Upper Branch Routes of the Thermohaline Conveyor Belt
  Abstract >> Problem >> Strategy >> Routes >> Three Paths >> Warm, Cold and Tepid Routes >> Comparison with other GCMs >> A17 Comparison


 
 

Strategy

We used

 • Numerical simulations of the Global Ocean (ORCA2) in order to recover the 3D dynamics varying in time

We chose to reconstruct a climatological picture of this circulation by using a numerical model of the global ocean whose tracer fields are constrained to remain close to the given hydrological climatology. Consequently, the model acts as a dynamical interpolation of the climatology, much in the sense of more classical inverse models.

&

• Quantitative Lagrangian diagnostics (ARIANE) in order to recover water masses pathways, transport & characteristics

We interpret the model's Eulerian Fields by means of a Lagrangian methodology linking together, dynamically, different oceanic sections

 

 

ORCA2: a global version of the OPA O.G.C.M.

ORCA2, the OPA global OGCM (Madec et al., 1998)

- global configuration from the Southern Ocean at 78°S, to 90°N;

- North Pole singularity removed by introducing a coordinate transformation with a double, numerical, inland pole;

- zonal resolution is 2° in the whole southern hemisphere and only slightly distorted in the northern hemisphere;

- meridional grid interval varies from 0.5° at the equator to a maximum of 1.9° at 20°N;

- 31 levels in the vertical, with the highest resolution (10 m) in the upper 150 meters;

- topography derived from the ETOPO5 data set; Beckmann and Döscher’s (1997) scheme for overflows;

- isopycnal lateral physics; TKE vertical mixing scheme (Blanke and Delecluse, 1993).

 

Forcing and restoring terms:

- daily climatology obtained from the ECMWF 1979-1988 re-analyses, and smoothed by a 11-day running mean;

- robust-diagnostic experiment: restoring term to the Levitus’ (1982) climatology added on temperature and salinity;

- restoring (Madec and Imbard, 1996) active everywhere except in the equator band, in the surface boundary layer, and in a 1000 km neighbourhood of the coastal boundaries; it also varies with depth.

 

ARIANE: a Lagrangian tool tracing water masses

The equations of motion of the OGCM are discretized on a C-grid. This differencing turns out to be ideal for the computation of successive analytical streamlines for a given velocity field sampled in time. This mass-preserving trajectory scheme is especially relevant for the tracing of ocean water masses. A water mass on a given geographic section is inseminated with tens or hundreds of thousands of particles, each of which is associated with an infinitesimal fraction of the incoming transport. For selected final destinations (another geographic section, or the fulfilment of a hydrological criterion) infinitesimal transports may be added, and directional transports can be produced.

Off-line diagnostics allow backward computations of trajectories and the joint use of backward and forward experiments gives access to the error made in computing directional transports, by determining any of them with two independent calculations. Off-line Lagrangian diagnostics also permit looping over a climatological year while calculating trajectories, without the constraint of the true length of the OGCM simulation for setting up the limits of the Lagrangian integration.

Circulation schemes are obtained by calculating the three-dimensional non-divergent transport field determined by the displacement of the particles and their associated transport, and by computing the horizontal streamfunction associated with the vertical integration of this transport field.

 

The ARIANE Lagrangian diagnostic more in detail HERE

 

Sabrina Speich: Sabrina.Speich@univ-brest.fr