Is There the Equatorial Water Mass in the Atlantic Ocean?

Using temperature and salinity profiles from the Argo data repository, a detailed volumetric temperature‐salinity diagram is compiled for the upper 2,000 m layer of the Atlantic Ocean. It is generally accepted that, unlike the Pacific and Indian Oceans where the Equatorial Water is present, there is no Equatorial Water in the Atlantic Ocean and its place is occupied by the South Atlantic Central Water (SACW). However, the detailed volumetric T‐S diagram shows that the main thermocline in the latitude range of 10°S–10°N is characterized by its own tight T‐S relationship which is relatively close to but clearly distinguishable from the tight T–S relationship of SACW in the latitude range of 10°S–40°S. We argue that the Atlantic Equatorial Water can be considered as a separate water mass which is probably formed by isopycnal mixing of SACW and the North Atlantic Central Water (NACW) in proportion approx. 3.5:1. Plain Language Summary: One of the most astonishing properties of water in the ocean is the presence of a so‐called tight temperature‐salinity relationship, when the vertical profiles of temperature and salinity, sampled over a distance of hundreds and thousands of kilometers, express similar dependence of temperature versus salinity. The volume of oceanic water expressing similar temperature‐salinity relationship is considered a separate water mass. In the World Ocean, the water masses differ depending on their geographic location and origin. Oceanic water masses have been actively studied since the 1930s, and it would seem that everything is known about them. However, in 1998 the Argo program was launched, that collects information from inside the ocean using a fleet of robotic instruments that drift with the ocean currents and move up and down between the surface and a mid‐water level. Re‐examination of water masses using previously unavailable high‐quality large volume Argo data allowed us to distinguish a formerly unnoticed water mass in the main thermocline of the Equatorial Atlantic and thereby complete the phenomenological pattern of basic water masses of the World Ocean. Key Points: Main thermocline in the equatorial Atlantic in the depth range of 150–500 m is characterized by its own distinct tight T‐S relationshipThe low thermoclinicity water body in the equatorial Atlantic thermocline is separated from SACW and NACW by thermohaline frontsThe newly introduced Atlantic Equatorial Water (AEW) can be thought of as a mixture of SACW and NACW in proportion 3.5:1 [ABSTRACT FROM AUTHOR]