ON THE STRUCTURE OF THE NORTH ATLANTIC CURRENT IN MAY – JUNE 1990

Abstract

“In the last quarter of the ХХ-th century, our country has implemented several major programs of experimental research of the World ocean. Among them, a special place was occupied by the huge in its scale and scientific significance the project SECTIONS aimed at studying the climatic interaction of the ocean and atmosphere. Currently, systematic research in this field has gained new momentum through regular Hydrophysical monitoring of the energy-active region in the North Atlantic in the annual expeditions by Shirshov Institute of Oceanology of RAS (Gladyshev et al., 2017). The results of some special Russian ocean expeditions of the past years, one of which is described for the first time in this article, can serve as a certain historical background for modern studies of the ocean climate evolution”.
In 1990 Russian oceanographers carried out a comprehensive hydrophysical study of the Newfoundland energy-active zone in the Northern Atlantic ocean, as part of the national project “Sections” included in the international program WOCE. Three research vessels (R/V) of the Shirshov Institute of Oceanology (SIO): “Vityaz“(cruise 19), “Professor Stockman“( cruise 26) and “Academician Kurchatov“ (cruise 50) together with additional 4 vessels of other institutions were engaged in the field study. Scientific management for general programme of the expedition, dubbed “ATLANTEX-90”, was carried out by Professor Yu. A. Ivanov. The main objective of the program was to study the space-time short-period variability of water dynamics in the large – scale ocean circulation system of Gulfstream–North Atlantic Current. To this aim, in May–June 1990 R/V “Academician Kurchatov” performed several sections crossing the main hydrological fronts of the Newfoundland energy-active zone (45–53°N., 36–45°W). Observations were conducted using the cable probe with sensors of temperature, conductivity and pressure (CTD) and expendable bathythermograph (XBT). All this equipment was special made and passed metrological certification in the design Bureau of Oceanological Engineering (BOE) of SIO. The equipment metrology fitted in whole with international standards at that time.
The sea surface temperature (SST) was recorded along the RV route as well. The current velocity was measured during about one month at 14 moorings deployed on a section along meridian 36°W, from 47 to 53°N. The measurements were conducted using electronically operated current meters of POTOK type of the BOE of SIO production. The meters were installed on the each mooring at the horizons of 100, 200, 1000, 2000, 3500 m. In addition to the data of own measurements, facsimile maps of SST from the nearest hydrometeorological observatories were received by radio communication channel during the whole period of observations.
Analysis of the obtained data showed that during the field study period the North Atlantic Current (NAC) divided into two branches (Central and Southern) roughly in the neighborhood of 47.5–48°N, where isobath 4500 m turned to the East at right angle to isobath 4000 m. After point of the bifurcation, the Central branch initially maintained a Northerly direction, then turned North-West along the isobath of 4000 m, and farther, turning East, crossed the meridian of 36°W between 51° and 52°N. Prior that stage, the Central branch sometimes approached the meridian 36°W at about 50°N, then deviated to the North–Northwest and finally turned North-East about 51,5°N. The Southern branch of NAC after a split of the main NAC flow followed approximately to isobath 4500 m, and crossed the meridian of 36°W at about 48°North latitude. East of 36°W it could be at times of East-North-East direction, but usually this branch unfolded to the South-South-West, forming the high ridge of the ocean surface dynamic height on the Eastern flank of the NAC.
Three return flows were observed in the section of 36°W. One of the flows is marked between the Central and Southern branches of the NAC, while the other two were recorded on the Northern and Southern edges of the section. This structure of the velocity field in fact remained unchanged through all June 1990. The basic zonal flow was observed in the entire water column within the depths from 100 m to 3500 m. The highest current speeds were typical for the upper part of this layer. At depths of 1000–2000 m the velocities were noticeably weakened, increasing again in some places near the bottom.
The distribution of meridional components of flow speed according to the measurements on the buoys allowed us to detect the presence of large-scale divergence, which was located along the section on 36°W. Direction of the meridional component of the current to the North and South of 49°N turned out to be the opposite, forming that divergence in the field of the velocity. Under the analysis of the observations it was taken into account that an important role in the formation of the structure of ocean circulation in the area of research could play a seamount (>2600 m), registered by the sounders of R/V “Academician Kurchatov” near 49°N, 36°W.
The results of measurements and calculations showed that the average over the entire observation period water transport of the Central branch of NAC through the 36°W section accounted for 62.4 Sv. This value is comparable to the transfer of NAC, assessed four years later by Lazer (1994) 50±23 Sv for approximately the same area where we conducted our work in 1990. Approximately the same average transfer (46,5 Sv) was found in two return flows (presumably North and South recirculations of the NAC Southern branch). In whole, the average water transport in the Eastern direction through the section on 36°W was as high as 111 Sv., and it was 60.9 Sv after subtracting reverse fluxes.