Circulations and currents

ocean circulation and currents

wind driven currents

results from

- wind dragg=skin drag + form dragg

-pressure gradient

-coriolis deflection

large scale wind systems set the ocean in motion and create circulation gyres

these also dictate the circulation gyres

Coriolis Deflection

the amount of deflection depends on the speed and latitude

ekmen transport

this is critical for wind driven circulation - with persistent winds the motion of the water created by drag is transferred downwards

speed decrease with depth with coriolis defelcetion at

all levels = ekmen spiral

upwelling and downwelling

important for surface water currents and not restricted to the edges of continents - this is driven by ekmen transport.

this is also known as divergence and convergence of rising and sinking of water

Geostrophic model of the ocean gyres

depends on winds, ekmen, coriolis, convergence/ pressure gradient

- dynamic equilibrium

-Geostrophic flow is a flow which the pressure gradient force and the coriolis force are in balance

-Gyres flow parallel to zonal winds due to Geostorphy not wind drag

Refinement of the model

landmass effects barriers = closed rotating circulation loop this leads to a asymetrical pattern ie the gulf stream and canary current

this is western boundary intensification

western boundary intensification

the coriolis force increase with latitude and this leads to an intensification of western boundary flows

Temperature and salinity

water and density

decrease

-heating via solar radiation and condensation

-freshwater addition via precipitation,river input and ice melt

increase

-cooling via cold winds and evaporation

-freshwater removal via evaporation and ice formation

temperature is a bigger effct then presure

these are slower wind driven currents

oceans show layered structure

warmer = on top

colder = underneath

exceptions occur at high latitudes where salinity effects are important

in summer there is a pronounce thermocline = shallow surface layer and in winter it almost disappears = deep surface layer

water column stability

this relates to the vertical density gradient

stabeling layer: density increase with depth

neutral layering: no density changes with depth

unstable layering: density decrease with depth

the presence of a cline indicates that the water column is stable and resists mixing

depth structure

Mixed surface layer (50m or 100m) - heated by solar radiation and mixed due to wind stirring and convection

main thermocline (100m - 1000m) - zone of rapid temperature change causing density changes and separates the layers

deep water - only gradual changes in temperature and salinity = little changes in density

density difference causes sinking = thermocline circulation

higher latitudes have more unstable layering

Thermohaline circulation

water temperature decrease with increased latitude and depth

so at high latitudes cold air cools the water causing it to raising in density - travels to the equator and as it meets warmer water they water descends underneath and travels horizontally

deep, subsurface currents arise from density differences - variations in temperature and salinity.

dense water underrides less dense water

water masses

in the deep water ocean:

-slow changes in salinity and temperature

-slow mixing of diff water masses

-water masses retain their distinct temperature and salinity values for long periods of times = quasi-conservative behaviour

climate change

deep water formation in the North Atlantic resulting from colling which daws warm water northward as a density- driven flow may be reduced by reduced cooling and more freshwater input - leading t weakening of the over turning circulation - leading to a 3 to 6 dgrees temperature drop in europe and america

salinity decrease in the NA - lower seawater density which may prevent deep water formation

Global ocean conveyor belt