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EVAT 554 OCEAN-ATMOSPHERE DYNAMICS

EVAT 554 OCEAN-ATMOSPHERE DYNAMICS. THERMOHALINE CIRCULATION (CONTINUED). LECTURE 20. Meridional Overturning Circulation. MORE REALISTIC MODEL (Marotzke et al, 1988). Assume the steady state horizontal momentum balance. Zonally averaging across a given basin yields,.

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EVAT 554 OCEAN-ATMOSPHERE DYNAMICS

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  1. EVAT 554OCEAN-ATMOSPHERE DYNAMICS THERMOHALINE CIRCULATION (CONTINUED) LECTURE 20

  2. Meridional Overturning Circulation MORE REALISTIC MODEL (Marotzke et al, 1988) Assume the steady state horizontal momentum balance Zonally averaging across a given basin yields,

  3. Meridional Overturning Circulation These can be combined to yield: Ignore explicit rotation, approximating the meridional momentum equation as, (Ad hoc “parameterization”) We then have,

  4. Meridional Overturning Circulation These can be combined to yield: Ignore explicit rotation, approximating the meridional momentum equation as, (Ad hoc “parameterization”) We then have,

  5. Meridional Overturning Circulation Invoke hydrostatic relationship (will need convective adjustment!) Invoke linear equation of state

  6. Meridional Overturning Circulation Define “meridional overturning” Streamfunction Note that there is no time dependence in this equation! The time dependence comes from the temperature and salinity equations

  7. Meridional Overturning Circulation Define “meridional overturning” Streamfunction Note that there is no time dependence in this equation! The time dependence comes from the temperature and salinity equations The last term in each case represents explicit convective adjustment

  8. Meridional Overturning Circulation Define “meridional overturning” Streamfunction ImposeBoundary Conditions and integrate forward in time Equilibrate with restoring surface boundary conditions kv T/z=K[T(y)- Ts] kv S/z=K[S(y)- Ss] Note that there is no time dependence in this equation! The time dependence comes from the temperature and salinity equations

  9. Meridional Overturning Circulation Pole Equator Pole Define “meridional overturning” Streamfunction ImposeBoundary Conditions and integrate forward in time Equilibrate with restoring surface boundary conditions kv T/z=K[T(y)- Ts] kv S/z=K[S(y)- Ss] Steady state circulation is symmetric under these boundary conditions

  10. Meridional Overturning Circulation Define “meridional overturning” Streamfunction ImposeBoundary Conditions and integrate forward in time Switch over to mixed boundary conditions kv T/z=K[T(y)- Ts] kv S/z=Q(y) Pole Equator Pole Symmetric circulation is unstable with respect to infinitesimal perturbations

  11. Meridional Overturning Circulation Even MORE realistic model (Wright and Stocker, 1991) Assume the steady state horizontal momentum balance Zonally averaging across a given basin yields,

  12. Meridional Overturning Circulation • More realistic parameterization Zonally averaging across a given basin yields, Even MORE realistic model (Wright and Stocker, 1991) • Resolve individual basins • Include surface windstress forcing • Non-linear equation of state • Equilibrate with mixed b.c.s

  13. Meridional Overturning Circulation • More realistic parameterization Even MORE realistic model (Wright and Stocker, 1991) • Resolve individual basins • Include surface windstress forcing • Non-linear equation of state • Equilibrate with mixed b.c.s

  14. Meridional Overturning Circulation Temperature Salinity Even MORE realistic model (Wright and Stocker, 1991)

  15. Meridional Overturning Circulation OGCM The most realistic ocean model is the ocean general circulation models (OGCM) Some OGCMs support the instability of the THC to future climate change

  16. Meridional Overturning Circulation OGCM Collapse of Thermohaline Circulation in Response to High-Latitude Freshening Associated with High-latitude Ice Melt

  17. Meridional Overturning Circulation Possible “Ice Age” consequences? Collapse of Thermohaline Circulation in Response to High-Latitude Freshening Associated with High-latitude Ice Melt OGCM

  18. Meridional Overturning Circulation Possible “Ice Age” consequences?

  19. 2xC02 Meridional Overturning Circulation 4xC02 GFDL COUPLED MODEL Possible “Ice Age” consequences?

  20. Meridional Overturning Circulation NORTH ATLANTIC OSCILLATION For the hemisphere on the whole, the warming or cooling due to the NAO is probably a zero-sum game, but regional influences are large Explains enhanced warming in certain regions of Northern Hemisphere in past couple decades

  21. Meridional Overturning Circulation NORTH ATLANTIC OSCILLATION North Atlantic Ocean and Atmosphere are Coupled

  22. Meridional Overturning Circulation Heat Flux and Surface Wind Anomalies Associated with Positive Phase of “NAO” NORTH ATLANTIC OSCILLATION Positive NAO implies increase in THC Delworth, T.L., and Dixon, K.W., Implications of the Recent Trend in the Arctic/North Atlantic Oscillation for the  North Atlantic Thermohaline Circulation, Journal of Climate: Vol. 13, No. 21, pp. 3721­3727, 2001.

  23. Meridional Overturning Circulation NORTH ATLANTIC OSCILLATION Positive NAO implies increase in THC THC response to Imposed NAO anomaly Delworth, T.L., and Dixon, K.W., Implications of the Recent Trend in the Arctic/North Atlantic Oscillation for the  North Atlantic Thermohaline Circulation, Journal of Climate: Vol. 13, No. 21, pp. 3721­3727, 2001.

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