Intraseasonal and Interannual Variability

1. Intraseasonal and Interannual Variability M. D. Eastin

2. Outline • Tropical phenomena • Madden Julian Oscillation (MJO) • Quasi-Biennial Oscillation (QBO) • Mid-Latitude phenomena with links to the Tropics • North Atlantic Oscillation (NAO) • Pacific Decadal Oscillation (PDO) • Comments on Global Warming and Data Records M. D. Eastin

3. Madden-Julian Oscillation (MJO) • Definition and Background • The MJO is an intraseasonal oscillation • between periods of enhanced and suppressed • tropical rainfall with a period of 30-60 days • Discovered by R. Madden and P. Julian in 1971 • Evolution • Each regime or phase (wet and dry) tends to • develop in the Indian Ocean and propagate • eastward at ~5 m/s into the central Pacific • More pronounced in the eastern Hemisphere • Upon crossing in the dateline, the convective • intensity decreases and the regimes begin • to propagate eastward at >10 m/s • Often hard to “track” in the western Hemisphere • Intensity is influenced by ENSO • Impacts • Affects the intensity and breaks periods of the • Asian (Indian) and Australian monsoon • Wet phase tends to enhance TC activity 30-60 Day Filtered OLR Anomalies (Blue = Convection Red = dry) M. D. Eastin

4. Madden-Julian Oscillation (MJO) • Structure • Wet phase is characterized by an “envelope” • of deep convection composed of numerous • higher frequency “events” (equatorial waves, • TCs, and tropical MCS) that propagate both • east and west through the envelope • Causes (or Forcing Mechanisms) • Unknown • Currently believed to result from both external • forcing and internal instabilities 30-60 Day Filtered OLR, Streamfunction, and Wind Anomalies shading → convection contours → streamfunction vectors → winds 200 mb Zonal Cross-Section 850 mb Wave motion Cool Cool Warm Moist Dry Dry Warm Cool Moist Cool SST Warm SST Cool SST Adapted from Kiladis et al. (2005) and Zhang (2005) M. D. Eastin

5. Madden-Julian Oscillation (MJO) Current Status: Let’s go to: http://www.cpc.noaa.gov/products/precip/CWlink/MJO/mjo.shtml M. D. Eastin

6. Quasi-Biennial Oscillation (QBO) • Definition and Background • The QBO is a quasi-periodic interannual oscillation of the tropical stratospheric zonal winds • between easterlies and westerlies with a mean period of 28-29 months • First recognized by R. J. Reed and R. A. Ebdon in 1960 • First referred to as the QBO by Jim Angell in 1964 • Each wind regime or phase (east and west) develops at the top of the stratosphere and • propagates downward at ~1 km per month until they dissipate at the tropical tropopause • The west phase (westerly winds) descends faster than the east phase • The east phase amplitude (20-30 m/s) is larger than the west phase amplitude (10-20 m/s) • The phases are coherent through the entire equatorial belt at any given time • Peak amplitudes are over the Equator at an altitude of 24 km (~30 mb) • Amplitudes decrease away from the equator (i.e. “equatorially trapped” → Why?) Time-Height (Pressure) diagram of Equatorial Zonal Wind East East East East East West West West West M. D. Eastin

7. Quasi-Biennial Oscillation (QBO) • Causes (or Forcing Mechanisms) • QBO is equatorially trapped →Forcing is from equatorial waves (also trapped) that verticallypropagate into the stratosphere → QBO physics described by Holton and Lindzen (1972), Plumb (1977), Plumb (1984), Baldwin et al. (2001) • Vertical Propagation of Equatorial Waves (the Basics) • Waves can propagate vertically until they reach a “critical layer” (which acts like a sponge) • Critical layers are determined by the mean flow characteristics and the wave type • When a wave encounters a critical layer, the wave is first damped and then dissipates • Damped waves deposit their propagation momentum into the mean flow, accelerating • the mean flow in the direction of wave propagation • “Critical Layer” • Wave Flow the wavesFlow the waves • PropagationCAN verticallycan NOT verticallyMomentum • Wave typeDirection propagate throughpropagate through Acceleration • Inertio-Gravity eastward westward strong eastward eastward • Kelvin • Inertio-Gravity • Rossby-Gravity westward eastward strong westward westward • Rossby M. D. Eastin

8. Quasi-Biennial Oscillation (QBO) QBO Physics: Switching from the East phase to the West phase in the lower stratosphere Time 0 Time 1 dissipate at upper levels start new west phase aloft westward flow max descends Eastward Flow eastward flow max descends pass unimpeded damped and accelerate mean flow Westward Propagating Waves Eastward Propagating Waves M. D. Eastin

