Mechanisms of ocean-atmosphere interactions

Arnaud CzajaSPAT PG lecture Nov. 2011

Atmosphere “feels” the ocean through:

• Momentum exchanges (friction)

• Surface heat exchanges

Q atmosphere

ocean

To

Atmosphere “feels” the ocean through:

• Momentum exchanges

• Surface heat exchange

Q atmosphere

ocean

To

Radiative+

“sensible”+

“latent”

Ocean – atmosphere heat exchange

• Radiative = solar (up and down) and longwave (up and down). Weakly dependent upon To and hence upon ocean currents, thermocline depth, etc.

• Sensible & latent: bring the surface atmosphere into thermodynamic equilibrium with the ocean (Ta=To and RH = 100%). Very strongly dependent upon To.

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 an 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

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)

“Gill’s response” to heating (1980)

Relevance of Hoskins/Karoly?

• Tropics: condensational heating occurs in cloud clusters and is balanced partly by upward motion in clouds and radiative cooling in the subsiding air in between clouds

hard to know what to choose for Q!

Relevance of Hoskins/Karoly?

• Midlatitudes: does not take into account the weather (travelling baroclinic waves)...

Friday session...