ENV 100 Chap.4 Ocean Circulation and Tides
Ocean Circulation and Tides
This chapter joins wind stress, rotation, basin geometry, temperature, salinity, and density into a circulation model. Surface currents, Ekman transport, upwelling, western boundary currents, and deep overturning redistribute heat. Tidal forcing begins with Moon-Sun geometry but local depth, friction, and coastlines determine the observed tide.
What this chapter covers
- 01
Wind-driven surface currents
- 02
Coriolis effect
- 03
Ekman transport and upwelling
- 04
Thermohaline circulation
- 05
Equilibrium tide model
- 06
Spring-neap and distance cycles
Worked example · free
Explain coastal upwelling
- +1Identify wind stress as the initial driver.
- +1State the direction of net Ekman transport.
- +1Use continuity to explain replacement from depth.
- +1Connect the rising water with temperature and nutrients.
Key terms
- Coriolis effect
- The Coriolis effect is the apparent turning of moving material caused by Earth's rotation.
- Ekman transport
- Ekman transport is the net movement of the wind-driven ocean layer at an angle to the wind.
- Upwelling
- Upwelling is the rise of deeper water that replaces surface water moved away.
- Thermohaline circulation
- Thermohaline circulation is ocean flow influenced by temperature and salinity density differences.
- Diurnal inequality
- Diurnal inequality is the difference in height between successive high or low tides.
Ocean Circulation and Tides FAQ
What drives surface ocean currents?
Wind supplies momentum, while rotation turns the flow and continents redirect it around basins. Friction and pressure gradients also matter, so a current is not a simple copy of the wind direction.
What drives deep ocean circulation?
Density differences linked to temperature and salinity contribute to sinking and deep flow. Winds, mixing, basin shape, and freshwater input also influence the connected overturning circulation.
Why do real tides differ from the bulge model?
Continents block movement, basins have finite depth, friction removes energy, and coastlines amplify or delay the tidal wave. Astronomical forcing sets the rhythm, while local geography shapes the response.
What causes coastal upwelling?
Wind-driven surface transport moves water away from a coast in the relevant configuration. Continuity then requires deeper water to replace it. The replacement water is commonly cooler and nutrient rich, but those properties should be checked with observations rather than assumed from direction alone.
How should tidal range be calculated?
Subtract the low-water height from the paired high-water height and retain the stated units. Then identify the date and location because range varies with astronomical geometry and local basin response. One calculation is a measurement for that interval, not a universal property of the coast.
Assessment move
Use separate diagrams for wind-driven circulation, density-linked overturning, and tidal forcing before connecting them. On the surface-current map, draw the wind direction, the net Ekman transport, the coast, and the replacement flow. State the hemisphere because rotation changes the turning direction.
Then convert the arrows into a causal paragraph that begins with wind stress and ends with the properties carried by upwelled water. On the overturning section, mark warmer and colder water, higher and lower salinity, density changes, sinking, deep return flow, and mixing. Avoid presenting the circulation as a rigid conveyor with one fixed speed.
Instead, list winds, buoyancy, basin geometry, freshwater input, and mixing as controls whose relative importance can vary. For tide revision, physically arrange three labelled points for Earth, Moon, and Sun. Build conjunction, opposition, and quadrature configurations, then predict whether the astronomical contribution tends to increase or reduce tidal range.
Add perigee and apogee as distance changes rather than confusing them with phase. Next, place the ideal forcing beside a real harbour response. Write how continents, water depth, friction, resonance, and coastline shape can alter timing and amplitude. Calculate tidal range from paired high and low water values and attach units; then explain why one range does not describe every tide in a mixed regime.
Retrieval practice should alternate a labelled sketch, a comparison table, and a short explanation. One useful drill gives only a wind arrow and coastline and asks you to infer surface transport and possible upwelling. Another provides Moon-Sun geometry and asks for the expected relative range plus one reason the local peak may differ from the ideal.
Review errors by classifying them as direction, driver, scale, or vocabulary errors. Before the test, write a compact answer in which every sentence performs one job: driver, turning influence, replacement, transported property, and local limitation. Check that gravity is described as forcing, while the basin and coast determine the observed response.
