Quasi-­‐Biennial Oscilla.on h1p://www.nwra.com/resumes/baldwin/ Observa.on of Stratospheric Easterlies a@er the erup.on of Krakatoa on 27 Aug 1883 Observa.on of Stratospheric Easterlies a@er the erup.on of Krakatoa on 27 Aug 1883 Hamilton, K., 2012: Sereno Bishop, Rollo Russell, Bishop’s Ring and the Discovery of the “Krakatoa Easterlies” Atmosphere-­‐Ocean 50 , 169–175 Observed structure of the QBO at equatorial la5tudes: •  alterna.ng easterly and westerly wind regimes propagate downward with .me; •  westerlies move down faster and more regularly than easterlies; •  the transi.on to easterlies is o@en delayed between 30 and 50 hPa; •  easterlies are generally stronger (30-­‐35 m/s) than westerlies (15-­‐20 m/s); •  maximum amplitudes of both phases typically occur near 20-­‐hPa; •  the average period is about 27 months; •  both period and amplitude considerably vary from cycle to cycle. Time-­‐height sec.on of monthly mean zonal winds (m/s) at equatorial sta.ons: Canton Island, 3°S/
172°W (Jan 1953 -­‐ Aug 1967), Gan/Maledive Islands, 1°S/73°E (Sep 1967 -­‐ Dec 1975) and Singapore, 1°N/
104°E (since Jan 1976). (From: h1p://www.geo.fu-­‐
berlin.de/en/met/ag/strat/produkte/qbo/) Thermally Damped Equatorial Kelvin Wave Wave structure Length propor.onal to wave amplitude Induced mean flow accelera.on (wave stress divergence) Phase propaga.on: EASTERLY Change in sign of COR at equator allow Kelvin waves to exist: U balanced by y-­‐dir PGF From Matsuno (1966) and Holton Thermally Damped Equatorial Rossby-­‐Gravity Wave Wave structure Zonal and ver.cal wind perturba.ons Change in sign of COR at equator allow Kelvin waves to exist: U balanced by y-­‐dir PGF From Matsuno (1966) and Holton Induced mean flow accelera.on (wave stress divergence) Meridional wind perturba.ons Sources of QBO Momentum u’ < 0 w’ < 0 u’w’ >0 u’ > 0 w’ > 0 u’w’ >0 Equatorial Kelvin Wave: Ver.cal flux of momentum a good representa.on of source of westerly momentum in QBO u’ > 0 w’ > 0 u’w’ >0 u’ < 0 w’ < 0 u’w’ >0 Rossby-­‐Gravity Wave: Can not use ver.cal momentum flux alone, v’T’ needs to be considered. Responsible for source of easterly QBO in a Tank : h1p://www.gfd-­‐dennou.org/library/gfd_exp/exp_e/exp/bo/1/app.htm momentum in QBO Sources of QBO Momentum Transi.on from E-­‐ly to W-­‐ly at 50 hPa Dunkerton (1990): QBO is somewhat synchronized to the annual cycle. Transi.on to easterlies at 50 hPa: NH late spring/summer Either transi.on: May – June period Transi.on from W-­‐ly to E-­‐ly at 50 hPa Sources of QBO Momentum Westerly Shear Easterly Shear Plum and Bell (1982) From: Baldwin, M.P. et al., 2001: The Quasi-­‐Biennial Oscilla.on. Reviews of Geophys., 39, 179-­‐229. 40 hPa QBOEly -­‐ QBOWly From: Baldwin, M.P. et al., 2001: The Quasi-­‐Biennial Oscilla.on. Reviews of Geophys., 39, 179-­‐229. 40 hPa QBOEly -­‐ QBOWly QBO Impacts: Mid and High La5tudes In the winter hemisphere in the lower stratosphere: •  Tropical winds alter the effec.ve waveguide for upward/equatorward propaga.ng planetary waves •  Easterly Phase of the QBO: o  Focus more wave ac.vity toward the pole, wave ac.vity converges à slows zonal mean winds o  Weaker polar vortex à impacts the troposphere QBO Impacts: Mid and High La5tudes Holton-­‐Tan Mechanism (1980)