Density and Stratification
Density and stratification provide the signatures of strong isopycnal flows and locally enhanced diapycnal mixing that together constitute the cold limb of the AMOC.
Figure 1. Meridional and zonal sections of potential density (panels a and b) sliced through the western basin along the lines depicted on the inset map. White background highlights reservoirs of low potential vorticity (< 2.0 e-12 m-1 s-1 ) where diapycnal mixing has weakened the stratification either locally (isopycnals dipping downward into topography) or upstream of the section. Arrows indicate the direction of flows over rugged topography. Circle/crosses and circle/dots indicate geostrophic flow into or out of the plane of the section. The color panel shows a section of silicate along 24°N depicting the upwelling of elevated nutrient concentrations (associated with southern ocean source waters) over the western flank of the Mid Atlantic Ridge and their circulation into the interior as diagrammed in panel b.
Old Views of the Deep Circulation
The prevailing view of the cold limb circulation has emphasized the role of large-scale recirculations to rationalize the observed near-doubling of the DWBC transports between the mid-latitudes and tropics. Aside from 1-2 Sv of locally enhanced upwelling, this picture implies that upward buoyancy flux in the western North Atlantic occurs at background diffusivity rates and timescales associated with advection in these large recirculations.
New Views of Circulation
Analysis of potential vorticity distributions in finely divided density layers from the seafloor up to ~1500 meters provides evidence that upwelling and water mass transformation along the MAR are the primary sources of interior flows and the boosted DWBC transport. Maps of layer thickness, H, identify locations of enhanced diapycnal mixing, while f/H contours trace the sources and sinks of the geostrophic interior flows.
In Figure 3 (below), composites of the finely divided layers identify 4 basic flow regimes in the cold limb circulation, and their corresponding density / temperature / watermass classes.
- In Layer 1, diapycnal mixing (blue spirals) and vortex stretching inflate the northward penetration of AABW.
- In Layer 2, AABW upwells along the MAR and flanks of Bermuda Rise, AABW and DSOW are mixed in the deep Gulf Stream.
- In Layer 3, mixing along the MAR drives interior flows of LNADW that entrain into the DWBC.
- In Layer 4, above the crest of the MAR, LSW flows equatorward in the DWBC and is modified by advective-diffusive mixing and eddies adjacent to the DWBC and Gulf Stream. Flows from the eastern basin (modified MOW) retroflect back to the northeast without entraining into the DWBC north of 20°N.
Fig 3. Top row : schematic diagrams of the mean geostrophic interior flows (solid lines, arrows), recirculations
adjacent to boundary currents (dashed ovals), and regions of locally enhanced diapycnal mixing (blue spirals)
in 4 layers spanning depths > 1500 m. Purple arrows depict the mean location of the deep Gulf
Stream. Potential density bounds, temperature and approximate depth ranges for each layer are labelled.
Each layer corresponds to a generalized water mass class:
Bottom row: vertical thickness of each layer. Thickness maxima correspond to regions of locally enhanced
diapycnal mixing: i) over rugged topography of the MAR (Layers 1-3), ii) in the deep Gulf Stream (Layer 2) ,
and iii) in the waters formed by convective mixing in the Labrador Sea (Layer 4) Click to enlarge.
Interior flows along the SE flank of Bermuda Rise are remarkably strong, exhibiting velocities of 20-30 cm/sec over 90 day periods at the base of moorings 1&2..The profiler data are still being processed, but a summary of temperature and velocity records from the VACMs, MicroCat CTDs and the MMPs that have been processed give ample evidence of vigorous near-bottom transports. These are primarily directed to the southwest at moorings 1, 2, 3 & 5, but episodically to the northeast at 4 & 6.