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 Chapters 1 & 2 of Tomzcak ’ s readings

What is Seawater?

• Seawater is 96.5% 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

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 r = f(S,T,p) = [kg m -3 ] r (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 Dr = +1 kg m -3 for D T = -5 C

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 • Salinity is in practical units (psu) • Often bottles are used

Salinity • Salinity varies from 32 to 37 psu • Good water mass tracer • Lower/higher values are unusual (riverine, huge evaporation, etc.) • Rule of thumb Dr = +1 kg m -3 for D S = +1 psu

Typical T & S Profiles Features Mixed layer Thermocline Halocline

CalCoFI

Temperature

Salinity

Line 80 Line 90 CalCoFI Cruise 9804

Line 90

Off Pt Arena Off San Clemente Off Punta Baja

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 • Rules of thumb 1 db pressure ~ 1 m depth Dr = +1 kg m -3 for D p = +100 db

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 q

Potential Temperature

Potential Temperature 0.05 C at 500 m

Ocean Distribution of q & S Mean ocean q ~ 4 C & S ~ 34.8 psu

Seawater Density • Equation of state r = f(S,T,p) = [kg m -3 ] r (S,T,p=0) range from 1020 to 1040 kg m -3 • Shorthand sigma-t: s t = r (S,T,0) - 1000 s t (S,T) ranges from 20 to 40 – Similarly, sigma-theta: s q = r (S, q ,0) - 1000

T-S Diagram (full range) Freezing Max density

T-S Diagram (typical range)

Density Profile

Temperature Inversion

Density Profile

Line 80 Line 90 CalCoFI Cruise 9804

Coastal Upwelling Changes Density

Density for Line 90

Review • Fundamental seawater properties – Salinity, temperature & pressure • Density is the important variable – in situ density – Sigma-t – Sigma q r (S,T,p) r (S,T,0) – 1000 r (S, q ,0) – 1000

Review • Rules of thumb -> Dr = +1 kg m -3 D T = -5C, D S = 1 psu or D p = 100 db • Global surface T & S driven largely by air-sea exchanges • Dense water sinks… now we're talking dynamics

Geostrophy • Geostrophy describes balance between horizontal pressure & Coriolis forces • Relationship is used to diagnose currents • Holds for nearly all large scale motions in sea • Need to know horizontal pressure gradients…

Horizontal Pressure Gradients Mass same @ stations A & B (bracket same isobars) q r A If r A > r B -> Height column @ B higher than @ A A Pressure gradient will push from B to A Measure by slope of sea surface – tan q B r

B

Horizontal Pressure Gradients • Horizontal density differences lead to horizontal pressure gradients • Provides the “ push ” for ocean circulations • Something must balance the pressure gradient push

Geostrophy • What balance HPF?

• Coriolis force!!!!

Earth Rotation

• Motions in a rotating frame will appear to deflect to the right (NH) • Deflection will be to the right in the northern hemisphere & to left in southern hemisphere • No apparent deflection on the equator

Geostrophic Relationship • Balance: Coriolis force = fu HPF = g tan q • Geostrophic relationship: u = (g/f) tan q • Know f (= 2 W sin f ) & tan q , calculate u

Estimating tan q • Need to slope of sea surface to get at surface currents • New technology - satellite altimeters can do this with high accuracy • Traditional methods use sea water properties – our focus

Mapped SSH Satellite altimeter Direction is along D ’ s in SSH NOT ACROSS!!

Mapped SSH Satellite altimeter Direction is along D ’ s in SSH NOT ACROSS!!

Hydrostatic Pressure • Hydrostatic pressure is simply the weight of water acting on a unit area at depth • Mass seawater in column = r A D – A = cross-sectional area of column [m 2 ] – D = depth of water column [m] • Weight column = ( r A D) * g – Mass * acceleration gravity (g = 9.8 m s -2 )

Hydrostatic Pressure • Hydrostatic pressure is the weight per unit area • p h = r g A D / A p h = r g D • It ’ s the D we want… p h = r g D D

Dynamic Height • Hydrostatics give us p h = r g D • Given isobars (p h ) & average r , D represents the dynamic height or D(0/1000 db) = p h / (g r (0/1000 db))

Dynamic Height • Dynamic height anomaly, D D(0/1500db)

Dynamic Height

Dynamic Height • California Cooperative Fisheries Investigations (CalCoFI) • Understand ocean processes in pelagic fisheries • Started in 1947

Dynamic Height • January 2000 - CalCoFI Cruise 0001

Dynamic Height • D D(0/500db) • Shows CA Current • Recirculation in the SoCal Bight

Geostrophy • Flow is along lines of constant dynamic height (light on the right) • Baroclinic portion (nearly all) can be diagnosed from CTD surveys • Quantifies the circulation of upper layers of the ocean

Geostrophy • Geostrophy describes balance between horizontal pressure & Coriolis forces • Relationship is used to diagnose currents • It is quantitative & valid for most large scale motions in sea

Ekman Transport • Ekman transport is the direct transport of seawater wind driven • Boundary layer process • Steady balance among the wind stress, vertical eddy viscosity & Coriolis forces • Story starts with Fridtjof Nansen [1898]

Fridtjof Nansen • One of the first scientist-explorers • A true pioneer in oceanography • Later, dedicated life to refugee issues • Won Nobel Peace Prize in 1922

Nansen ’ s Fram • Nansen built the Fram to reach North Pole • Unique design to be locked in the ice • Idea was to lock ship in the ice & wait • Once close, dog team set out to NP

Fram Ship Locked in Ice

1893 -1896 - Nansen got to 86 o 14 ’ N

Ekman Transport • Nansen noticed that movement of the ice locked ship was 20-40 o to right of the wind • Nansen figured this was due to a balance among friction, wind stress & Coriolis forces • Ekman did the math…

Ekman Transport Motion is to the right of the wind

Ekman Transport • The ocean is like a cake • Each layer is accelerated by the one above it & slowed by the one beneath it • Top layer is driven by wind • Transport of momentum into interior is inefficient

Ekman Spiral • Top layer balance of t w , friction & Coriolis • Layer 2 dragged forward by layer 1 & behind by layer 3 • Etc.

Ekman Spirals • Ekman found an exact solution to the structure of the Spiral • The layer of frictional control is called the Ekman layer (~50 m) • The details of the spiral do not turn out to be important

Ekman Transport • Ekman transport describes the direct wind-driven circulation • Ekman transports will be right of wind in the northern hemisphere • Simple & robust diagnostic calculation

Wind Driven Circulation in the California Current • Seasonal changes of equatorward winds drive circulation of the CA Current • Winds alter offshore Ekman transports • This in turn drives upwelling processes • AND … geostrophic flows

Seasonal Winds

Seasonal Wind Stress

Seasonal Wind Stress

Coastal Upwelling

Coastal Upwelling • Equatorward winds along a coastline lead to offshore Ekman transport • Mass conservation requires these waters replaced by cold, denser waters • Brings nutrients to the surface increasing rates of primary production • Creates dynamic height gradients - currents

Coastal Upwelling

Coastal Upwelling

Coastal Upwelling

CalCoFI Cruise 9804

Coastal Upwelling Changes Density Line 90

Density & Currents for Line 90 Negative currents to south

Seasonal Wind Stress

Seasonal Circulation

Seasonal Circulation

Seasonal Circulation

Seasonal Circulation

Seasonal Circulation

Seasonal Circulation