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 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 Sigma t Sigma S T p S T 0 1000 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