Atmosphere-ocean interactions
A. Pozzer
Int. Centre for Theoretical Physics - Max Planck Inst. for Chemistry
14/10/2011 Synopsis
Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange Coupling of Introduction GCMs
Conclusions Heat exchange Momentum exchange Mass exchange Models coupling Conclusions
A. Pozzer 14/10/2011 The Atmosphere:
Introduction The physical settings Heat exchange
Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Blue planet
Introduction The physical settings Heat exchange 29/01/1996, Galileo mission, Pacific Ocean view
Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Introduction The physical settings Ocean definition Heat exchange An ocean is a major body of saline water, and a principal Momentum exchange component of the hydrosphere. Only one ocean, divided in
Chemical air-sea oceans and seas: exchange Oceans: Coupling of GCMs Pacific ocean. Conclusions Atlantic ocean. Indian ocean. Southern ocean. Seas: Mediterranean (mostly surrounded by land, as the Artctic and the Carribean). Marginal (defined only by indentation, as Arabian sea).
A. Pozzer 14/10/2011 The physical settings
Introduction The physical settings Heat exchange
Momentum exchange
Chemical air-sea exchange
Coupling of GCMs The Oceans: Conclusions Cover 70.8% of the surface of earth. have horizontal dimensions (∼1500km-13000km) much larger than vertical dimensions (∼ 3-4km)
A. Pozzer 14/10/2011 Introduction The physical settings Heat exchange 180:1 and 30:1 cross sections of South Atlantic Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Introduction The physical settings Ocean gravity map Heat exchange
Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
Characteristics: Basin Mid Ocean Ridge Shelf Trench A. Pozzer 14/10/2011 Time scale
Introduction The physical settings Heat exchange
Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Atmosphere-Ocean interactions
Introduction The physical settings The sun and the atmosphere drive directly or indirectly Heat exchange almost all process in the ocean. Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Atmosphere-Ocean interactions
Introduction The physical settings Heat exchange
Momentum exchange
Chemical air-sea exchange
Coupling of GCMs The ocean is hence important for exchange of: Conclusions Heat Momentum Mass
A. Pozzer 14/10/2011 Introduction
Heat exchange Heat storage Heat transport Energy/Radiation at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange Exchange of heat Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Atmosphere-Ocean interactions
Introduction
Heat exchange Heat storage Heat transport Energy/Radiation at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Heat exchange
Introduction
Heat exchange Heat storage Heat transport Energy/Radiation at TOA Energy at Ocean’s surface Conclusions Momentum exchange The ocean is important because: Chemical air-sea exchange Large storage of heat in the ocean compared with the Coupling of land. GCMs
Conclusions Transport of heat polewards.
A. Pozzer 14/10/2011 Heat storage
Introduction
Heat exchange The specific heat of sea water Cp is ' 5 times the specific Heat storage Heat transport heat of soil/rocks. For the same variation in temperature the Energy/Radiation at TOA ocean stores ' 100 times more energy. Energy at Ocean’s surface Conclusions Momentum ∆E = Cpm∆T exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Earth’s energy balance
Introduction
Heat exchange Heat storage Heat transport Energy/Radiation at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Radiation at the Top of Atmosphere (TOA)
Introduction
Heat exchange NASA Earth Radiation Budget Experiment (ERBE) Heat storage Heat transport Energy/Radiation at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
Incoming radiation (SW) and outgoing radiation (LW) must A. Pozzer be balance in a solar year 14/10/2011 Radiation at the Top of Atmosphere (TOA)
Introduction
Heat exchange Heat storage Heat transport −2 Energy/Radiation Annual mean of incoming shortwave (net) [Wm ] at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Radiation at the Top of Atmosphere (TOA)
Introduction
Heat exchange Heat storage Heat transport −2 Energy/Radiation Annual mean of outgoing longwave [Wm ] at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Radiation at the Top of Atmosphere (TOA)
Introduction
Heat exchange Heat storage Heat transport Energy/Radiation at TOA Annual mean radiation (ERBE observations) Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
The differential heating between the low and the high latitude is the primary driving force of the atmospheric circulation.
A. Pozzer 14/10/2011 Radiation at the Top of Atmosphere (TOA)
Introduction
Heat exchange Meridional Heat Transport Heat storage Heat transport The poleward heat transport HT at a latitude φ can be Energy/Radiation at TOA estimated by integrating the net radiative balance at the Energy at Ocean’s surface Conclusions TOA from the South Pole to φ Momentum exchange Z 2π Z φ 2 Chemical air-sea HT (φ) = R (λ, φ) a cosφdλdφ = exchange 0 −π/2 Coupling of GCMs Z φ 2 Conclusions 2πa R (φ) cosφdφ −π/2
R (φ)
A. Pozzer 14/10/2011 Radiation at the Top of Atmosphere (TOA)
Introduction
Heat exchange Meridional Heat Transport Heat storage Heat transport Energy/Radiation φ at TOA Z Energy at Ocean’s 2 surface HT = 2πa R (φ) cosφdφ Conclusions −π/2 Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
BUT: This is the overall transport!!! What is the role of the Ocean A. Pozzer 14/10/2011 and the Atmosphere? Radiation at the Top of Atmosphere (TOA)
Introduction
Heat exchange Heat storage Heat transport Energy/Radiation at TOA Energy at Ocean’s surface Three methods available Conclusions Momentum Direct method, via observations of heat fluxes. exchange Residual method, calculating the atmospheric or oceanic Chemical air-sea exchange heat transport with numerical model or observations and Coupling of obtain the other with subtraction from the total heat GCMs
Conclusions transport at TOA. Surface flux method, calculating the radiation and heat terms at the ocean’s surface using the same method as the TOA. Estimates ocean’s heat transport.
