Ocean Currents

Ocean Currents There are two types of ocean currents: Surface Currents: are wind driven and affect about 10% of the ocean water, but cover the entire ocean surface. Thermohaline Currents: (heat/salt) = density driven currents. They occur deeper in the ocean and affect ~ 90% of the ocean water.

Surface Currents Surface currents are mostly horizontal currents. These currents follow the pattern of the wind e.g. the Westerlies/ Trade winds They occur from the surface down to a few hundred meters ~ 660ft. Generally travel clockwise in the northern hemisphere and counter clockwise in the southern hemisphere.

Significance of surface currents Surface currents cause the general “mixing” of the worlds oceans and waters move nutrients from more productive areas to less productive areas of the oceans. Important role in migration and the movement of many less active (or passive) organisms (plankton, jellyfish).

Deep currents Thermohaline currents are driven by density and temperature changes in water. Thermo- = heat -haline = salt These currents are also known as the “ocean’s conveyor belt” They flow under the surface of the ocean and move significant amounts of ocean water vertically.

“The Great Conveyor Belt” A global "conveyor belt" is set in motion when cold water forms in the Northern Atlantic, sinks, moves south, and circulates around Antarctica, and then moves northward to the Indian, Pacific, and Atlantic basins. It can take a thousand years for water from the North Atlantic to find its way into the North Pacific.

Conveyor Belt

Significance of deep currents Form upwelling: the drawing up of cold water from deeper regions to shallow waters throughout the Ocean Result: all ocean nutrients don’t end up settling on the bottom: surface organisms can benefit from decomposers, cycle nutrients and sustain life. So what is downwelling? 

DENSITY DRIVEN WIND DRIVEN sunlight Upwelling Downwelling 0m Surface Mix Layer Deep Water Layer 4000m 100-200m 0m Upwelling Downwelling WIND DRIVEN sunlight DENSITY DRIVEN

Upwelling & Downwelling Convergence: water masses collide and sink ® downwelling Divergence: Surface waters move APART. Deep water rises up to fill the gap ® upwelling Upwelling brings NUTRIENTS (from DECAYED organic matter) back up to the surface. Sverdrup et al., Introduction to the World’s Oceans, 8th edition, McGraw Hill, Fig. 9.3

Upwellings will occur along the coast to fill void When wind blow from the land toward the ocean, it pushes water away from the coast Upwellings will occur along the coast to fill void NE Tradewinds/ California Coast Upwellings

Density Stratification Thermocline – Heat Halocline – Salinity Pycnocline – Density Pycnoclines can prevent mixing between layers

Scripps Video - Pycnoclines https://www.youtube.com/watch?v=TxdiU3LJlZ8 Scripps Video - Pycnoclines

REVIEW Winds move water a net of 90 degrees to right of the wind (surface 45 degrees to rt) Ekman spiral – top 600ft water moved by wind – in spiral shape as water pulls water Where water converges due to wind = downwellings Where water diverges due to wind = upwellings

REVIEW Upwellings and Downwellings necessary to mix oceans Bring cold, nutrient rich waters back to surface Take warm, nutrient depleted waters down to ocean bottom Currents N and S of equator tend to move in circular patterns called GYRES

Gyres Deflected by the continents and driven by surface winds, they bend and create large current loops called circulation gyres. The centers often collect floating mats of seaweed and garbage. (Sargasso Sea and the N. Pacific gyre garbage patch.)

Gyre Formation CLOCKWISE Warm Cool COUNTER CLOCKWISE

Five major gyres and the Antarctic circumpolar current

North Atlantic Gyre Notice how the winds are helping the currents. This also shows the effects of the Ekman spiral

Sargasso Sea “Sea of Weeds”

Sargasso Sea…home to many

North Pacific Garbage Patch http://www.buzzfeed.com/summeranne/youll-never-litter-again-after-you-watch-this-video

Comparing Currents… Are all currents similar? NO! Eastern or Western side of ocean Warm or Cold Fast or Slow Deep or Shallow Wide or Narrow High latitudes toward Low Latitudes or vice versa

Why are some currents FAST and other currents SLOW Why are some currents FAST and other currents SLOW? What is the difference between Eastern AND Western Boundary Currents?

What happens when you push in on a plastic container of water?

The container moves first, and water moves a fraction of a second later.

Water sloshes up again the left-hand side, creating a pile

As Earth rotates the continents smack into the oceans Earth’s rotation The Americas hit the Atlantic Ocean Asia, Australia hit the Pacific Ocean

A West to East Cross Section of the mound of water Continents crash into the mound of water. The mound is asymmetrical. This leads to Western Intensification – water piles up on the coast of North America West East North America 1m Only ~ 1m high, but that’s enough to create BIG differences in the currents.

Western Intensification East North America Gravity is trying to pull this down and out to flatten the ocean. The flow is constricted on the WEST side and spread out on the EAST

Western Intensification Gulf Stream (Flowing into screen) Narrow and fast East West Canary Current (Flowing OUT of screen) diffuse, slow North America Think about constricting flow out of a garden hose by covering half the opening with your thumb. The constricted flow moves FASTER. WHY? SAME AMOUNT of water forced to move through smaller opening