Currents & Gyres Notes
Current A river of water flowing in the ocean. 2 Types of Currents Surface Currents wind-driven currents that occur in the top 100m or less Deep Currents density-driven currents that creep along the bottom of the ocean.
Surface Currents Surface currents spread heat from the tropics & cold water from the poles.
Ben Franklin & the Gulf Stream In the 1750s, when Postmaster for the Colonies, Ben Franklin & Capt. Timothy Folger created the 1st map of the Gulf Stream to help speed up delivery of mail to & from Britain.
Matthew Fontaine Maury Did the first systematic study of currents based on logbooks from the US Navy.
Winds and Surface Water Rule: the Coriolis effect is more pronounced the slower an object moves. 1. Wind blows only on the surface water layer (but it moves slower than the wind). 2. This layer of water will drag the underlying water, but w/ less force (even slower). 3. This proceeds lower & lower (each layer is even slower). 4. This is an Ekman Spiral.
What is this Coriolis Effect? Click here to find out.
Ekman Spiral A spiraling pattern of water direction caused b/c slower substances are more greatly influenced by the Coriolis effect.
Ekman Spiral Nansen & other Arctic explorers noticed that ice & surface currents move at an angle to the wind direction. Ekman first explained the mathematics of this phenomena in 1905, & why it decreases with depth to produce the spiral pattern.
The overall movement of a current is 90 to the right or left of the global wind direction. Ekman Transport
Surface Current Formation 2 1 2 5 3 4
Gyres A circular pattern formed by the connection of several surface currents. Northern Hemisphere 2 gyres - clockwise rotation Southern Hemisphere 3 gyres counter-clockwise rotation Gyres are named for the ocean & hemisphere they are found in.
Parts of a Gyre Western Boundary Currents 1. Form on the western sides of oceans. 2. Fast, deep, narrow currents that form eddies. 3. Warm currents b/c they flow from the equator. 5 WBCs: Gulf Stream largest Japan Current (aka Kuroshio c.) Brazil Current Agulhas Current East Australian Current
Parts of a Gyre Eastern Boundary Currents 1. Form on the eastern sides of oceans. 2. Slow, shallow, broad currents with no eddies. 3. Cold currents b/c they flow from the poles. 5 EBCs: Canary Current California Current Peru Current Benguela Current West Australian Current
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? 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 Asia, Australia hit the Pacific Ocean The Americas hit the Atlantic 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 I ntensification water piles up on the coast of North America West East North America 1m Only ~ 1m high, but that s enough to create BI G differences in the currents.
Western I ntensification West 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
West Western Intensification Gulf Stream (Flowing into screen) Narrow and fast East North America Canary Current (Flowing OUT of screen) diffuse, slow 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
Currents going from the warm water currents. Current going from the cold water currents.
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.)
Sargasso Sea Sea of Weeds
Sargasso Sea home to many
North Atlantic Garbage Patch https://www.youtube.com/watch?v=hfluoqdidt Q
Currents Lab N. Pacific N. Equatorial S. Equatorial N. Equatorial Equatorial Counter S. Equatorial Equatorial S. Equatorial West Wind Drift West Wind Drift West Wind Drift
Density-Driven Ocean Circulation Cold, dense water sinks below warmer water. Salty, dense water sinks below less salty water. Just like air, water will move to areas of lesser concentration. There are different density layers found in the ocean.
Water Masses Bodies of water that are fairly uniform in their densities. Identifiable from their temperature, salinity and other characteristics.
Mixed/Surface Layer The top few hundred meters. Here the water is mixed around by wind, waves & surface currents. Has a relatively uniform temp. & salinity throughout.
Intermediate Layer Immediately below the mixed layer. It is more dense than the mixed layer due to temp. and/or salinity.
North Atlantic Deep Water (NADW) From below the intermediate water to the ocean bottom. Very cold water w/ higher salinity than the above layers.
Antarctic Bottom Water (AABW) In contact w/ the seafloor. Extremely cold water. Typically nutrient rich. Can travel around the globe for years.
Great Ocean Conveyor Belt Combination of deep ocean currents & surface currents
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.
Significance of deep currents Form upwellings: 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?
WIND DRIVEN 0m DENSITY DRIVEN sunlight Upwelling Surface Mix Layer Deep Water Layer Downwelling 100-200m 100-200m 4000m
Upwelling brings NUTRIENTS (from DECAYED organic matter) back up to the surface. Upwelling & Downwelling Convergence: water masses collide and sink downwelling Divergence: Surface waters move APART. Deep water rises up to fill the gap upwelling
Density Stratification Thermocline Heat Halocline Salinity Pycnocline Density Pycnoclines can prevent mixing between layers
https://www.youtube.com/watch?v=txdiu 3LJlZ8 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