LABRADOR SEA-WATER IN THE EASTERN NORTH-ATLANTIC .1. A SYNOPTIC CIRCULATION INFERRED FROM A MINIMUM IN POTENTIAL VORTICITY

A synoptic distribution of Labrador Sea Water (LSW) in the eastern Nor th Atlantic is determined from a regularly sampled, but sparse (3 degr ees resolution), survey covering 39 degrees to 54 degrees N, 11 degree s to 34 degrees W during spring 1991. The core of LSW can be defined b y 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 a re able to be made of LSW vintages. The authors then compared the syno ptic circulation to historical data. Youngest, 1986 vintage, LSW cross es the Mid-Atlantic Ridge to the eastern basin between 48 degrees and 51 degrees N at 34 degrees W. This water then flows north to the Icela nd Basin and eastward to the Rockall Trough, where it was found to be of 1978 vintage. Tongues of low salinity, low temperature, and high ox ygen extend southward on the eastern side of the Mid-Atlantic Ridge, i ndicating that LSW also flows southward in the eastern basin. At the s outhern 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 ha ving much lower PV and 2) that the core of LSW is significantly freshe r. These differences show that climate variability, which affects thes e properties at the source, has a dramatic impact on tracer distributi on at middepth in the eastern North Atlantic. Mediterranean Water is s hown to overlap the LSW in a band 600 km wide spanning the eastern Nor th Atlantic. Staircase structures on salinity profiles are not observe d in the region, indicating that salt fingering if present, must be in termittent. This is contrasted with the work of Schmitz and McCartney who show that salt fingering is active south of 39 degrees N. In Part II of this paper, the authors examine the anomalies inherited from the boundary condition variability and examine the advective/diffusive ba lance for LSW.