9. Quasi-Biennial Oscillation (QBO) QBO Physics: Switching from the East phase to the West phase in the lower stratosphere Time 1 Time 2 Westward Flow damped and accelerate mean flow westward flow max descends Eastward Flow pass unimpeded eastward flow max descends damped and accelerate mean flow Westward Propagating Waves Eastward Propagating Waves M. D. Eastin

10. Quasi-Biennial Oscillation (QBO) QBO Physics: Switching from the East phase to the West phase in the lower stratosphere Time 2 Time 3 Westward Flow westward flow max descends damped and accelerate mean flow damped and accelerate mean flow pass unimpeded Westward Propagating Waves Eastward Propagating Waves thin eastward max destroyed by viscous diffusion M. D. Eastin

11. Quasi-Biennial Oscillation (QBO) QBO Physics: Switching from the West phase to the East phase in the lower stratosphere Time 3 Time 4 dissipate at upper levels start new east phase aloft Westward Flow eastward flow max descends westward flow max descends pass unimpeded damped and accelerate mean flow Westward Propagating Waves Eastward Propagating Waves M. D. Eastin

12. Quasi-Biennial Oscillation (QBO) QBO Physics: Switching from the West phase to the East phase in the lower stratosphere Time 4 Time 5 Eastward Flow damped and accelerate mean flow eastward flow max descends westward flow max descends Westward Flow pass unimpeded damped and accelerate mean flow Westward Propagating Waves Eastward Propagating Waves M. D. Eastin

13. Quasi-Biennial Oscillation (QBO) QBO Physics: Switching from the West phase to the East phase in the lower stratosphere Time 5 Time 6 Eastward Flow eastward flow max descends damped and accelerate mean flow damped and accelerate mean flow pass unimpeded Westward Propagating Waves Eastward Propagating Waves thin westward max destroyed by viscous diffusion M. D. Eastin

14. Quasi-Biennial Oscillation (QBO) QBO Physics: Switching from the East phase to the West phase in the lower stratosphere Time 0 or Time 6 Time 1 or Time 7 dissipate at upper levels start new west phase aloft westward flow max descends Eastward Flow eastward flow max descends pass unimpeded damped and accelerate mean flow Westward Propagating Waves Eastward Propagating Waves M. D. Eastin

15. Quasi-Biennial Oscillation (QBO) • Why is the QBO Important? • The phase of the QBO affects Atlantic TC activity • West phase → Increased TC activity • East phase → Decreased TC activity • West Pacific TC frequency increases during the West phase • Southwest Indian Ocean TC frequency increased during the East phase • Believed to impact the onset and intensity of monsoon circulations • Used by some statistical predictions of ENSO to forecast strength and timing of events • Believed to impact west African rainfall • Decay of aerosol loading from volcanic eruptions depends on QBO phase M. D. Eastin

16. North Atlantic Oscillation (NAO) • Definition and Background • The NAO is an interannual meridional oscillation of mass within the North Atlantic • defined by the normalized surface pressure difference between the Iberian Peninsula • and (minus) Iceland • First recognized by the Danish in the 1770s • First researched and defined by Sir Gilbert Walker in the 1920s • Primarily influences the strength of mid-latitude westerly winds and storm tracks • Influence is most prominent in the winter months (Nov-Apr) • Impacts the seasonal climate from eastern North America to central Asia and • from tropical to polar latitudes Winter normalized NAO index: 1900-2012 Source: http://www.cgd.ucar.edu/cas/jhurrell/indices.html M. D. Eastin

17. North Atlantic Oscillation (NAO) • Positive NAO Index • Low pressure anomaly over Iceland (a • stronger Icelandic Low) and a high • pressure anomaly over the subtropical • Atlantic (a stronger Azore High) • Enhanced mid-latitude westerlies • Enhanced tropical trade winds • More intense SAL outbreaks • Warm over Europe and the eastern U.S. • Cold over the polar North Atlantic and • the tropical North Atlantic • Wet from Iceland to Scandinavia • (contributes to advancing glaciers) • Dry over southern Europe • More intense winter storms in • northern Europe • Milder winters in eastern U.S. • Dominant phase from the early 1980s • through the mid-2000s (will it continue?) M. D. Eastin

18. North Atlantic Oscillation (NAO) • Negative NAO Index • High pressure anomaly over Iceland (a • weaker Icelandic Low) and a low • pressure anomaly over the subtropical • Atlantic (a weaker Azore High) • Suppressed mid-latitude westerlies • Suppressed tropical trade winds • Less intense SAL outbreaks • Cold over Europe and the eastern U.S. • Warm over the polar North Atlantic • (enhanced ice sheet melting) and • the tropical North Atlantic • Dry over Iceland to Scandinavia • Wet over southern Europe and • eastern North America • More intense winter storms in • the eastern U.S. • Dominant phase from the mid 1950s • to the late 1970s M. D. Eastin

