Oceanography and the Hydrosphere
This domain covers ocean basins, seawater properties, circulation, waves, tides, coasts, groundwater, watersheds, and the water cycle. Items often require interpreting a circulation pattern or calculating a water-residence time.
Core rules and concepts
- Seawater salinity averages about 35 parts per thousand; inputs from rivers and removal by evaporation, precipitation, and sea ice formation balance over time.
- Thermohaline circulation is driven by temperature and salinity differences; cold, salty water sinks in high latitudes.
- Surface currents are driven mainly by winds and deflected by the Coriolis effect-right in the Northern Hemisphere, left in the Southern.
- Tides result from gravitational pull and orbital motion, producing spring tides at new and full moons and neap tides at quarter moons.
- Upwelling brings nutrient-rich deep water to the surface, supporting high productivity.
- Groundwater flows from high hydraulic head to low head; aquifers recharge slowly and can be depleted.
Worked example
Estimate ocean water residence time: ocean volume ≈ 1.35 × 10^9 km³; river discharge ≈ 3.7 × 10^4 km³/yr. Residence time = volume ÷ input = 1.35 × 10^9 ÷ 3.7 × 10^4 ≈ 36,500 years. This long residence time helps explain why ocean salinity is relatively stable: small imbalances persist for thousands of years.
Traps that cost marks
- Claiming tides are caused only by the Moon's gravity; the Sun and Earth-Moon orbital motion also matter.
- Reversing Coriolis deflection between hemispheres.
- Confusing upwelling (deep water rising) with downwelling.
- Treating groundwater as an unlimited resource; recharge rates are often much slower than pumping.