A. Pozzer 14/10/2011 Earth’s energy balance
Introduction
Heat exchange Heat storage Heat transport Energy/Radiation at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Energy at Ocean’s surface
Introduction Changes in energy stored in the upper ocean result from Heat exchange Heat storage difference between input and output of heat through the sea Heat transport Energy/Radiation surface (heat flux). at TOA Energy at Ocean’s The total flux of energy into and out of the ocean must be surface Conclusions zero. Momentum The sum of the heat fluxes into or out of a volume of water is exchange the heat budget. Chemical air-sea exchange
Coupling of Major terms in the heat budget at the sea surface GCMs Insolation QSW , the flux of solar energy into the sea; Conclusions Net Infrared Radiation QLW , net flux of infrared radiation from the sea;
Sensible Heat Flux QS , the flux of heat out of the sea due to conduction;
Latent Heat Flux QL, the flux of energy carried by evaporated water;
A. Pozzer Advection QV , heat carried away by currents. 14/10/2011 Energy at Ocean’s surface
Introduction
Heat exchange Heat storage Heat transport Energy/Radiation at TOA Energy at Ocean’s surface Conclusions Q = QSW + QLW + QS + QL + QV Momentum exchange This Q is what defines the change in temperature in the Chemical air-sea Ocean’s surface. exchange
Coupling of 2 GCMs All unit in : W /m Conclusions The total flux of energy into and out of the ocean must be zero. Hence, for a year average we must have :
Q = 0
A. Pozzer 14/10/2011 Energy at Ocean’s surface
Introduction Heat exchange Incoming radiation (Insolation). Heat storage Heat transport Energy/Radiation at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
The number of standard atmosphere masses is designated by m. Hence m = 2 is applicable for sunlight when the sun is 30◦ above the horizon (sin(30◦) = 1/2). A. Pozzer 14/10/2011 Energy at Ocean’s surface
Introduction
Heat exchange Heat storage Heat transport Energy/Radiation at TOA Energy at Ocean’s surface Conclusions Factor influencing incoming radiation Momentum exchange Height of the Sun above horizon depending on latitude, Chemical air-sea season and time of the day. exchange Attenuation which depends on: Coupling of GCMs Clouds, which absorb and scatter radiation. Conclusions Path length through the atmosphere. Gases and aerosols which absorb and scatter radiation. Reflectivity of the surface (albedo).
A. Pozzer 14/10/2011 Energy at Ocean’s surface
Introduction
Heat exchange Heat storage Outgoing thermal radiation. Heat transport Energy/Radiation at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
Factor influencing outgoing radiation Cloud thickness/height. Atmospheric heat water content (more humid, less heat out). Surface (water) temperature. The blackbody irradiate (power emitted per unit area) as T 4 (Boltzmann’s law).
A. Pozzer 14/10/2011 Energy at Ocean’s surface
Introduction
Heat exchange Heat storage Heat transport Energy/Radiation at TOA Energy at Ocean’s Outgoing thermal radiation. surface Conclusions Water vapour and clouds influences the net loss of infrared Momentum exchange radiation more than surface temperature.
Chemical air-sea exchange
Coupling of Water vapour can change the net emitted radiance by GCMs 200%. Conclusions Change in temperature are in the order of 25◦ 273K < T < 298K Hence: 298/273 = 1.092 → (1.092)4 = 1.42 ONLY 42 % difference.
A. Pozzer 14/10/2011 Energy at Ocean’s surface
Introduction
Heat exchange Heat storage Heat transport Energy/Radiation at TOA Energy at Ocean’s surface Factor influencing latent heat fluxes: Conclusions wind speed. Momentum exchange relative humidity. Chemical air-sea exchange In the arctic, most of the heat lost from the sea through Coupling of GCMs leads (ice-free areas).
Conclusions Factor influencing sensible heat fluxes: wind speed. air-sea temperature differences.
A. Pozzer 14/10/2011 Energy at Ocean’s surface
Introduction Heat exchange Can we measure radiations? Heat storage Heat transport Energy/Radiation Easy with radiometers, sensitive from 0.3µ m to 50µ m with at TOA Energy at Ocean’s a 3% accuracy. They must be well calibrated and maintained. surface Conclusions Momentum exchange Can we measure latent and sensible heat fluxes? Chemical air-sea We need direct measurements of turbulent quantities in the exchange boundary layer (PBL). They must be: Coupling of GCMs On the surface layer (in the PBL). Conclusions By fast responds instrument (gust-probe). Must be measured all three wind components plus temperature and humidity. But they are expensive, and not valid for large areas.
