PHYSICAL OCEANOGRAPHY OCP 6050
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This 13 page Class Notes was uploaded by Dr. Estrella Hessel on Friday September 18, 2015. The Class Notes belongs to OCP 6050 at University of Florida taught by Valle-Levinson in Fall. Since its upload, it has received 24 views. For similar materials see /class/206651/ocp-6050-university-of-florida in Physical Oceanography at University of Florida.
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Date Created: 09/18/15
Lecture 3 Properties of Sea Water In this section we will discuss the properties of sea water The properties of sea water include the pressure p temperature T salinity S density 9 sound velocity heat capacity optical characteristics and compressibility All are important to various aspects of physical oceanographers This part of oceanography is very strictly defined Probably more so than any part of the eld you will encounter Many properties are strictly defined by an international body called International Association for the Physical Sciences of the OceanIAPSO Typical ranges of T S p and sound velocity in the ocean will be discussed later Many of the physical chemical studies that have lead to the standard equations were limited to the typical ranges found in the open ocean One should always check when working in unusual environments that the tables are appropriate TEMPERATURE A typical temperature profile in the ocean shows maximum values at the surface that decrease with depth Fig 1 In high latitudes like in the Arctic and around Antarctica the temperature changes little with depth The maximum vertical variations are usually seen in the tropics where strong thermoclines can be identified The thermocline is a region not only one given depth but a range of depths where temperature changes most rapidly How do we measure temperature Methods range from the mercury thermometer to platinum resistance thermometers and thermistors Most conductivitytemperaturedepth CTD systems Fig 2 have a highly accurate platinum resistant sometimes matched with a fast response thermistor sensor Reversing mercury thermometers and digital thermometers are used for ultimate calibration that connects the field instrument to the nationalintemational standards The Celsius temperature T is the difference between the thermodynamic temperature To and the temperature To 27315K Temperature is measured using the International Temperature Scale established in 1990 ITS90 Previous to 1990 temperature measurements were made using the IPTS68 scale With this change the Joint Panel on Oceanographic Tables and Standards recommended that the following conversion be used for oceanographic measurements T68 100024 T90 1 This is an important conversion because the computation of practical salinity PSS78 and density EOS80 requires use of the IPTS68 scale but temperature measurements are made with the ITS90 scale The temperature differences between the two scales ranges from 0002 at 10C to 0010 at 40C As was noted in a 1991 Unesco report the main effect will be on salinity calculations with a lesser effect on density calculations and essentially no effect on other properties A typical crosssection along the ocean from north to south reveals the warmest waters located near the equator at a relatively thin layer Fig 3 Most of the ocean features cold water lt 4 C Again a strong thermocline is usually found in tropical latitudes These thermoclines exhibit strong seasonal variability in temperate latitudes Fig 4 Summer months feature the shallowest thermocline which deepens from autumn cooling and wind forcing increase Late in winter the thermocline is practically nonexistent and begins to develop again in the spring when heating increases and wind forcing tends to decrease These midlatitudes also show great seasonal variability in surface temperature animation of Fig 5 This is most evident over the western part of the oceans as the surface current systems throughout the ocean move warm tropical waters to the north This process continues in winter but the cold air masses that move from land to the ocean cool down the ocean very rapidly PRESSURE The great depths of the ocean impart considerable pressure on the water and the instruments we lower into it Pressure is defined as the force per unit area resulting from the water overlying a specific point The units are Nm2 l Pascal 10395 bar 10 dyne cm39z Pressure is usually measured with a strain gauge transducer When working in shallow waters the wire out can be used to approximate depth but care must be taken to know the angle of the wire in the water Quite often we measure pressure but report it as depth assuming that l m depth equals about 1 decibar or 10quot Pa The total hydrostatic pressure at any location is I P2 pz g dz 2 4 Figure 6 illustrates hydrostatic pressure for different water column Given a water density of 1020 kg m39z a depth increment of l m and letting g 98 m s392 yields a pressure of 102 X 104 Pa Pressure sensors are calibrated by checking the wire out depth to pressure reported on deck and by sending to calibration service who will calibrate it using a dead weight tester An error of 25 dbar causes a 0001 error in the salinity calculation Consider what the errors are in the depth measurement caused by ship roll and other motion of a sensor suspended by a wire sometimes miles long DEPTH Depth is the linear distance in the vertical to a point below the surface Units m We often assume 1 m l decibar 104 Pascal SALINITY A DETERMINED QUANTITY Salinity was originally de ned as quotthe salinity of a sample of