Study guide: Ocean Dynamics and Climate

📋 Course Outline

1. Ocean Geographic and Physical Properties
2. Salinity Balance and Dissolved Salts
3. Ocean Stratification and Mixing
4. Thermohaline Overturning Circulation
5. Wind-Driven Surface Circulation
6. Ocean Gyres and Vertical Motion
7. Physical and Biological Coupling
8. Ocean and Climate Change

📖 1. Ocean Geographic and Physical Properties

★ Must-know

• The oceans cover 71% of Earth's surface.

• The ocean's average salinity is about 34.5 g/kg.

Further detail

• The ocean has an average depth of 3,700 m.

• The ocean's average temperature is about 3.0 °C.

📖 2. Salinity Balance and Dissolved Salts

★ Must-know

• Surface salinity rises when evaporation removes freshwater and concentrates dissolved salts, and falls when precipitation, river discharge, or melting ice adds freshwater.

📐 Formula - The net surface freshwater loss is expressed as $F = E - (P + R)$, where E is evaporation, P is precipitation, and R is freshwater input from rivers.

• Dissolved elements enter the ocean through continental waters, volcanoes, and hydrothermalism; sodium and chlorine do not have the same origin.

Further detail

• Freshwater sources include:

  • the Amazon
  • the Congo
  • major Asian rivers entering the Bay of Bengal
  • Siberian rivers
  • melting ice

• The intertropical convergence zone has maximum rainfall, while regions under five atmospheric anticyclones have weak precipitation and high salinity.

• Ocean sinks include: biology and sedimentation, which remove SiO2 and CaCO3, hydrothermalism, which removes NaCl, MgSO4, and KCl, evaporites, which remove S and Mg

💡 Memory Hook

Evaporation concentrates salts; freshwater inputs dilute them.

📖 3. Ocean Stratification and Mixing

🔑 Key Concepts & Definitions

• Mixed layer: a surface layer homogeneous in density, and therefore in salinity and temperature
• Thermocline: a zone of rapid temperature transition between surface and deep waters; it can be permanent or seasonal
• Density anomaly: A density anomaly is $\sigma = \rho - 1000$, where ρ is the mass per cubic metre of seawater in kg/m³.

★ Must-know

📌 Strong stratification at low latitudes prevents vertical water mixing, whereas weak temperature contrasts at the poles permit vertical mixing.

Further detail

• Ocean seawater has a mass density of 1,024–1,027 kg/m³.

• In the permanent thermocline, the largest temperature variation occurs between 200 and 1,000 m, where temperature ranges from 4 to 25 °C; across a 3,000 m water column, the temperature variation is 0–3 °C.

• In the Northern Hemisphere, August has a maximum seasonal thermocline and a shallow mixed layer, whereas March has no seasonal thermocline and a deep mixed layer.

• High temperature and precipitation correspond to low surface density, while very low temperature combined with high salinity corresponds to high surface density.

💡 Memory Hook

Strong tropical stratification suppresses vertical mixing; weak polar stratification allows it.

📖 4. Thermohaline Overturning Circulation

🔑 Key Concepts & Definitions

• Thermohaline circulation: or meridional overturning circulation, is the large-scale movement of ocean waters in which currents sink into the deep ocean and later return to the surface, driven by density variations due to temperature and salinity

★ Must-know

• In the North Atlantic, cooling and/or increasing salinity makes water dense enough to sink to depths of 2,000–4,000 m, forming North Atlantic Deep Water.

Further detail

• In the Southern Ocean, dense water sinks to depths greater than 4,000 m and forms Antarctic Bottom Water.

• Thermohaline circulation represents a highly schematized, millennial-scale journey of ocean waters.

💡 Memory Hook

Dense water sinks, travels at depth, then returns toward the surface.

📖 5. Wind-Driven Surface Circulation

★ Must-know

• Friction and the Coriolis effect make the surface current direction approximately 45° from the wind direction in the classical Ekman model, producing Ekman spiral and transport.

• Ekman transport creates sea-level pressure gradients that drive currents; the difference between sea level and the geoid defines dynamic topography, which can be used to calculate current speed.

Further detail

• The direct effect of wind on surface currents is limited to approximately the upper 50–100 m.

• The Coriolis force decreases toward the equator and becomes zero there.

💡 Memory Hook

Wind and Coriolis drive Ekman transport, which creates sea-level gradients and currents.

📖 6. Ocean Gyres and Vertical Motion

★ Must-know

• At the equator, converging trade winds associated with the intertropical convergence zone drive surface-current divergence, creating a surface depression and upwelling.

• Ekman transport causes water to converge in the centers of subtropical gyres beneath atmospheric high-pressure regions, producing downwelling.

• Surface circulation comprises five anticyclonic subtropical gyres and four cyclonic subpolar gyres; the subtropical gyres have intense western boundary currents, and three subpolar gyres are sites of deep dense-water formation.

Further detail

• Debris accumulated in:
  • the central North Pacific
  • the central South Pacific
  • the central North Atlantic
  • the central South Atlantic
  • the central Indian Ocean

💡 Memory Hook

Subtropical convergence drives downwelling; equatorial divergence drives upwelling.

📖 7. Physical and Biological Coupling

★ Must-know

📌 Photosynthesis occurs only at the ocean surface, whereas respiration occurs both at the surface and at depth.

• Eastern-boundary upwelling zones account for approximately 17% of world fisheries while covering approximately 0.02% of ocean surface area.

Further detail

• In temperate regions, phytoplankton blooms are limited first by light and later by nutrients as seasonal conditions change.

• The named eastern-boundary upwellings are:

  • Canaries and Mauritania
  • Benguela
  • California
  • Peru–Chile (the Humboldt Current)

📖 8. Ocean and Climate Change

📝 Essential Points

• Since 1970, the ocean has absorbed 91% of excess energy that cannot escape at the top of the atmosphere because of greenhouse gases. - IPCC, AR6, WG1, 2021

• Thermal expansion of the ocean contributes approximately 50% of sea-level rise.

• During 2011–2020, the ocean absorbed approximately 25% of anthropogenic CO2 emissions, equivalent to 2.9 GtC per year.

📊 Synthesis Tables

Ocean Circulation and Vertical Motion

Setting	Surface motion	Vertical motion
Equator	Current divergence	Upwelling
Subtropical gyre centers	Water convergence	Downwelling
North Atlantic	Dense water sinks	Deep-water formation
Southern Ocean	Dense water sinks	Bottom-water formation