Oceanography and hydrosphere
Oceanography and hydrosphere
Questions come from ocean-floor relief, salinity distribution, temperature structure, currents, tides and coral reefs - usually as a "why here and not there" reasoning task rather than simple recall.
Core rules
- Relief divisions in order: continental shelf → continental slope → continental rise → abyssal plain, with trenches at subduction zones and mid-oceanic ridges at divergent margins. The shelf is shallow (to about 200 m), gently sloping and biologically rich.
- Mean ocean salinity ≈ 34.7 ‰; the rule is salinity rises where evaporation exceeds precipitation and falls where precipitation, river discharge or ice-melt dominate.
- A three-layer temperature structure: warm mixed layer, thermocline (rapid fall) and cold deep layer.
- Surface currents form gyres driven by prevailing winds, deflected by Coriolis and shaped by coastlines; cold currents occur on the western margins of continents in low latitudes (upwelling), warm currents on the eastern margins.
- Spring tides at new and full moon (sun and moon in line); neap tides at the quarters.
- Coral needs warm (about 20–29 °C), shallow, clear, saline, well-oxygenated water - so it avoids river mouths and cold currents.
Worked example
Why is surface salinity near the Amazon mouth low but high near 25°N in the same Atlantic? Reasoning: at the river mouth, dilution by huge freshwater discharge outweighs evaporation, so salinity drops well below 35 ‰. Near 25°N, the subtropical high gives clear skies and strong evaporation with little rainfall (E > P), so salinity exceeds 36 ‰. The controlling variable is the evaporation–precipitation balance plus river inflow, not latitude alone.
Traps that cost marks
Reversing spring and neap tides; treating tides as currents; giving coral growth in turbid estuarine water; placing the continental slope before the shelf; quoting salinity without the ‰ sign or as a percentage.