Distribution of intermediate water masses in the subtropical northeast Atlantic

Abstract

This work presents the quantitative study of climatological distributions of mid-depth Source Water Types in the NE Atlantic by the Optimum Multiparameter analysis (OMP), merging a~number of regional results from particular synoptic sections. The cores of the Mediterranean Water (MW), the modified Antarctic Intermediate Water (mAAIW) and the Subarctic Intermediate Water (SAIW) are detected and spatial variations of their depth/density are obtained: as expected, spreading of the source water types is predominantly isopycnic and follows the major mid-depth circulation patterns. In some areas the turbulent transport should also be considered. The MW in the Atlantic spreads as 3 cores of different density: the upper MW core (northwest of the first transition line between 28° W, 35° N and 14° W, 44° N) is found in the neutral density range of 27.65–27.70 kg m−3 and depths of 900–1000 m; the main MW core (northwest of the second transition line between 35° W, 28° N and 10° W, 37° N) has neutral density around 27.75 kg m−3 and is found at 1000–1100 m; the lower MW core (southeast of the second transition) has neutral density around 27.80 kg m−3 and is found at 1250–1350 m. The upper MW core has comparatively low MW contents (below 30%) and is speculated to be transported by the mean flow from the northern Iberian Peninsula and the Bay of Biscay to the northern Azores. The main MW core contains the most of the MW. It primarily originates from the MUC between Cape St. Vincent and Estremadura Promontory, where the strongest local decrease of the topographic β-effect is detected and is transported west by a flow at around 39° N. The lower MW core originates in the Gulf of Cadiz and is translated southwestwards by dominating flows. The SAIW (the core between 27.70 and 27.75 kg m−3) is found to spread south along both slopes of the MAR. The SAIW east of the MAR mixes with the upper and the main MW cores and re-circulates in a cyclonic gyre at 15–25° W and 34–39° N as far south as the Azores Current. The northernmost spreading limit of the mAAIW (the core between 27.60 and 27.65 kg m−3) is at 25–29° N, but its influence reaches 32° N east and west of the Canary Islands. Its maximum concentration is found south of the Canaries, from where the mAAIW is transported westwards, parallel to the westward transport of the deep fraction of the MW.