sea water represents the total mass of solid material dissolved in a sample of sea water divided by the mass of this sample when all the carbonate has been converted into oxide the bromine and iodine replaced by chlorine and all organic matter completely oxidizedquot Sorensen and Knudsen date With the development of conductivity sensors and the need for more accurate and comparable salinity measurements two new de nitions were developed They are the absolute and practical salinity The absolute salinity is defined as follows quotAbsolute Salinity is de ned as the ratio of the mass of dissolved material in sea water to the mass of sea water In practice this quantity cannot be measured directly and a Practical Salinity is de ned for reporting oceanographic observations Reference Practical salinity is de ned as The Practical Salinity symbol S of a sample of sea water is defined in terms of the ratio K15 electrical conductivity of the sea water sample at the temperature of 15C and the pressure of one standard atmosphere to that of a potassium chloride KCL solution in which the mass fraction of KCl is 324356 x 10393 at the same temperature and pressure The K15 value exactly equal to 1 corresponds by de nition to a Practical Salinity exactly equal to 35 The practical salinity is defined in terms of the ratio of K15 by the following equation 12 32 2 52 S a0 alKls a2K15 a3K15 a4K15 asKls where a0 00080 a1 1692 a2 253851 a3 140941 a4 70261 a5 27080 21ai 35000 This equation is good for 2 lt S lt 42 salinity in full and not salinity However once all r 39 of has J Fr after a suf cient lapse of time or when it becomes quite clear from the text that practical salinity is concerned it will be quite acceptable to speak simply of salinity This quantity has only one symbol S and for considerable time it should be called practical 3 Salinity has become a very simple quantity to determine however the measuring devices CTDs must be frequently checked and calibrated Simple calibrations to check for drift and other offsets should be done daily while calibrations done by professional groups should be done yearly Typical profiles of salinity in the open ocean at di erent latitudes are shown in Figure 7a In high latitudes the freshwater from rivers or ice thawing produces a profile with minimum salinity at the surface and increasing values with depth In temperate latitudes water vapor loss produces highest salinity at the surface that decreases with depth down to 1000 m and then changes little according to different major water masses The highest surface salinity in the open ocean is observed at the subtropical latitudes where intense solar radiation and steady winds produce large evaporation rates that concentrate salinity Similarly to temperate latitudes the salinity profile at subtropical latitudes shows decreasing salinity down to 1000 m and little change below that depth A crosssection from north to south in the ocean Fig 7b shows the different water masses with different salinities that are re ect the typical pro les A global distribution of surface salinity Fig 8 offers a good idea on where evaporation processes dominate over precipitation namely in the tropics and where precipitation overwhelms evaporation ie tropics and high latitudes This is also re ected by the latitudinal distribution of surface salinity along a meridian or line of longitude and the difference between evaporation and precipitation Fig 9 Density and Speci c Volume A typical profile of seawater density shows increasing values regardless of where you are in the ocean Fig 10 This makes sense because you cannot have a situation of heavy water over light water as it represents an unstable situation that quickly reverses to stable In the open ocean the density profile closely resembles that of temperature but in coastal waters in uenced by freshwater the density profile will be more similar to that of salinity Seawater density is determined using the Equation of State EOS80 This equation was developed after the redefinition of salinity EOS80 is a lengthy polynomial expression The code is available in the reference reports listed or from most oceanographic institutions computer centers Speci c volume is defined as VS TP VS T 01 PKS T 41 Density is defined as 1 2 pS T 0 1vS T 0 A BS CS DS2 where 3 2 3 KS Tp E FS GS H IS JS2p MNSp2 The coefficients A through N are temperaturerelated coef cients Salinity is in PSS78 scale Temperature is in OC Pressure is in bars Density 9 is in kg m393 Speci c volume 1 is in m3 kg39l Density and Speci c Volume Steric Anomalies Since density in the ocean varies little is convenient to de ne density and specific volume anomalies The density anomaly is defined as YS Tp PSTp1000 kgm3 and speci c volume or steric anomaly is de ned as 55 T P VS T P 39 V35 0 P Inakg Note that while gamma is simply scaled against 1000 kg m393 the speci c volume anomaly is scaled against a standard sea water of observed salinity and pressure but zero temperature Until recently the term a speci c gravity anomaly quotsigmatquot was used as a similar anomaly Because it was based on the older methods of determining density its relation to the new density measurements are at 103ppm 1 where pm is the maximum density of pure water that was at the time accepted to be 1 gcm3 Since the new density is expressed as shown in the previous equations the relation between the old and new density anomalies are 7 103 pm at pm 1000 Using the new standards for maximum density of ocean water the relation is y 0999975 01 0025 Thus y is consistently 0025 kg m393 lighter than 0 t They cannot be