Core Lecture 2 - Why does the ocean have layers and why does it move?

Where are warmest sea surface temps

near equator

what does seawater experience at surface of ocean

• heating from sun

• cooling by conduction, radiation, and evaporation

• freshwater exchange

• wind stress

heating from sun

• more heat, molecules have more kinetic energy and move faster

• causes seawater to expand

• decreases its density

• happens more at tropics than poles

cooling by conduction, radiation, evaporation

• less heat, molecules move slower

• causes water to contract

• increases its density

freshwater exchange

• precipitation and river runoff → fresher surface layer

• evaporation → increases salinity

• freezing/melting sea ice change salinity

wind stress

energy transferred from winds to the ocean can move water

what causes thermohaline circulation

• heating from sun

• cooling by conduction, radiation, and evaporation

• freshwater exchange

how does temp affect liquid water density

• warm water is less dense, floats on cold water

• since cold water is denser, sinks in warm water

global distribution of average sea surface salinity

• strong gradients from eq to poles

• highest in tropics, lower at equator because its rainier

• Mediterranean → hot, really dry so lots of evaporation → saltier

what is practical salinity scale

• in 1000 g of seawater there are 25g of salt

• pss is reported as just 35

• unitless

how does salinity affect water density

• adding salt increases the total mass of the liquid while barely increasing volume

• makes water denser

how do temperature and salinity vary with depth

• temp decreases

• salinity increase

• relatively thin surface layer overlies large deep layer

what is stable stratification

light water lies above denser water

thermocline

depth range where temp changes most rapidly

halocline

depth range where salinity changes most rapidly

stratification

the natural unperturbed state of lighter fluid overlying denser fluid

mixing

the process of stirring and exchanging of seawater properties down to a molecular level to reach some combined mixed state

why does stratification matter

• limits the exchange of properties between layers

• particularly between deep ocean and surface layer

ocean station papa history

• one of oldest oceanographic sampling programs

• started as ocean weather station from 1949-1981 to support weather forecasting

• pre-satellite era

ocean station papa modern use

oceanographic survey line of 26 stations (line p) managed by fisheries and oceans canada

sigma t

• oceanographers report density as units of kg/m³

• actual density - 1000kg/m³

summer conditions along line p

• surface heating increases stratification

• produces shallow mixed layer

winter conditions along line p

• surface cooling and stronger wind mixing reduce stratification

• produces deeper mixed layer

seasonal changes of stratification

• more changes as you go further latitudes

• in tropics - not as much difference

thermohaline circulation

• density driven circulation

• redistributes heat globally

• moves surface water into deep ocean

how do differences in density make the ocean move

• if surface water is denser, it sinks

• situations with cold water over warm or salty water over fresh can lead to sinking and mixing

how does thermohaline circulation work

• heating at equator causes surface water to flow toward poles

• deep water must return to surface by slow vertical mixing

• pushes from behind, water moves slowly through deep ocean

• surface water becomes cold in polar regions and sinks

• where sea-ice forms, you get very cold and salty water

how long does it take for a parcel of water to complete entire global circuit

1000-2000 years

flow of thermohaline circulation

• weak

• 1-5 cm/s

solar radiation

the distribution of heat from the sun is not equal across the earth’s surface

solar radiation - winds

• hot air rises at the eq, flows to poles and sinks

• area of distribution is bigger at poles than eq

atmospheric circulation on non-rotating earth

• simple eq to pole circulation

coriolis force

• deflects rising and poleward moving air masses

• turns them into prevailing winds like trade winds and westerlies

• NH: deflection ro right

• SH: deflection to left

• eq: no deflection

global wind patterns

• prevailing winds exert stress on the ocean surface

• transfers momentum to the water

5 main ocean gyres

• large systems of rotating ocean currents driven by global winds and shaped by earth’s rotation and continents

• continents block flow, except around antarctica

• north pacific, indian, south pacific, north atlantic, south atlanticwhy

why do gyres rotate clockwise in Nh and counterclockwise in SH

• combination of coriolis, global wind patterns, and positions of continents