Labrador Sea Water in the Eastern North Atlantic. Part I: A Synoptic Circulation Inferred from a Minimum in Potential Vorticity

Abstract

A synoptic distribution of Labrador Sea Water (LSW) in the eastern North Atlantic is determined from a regularly sampled, but sparse (3° resolution), survey covering 39° to 54°N, 11° to 34°W during spring 1991. The core of LSW can be defined by a minimum in potential vorticity (PV). Using property values at this minimum the authors infer the circulation of LSW. In addition, using a known source function for salinities at the core of LSW, estimates are able to be made of LSW vintages. The authors then compared the synoptic circulation to historical data. Youngest, 1986 vintage, LSW crosses the Mid-Atlantic Ridge to the eastern basin between 48° and 51°N at 34°W. This water then flows north to the Iceland Basin and eastward to the Rockall Trough, where it was found to be of 1978 vintage. Tongues of low salinity, low temperature, and high oxygen extend southward on the eastern side of the Mid-Atlantic Ridge, indicating that LSW also flows southward in the eastern basin. At the southern edge of the survey the salinity and density of LSW increases. Compared to historical data of Talley and McCartney for the years 1957?1964 the authors found 1) no coincident values of PV, with LSW now having much lower PV and 2) that the core of LSW is significantly fresher. These differences show that climate variability, which affect these properties at the source, has a dramatic impact on tracer distribution at middepth in the eastern North Atlantic. Mediterranean Water is shown to overlap the LSW in a band 600 km wide spanning the eastern North Atlantic. Staircase structures on salinity profiles are not observed in the region, indicating that salt fingering if present, must be intermittent. This is contrasted with the work of Schmitz and McCartney who show that salt fingering is active south of 39°N. In Part II of this paper, the authors examine the anomalies inherited from the boundary condition variability and examine the advective/diffusive balance for LSW.