1. The Earth’s Climate System: Variability and change Kevin E. Trenberth NCAR The Earth’s Climate System: Variability and change Kevin E. Trenberth NCAR

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3. Energy on Earth The incoming radiant energy is transformed into various forms (internal heat, potential energy, latent energy, and kinetic energy) moved around in various ways primarily by the atmosphere and oceans, stored and sequestered in the ocean, land, and ice components of the climate system, and ultimately radiated back to space as infrared radiation. An equilibrium climate mandates a balance between the incoming and outgoing radiation and that the flows of energy are systematic. These drive the weather systems in the atmosphere, currents in the ocean, and fundamentally determine the climate. And they can be perturbed, with climate change.

6. WaterVapor60% CarbonDioxide26% O 3 8% CH 4 N 2 0 6% 6% The Natural Greenhouse Effect: clear sky Clouds also have a greenhouse effect Kiehl and Trenberth 1997 The Earth would be -19°C (-2°F) without atmosphere. 99% of the atmosphere is nitrogen and oxygen which are transparent to radiation

7. Net observed radiation TOA Trenberth & Stepaniak, 2003

8. The role of the climate system Atmosphere: Volatile turbulent fluid, strong winds, Chaotic weather, clouds, water vapor feedback Transports heat, moisture, materials etc. Heat capacity equivalent to 3.2 m of ocean Ocean: 70% of Earth, wet, fluid, high heat capacity Stores, moves heat, fresh water, gases, chemicals Adds delay of 10 to 100 years to response time Land: Small heat capacity, small mass involved (conduction) Water storage varies: affects sensible vs latent fluxes Wide variety of features, slopes, vegetation, soils Mixture of natural and managed Vital in carbon and water cycles, ecosystems Ice: Huge heat capacity, long time scales (conduction) High albedo: ice-albedo feedback Fresh water, changes sea level Antarctica 65 m (WAIS 4-6m), Greenland 7m, other glaciers 0.35m

9.  The atmosphere is the most volatile component of climate system  Winds in jet streams exceed 100 mph or even 200 mph; winds move energy around.  Thin envelope around planet 90% within 10 miles of surface 1/400 th of the radius of Earth; clouds appear to hug the surface from space.  The atmosphere does not have much heat capacity  “Weather” occurs in troposphere (lowest part)  Weather systems: cyclones, anticyclones, cold and warm fronts tropical storms/hurricanes move heat around: mostly upwards and polewards  The atmosphere is the most volatile component of climate system  Winds in jet streams exceed 100 mph or even 200 mph; winds move energy around.  Thin envelope around planet 90% within 10 miles of surface 1/400 th of the radius of Earth; clouds appear to hug the surface from space.  The atmosphere does not have much heat capacity  “Weather” occurs in troposphere (lowest part)  Weather systems: cyclones, anticyclones, cold and warm fronts tropical storms/hurricanes move heat around: mostly upwards and polewards The role of the atmosphere in energy

10. George Hadley (1685-1768), English lawyer and scientist. “I think the cause of the general Trade-winds have not been explained by any of those who have wrote on that subject” (1735) The overturning Hadley cells are the main way the atmosphere transports energy polewards in low latitudes

11. “Extratropical Storms” Source: USA TODAY research by Chad Palmer, Graphic by Chuck Rose Cyclones and anticyclones are the main way of transporting energy polewards in extratropics. Winds converging into the low, pull cold air from the poles toward the equator, and warm air from the equator to the poles. Where they meet is where we find fronts, bringing widespread precipitation and significant weather, like thunderstorms.

