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Jaime B. Palter, M. Susan Lozier, and Kara L. Lavender

Abstract

Labrador Sea Water (LSW), a dense water mass formed by convection in the subpolar North Atlantic, is an important constituent of the meridional overturning circulation. Understanding how the water mass enters the deep western boundary current (DWBC), one of the primary pathways by which it exits the subpolar gyre, can shed light on the continuity between climate conditions in the formation region and their downstream signal. Using the trajectories of (profiling) autonomous Lagrangian circulation explorer [(P)ALACE] floats, operating between 1996 and 2002, three processes are evaluated for their role in the entry of Labrador Sea Water in the DWBC: 1) LSW is formed directly in the DWBC, 2) eddies flux LSW laterally from the interior Labrador Sea to the DWBC, and 3) a horizontally divergent mean flow advects LSW from the interior to the DWBC. A comparison of the heat flux associated with each of these three mechanisms suggests that all three contribute to the transformation of the boundary current as it transits the Labrador Sea. The formation of LSW directly in the DWBC and the eddy heat flux between the interior Labrador Sea and the DWBC may play leading roles in setting the interannual variability of the exported water mass.

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Kara L. Lavender, Russ E. Davis, and W. Brechner Owens

Abstract

The occurrence and extent of deep convection in the Labrador Sea in winters 1996/97 and 1997/98 is investigated from measurements of over 200 neutrally buoyant subsurface Profiling Autonomous Lagrangian Circulation Explorer (PALACE) and Sounding Oceanographic Lagrangian Observer (SOLO) floats. In addition to providing drift velocity data and vertical profiles of temperature and salinity, 55 floats are equipped with vertical current meters (VCMs). Time series of vertical velocity (derived from measured pressure and vertical flow past the float) and temperature are obtained from the VCM floats. Mixed layer depths estimated from profile measurements indicate that convection reached depths greater than 1300 m in 1997, but no deeper than 1000 m in 1998. Deep mixed layers were concentrated in the western basin, although a number of deep mixed layers were observed southwest of Cape Farewell and also north of 60°N. The highest variance in vertical velocity and the lowest mean temperatures were found in the western basin, suggesting that this area is the main site of deep convection. Deep mixed layers and large vertical velocities were observed as late as April and May, despite the fact that surface forcing appears to have ceased. Estimates of mean vertical velocity appear to be affected by a float sampling bias, whereby floats preferentially sample convergent regions. The effect of this bias, which is dependent on the float depth within the convective layer, is to sample upward flow in early winter and downward flow in late winter when the convective layer has deepened. A one-dimensional heat balance model is examined, whereby the winter surface heat flux, estimated from temperature profiles, is balanced by the turbulent vertical heat flux associated with deep convection, estimated from time series measurements. The plume-scale vertical heat flux can only account for roughly −80 of −350 W m−2 measured at 400-m depth. The vertical heat flux at longer timescales is investigated, but cannot be resolved with this dataset. Failure to balance the surface heat flux by plume-scale motions, combined with an observed high variance of w and T at low frequencies, suggests that motion at these longer timescales contributes to the one-dimensional heat budget in winter.

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