What is DynAMITE?

Fig 1. Map of potential density (relative to 4000 db)
along the seafloor (color) with schematic flows of AABW,
DSOW and LNADW. Click to Enlarge.

The densest waters in the AMOC — Antarctic Bottom Waters (AABW) and Nordic Seas Overflow Waters (NSOW) — undergo a diapycnal transformation into warmer, lighter density classes – North Atlantic Deep Waters (NADW) — that is remarkable for its amplitude (e.g. 7-8 Sv across the 1.8° isotherm between latitudes 15° - 40°N). This upward transfer of mass and buoyancy gain -- the North Atlantic’s Abyssal Upwelling Cell (AUC) -- is primarily driven by diapycnal mixing along the Mid Atlantic Ridge (MAR) and in the deep Gulf Stream, with secondary contributions from lateral recirculations adjacent to the western boundary. Entrainment of overlying waters warms the bottom layers in the direction of the basin-scale circulation; vortex stretching enhances northward penetration of AABW properties. As the MAR topography steepens, diapycnal flows veer uphill and then westward, boosting DWBC transports by 10-15 Sv near Cape Hatteras and the Bahamas. Mixing weakens the stratification creating massive reservoirs of low potential vorticity waters – an abyssal analog to water masses formed by deep convection and buoyancy loss at high latitudes.

DynAMITE is an investigation of the processes underlying the AUC in the western North Atlantic. It is comprised of 3 components:

1) Analysis of basin-scale PV distributions from climatology.

2) A moored transport array of profiling CTDs and current meters to measure the interior flows at depths between 1500 - 6000 m.

3) Microstructure measurements (using the High Resolution Profiler) between Bermuda and the MAR where upwelling is postulated to feed the interior flows

Results from this program underscore the importance of the AUC’s contribution to budgets of mass, heat and tracers in the AMOC, and in setting the deep interior layers in motion.

The Field Program

Fig 2. Locations of the DynAMITE moorings
(yellow squares) and microstructure survey
(red lines) relative to existing full water
column profiles (black circles). Click to enlarge.

The field program was conducted between 20-30N, Bermuda and the MAR.

1) A moored array of profilers was installed down the southeast flank of Bermuda Rise in September 2010 and recovered in June 2012. These instruments returned profiles of velocity, pressure, temperature and salinity; and are being used to construct estimates of interior transports as a function of density.

2) A microstructure survey was conducted in May / June 2011 aboard R/V Knorr using the High Resolution Profiler (HRP). These measurements have provided estimates of vertical diffusivities across different geographical regimes in the study area. Full water column profiles of P,T,S and velocity filled significant sampling gaps and these have greatly clarified the structure of the regional flow field.

Moored Profilers

An array of moored instruments, deployed for 1.5 years, measured the property and flow fields on the southeastern flank of Bermuda Rise (a primary pathway for the interior circulation). The 6 moorings each supported 2 moored profilers plus 3 VACMs and fixed T/S sensors (MicroCats). These directly measured pressure, temperature, salinity and velocity to quantify the interior transports through the array. Shipboard hydrography and sampling for tracers ( CFCs, Iodine-129 and oxygen) provided evidence regarding the origins of these flows.

Fig 3. Diagram of the DynAMITE moored array superimposed on potential
temperature. Each mooring was equipped with 2 McLane Moored Profilers (MMP)
and 3 sets of fixed point instruments (current meters and CTDs) at the top,
middle and bottom of the mooring. Section location is down the mooring line
and zonal along 28N to the MAR (see Fig 2). All 6 bottom profilers returned
full records over the deployment. Only 3 of the top MMPs (M2,M3,M6) returned
usable measurements. Click to enlarge.

Microstructure Survey

Fig 4. HRP (figure 4) is an untethered
instrument that descends to the
seafloor, drops its weights at a
preprogrammed height above the
bottom and ascends buoyantly
to the surface.

In May 2011, Knorr 200‐6 conducted a survey along the trackline shown in Fig 2 above using SeaBeam to map seafloor topography, CTD casts to capture water samples, and the High Resolution Profiler (HRP) to measure water column properties and velocity at very small vertical scales.

The specific goals were to determine:

1) where and how much turbulent mixing occurs in the study region in relation to varying bathymetric characteristics,

2) the sources of energy driving that mixing,

3) how the dense waters navigate past a topographic ridge at 20°N, and

4) to acquire a detailed bathymetric survey and water samples along the moored array deployed last September.

HRP (Fig 4) is an untethered instrument that descends to the seafloor, drops its weights at a preprogrammed height above the bottom and ascends buoyantly to the surface. Recovery involves maneuvering the ship alongside the instrument in order to hook a line onto it, and using a specially designed winch and lifting rig system to hoist the package back onto deck at the stern. A system of tracks was used to move the package from the stern to a protected area near the aft hangar. CTD casts were conducted simultaneously with HRP operations, but not on every HRP dive. Although 100 HRP dives and 60 CTD casts were originally planned, the package hit bottom and stuck in the mud on dive #34. Although the package was finally recovered after dragging for it over a 6-day period, sampling time was consequently reduced to 48 HRP and 41 CTD casts.