To calculate the fluxes from practical measurements, we use observed correlations between fluxes and variable that can be A. Pozzer measured globally. 14/10/2011 Energy at Ocean’s surface
Introduction
Heat exchange Bulk formulas Heat storage Heat transport For fluxes of sensible and latent heat, the correlations are Energy/Radiation at TOA called bulk formulas. Energy at Ocean’s surface Conclusions QS = ρaCpCS U10 (Ts − Ta) Momentum exchange Chemical air-sea QL = ρaLE CLU10 (qs − qa) exchange
Coupling of GCMs ρ = density of air. Conclusions a −2 −1 Cp = specific heat capacity of air (1030 Jkg K ). −3 CS = sensible heat transfer coefficient (1.0 × 10 ).
U10 = wind speed at 10 meters.
Ts / Ta = temperature at 10 meters and at the surface. 6 LE = latent heat of evaporation (2.5 × 10 J/Kg). −3 CL = latent heat transfer coefficient (1.2 × 10 ).
A. Pozzer qa / qs = specific humidity of air at 10m and at sea level. 14/10/2011 Energy at Ocean’s surface
Introduction
Heat exchange Heat storage Heat transport Energy/Radiation at TOA Energy at Ocean’s surface Data sources/dataset available: Conclusions Momentum direct observations compilations for wind, sea surface exchange temperature (SST). Chemical air-sea Most known dataset is the International Comprehensive exchange Ocean-Atmosphere Data Set (ICOADS) Coupling of GCMs satellite observations: Conclusions insolation (with the assumption that the atmosphere has a constant absorption of short wave fluxes). wind speed and stress (Scatterometers via Braggs’ law). numerical models for any needed variable.
A. Pozzer 14/10/2011 Numerical model
Introduction Heat exchange All work performed with the EMAC model Heat storage Heat transport Energy/Radiation ECHAM5/MESSy for Atmospheric Chemistry at TOA Energy at Ocean’s surface www.messy-interface.org Conclusions Momentum exchange Main characteristics: Chemical air-sea exchange Basemodel: General circulation model ECHAM5 Coupling of (developed at the MPI for Meteorology in Hamburg). GCMs
Conclusions Chemistry submodels : MESSy, Modular Earth Submodel System (developed at the MPI for Chemistry in Mainz).
Hence: Global scale studies. Flexible for different studies on atmospheric chemistry. A. Pozzer 14/10/2011 Energy at Ocean’s surface
Introduction
Heat exchange Heat storage Heat transport −2 Energy/Radiation Annual mean of incoming shortwave [Wm ] at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Energy at Ocean’s surface
Introduction
Heat exchange Heat storage Heat transport −2 Energy/Radiation Annual mean of outgoing longwave [Wm ] at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Energy at Ocean’s surface
Introduction
Heat exchange Heat storage Heat transport −2 Energy/Radiation Annual mean of latent heat [Wm ] at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Energy at Ocean’s surface
Introduction
Heat exchange Heat storage Heat transport −2 Energy/Radiation Annual mean of sensible heat [Wm ] at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Energy at Ocean’s surface
Introduction
Heat exchange Heat storage Heat transport −2 Energy/Radiation Annual mean of NET HEAT FLUX [Wm ] at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Energy at Ocean’s surface
Introduction
Heat exchange Heat storage Heat transport −2 Energy/Radiation Zonal average of heat trasfer to the ocean [Wm ] at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Heat transport
Introduction
Heat exchange Heat storage Heat transport Poleward heat transport from EMAC model Energy/Radiation at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Conclusions
Introduction
Heat exchange Heat storage Heat transport Energy/Radiation at TOA Energy at Ocean’s Sunlight is absorbed by the tropical ocean. surface Conclusions Most of the heat is released as water vapor, which heat Momentum exchange the atmosphere when condenses. Chemical air-sea exchange The atmosphere transports most of the heat needed to ◦ Coupling of warm latitudes higher than 35 . The oceanic meridional GCMs transport is comparable to the atmospheric one only in Conclusions the tropics. Heat release by rain and absorbed infrared radiation from the ocean are the primary drivers for the atmospheric circulation.
A. Pozzer 14/10/2011 Introduction
Heat exchange Heat storage Heat transport Energy/Radiation at TOA Energy at Ocean’s surface Conclusions Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Introduction
Heat exchange
Momentum exchange Equations of motion Inertial currents Ekman layer Conclusions Chemical air-sea exchange
Coupling of GCMs Momentum exchange Conclusions
A. Pozzer 14/10/2011 Atmosphere-Ocean interactions
Introduction
Heat exchange
Momentum exchange Equations of motion Inertial currents Ekman layer Conclusions Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Introduction
Heat exchange
Momentum surface currents exchange Equations of motion Inertial currents Ekman layer Conclusions Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Equations of motion
Introduction
Heat exchange
Momentum exchange Equations of motion Inertial currents Newton’s second law (per unit mass): Ekman layer Conclusions Dv F Chemical air-sea = = fm exchange Dt m Coupling of GCMs Forces acting on a parcel of fluid: Conclusions 1 Pressure − ρ 5 p. Coriolis −2Ω × v. Gravity g
Friction Fr
A. Pozzer 14/10/2011 Equations of motion
Introduction Condensing one year course in one slide Heat exchange Momentum Navier-Stokes equations exchange Equations of motion Du 1 ∂p Inertial currents Ekman layer = − + f υ + Fx Conclusions Dt ρ ∂x Chemical air-sea exchange Dυ 1 ∂p = − − fu + Fy Coupling of Dt ρ ∂y GCMs Dw 1 ∂p Conclusions = − − g + F Dt ρ ∂z z f = 2Ω sin φ w << υ and 2Ω cos φ << g
Continuity equation From Boussinesq approximation (i.e. incompressible flows)
∂p ∂p ∂p A. Pozzer + + = 0 14/10/2011 ∂x ∂y ∂z
Four equations for four unknowns p,u,υ,w Equations of motion
Introduction
Heat exchange
Momentum exchange Equations of motion Inertial currents Ekman layer Conclusions Chemical air-sea exchange These equations are almost impossible to solve!
