MSCI3001 Physical Oceanography MSCI5004 Oceanographic ...

25/07/11

Course Information

MSCI3001 Physical Oceanography MSCI5004 Oceanographic Processes Alex Sen Gupta Laura Ciasto Climate Change Research Centre (CCRC)

Dr. Alex Sen Gupta Room: 454 CCRC Contact hours: Tue & Wed Email: [email protected] Dr. Laura Ciasto tutorials & computer labs Room: 451 CCRC Contact hours: Thu 1-5pm Email: [email protected] Lecture Tuesday 2 – 4pm. CCRC Seminar Room, Mathews level 4 (weeks 2-7, 8-13) Lab Friday 9 – 11am. BioSc, Room G11 (weeks 1,3,5,7,9,11) Tute Friday 9 – 11am. CCRC Seminar Room, Mathews level 4 (weeks 2,4,6,8,10,12)

Assessment Course Webpage All the information for the course will be available at http://web.maths.unsw.edu.au/~alexg/course_msci3001.html Or http://web.maths.unsw.edu.au/~lauraciasto/teaching/MSCI3001.html Course Outline Course notes Matlab tutorial notes Matlab tutorial data Lecture slides (will be put up a few days prior to class) Please check this site once a week for important information If you want email reminders please email me with MSCI3001 or MSCI5004 in the subject ([email protected]; [email protected])

The final mark for MSCI3001 will be calculated as follows: 3 Comp Labs 4+5+5% each (14% in total) Assignment 1 12% Assignment 2 16% Research Project/presentation 13%/ 5% Final examination 40% The final mark for MSCI5004 will be calculated as follows: 3 Comp Labs 5+5+6% each (16% in total) Assignment 1 13% Assignment 2 17% Research Project/presentation 13%/ 5% Final examination 35% **If not otherwise stated, all pieces of assessment are due by Friday noon of the week indicated.** They should be put into submission box or handed in to Simone Purdue or Stephen Grey, CCRC reception, Mathews level 4 **To pass this course satisfactory performance across all components of the course is required**

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Textbooks

Computer Labs Bring a memory stick

• Introductory oceanography, H.V. Thurman • An introduction to the world s oceans, A.C. Duxbury and A. Duxbury • Descriptive physical oceanography, G.L. Pickard and W.J. Emery • Introductory dynamical oceanography, Pond and G.L. Pickard How the Ocean Works. Mark Denny • Regional oceanography: an introduction, M. Tomczak and J.S. Godfrey: http://www.es.flinders.edu.au/~mattom/regoc/pdfversion.html • Ocean Circulation (Open University) • Waves, tides, and shallow-water processes (Open University) • Introduction to Physical Oceanography, J.A. Knauss (Prentice Hall) http://oceanworld.tamu.edu/resources/ocng_textbook/

Research Project/Presentation • Top 3 choices, by the end of this week to [email protected] • Planning stage. A half –one page outline by Friday week 6 (27th August). bullet points at least 3 journal articles • Written report. Maximum of 3 typed pages + figures • Presentation. 6 minute talk (+ 2 minutes of Q&A) using powerpoint (or equivalent). • Workshop Environment (if possible)

• Possible oceanography on Europa, could this mean life on another planet? • Accelerated warming of the Tasman Sea, physical reasons and biological consequences. • ENSO and Fisheries • Understanding ENSO using simple models • Global warming changes to the physical and biological pumps – what does it mean for carbon capture. • Pros and cons of iron fertilisation • The Southern Annular mode – effects on the ocean and its biology • Marine life’s influence on the evolution of the atmosphere • Can salps counter Global Warming • Is there any observational evidence for Global Warming affecting marine life? • Ocean eddies and biological productivity • How productivity is enhanced along ocean fronts • Life and sea-ice • The physical and biological oceanography of lake Vostock • Ocean acidification and coral reefs • Ocean acidification and high-latitude ecosystems • NPZ modelling – combining ocean physics and ecosystems • Seasonality of the Californian upwelling system • Can satellites provide us with accurate measures of ocean productivity • Challenges of hydrothermal vent ecosystems • What limits ocean productivity – how do different regions get their nutrients • Hurricanes and primary productivity • Ocean circulation and reproductive biology/life cycles • Ocean circulation and biogeography / species distribution / invasive species (modelling) • Aeolian dust input into the ocean across different timescales • Rubber ducks, sport shoes & ocean circulation – early/historic oceanographic discoveries • How has remote sensing revolutionised oceanography • How have ARGO floats revolutionised oceanography • Thermohaline circulation changes due to Global Warming and implications for the marine ecosystem • Trends in the Southern Annular Mode and its effect on CO2 Feedbacks • Subtropical Gyres and Garbage • Is it Climate Change or just Natural variability? The Pacific Decadal Oscillation

