Transcript Slide 1

Midterm Review
Geography 163 Spring 2010 Midterm Study Guide
The following is a list of some concepts we have covered so far this
quarter. Keep in mind that this list is not everything we’ve covered and
some may or may not be on the midterm exam. If you’ve been doing the
assigned readings, have attended lecture, and have put effort into doing
the homework you should do well. I suggest going over your notes and
the lecture notes posted online.
http://www.icess.ucsb.edu/~davey/Geog163/
The Midterm is Tuesday 05/11/2010
BRING A CALCULATOR!!!
Sea Water Properties
Pure water (96.5%);
Dissolved salts, gases, organic substances, and particles (3.5% );
Physical properties are mainly determined by pure water.
Hydrogen Bonding:
• Ice crystals are less dense than liquid water;
• Maximum density is water at 4°C.
As lakes cool they reach temperature of maximum density (4°C) & overturn;
Later ice forms at the surface, sheltering the interior from winter conditions;
This allows fish over winter under the ice.
Fundamental seawater properties:
• Salinity, temperature & pressure.
Density is the important variable.
Sea Water Properties
Salinity :
• [mass “salts”]/[mass seawater]
• The “salts” (Cl-, SO4-2, Na+, K+, etc.) are in approximate constant proportion
• Law of salinity (residence time is huge)
• Measure one ion [Cl-] - estimate salinity
• Units are “practical salinity units” (psu)
Temperature:
• Generally decreases with depth in the ocean
• Except where ice is formed, temperature changes primarily regulate density
• Rule of thumb: Dr = +1 kg m-3 for DT = -5 C
Pressure:
• weight of sea water lying above a depth (hydrostatic)
• Pressure varies from 0 to >5000 db
• p = 0 is atmospheric pressure
• Note: 1 db pressure ~ 1 m depth
Features:
• Mixed layer
• Thermocline
• Halocline
Density (the key property)
Changes in vertical - inhibit mixing
Changes in horizontal - drive currents
Controled by:
• temperature
• salinity (dissolved salt content)
• pressure (related to depth)
in situ density
Sigma-t
Sigma-q
r(S,T,p)
r(S,T,0) – 1000
r(S,q,0) – 1000
Rules of thumb  Dr = +1 kg m-3  DT = -5C, DS = 1 psu or Dp = 100 db
Mixing & Turbulence
Mixing leads to a homogenization of water mass properties
Mixing occurs on all scales in ocean
• molecular scales (10’s of mm)
• basin scales (1000’s of km)
Turbulence interactions cascade energy from big to small scales
10 cm eddies
• Small-scale turbulence
• Shear-driven
200 km eddies
• Mesoscale
• Geostrophic
Buoyancy
Dense water sinks - light water floats
Density profile will increase with depth
Upward force due to D’s in r is called the buoyancy force
Buoyancy restricts vertical mixing of water masses
Buoyancy is important to vertical mixing:
• Asymmetric mixing in ocean interior
• Convection
Waters of same r mix easily, waters of different don’t (oil & vinegar)
Potential energy differences must be overcome by mechanical energy inputs
Mixing along isopycnal surfaces will be >>> than mixing across them
Convection:
• Air-sea cooling & evaporation creates cool & saline surface waters
• These waters are then denser than those just beneath them and they sink
• Annual & diurnal time scales
Convection & the Conveyor Belt
• NADW production drives the conveyor
The Atmosphere
Wind Field: Drives upper layer flows of the major gyres
Net Heat & Freshwater Exchanges: Drives buoyancy flows (like the conveyor belt)
Convergence of trades leads to ITCZ:
• Ascending moist air at equator
• Drying & subsidence high pressure over the subtropical ocean
Location of ITCZ shifts seasonally
• Driven in large degree by greater seasonal heating on the land
Winds blow from high to low pressure
Earth’s rotation  apparent force called the Coriolis force  turns the winds to the
right (left) in the northern (southern) hemisphere.