used interchangeably and the difference must be accounted for The difference between steric anomalies calculated using the old and new density formula are insignificant and geostrophic calculation as will be carried out later in the course are not affected by the differences Freezing point of seawater The freezing point of seawater is defined as UNESCO Report No 28 1978 3 2 T aOSalS 2 f a25 bp where do 00575 a1 1710523x10393 a2 2154996x10quot39 and b 753x10quot39 Sis the salinity andp is the pressure in decibars At S 40 andp 500 Tf 2588567 C Figure 12 shows the freezing point of seawater and the temperature of maximum density on a T S plot Note that at low salinities the water acquires its maximum density before it freezes Therefore the water column is tumed over before freezing In contrast at high salinities lt 247 water freezes before it acquires its maximum density and therefore the water column tends to remain strati ed as it freezes TS Plots TS plots were developed by HellandHansen 1918 as a way to meaningfully plot oceanographic data This came about when observations made clear that ocean waters had distinctive temperature and salinity characteristics that were not dependent on depth They were finding that characteristic water masses were formed in various parts of the ocean and owing to other parts along density surfaces in the ocean The T S plot Fig 13 was well suited to demonstrate the types of water masses at any location The T S plot is an excellent way to conceptually visualize the structure of ocean waters and the manner in which they mix The T S plot is a useful way to examine the vertical T and S properties of a location in the ocean The plot in which we look at the ocean in T S space also shows the relationship of T and S to density or steric values TEMPERATURE SALINITY AND PRESSURE EFFECTS ON DENSITY The vertical stability of the ocean depends on the vertical gradient of temperature salinity and pressure The effect of temperature pressure and salinity on density are called the volumetric coef cients They are as follows Coefficient of Thermal Expansion 16p a p6T Coefficient of Saline Contraction Adiabatic Lapse Rate where T is the absolute temperature and Cp is the specific heat of seawater Figure 14 shows the variation of the thermal and haline contraction coefficients over ranges of temperature and salinity Density increases about 80 sigmat units for each 1 unit of salinity increase Density decreases by about 2 sigmat units for each one degree increase in temperature Note that at low temperatures and low salinities temperature has little effect on density while salinity changes stay approximately the same throughout the normal salinity and temperature ranges POTENTIAL TEMPERATURE and ADIABATIC ADJUSTMENT While potential temperature is defined classically as the temperature a parcel of seawater would have if it was raised to the surface with no change in salinity A more strict definition would be quot the potential temperature can be defined as the temperature resulting from an adiabatic displacement to a reference pressure pI that may be greater or less than the initial pressure p quotUNESCO Report 44 The potential temperature can be computed from the adiabatic lapse rate using the following equation P1 esTpp T frsT pdp P where STSp I an adiabat The complete equation numerical code is given in UNESCO Report 44 If you are looking at water that is moving over considerable depths ranges you may want to eliminate the effect of adiabatic heating and cooling Then we use ya pS 00 1000 Figure 15 shows an example of a station in an oceanic trench DENSITY RATIO The coefficients we just discussed are important especially in coastal environments or when considering the causes of stratification or destratification or in the interleaving of different water masses By examining the vertical temperature and salinity gradients we can determine whether the temperature or salinity gradient is most important in stratification The density ratio R expresses the relative importance of thermal expansion and haline contraction and is defined as the following R dTdp p 3 IEdz Exercise An example of the use of RF is shown in the following table Depth Temp Salinity Depth Temp Salinity 0 20 25 0 20 3 5 5 19 28 5 17 34 9 10 1 8 3 1 10 14 34 8 15 17 3 4 15 1 1 3 4 7 The data set on the left is from an inshore area where salinity is decreased because of river ow while offshore the river ow is unimportant but the water column is vertically stratified Determine the vertical temperature and salinity gradients obtain approximate values for ac and 3 and determine Rp for each station Vertical Stability and the Buoyancy Brunt Viiis il i Frequency The vertical stability of the water column is an important parameter relevant to many aspects of oceanography Stability was originally de ned by Hesselberg and Sverdrup as IT 1539 3E pg2a I B N2 dp dp where g is the acceleration of gravity and E 1p dpdz E is the stability parameter simply expressing the vertical salinity gradient N is called the Buoyancy or Brunt Vais il i Frequency and vertical stability is proportional to the square of it The Buoyancy or Brunt Vaisala Frequency is the natural frequency of oscillation a water parcel would have if raised or lowered from its rest position to denser or lighter waters below or above A simpli ed expression forN is 161 N gpaz Frequency NZT and period 2 N Speci c Heat The speci c heat of seawater Cp is the amount of heat required to increase the temperature of one kilogram of seawater one degree C at constant pressure Cp is a function of S T and p and the