This file includes all 14 lectures given in the course in . The course supervisor was Roderik van der Wal. It includes all the information from the PowerPoints and a lot of information that was only told during the lectures (I made very elaborate notes). It also includes additions that were importa...
Introduction to Physical Oceanography –
Summary
Lecture 1 – Introduction to our oceans
Different scales of circulation in the ocean, transport in many directions. There is chaos!
Physical oceanography = field of study that deals with the physical properties and dynamics of the
ocean and its influence on climate and atmosphere.
Using : Observations, Numerical model output, Theoretical studies, Process-based numerical model
experiments
Ocean properties and dynamics → play a key role. Many feedbacks between:
Economy:
- Transport → transport of water but also of boats etc
o Thermohaline, gyres (localized circulation in a bigger system)
o Driven by temperature differences, wind circulation and topography
Energy transition → wind farms, tidal systems, osmosis
- Tourism → Ocean plays a role in distributing plastic
Ecosystems:
- Habitat → Largest habitat on earth!
- Food security → fisheries and aquaculture (e.g. seaweeds)
- Natural defence → mangroves and coral reefs pose protection
Climate:
- Heat uptake and transport → Surplus of radiation in equatorial regions is transported away
- Carbon, nutrient and hydrological cycle
- Forcing for ice sheets
- Atmospheric composition & circulation
Ocean and climate change
Energy take up, (not carbon dioxide → ENERGY) → 95%
- Total energy added to the climate system is 250 ZJ = 250.000.000.000.000.000.000.000 Joule
- Between 1970 and 2010 this is 104 million atomic bombs added into the ocean (like
Hiroshima), 3.3 bombs s-1
Ocean’s role in taking up CO2
- 25% of emitted CO2 goes into the ocean (25% on land, 50% in atmosphere)
- Inter-annual variability between land and atmosphere due to seasonality of the biosphere
- To stay below 1.5 degree warming we need CO2 removal
Past and projected changes in the ocean
- Temperature (Arctic sea ice is directly related), marine heatwaves, decrease of pH, oxygen
decrease, increase heat content
- Sea level rise: expansion of sea water and melting of land ice.
Ocean dynamics → ocean releases energy into the atmosphere causing storms (cyclones, hurricanes)
, - Forces: e.g. salinity and wind
- Properties
- Domain: set by continents, in this course set as constant.
Ocean topography
- Only 23% of the ocean is mapped (large uncertainty)
- 71% of earth surface
- Sea surface slope measurement from satellites can be transformed to seafloor bathymetry
Topographic details matter!
- Impacts strength, pathway & stability of ocean currents → bottom roughness affects mixing
- Mapping of seafloor important for plate tectonics → may reveal important areas for
generation of earthquakes and tsunamis.
Ocean basins
Using clustering techniques to find regions that are characterised by distinct flow properties
Properties of sea water
Temperature
- Reflects amount of heat held and transported by ocean, plays role in circulation via density
- -2°C at poles to >28°C at equator
- Primarily influenced by heating at air-sea interface
Horizontal distribution: surface heat flux, circulation, mixing and upwelling
Vertical distribution: thermocline. Heat flow from high to low (in or out of the ocean)
- Surface ocean T: reflected by heat influx (e.g. in summer) or heat outflux
o Constant T at the surface due to mixing by surface (wind) waves and/or instability of
the water column due to cooling/evaporation
, - Deep ocean T: quite consistent (even at equator!)
- Global warming → warming will eventually also warm deeper ocean
Difference between temperature and heat
- The heat content of seawater is its thermodynamic energy, related to T but not the same
o Units are Joule = kg m2/s2
- Heat content
o is specific heat (~ 4000 J/kg/K) is mass (kg) is temperature (K)
o Heat is zero at absolute zero temperature (on Kelvin scale)
- Heat change per unit time = is in Watt (W) → 1 W = 1 J/s
- Heat flux = is amount of heat entering a system per unit time per unit area → W/m2
o Order of magnitude important in understanding what is important and what is less
o Ocean: 50 W/m2
o On average negative fluxes: looses heat to atmosphere
Potential temperature = θ
Pressure increases with depth, water is compressible → when water is moved down pressure
increases and water is heated up (reverse from rising and cooling/expanding air) → T increases even
if no extra heat is added, no measure for heat content → use potential temperature!
- = T of sea water if it is moved to the surface (p=0) without exchange of heat salt, or
dissipation of kinetic energy
o Removes effect of adiabatic (without changes in amount of heat) warming
- Potential temperature < Temperature
o Larger difference with depth
- Conservative Temperature = Θ = embeds sea water heat capacity with p, θ, and S
o Differences small but important
- THUS because of the compressibility of sea water we use conservative or potential T
Salinity
, - Atlantic most saline. Poles more fresh water input. Gyres more saline. Fresh at river outlets
- Climate change: fresh gets fresher, saltier gets saltier
- Plays important role in circulation via density
- At depth less saline because at surface there is evaporation
- High salinity at subtropics → more evaporation than precipitation → output of fresh water
→ salt more concentrated
- Lower salinity at tropics → more P than E → input fresh water → salt more diluted
Absolute salinity = fraction of non-H2O in water (all dissolved material, mainly sodium chloride)
- Often expressed in g/kg or psu
- Sea water on average 35 g/kg
Practical Salinity = unitless measure of sea salt using conductivity
Density = ρ → mainly influenced by pressure → linear increase of density with depth
- Mass of 1m3 of sea water ranges 1020-1039 kg/m3 → slightly higher than pure water
- Often expressed with 1000 removed
- A (very!) complicated function of temperature, salinity and depth (pressure)
- Ocean is mostly in the grey bar
- Fresh water → T of max density is 4°C → water of 4 degrees at bottom of fresh water lake
- Ocean water → T of max density is below freezing point → thus at bottom is coldest water
Important because:
- Determines what sinks or floats → plastic, sediment, plankton/biomass (carbon)
- Determines how water circulates → against density costs energy
- Important for dynamical feature like mixing, convection and internal waves
Pressure = p
- Pressure differences main drivers of ocean circulation
- P = ocean pressure = the weight of seawater per square meter – force F per unit area A
o (g = gravitational acceleration = 9.81 m/s2)
- Almost linearly related to depth and density
- P increases with 1 atm (1 bar = 105 N/m2) per 10 m water → 1dbar per 1 m water
o Increases so rapidly because density of water is a factor of 103 larger than that of
atmosphere
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