Mid-latitude storms do most of the atmospheric heat transport
Cyclones: low pressure & CCW (NH) rotation
Anticyclones: high pressure & CW rotation
Ekman Transport
Wind stress (tw)  input of momentum into the ocean by the wind
•tw is a tangential force per unit area (N m-2 = kg m-1 s-2)
Fridtjof Nansen (Pioneer in oceanography)
•Nansen built the ship “Fram” to reach North Pole;
•Lock ship in the ice & wait  set out to NP;
•Nansen noticed that movement of the ice-locked ship was 20-40° to right of
the wind
•Nansen figured this was due to a steady balance of friction, wind stress &
Coriolis forces
•Ekman did the math
Ekman Transport
A ocean layer is accelerated by the one above it & slowed by the one beneath it
Top layer is driven by tw  Transport of momentum into interior is inefficient
Top layer balance of tw, friction & Coriolis
Layer 2 dragged forward by layer 1 & behind by layer 3
Depth of frictional influence defines the Ekman layer
Typically 20 to 80 m thick
Boundary layer process
•Typical 1% of ocean depth (a 50 m Ekman layer over a 5000 m ocean)
Ekman transport describes the direct wind-driven circulation
Only need to know tw & f (latitude)
Ekman current will be right (left) of wind in the northern (southern) hemisphere
Simple & robust diagnostic calculation
Inertia Currents
Ekman dynamics are for steady-state conditions
if the wind stops  Coriolis will be the only force
Inertial motions will rotate CW in NH & CCW in the SH
Important in open ocean as source of shear at base of mixed layer
•A major driver of upper ocean mixing
•Dominant current in the upper ocean
Pressure
Hydrostatic pressure  the weight of water acting on a unit area at depth
Total pressure = hydrostatic & atmospheric (pt = ph + pa)
Hydrostatic pressure:
•ph = r g D
•Links water properties (r) to pressure
•Given r(z), we can calculate ph
•Rule of thumb: 1 db pressure ~ 1 m depth
Horizontal Pressure Gradients
Pressure changes provide the push that drive ocean currents ;
Geostrophy:
• balance between horizontal pressure & Coriolis forces
• Relationship is used to diagnose currents
1. u = (g/f) tanq; where f = Coriolis parameter (= 2W sinf)
• Holds for most large scale motions in sea
Need to slope of sea surface to get at surface currents
Satellite Altimeters:
• measures distance between satellite and ocean surface;
• sea surface height (SSH)  SSHelli = SSHcirc + SSHtides + Geoid
• Satellite altimeters can estimate the slope of the sea surface
• Only surface currents are determined
Dynamic Height
• Dynamic height anomaly, DD(0/1500db)
Barotropic Conditions
Current velocity is NOT a function of depth  u ≠ f(x)
Holds for r = constant or when isobars & isopycnals coincide
• Isobar depths are parallel to sea surface
• tanq = constant WRT depth
• changes will be small
Baroclinic Conditions
Isobars & isopycnals can diverge
Density can vary enabling current velocity to vary  u = f(x)
Baroclinic flow:
Density differences drive HPF’s -> u(z)
Changes in the mean r above an isobaric surface will drive changes in D (=Dz)
Changes in D (over distance Dx)  tanq to predict currents
Density can be used to map currents following the Geostrophic Method
Flow is along isopycnal surfaces not across (“Light on the right”)
Current velocity decreases with depth
Baroclinic vs. Barotropic
Divergence and Convergence
Divergence leads to upwelling;
Convergence leads to downwelling;
Ekman pumping:
Convergence of surface Ekman transports piles the water up
Geostrophy pushes it around the circle  anticyclonic circulation;
Little water is moved by Ekman transport (boundary layer)
Downwelling in gyre interior
•displacing thermocline & lowering density;
•lowers nutrient availability & algal biomass;
Between trades & westerlies
• Convergence of Ekman transports
• Downwelling
• Subtropical gyres
Between westerlies & easterlies
• Divergence & Upwelling
• Subarctic gyres
The Gulf Stream
Gulf Stream is a western boundary current
Important contributor to poleward heat transport & the climate of Europe
Also important as a trade route & for animal migrations
Western Boundary Current (WBC):
• WBC’s are found in all subtropical gyres
• Gulf Stream, Brazil Current, Kurishio
• Creates asymmetric gyres
• WBC’s have need to “rub up” to continent
Vorticity
Measure of angular momentum for a fluid (Tendency of a parcel to rotate)
Important for understanding western boundary currents
Relative vorticity “ z “ (angular momentum in rotating frame ):
z = Dv/Dx - Du/Dy
Planetary vorticity “ f ” (rotation of the frame):
• The planet also rotates about its axis
• Objects are affected by both planetary & relative vorticity components
• f = 2 W sin f  2 W @ north pole; 0 on equator; - 2 W @ south pole
Total vorticity:
• Only the total vorticity (f + z) is significant
• For flat bottom ocean  uniform r & no friction  total vorticity is conserved
• Water transported north will decrease its z to compensate for changes in f
• Water advected south will increase its z
• Potential vorticity: (f + z) / D
• PV is conserved except for friction
• If f increases, a water spin slower (reduce z) or increase its thickness D
• Typically, PV is approximated as f/D (z << f)
Western Intensification
Subtropical gyres are asymmetric & have intense WBC’s
Western intensification is created by the conservation of angular momentum in gyre
Friction driven boundary current is formed along the western sidewall
Maintains the total vorticity of a circulating water parcel
Stommel’s experiments
• Includes rotation and horizontal friction
Conservation of potential vorticity (f + z)/D
Assume depth D is constant (barotropic ocean)
Friction can alter (f + z)
In the absence of friction:
• Southward parcels gain z to compensate reduction in f
• Northward parcels lose z to compensate increase in f
In an asymmetric gyre:
• Southward: wind stress input of -z is balanced +z inputs by D’s in latitude &
sidewall friction
• Northward: D’s in latitude result in an input of -z along with the wind stress
input of -z
Coastal Upwelling
Equatorward winds along a coastline lead to offshore Ekman transport;
Mass conservation requires these waters replaced by cold, denser waters;
Brings nutrients into surface waters creating blooms;
• Euphotic zone:
• Defined as the depth where the light = 1% of the surface value
• A function of plant biomass or chlorophyll concentration
• Varies from 10 to 130 m
Creates dynamic height gradients – currents
Geog 163 – Ocean Circulation
TA: Rodrigo Bombardi (Rod)
[email protected]
Office hours:
Wed. 2:00 – 2:50, Thurs. 2:00 – 2:50
Office: 4832 Ellison
Good Luck!