mechanisms of ocean-atmosphere interactions

Mechanisms of ocean-atmosphere interactions

Arnaud CzajaSPAT PG lecture Oct. 2012

Atmosphere “feels” the ocean through:

• Sea surface friction

• Sea surface heating/cooling:

Q atmosphere

ocean

To

Atmosphere “feels” the ocean through:

• (Sea surface friction)

• Sea surface heating/cooling:

Q atmosphere

ocean

To

Radiative+

“sensible”+

“latent”

Ocean – atmosphere heat exchange

• Radiative = solar (up and down) and longwave (up and down). Net heating of the ocean.

• Sensible & latent: bring the surface atmosphere into thermodynamic equilibrium with the ocean (Ta=To and RH = 100%). Net cooling of the ocean.

NB: following slides show these fluxes from the ERA40 atlas website (very useful web resource)

Net longwave ↓+↑ flux climatology (Wm-2, positive downwards)

Annual mean

DJF JJA

Net shortwave ↓+↑ flux climatology (Wm-2, positive downwards)

Annual mean

DJF JJA

Sensible heat flux climatology (Wm-2)

Annual mean

DJF JJA

Latent heat flux climatology (Wm-2)

Annual mean

DJF JJA

Net heat flux climatology (Wm-2)

Annual mean

DJF JJA

Over which atmospheric layer is Q=Qlat+Qsen felt?

• For given To, destabilize atm by advecting a thermodynamically unbalanced air mass

To

advection

Buoyancy offree atm (bf)

Buoyancy ofatm affected

by Q(bs)QTa, qa

Over which atmospheric layer is Qlat+Qsen felt?

• At same To, could observe very different atmospheric responses

advection

Situation whenbs ≈bo << bf

Ta, qa

To

Shallow perturbation

Over which atmospheric layer is Qlat+Qsen felt?

• At same To, could observe very different atmospheric responses

advection

Situation when bs ≈ bo ≈ bf

Ta, qa

To

Deep perturbation

Atm. heating rate climatology (K/day)

Annual mean

DJF JJA

Global Infrared image

Tropics: “spotty” (convection organized in random clusters)Extra-Tropics: “wavy” (convection organized by Jet Stream meanders)

DJF2004

JJA2004

FracFraction of days with (Stp-Ssb < 0) in color & time mean SST (CI = 2⁰C)

DJF 2003-04

JJA 2004ERAint data

Response of the atmosphere to a prescribed heating / cooling

• Linear calculations on the board

• Follows the classic paper by Hoskins and Karoly (J. Atm. Sci., 1981)

Q6

Response of the atmosphere to a prescribed heating / cooling

“Gill’s response” to heating (1980)

Q7

Friday session...