Coupling of Analitical solutions in very few cases GCMs with extreme semplifications! Conclusions
They must be solved numerically!
A. Pozzer 14/10/2011 Surface currents
Introduction
Heat exchange Solution of equation of motion, after an impulse.
Momentum exchange Equation of motion Equations of motion Inertial currents Du 1 ∂p Ekman layer = +f υ − + Fx Conclusions Dt ρ ∂x Chemical air-sea exchange Dυ 1 ∂p = −fu − + F Coupling of y GCMs Dt ρ ∂y Conclusions Dw 1 ∂p = −g − + F Dt ρ ∂z z
Hypothesis of no forces acting on the water (inertial motion):
No friction. No horizontal pressure gradient. Horizontal flow. A. Pozzer We search simple solution of these equations. 14/10/2011 Surface currents
Introduction
Heat exchange
Momentum exchange Equations of motion Solving the equations: Inertial currents 2 Ekman layer du 1 d υ Conclusions = − 2 Chemical air-sea dt f dt exchange d2υ Coupling of + f υ = 0 GCMs dt2 Conclusions Final solutions: u = V sin ft υ = V cos ft V 2 = u2 + υ2
A. Pozzer 14/10/2011 Surface currents
Introduction
Heat exchange Momentum Inertial current or inertial oscillation: exchange Equations of motion Inertial currents Ekman layer u = V sin ft Conclusions Chemical air-sea exchange υ = V cos ft Coupling of 2 2 2 GCMs V = u + υ Conclusions D = 2V /f , T = (2π)/f
Inertial oscillation for V = 20cm/s
latitude (ψ) D(km)T(hr) 90◦ 2.7 11.97 35◦ 4.8 20.87 10◦ 15.8 68.93
A. Pozzer 14/10/2011 Surface currents
Introduction
Heat exchange
Momentum exchange Equations of motion Inertial current or inertial oscillation: Inertial currents Ekman layer Conclusions Chemical air-sea exchange
Coupling of GCMs
Conclusions
Inertial currents in the North Pacific in October 1987
measured by drifting buoys drogued at a depth of 15 meters.
A. Pozzer 14/10/2011 Surface currents
Introduction
Heat exchange
Momentum exchange ◦ ◦ Equations of motion It is known that ice drift at an angle between 20 and 40 to Inertial currents Ekman layer the right of the blowing wind in the Arctic. Conclusions Chemical air-sea Is it possible to explain this behaviour with the equation of exchange motion? Coupling of GCMs Conclusions What if we have a steady wind? Three forces are important: Wind stress (W). Friction (F). Coriolis Force (C).
A. Pozzer 14/10/2011 Surface currents
Introduction
Heat exchange
Momentum Forces characteristics exchange Equations of motion F is opposite the ice’s direction. Inertial currents Ekman layer Conclusions C is perpendicular to the velocity. Chemical air-sea The forces are balanced (steady flow):W + F + C = 0 exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Surface currents
Introduction
Heat exchange Equation of motion Momentum exchange Equations of motion Du 1 ∂p ∂u Inertial currents 0 = +f υ − + Fx + ρAz Ekman layer Dt ρ ∂x ∂z Conclusions Dυ 1 ∂p ∂υ Chemical air-sea 0 = −fu − + F + ρA exchange Dt ρ ∂y y z ∂z Coupling of GCMs Dw 1 ∂p 0 = −g − + F Conclusions Dt ρ ∂z z
Hypothesis of steady flow with friction: Steady flow. Horizontal flow. ∂u ∂υ Friction in the form: Fxz = ρAz ∂z , Fyz = ρAz ∂z Homogenoeous flow.
A. Pozzer We search simple solution of these equations. 14/10/2011 Surface currents
Introduction Heat exchange Equation of motion simplified: Momentum 2 exchange ∂ u Equations of motion f υ + ρAz = 0 Inertial currents ∂z Ekman layer Conclusions ∂2υ Chemical air-sea −fu + ρAz = 0 exchange ∂z Coupling of GCMs The solutions Conclusions If the wind is blowin to the north: Fyz = F and Fxz = 0
u = V0 exp(az) cos (π/4 + az)
υ = V0 exp(az) sin (π/4 + az) with F r f V0 = 2 and a = A. Pozzer ρ fAz 2Az 14/10/2011 Surface currents
Introduction
Heat exchange
Momentum exchange Equations of motion The solutions are: Inertial currents Ekman layer Conclusions u = V0 exp(az) cos (π/4 + az) Chemical air-sea exchange υ = V0 exp(az) sin (π/4 + az) Coupling of GCMs at the sea surface (z = 0 → exp(z) = 1) Conclusions
u(0) = V0 cos (π/4)
υ(0) = V0 sin (π/4) The flow is 45◦ rotate with respect to the wind! Below the surface the velocity decay exponentially.
