropical Atlantic Ocean. Deep-Sea Res. Part II 46: 279-303. http://dx.doi.org/10.1016/S0967-0645(98)00109-X

Thorpe S.A. 1977. Turbulence and Mixing in a Scottish Loch. Phil. Trans. Roy. Soc. London A 286: 125-181. http://dx.doi.org/10.1098/rsta.1977.0112

Wunsch C., Heimbach P. 2006. Estimated Decadal Changes in the North Atlantic Meridional Overturning Circulation and Heat Flux 1993–2004. J. Phys. Oceanogr. 36: 2012-2024. http://dx.doi.org/10.1175/JPO2957.1

You Y. 2002. A global ocean climatological atlas of the Turner angle: implications for double-diffusion and water-mass structure. Deep-Sea Res. Part I 49: 2075-2093. http://dx.doi.org/10.1016/S0967-0637(02)00099-7

Downloads

Published

2012-09-30

How to Cite

1.
San Antolín Plaza M Ángel, Pelegrí JL, Machín FJ, Benítez Barrios V. Inter-decadal changes in stratification and double diffusion in a transatlantic section along 7.5°N. Sci. mar. [Internet]. 2012Sep.30 [cited 2024Mar.29];76(S1):189-207. Available from: https://scientiamarina.revistas.csic.es/index.php/scientiamarina/article/view/1384

Issue

Section

Articles

Most read articles by the same author(s)

1 2 > >>