Structure of intermediate and deep waters in the tropical Atlantic

The purpose of this work is to reveal structural features of waters in the tropical Atlantic in the deep and intermediate layers. Based on the data set expanded in recent years, the content of deep and intermediate waters was calculated from conservative chemical variables.

The work includes data obtained from 1873 to 2023 (GLODAPv2.2022, eWOCE, WODB18 databases). Data from expeditions of the MSU Faculty of Geography from 2019 to 2023 were also used.

The following parameters were used to calculate the water mass content:

Results:

1) Broecker calculated the fraction of deep water in the Atlantic using the PO₄*. It was found that the best agreement with the content calculated by PO₄* was shown by the PO parameter with a deviation of 5-10%.

a)  b)   

Fig.1. North Atlantic Deep Water (NADW) distributions calculated by PO₄*(a), PO (b).

2) Deep water contents calculated using PO₄* on the sections were compared with water mass boundaries determined mainly using hydrophysical parameters.

NADW in the western tropical Atlantic is divided into three components: Upper NADW, Middle NADW and Lower NADW. It was found that in most of the analyzed sections, the lower boundaries of MNADW and LNADW practically coincide with the isolines of 85% and 60% of the NADW content.

In addition to large gradients of hydrophysical characteristics, the upper boundary of Antarctic Bottom Water (AABW) is determined by the Si/P=33 ratio (Arzhanova, Artamonova, 2014). In the western Atlantic it most often passes along the isoline of 25% AABW content, in the eastern Atlantic - along the isoline of 15% AABW content.

3) The distribution of AABW is of particular interest because it is transformed as it flows from the western basin to the eastern basin through the Mid-Atlantic Ridge faults. It was decided to refer to the transformed AABW as Northeast Atlantic Bottom Water (NEABW). It has been shown that NEABW is 50% composed of waters entering the eastern Atlantic through the Vema Fracture Zone, and 30% of these waters are “pure” AABW.

4) The PO parameter was used to determine the fraction of Antarctic Intermediate Water (AAIW) and Mediterranean Water (MW):

 

Fig. 2. Examples of obtained distributions of intermediate waters.

This work was supported by the Russian Science Foundation grant № 23-17-00032.

References:

1) Broecker et al. Radiocarbon decay and oxygen utilization in the Deep Atlantic Ocean // Global geochemical cycles. 1991. V 5, №1. Pp 87-117.

2) Broecker W. "NO" a conservative water-mass tracer // Earth and Planetary Science Letters. V 23. Pp 100-107.

3) Broecker et al. Sources and Flow Patterns of Deep-Ocean Waters as Deduced From Potential Temperature, Salinity, and Initial Phosphate Concentration // J. Geophys. : Oceans.1985. V 90, № C4. Pp 6925-6939.

4) V. Arzhanova, K. V. Artamonova. Hydrochemical structure of water masses in areas of the Antarctic Krill (Euphausia Superba Dana) fisheries // Proceedings of VNIRO. 2014. V 152. Pp. 118-132.