Eastern and Western Boundary Currents in the Labrador Sea, 1995-2008

Since 1995, the annual occupation of AR7W in the Labrador Sea has usually included LADCP data in addition to hydrographic measurements and tracers. We have previously presented results discussing the section-wide circulation for particular years, comparison with geostrophic velocities, and heat flux as determined from individual as well as composite sections. In this work, we present boundary current transports for a sampling of AR7W sections from 1995 through 2008. Both eastern and western boundary currents (EBCs, WBCs) are examined by combining LADCP data with density (or hydrographic) measurements from ships and profiling floats (Argo, PALACE). The transport estimates from LADCP data are also compared with the currents based on along-track multi-mission altimetry and with the lagrangian velocities from historic float and drifter trajectories. We find that WBC transports are mostly weaker than EBC transports, with slightly less variability year to year. Transports may be underestimated by 2 - 4 Sv when LADCP data are not available far enough onshore, so we extrapolate the velocities (carefully) to improve our estimate. Geostrophic velocities from hydrographic data can also be used to extend the coverage. WBC transports are about 32 Sv in the mean, but range from 22 to as much as 55 Sv. EBC transports range from 26 to 70 (!) Sv, with a mean of about 45 Sv. Higher transports, year to year, result from a combination of greater current width and faster velocities. Baroclinic transport relative to 1500 dbars for the upper level current ranges from 2.5 - 4.6 Sv, in good agreement with Lazier and Wright (1993). Using hydrographic data to determine transport in prescribed density layers, we find reasonable agreement with other recent observations in the Labrador Sea: for the western boundaries, Fischer et al. (2004) and Dengler et al. (2006) (CTD, LADCP and moored array data near 53 N and 56 N, respectively); for the eastern boundaries, Holliday et al. (2009), their Labrador Sea section near 59 N (CTD and LADCP data). For our sections, transport in density layers (in the mean) is conserved only below sigma-theta = 27.8, that is, in the overflow waters of the DWBC. References: Dengler, M., J. Fischer, F. A. Schott and R. Zantopp (2006), Deep Labrador Current and its variability in 1996-2005, Geophys. Res. Letters, vol. 33, L21S06, doi: 10.1029/2006GL026702. Fischer, J., F. Schott, and M. Dengler (2004), Boundary circulation at the exit of the Labrador Sea, J. Phys. Oceanogr., 34, 1548-1570. Holliday, N. P., S. Bacon, J. Allen, and E. L. McDonagh (2009), Circulation and transport in the western boundary currents at Cape Farewell, Greenland, J. Phys. Oceanogr., 39, 1854-1870. Lazier, J. R. N., and D. G. Wright (1993), Annual velocity variations in the Labrador Current, J. Phys. Oceanogr., 23, 659-678.