Teaching · physical fluxes

Physics decides when and where biology can use nutrients

Physical fluxes move nutrients and carbon between compartments: eddies and mixing, upwelling and downwelling, advection, and air–sea exchange. This is a multi-process story — not eddies alone — with Eastern Tropical Pacific teaching notes on Tehuantepec wind jets and mixed-layer depth seasonality.

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Process literacy for COP visitors · teaching approximation, not a forecast model.

Four processes that move the boxes

Eddy kinetic energy is a supply pathway, not a carbon credit.

  • Eddies and mixing

    Mesoscale eddies stir tracers, modulate nutricline depth, and create phytoplankton patchiness. Vertical mixing and entrainment renew mixed-layer nutrients when MLD deepens into the nutricline.

  • Upwelling and downwelling

    Coastal and open-ocean upwelling lifts macronutrient-rich (and sometimes Fe-variable) water into the euphotic zone. Downwelling and subduction export surface properties and can remove biomass from the sunlit layer.

  • Advection

    Horizontal currents relocate nutrients, larvae, and organic matter across analysis boxes. A fixed box is not a closed bathtub — lateral exchange matters (Assumption for the v1 slider).

  • Air–sea exchange

    CO₂ and O₂ gas exchange couple physics to chemistry (DIC) and biology O₂ stop hooks. Gas-transfer velocity scales with wind; solubility with temperature and salinity. Explorer uses a simplified piston-velocity style term — Assumption, not a bulk-formula credit model.

Eastern equatorial / tropical Pacific upwelling is a classic HNLC-adjacent teaching region: Moore et al. (2013) map Fe limitation where subsurface macros are supplied.

Fact (Li et al. 2025, paper result) — Anticyclonic eddies enhance oceanic CO₂ uptake on average; primary mechanism is downward DIC pumping; cyclonic eddies marginally diminish uptake; asymmetric biological responses contribute to the net imbalance.

Teaching only. Not a credit calculator, not export tonnes, and not an ETP default to copy KE/Gulf Stream percentages into (Assumption if transplanted). Heterogeneous eddy–CO₂ responses elsewhere warn against one global mean (Salinas-Matus et al. 2025).

One-line: eddies are not one-signed “CO₂ machines” — polarity, DIC pumping, and biology jointly set uptake anomalies. Explorer: no credit / CDR tonnes from physics teaching terms.

ETP teaching — Tehuantepec and MLD seasonality

Fact Gap-wind jets through the Chivela Pass force the Gulf of Tehuantepec: strong mixing, thermocline response, and eddy generation, peaking in boreal winter (roughly Nov–Feb/Mar). Process existence and seasonality order of magnitude are documented in eddy-census and wind-forcing literature such as Palacios & Bograd (2005).

Mixed-layer depth shoals in calm or warm periods and deepens under strong winds — nutrient entrainment is seasonal, not continuous.

Explorer implication: use N_active_days with default 180 d yr⁻¹-equivalent rather than blind ×365.25 continuous areal paint (Assumption, aligned with the Flux Explorer scaffold).

Derived teaching fluxes

Show the idea; do not free-slide these as independent knobs.

  1. Nutrient entrainment supply ~ (dC/dz) × max(∂MLD/∂t, 0) — Assumption schematic.
  2. Upwelling supply ~ wup × Cdeep — Assumption.
  3. Diffusive supply ~ Kz × (ΔC / Δz) — Assumption.

Physical supply sets how fast surface [NO₃⁻], [PO₄³⁻], and dFe can be renewed against biological uptake. N_active_days scales seasonal inventory framing on the shared equation sheet.

Flux Explorer — physics inputs

Six teaching sliders. Excluded: operational forecast claims, credit tonnes from gas exchange alone, dosing setpoints.

Symbol Label Unit Default Status
MLD Mixed-layer depth m 30 Assumption ETP open-coast order; Tehuantepec wind events can deepen locally
wup Effective upwelling velocity m d⁻¹ 0.5 Assumption teaching upwelling supply — not a mooring mean
Kz Vertical eddy diffusivity (nutricline) m² s⁻¹ 1×10⁻⁴ Assumption open-ocean upper-ocean order; highly variable
Uadv Lateral exchange / advection scale m d⁻¹ 5 Assumption box-flushing teaching proxy
kgas Air–sea gas-transfer velocity (CO₂ proxy) m d⁻¹ 3 Assumption wind-dependent order
Nactive_days Active-season days (Tehuantepec / MLD aware) d yr⁻¹-eq. 180 Assumption avoid continuous year paint; aligns Flux Explorer scaffold

Run the knobs in the Flux Explorer. Supply timing pairs with chemistry pools and biology uptake.

Key citations

  1. Moore, C. M., et al. (2013). Processes and patterns of oceanic nutrient limitation. Nature Geoscience, 6, 701–710. https://doi.org/10.1038/ngeo1765
  2. Palacios, D. M., & Bograd, S. J. (2005). A census of Tehuantepec and Papagayo eddies from the altimeter record. Geophysical Research Letters. https://doi.org/10.1029/2005GL024324
  3. Tagliabue, A., et al. (2017). The integral role of iron in ocean biogeochemistry. Nature, 543, 51–59. https://doi.org/10.1038/nature21058
  4. Li, Z., et al. (2025). Oceanic uptake of CO₂ enhanced by mesoscale eddies. Science Advances, Article eadt4195. https://doi.org/10.1126/sciadv.adt4195 — teaching only; not credits
  5. Salinas-Matus, et al. (2025). Mesoscale eddies heterogeneously modulate CO₂ fluxes… Biogeosciences. https://doi.org/10.5194/bg-22-7519-2025

Fiedler & Talley (2006) ETP hydrography is noted for Phase B full-cite lock.

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Teaching approximation. Not a forecast model. Not credits. See Disclaimer and Support.