ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY Lecture 4

ATOC 5051 INTRODUCTION TO PHYSICAL

OCEANOGR

APHY

Lecture 4

Observational methods:

e) Currents: direct measurement

Reading assignment: chapter 2

Recommended reading: chap S16 of Talley et al. 2011

Learning objectives:

• How are the ocean

properties measured?

• How have the

methods evolved with the advancement of technology?

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.

Observed temperature at (0N,110W)

by the Tropical

Atmosphere Ocean (TAO) array

Observed temperature

at (0N,110W) by the tropical

atmosphere ocean (TAO) array

Critical thinking: Q1:Definition of

mixed layer depth: are we missing

something?

Q2: What signals are showing for what event?

Q3: T (z,t): today’s class

MLD

Thermocline:

a) Measurements of depth

• Variables T, S, V are

functions of time & space, 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

• Measure Pressure and then calculate depth.

Hydrostatic equation:

Measure P & then z

• Protected and unprotected reversing

thermometer

; Mercury-in-glass

thermometers attached to a water sampling

bottle: one is protected from the seawater

pressure by vacuum and the other is

exposed to seawater pressure.

Depth accuracy: 0.5% or 5m, whichever is

greater.

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%.

• Quartz crystal:

• Very accurate pressure measurements can be made using a quartz crystal, whose frequency of oscillation depends on pressure. Used in modern conductivity, temperature &

depth (CTD) measurement. The accuracy is 0.01% and precision is 0.0001% of full-scale values. (For more details see chapter S16 of Talley textbook.)

• 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, …140m, 270m)

Bathythermograph Metal point (inside)

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). • 0-200~1800m. (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

Dashed: with bias; solid: after bias correction for

XBTs falling rate

Measurement of temperature

(actually Pressure). T is measured by a thermistor mounted close to the conductivity sensor.

• Accuracy: 0.005K.

_{S. Carolina--St Lucia}

NOAA Ship.

Water sampling bottles; CTD is at the bottom of these bottles

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.

Measurements of temperature:

Satellite

•Satellite. AVHRR

(Advanced very high resolution radiometer) on NOAA satellite (1985-present), accuracy of 0.1-0.3K. Multi-channel: 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.

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.)

• Note that “conservative temperature” is defined in TEOS-10, which also takes into account of heat capacity change due to T & S

variations (in addition to adiabatic compression & expansion effect). The difference between potential temperature and conservative

a) Profiles of in-situ temperature (t), potential temperature (𝜃), and conservative temperature (𝛩); b) Practical salinity S_P (based on measurement of conductivity), reference salinity S_R and absolute salinity S_A (discussed below).

𝑆_!( 𝑔

𝑘𝑔) =

35.16504 35 𝑆"

𝑆

= 𝑆

+ 𝛿𝑆

In open ocean, there

are atlases for 𝛿𝑆

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=1.80655Cl

. Accuracy: 0.025.

Units: ppt

• Measure conductivity. Accurary:0.001-0.004.

(psu; or unit less).

Conductivity (salinity) measurement

• Conductivity: depends strongly on T, then on S; • (1) Seawater sample (lab): autosalinometer; =>

ratio of seawater conductivity against seawater standard (KCl - potassium chloride); [control lab T, important];

• (2) CTD. Electronic instrument, inductive or capacitance cells are used;

• For known Conductivity and Temperature at certain Depth, => salinity. Accuracy: 0.005.

• TEOS10 (Thermodynamical equation of seawater 2010) -new standard: use Practical Salinity (S_P) measured by

conductivity, to derive Absolute salinity (S_A), the latter has g/kg unit – mass (or salt content); see Figure on slide 18. • The S_A takes into account of the varying composition of seawater (dissolved materials) in different regions of the world’s oceans, instead of constant compositions.

• Requirement of this course:

• Satellite. NASA Aquarius mission: Launched June 2011:

http://aquarius.nasa.gov/: 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.

Aquarius: 2011-2015; NASA’s soil moisture active passive (SMAP): 2015-present; ESA’s SMOS (Soil Moisture and Ocean Salinity): 2010-present

d) Measurement of Density

(reading assignment)

• Standard laboratory method: weighing

bottle. Not practical.

• Calculated from equation of state:

For more detailed equation, see Gill’s book,

Page 599.

Where

_{is the density of pure water}

with S=0.

(see Gill, appendix 3 for the equation at

pressure p).

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),

10-100 cm/s

(equatorial

current system),

200cm/s

western boundary

current.

• Vertical speed:

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).

Direct measurements

• Surface drifters

• Subsurface floats

• Current meters

• Acoustic Doppler Current Profiling (ADCP)

Indirect:

geostropic method (next week).

Direct:

Surface drifters: Lagrangian method

• Ship drift: earliest circulation map.

• Drifting pole:

Near land mark

A

B

current

ship

C

(begin)

(end)

• Drifting buoy:

extend the pole idea to open

ocean; radio transmitor or

tracked by satellite.

Subsurface drogues:

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: 800-1000m.

Eulerian methods

a). Current meter

Rotor current

Meter (RCM)

Accuracy~a few

cm/s

b) Acoustic Doppler Current

Profiling (ADCP)

Using sound

Wave’s Doppler

Shifting effect

ADCP:

F

F reflect

source

Frequency shift:

WOCE: 75khz-150khz; up-look, down-look

V (z)

Question received from last class:

Do ocean currents affect seafloor mapping by ecosounder – single beam or multibeam?

Marine geology:

Research article: An uncertainty model for deep ocean single beam and multibeam echo sounder data:

https://link.springer.com/article/10.1007/s11001-008-9060-y?shared-article-renderer