units are J kg C391 In normal oceanic conditions Cp increases with T and decreases with S and P The equation for Cp is i 2 CpST0 Cp0T0 AS BS where CPOT0 C0 CIT CZT2 C3T3 C4T4 The coefficients are given in the UNESCO report 44 The pressure dependence of Cp is computed from the expression T 6p 6T2 where V is the speci c volume T the absolute temperature and p the pressure The solution of this and the equation for the fully T S and p depended Cp is given in the UNESCO Report 44 Sound Sound energy unlike light energy is transmitted quite efficiently through seawater Because of this sound is used both actively and passively in oceanography The thermodynamic form of the sound speed equation is as follows 1 a 62 617 where c is the sound speed rho is the density of sea water p is pressure and eta is the entropy A simpler engineering formula is as follows ccstzcoa0T 1050T 102y0T 18250S 3560T 18S 3S60z Where c0 1493 050 30 30 006 yo 04 50 12 60 001 000164 Or an even more simple equation is c 1449 46T 055T2 14S 35 0017D mS Sound velocity varies about 4 ms for each degree change in temperature about 14 ms for each unit change in salinity and about 17 ms for each 1000 m of depth Typical sound pro les are shown in Figure 16 Misc 2D and 3D measurements Time Dependence X Y and Z constant and time varies tide gauge anchor station beach station X and Y constant Z varies and time varies anchor station or mooring with more than one sensor at different depths Multiple X Y and Z and time varying many mooring or array ships or satellite Data De nition All data must be located in space and time Space is latitude longitude and depth Routinely we use GPS and echo sounders for these measurements Time is UCT Universal Coordinated Time but we us GMT in practice Always note the time zone and whether you are using local and or daylight time or not It is best to note the time difference from UCT for clarity For plotting data it is usually easiest to go to a numerical time system That is usually referred to as Julian date However there are the following differences Julian date number of days since a specific fixed time military Julian days since 1900 Year day is the simple day of the year from 1 to 365 Scales of Temporal Variability Diumal 24h daily heating and cooling cycle sea breeze Storm cycle 3 14 days winter storms jet stream variability Springneap tidal cycle 14 days Seasons several months movement of ITCZ Monsoon annual cycle Annual cycle ENSO 7 10 years Solar 21 year and TIDAL Temporal Variability and the Thermoclines Most of the global ocean remains stratifred to some degree throughout the year because mixing forces do not overcome buoyancy forces In coastal waters and higher latitudes this may not be the case because of additional mixing forces Figure xx Shows a schematic of the seasonal progression of the thermocline Note that surface heating causes the surface layer to become lighter gain buoyancy and thus restricts the downward penetration of heat With the coming of fall wind mixing and cooling cause the warmer surface waters to mix downward The result is a deepening thermocline in the fall and a maximum heat ux in the fall at depth rather than in the summer A look at the whole ocean shows the seasonal thermocline and the main thermocline The main thermocline represents the boundary between the surface waters formed in the subtropics and tropics and the deeper waters formed in high latitudes such as NADW Measurements and Errors Since observations are so very important to our science and whether we are making them ourselves or use exiting data we must understand the principles of measurement and errors associated with measurement Some defrnitions Determination a direct measurement such as length weight temperature Estimation calculated from knowledge of determined variables Ie Salinity ftcp Now to talk a bit about statistics of measurements As the VG shows there is accuracy and precision Accuracy Difference between the determination or estimation and the TRUE VALUE This is why we calibrate The accuracy will be as good as the calibration Precision the r J quot quotquot ofthe The 139 between one result and others The random error By taking additional samples we can improve the precision This has nothing to do with accuracy Systematic error effects results because of basic fault with method causing values to consistently differ from true value Calibration can determine the systematic error The basic properties of seawater are one of the few things in the ocean that can be measured easily and with great accuracy There are international standards and agreed upon procedures These data are routinely exchanged thus there is a great need to assure the data are of the highest quality Also since many of the parameters are watched over long times to note global change 12 effects the measurement must be reliably made Reference Documents quotProcessing of Oceanographic Station Dataquot 1991 JPOTS editorial panel Unesco Paris 138 PP quotBackground papers and supporting data on the International Equation of State of Seawater 1980quot Technical Papers in Marine Science N0 38 quotAlgorithms for computation of fundamental properties of seawaterquot UNESCO Technical Papers in Marine Science No44 quotThe International System of Units S1 in Oceanographyquot UNESCO Technical Papers in Marine Science N0 45 quotThe acquisition calibration and analysis of CTD dataquot Unesco technical papers in marine science N0 54 You can order these from Division of Marine Sciences UNESCO Place de Fontenoy 75700 Paris FRANCE
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