The general circulation of the atmosphere revisited

1. The general circulation of the atmosphere revisited Kevin E. Trenberth NCAR Boulder, CO

2. At the Fifth World Meteorological Congress, Professor Ed Lorenz presented the IMO lecture entitled "The Nature and Theory of the General Circulation of the Atmosphere," characterised by the Congress as a "brilliant lecture" and published as a WMO monograph. For those of us not there: Professor Ed Lorenz wrote a wonderful classic book “The nature and theory of the general circulation of the atmosphere” published by WMO in 1967.

3. Professor Ed Lorenz wrote a wonderful classic book “The nature and theory of the general circulation of the atmosphere” published by WMO in 1967.

4. The book set up the dynamical framework for; • the atmospheric general circulation, • the governing equations and their approximate forms, • the energetics, • the observations, • the processes revealed, • laboratory, theoretical and numerical models. • The historical review and the background framework are every bit as useful today, as are the many insights into processes. • What have changed are the observations, the ability to analyze those into global fields, and the computers and modeling capabilities.

5. Of course Ed Lorenz has done much more than this: all of his work on chaos, for example, but others will deal with that. He also has many other awards and honors, including the Kyoto Prize, the Roger Revelle Medal, the Holger and Anna-Greta Crafoord Prize and others.

6. In this tribute to Ed Lorenz, I provide a more up to date (but superficial) view of the energy cycle in the atmosphere and its role in the larger climate system. Professor Lorenz was my doctoral thesis advisor when I was a graduate student at M.I.T. in the late 1960s early 1970s. My work and perspective has clearly been greatly influenced by the education I received at M.I.T. I consider myself exceptionally fortunate to have been one of Ed’s students.

7. Atmospheric energetics became a vigorous area of study at MIT in the 1960s under Victor P. Starr: Planetary Circulations Project. • The incentive and focal point was the availability of the upper air soundings following the IGY. These studies were based on station statistics which were mapped over the domain: mainly extratropics NH. • Disadvantages of the station based approach: • Missing data and huge gaps over oceans. • Also, inability to deal with vertical motion and divergent part of flow that is linked to diabatic processes that force the system.

8. Expanded into the Tropics and globally by Newell, Kidson, Vincent & Boer (1972, 1974, MIT Press) “The general circulation of the tropical atmosphere and interactions with extratropical latitudes” Vols 1 and 2 and more recent summary by Peixoto and Oort (1992).

9. Use of global reanalyses overcomes some of these disadvantages: global coverage and no missing data. Also multivariate, consistent method of analyses, with reasonable (much improved) divergent flow. Problems remain from the changing observing system. Now possible to reconcile implied ocean heat transports from estimated atmospheric transports plus TOAradiation (from satellites) with direct ocean measurements. Hence we can explore atmospheric portion in detail and partition into components. Trenberth and Caron 2001

10. The MIT framework defined the “transient” contribution to be the departure from a mean (we use a month) h = h + h’ “Quasi-stationary” : long-term mean plus the interannual and inter-monthly variability. Hence it includes the Hadley and Walker circulations in the tropics: part of “global monsoon”. And it includes quasi-stationary planetary waves (mainly a factor in NH extratropics in winter) ¯

11. TE = PE + IE + LE + KE total energy = potential + internal + latent + kinetic Now the first two terms can be combined into the Total Potential Energy (TPE) which, through a major insight from Lorenz, can in turn be partitioned: TPE = UPE + APE Where APE is the Available Potential Energy and UPE is the Unavailable Potential Energy. Lorenz points out how friction is positive definite in terms of heating and thus must contribute to UPE. Note these do NOT relate simply to Dry Static Energy.

12. In terms of transports though we partition Total Energytransports into Dry Static Energy (DSE), Latent Energy (LE), and Kinetic Energy (KE), but the latter is small. Transport of energyalso involves work done: Dry static energy DSE = SH + PE sensible heat+geopotential Moist static energyMSE = DSE + LE DSE+latent Total energy FATE = MSE + KEKinetic energy (small) Divergence of transports balanced by diabatic forcings, ignoring tendencies and friction heating (small) .TEv = Q1 – Q2 = atmospheric diabatic heating + column moistening

13. Q1 = RT + Fs + L(P-E) • Q2 = L(P-E) • .FA = Q1-Q2 = RT+ Fs • = (RT – Rs) + LE +Hs .FA Hs LE Rs LP Fs .FO RT

14. Mechanisms for poleward heat transports in the atmosphere vary: In Tropics: large-scale overturning by the global monsoon and its embedded Hadley and Walker circulations. In extratropics: baroclinic eddies (cyclones and anticyclones and associated cold and warm fronts) Plus quasi-stationary planetary waves In NH: Aleutian Low, Siberian High, Icelandic Low

