summary from last class… importance of large-scale ocean circulation –climate, biogeochemistry,...

Summary from last class…

• Importance of large-scale ocean circulation– climate, biogeochemistry, marine resources

• Characteristic “Types” of Ocean Circulation – Patterns of circulation can be generalized for different regions

– Suggests that dynamical processes are the same

– Wind-driven, Buoyancy-driven & Coastal

• Scales - “spit on a basketball”

Sea Water Properties

• Water mass characteristics

– Salinity, temperature, nutrients, oxygen

• Key property is seawater density

– Changes in vertical - inhibit mixing

– Changes in horizontal - drive currents

– See Lecture 3 of Tomzcak’s web coursegyre.umeoce.maine.edu/physicalocean/Tomczak/IntroOc/lecture03.html

What is Seawater?

•Seawater is 96.5% pure water

•About 3.5% is other materials

dissolved salts, gases & organic

substances as well as particles

•Physical properties are mainly

determined by pure water

A Water Molecule

•Water is a non-polar molecule

•Weak hydrogen bonding

Consequences of Water’s Hydrogen

Bonding•Water forms a lattice or

aggregation of many

molecules (polywater)

Consequences of Water’s Hydrogen

Bonding•High specific heat (break the lattice!!)

•High latent heat for phase changes

•Great solvent

Consequences of Water’s Hydrogen

Bonding•Ice crystals take up more

space than liquid water

•Ice Floats!! (rare for liquids)

–Maximum density is water at 4C

•Critical for freshwater

systems

More about ice...

•Seasonal turnover in lakes

–As lakes cool they reach temperature of maximum density (4C) & overturn

–Later ice forms at the surface, sheltering the interior from winter

conditions

–This allows fish over winter under the ice

More on Hydrogen Bonding

Back to Oceans...

Density of seawater is controlled by

–temperature

–salinity (dissolved salt content)

–pressure (related to depth)

Equation of state = f(S,T,p) = [kg m-3]

(S,T,p=0) range from 1020 to 1030 kg m-3

Temperature

•Temperature generally decreases

with depth in the ocean

•Except where ice is formed,

temperature changes primarily

regulate density

•Rule of thumb

= +1 kg m-3 for T = -5 C

Temperature

Equatorial Pacific - WOCE150W

Temperature

60S - WOCE150W

Pacific Temperature

eWOCE gallery – www.ewoce.org

Salinity

Ocean waters are “salty”

Salinity ~ [mass “salts”]/[mass seawater]

The “salts” (Cl-, SO4-2, Na+, K+, etc.) are in approximate constant

proportion

–Law of salinity (residence time is huge)

–Measure one ion [Cl-] - estimate salinity

Salinity

Salinity is measured

electrically now

Units are “practical

salinity units” (psu)

Often bottles

are used

Salinity

Salinity varies from 32 to 37 psu

Atlantic is saltier than the Pacific…

–Why??

Good water mass tracer

Lower/higher values are unusual (riverine, huge evaporation, etc.)

Typical T & S

Profiles

Features

Mixed layer

Thermocline

Halocline

Pacific Salinities

eWOCE gallery – www.ewoce.org

Atlantic Salinity

Ocean Distribution of & S

Mean ocean ~ 4 C & S ~ 34.8 psu

Pressure

Pressure is due to the weight of sea

water lying above a depth (hydrostatic)

Pressure varies from 0 to >5000 db

p = 0 is atmospheric pressure

Note: 1 db pressure ~ 1 m depth

Potential Temperature

Hydrostatic pressure will heat a water

parcel as descends within the ocean

Adiabatic lapse rate is ~0.0001 C/m

A surface parcel (T=0 & S=35) will

heat ~0.3C if moved to 3000 m depth

Defines potential temperature or

Potential Temperatu

re

World OceanAtlas 1994

Global SST

Annual Change in SST

Annual Change in SST

January July

Net Air-Sea Heat FluxesRed = Heat In Blue = Heat Out

Interannual SST Changes

March 1998relative toaverage March

World OceanAtlas 1994

Global Salinity

Global Salinity

Global Salinity & Air-Sea Fluxes

Global Salinity

1 Sverdrup = 106 m3 s-1

Why is the Atlantic so salty?

Seawater Density

Equation of state = f(S,T,p) = [kg m-

3]

(S,T,p=0) range from 1020 to 1040 kg m-3

Shorthand sigma-t: t = (S,T,0) - 1000

t(S,T) ranges from 20 to 40

–Similarly, sigma-theta: = (S,,0) -

1000

Seawater Density

Density of seawater is controlled by

Temperature, salinity & pressure

= f(S,T,p)

Rule of Thumb => = +1 kg m-3

IF S = +1 psu OR T = -5C OR p = +100 db

T-S Diagram (full range)

Max density

Freezing

Values of t

T-S Diagram (typical range)

Density Calculator

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

http://gyre.umeoce.maine.edu/physicalocean/Tomczak/Utilities/index.html

Review

Fundamental seawater properties

–Salinity, temperature & pressure

Density is the important variable

–in situ density (S,T,p)

–Sigma-t (S,T,0) – 1000

–Sigma- (S,,0) – 1000

Review

Rules of thumb -> = +1 kg m-3

T = -5C, S = 1 psu or p = 100

db

Global surface T & S driven largely

by air-sea exchanges

Dense water sinks… now we're

talking dynamics