A. Pozzer 14/10/2011 Surface currents
Introduction Heat exchange In general Momentum ◦ exchange surface currents is 45 to the right of the wind when Equations of motion Inertial currents looking downwind in the Northern Hemisphere. Ekman layer Conclusions surface currents is 45◦ to the left of the wind when Chemical air-sea exchange looking downwind in the Southern Hemisphere.
Coupling of GCMs
Conclusions
Ekman current generated by a 10m/s wind at 45◦ N A. Pozzer 14/10/2011 Surface currents
Introduction
Heat exchange
Momentum exchange Ekman layer Equations of motion Inertial currents Ekman layer Arbitrary definition as velocity decreases exponentially Conclusions Chemical air-sea r exchange π 2πAz DE = = Coupling of a f GCMs Conclusions Typical Ekman Depths
U10 latitude 15◦ 45◦ 5 75 m 45 m 10 150 m 90 m 20 300 m 180 m
A. Pozzer 14/10/2011 Ekman layer
Introduction
Heat exchange
Momentum exchange Ekman layers are locations where frictional force and coriolis Equations of motion forces are equivalent. Inertial currents Ekman layer Hence other few Ekman layer exists: Conclusions Chemical air-sea Ekman layer at the bottom of the atmosphere (also exchange called Planetary Boudary Layer). Coupling of GCMs Ekman layer at the bottom of the Ocean. Conclusions
PBL Velocity goes zero at the boundary surface winds are 45◦ to the left of the free troposphere flow in the Northern Hemisphere surface winds are 45◦ to the right of the free troposphere flow in the Southern Hemisphere
A. Pozzer 14/10/2011 Ekman layer
Introduction
Heat exchange
Momentum exchange Ocean/Atmosphere flows in the surface Ekman layer Equations of motion Inertial currents Let’s take the Northern Hemisphere: Ekman layer Conclusions Winds above the planetary boundary layer are Chemical air-sea exchange perpendicular to the pressure gradient in the atmosphere Coupling of and parallel to lines of constant surface pressure. GCMs ◦ Conclusions surface winds are 45 to the left of the free troposphere flow in the Northern Hemisphere surface currents is 45◦ to the right of the wind when looking downwind in the Northern Hemisphere. Currents at the sea surface are expected to be nearly in the direction of winds above the planetary boundary layer and parallel to lines of constant pressure.
A. Pozzer 14/10/2011 Ekman layer
Introduction
Heat exchange
Momentum exchange Equations of motion Inertial currents Ekman layer Conclusions Chemical air-sea exchange
Coupling of GCMs
Conclusions
Trajectories of surface drifters in April 1978
together with surface pressure in the atmosphere averaged for the month.
A. Pozzer 14/10/2011 Application of Ekman theory
Introduction
Heat exchange
Momentum exchange Equations of motion Costal upwelling Inertial currents Ekman layer Conclusions Chemical air-sea exchange
Coupling of GCMs
Conclusions
The winds produce a mass transport away from the shore everywhere along the shore. The water pushed offshore can be replaced only by water from below the Ekman layer. Because the upwelled water is cold, the upwelling
leads to a region of cold water at the surface along the coast.
A. Pozzer 14/10/2011 Costal Upwelling
Introduction Heat exchange Climatic impact Momentum exchange Upwelled water is colder than water normally found on Equations of motion the surface. Inertial currents Ekman layer Conclusions Cold water along the coast leads to a thin, cool Chemical air-sea atmospheric boundary layer. exchange
Coupling of As the air cools, fog forms along the coast which is GCMs blown over land (San Francisco Fog). Conclusions Thin layer of cold air tapped by warm air inhibits convection, and rain is rare.
Biological impact Upwelled water is richer in nutrients. The nutrients starts the food chain. These regions are productive waters / major fisheries
A. Pozzer (Peru, California, Morocco and Namibia). 14/10/2011 Costal Upwelling
Introduction
Heat exchange
Momentum exchange Equations of motion Inertial currents Ekman layer Conclusions Chemical air-sea exchange
Coupling of GCMs
Conclusions
Sea Surface Temperature from satellite observations (NASA, with MODIS data).
A. Pozzer 14/10/2011 Conclusions
Introduction
Heat exchange
Momentum exchange Equations of motion Inertial currents Ekman layer Conclusions Chemical air-sea exchange Equations of motion are called “Navier-Stokes
Coupling of equations” GCMs Solutions are possible only in few (simplified) cases. Conclusions Numerical model are essential to estimate a “solution” for such equations.
A. Pozzer 14/10/2011 Conclusions
Introduction
Heat exchange
Momentum Wind effect on the suface of the ocean:: exchange Equations of motion Changes in wind stress produce transient oscillations in Inertial currents Ekman layer the ocean called inertial currents (very common in the Conclusions Chemical air-sea ocean). exchange Steady wind produce a thin boundary layer (Ekmann Coupling of GCMs layer) at the top of the ocean. Conclusions The Ekman layer in the atmosphere is called Planetary Boundary Layer (PBL) At the surface the Ekman layer has: currents 45◦ to the right of the wind, looking downwind in the Northern Hemisphere. 1-2.5% of wind speed depending on latitude. approximately 40-300 meters, depending on latitude and wind.
