The relative contributions of advection and isopycnal and diapycnal mixing below the subtropical salinity maximum

Abstract

Abstract--The spatial distribution of the subtropical salinity maximum is identified using historical and recent data from the eastern North Atlantic. In the regions with high frequency of occurrence of the salinity maximum, the relative contributions of advection, eddy diffusion and double diffusion to the salt balance below the maximum salinity layer are determined. McDoUGALL's (1984, Journal of Physical Oceanography, 14, 1577-1589 salt balance equation for neutral surfaces is used in this analysis. The data base consists of two meridional CID sections along 33* and 270W between 10 ° and 35°N, mean temperature-safinity profiles in 5* x 5* squares presented by EMERY and DEWAR (1982), and mean velocity profiles in 3 ° x 3 ° squares evaluated by STRAMMA (1984, Journal of Marine Research, 42, 537-558).The tropical salinity maximum tongue is found to be quite persistent in its salinity value and its geographic distribution, but less clearly in its vertical or isopycnal position. Double diffusion due to salt-fingering appears to be an important process for the salt balance below the salinity maximum layer. An approximate estimate of the double-diffusive salt flux is obtained. Near the subtropical source region, the double-diffusive salt flux is balanced primarily by isopycnal advection; further to the south it is also balanced by isopycnal eddy diffusion. Maximum doublediffusive fluxes correspond in magnitude to the mean salt flux caused by the excess in evaporation at the surface in the central subtropics. The resulting isopycnal and diapycnal eddy-mixing coefficients derived by a linear inversion technique have the reasonable values of K i = (11 _+ 5) x 102 m2s -1 and Kd = (4 + 2) X 10-5 m 2 s -1. Considering the intermittency of the double-diffusive process, limiting values for the mean eddy-mixing coefficients are determined by neglecting the contribution of the double-diffusive salt fluxes. This leads to Ki = (5 + 2) x 102 m 2 s -1 and Kd = (5 --+ 1) X 10 -5 m 2 s -1 for the isopycnal and diapycnal mixing coefficients, respectively.

Meridional changes in water mass distributions off NW Africa during November 2007/2008

“…This corresponds to waters formed at the sea surface, in areas where evaporation exceeds precipitation, that sink due to Ekman transport and winter vertical convection. Throughout spring and summer there is surface capping of this salinity anomaly, which then spreads horizontally over long distances (Bauer and Siedler 1988).…”

Cape Verde Frontal Zone

“…In the northwest, we recognize the typical subtropical Salinity Maximum Water of the North Atlantic. This water mass is formed in the inner subtropical gyre where evaporation exceeds precipitation, and has recently been studied hy BAUER and SIEDLER (1988).…”

An unexpected nitrate distribution in the tropical North Atlantic at 18°N, 30°W—implications for new production

“…Recently, HAMILTON et al, (1989) suggested that salt fingers and associated double diffusion might be an important process in the vertical flux of nitrate. Diapycnal mixing due to salt fingering has been observed when warm and more saline water overlies cold and less saline water layers (BAUER and SIEDLER, 1988). Such a situation was given after intrusion of warm and saline Smax over cold and less saline Central Water during our drift study (PODEWSKI et al, 1993).…”

“…The depth of the Sma × layer is critical for the distribution of nitrate within ~ "potential euphotic zone" of 115-125 m. Usually, the core of the Smax water in the tropical North Atlantic is observed between 80 and 150 m (WORTHINOTON, 1976;BAUER and SIEDLER, 1988), however, it rises towards the Equatorial Counter Current, where it can bc found at 50 m depth (PETERS, 1978;BAUER and SIEDLER, 1988). WORTHINGTON (1976~ showed that the Smax water keeps its oxygen signature of about 100% saturation for a great distance from its formation at the surface.…”

“…From the source region, the subsurface salinity maximum water spreads out mainly in a southwest-directed tongue (WORTHINGTON, 1976;KAWASE and SARMIENTO, 1985), and has been observed even in Caribbean Sea waters (LAMBERT and STURGES, 1977). In the eastern North Atlantic it has been found between 20 and 8°N, 35 and 17°W, respectively (DEFANT, 1936;WILLENBRINK, 1982;BAUER and SIEDLER, 1988).…”