Source waters contribution to the tropical Atlantic central layer: New insights on the Indo-Atlantic exchanges

This study reveals the influence of the Subtropical Mode Water (STMW) varieties and their spatiotemporal variability on the upper thermocline of the tropical Atlantic Ocean. Optimum Multiparameter water mass analysis was applied using the hydrographic and chemical datasets available from the World Ocean Database 2018 and The Global Ocean Ship-Based Hydrographic Investigations Program. Eighteen Degree Water (EDW) and Madeira Mode Water (MMW), sourced in the northwest and northeast North Atlantic, respectively, contribute to the North Atlantic Central Water (NACW) layer, which follows the North Equatorial Current and setting up the limit with the South Atlantic Central Water (SACW) at about 10ºN. The contribution of the EDW to the NACW spreads at the surface layer (excluding the first 100 m) until the 26.6 kg m⁻³ isopycnal (∼250 m). MMW contributes to denser levels of the upper layer, mainly between 26.8 and 27 kg m⁻³ (300–700 m). In the southern hemisphere, mode waters formed in the eastern South Atlantic (STMW₁₈) and along the edge of the subtropical front (STMW₁₂) were the main contributors to the surface and denser levels, respectively. The variety formed in the Brazil-Malvinas confluence (STMW₁₄) was restricted to the southwestern side of the South Atlantic gyre, while the Subtropical Indian Mode Water (STIMW; mode water advected from the Indian Ocean into the South Atlantic Ocean) influenced the SACW structure between 20°S and 30°S with an averaged mixing fraction of ∼34% ± 20%, mainly at isopycnals of 26.2 and 26.4 kg m⁻³ (100–400 m). The main source of the STIMW, the Agulhas eddies, act as an interocean exchanges structure that transport an average of 5.5 ± 3.2 Sv (1 Sv = 10⁶ m³ s⁻¹) of STIMW to the Atlantic, even after they have been altered by splitting and/or merging processes. In addition, a smaller contribution of ∼20% of STIMW was associated with the North Brazil Current at ∼350 m, showing the interhemispheric connection between the Indian Ocean to the upper branch of the Atlantic Meridional Overturning Circulation. Therefore, due to the previously reported increase of the Agulhas Leakage and the dynamic of the bifurcation of the southern Equatorial Current, the heat and salt fluxes to the northern hemisphere may likely increase in the following years.

Figure 8. Schematic representation of the main (a) ocean currents, (b) horizontal and vertical distribution of the different Subtropical Mode Water (STMW) varieties to the central layer (100 to 1000 m) of the tropical Atlantic. Main ocean currents in the study region are the North Brazil Current (NBC), North Equatorial Current (NEC), Brazil Current (BC), southern South Equatorial Current (sSEC), and Benguela Current. The thick arrow in (a) depicts the rotation view for panel (b). The source water masses are: Antarctic Intermediate Water (AAIW), Eighteen Degree Water (EDW), Madeira Mode Water (MMW), STMW of 18ºC (STMW₁₈), STMW of 14ºC (STMW₁₄), STMW of 12ºC (STMW₁₂), and Subtropical Indian Mode Water (STIMW). From bottom to top, the limits of STMW varieties are defined by lines: spaced dashed line for AAIW, dotted line for STMW₁₄, dotted and dashed for STMW₁₂, dashed line for MMW, and full line for STMW₁₈ (EDW is limited by STMW₁₈ and MMW). Red arrows depict the upwelling zones, and the shaded area represents the continent.