Lecture 2: physical characteristics of the ocean

Atmosphere, Ocean, Climate Dynamics

EESS 146B/246B

Physical characteristics of the ocean

• Properties of seawater• Hydrostatic balance• Equation of state of seawater• Stratification and buoyancy• Vertical structure of the ocean

Properties of seawater•The properties of seawater that are most important for the dynamics of the circulation and the ocean-atmosphere climate system are its

•Large density (around a thousand times that of air)

•Temperature and salinity, which modifies its density (cold and/or salty water is more dense than warm and/or fresh water).

•Specific heat per unit volume (J K-1 m-3) is 4000 times greater than that of air.

Total amount of heat in a volume of water

(J K-1 kg-1)

Specific heat per unit mass

How temperature and salinity are measured in the ocean

Conductivity-Temperature-Depth Sensor (CTD)

•The electrical conductivity of seawater is a function of salinity and temperature.

•Conductivity and temperature measurements can be used to infer salinity.

•Salinity is measured in terms of the practical salinity unit which is nearly equivalent to g/kg.

World Ocean Circulation Experiment

• Between 1990 and 1998, nearly 30 countries joined together to make hydrographic measurements across the world’s oceans.

• These observations can be easily explored using the program Ocean Data View.

Temperature and salinity profiles

1 db=0.1 of an atmosphere

500 atmospheres=7250 pounds per square inch

Hydrostatic pressure

•Vertical force balance: the weight of column of water is balanced by difference in pressure forces on the bottom and top faces.

•This balance is called the hydrostatic balance

Equation of state for seawater

•The density of seawater is a nonlinear function of salinity, temperature, and pressure.

ICE

Freezing point of water

Density anomaly

Equation of state for seawater

•Seawater is slightly compressible (its volume decreases under pressure), thus its density increases with pressure.

Equation of state for seawater

•The density of seawater is a nonlinear function of salinity, temperature, and pressure.

Linear equation of state

•The equation of state can be approximated using a linear function of temperature and salinity

=(35,15,26)

Linear equation of state

•The thermal expansion and haline contraction coefficients are functions of temperature and salinity.

=(31,0,24.9)

Linear equation of state

=(31,0,43)

•The coefficients of thermal expansion and haline contraction change with pressure as well.

In situ density and temperature

•Increasing the pressure causes the in situ temperature to increase (adiabatic warming) and the in situ density to increase.

Potential density and temperature

•Removing the effects of the pressure on temperature and density, yields the potential temperature and density, which is the temperature and density that is only relevant to the dynamics of the circulation.

Stratification

50 S Equator 50 N

Dep

th (m

)

North-South section of potential density in the Pacific, 170 W

isopycnals

•The ocean is stratified, i.e. layered in density, with density increasing with depth.

Stratification and static stabilityparcel of water

(mass of parcel)*(acceleration)=sum of forces

Hydrostatic balance

Stratification and static stability

IN SITUPOTEN

TIAL

zparcel of water

Change in density due to pressure

Parcel experiences same pressure change as environment

BUOYANCY

Stratification and static stability

IN SITUPOTEN

TIAL

zparcel of water

BUOYANCY

•The potential density is conserved following parcel

•For small displacements

N is the buoyancy frequency and is a measure of the stratification The stratification provides a restoring force that

tries to keep parcels from crossing isopycnal

Vertical structure of the ocean

MIXED LAYER

THERMOCLINE/ PYCNOCLINE

ABYSS

BUOYANCY FREQUENCY

•Three regions: mixed layer near the surface with weak stratification, thermocline/pycnocline with strong stratification, and the abyss with weak stratification.

April

N. Atlantic

The mixed layer

•The mixed layer is in direct contact with the atmosphere and is where heat, salt, and dissolved gases are fluxed into/out of the ocean.

•The depth of the mixed layer is governed by processes that change the stratification: radiative heating/cooling, heat loss and evaporation, and wind-driven turbulence.