Ocean Currents

Why is Ocean Circulation Important? Transport ~ 20% of latitudinal heat –Equator to poles Transport nutrients and organisms Influences weather and climate Influences commerce

Convection cell model Non-rotating Earth

Add rotation and add landmasses unequal heating and cooling of the Earth

Physical properties of the atmosphere: Density Warm, low density air rises Cool, high density air sinks Creates circular- moving loop of air (convection cell)

Physical properties of the atmosphere: Water vapor Cool air cannot hold much water vapor, so is typically dry Warm air can hold more water vapor, so is typically moist Water vapor decreases the density of air

Physical properties of the atmosphere: Pressure

ITCZ intertropical convergence zone= doldrums Low pressure, wet climate High pressure, dry climate Low pressure, wet climate 30 o 60 o 90 o 0 o High pressure, dry climate

The Coriolis effect –Is a result of Earth’s rotation –Causes moving objects to follow curved paths: In Northern Hemisphere, curvature is to right In Southern Hemisphere, curvature is to left –Changes with latitude: No Coriolis effect at Equator Maximum Coriolis effect at poles

The Coriolis effect on Earth As Earth rotates, different latitudes travel at different speeds The change in speed with latitude causes the Coriolis effect

equator Quito Buffalo 79 o W North Pole South Pole N Quito Buffalo equator Buffalo moves 783 mph Quito moves 1036 mph 15 o

A)Idealized winds generated by pressure gradient and Coriolis Force. B)Actual wind patterns owing to land mass distribution..

Surface Currents The upper 400 meters of the ocean (10%). Deep Water Currents Thermal currents (90%) Ocean Currents

Surface Currents Forces 1.Solar Heating (temp, density) 2.Winds 3.Coriolis

Wind-driven surface currents

Wind-Driven and Density-Driven Currents Wind-driven currents occur in the uppermost 100 m or less Density differences causes by salinity and temperature produce very slow flows in deeper waters.

Sailors have know about ocean currents for centuries Sailors have know that “rivers” flow in the seas since ancient times. They used them to shorten voyages, or were delayed by trying to stem them. If navigators do not correct to deflection by currents, they may be far away from where they think they are and meet disaster.

Ben Franklin and the Gulf Stream

Matthew Fontaine Maury The first systematic study of currents was done by Maury based on logbooks in the US Navy’s Depot of Charts and Instruments. His charts and “Physical Geography of the Sea” assisted navigators worldwide.

Winds and surface water Wind blowing over the ocean can move it due to frictional drag. Waves create necessary roughness for wind to couple with water. One “rule of thumb” holds that wind blowing for 12 hrs at 100 cm per sec will produce a 2 cm per sec current (about 2% of the wind speed)

Top-down drag Wind acts only on the surface water layer. This layer will also drag the underlying water, but with less force. Consequently, there is a diminution of speed downward. Direction of movement is also influenced by the Coriolis Effect and Ekman Spiral

Ekman spiral Ekman spiral describes the speed and direction of flow of surface waters at various depths Factors: –Wind –Coriolis effect

Ekman transport Ekman transport is the overall water movement due to Ekman spiral Ideal transport is 90º from the wind Transport direction depends on the hemisphere

Ekman Transport Water flow in the Northern hemisphere- 90 o to the right of the wind direction Depth is important

Currents in the “Real” Ocean Currents rarely behave exactly as predicted by these theoretical explanations due to factors such as Depth—shallow water does not permit full development of the Ekman spiral Density—deeper currents moving in different directions influence the overlying surface movement

Geostrophic Flow Surface currents generally mirror average planetary atmospheric circulation patterns

Gyres are large circular-moving loops of water Five main gyres (one in each ocean basin): North Pacific South Pacific North Atlantic South Atlantic Indian Generally 4 currents in each gyre Centered about 30 o north or south latitude Current Gyres

Geostrophic flow and western intensification Geostrophic flow causes a hill to form in subtropical gyres The center of the gyre is shifted to the west because of Earth’s rotation Western boundary currents are intensified Figure 7-7

Western intensification of subtropical gyres The western boundary currents of all subtropical gyres are: –Fast –Narrow –Deep Western boundary currents are also warm Eastern boundary currents of subtropical gyres have opposite characteristics

Boundary Currents in the Northern Hemisphere Type of Current General Features Speed Special Features Western boundary Currentswarmswift sharp boundary Gulf Stream, Kuroshionarrow w/coastal circulation, deep little coastal upwelling Eastern Boundary Currentscoldslow diffuse boundaries California, Canarybroad separating from coastal shallow currents, coastal upwelling common

Pacific Ocean surface currents

“Hills and Valleys” in the Ocean A balance between the Ekman transport and Coriolis effect produces “hills” in the center of the gyres and “valleys” elsewhere Gravitational effects from sea floor features also produce variations in sea surface topography

What do Nike shoes, rubber ducks, and hockey gloves have to do with currents?

