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 northeast (NE) Atlantic using the optimum multiparameter analysis (OMP). It generalises 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 water types is predominantly isopycnic and follows the major mid-depth circulation patterns. In some areas the turbulent transport also makes a considerable contribution. MW in the Atlantic spreads in three cores of different density: the upper MW core (northwest of the line 28° W, 35° N–14° W, 44° N) is found in the neutral density range of 27.65–27.70 kg m−3 at depths of 900–1000 m; the main MW core (between the line above and the line 35° W, 28° N–10° W, 37° N) has neutral density of around 27.75 kg m−3 and is found at 1000–1100 m; the lower MW core (southeast of the line 35° W, 28° N–10° W, 37° N) has neutral density of around 27.80 kg m−3 and is found at 1250–1350 m. The upper MW core has a comparatively low MW content (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 majority of the outflowing MW. It primarily originates from the Mediterranean undercurrent around Estremadura Promontory, where the Joint Effect of Baroclinicity and Bottom Relief (JEBAR) overrides the topographic β effect. It is transported west to the Azores Islands mostly along 39° N. The lower MW core originates in the Gulf of Cádiz, from where it is transported by the dominating flows southwestwards. The SAIW core is detected between 27.70 and 27.75 kg m−3. It is found to spread south along both slopes of the Mid-Atlantic Ridge (MAR). 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, penetrating as far south as the Azores Current. The mAAIW core is detected between 27.60 and 27.65 kg m−3. Its northernmost spreading limit is between 25 and 29° N, but its influence can be observed along the African coast and immediately west of the Canary Islands up to 32° N. The maximum concentration of the mAAIW core is found south of the Canary Islands, from where mAAIW is advected westwards, along with the westward spreading of the deep fraction of MW.