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The Labrador current as represented by the Mariano Global Surface Velocity Analysis (MGSVA). The Labrador Current is the "southward" flowing component of the N. Atlantic subpolar gyre. It transports cold waters into the warmer Gulf Stream ring and meander region. Click here for example plots of seasonal averages. | |
The transport of the Labrador Current is thought to contain a large barotropic component. Early estimates indicated that the current may be 30% stronger than geostrophic calculations indicated as a result of a significant barotropic flow component (Hayes and Robe, 1978). Greenberg and Petrie (1988) calculated a total transport of 7.6 Sv while the geostrophic transport was calculated to be just 4.1 Sv (based on IIP sections). With a 30% increase (due to barotropic flow) the transport is only 5.3 Sv so, the high transport values are thought to largely from the inclusion of deep currents indicated by a deep water mooring. Speeds for the Labrador Current are about 0.3-0.5 ms-1 along the shelf edge (Greenbergand Petrie, Reynaud et al, 1985. Current speeds of 0.3-0.5 ms-1 were found by Reynaud et al. (1995) for the Labrador Current. Including the barotropic component, they estimate a value of 3Sv for the continental shelf branch of the Labrador Current and 16 Sv transport for the slope branch of the Labrador Current. The inshore branch of the Labrador Current is approximately 100 km wide and 150 m deep and it passes through Avalon Channel and the splitting of the Labrador Current around Flemish Cap can be seen in the tracks of satellite tracked drifters (Petrie and Anderson, 1985). Within Flemish Pass, Petrie and Anderson (1985) report that the width of the Labrador Current is reduced to 50 km with a speeds of 25 ms-1 which they believe is actually 30 ms-1.
The seasonality of the Labrador current has been questionable since it's initial descriptions. Smith et al. (1937) found no evidence for a systematic variation of the geostrophic transport. However, the commonly held belief at the time was that there was a distinct seasonal cycle in which a maximum was obtained in May and the Labrador Current then decreased in flow, perhaps even disappearing in late summer. The possibility of a seasonal cycle was raised again by Bullard et al. (1961) who put forth a possible transport decrease from 5.5 to 3.5 from April to July. However their transports are highly variable with a mean of 4.1 Sv and a standard deviation of 2.0 Sv and no apparent systematic change flow. Mountain (1980) used measurements from a current meter mooring near the core of the Labrador Current and found no evidence of seasonal transport decrease. Their velocities of 45.7 and 18.3 cm s-1 (110 and 380 m respectively) were higher than geostrophic velocities of 30 and 6 cm s-1 obtained from hydrographic sections over the mooring site in spring (1972-1976).
Lazier and Wright (1993) found a transport of 3.8 ± 0.9 Sv using CTD sections referenced to 1500 m in the Main branch of the Labrador Current 85% of this flow was concentrated in a 50 km wide jet between 400 and 1200 m. Additionally there was a baroclinic component with speeds reaching .09 m/s at 1000 m which adds ~ 7,2 Sv to the baroclinic transport estimate. They found seasonal variation in upper level circulation (400 m and up) but not deeper (1000m) levels. Velocities varied by factors of 2 at 400 m and 3 at 200m on the same mooring line believing the variation at the surface to be roughly twice that seen at 200 m. Minima were in March and April while maxima were in October with a range in annual variation of about 4 Sv. Buoyancy forcing is believed to be the cause of these differences rather than wind forcing (Thompson et al., 1986; Greatbach and Goulding, 1989). Lazier and Wright also report the presence of the an additional current regime seaward of the Labrador Current which they call the "Deep Labrador Current". The "Deep Labrador Current" was found over the lower continental shelf and is strongly barotropic. Minimum speeds are observed in the "Deep Labrador Current" from September to October while maximum speeds are seen from February to March.