A. Pozzer 14/10/2011 Conclusions
Introduction
Heat exchange
Momentum exchange Equations of motion Inertial currents Ekman layer Conclusions Ekman’s theory consequences Chemical air-sea exchange Winds blowing toward the equator in the west coasts Coupling of produces upwelling along coasts, leading to cold GCMs productive water about 100 km of the shore. Conclusions Ekman pumping (driven by spatial variability of winds) drives vertical currents which drives the interior geostrophic circulation of the ocean.
A. Pozzer 14/10/2011 Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange Air-sea exchange model Trasfer velocities Concentration difference Ocean as Exchange of mass : air sea source/sink of tracer conclusions Coupling of exchange of gases GCMs
Conclusions
A. Pozzer 14/10/2011 Atmosphere-Ocean interactions
Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange Air-sea exchange model Trasfer velocities Concentration difference Ocean as source/sink of tracer conclusions Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange Henry’s law: Air-sea exchange model Trasfer velocities The solubility of a gas in a liquid at a particular temperature Concentration difference is proportional to the pressure of that gas above the liquid. Ocean as source/sink of tracer conclusions Coupling of Dimensionless Henry’s law number GCMs Cw = concentration in water phase. Conclusions Cg = concentration in gas phase.
At equilibrium : H = Cw /Cg .
A. Pozzer 14/10/2011 Chemical air-sea exchange model
Introduction
Heat exchange Two layer model
Momentum exchange
Chemical air-sea exchange Air-sea exchange model Trasfer velocities Concentration difference Ocean as source/sink of tracer conclusions Coupling of GCMs
Conclusions
Fick’s law The transfer flux across each film is : ∂C F = −D × A. Pozzer ∂z 14/10/2011 Chemical air-sea exchange model
Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange Adding the hypothesys of: Air-sea exchange model Trasfer velocities Steady state (fluxes are the same in gaseous an liquid Concentration difference phase) Ocean as source/sink of tracer conclusions No strong chemical reactions between the layers Coupling of GCMs
Conclusions and using:
Henry’s law : HCg = Cw a bit of math
A. Pozzer 14/10/2011 Chemical air-sea exchange model
Introduction
Heat exchange
Momentum exchange We end up with: Chemical air-sea exchange 1 Air-sea exchange F = Hz × (Cw − HCg ) model zw + g Trasfer velocities Dw Dg Concentration difference Ocean as source/sink of tracer conclusions where:
Coupling of Dg GCMs gas phase trasfer velocity K = g zg Conclusions water phase trasfer velocity K = Dw w zw 1 total transfer velocity Ktot = Hz zw + g Dw Dg 1 or better Ktot = 1 + H αKw Kg
A. Pozzer 14/10/2011 Chemical air-sea exchange model
Introduction
Heat exchange
Momentum exchange
Chemical air-sea Two layer model exchange Air-sea exchange 2 model Flux in mol/m s Trasfer velocities −1 Concentration 1 H difference Transfer velocity in m/s : Ktot = + Ocean as αKw Kg source/sink of tracer 3 conclusions Concentration difference in mol/m Coupling of GCMs
Conclusions F = Ktot × (Cw − HCg )
Basic modeling equation !
A. Pozzer 14/10/2011 Trasfer velocities
Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange −1 1 H Air-sea exchange Transfer velocity in m/s : K = + model tot αKw Kg Trasfer velocities Concentration difference Kw << Kg /H : the flux is controlled by the water film Ocean as source/sink of tracer transfer conclusions Coupling of Kw >> Kg /H : the flux is controlled by the gas film GCMs transfer Conclusions The water side transfer velocity (Kw ) is generally three orders of magnitude lower than the air side transfer velocity (Kg ).
A. Pozzer 14/10/2011 Trasfer velocities
Introduction
Heat exchange
Momentum However : exchange having H as dimensionless Henry’s law coefficient Chemical air-sea exchange 5 Air-sea exchange H > 10 (soluble gases): Ktot is dominated by Kg . model Trasfer velocities 5 Concentration 10 < H < 10 : Kg ' Kw both have to be considered in the difference Ocean as calculations. source/sink of tracer conclusions H < 10 (non soluble gases) : Ktot is dominated by Kw . Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Kg : air side transfer velocity
Introduction
Heat exchange
Momentum exchange Thanks to tracer deposition studies we have that:
Chemical air-sea exchange 1 Air-sea exchange Kg = model Ra + Rqbr Trasfer velocities Concentration difference Ocean as where : source/sink of tracer conclusions Ra (in s/m) the aerodynamic resistance Coupling of GCMs Rqbr (in s/m) the quasi-laminar boundary layer Conclusions resistance in addition:
Ra is a function of the physical state of the atmosphere.
Rqbr (X ) is controlled by molecular diffusion.
A. Pozzer 14/10/2011 Kw : water side transfer velocity
Introduction Heat exchange Kw is the most important for many gases of environmental Momentum interest. exchange
Chemical air-sea exchange Air-sea exchange model Trasfer velocities Concentration difference Ocean as source/sink of tracer conclusions Coupling of GCMs
Conclusions
Wave simulator at FUB
Following laboratories studies, it has been show that Kw can be influenced by: wind A. Pozzer 14/10/2011 bubbles surfactants rain temperature/humidity (skin effect) However It is extremely difficult to use these data to extrapolate results to coastal seas and oceans, due to more complex mechanism (missing direct/immediate wave-wind connection). Kw : water side transfer velocity
Introduction
Heat exchange Where is the thin diffusion layer ?
