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Climate System Dynamics and Feedback Mechanisms

Explore the different types of climate systems, feedback processes, and their impacts on global climate. Learn about the Walker circulation, El Niño and La Niña, Southern Oscillation, Pacific Decadal Oscillation, extreme climate anomalies, challenges in climate simulation, cloud radiative effects, cloud-climate feedback, cloud formation, precipitation, and aerosol feedback.

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Climate System Dynamics and Feedback Mechanisms

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  1. 1. Systems • Open System: Energy and Matter can be exchanged between systems • Closed System: Exchange of Matter greatly restricted, but may allow exchange of energy • Isolated System: No Energy or Matter can be transferred in or out of the system • Stable System: resists change and reverts back to this state when disturbed • Unstable System: Once disturbed the system cannot return to the original state • Metastable System: Can have several stable states.

  2. 2. Feedback • Processes in one system influences processes in another interconnected system by exchange of matter and energy. The exchange is called feedback. • Positive Feedback: Change in one system causes similar change in the other system. Can cause runaway instability • Negative Feedback means a positive change in one system causes a negative change in the other Changing CO2 induces positive water vapor feedback Changing CO2 induces positive albedo feedback

  3. 3. Low frequency climate variability: sub-seasonal variation, seasonal variation, annual variation, and interannual variation. 4. Walker circulation The Walker Circulation refers to an east-west circulation of the atmosphere above the tropical ocean in the zonal and vertical directions, with air rising above warmer ocean regions (normally in the west), and descending over the cooler ocean areas (normally in the east). Its strength fluctuates with the change in sea surface temperature.

  4. 5. El Niño and La Niña El Niño is characterized by unusually warm ocean temperatures in the Equatorial Pacific, as opposed to La Niña, which characterized by unusually cold ocean temperatures in the Equatorial Pacific. El Niño is an oscillation of the ocean-atmosphere system in the tropical Pacific that is closely related to the change in the Walker circulation and has important consequences for weather and climate around the globe. La Niña condition Stronger Walker circulation Weakened Walker circulation El Niño condition

  5. 6. Southern Oscillation Darwin Tahiti 1958-1998 The Southern Oscillation is the atmospheric component of El Niño/ La Nina. This component is an oscillation in surface air pressure between the tropical eastern Pacific and the western Pacific Ocean waters. El Niño/La Niña-Southern Oscillation (ENSO)

  6. 7. Impact of ENSO on Global Climate Teleconnections via atmospheric Rossby waves

  7. 8. ENSO and hurricane • Less hurricane days during El nino years mainly due to stronger vertical wind shear • More hurricane days during La nina years mainly due to weaker vertical wind shear.

  8. 9. Pacific Decadal Oscillation (PDO) PDO is a long-lived ENSO-like pattern of Pacific climate variability usually persisting for a long time period about 20-to-30 years. ENSO and PDO are not the independent anomalies but are somehow linked phenomena.

  9. 10. Some extreme climate anomalies (a) A decade of western North American drought could be related to both human activities and natural climate anomalies, such as ENSO. (b) A possible cause for the 2003 European heat wave is the polarwaord migration of polar jets in a warm climate. (c) The vanishing snow of Kilimanjaro may be due to the fact that the maximum warming occurs in the mid troposphere over the Equator.

  10. 11. Challenges of numerical simulation of climate • Insufficient observations – leading to inaccurate initial conditions; • Chaotic nature of the atmospheric and oceanic system; • Inherent deficiency of numerical models with limited resolution that fails to resolve sub-grid physical processes. Our answers to face the challenges: • Data assimilation; • Ensemble forecast; • Parameterization.

