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,

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

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

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

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

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

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 Impose Boundary Conditions and integrate forward in time Equilibrate with restoring surface boundary conditions  v  T/  z=K[T(y)- T s ]  v  S/  z=K[S(y)- S s ]

Meridional Overturning Circulation Define “meridional overturning” Streamfunction Impose Boundary Conditions and integrate forward in time Equilibrate with restoring surface boundary conditions  v  T/  z=K[T(y)- T s ]  v  S/  z=K[S(y)- S s ] Steady state circulation is symmetric under these boundary conditions Pole Equator Pole

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

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

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

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

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

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

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

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

Meridional Overturning Circulation Possible “Ice Age” consequences?

Meridional Overturning Circulation Possible “Ice Age” consequences? 2xC0 2 GFDL COUPLED MODEL 4xC0 2

Explains enhanced warming in certain regions of Northern Hemisphere in past couple decades 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 NORTH ATLANTIC OSCILLATION Meridional Overturning Circulation

NORTH ATLANTIC OSCILLATION North Atlantic Ocean and Atmosphere are Coupled

Meridional Overturning Circulation NORTH ATLANTIC OSCILLATION Positive NAO implies increase in THC Heat Flux and Surface Wind Anomalies Associated with Positive Phase of “NAO” 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.

Meridional Overturning Circulation 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, THC response to Imposed NAO anomaly