Dianeutral exchange between intermediate and deep water in the tropical Atlantic

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In the tropical Atlantic the vertical distance between Antarctic Intermediate Water (AAIW) and upper North Atlantic Deep Water (uNADW) cores (defined by their salinity extrema) exceeds 1000 m. In this thick layer, two additional water mass signals, characterized mainly by temperature extrema, are identified in the western boundary region. They are the intrusive flows of upper Circumpolar Deep Water (uCDW) and shallow uNADW (suNADW). The latter is considered a water type of uNADW. Since uCDW is a northward flow as a lower phase of AAIW and suNADW is a southward flow as an upper phase of NADW, a reference of level of no motion (LNM) is defined between uCDW and suNADW at the neutral density surface σN = 27.70 (at about 1150 m). This surface is coincident with a minimum of dianeutral velocity and measured velocity by lowered acoustic Doppler current profiler. Dianeutral motion processes are examined across the five selected neutral surfaces σN = 27.45, 27.60, 27.70, 27.80, and 27.90 that span the depth range between AAIW and uNADW and follow the major water mass signals. Dianeutral transport shows a decreasing tendency toward the middle σN = 27.70 neutral surface and reaches a minimum of 0.6 Sv (1 Sv = 106 m3 s−1), integrated in the tropical region between 10°N and 10°S. Thus the weakening of dianeutral motion and required minimum of epineutral advection at LNM seem to be relevant.