EVAT 554OCEAN-ATMOSPHERE

DYNAMICS

THERMOHALINE CIRCULATION (CONTINUED)

LECTURE 20

Meridional Overturning Circulation

MORE REALISTIC MODEL (Marotzke et al, 1988)

2

2v/ˆ1u

zp

af V

2

2/

cosˆ1v

z

upa

f V

Assume the steady state horizontal momentum balance

20

2

00

v/

ˆ1u

zp

af V

Zonally averaging across a given basin yields,

20

2

0

u

cosˆ

)()(v

za

ppf V

WE

Meridional Overturning Circulation

These can be combined to yield:

yzzWE p

a

ppff VV

zzzz 002

02

cosˆ

)()(vv

20

2

00

v/

ˆ1u

zp

af V

20

2

0

u

cosˆ

)()(v

za

ppf V

WE

Ignore explicit rotation, approximating the meridional momentum equation as,

(Ad hoc “parameterization”)

20

2

0

v/

ˆ1

zAp

a

yzzzzzzpA

00v

We then have,

Meridional Overturning Circulation

These can be combined to yield:

yzzWE p

a

ppff VV

zzzz 002

02

cosˆ

)()(vv

Ignore explicit rotation, approximating the meridional momentum equation as,

(Ad hoc “parameterization”)

20

2

0

v/

ˆ1

zAp

a

yzzzzzzpA

00v

We then have,

yzzzzpA

00v

Meridional Overturning Circulation

yzzzzpA

00v

Invoke hydrostatic relationship (will need convective adjustment!)

gpz 00

ygA

0

0zzzv

)1(0

ST

Invoke linear equation of state

yS

yT

Ag

zzz0v

yAg

0

0zzzv

Meridional Overturning Circulation

yS

yT

Ag

zzz0v

Define “meridional overturning” Streamfunction

0vz

0wy

yS

yT

Ag

zzzz

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

0vz

0wy

yS

yT

Ag

zzzz

Note that there is no time dependence in this equation!

The time dependence comes from the temperature and salinity equations

]conv[//v/20

2

00000q

z

TkdzdTwdydTdtT

]conv[//v/20

2

00000q

z

SkdzdSwdydSdtS

The last term in each case represents explicit convective adjustment

Meridional Overturning Circulation

Define “meridional overturning” Streamfunction

0vz

0wy

yS

yT

Ag

zzzz

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)- Ts]

v S/z=K[S(y)- Ss]

]conv[//v/20

2

00000q

z

TkdzdTwdydTdtT

]conv[//v/20

2

00000q

z

SkdzdSwdydSdtS

Meridional Overturning Circulation

Define “meridional overturning” Streamfunction

0vz

0wy

Impose Boundary Conditions and integrate forward in time

Equilibrate with restoring surface boundary conditions

v T/z=K[T(y)- Ts]

v S/z=K[S(y)- Ss]

Steady state circulation is symmetric under these

boundary conditions

Pole Equator Pole

Meridional Overturning Circulation

Define “meridional overturning” Streamfunction

0vz

0wy

Impose Boundary Conditions and integrate forward in time

Switch over to mixed boundary conditions

v T/z=K[T(y)- Ts]

Pole Equator Pole

v S/z=Q(y)

Symmetric circulation is unstable with respect to infinitesimal

perturbations

Meridional Overturning Circulation

2

2v/ˆ1u

zp

af V

2

2/

cosˆ1v

z

upa

f V

Assume the steady state horizontal momentum balance

20

2

00

v/

ˆ1u

zp

af V

20

2

0

u

cosˆ

)()(v

za

ppf V

WE

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 ddppp

WE/)()(

0

•Resolve individual basins

•Include surface windstress forcing

•Non-linear equation of state

•Equilibrate with mixed b.c.s

20

2

00

v/

ˆ1u

zp

af V

20

2

0

u

cosˆ

)()(v

za

ppf V

WE

Zonally averaging across a given basin yields,

Meridional Overturning Circulation

Even MORE realistic model (Wright and Stocker, 1991)

•More realistic parameterization ddppp

WE/)()(

0

•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 CirculationThe 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 CirculationPossible “Ice Age” consequences?

Meridional Overturning CirculationPossible “Ice Age” consequences?

2xC02

GFDL COUPLED MODEL

4xC02

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 OSCILLATIONMeridional Overturning Circulation

Meridional Overturning CirculationNORTH ATLANTIC OSCILLATION

North Atlantic Ocean and Atmosphere are Coupled

Meridional Overturning CirculationNORTH 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. 3721 3727, 2001.

Meridional Overturning CirculationNORTH 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. 3721 3727, 2001.

THC response to Imposed NAO anomaly