Rotating Fluid -Part II A “GFD view” of the Ocean and the Atmosphere (a follow up Raymond’s Lectures) Arnaud Czaja
–see Raymond’s lectures Source / sink flows –see Raymond’s lectures “Basin” “Channel”
–see Raymond’s lectures Source / sink flows –see Raymond’s lectures “Basin” “Channel” No distinction between Ocean & Atmosphere…
Central idea Constraint 1: Ocean & Atmosphere are rapidly rotating fluids: geostrophy is the leading order dynamics. Constraint 2: The two fluids must transport energy poleward (cold parcels move equatorward and warm parcels poleward)
Central idea This brings a key distinction between basins (~ocean) and channel (~atmosphere)’s geometry: Basins: walls provide dP/dx and a large scale (eddy free) geostrophic heat transport is possible. Channels: no zonally integrated dP/dx and the heat transport must involve eddies and / or ageostrophic effects (e.g., Hadley cell).
Outline The energy constraint Basin dynamics Channel dynamics
The energy constraint
impinging at low than high The energy constraint Geometry: more energy impinging at low than high latitudes
Assume infra-red radiation and albedo is uniform Observations ASR IR Assume infra-red radiation and albedo is uniform Observations Stone, 1978.
The energy constraint
The energy constraint Poleward motion in ocean & atmosphere
Basin: Northern Oceans, Atmosphere Background Geostrophic mass transport calculation Heat transport Complications…
oxygen distribution at 2500m A classic: oxygen distribution at 2500m (from Wüst, 1935).
oxygen distribution at 2500m A classic: oxygen distribution at 2500m (from Wüst, 1935). -Spreading from high latitude North Atlantic source region -Large spatial scale of `tongue’ considering the narrowness of ocean currents
More recent section along the `great tongue’
The “great oceanic conveyor belt”
The “great oceanic conveyor belt”
Broecker, 2005 NB: 1 Amazon River ≈ 0.2 Million m3/s
Atlantic ocean’s meridional overturning streamfunction NB: From an OGCM constrained by data (Wunsch, 2000)
Can we measure the ocean circulation in basins using the Geostrophic calculation? All you need is the thermal wind: Coriolis parameter East-west density gradient North-South velocity Gradient with height
Global “inverse” ocean circulatioin and heat transport Ganachaud and Wunsch, 2003
RAPID – WATCH array at 26N
RAPID array calculation
RAPID array calculation
Blackboard calculations…
Heat Transport Up Warm water North Cold water 26N East
yields Ho≈1PW as required Heat Transport Up Mo ≈ 20 Sv & Δθ≈10K yields Ho≈1PW as required Warm water North Cold water 26N East
Are there basins in the atmosphere? Z Density profile H~7km X OCEAN ATMOSPHERE
Different situation in the Tropics Trade wind inversion 2-3km … “isolated” low level layer
East-African Highlands & the Indian Monsoon Orography Northward flow across the equator
Low level winds climatology (June-August) ERA40 Atlas
Channel: Atmosphere, Southern Ocean How to satisfy the energy constraint In a geometry in which <dP/dx> = 0? Hadley cell Oceanic & atmospheric eddies
Zonally averaged atmospheric circulation (annual mean) ~100Sv NB: Ocean: ~10-20Sv
Zonally symmetric motions are the key energy carriers in the Tropics Total Zonally symmetric motions are the key energy carriers in the Tropics Transient eddies Stationnary eddies Axisymmetric motions
Zonally averaged atmospheric circulation (annual mean) Ω Eq df/dy max at equator Frictional effects dominate
Zonally averaged atmospheric circulation (annual mean) Inertial effects dominate
Critical (moist) temperature distributions leading to the onset of Hadley cell Emanuel (1995)
Poleward heat transport in Hadley cell –see Q3 High gz Low gz
Eumetsat/MetOffice infrared picture (daily composite)
Eddy motions are the key energy carriers in midlatitudes Total Eddy motions are the key energy carriers in midlatitudes Transient eddies Stationnary eddies Axisymmetric motions
Ocean eddies: the Movie
Ocean eddy heat transport from a ¼ º ocean GCM Total heat transport Eddy heat transport From Jayne & Marotzke (2002)
P T V “Shallow” Ocean (heat trspt ≠0) “Deep” Ocean (heat trspt=0) Height V Longitude