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MSCI3001 Physical Oceanography

Oceanography – why do we care

MSCI5004 Oceanographic Processes Alex Sen Gupta

Introduction •  Why do we care about the oceans? •  Basic Properties: §  Temperature §  Salinity §  Density §  Pressure §  Ocean Tracers §  Light •  Maths Basics

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Oceans: why do we care?


Tsunami: •  Shallow water wave •  Physics equivalent to swell close to beach •  Wavelength can be many hundreds of kms



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Oceanography –


why do we care

La Nina

El Nino


•  •  •  • 

Weather Prediction Climate Prediction Climate Change ENSO


Dispersion of jellyfish larvae (Dawson et al. 2005)

http://www.osdpd.noaa.gov/PSB/EPS/SST/climo.html

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•  •  •  • 

Weather Prediction Climate Prediction Climate Change ENSO

Oceanography –

Oceanography –

why do we care

why do we care

http://www.osdpd.noaa.gov/PSB/EPS/SST/climo.html

Oceanography – Atmospheric CO2 Concentra?on


why do we care

What we expect

What we measure

The rest must be sucked up by the Ocean and the Land

Map of Ocean CO2 Uptake (IPCC)

Maximum yearly mixing depth [m]

Every year the ocean absorbs ~1/3 of human CO2 emissions

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Oceanography –

Oceanography –

For a 0.5m rise in sea level - in Sydney at 1 in a 100 year flooding event will likely occur every year

For a 0.5m rise in sea level - in Sydney at 1 in a 100 year flooding event will likely occur every year

why do we care

Courtesy of John Hunter UTAS

Barrier to withstand 1:10 yr event


why do we care

Courtesy of John Hunter UTAS

Barrier to withstand 1:10 yr event

Geophysical Fluid Dynamics

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Oceanic time scales & length scales

Oceanic and Atmospheric Phenomena

Time Scale Climate Change

100 y

El Nino Boundary Fronts currents Eddies

Month

Upwelling

days mins

N. Atlantic Oscillation

Deep water formation

2 yrs

Turbulence Molecular cms

Weather

Diurnal cycle

Hail storm Waves 10 m

1 km

100 km

10000 km

Spatial Scale

Oceanic and Atmospheric Phenomena

Properties of the oceans

Water covers ~71% of world surface Average depth 3800m Pacific Ocean 46% Atlantic Ocean 23% Indian Ocean 20% Others 11% Q. What is the volume of the ocean? Q. What is the aspect ratio of a piece of paper? (a 500 sheet ream of paper has a width of ~5cm)? Q, What is the aspect ratio of the ocean (i.e. a typical depth H divided by a typical length, L)? The aspect ratio turns out to have important consequences for the magnitude of typical velocities in the horizontal compared to vertical directions

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Properties of the oceans Measuring the Ocean Echo sounders (time taken to bounce a sound wave) Gravity measurements – the slope of the surface is affected by the gravitational attraction of underlying bathymetry features (see Giod)

The World Ocean Floor

Gravity Measurement

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Properties of the Ocean

Global Sea Surface Temperature

Temperature (T) • Temperature is important because it reflects the amount of heat held and transported by the ocean. • Plays important role in circulation via density • The temperature range in the ocean varies from -2°C (http://www.csgnetwork.com/h2ofreezecalc.html) at the poles to >28°C at the equator. • The temperature of the ocean is primarily influenced by the heating at the air-sea interface.