Momentum exchange
Chemical air-sea exchange Air-sea exchange model Trasfer velocities Concentration difference Ocean as source/sink of tracer conclusions Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 Kw : water side transfer velocity Introduction Despite the importance, measuring air-sea tracer exchange in Heat exchange situ is extremely difficult..... Momentum exchange
Chemical air-sea exchange Air-sea exchange model Trasfer velocities Concentration difference Ocean as source/sink of tracer conclusions Coupling of GCMs
Conclusions
NOAA/Equatorial air-sea exchange experiment Methodologies used: Large scale techniques 14 Radiocarbon ( CO2)
A. Pozzer Oxygen/Nitrogen ratios 14/10/2011 Local scale techniques Mass balance Radon
Deliberate tracers experiment (e.g. SF6) Eddy correlation technique Relaxed eddy accumulation Atmospheric profiles Kw : water side transfer velocity
Introduction
Heat exchange Kw parametrisations Momentum exchange Reference relationship 0.17U10 (U10 < 3.6m/s) Chemical air-sea Liss and Merlivat 1986 2.85U10 − 9.65 (3.6 < U10 < 13m/s) exchange 5.9U10 − 49.3 (13m/s < U10) 2 Air-sea exchange Wanninkhof 1992 0.31(U10) model Wanninkhof-Mc-Gills 1999 0.0283(U )3 Trasfer velocities 10 2 Concentration Nightingale 2000 0.333(U10) + 0.222(U10) difference 2 Ho 2006 0.266(U10) Ocean as source/sink of tracer conclusions Coupling of GCMs
Conclusions
Uncertainties of more than a factor of 2! A. Pozzer 14/10/2011 Concentration difference
Introduction
Heat exchange
Momentum exchange
Chemical air-sea Basic modeling equation : exchange Air-sea exchange model Trasfer velocities F = Ktot × (Cw − HCg ) Concentration difference Ocean as source/sink of tracer conclusions ∆C = (Cw − HCg ) Coupling of GCMs
Conclusions ∆C defines the flux direction
Cw > HCg : water is supersaturated.
Cw < HCg : water is undersaturated.
A. Pozzer 14/10/2011 Henry’s law number
Introduction Heat exchange H can be influenced by: Momentum exchange salinity (higher salinity ⇒ higher solubility) Chemical air-sea temperature (higher temperature ⇒ lower solubility) exchange Air-sea exchange model Trasfer velocities Concentration Temperature effect difference Ocean as source/sink of tracer conclusions Coupling of GCMs
Conclusions
Origin of the solubility pump.
A. Pozzer ∆C = (Cw − HCg ) 14/10/2011 Cw : how can it be estimated?
Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange Air-sea exchange model Trasfer velocities Mechanism of formation of tracer in seawater Concentration difference Ocean as Phytoplankton (DMS) source/sink of tracer conclusions Photochemistry/Photodissociation (NMHC) Coupling of GCMs Bacterial (CH4) / Microbial (N2O) Conclusions In addition: tracers are transported in the ocean...
A. Pozzer 14/10/2011 Chlorophyll-a, a proxy for Phytoplankton
Introduction
Heat exchange
Momentum Winter 2006/2007, Acqua MODIS Level 3 data. exchange
Chemical air-sea exchange Air-sea exchange model Trasfer velocities Concentration difference Ocean as source/sink of tracer conclusions Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011 How to model the concentration difference?
Introduction
Heat exchange Interpolated global map of observations
Momentum Satellite observation of proxies exchange Model simulation Chemical air-sea exchange Kettle et al. (1999) Air-sea exchange model Trasfer velocities Concentration difference Ocean as source/sink of tracer conclusions Coupling of GCMs
Conclusions
Kettle et al.
A. Pozzer 14/10/2011
(1999) Palmer and Shaw (2005)
Vichi and Masina
(2009) Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange Tracer in seawater and their atmosphere-ocean flux Air-sea exchange model Atmospheric Main production Net annual flux % of atmopsheric Gas Trasfer velocities role mechanism to the atmospehre source/sink Concentration difference DMS CCN + acidity Phytoplankton 15-22 TgS 80 Ocean as COS CCN + acidity Photochemistry -0.1-0.3 Tg 40 source/sink of tracer CH3I Oxydation capacity Phytoplankton 0.13-0.36 Tg 10(?) conclusions CH3Cl Ozone depletion ? 0.2-0.4 Tg 7-14 Coupling of N2O GHG, ozone depl. (De)nitrification 11-17 Tg N 60-90 GCMs CH4 GHG, oxydation Bacteria 15-24 Tg 3-5 CO2 GHG Respiration -1.7±0.5 PgC -30 Conclusions O3 GHG, oxydation - -300 Tg -30 CFCs GHG, ozone depl. - ? ? CO Oxydation capacity Photochemistry 10-650 TgC 3-20 NMHC Oxydation capacity Photochemistry 2-3 Tg 1 OVOC Oxydation capacity Photochemistry ? ?
