Temporal and Spatial Scales of Labrador Sea Water Formation 
R. Allyn Clarke 
Ocean Circulation Division, Atlantic Oceanographic Laboratory 
Bedford Institute of Oceanography 
P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2 
Labrador Sea Water is an intermediate water found at the same density and depth range in 
the North Atlantic as the Mediterranean water. It is formed by convection from the sea surface 
to depths greater than 2 km in winter in the Western Labrador Sea. 
The processes leading to deep convection begin with the formation of a 200 km scale cyclo- 
nic circulation about denser than average upper layer water in the Western Labrador Sea (Figure 
1). This circulation pattern is hypothesized to be driven by an ocean/atmosphere heat exchange 
that has its maximum in this region (Clarke and Gascard, 1983). 
By early March, if deep convection is taking place, one sees that this body of denser 
upper waters penetrates to the top of the deep temperature/salinity maximum marking the core of 
the North Atlantic Deep Water (Figure 2). We note that the horizontal scale of this body is still 
100-200 km normal to the coastline. 
If we examine the details of the kinematics taking place within the cyclonic circulation, we 
find that there are two scales of motion, a mesoscale structure with a radius of 30-40 km and an 
eddy scale feature that is the order of 10-20 km (Figure 3). The mesoscale structures have a 
rotational period of the order of 10 days with the eddy scale periods 1 to 2 days. It is hypothe- 
sized that an instability of the mesoscale features forms the eddy scale features. The densest 
and most homogeneous water columns were found within the eddy scale features. The features 
attain their anti-cyclonic circulation by greatly depressing the pycnocline found at the top of the 
North Atlantic Deep Water. By the end of March, 1976, the homogenous water columns in the 
center of the eddies reach depths greater than 2.2 km. The pynocline outside of the eddies and 
mesoscale was elevated to 1.2 km. 
The actual vigorous deep convection which takes place within these eddies, occurs on 
short time and space scales in response to strong atmospheric forcing. Our vertical current 
meter results (Gascard and Clarke, 1983) suggest space scales of a kilometer or less and the 
same time scale as that of the storm causing the vigorous convection (12 hours). The dynamics 
of the eddy also responded to these periods of intense downwelling. 
64° 60° 56° 52° 48° 4ae 40° 
60° 60° 
58° pon 
se° se° 
sae sae 
52° 
ERE 50° 
64° 60° 56° 52° ape aae 40° 
Fig. 1. Potential Density at 100 dbars in the Western Labrador Sea during February, 1978. 
