Midterm ReviewGeography 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 doingthe 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 PropertiesPure 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 PropertiesSalinity :

• [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 mixingChanges 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) – 1000Sigma- (S,q,0) – 1000

Rules of thumb = +1 kg m-3 T = -5C, S = 1 psu or p = 100 db

Mixing & TurbulenceMixing leads to a homogenization of water mass propertiesMixing 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

BuoyancyDense water sinks - light water floatsDensity profile will increase with depthUpward force due to ’s in is called the buoyancy forceBuoyancy 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 inputsMixing 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 AtmosphereWind Field: Drives upper layer flows of the major gyresNet 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 pressureEarth’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 transportCyclones: low pressure & CCW (NH) rotationAnticyclones: high pressure & CW rotation

Ekman TransportWind 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 TransportA ocean layer is accelerated by the one above it & slowed by the one beneath itTop layer is driven by tw Transport of momentum into interior is inefficientTop layer balance of tw, friction & CoriolisLayer 2 dragged forward by layer 1 & behind by layer 3

Depth of frictional influence defines the Ekman layerTypically 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 circulationOnly need to know tw & f (latitude)Ekman current will be right (left) of wind in the northern (southern) hemisphereSimple & robust diagnostic calculation

Inertia CurrentsEkman 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

PressureHydrostatic pressure the weight of water acting on a unit area at depthTotal 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 GradientsPressure 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) tanwhere 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 ConditionsCurrent 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 currentsDensity 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 ConvergenceDivergence leads to upwelling;Convergence leads to downwelling;

Ekman pumping:Convergence of surface Ekman transports piles the water upGeostrophy 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 StreamGulf Stream is a western boundary currentImportant contributor to poleward heat transport & the climate of EuropeAlso 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

VorticityMeasure 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 IntensificationSubtropical gyres are asymmetric & have intense WBC’sWestern intensification is created by the conservation of angular momentum in gyreFriction driven boundary current is formed along the western sidewallMaintains the total vorticity of a circulating water parcel

Stommel’s experiments• Includes rotation and horizontal friction

Conservation of potential vorticity (f + )/DAssume 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 UpwellingEquatorward 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)

bombardi@geog.ucsb.edu

Office hours:

Wed. 2:00 – 2:50, Thurs. 2:00 – 2:50

Office: 4832 Ellison

Good Luck!