Oceanography and Atmospheric Science
This section covers oceanography and atmospheric science. The exam tests whether you can explain circulation, upwelling, air pressure and climate variability using rotating-fluid physics and conservation principles.
Atmosphere and ocean circulation
- Atmospheric pressure falls with altitude; the environmental lapse rate is about 6.5 degrees Celsius per kilometre in the troposphere, while adiabatic rates describe rising and sinking air.
- The Coriolis effect deflects moving air and water to the right in the Northern Hemisphere and left in the Southern Hemisphere; it is zero at the equator.
- Geostrophic flow balances pressure-gradient and Coriolis forces aloft; near the surface, friction turns flow across isobars.
- Hadley, Ferrel and polar cells produce the trade winds, westerlies and polar easterlies.
- Thermohaline circulation is driven by temperature and salinity contrasts; wind-driven Ekman transport moves surface water about 90 degrees to the wind.
Worked example
On a Southern Hemisphere coast, wind blows equatorward along the shore. Ekman transport is 90 degrees left of the wind, so surface water moves offshore. Deeper, nutrient-rich water upwells to replace it. Reasoning: wind stress plus Coriolis produces net transport to the left in the Southern Hemisphere; offshore transport creates divergence at the coast and upward replacement.
Traps
- Calling Coriolis a force rather than an apparent effect in a rotating frame.
- Forgetting that Coriolis deflection is zero at the equator.
- Confusing wind direction with Ekman transport direction.
- Treating El Nino as stronger trade winds; it involves weakened or reversed trade winds and reduced equatorial upwelling.
- Applying the Northern Hemisphere deflection rule in the Southern Hemisphere.