ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY

Transcription

1 ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY Lecture 4 Learning objective: understand how the ocean properties are measured & how the methods have been improved Observational methods: a) Depth; b) Temperature; c) Salinity (reading assignment); d) Density (reading assignment); e) Currents: direct measurement

2 Observational method Definitions. Accuracy: The difference between the results obtained and the true value. Precision. Ability (instrument) to measure consistently within a given data set.

3 Observed temperature at (0,110W) by the tropical atmosphere ocean (TAO) array Critical thinking: Q1:Define mixed layer; Q2: What signals are showing for what event? Q3: T (z,t): today s class

4 a) Measurements of depth current Variables T, S, V are functions of space and time, and thus functions of depth, => measure depth. Meter wheel. Old method. A wire is passed over a meter wheel, which is simply a pulley of known circumference with a counter attached to the pulley. Accuracy: calm ocean is ok

5 Measure Pressure and then calculate depth. Hydrostatic equation: By measuring P we can calculate z.

6 Measure P & then z Protected and unprotected reversing thermometer; Mercury-in-glass thermometers attached to a water sampling bottle: one is protected from the sea water pressure by vacuum and the other is exposed to sea water pressure. Depth accuracy: 0.5% or 5m, whichever is greater.

7 Measure P and then calculate z Electrical strain-gauge pressure transducer. Uses the change of electrical resistance of metals with mechanical tension. Resistance wire - connected to a flexible diaphragm, to one side the in situ hydrostatic pressure is applied. Accuracy 0.1%.

8 b) Measurement of temperature Old. Bathythermograph: a liquid-in-metal thermometer=> metal point to move in one direction over a smoked or gold plated glass slide, move at right angle by a pressure sensitive bellow. Continuous T(z) curve but less accurate. (at depths of 60m, 270m ) Bathythermograph Metal point (inside)

9 Measurement of temperature Expendable Bathythermograph (XBT). Widely used. Uses a thermistor as T sensitive element, which is in a small streamlined weighted casing. Fine wire connected to a recorder on ship=>t(z) ~1800m. (major source: before 1970s; falling rate bias correction) A thermistor is a type of resistor whose resistance varies significnatly with temperature more so than in standard resistors.

10 Dashed: with bias; solid: after bias correction

11 Measurement of temperature CTD-Conductivity, Temperature, and Depth (actually Pressure). T is measured by a thermistor mounted close to the conductivity sensor. Accuracy: 0.005K. S. Carolina--St Lucia NOAA Ship. CTD bottles

12 Measurements of temperature Protected reversing mercury thermometer. (Negretti and Zamba 1874). Attached to water sampling bottle. Accuracy 0.004C. Thermistors chains. The best quality one: Accuracy 0.002C. Buoy (moored buoy and drifting buoy).

13 TAO Buoy and ship

14 Measurements of temperature: Satellite Satellite. AVHRR (Advanced very high resolution radiometer) on NOAA satellite (1985-present), accuracy of K. Multichannel: visible & infra-red. Problem: Cloud vapor absorption and cloud reflection. Satellite. TRMM: Tropical Rainfall Measuring Mission (TRMM)-Microwave Imager (TMI); 1997-present; Penetrate clouds, but affected by strong rainfall; measuring SST; 0.2C difference with buoy. Acoustic tomography. ( research/atoc.html)

15 Potential temperature Potential temperature is the temperature a water parcel has when it is moved adiabatically to the sea surface. [Remove T change due to adiabatic compression and expansion effects.]

16 c) Measurement of salinity (reading assignment) Laboratory (Old). Evaporate and weigh residual; units: g/kg Lab. Classical (Knudsen) method (1957). Determine amount of chlorine, bromine and iodine => chlorinity (via silver nitrate titration). S= Cl. Accuracy: Units: ppt Measure conductivity. Accurary: (psu; or unit less);

17 Conductivity (salinity) measurement Conductivity: depends strongly on T, then on S; (1) Sea water sample (lab): autosalinometer; => ratio of sea water conductivity against sea water standard (KCl); [control lab T, important]; (2) CTD. Electronic instrument, inductive or capacitance cells are used; For known Conductivity and Temperature at certain Depth, => salinity. Accuracy:

18 TEOS-10 (Thermodynamical equation of seawater 2010) - new standard: use Practical Salinity (Sp) measured by conductivity, to derive Absolute salinity (Sa), the latter has g/kg unit mass (or salt content). Satellite. NASA Aquarius mission: Launched June 2011: measure sea surface salinity (SSS) -the salinity sensor detects the microwave emissivity of the top 1 to 2 centimeters (about an inch) of ocean water a physical property that varies depending on temperature and saltiness. The instrument collects data in 386 kilometer-wide (240- mile) swaths in an orbit designed to obtain a complete survey of global salinity of ice-free oceans every seven days.

19 d) Measurement of Density (reading assignment) Standard laboratory method: weighing bottle. Not practical. Calculated from equation of state: T, ; S For more detailed equation, see Gill s book, Page 599.

20 At one standard atmosphere (effectively p=0) is: Where is the density of pure water with S=0. (see Gill, appendix 3 for the equation at pressure p).

21 e) Measurement of currents

22 Measurement of currents Goals: 3-dimensional circulation => heat & salt, nutrient transport => climate, biology; Typical horizontal currents vary from a fraction of 1 cm/s (deep ocean and much of near surface), cm/s (equatorial current system), 200cm/s western boundary current. Vertical speed: Critical thinking: Why?

23 Measurement of currents Lagrangian methods: follows fluid parcels. Eulerian methods: velocity is measured at every point in the fluid. (instrument is fixed in a space).

24 Direct measurements Direct: Surface drifters Subsurface floats Current meters Acoustic Doppler Current Profiling (ADCP) Indirect: geostropic method (next week).

25 Surface drifters: Lagrangian method Ship drift: earliest circulation map. current C B (end) Drift pole: Near land mark ship A (begin)

26 Drift buoy: extend the pole idea to open ocean; radio transmitor or tracked by satellite. Subsurface drogues: TOGA WOCE drifter: 15m

27 Subsurface floats SOFAR floats (sound source, moored receiver) RAFOS floats (sound receiver, moored sound source) Pop up floats: pop up regularly to communicate with the satellite (bladder). WOCE subsurface floats: m.

28 RAFOS float

29 1. Current meter Rotor current Meter (RCM) Accuracy~a few cm/s Eulerian methods

30 Acoustic Doppler Current Profiling (ADCP) Using sound Wave s Doppler Shifting effect

31 ADCP: Frequency shift: source F F reflect WOCE: 75khz-150khz; up-look, down-look V (z)

32 Gliders (current, T, dissolved O2, etc.) from deep ocean to surface