Hydrosphere: Ocean Currents, Tides and Water Resources
This domain covers the movement of ocean water and the freshwater budget, and expects you to link circulation to climate and human use.
Ocean currents
Surface currents are driven by prevailing winds, deflected by the Coriolis force and shaped by coastlines; deep circulation is thermohaline, driven by temperature and salinity. Warm currents include the Gulf Stream, Kuroshio and North Atlantic Drift; cold currents include the Labrador, Canary, Benguela, Humboldt and Oyashio. Where warm and cold currents meet, fog forms and plankton-rich fishing grounds develop, as at the Grand Banks. Cold currents along tropical west coasts chill the air, suppress convection and cause coastal deserts such as the Atacama.
Tides
Tides result mainly from the moon's gravitational pull, which is about twice the sun's, plus centrifugal force. Most coasts have two high tides in a lunar day of 24 h 50 min, so tide times shift roughly 50 minutes later daily. Spring tides occur at new and full moon (sun, moon and earth in line - syzygy) and are highest; neap tides occur at quadrature and are lowest.
Water resources
Groundwater lies below the water table in aquifers; a watershed is the catchment of a river system. Rainwater harvesting, check dams and wetland protection are key conservation measures. Assam depends on the Brahmaputra and on beels (oxbow wetlands), while Punjab and Haryana face falling water tables from over-extraction.
Worked example
If high tide is at 06:00 today, tomorrow's equivalent tide is about 06:50, because the lunar day exceeds the solar day by 50 minutes.
Traps
- Reading "spring tide" as a seasonal tide; it is a monthly alignment tide.
- Reversing the effect of cold currents on adjacent coasts.
- Writing that tides are caused by winds.