19. North Atlantic Oscillation (NAO) • Causes (or Forcing Mechanisms) • Thought to be driven by “natural internal atmospheric processes” (i.e. unknown) • High correlation with Atlantic SST and ice variability, but it’s unclear which causes which • Interesting “Relationship” to Global Warming • The negative phase (cold North America and Europe) was dominant 1955-1979 • The positive phase (warm North America and Europe) has dominated since (1980-2005) • The majority of “reliable” global surface observations are made over land • (in particular over North America, Europe, and Asia) • Could the recent “global warming” be an artifact of the NAO and our observation networks? Global Climate Observing Network 981 total stations M. D. Eastin

20. Pacific-Decadal Oscillation (PDO) • Definition and Background • The PDO is an multi-decadal (20-30 year) oscillation of SST within the North Pacific • defined by normalized (and de-trended) SST anomalies north of 20ºN • First recognized by Steven Hare (a fisheries scientist) in 1996 • First related to climate by Yuan Zhang in 1997 • Influence is most prominent in the winter months (Oct-Mar) • Impacts the seasonal climate from eastern Asia and North America • Climate impacts that occur during each phase are similar to the ENSO impacts • but are less intense and occur over a longer timescale • The cause of the PDO is unknown Monthly values of the normalized PDO index: 1900-2012 Source: http://jisao.washington.edu/pdo/ M. D. Eastin

21. Pacific-Decadal Oscillation (PDO) • Positive PDO index (Warm phase) • Colder SSTs in the central North Pacific • Warmer SSTs along the west coast of • North America and in the equatorial • central Pacific (similar to El Nino) • Low pressure anomaly over North Pacific • Warm winters in the Pacific Northwest • Cold winters in the southeast U.S. • Wet winters in western U.S. • Dry winters in U.S. Midwest Anomalous SST, Pressure, and Winds Warm Phase of PDO Correlation of Winter Surface Temperature and Precipitation with the PDO Index Temperature Precipitation M. D. Eastin

22. Pacific-Decadal Oscillation (PDO) • Negative PDO index (Cold phase) • Warmer SSTs in the central North Pacific • Colder SSTs along the west coast of • North America and in the equatorial • central Pacific (similar to La Nina) • High pressure anomaly over North Pacific • Cold winters in the Pacific Northwest • Warm winters in the southeast U.S. • Dry winters in western U.S. • Wet winters in U.S. Midwest Anomalous SST, Pressure, and Winds Cold Phase of PDO Correlation of Winter Surface Temperature and Precipitation with the PDO Index Temperature Precipitation M. D. Eastin

23. Global Warming? • Reliable global observations: • It is well accepted that reliable • global weather observations • began in the late 1940s • Hence → global warming “evidence” Winter normalized NAO index: 1900-2012 Monthly values of the normalized PDO index: 1900-2012 M. D. Eastin

24. Global Warming? • Reliable global observations: • Notice how the NAO and PDO exhibit • a negative trend from 1920-1960 • Could the recent trends in ENSO • NAO and PDO be an oscillation? ? ? Winter normalized NAO index: 1900-2012 Monthly values of the normalized PDO index: 1900-2012 ? M. D. Eastin

25. Intraseasonal and Interannual Variability • Summary: • Madden-Julian Oscillation (MJO) • Definition and Evolution • Causes / Forcing • Impacts • Quasi-Biennial Oscillation (QBO) • Definition and Basic Characteristics • Causes / Physics • Impacts • North Atlantic Oscillation (NAO) • Definition • Impacts during each Phase • Causes / Forcing • Pacific Decadal Oscillation • Definition • Impacts during each Phase • Causes / Forcing M. D. Eastin

26. References Baldwin, M. P., and Coauthors, 2001: The Quasi-Biennial Oscillation. Reviews of Geophysics, 39, 179-229. Bond, N. A., and D. E. Harrison (2000): The Pacific Decadal Oscillation, air-sea interaction and central north Pacific winter atmospheric regimes. Geophys. Res. Lett., 27(5), 731-734. Climate Diagnostic Center’s (CDCs) Interactive Plotting and Analysis Webage ( http://www.cdc.noaa.gov/cgi-bin/PublicData/getpage.pl ) Hurrell, J. W., Y. Kushnir, M. Visbeck, and G. Ottersen, 2003: An Overview of the North Atlantic Oscillation. The North Atlantic Oscillation: Climate Significance and Environmental Impact, J.W. Hurrell, Y. Kushnir, G. Ottersen, and M. Visbeck, Eds. Geophysical Monograph Series, 134, 1-35. Trenberth, K. E. and J. W. Hurrell, 1994: Decadal atmosphere-ocean variations in the Pacific. Clim. Dyn. 9, 303-319. Zhang, C., 2005: Madden-Julian Oscillation, Reviews of Geophysics, 43, 1-36. M. D. Eastin