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1 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 differences in density below the surface zone Heated unevenly Evaporated unevenly Ocean Currents Ocean water moves constantly sideways, up, down because it is: Affected by Earth’s rotation (Coriolis) Pulled by sun & moon CURRENTS Currents are important because they moderate climates, mix nutrients and oxygen, and transport larvae and nekton. They are caused by the: 1. Spin of the Earth on its axis and the Coriolis effect 2. Warm air from equator flows toward poles, drives air circulation, makes wind 3. Continents deflect east-west water movement, but not air NASA NOAA 5 Coriolis Effect Causes currents to move Clockwise in the Northern Hemisphere Counterclockwise in the Southern Hemisphere Causes westward moving currents to be intensified (move much quicker) 7 EKMAN TRANSPORT 8 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 characteristics of a current depend upon where the water is coming from in the gyre. In the Northern hemisphere the gyre is (right) clockwise, and in the southern hemisphere the currents are (left) counter clockwise. 12 East Coast of United States Eastern boundary currents are located on the eastern side of the OCEAN - not the continent. Gulf Stream in red Florida Western boundary currents are located on the western side of oceans. 16 Surface Currents 17 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 (from deeper to shallower water) Brings nutrients Vertical Movement Downwelling – downward movement of water – caused by: Brings dissolved gases and nutrients to deeper ocean EL NIÑO Normally, trade winds blow warm surface water toward the western Pacific. When the Trade Winds falter, warm water piles up in the western Pacific about 1 & ½ feet higher than the average ocean surface due to wind and Ekman transport. This causes the thermocline to rise near the surface in the eastern Pacific. 25 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 26 El Niño describes abnormal oceanic and atmospheric conditions. First noticed off the coast of Peru around Christmas time, the condition was called El Niño referring to the Christ child. 28 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. 29 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. 31 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. 32 Comparison of El Niño and La Niña El Niño 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 33 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 34 Thermohaline currents are 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 35 Density Structure of the Ocean Seawater 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 80% of all ocean water Density stratificaiton in the ocean Temperature, Salinity & Stratification Surface zone well mixed, little temperature change with depth Pycnocline temperatures drop rapidly, density increases, temperature decreases Deep zone, little temperature change with depth – a cold, stable zone Below pycnocline/thermocline water very cold 1-3 deg C Average temperature of ocean 3.9 deg C Density Stratification & Water Movement Water mass – body of water with same temperature and salinity Even deepest water originates at surface Evaporation makes dense salty water Dense water mass trapped at depthocean is stratified Density Stratification & Water Movement Dense water masses form: 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 Density Stratification & Vertical Water Movement Vertical movement possible when surface-water and deep-water density similar Water in tropics very stable (no exchange) Water in northern polar ocean very unstable (due to salinity differences) 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 and 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
