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 course gyre.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 –

Salinity Ocean waters are “salty” Salinity ~ [mass “salts”]/[mass seawater] The “salts” (Cl -, SO 4 -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 –

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 ~ C/m A surface parcel (T=0 & S=35) will heat ~0.3C if moved to 3000 m depth Defines potential temperature or 

Potential Temperature

World Ocean Atlas 1994 Global SST

Annual Change in SST

January July Net Air-Sea Heat Fluxes Red = Heat In Blue = Heat Out

Interannual SST Changes March 1998 relative to average March

World Ocean Atlas 1994 Global Salinity

Global Salinity & Air-Sea Fluxes

Global Salinity

1 Sverdrup = 10 6 m 3 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)  t (S,T) ranges from 20 to 40 –Similarly, sigma-theta:   =  (S, ,0)

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

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