Temperature distribution with depth

Temperature distribution with depth

surface

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Potential Temperature (θ)

Why Potential Temperature?

•  The temperature a water parcel would have at the surface, if it were raised to the surface adiabatically. •  Adiabatically: No heat is transferred to or from the fluid. •  From chemistry, VT = constant. •  Thus, as pressure increases, (volume decreases) and temp increases. à θ removes the effect of pressure on temperature

Atlantic Potential Temp

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Properties of the Ocean Salinity •  Total dissolved solids (mainly sodium chloride, or table salt ) •  About 3.5% by weight (i.e 35kg of salt in 1000kg of pure water on average) •  Usually expressed as 35psu (practical salinity units, psu, or ppt) What sets the salinity in the ocean?

Surface Salinity

Properties of the Ocean Salinity •  Total dissolved solids (mainly sodium chloride, or table salt ) •  About 3.5% by weight (i.e 35kg of salt in 1000kg of pure water on average) •  Usually expressed as 35psu (practical salinity units, or ppt parts per thousand – they are the same) •  Varies geographically according to Evaporation, precipitation, rivers, ice formation and ice melt. •  Plays an important part in ocean circulation, through influence on density

E-P

à Surface salinity determined by surface Freshwater Flux

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Inter Tropical Convergence Zone (ITCZ)


Salinity changes with depth: Polar Temperate Tropical

Ganges Delta

Amazon River

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Density in the ocean (ρ) •  Density is the mass of sea water per unit volume.

Kg / m3 •  Density depends on salinity, temperature and pressure. - Density increases with increasing salinity - Density increases with decreasing temperature

Density (continued) •  Density increases with pressure, as the pressure force squashes water into a smaller volume. •  Lighter water is •  Denser water is

- warmer - fresher - colder - more saline

•  Generally for stability, less dense water overlies more dense water – otherwise convection

•  Seawater density ranges from 1020 – 1030 kg/m³, the average density is 1025 kg/m³

Density (continued)

The Equation of State For regions where T and S vary little, we may assume a linear equation of state.

http://www.phys.ocean.dal.ca/~kelley/seawater/density.html

At T=30, S=40, P=0 (e.g. Dead Sea), ρ=1025.48 1.  What is the density if we increase the temp by 2°C (sun) ? 2.  What is the density if we decreasing the salinity 1psu (rain) ? 3.  What is the density if we decrease the temp 2°C (night) ? 4.  What is the density if we increase the salinity 0.5psu (wind/ evaporation) ?

Where alpha and beta are nearly constant expansion coefficients for T and S.

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Types of Density For most applications we can use a simplified linear Equation of State: ρ~ = ρo[1-α(T-To)+β(S-So)] Where ρo is a reference density, and α & β are nearly constant expansion coefficients. ρo = 1028 kg/m3 α = 1.7 x 104 K-1 β = 7.6 x 104 T = 10°C S = 35 psu

Changes with depth

•  In-situ

σ = ρs,t,p –1000

•  Sigma–t

σt = ρs,t,0 –1000

•  Is density parcel would have if raised to the surface •  Removes pressure s HUGE effect on compressibility

•  Potential

σθ = ρs,θ,0 –1000

•  Is density parcel would have if raised to the surface adiabatically •  Removes Pressure s HUGE effect on compressibility •  Removes Pressure s much smaller effect on temperature

Atlantic Potential Density

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Density Structure of the Ocean


Density (continued)

•  Density increases with pressure, as the pressure force squashes water into a smaller volume. •  Lighter water is •  Denser water is

- warmer/fresher - colder/more saline

•  Generally for stability, less dense water overlies more dense water – otherwise convection

Pressure •  Ocean pressure is the weight of seawater per unit area (force per unit area).


