Oceanography and the Hydrosphere
Oceanography and the Hydrosphere
This domain tests seawater properties, ocean circulation, waves and tides, coastal processes, groundwater, and the water cycle.
Core concepts
- Seawater averages about 35 psu salinity. Density increases with salinity and pressure and decreases with temperature; cold, salty water sinks and drives thermohaline circulation.
- Surface currents are wind-driven and deflected by the Coriolis effect: clockwise gyres in the Northern Hemisphere, counterclockwise in the Southern. Ekman transport moves surface water about 90° to the right of the wind in the Northern Hemisphere and left in the Southern.
- Upwelling occurs when Ekman transport moves surface water offshore, allowing nutrient-rich deep water to rise.
- Waves are generated by wind; speed equals wavelength divided by period. Shallow-water waves feel bottom when depth is less than half the wavelength.
- Tides are caused mainly by the Moon's gravity, with the Sun modifying range. Spring tides occur at new and full moons when the bodies align; neap tides at quarter moons.
- Groundwater flows from high hydraulic head to low head through permeable rock; porosity and permeability control storage and movement.
Worked example
At full moon, the Sun, Earth, and Moon are aligned, so solar and lunar tidal bulges reinforce each other. The result is a spring tide with a larger-than-average range. Reasoning: alignment produces constructive addition of tidal forces; quarter moons produce destructive interference and neap tides.
Traps
Coriolis deflection is to the right in the Northern Hemisphere, not the left. Spring tides are not a seasonal event. Density depends on both temperature and salinity, so warm freshwater can float above cold salty water. Groundwater does not always flow downhill at the surface; it flows down hydraulic head. High porosity does not guarantee high permeability.