Lecture 9: The hydrosphere and cryosphere

Lecture 9: The hydrosphere and cryosphere

1. Global thermohaline circulation

   • Driven by wind stress and tidal mixing which exploit the potential energy of density contrasts (caused by variation in temperature and salinity)
   • A) As warm water in the North Atlantic Conveyor travels N from the USA towards the UK, water evaporates, causing salinity to increase and temperature to decrease.
     • The Gulf Stream, which is a warm current within the North Atlantic Conveyor, carries water from the east coast of the USA to Canada.
   • B) When this high-density, cooler water sinks, it forms North Atlantic Deep Water.
   • C) This deep water then travels S.
   • D) When deep water eventually upwells, it carries lots of nutrients to the sea surface, promoting productivity.
     • E.g. at sites of deep overturning.

2. Bottom water production

   • Sea ice has very low salinity
     • Therefore, when sea ice forms, high-salinity, high-density 'brine' forms around it.
       • This 'brine' sinks below NADW, as it is more dense, forming Antarctic Bottom Water (AABW).
       • This process occurs more in the S pole.

3. The hydrological cycle

   • An aquifer is a rock system containing freshwater.
   • Evaporation is a form of distillation.

4. Global precipitation distribution

   • Low precipitation occurs mostly between 15-30 degrees N and S.
     • E.g. the Sahara Desert, north Africa.
     • Caused by high pressure air, which carries water vapour away from the equator.

5. The flood hydrograph

   • Aka. storm hydrograph
   • Records the volume of precipitation, and the runoff and discharge of a river following a rainfall event.
   • A measure gage can be used to measure the height of a river system following a rainfall event to estimate the volume of water added in the form of runoff.
   • Shape varies depending on...
     • Relief
     • Rock type
     • Soil type
     • Type of natural vegetation
     • Land use
     • Drainage density

6. The cryosphere

   • Ice shelves - freezing of water on land.
   • Sea ice - freezing of water on oceans.
   • Permafrost - freezing of water in rock pores.

7. Ice sheets

   • Are the largest component of the cryosphere.

8. Mass balance of ice sheets

   • States that the mass of an ice sheet or glacier is a function of its inputs and outputs.

9. Cryospheric control of global sea level

   • Via the glacio-eustatic mechanism
     • Which controls the amount of water stored in glaciers and ice sheets.
   • For instance, during the Last Glaciation, eustatic sea level was much lower because more water was stored as ice.
     • The great weight of the ice caused isostatic subsidence, which meant that when the ice eventually melted, the crust rebounded, causing land to rise once again.

10. Glaciers

    • Inputs
      • Accumulation of snow
      • Influenced by albedo
    • Outputs
      • Ablation (when snow/ice is lost from the bottom of glaciers)
      • Controlled by atmospheric and ocean temperatures
        • E.g. Ice sheets melt in West Antarctica due to ocean heat.
    • Light is reflected from fresh ice/snow, preserving cold temperatures.
    • Glacial collapse is the main cause of glacier retreat.
      • When a large block of ice collapses, it sends an internal wave through the remaining glacier, injecting warm water into it and causing more warming.
      • It causes a cascade effect.