Lecture 08 Short-term Climate Variability 短期氣候變異

1. Lecture 08 Short-term Climate Variability 短期氣候變異

2. Climate: 30 years interval （WMO: 1951-1980） Climate variability: fluctuations within 30 years Climate change: differences between intervals Time scales: Climate change: a hundred- to a thousand year Climate variability: interannual to decadal scales

3. UNFCCCmakes a distinction between “climate change” attributable to human activities altering the atmospheric composition, and “climate variability” attributable to natural causes (both internal and external). United Nations Framework Convention on Climate Change

4. Natural Climate Variability An act of God BothInternalandexternal Internalclimate processes and feedbacks may cause variations in the radiative balance by their impact on the reflected solar radiation or emitted infrared radiation, but such variations are not considered part of radiative forcing. Externalforcings, such as the solar radiation or the large amounts of aerosols ejected by volcanic eruption into the atmosphere, may vary on widely different time-scales, causing natural variations in the radiative forcing.

5. El Nino(聖嬰) andLa Nina(辣妞)

7. What is El Niño? 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

8. ENSO is a natural phenomenon. It simply just happens.

9. Normally, the trade winds and equatorial currents flow toward the west. At the same time, an intense Peruvian current causes upwelling of cold water along the west coast of South America.

10. The Walker Cells The East-West SST gradient in ocean is tied to the pressure fluctuation in air.

11. Annual march of Sea Surface Temperature

12. Originally, the term “El Nino” was used to describe the episodic appearance of warm water by the end of year (Christmas) in the far east tropical Pacific.

13. Why studying El Nino ? Increased SSTs cause Enhanced convection Upper troposphere: outflow, divergence Anticyclonic pair Wave-train Storm tracks: southward, enhanced northward, diminished Increased SSTs cause Enhanced convection Upper troposphere: outflow, divergence Anticyclonic pair Wave-train Storm tracks: southward, enhanced northward, diminished Tele-connection Rossby wave train (Trenberth et al. 1998)

14. Typical Jan-March weather anomalies and atmospheric circulation during El Nino and La Nina. Each El Nino (or La Nina) event is unique.

17. El Nino and La Nina or the El-Nino Southern Oscillation (ENSO) is a quasi-periodic climate pattern occurring across the Pacific Ocean. It is a pattern that affects weather around the world. In this pattern there are two oscillating phases: El Nino, which is characterized by warmer weather, and La Nina, which is characterized by cooler weather. ENSO generally occurs alternatively every 3 to 5 years, however it has historically varied at the outskirts 2 to 7 years.

18. Various Nino indices

19. El Nino conditions (warm ENSO phase) Weakened annual cycle

20. La Nina conditions (cold ENSO phase) Enhanced annual cycle

21. El Nino trade winds weaken trade winds strengthen upwelling slows upwelling intensifies Eastern Pacific warms Eastern Pacific cools Bjerknes Feedback loop La Nina

22. El Niño theories • Previous El Niño sows the seeds for the next • Leads to quasi-cyclic behavior • Bit like a pendulum • El Niños occur 31% of time, La Niñas 23% • Ocean is “primed” for an event to occur • But the trigger may be “random weather” • “Delayed Oscillator” • Ocean waves are a key: • Rossby (off equatorial, westward traveling) • Kelvin (equatorial, eastward traveling) Waves reflect off boundaries and return altered

23. Q’SW<0 Q’L<0 Q’L>0 Cooling SST tendency Warming SST tendency West East “random weather” as a triggering mechanism Warm SST’ Westerly winds

25. Subsurface ocean temperature change along equator during the development of 1997-98 ElNiño 1997. 8 1996. 12 1997. 4 1997. 12 There is an embedded precursor in the equatorial western Pacific warm pool one year ago !

26. Is global warming contributing to changes in El Niño? Likely, yes, to some extent. But, which part is natural/anthropogenic variability? How will El Niño events change with global warming?

27. 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 is expected to 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.