Surface Currents

Question 1

Types of ocean circulation

Answer 1

Surface ocean:
- currents driven by winds
- water above pycnocline, upper 1000 meters

Deep ocean:
- currents driven by density differences (from changes in T & s)
- water at or below pycnocline

Question 2

Coriolis force

Answer 2

• to the right in norhtern hemisphere
• to the left in southern hemisphere

Question 3

Understanding wind-driven gyres

Answer 3

• gyre = a large system of circular ocean currents
• driven by global winds (westerlies & trade winds) + earth’s rotation

Question 4

Voyage of the Fram (1893 - 1896)

Answer 4

Led by Norwegian Fridtjof Nansen, intending to reach North Pole

Question 5

Ekman Explained Nansen’s Observations

Answer 5

Showed mathematically why:
- ocean velocity spirals with depth
- surface current & icebergs move 45 degrees to the right of the wind
- net surface transport is 90 degrees to the right of the wind

Question 6

Global scale of westerlies and trade winds

Answer 6

What direction is ocean transport in response to those winds? southeast

Question 7

Ekman transport

Answer 7

• convergence, pile up water in gyre, creating a bulge in the sea surface
• water wants to flow “downhill”, but it’s deflected by Coriolis
• Coriolis deflects the downhill flow so water flows perpendicular to the slope (pressure gradient)
• causes accumulation of plastic debris in the ocean

Question 8

geostrophic flow

Answer 8

balance between pressure gradient force and coriolis force

Question 9

Ocean gyre ciruclarion

Answer 9

• driven by winds
• deflected by coriolis
• occurs in all major ocean basins

Question 10

Western intensification of currents in a gyre

Answer 10

at high lats:
- coriolis is stronger
- turns flow towards the equator sooner, leading to currents on the eastern side of the gyre that are spread out and weak

at low lats:
- coriolis force is weaker
- water doesn’t turn rowards the poles until it hits the continent, leading to a very strong current on the western side of the gyre

speed of current proportional to slope of hill

Question 11

Boundary currents

Question 12

Western boundary currents

Answer 12

• in both northern and southern hemisphere
• warm water + flow toward the poles = poleward heat-transport mechanism

Question 13

global sea surface temp

Answer 13

temp of current reflects region of origin

Question 14

Effects of currents on regional climate

Answer 14

Western coundary curretns
- warm currents from equator -> warm, humid air -> humid climate on land
Eastern boundary currents:
- cold currents from poles -> cool, dry air -> dry climate on land

Question 15

Case study: north atlantic gyre

Question 16

Gulf stream eddies

Answer 16

Since the gulf stream is a strong, fast current, it creates lots of eddies
two types of eddies:
- warm core eddies to the north of the gulf stream
- cold core eddies to the south of the gulf stream

Question 17

Eddies in the ocean

Answer 17

Strong/fast western boundary currents, like the gulf stream, generate _____

Question 18

Case study: north atlantic gyre
eastern boundary current: the canary current

Answer 18

• wide and slow moving current
• bring cold water south
• creates dry clear weather = good vacationing all year long

Question 19

Take home messages

Answer 19

• ocean gyres have circular circulation patterns due to a combined effect of wind forcing and coriolis effect
• currents circulate roughly in same directon as winds, but are not simply going downward

Question 20

What happens when the wind blows near a coast?

Answer 20

Upwelling and downwelling
- along-shore wind can either
- push surface water awa from the coast, which upwells deeper water from below
- push water towards the coast, downwelling

Question 21

Upwelling

Answer 21

• brings nutrients to the surface where they can be consumed by phytoplankton, which feed the entire food web
• regions of upwelling tend to have productive fisheries

Question 22

Biological productivity enhances in upwelling regions

Answer 22

Upwelling of nutrient rich water (nitrate, phosphate) into euphotic (light available) zone where phytoplankton photosynthesis can occur

Question 23

Vertical velocities

Answer 23

Small but mighty
Horizontal velocities are much bigger
Vertical startification&raquo_space; horizontal statification
So a small vertical velocuty can have a big imoact on transport & structure of ocean preoperties (temp, nutrients, etc)

Question 24

Measuring ocean currents

Answer 24

• Incidental drifters: debris tracked following earthquake
• drift current meter afloat in ocean (gps, radio reciever) for surface currents
• a propellor-type flow meter, see how fast water spins based on flow
• ADCP = acoustic doppler current profilers (Doppler flow meter) - sound signals, accoustic beams bounce off particles in water, stretched = lower frequency
• -ARGO floats: profile from surface to deep ocean, sink or rise, collect temp/salinity. Park at some depth and drift for 9 days, get avg ocean current in deep layer
• Indirect: inferring surface velocities from sea surface height

Question 25

Geostrophic balance

Answer 25

coriolis balancing pressure gradient, flow goes perpendicular to pressure gradient
If surface velocity is in geostrophic balance, then flow goes along contours of constant height

Question 26

Indirect: inferring sub-surface velocty from density structure

Answer 26

If velocty is in geostrophic balance, the sub-surface velcoty can be predicted from the density field