Ocean Currents Ocean water circulates in currents caused by wind and by density differences Currents are the flow of water between areas of different surface levels or different densities
Ocean Currents Ocean water moves constantly sideways, up, down because it is: Blown by wind Moved by density differences Heated unevenly Evaporated unevenly Affected by Earth’s rotation (Coriolis) Pulled by sun & moon The Ocean: A Driving Force for Weather and Climate
CURRENTS Currents are important because they: moderate climates (transfer heat) mix nutrients and gases transport larvae and plankton Warm air from equator flows toward poles, drives air circulation, makes wind, moves water Continents deflect east-west water movement, but not air NASA NOAA
Coriolis Effect Water near ocean surface moves to the right of wind direction in Northern Hemisphere Water near ocean surface moves to the left of wind direction in Southern Hemisphere
Coriolis Effect Coriolis Effect Causes currents to move Clockwise in the Northern Hemisphere Counterclockwise in the Southern Hemisphere Coriolis Effect
Surface Currents Affect the uppermost 10% of world ocean Some rapid & riverlike, some slow and diffuse Largest organized into gyres
The circular patterns of surface water currents are called gyres The circular patterns of surface water currents are called gyres. The characteristics of a current depend upon where the water is coming from in the gyre.
Western boundary currents are located on the western side of oceans. Eastern boundary currents are located on the eastern side of the OCEAN - not the continent. East Coast of United States Gulf Stream in red Western boundary currents are located on the western side of oceans. Florida
Comparison of Boundary Currents Western Eastern currents bring warm water from the equator 1. currents bring cold water from the poles 2. high salinity; low oxygen content 2. low salinity; high oxygen content 3. nutrient poor, infrequent upwelling 3. nutrient rich due to upwelling 4. lower biomass 4. high biomass
Surface Currents
Effect of Surface Currents on Climate Surface currents distribute heat worldwide Warm water flows to higher latitudes transfers heat to the air and cools Moves back to low latitudes Absorbs heat Repeats
Vertical Movement Upwelling – upward movement of water Can be caused by wind blowing parallel to shore or offshore (Coastal Upwelling) Brings nutrients
Vertical Movement Downwelling – downward movement of water – caused by: Increase in surface water density Wind blowing parallel to shore or onshore (Coastal Downwelling) Brings dissolved gases and nutrients to deeper ocean
DENSITY DRIVEN CURRENTS AKA THERMOHALINE CURRENTS
Thermohaline currents found on the ocean floor. “Thermo” refers to temperature and “haline” refers to salinity. NASA Water slows down below the pycnocline (area defining water masses of different densities) to 1-2 meters/day. Cold water sinks at the poles and crawls toward the rising warmer water at the equator. NASA
Density Structure of the Ocean Salt water weighs 2-3% more than pure water Cold, salty water is more dense than warm, less salty water Much of the ocean can be divided into three density zones: surface, pycnocline & deep
Density Zones Surface or Mixed Zone Temperature, salinity & density constant due to currents and waves In contact with atmosphere Exposed to sunlight Least dense water 2% all of ocean water ~150 meters deep
Density Zones Pycnocline aka Thermocline aka Halocline Density increases with depth, temperature decreases, salinity increases Isolates surface water from denser layer below 18% of all ocean water
Density Zones Deep Zone Below pycnocline Depth below 1000 m Little change with depth Very cold (1-3C) and stable 80% of all ocean water
Density stratification in the ocean – surface layer does not mix with bottom layer