Model Development: WOMBAT

New developments to the World Ocean Biogeochemistry And Trophic dynamics (WOMBAT) model.

The original version of WOMBAT was a classic NPZD model (Nutrient, Phytoplankton, Zooplankton and Detritus). This version had 10 tracers, used nitrogen as the base element that was tracked through the ecosystem component, and simulated growth by phytoplankton based on a classic Eppley curve scaled down by limitation by light and two nutrients (Phosphate and dissolved Fe). Dissolved Fe was a simple cycle with strong restoring based on Johnson et al., 1997. CaCO3 was considered to form based on a constant “rain ratio” with organic carbon (the detrital pool) of 0.06.

A new version of WOMBAT (called WOMBAT-mid) has been expanded to account for

  • multiple functional types of phytoplankton (nanophytoplankton, microphytoplankton and diazotrophs), zooplankton (micro- and meso-zooplankton) and detritus (slow and fast sinking),
  • additional nutrients including nitrate, silicate and ammonium that limit the growth phytoplankton types to differing degrees,
  • explicit chlorophyll concentrations within phytoplankton types to account for photoacclimation and which alter the light limitation term on growth (affect specifically the photosynthesis-irradiance curve),
  • consideration of photosynthetically active radiation into three wavelengths (red, green and blue), each of which is absorbed differently by the different phytoplankton types,
  • additional inputs of nutrients to the ocean, such as from hydrothermal vents (iron) and rivers (all nutrients),
  • major updates the oceanic iron cycle, including variable Fe:C ratios that are tracked through all ecosystem actors, chemical equilibrium speciation between Fe bound to ligands and free Fe, and scavenging based on how soluble free iron is in solution,
  • permanent burial of elements in in the ocean sediments
  • an active marine nitrogen cycle with biologically mediated sources and sinks (nitrogen fixation and denitrification).

However, as part of the development of WOMBAT-mid, we have also updated WOMBAT-lite. With the addition of only a small number of tracers (from 10 to 14) we can improve the representation of the iron cycle and patterns of primary production. WOMBAT-lite will be useful for ensemble simulations using the ACCESS-ESM for its computational efficiency.

For evaluation of WOMBAT-lite and WOMBAT-mid, keep an eye on the COSIMA forum, under “Biogeochemistry”, and then under “New WOMBAT evaluation”.