7.1 How are ocean currents measured?

  • ocean currents are either wind driven OR density driven

  • moving air masses, particularly major wind belts of the world, set wind-driven currents in motion

    • move water horizontally and occur primarily in ocean’s surface waters = surface currents

  • density driven circulation: moves water vertically and accounts for thorough mixing of deep masses of ocean water

    • some surface waters become high in density through low temp and/or high salinity, and sink beneath surface = deep currents

Surface Current Measurement

  • surface currents are measured by wind —> rarely flow in the same direction and @ same rate for very long —> measuring avg flow rates can be difficult

  • SOME consistency exists in the overall surface current pattern worldwide

Direct Methods

  1. floating device released into the current and its position is tracked through time. typically, radio-transmitting float bottles or other devices are used but other accidentally released items also make good drift meters.

  2. current-measuring device i.e. propeller flow meter, which is deployed from a fixed position, such as a pier or a stationary ship. a propeller device can also be towed behind a ship, w/ ship’s speed subtracted to determine current’s true flow rate

Indirect Methods

  1. pressure gradients: slopes caused by large-scale bulges and depressions in the ocean’s surface. water flows parallel to a pressure gradient (downhill) —> method determines internal distribution of density and corresponding pressure gradient across an area of the ocean

  2. radar altimeters: determine lumps and bulges at ocean surface, which are a result of the shape of the underlying sea floor as well as current flow. from this data, dynamic topography maps can be produced that show the speed and direction of surface currents.

  3. doppler flow meter: transmits low-frequency sound signals through the water. flow meter remains stationary and measures shift in frequency between sound waves emitted and those backscattered by particles in the water to determine current movement.

Deep Current Measurement

  • great depth at which deep currents exist make them harder to measure than surface currents

  • typically mapped using underwater floats that are carried w/in deep currents

  • Argo: global array of free-drfiting profiling floats that move vertically and measure temp, salinity, and other water characteristics of the upper 2000m of ocean

    • once deployed, each float sinks to a particular depth, drifts for up to 10 days collecting data, then resurfaces and transmits data on its location and ocean variables, which are made publicly available within hours. each float then sinks back down to a programmed depth and drifts for another 10 days, collecting more data before resurfacing and repeating the cycle.

    • program allows oceanographers to develop a forecasting system for the oceans analogous to weather forecasting on land, and also to track changes in ocean properties as a result of human-caused climate change.

  • identifiying distinctive temp and salinity characteristics of a deep-water mass and tracking telltale chemical tracers. some tracers are naturally present in seawater, while some are intentionally added. some useful tracers that have been inadvertently added to seawater include tritium and chlorofluorocarbons

RECAP

  1. compare the forces that are directly responsible for creating horizontal and deep vertical circulation in the oceans. what is the ultimate source of energy that drives both circulation systems?

horizontal (surface) = wind-driven

vertical (deep) = density-driven

both = the sun - the sun

  1. Describe the different ways in which currents are measured.

Surface:

direct: (1) floating device is released into current and its position is tracked over time, typically a radio-transmitting float bottle is used, (2) current-measuring device i.e. propeller flow meter that is deployed from a fixed position i.e. pier or stationary ship, can also be towed behind a ship

indirect: (1) pressure gradients - water flows parallel to pressure gradient, so method determines internal distribution of density and corresponding pressure gradient across an area of the ocean, (2) radar altimeters - determines lumps and bulges at ocean surface which are a result of underlying sea floor and currents, (3) doppler flow meter - transmits low-frequency sound signals through the water. flow meter remains stationary and measures the shift in frequency between the sound waves emitted and those backscattered by particles in the water to determine current movement.

Deep:

(1) Argo: global array of free-drifting profiling floats that move vertically and measure temp, salinity, and other water characteristics of upper 2000m

(2) identifying distinctive temp and salinity characteristics of a deep-water mass and tracking telltale chemical tracers.