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

14. Role of Oceans  The oceans cover 70.8% of the Earth’s surface.  The oceans are wet: water vapor from the surface provides source for rainfall and thus latent heat energy to the atmosphere.  The heat capacity of the atmosphere is equivalent to that of 3.5 m of ocean. The oceans slowly adjust to climate changes and can sequester heat for years.  The ocean is well mixed to about 20 m depth in summer and over 100 m in winter. An overall average of 90 m would delay climate response by 6 years.  Total ocean: mean depth 3800 m.  Would add delay of 230 years if rapidly mixed. In reality, the response depends on rate of ventilation of water through the thermocline (vertical mixing).  Estimate of delay overall is 10 to 100 years.  The ocean currents redistribute heat, fresh water, and dissolved chemicals around the globe.  The oceans cover 70.8% of the Earth’s surface.  The oceans are wet: water vapor from the surface provides source for rainfall and thus latent heat energy to the atmosphere.  The heat capacity of the atmosphere is equivalent to that of 3.5 m of ocean. The oceans slowly adjust to climate changes and can sequester heat for years.  The ocean is well mixed to about 20 m depth in summer and over 100 m in winter. An overall average of 90 m would delay climate response by 6 years.  Total ocean: mean depth 3800 m.  Would add delay of 230 years if rapidly mixed. In reality, the response depends on rate of ventilation of water through the thermocline (vertical mixing).  Estimate of delay overall is 10 to 100 years.  The ocean currents redistribute heat, fresh water, and dissolved chemicals around the globe.

15. The great ocean conveyer: of heat and salts

16. OCEAN-ATMOSPHERE TRANSPORTS The latest best estimate of the partitioning of meridional transports by the atmosphere and ocean. Trenberth and Caron, J. Clim. 2001

17. Role of Land  Land has enormous variety of features: topography, soils, vegetation, slopes, water capacity.  Land systems are highly heterogeneous and on small spatial scales.  Changes in soil moisture affect disposition of heat: rise in temperature versus evaporation.  Changes in land and vegetation affect climate through albedo, roughness and evapotranspiration.  Heat penetration into land with annual cycle is ~2 m.  Heat capacity of land is much less than water:  Specific heat of land 4½ less than sea water  For moist soil maybe factor of 2  Land plays lesser role than oceans in storing heat. Consequently:  Surface air temperature changes over land are large and occur much faster than over the oceans.

18. Kansas 2001 Irrigation circles: Corn, sorghum green, Wheat gold Kansas 2001 Irrigation circles: Corn, sorghum green, Wheat gold

19. The Role of Ice Major ice sheets, e.g., Antarctica and Greenland. Penetration of heat occurs primarily through conduction.  The mass involved in changes from year to year is small but important on century time scales. Unlike land, ice melts  changes in sea level on longer time-scales. Ice volumes: 28,000,000 km 3 water is in ice sheets, ice caps and glaciers. Most is in the Antarctic ice sheet which, if melted, would increase sea level by  65 m, vs Greenland 7 m and the other glaciers and ice caps 0.35 m. In Arctic: sea ice ~ 3-4 m thick Around Antarctic: ~ 1-2 m thick Ice is bright: reflects the solar radiation  ice-albedo feedback Ice   radiation reflected  cooler  Ice  The West Antarctic Ice Sheet (WAIS) partly grounded below sea level.  Warming could alter grounding of the ice sheet, making it float, and vulnerable to rapid (i.e. centuries) disintegration.  rise in sea level of 4-6 m. May be irreversible if collapse begins. Ice is bright: reflects the solar radiation  ice-albedo feedback Ice   radiation reflected  cooler  Ice  The West Antarctic Ice Sheet (WAIS) partly grounded below sea level.  Warming could alter grounding of the ice sheet, making it float, and vulnerable to rapid (i.e. centuries) disintegration.  rise in sea level of 4-6 m. May be irreversible if collapse begins.

20. What about variability and change?

21. El Niño

22. What is El Niño? Warming of sea surface waters in the central and eastern tropical Pacific Ocean. El Niño: the ocean part: Warm phase of ENSO: El Niño - Southern Oscillation Southern Oscillation: the atmospheric part; a global wave pattern La Niña: is the cold phase of ENSO: Cool sea temperatures in tropical Pacific EN events occur about every 3-7 years

23. ENSO A natural mode of the coupled ocean- atmosphere system ENSO: EN and SO together: Refers to whole cycle of warming and cooling. ENSO events have been going on for centuries (records in corals, and in ice layers in glaciers in South America) ENSO arises from air-sea interactions in the tropical Pacific

25. SSTs and SST anomalies There is a close link between tropical SSTs and precipitation Not simple: involves total SST and its gradients Winds blow down pressure gradient toward warmest waters Convergence over/near warmer waters Subsidence in cooler regions For the most part: High SSTs determine where the action is! Low SSTs determine where it isn't!

