Distribution of Antarctic Bottom Water in the Western Gap (Northeast Atlantic)

Based on the data obtained during the 59th cruise of the R/V "Akademik Ioffe", the comprehensive study of modern hydrological and hydrochemical conditions in the near-bottom layer of the Western Gap of the Azores-Gibraltar Fracture Zone was made for the first time. Eleven stations were performed in the study area. They were located to the south of the gap, at the entrance and exit sills, in the central part of the gap basin, as well as in the Iberian abyssal plain. Thermohaline parameters, characteristics of currents, the content of dissolved oxygen and nutrients (silicon, phosphorus) were measured at the stations.

There was water with a potential temperature less than 2°C, high in oxygen, silicon, and phosphorus deeper than 4558 m south to the gap. The current in this layer had a predominantly northeasterly direction with velocities ranging from 8 cm/s at the upper boundary to 2–3 cm/s near the bottom.

Water with θ <2 °С was found in the central part of the entrance sill —  in the bottom layer of 20–85 m thick and in the northeastern part at the depth of 4450–4560 m. The current flowed inside the gap and had high velocities: 10–20 cm/s in the central part and 27–30 cm/s in the northeastern part of the sill. The transport of water with θ<2°С through the transect was 0.097 Sv. Hydrochemical parameters in this section had elevated concentrations.

The near-bottom videorecording performed at the southern slope of the gap basin showed pronounced signs of erosion, which suggested a constant strong AABW flow directed along the slope into the Western Gap. Direct measurements showed that in the 200 m thick bottom layer, the current was directed northward, and its average velocity was 29 cm/s. The water in this layer had an average potential temperature of 1.998 °C and was rich in oxygen, silicon and phosphorus.

There was no water with θ<2 °С detected at the stations in the central part of the gap, at the exit sill and in the Iberian Abyssal Plain

Thus, the AABW corresponding to the classical definition crosses the entrance sill and moves along the southern slope of the Western Gap basin. However, this water does not enter the central part of the gap and does not propagate further. It can be assumed that the flow on the southern slope of the basin under the action of the Coriolis force turns to the right and mixes up, recirculating in the eastern part of the basin or propagating further to the east.

Acknowledgements

The expedition financing and the primary processing of the data obtained on the 59^th cruise of the R/V "Akademik Ioffe" were carried out at the expense of State Assignment of the Shirshov Institute of Oceanology, project № 0128-2021-0012. The analysis and interpretation of the data were supported by the Russian Science Foundation (project no. 21-77-20004).