15. Transports Atmosphere Annual Divergence 7°N Substantial divergence out of subtropics Note: seamless total northward transports and divergence, but structure in components. Also DSE and LE opposite for stationary but additive for transients. Trenberth & Stepaniak 2003

16. Why is there a Hadley circulation? Fundamentally it is the most efficient way to transport heat (energy) polewards in the Tropics. It is primarily driven by latent heating in upward branch. But the moisture is evaporated in subtropics and is transported by the circulation into upward branch, so this is not fundamental but is rather a secondary result. Often also thought to be driven by radiative cooling to space in subtropics. This is partly a MYTH! Instead there has to be a link with extratropical poleward energy transport by baroclinic eddies and quasi-stationary waves.

17. Hadley circulation theories • Schneider (1977) zonally symmetric steady state model • Held and Hou (1980), Lindzen and Hou (1988), Hou and Lindzen (1992) included effects of distribution of heating related to latent heat feedback, and displacement of upward branch away from equator (summer, winter scenarios) • Schneider (1987) and Emanuel (1995) included non-zonal monsoonal effects • Fang and Tung (1999) included time-varying heating that enhanced strength of time mean. • Theories successfully account for several features of Hadley circulation: • Width of circulation, position of subtropical jet which are controlled by geostrophy and conservation of heat and momentum • All use radiative-convective basic state, often with Newtonian relaxation to produce heating and cooling.

18. Monsoons and anticyclones • Zonal asymmetries important: • Trenberth et al. (2000): Global monsoon, variability on multiple time scales. • Rodwell and Hoskins (1996, 2001), Chen et al. (2001) also used idealized models and relaxation to basic state. • Downward branch important for water vapor: drying of subtropics above the boundary layer. • Pierrehumbert 1995, 1999; Spencer and Braswell 1997; Lindzen et al. 2001; Salathe and Hartmann (1997): latter shows paths of moisture swirl and do not correspond to zonal mean Hadley circulation. • Large scale subsidence driven by radiative cooling to space regarded as dominant process in subtropics. • Iris effect: window to space for OLR.

19. Hadley Circulation zonal mean Pacific DJF Annual Trenberth et al. 2002

20. Difference due to ocean transports Trenberth and Stepaniak 2003 Diabatic heating atmosphere Q1 Column latent heating Q2 Total heating Q1-Q2 ASR OLR NET RT Radiation TOA

21. Divergence of total atmospheric energy Note how transient and stationary components almost exactly compensate in extratropics: Divergence by transients in subtropics is compensated by subsidence and hence convergence by stationary component and, at same time, values of opposite sign to north and south. Trenberth and Stepaniak 2003

22. Divergence of: DSE LE Note strong compensation locally in stationary component. Closer relation with DSE and LE transients. T&S 2003

23. The Hadley circulationis driven mostly from the subtropics throughcooling by transient baroclinic waves in storm tracks at mid-lats. This is reasonHadley circulationreverses withannual cycle. The cooling drives the downward branch of the Hadley circulation, clears the skies to allow OLR to contribute, and allows solar radiation through to surface where it provides moisture through evaporation. Tropical SSTs determine where the upward motion is favored, and the upward motion is driven by latent heating. But the moisture comes mostly from the subtropics, transported by the Hadley circulation itself. The subtropical OLR and the tropical latent heating are secondary consequences of the more fundamental drivers.

24. Evaporation Moisture transport Moisture transport Hadley circulation and heat budget in subtropics Radiation solar down infrared up Latent heating in convective rain Dynamical warming by subsidence Dynamical cooling by advection Heat transport by transients warm Ocean heat transport Trenberth and Stepaniak 2003

25. In Tropics: Global monsoon • TE transport is small residual of DSE and LE. • Solar radiation in clear skies heats ocean, cooled by evaporation: moisture transported into upward branch, feeds DSE. • Circulation that provides transport, supplies LE 2. In extratropics: transient baroclinic waves LE and DSE additive, moisture more prominent in low-mid-latitudes. 3. Subtropics: substantial cooling by baroclinic waves Coordinated with Hadley circulation adiabatic warming; and upward motion near equator. I.e. Hadley circulation and mid-latitude storm tracks directly linked. Gives seamless total energy transport: on seasonal time scales

26. Ed Lorenz is having a great career, and it has been my privilege to have you, Ed, as a thesis advisor and a friend. Photo courtesy H. Bluestein 26 July 1990 N of Estes Park