Biogeochemical Tracers in a High Resolution Simulations of Ocean Circulation in the North Atlantic

Mesoscale eddies, the oceanic analogue of atmospheric weather systems, play an important role in modifying the hydrographic structure and current systems of the oceans. Numerical models of the oceans, however, have traditionally not resolved the mesoscale but have had to rely on parameterizations to deal with sub-gridscale effects. Due to the rapid advances in computational facilities it is now becoming possible to conduct regional ocean modeling experiments which resolve both the broad, basin scale flow and the mesoscale on timescales of interest for climate and carbon cycle studies.

 Mesoscale eddies will have an impact on the circulation through various eddy-mean  flow interactions such as in intergyre exchange and subtropical thermohaline processes.  Eddies can also have  dramatic effects on the nutrient and plankton distributions in the upper ocean - and are therefore essential for estimates of primary production. These mesoscale processes may also be important in the deep circulation and the fate of tracers at depths.

1/6 degree model of the North Atlantic

Temperature (C) and the velocity vectors for the second level (37.5m) after 300 days of integration

 
 
 
Converging merdians in a spherical coordinate system lead to issues of numerical stability and timesteps. In a novel technique to increase the minimum timestep needed over the domain  of the North Atlantic, we integrate the model in a rotated grid system - one where the poles  lie within the plane of the equator. 
Figures shown here are oriented back onto a grid of latitude and longitude. The domain extends from 20S to 74N and from 100W to 20E, with resolution of 1/6 degree (maximum north-south
grid length is 18.5km and minimum is 9km).  Vertical resolution increases from 25m in the surface waters to 500m in the deep ocean. The domain size is 576x672x21. 
The model is initialized with Levitus climatology and forced by surface winds. At the Southern boundary, climatological datasets are used to set water properties. 
Temperature at 37.5m 

This model captures the effects of the Gulf Stream, deep water formation in high latitudes and vigorous Equatorial upwelling. 

 
 
 Click for  animation  (Kbytes) of the early development of the model 

The development and propagation of anomalously warm and cold eddies can be clearly been seen in the Gulf Stream region. The transport of warm water northward by the mean flow of the Gulf Stream is also clearly visible. Both processes, transport significant heat and so affect regional climate. They also impact nutrient distribution in the ocean, which affects all forms of marine life. 

 
 
 

 Bathymeric details can have profound influence on the ocean mean circulation. As an additional improvement to the model we more faithfully represent the topography with the use of ``lopped'' cells. Here the bottom cell of each column of water is determined to have a depth of the average found in the cell, rather than having the depth dictated by the vertical resolution. Below we show two representations of the bathymetry (taken from ETOPO5 data): the first has full cells, the second has lopped cells. The difference between the two is also shown.
 
 

Full Cell Bathymetry
Lopped Cell Bathymetry
Difference

 
 


 Authors: Stephanie Dutkiewicz, Chris Hill, Alistair Adcroft, John Marshall and Mick Follows.
                 Department of Earth, Atmospheric and Planetary Sciences, MIT

These simulations are being run on the Origin 2000, SGI Power Challenge Array, part of the Scientific Computing Facilities and housed at Boston University, on the NERSC DOE computing facilities and on local cluster computers at MIT.