Water mass interaction at intermediate depths in the southern Rockall Trough,northeastern North Atlantic

Mooring time series, CTD data, and Argo float data were analysed to investigate some aspects of the circulation at the southern entrance to the Rockall Trough during 2003–2004. The in situ data are used to describe the distribution at intermediate levels of Sub-Arctic Intermediate Water (SAIW) and Mediterranean Water (MW), as well as the temporal variability in the presence of these two water masses. Salty, MW-influenced water was found in the southeastern part of the study area, near the continental slope as far north as Porcupine Bank, consistent with earlier reports. Apart from the main tongue, the distribution of MW is patchy, and MW parcels were found not only adjacent to the slope but also offshore. Further north and west, water at intermediate depths was influenced by the fresher SAIW. Unlike in some earlier studies, SAIW did not extend as far east as the continental shelf. The year-long hydrographic and current time series from a mooring on the western slope of the entrance, at the southern end of Feni Ridge, showed pulses of SAIW influenced water throughout winter and spring. In late spring, the fresh pulses almost completely ceased; throughout summer only a few weak and shortlived fresh anomalies appeared. The weakening of the SAIW signal did not seem to be caused by winter convection, which did not extend to sufficient depth that winter. The relatively weak SAIW presence during most of the study period may be linked to the near neutral state of the NAO index. The warm and saline conditions observed at the southern entrance to Rockall Trough were in agreement with the rising temperatures and salinities found over large parts of the subpolar North Atlantic in the late 1990s and early 2000s.