Overturning Circulation

• stirring


  input of mechanical energy
• stirring can lead to turbulence
• turbulence


  violent of unsteady motion
• turbulence can lead to mixing
• viscosity 


  resistance of a fluid
• turbulence can lead to mixing if there is something to mix
• turbulence and mixing drives large scale movement
• mixing can be driven by the tide
• meridional overturning circulation


  water that circulates from north to south and back in long cycles
• Meridional overturning current

  
• meridional imbalance between surface radiation
• more heat at the equator then at the poles
• horizontal heat fluxes help move cold and warm water
• sandstroms theorem 


  unless there is an input of mechanical energy, thermal circulation can cause vigorous, steady circulation only if heating occurs at greater depths than cooling
• heat is mixed downwards and water mixed upwards
• the flow of water upwards must be balanced by the horizontal and vertical flow elsewhere
• 2 terra watts of energy required to lift water upwards
• 10^4 m2/s needed to maintain stratification
• half of energy come from wind, other half comes from tides
• deep water is made at poles
• water at poles is salty and sinks and spreads out and equatorwards
• mixing drives vertical flow to close the loop
• main heat source in the atmosphere is in the ocean
• adding of freshwater from glacial melt
• dropping long term temperature by 4C will cause glaciation
• the upper and intermediate layers of water in the Labrador sea directly determine the rate of the main atlantic gyre circulation
• Labrador Sea is increasing in freshwater causing the steric height to increase by 8-10cm
• labrador sea has a deepening convection due to input of freshwater
• salinities have decreased there almost linearly by 0.01 per decade since the mid 1970s in the north atlantic deep water
• freshening rate of eastern overflow maintained downstream by mixing waters that were freshening at an equal or greater rate
• faroe bank overflow has become warmer and less saline
• faroe weaking weakened global thermohaline circulation and reducing of Atlantic water to the Nordic seas
• Faroe Bank channel overflow has become less intensive and less dense
• AMOC has been decreasing by 0.5 Sv per 10 years