Simulation

One-dimensional simulation in a Virtual Mesoscosm

The environment

The plankton ecosystem occurs largely in the seasonal boundary layer of the ocean, which is controlled to first order by vertical fluxes, to second order by the mean current (averaged over, say, the top 100m), to third order by the baroclinic motion (i.e. deviations from the mean current). The one-dimensional virtual ecosystem takes account of the first two orders.

The plankton

By definition, plankton cannot swim fast enough to change their ambient environment by migrating horizontally. However, many do usefully migrate vertically, both diurnally and seasonally.

Mesoscosm

The virtual mesoscosm is a vertical tube with the upper end at the sea surface and the lower end at a depth of 500m. It is assumed that the ocean is deeper. So it is not necessary to model seabed processes (but see page 43).

The horizontal area is nominally one square kilometre. The virtual mesocosm has no side wall, but the horizontal flux divergences of all variables are zero at every depth. Particles move vertically, and cannot therefore pass through the virtual side wall. The vertical profiles of the environmental and demographic variables are described by values in layers that are typically one metre thick.

For simplicity we shall refer to mesocosms, without the adjective "virtual".

Advection

The mesocosm drifts around the ocean under the influence of the currents. They are derived from a global ocean circulation model. The drift velocity is typically specified to equal the vertically-averaged current in the top hundred metres.

Track

The Workbench automatically computes the track of the mesocosm, when you specify a starting location and date, and the duration.

Track of a mesoscosm, which drifts for eight years, starting off Hispaniola and ending at Bermuda. The yellow crosses mark January 1st each year. The red cross marks the termination on 1 March in year 8. The mesocosm drifts with the mean current in the top 100m according to the OCCAM circulation kindly provided by Southampton Oceanography Centre.

Geographically-Lagrangian Integration

This procedure is called “Geographically-Lagrangian integration”. GLI allows us to discover how the virtual ecosystem is affected by ocean circulation, at least to first order. Errors occur in emergent properties because we neglect (baroclinic) flow relative to the drifting mesocosm. However, they are normally negligible because the gradients of environmental variables are weak and orthogonal to the baroclinic vector. (But see below.)

Surface forcing

The ecosystem is driven by surface fluxes through the top of the virtual mesocosm. These fluxes comprise: (1) solar radiation, (2) sensible heat, (3) latent heat, (4) IR radiation, (5) carbon dioxide, and other gaes and particles. The fluxes are computed from astronomical formulae and meteorological data. The latter are extracted from the ERA40 global re-analysis computed by the European Centre for Medium-range Weather Forecasting. The illustration, taken from the VEW Scenario GUI, shows ERA40 surface winds in European seas at noon GMT on 12 April 1958.

Mesoscale patchiness

The main omission from a one-dimensional simulation is mesoscale turbulence. Its kinetic energy occurs largely in transient semi-geostrophic jets, which meander unstably, giving a complex pattern of vertical motion. The local upwelling and downwelling speed can reach tens of metres per day in patches a few kilometres wide. That modulates the nutrient supply to primary production, giving rise a patchiness in ocean colour images. The horizontally-averaged primary production differs significantly from that predicted by a one-dimensional simulation. A three-dimensional version of the Virtual Ecology Workbench is being developed to address this problem (see "Developing a three-dimensional Virtual Ecology Workbench").