Lost at Sea

January shipwrecked in the Pacific Ocean, off the coast of China November half had drifted north to the Bering Sea and Alaska; the other half went south to Indonesia and Australia 1995 to spent five years in the Arctic ice floes, slowly working their way through the glaciers the duckies bobbed over the place where the Titanic had sunk they were predicted to begin washing up onshore in New England, but only one was spotted in Maine a couple duckies and frogs were found on the beaches of Scotland and southwest England. Duckie Progress

Barber’s Point

“Great Pacific Garbage Patch” Estimate: 46,000 pieces of floating garbage/mi 2. North Pacific Subtropical Gyre

135° to 155°W and 35° to 42°N

North Pacific Subtropical Gyre Great Pacific Garbage Patch- Good Morning America

Eddy Warm core ring 1.Rotates clockwise 2.Found on the landward side of the current Cold core ring (cyclonic eddy) 1.Rotates counterclockwise 2.Forms on the ocean side of the current A circular movement of water formed along the edge of a permanent current In an average year, rings are formed km in diameter Speed 1 m/sec

Sargasso Sea

Upwelling and downwelling Vertical movement of water (  ) –Upwelling = movement of deep water to surface Hoists cold, nutrient-rich water to surface Produces high productivities and abundant marine life –Downwelling = movement of surface water down Moves warm, nutrient-depleted surface water down Not associated with high productivities or abundant marine life

upwelling downwelling

Langmuir Circulation

Satellite Observations TOPEX/Poseidon, Jason 1, and other satellites have observed patterns of change over the past few years Animation of seasonal and climatically- influence shifts available at /Topex_Dynamic_Ocean_Topography.mpg

El Niño-Southern Oscillation (ENSO) El Niño = warm surface current in equatorial eastern Pacific that occurs periodically around Christmastime Southern Oscillation = change in atmospheric pressure over Pacific Ocean accompanying El Niño ENSO describes a combined oceanic- atmospheric disturbance

El Niño Oceanic and atmospheric phenomenon in the Pacific Ocean Occurs during December 2 to 7 year cycle Sea Surface Temperature Atmospheric Winds Upwelling

Normal conditions in the Pacific Ocean

El Niño conditions (ENSO warm phase)

La Niña conditions (ENSO cool phase; opposite of El Niño)

El Ni ñ oNon El Ni ñ o 1997

Non El Niño El Niño thermocline upwelling

El Niño events over the last 55 years El Niño warmings (red) and La Niña coolings (blue) since Source: NOAA Climate Diagnostics CenterNOAA Climate Diagnostics Center

mations/26_NinoNina.html El Nino Animation World Wide Effects of El Niño Weather patterns Marine Life Economic resources

Effects of severe El Niños

Surface and Deep-Sea Current Interactions Unifying concept: “Global Ocean Conveyor Belt”

Heat Transport by Currents Surface currents play significant roles in transport heat energy from equatorial waters towards the poles May serve as “heat sources” to cooler overlying air, “heat sinks” from warmer Evaporation and condensation participate in latent heat exchanges

Matter Transport and Surface Currents Currents also involved with gas exchanges, especially O 2 and CO 2 Nutrient exchanges important within surface waters (including outflow from continents) and deeper waters (upwelling and downwelling) Pollution dispersal Impact on fisheries and other resources

Global ocean circulation that is driven by differences in the density of the sea water which is controlled by temperature and salinity.

White sections represent warm surface currents. Purple sections represent deep cold currents

What effect does global warming play in thermohaline circulation?

North Atlantic regional cooling Global climate interconnections CO 2 fossil fuel combustion Atmospheric and ocean temp Subtropical evaporation High latitude precipitation & runoff Deep water formation & thermohaline circulation Nordic seas salinity & deep convection Potential feedback of increased tropical salinity

1.What is a convection cell? 2.Which direction do currents get deflected in the Southern Hemisphere? 3.What depth should the water be for an Ekman spiral to occur? 4.How are surface currents created? 5.What is a gyre? 6.How can an El Nino impact upwelling? 7.Coriolis Effect is strongest near the _____? Inquiry