Pressure

P=F/A •  It is mainly a function of depth (also depends on density). P=F/A = mg/A = ρgh •  Pressure in the ocean increases at a rate of about 1 atmosphere OR bar per 10 m of water. •  Or pressure increases by 1 dbar per 1 m of water. •  Differences in pressure one of major drivers of ocean circulation

P1

P2

Stiletto vs Elephant Press = depth x density x g

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Other Properties

Pressure

•  Can use other properties as ‘tracers’ showing source of water. •  Dissolved phosphate, silicate, nitrate, and nitrites, indicative of e.g. upwelling, biological activity etc. •  Dissolved Oxygen (ml/L) – tells us when water was last in contact with the atmosphere. P1

>

P2

•  Radioactive elements such as tritium and radiocarbon and CFCs were introduced to the atmosphere by humans. We can use them as tracers in the ocean. Press = depth x density x g

watermass formation

Oxygen

Cross-section of oxygen content along 135°W in Pacific Ocean (WOCE).

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Carbon-14

= measure of age of the water

Cross-section of carbon-14 (14C) content along 150°W in Pacific Ocean (WOCE).

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Anthropogenic compounds and chemicals can act as tracers in the ocean. e.g. CFCs Recently overturned waters carry a higher signature of CFC than older water.

Rapid increase in CFCs since introduction in 1930s

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Light in the Ocean The vertical penetration of light decreases with depth: - Scattering of molecules and absorption by molecules and particulate matter - Light intensity (I) decreases as a function of depth

I ( z ) = I o e − kz

He comes from the earth s mantle and is emitted at plate boundaries, i.e. at mid – ocean ridges.

Where Io is the surface intensity, and k is the attenuation coefficient - The length scale z = k-1 is the e-folding scale of penetration

àUsed to track water plumes from hydrothermal vents

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Light (cont’d)

Light intensity with depth

•  For seawater the coefficient k varies with wavelength. •  For clear ocean, k is a minimum at 0.45 µm i.e blue light is attenuated least. •  At both shorter and longer wavelengths the attenuation is greater (hence penetration of light is less)

Light intensity with depth and light required for biological processes (Sarmiento & Gruber 2006) Light absorption with depth (Mann & Lazier 1996)

Thermocline Halocline Pycnocline

à Nearly all the energy is absorbed in the surface waters

– Sharp temperature gradient – Sharp salinity gradient – Sharp density gradient

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Coordinate systems and Notation

What can you say about the gradient?

y

v

u

Oceans

du >0 dx

North

x East

w

z Down


y

v

u

North


y

v

u

x East

w

w

z

z

Down

Down

North

x East

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Gradients A Gradient is a measure of how a quantity (pressure, salt, temperature, density) changes with direction: e.g

ρ2 – ρ1 Δρ x2 – x1 = Δx

Δρ , Δρ , Δρ Δx Δy Δz

Where is the temperature gradient bigger? 1000m

100m

In most applications in oceanography, the gradient term in an equation, such as: dT d"

dx Can be approximated as

,

dy

, etc

Change

dT ΔT ≅ dx Δx

!

dT/dx ~ 2oC/1000m = 0.002 oC m-1

Where is the temperature gradient bigger?

Units Force N (Newtons)

1000m

1 bar = 100 Pa Pa = Nm-2

dT/dx ~ 2oC/ 100m = 0.02 oCm-1 100m

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Questions A

•  •  •  • 

B

Next week: Thermohaline Circulation Friday: Tute with Laura in CCRC Send me an email so I can put you on the mailing list Email me your research project title by the end of the week

What is dT/dz in region A, B and C? What you think the corresponding density profile would look like?

C

Name 2 processes that might give rise to the homogeneous temperature at A?

Questions If the light intensity, I, at depth z is given by:

I(z) = Ioe"0.5(z) At what depth will intensity drop of by 50%? You will need to use the ln function on you calculator. No calculator write down how you would do it.

!

If the density of water is 1027kgm-3 calculate the weight of water above a 1m2 horizontal area at 100m depth. Now calculate what the pressure exerted on that area is. Remember density =mass / volume weight (or force) = mass x g Pressure = force / area

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