26. Darwin and Tahiti

27. El Niños are red La Niñas are blue They follow in sequence Every year or two.

28. El Niños are red La Niñas are blue But with global warming The latter are few?

29. SOI Temperature 1951-2004 Precipitation 1979-2004

30. ENSO Impacts Occur around the world Droughts (e.g. Australia, Africa, Brazil, Indonesia) Floods (e.g. Peru, southern USA)

31. El Niño: For El Niño there is a build up of heat in the tropical Pacific prior to and during the event, which is then moved out of the tropical Pacific to higher latitudes in the ocean and into the atmosphere through latent heating (evaporation). This causes a mini-global warming in the atmosphere that peaks several months after the Niño SSTs peak.

32. El Niño: Location of convection controlled by SST patterns and region of warmest water. Currents Upwelling Sea Surface Temperatures Surface winds Atmospheric Circulation Evaporation Pacific Ocean Circulation Teleconnections Warm Pool Tropical Rain Heat from ocean goes into atmosphere mainly through evaporation. Teleconnections in atmosphere alter the stationary waves and hence the storm tracks.

33. Quelccaya Ice Cap Peru Annual layers of ice reveal El Niños Courtesy Lonnie Thompson

34. How will El Niño events change with global warming? El Niño involves a build up and depletion of heat as well as major redistribution of heat in the ocean and the atmosphere during the course of events. Because GHGs trap heat, they interfere. Possibly expand the Pacific Warm Pool. Enhance rate of recharge of heat losses. More warming at surface: enhanced thermocline  enhanced swings More frequent El Niños? Some models more El Niño-like with increased GHGs. But models do not simulate El Niño well Nor do they agree The hydrological cycle may speed up with increased GHGs. Increased evaporation enhances the moisture content of the atmosphere which makes more moisture available for rainfall. ENSO-related droughts are apt to be more severe and last longer, while floods are likely to be enhanced. El Niño involves a build up and depletion of heat as well as major redistribution of heat in the ocean and the atmosphere during the course of events. Because GHGs trap heat, they interfere. Possibly expand the Pacific Warm Pool. Enhance rate of recharge of heat losses. More warming at surface: enhanced thermocline  enhanced swings More frequent El Niños? Some models more El Niño-like with increased GHGs. But models do not simulate El Niño well Nor do they agree The hydrological cycle may speed up with increased GHGs. Increased evaporation enhances the moisture content of the atmosphere which makes more moisture available for rainfall. ENSO-related droughts are apt to be more severe and last longer, while floods are likely to be enhanced.

35. Can we explain the linkages between climate variability and trends in patterns? Cold Hot PDO? NAO?

36. H H L L cold & dry warm & wet The NAO: North Atlantic Oscillation Most of the variability arises from processes internal to the atmosphere and is thus random and unpredictable. However, lower frequency variations are apparent, including a trend toward the positive index phase during the last half century Is this related to climate change and what is the role of the ocean? Winter Index 1864-2000

37. Thompson et al., J. Climate, 2000. NAO change Trends in the AO have affected the structure of SAT and precip trends over Europe and elsewhere. AO sfc T anoms 1950-96 °C

38. Global increases in SST are not uniform. Why?  Coupling with atmosphere  Tropical Indian Ocean has warmed to be competitive as warmest part of global ocean.  Tropical Pacific gets relief owing to ENSO?  Deeper mixing in Atlantic, THC. This pattern is NOT well simulated by coupled models! Relates to ocean uptake of heat, heat content & transport. Global increases in SST are not uniform. Why?  Coupling with atmosphere  Tropical Indian Ocean has warmed to be competitive as warmest part of global ocean.  Tropical Pacific gets relief owing to ENSO?  Deeper mixing in Atlantic, THC. This pattern is NOT well simulated by coupled models! Relates to ocean uptake of heat, heat content & transport. INDIAN OCEAN WARMING relative to 1951-80 mean GISS INDIAN OCEAN WARMING relative to 1951-80 mean GISS

39. Time series of monthly SST anomalies over the equatorial Indian and western Pacific Oceans (lower left) and eastern equatorial Pacific Ocean (lower right). J. Hurrell et al. 2004. NAO: Linear trends (JFM, 1950-99) of observed and simulated (multi- model Global Ocean-Global Atmosphere (GOGA) ensemble mean) 500 hPa heights (top) and observed tropical SST (middle).