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. 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:  = +1 kg m -3 for  T = -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  (S,T,p) Sigma-t  (S,T,0) – 1000 Sigma-   (S,q,0) – 1000 Rules of thumb   = +1 kg m-3   T = -5C,  S = 1 psu or  p = 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  ’s in  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  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 (  w)  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 =  g D Links water properties (  ) to pressure Given  (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) tan  where f = Coriolis parameter (= 2  sin  ) 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,  D(0/1500db)

Barotropic Conditions Current velocity is NOT a function of depth  u ≠ f(x) Holds for  = constant or when isobars & isopycnals coincide Isobar depths are parallel to sea surface tan  = 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  above an isobaric surface will drive changes in D (=  z) Changes in D (over distance  x)  tan  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 “  “ (angular momentum in rotating frame ):  =  v/  x -  u/  y 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  sin   2 north pole; 0 on equator; - 2 south pole Total vorticity: Only the total vorticity (f +  ) is significant For flat bottom ocean  uniform  & no friction  total vorticity is conserved Water transported north will decrease its  to compensate for changes in f Water advected south will increase its  Potential vorticity: (f +  ) / D PV is conserved except for friction If f increases, a water spin slower (reduce  ) 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 +  )/D Assume depth D is constant (barotropic ocean) Friction can alter (f +  ) In the absence of friction: Southward parcels gain  to compensate reduction in f Northward parcels lose  to compensate increase in f In an asymmetric gyre: Southward: wind stress input of -  is balanced +  inputs by D’s in latitude & sidewall friction Northward: D’s in latitude result in an input of -  along with the wind stress input of - 

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) Office hours: Wed. 2:00 – 2:50, Thurs. 2:00 – 2:50 Office: 4832 Ellison Good Luck!