  11. 12. Cloud radiative effect Cooling effect: reflecting solar radiation Warming effect: absorbing and emitting longwave radiation Current climate: Shortwave cloud forcing: -50 W/m2 (cooling) Longwave cloud forcing: 30 W/m2 (warming) Net cloud forcing ΔCRF: -20 W/m2 (cooling)

  12. 13. Cloud-climate feedback The impact of clouds on global warming depends on how the net cloud forcing changes as climate changes. 14. Cloud radiative effects depend on height. Low cloud High cloud SW cloud forcing dominates, cooling effect LW cloud forcing dominates, warming effect

  13. 15. In general circulation models (GCMs), clouds are the sub-grid scale processes and are not resolved. They are represented parametrically in models. The cloud-climate feedback is one of the largest uncertainties in climate simulations. 16. Cloud formation Two processes, acting together or individually, can lead to air becoming saturated: cooling the air or adding water vapor to the air. But without cloud nuclei, clouds would not form. 17. Precipitation Cloud droplets need to grow up to a certain size in order to fall to the surface due to gravity

  14. 18. Aerosol feedback Direct aerosol effect: scattering, reflecting, and absorbing solar radiation by particles. Primary indirect aerosol effect (Primary Twomey effect): cloud reflectivity is enhanced due to the increased concentrations of cloud droplets caused by anthropogenic cloud condensation nuclei (CNN). Secondary indirect aerosol effect (Second Twomey effect): 1. Greater concentrations of smaller droplets in polluted clouds reduce cloud precipitation efficiency by restricting coalescence and result in increased cloud cover, thicknesses, and lifetime. 2. Changed precipitation pattern could further affect CCN distribution and the coupling between diabatic processes and cloud dynamics.

  15. 19. Climate Scenarios and Emissions Scenarios What is a scenario? • Image of future • Neither forecast nor prediction • Each scenario is one possible future • Useful tool for not fully understood complex systems, whose prediction is impossible • Emission scenario ≠ climate scenario Main driving forces of future emissions: 1. Population prospects 2. Economic development 3. Energy intensities and demand, structure of its use 4. Resource availability 5. Technological change 6. Prospects for future energy systems 7. Land-use changes

  16. 20. Storylines of scenarios A1: • Rapid economic growth. • Peak population mid-21st century, then, declining. • Rapid introduction of new and more efficient technologies. • Substantial reduction of regional difference in per-capita income. A1FI: Fossil fuel intensive A1B: Balanced emphasis on all energy sources A1T: Non-fossil fuel intensive • Regional solutions to environmental and social equity issues. • Continuously rising world population. • Slow per-capita income growth technological development A2: • Rapid changes in economic structures. • Peak population mid-21st century, then, declining, as in A1. • Reduction in intensity of demand for materials. • Introduction of clean and resource efficient technologies. • Global solutions to environmental and social equity issues. B1: B2: • Intermediate economic development. • Moderate population growth. • Less rapid and more diverse technological change than in the B1 and A1. • Regional solutions to environmental and social equity issues.

  17. 21. Uncertainties associated with Scenario Analysis and climate change projection • Three types of uncertainties: • data uncertainties, • modeling uncertainties, • completeness uncertainties. 22. Carbon-cycle feedbacks a. Warmer land b. Warmer ocean c. Ocean acidification d. Pump problems e. A sluggish ocean f. Rock weathering The above processes can induce either positive or negative carbon-cycle feedbacks. But overall, positive feedbacks prevail!

  18. 23. Stabilizing atmospheric CO2 level a. The lower the stabilization target, the sooner peak emission of CO2 must occur, or we must cut back on fossil-fuel use, e.g., to stabilize CO2 level at 450 ppm, we would reach peak usage before 2020. b. Lower stabilization levels can be achieved only with lower peak emission. c. All stabilization targets require sharp reductions in CO2 emission after the peak. Low stabilization targets require that the emission rates fall below the current rates within a few decades.

  19. 24. Changes in the Oceans due to global warming Melting glaciers Changes in deep-ocean circulation (slowing down) Warmer surface waters 25. Polar Ice Melting Loss of ice = enhanced warming due to lower albedo Arctic ice melting affects polar bear survival. Food sources are dwindling for Arctic dwellers. Sea level rise

  20. Organisms threatened by Increased Marine Acidity • Coccolithophores • Foraminifers • Sea urchins • Corals • 27. Rising Sea Level – already occurring • Main contributors: • Melting of Antarctic and Greenland ice sheets (most important) • Thermal expansion of ocean surface waters • Melting of land glaciers and ice caps • Thermal expansion of deep-ocean waters 26. Ocean acidity increase Some atmospheric carbon dioxide dissolves in ocean water. ----- Acidifies ocean

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