8. Ekmann Upwelling etc Flashcards
Wind-driven circulation happens in the upper __-__m of the ocean
500-1000m
Winds generate movement in the ocean in 2 ways:
- by transferring its energy to the surface water through friction
- by blowing/ pushing the surface waters in a given direction, creating a slope on the sea sfc –> pressure gradient
wind energy transfer (via ____) generates currents and waves
friction
Generally, about __-__% of the wind energy is converted to ocean kinetic energy
For example, 60km/hr winds give __-__km/hr ocean currents
1-3%
1-2km/hr
T/F
Ocean speed generally decreases with depth
true
b/c as subsequent layers interact, only a fraction of the kinetic energy is transferred down
____ observed that the surface layer of the ocean is deflected __-__ degrees to the right of the wind
____ developed the circulation model to explain this observation
Nansen
20-40
Ekman
Ekman spiral states:
The surface current will deflect 45 degrees
(to the right in NH and left in SH).
The deflection increases as you go deeper (at a certain depth, current can go opposite direction of the wind)
In practice, does the surface current deflect exactly 45 degrees via Ekman?
No, it’s usually less than 45 degrees
What’s the total transport of water when you average all the currents (at different depths)?
The total transport of water is 90 degrees
(to the right in NH and left in SH)
Give 3 conditions that must hold true for the Ekman motion to be set up
- the water column must be nearly uniform in density over the spiral
- the winds must blow at a constant speed for at least 1-2 days
- if the water is too shallow, friction with the bottom stops the establishment of the ekman spiral
T/F
Interactions with the bottom (friction) set up another ekman spiral at the bottom of the oceans
true!
Explain how winds can drive vertical movement of water in the ocean
- When winds blow away from an area, they cause divergence, which lets water UPWELL from below (cold water from below the pycnocline)
- When winds blow towards an area (eg landmass), = convergence of ocean currents= downwelling
Explain how ekman can cause upwelling
If the offshore winds are parallel to the shore, the resulting water movement is either outward or inwards, depending on wind direction (deflection to right in NH). Water then upwells or downwells to fill that space
Give 3 causes of upwelling/ downwelling
- offshore winds
- seafloor obstructions
- sharp bends in coastlines
t/f
downwelling areas have high productivity
false
low
Why might the mean flow in deeper layers oppose the Ekman transport?
Pressure gradient transfer!
Flows are accelerated and deflected until the pressure gradient is balanced by the ________ ______. This is called a _______ _______
Coriolis force
Geostrophic current
Geostrophic currents are __ degrees to the ___of the pressure gradient in the northern hemisphere
90
right
Ideally, geostrophic currents flow parallel to the ___
isobars
In the open ocean, surface currents are initiated by ___ ___, and then maintained as ______ _______
Ekman transport
Geostrophic currents
The larger the pressure gradient, the ___ the geostrophic current
larger
What is the typical ocean unit of volume transport?
Equivalent to:
Sverdrup (Sv)
1Sv= 10^6m^3/sec
Ekman transport is based on the large-scale ____ circulation
This leads to a band of convergence at __ degrees and divergence at __ degrees. What does this create?
atmospheric
convergence= 30 degrees
divergence= 60 degrees
Large-scale geostrophic currents that flow around the earth
Gyre=
closed circulation loop within the ocean
Only 1 current can flow all the way around the world, unobstructed by land:
Antarctic Circumpolar Current
Do gyres spin at a constant speed?
Nearly constant, yes
It reflects the average wind input to the gyre
Doesn’t stop
Subtropical gyres have a ____(lowered/raised) sea surface height in the middle. Why?
raised
Coriolis deflection of boundary currents towards the center + ekman transport to right (NH)= subtropical convergence of water in the middle of the gyre
- balanced by PGF pointed outwards
Do subpolar gyres have a raised or depressed sea surface height in the middle?
depressed
Sea surface height maximum appears shifted to the ___(east/west) side of subtropical gyres
west
