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

EL NIÑO

NORMAL Normally, trade winds blow warm surface water toward the western Pacific. This causes the thermocline to rise near the surface in the eastern Pacific.

Notice the warm water is in a pool in the western Pacific Ocean. Cold, nutrient-rich water wells up from the thermocline to the surface waters off the coast of Peru - good for fishing. This image shows normal sea surface temperature in the Equatorial Pacific Ocean in January 1997. Notice the warm water is in a pool in the western Pacific Ocean. NOAA

EL NIÑO El Niño describes abnormal oceanic and atmospheric conditions. When the Trade Winds falter, warm water piles up in the western Pacific about 1 & ½ feet higher than the average ocean surface First noticed off the coast of Peru around Christmas time, the condition was called El Niño referring to the Christ child.

El Niño’s weaker trade winds blow warm water eastward, across the equatorial region of the Pacific. NOAA This depresses the thermocline across the Pacific, and stops upwelling.

Because warm water evaporates faster, the air fills with more water vapor and it produces more rain. El Niño occurs primarily in the tropical Pacific Ocean but it affects global weather.

LA NIÑA NOAA When too much westerly wind blows surface water near the Americas, it has an effect on the weather that is often opposite to El Niño, called La Niña.

Comparison of El Niño and La Niña warmer ocean temperatures colder ocean temperatures occurs every 3-7 years frequency unknown wetter than normal summers drier than normal summers weak trade winds strong trade winds decreases hurricanes forming in N. Atlantic

1997 was the warmest year of the century and the strongest El Niño in 40 years. Oddly, five of the century’s warmest years have been in the last decade. NASA

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 (1-3C) and stable 80% of all ocean water

Density stratification in the ocean – surface layer does not mix with bottom layer

Density Stratification & Water Movement Even deepest water originates at surface Dense water forms: near poles (as water freezes) in enclosed areas (where water evaporates)

Density Stratification & Water Movement Water masses below pycnocline do not mix – no energy – too cold

Thermohaline Circulation 90% of ocean water below surface zone gravity driven Dense water sinks, less dense water rises Density is a function of temperature and salinity – movement of water due to density differences called thermohaline circulation Can take hundreds of years to circulate

Water masses form at the surface Densest & deepest form where water becomes very cold and salty (at the poles)

Circulation of the Atlantic Ocean

The Global Heat Connection Because they transfer large quantities of heat, ocean currents affect world weather and climate The transport of tropical water to the polar regions is part of the global conveyor belt for heat transfer

Ocean Flow The slow, steady flow of water distributes dissolved gases and solids mixes nutrients transports juvenile marine organisms among ocean basins