Oceanic Heat Flux, Biogeochemistry, and Circulation Dynamics
Oceanic Heat Flux and Energy Balance
Heat flux (W/m²) components include short-wave radiation, long-wave radiation, sensible heat flux, and latent heat flux. Short-wave radiation is the dominant heat source, strongest in summer and dependent on cloud cover. Latent heat flux, the largest sink, is proportional to evaporation and strongest in winter. Net heat flux increases with latitude and varies seasonally. The formula H=Q/A, where Q=ρCpT, describes heat flux. Salinity seasonality reflects the imbalance of evaporation and precipitation. Buoyancy (b) is controlled by temperature (T) and salinity (S), where b=-gρ/ρ₀.
Wind stress (τ-N/m²), the net effect of wind-driven momentum, is usually stronger in winter. Monsoon winds are driven by seasonal land-ocean temperature gradients, with τ=cDρu₁₀². Heat flux and wind stress regulate ocean seasonality. Cooling increases surface water density, causing it to sink and deepen the mixed layer. Heating makes surface water lighter, shrinking the mixed layer. Vertically integrated density changes balance surface heat loss or gain. Wind stress intensifies surface currents and near-surface turbulence, deepening the mixed layer. Vertically integrated buoyancy is conserved.
Potential Energy (PE) is given by PE=ρgz. For a well-mixed layer of thickness h, PE = ½ρgh². For a stratified layer (Δρ) with equal thickness h/2, PE = ½ρgh²-¼Δρgh². The well-mixed state has higher potential energy. The transition from stratified to well-mixed requires energy. ΛΔbh(∂h/∂t) = 2mu*³-βh, represents potential energy gain due to mixed layer deepening, wind kinetic energy input, and potential energy input due to buoyancy loss. Sea surface temperature (SST) integrates heat flux, with maximum SST lagging behind maximum heating and cooling. Mixing of thermocline water cools SST in fall and winter. Mixed layer deepening occurs rapidly from surface heating in spring/summer and gradually from subsurface mixing.
Phytoplankton and Marine Productivity
Phytoplankton are responsible for ~50% of Earth’s photosynthesis, using light, CO₂, and nutrients to produce organic material: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The Redfield ratio is P:N:C = 1:16:106. Photosynthesis is regulated by nutrients and light. High productivity occurs in high latitudes (summer), tropics, and coastal regions; low productivity in subtropics and high latitudes (winter). Nutrient uptake has two regimes: low nutrient levels (diffusion) and high nutrient levels (enzyme kinetics). At low N, uptake is proportional to radius (R); at high N, uptake is proportional to R². Smaller cells are advantageous at low N; larger plankton can store N at high N.
- Pico-phytoplankton: Tiny (<1μm) single-cell bacteria, prokaryotes, easily grazed and recycled, efficient at low nutrient levels.
- Diazotrophs: Nitrogen-fixing bacteria, using N₂ as a nitrogen source.
- Diatoms: Large (2-200 μm) photosynthetic eukaryotes, silica-based shells (frustules), sinkers, abundant in upwelling regions.
- Coccolithophorids: Calcium carbonate shells, involved in CaCO₃ cycling, sinkers.
- Flagellates: Motile, mixotrophic, can rapidly grow and bloom, causing harmful algae blooms.
Phytoplankton contain light-harvesting chlorophyll pigments, producing more under low light. Seawater absorbs light exponentially with depth, I = I₀e^(z/zλ). Red light is readily absorbed; blue light penetrates deeper. The clearest water is deep blue. Subpolar productivity shows spring/summer blooms. Upper ocean stratification regulates productivity: spring/summer shoaling of the mixed layer and wind-induced mixing. Gross Primary Production (GPP) is the total photosynthesis; Net Primary Production (NPP) = GPP – Ra (autotrophic respiration); Net Community Production (NCP) = NPP – Rh (heterotrophic respiration).
Ocean productivity is observed through ship-based measurements (radioactive Carbon-14 incubation, nutrient mass balance, oxygen mass balance) and satellite ocean color (remote sensing reflectance). Marine particle concentration is estimated using algorithms. Nitrogen for phytoplankton is available as urea, ammonium, and nitrate (NO₃). Recycled production uses urea and NH₄; new production uses nitrate. New production is ~ export production and NCP. Sinking particles include organic and mineral matter. Sediment traps capture falling particles. Aggregation and fragmentation processes transfer material between particle sizes. The e-ratio = C export/NPP. Smaller particles are more numerous. Surface water is depleted of nutrients; deep water is enriched. New deep water has less nutrients; old deep water has high nutrients. Organic matter decomposition in deep water releases N and CO₂. Phytoplankton need trace elements like iron (Fe), which comes from mineral dust, continental shelf sediment, hydrothermal vents, and Fe-oxides. Dissolved Fe is very low. Marine phytoplankton Fe demand is 0.5% to 2% solubility. Fate of mineral Fe includes hydrolysis, scavenging, binding with organic ligands, and photochemical reactions.
Ocean Carbon Cycle and Circulation
Dissolved Inorganic Carbon (DIC) is composed of [CO₂] <1%, [HCO₃⁻] 90%, and [CO₃²⁻] 10%. Downward organic flux (J) is balanced by vertical transport at steady state. CO₂ reacts with water to form bicarbonate and carbonate ions. Colder SST enhances CO₂ solubility. Subtropical water is more alkaline due to excess evaporation, enhancing buffering capacity. Subtropical water can absorb more carbon than higher latitudes. The Revelle buffer factor (β) is ~10 in tropics/subtropics and ~18 in polar oceans. Anthropogenic CO₂ increase leads to different DIC responses depending on β. β≈[HCO₃⁻]/[CO₂⁻]. Seawater is transported into the interior ocean. Vertical circulation replaces subsurface water via ocean currents and mixing. Average pH is ~8, maintained by the balance between CO₂ and alkalinity: CO₂ + CO₃²⁻ + H₂O ↔ 2HCO₃⁻. In a CO₂-rich ocean, the balance shifts right. [H⁺] = K₂[HCO₃⁻]/[CO₃²⁻] = k₂(2DIC-Ac)/Ac-DIC. Calcifiers produce CaCO₃, consuming 2 mol of alkalinity and 1 mol of DIC: Ca²⁺ + CO₃²⁻ → CaCO₃. Sinking CaCO₃ shifts the balance left, increasing CO₂ in seawater, leading to degassing into the atmosphere. Carbon pumps maintain the vertical gradient of DIC.
The organic pump, driven by sinking organic particles, increases thermocline/deep nutrients and carbon, consuming oxygen. The solubility pump is driven by thermal stratification. The carbonate pump, driven by sinking CaCO₃ particles, increases thermocline/deep alkalinity and carbon, decreasing surface buffering capacity, increasing atmospheric CO₂. Winter cooling increases solubility, lowering pCO₂; summer heating decreases solubility, raising pCO₂. Spring/summer blooms export C to the deep ocean, lowering surface C and pCO₂. Winter entrainment of thermocline waters returns C and N to the mixed layer, increasing C and pCO₂.
Wind stress curl measures the spin of wind stress; vertical velocity depends on wind stress curl. The North Atlantic Subtropical gyre includes the Gulf Stream and Canary Current; the North Atlantic Subpolar gyre includes the Labrador Current; the South Atlantic Subtropical gyre includes the Brazil Current and Benguela Current. Three dynamic regimes are: Western Boundary (Gulf Stream & Brazil Current), Eastern Boundary (Canary & Benguela Current), and Interior Gyre. Positive wind stress curl causes northward motion of the water column. Sverdrup circulation is equatorward in the subtropics (wind stress curl < 0).