3D

Developing a VEW for 3D Virtual Ecosystems

The one-dimensional simulation in a mid-ocean virtual mesocosm provides valuable insights into the working of the plankton ecosystem. But it has limitations. It neglects baroclinic flow relative to the drifting mesocosm. That flow is important at fronts, the quintessential feature of mesoscale turbulence. Three-dimensional simulation is also essential for coastal waters. Here we see how the LEI-IBM metamodel can be implemented in three-dimensions.

Ocean circulation

Ocean circulation can be divided into three parts: permanent, eddies and mesoscle turbulence.

Permanent circulation
First is the large-scale permanent circulation,. This includes the anti-cyclonic gyres and the embedded western boundary currents, like the Sargasso Sea gyre and Gulf Stream. A representation of this global circulation is stored in the VEW. It is used to advect the virtual mesocosm. The perament circualtion is not steady. The velocity fi eld in the workbench has velocity vectors every fi fteen days for every degree of latitude and longitude. So it resolves the annual reversal of the Somali current in the In dian Ocean.

The eddies
The permanent circulation is unstable. It loses energy to transient eddies with horizontal scales of tens of kilometres. These are the oceanic equivalent of storms or depressions in the atmosphere. Sea level is depressed by a few decimetres at the centre of ocean eddies; this depression can be measured from space by satellite altimeter. The transient pattern of motion asssociated with the eddies contains all but a few percent of the kinetic energy in the ocean circulation. The waxing and waning of the eddies invovles patterns of upwelling and downwelling which modulate the nutrient fl ow to the euphotic zone. Ocean colour images show that the ecosystem is modulated by the eddies.

Large-scale fronts
The permanent circulation redistributes the heat stored in the seasonal boundary layer. The horizontal gradients of temperature, salinity, and nutrients are increased in regions of horizontal confl uence between gyres. The temperature gradients are enhanced by seasonal heating to create a large-scale pattern of summer fronts repeated each year. We can compute the consequent geographical distribution of density stratifi cation, and its dynamically-important sibling, potential vorticity. The seasonally-varying geographical patterns of the isopycnic gradients of potential vorticity and nutrient concentration provide the environment which allows mesoscale turbulence to fl ourish and modulate the plankton ecosystem.

Mesoscale turbulence
The transient velocity fi eld associated with the eddies includes local regions of horizontal confl uence a few tens of kilometres across. These concentrate horizontal gradient of potential vorticity creating mesoscale jets that are a few kilometres wide and a few tens of kilometres long. The jets are geostrophic to fi rst order. Mesoscale turbulence is a tangled fi eld of these transiet jets. They are unstable and soon develop meanders that grow and sometimes pinch off into tiny transient eddies. The accelerating jets and groeing meanders require an ageostrophic circulation that includes powerful upwelling and downwelling in patches a few kilometres across. This vertical motion is responsible for the large amplitude variation of plankton concentration called mesoscale patchiness.

The challenge

Strategy

ICOM

Advanced dynamics - Non-hydrostatic
Adaptive mesh

Seabed processes

Optics in shallow water
Turbulent boundary layer on the seabed
Sedimentation and re-suspension of plankton and inorganic particles.

Three-dimensional VEW

VGW