A. Pozzer 14/10/2011 Final considerations
Introduction
Heat exchange
Momentum exchange
Chemical air-sea The key assumption for the thin-film layer model are exchange Air-sea exchange The main bodies of air and water are well mixed. model Trasfer velocities Concentration Production or removal processes in the thin film are slow difference Ocean as compared to the transport itself. source/sink of tracer conclusions Coupling of GCMs The direction of the air-sea exchange Conclusions depends on atmospheric and oceanic concentrations at the interface.
WARNING: each tracer is different!
A. Pozzer 14/10/2011 Future study direction
Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange Parametrisation (from laboratory studies) of tracer Air-sea exchange model Trasfer velocities concentrations via ”easy to measure” proxies. Concentration difference Large observational dataset of gases concentration in the Ocean as source/sink of tracer water is missing! These large scale observations requires conclusions Coupling of international collaboration. Hence growth of many GCMs international projects: Conclusions JGOFD, Joint Global Ocean Flux Study. SOLAS, SURface Ocean Lower Atmosphere Study.
A. Pozzer 14/10/2011 Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange
Coupling of GCMs Conclusions Conclusions Coupling of General Circulation Models
A. Pozzer 14/10/2011 Introduction
Heat exchange
Momentum What is a GCM exchange
Chemical air-sea A General circulation Model (GCM) is a mathematical model exchange of the general circulation of a planetary atmosphere or ocean Coupling of GCMs and based on the Navier-Stokes equations on a rotating Conclusions sphere with thermodynamic terms for various energy sources Conclusions (radiation, latent heat).
What is a AO-GCM Coupled atmosphere-ocean general circulation models (AO- GCMs) comprise an atmospheric general circulation model (A-GCM), also including a land-surface component, and an ocean model (an Ocean General Circulation Model, O- GCM), also including a sea-ice component.
A. Pozzer 14/10/2011 Information exchange between models
Introduction The models need to exchange informations (i.e. boundaries) Heat exchange to each other Momentum exchange With Bulk’s formula both model can estimates latent and Chemical air-sea sensible heat fluxes! exchange Coupling of From Atmosphere to Ocean GCMs Conclusions wind stress over ice and water Conclusions solid/liquid freshwater flux heat fluxes at the ocean’s surface radiations at the ocean’s surface wind at 10 meters / air temperature
From Ocean to Atmosphere SST ice thickness/compactness
A. Pozzer water velocity 14/10/2011 Information exchange between models
Introduction
Heat exchange Different grid:
Momentum exchange Atmosphere GCM Ocean GCM Chemical air-sea exchange
Coupling of GCMs Conclusions Conclusions
Fluxes Must be conserved globally and locally ! Sophisticate algorithm for estimating the fluxes between different grids (divergence theorem)
(5 · F) dV = (F · n) dS A. Pozzer y { 14/10/2011 V S Information exchange between models
Introduction
Heat exchange Coupler methods:
Momentum exchange
Chemical air-sea exchange
Coupling of GCMs Conclusions internal/serial Conclusions coupler
external/parallel coupler
A. Pozzer 14/10/2011 Time scale is different between Atmosphere and Ocean Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange
Coupling of GCMs Conclusions Conclusions
The coupling is not every time step! A. Pozzer 14/10/2011 Applications:
Introduction
Heat exchange
Momentum exchange IPCC simulations Chemical air-sea exchange
Coupling of GCMs Conclusions Conclusions
A. Pozzer 14/10/2011 Applications:
Introduction Heat exchange IPCC simulations: sea level rise Momentum exchange
Chemical air-sea exchange
Coupling of GCMs Conclusions Conclusions
A. Pozzer 14/10/2011 Applications:
Introduction
Heat exchange Momentum IPCC simulations: sea level rise exchange
Chemical air-sea exchange
Coupling of GCMs Conclusions Conclusions
A. Pozzer 14/10/2011 Conclusions:
Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange
Coupling of GCMs Models/numerics are essential to simulate Erath’s Conclusions Conclusions climate. A GCM and OGCM have been developed separately and only later coupled. The time scale of atmosphere and ocean is very different.
A. Pozzer 14/10/2011 Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange
Coupling of GCMs Conclusions Conclusions
A. Pozzer 14/10/2011 The Ocean:
Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange
Coupling of GCMs Conclusions covers 3/4 of our globe. is essential for our climate. has strong impact on the atmospheric composition.
A. Pozzer 14/10/2011 Main concepts
Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange Oceans contribute to redistribute incoming energy. Coupling of GCMs Oceans release most of the heat as latent heat via Conclusions evaporation. Numerical models are essential to study heat exchange.
Winds are essential for the ocean circulation. Wind induced Ocean transport has strong climatic effects.
A. Pozzer 14/10/2011 Main concepts
Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange Air-sea transfer of chemical species: Coupling of GCMs Requires expertises from different fields: Conclusions Micrometeorology (Ocean and Atmosphere). Large scale meteorology (Ocean and Atmosphere). Biology. Present many uncertainties. Rely strongly on parametrisation. Is it still a “terra incognita”.
A. Pozzer 14/10/2011 Thank you!
Introduction
Heat exchange
Momentum exchange
Chemical air-sea exchange
Coupling of GCMs
Conclusions
A. Pozzer 14/10/2011