Oceanic biogeochemistry modelling: teaching numerical oceans to breathe

Transcription

1 Oceanic biogeochemistry modelling: teaching numerical oceans to breathe Eric Galbraith McGill University, Montreal, Canada

2 Overview Types of models General Circulation Models Coupling biogeochemistry with circulation New ideas

3 General idea Biogeochemistry and ocean physics are linked Processes are heterogeneous in space and time, 4- dimensional Biogeochemical ocean models combine basic ecological principles with ocean physics to compute global fluxes Light Mixed layer depth Advection and mixing Temperature Light absorption profile Trace gases

4 Why bother? A lot of time in front of a computer, not much fresh air, exercise, or photo opportunities Great for synthesizing concepts, developing hypotheses, illustrating complex phenomena Especially good at combining several wellunderstood processes in order to come up with emergent interactions

5 Transparency There are all kinds of models Single equation Box model Complexity Earth System Model

6 General Circulation Models (GCMs) Solve primitive equations for conservation of mass/volume, momentum, etc. Provide a self-consistent solution for ocean circulation at each time step Always wrong, but getting less wrong over time Vary in terms of resolution and simplifications

7 Modeling ocean biogeochemistry

8 What the model actually is Usually written in high-tech FORTRAN (one of the oldest computing languages) UNIX is the standard working environment Code is edited in a text editor, compiled, and executed using scripts Ocean model code can be hundreds of thousands of lines, in dozens of modules - biogeochemical code is usually much less

9 GCM Resolution Typically given in units of degrees, 1 degree~100km (on average) Usually can t resolve anything that is smaller than 3 grid cells Coastal upwelling (~100km) requires 1/3 degree resolution Time resolution generally scales with spatial resolution shorter time steps are required with smaller grid sizes. Some models are run on a desktop in an afternoon (e.g. GENIE), some are run using thousands of processors on supercomputers over months (e.g. IPCC Earth System Models)

10 GCM simplifications Detail of seafloor topography Rigid lid fixed ocean volume Complexity of imposed mixing processes Treatment of sub-gridscale features, like unresolved eddies and downslope currents

11 Example GCMs MOM3 (1990s): 5-degree resolution (can resolve ~1500km features). Ocean only. Sarmiento et al., Nature CM2.4: 0.2-degree resolution (can resolve ~30km features). Coupled ocean-atmosphere.

12 Modeling ocean biogeochemistry Nutrients CO 2 O 2 OM

13 GCM provides a physical environment where biology can be planted Growth (all phytoplankton, or functional groups ) Remineralization (suspended/dissolved and sinking organic matter) Associated gas transformations

14 Growth dependence on environment Temp ( C) 2 20 Parameters: m 0 = 1 d -1

15 Temperature

16 Growth dependence on environment Temp ( C) e kt Irr (W m - 2 ) Parameters: m 0 = 1 d -1 k = 0.063

17 Light 1 - exp (- IRR / IRRk) IRR / (IRRk 2 + IRR 2 ) 0.5 IRR / (IRRk + IRR) IRRk = 30 W/m 2

18 Growth dependence on environment Temp ( C) e kt Irr (W m - 2 ) 1-e -Irr/Ik N (mm) Parameters: m 0 = 1 d -1 k = Ik = 20 W m -2

19 Nutrients N 2 / (N 2 + Nk 2 ) N / (N k + N) Michaelis- Menton N k = half-saturation = 0.2 mmol/m 3

20 Temp ( C) Growth dependence on environment e kt Irr (W m - 2 ) 1-e -Irr/Ik N (mm) N/ (k N +N ) m (d -1 ) Parameters: m 0 = 1 d -1 k = Ik = 20 W m -2 k N =.5 mm

21 Growth effect on dissolved nutrients N Uptake = biomass * m Instructions: Run ocean physics, calculate growth rate, calculate biomass, calculate N uptake, rinse and repeat

22 GCM physics moves tracers Tracers are water mass properties, e.g. temperature and salinity Ocean model moves them around each timestep, according to advection and mixing Includes inorganic nutrients and gases, as well as organic components dc dt = Advection + mixing + (sources sinks)

23 Organic components Not required for biogeochemistry, but usually included in some combination of: Dissolved Organic Matter (DOM), often multiple types Undifferentiated phytoplanton (P) Phytoplankton function types (PFT) Zooplankton (Z), often multiple types

24 Sinking particles Remineralize as a function of sinking rate (may change with depth) and temperature. Usually approximate the Martin curve. It must be more complicated than this? Sinking flux

25 Three biogeochemical models in one circulation model Ideal nutrient: temperature, light, 1 nutrient, no OM, magical remineralization ibgc: temperature, light, 1 nutrient, DOM TOPAZ: temperature, light, 4 nutrients (N,P,Si,Fe), 2 DOM, 4 PFT Circulation model: MOM4, 3-degree

26 2 Observed PO 4 TOPAZ PO 4 ibgc PO 4 Ideal nutrient

27 Adding gas Dissolved gases are tracers Most have internal ocean sources/sinks due to biogeochemistry (O 2, CO 2, N 2 O) Also exchange with atmosphere, usually as a function of partial pressure, solubility f(t,s) and windspeed

28 Cutting edges Adding more moving parts: more elements, more PFTs, more upper trophic levels Improving parameterizations: iron cycling, interactions with sediments, remineralization Understanding what s happening within the models: complex results are often difficult to translate into meaningful new understandings Evolving models

29 Theoretical tracers Preformed nutrients, disequilibrium carbon, nutrient source regions, etc. Allow quantification of things that are not directly measurable in the real ocean Can be extremely useful for developing conceptual understanding

30 Example Sarmiento et al., Nature 2004

31 S N

32 Total export Northern high latitude Southern high latitude Low latitude

33 Follows Darwin model Introduce many random phytoplankton species (different parameters) to an ocean model Allow the species to compete Results in certain species extinctions, certain successful species in clear niches

34 Follows Darwin model

35 Summary Models are an indispensable tool, especially if you understand what they are doing Ocean GCMs offer realistic ocean environments in which to build ecosystems and experiment with biogeochemical cycling Help with refining hypotheses but useless without observations to test with