The Arctic Polar-night Jet Oscillation by Peter Hitchcock A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Graduate Department of Physics University of Toronto Copyright (cid:13)c 2012 by Peter Hitchcock Abstract The Arctic Polar-night Jet Oscillation Peter Hitchcock Doctor of Philosophy Graduate Department of Physics University of Toronto 2012 The eastward winds that form each winter in the Arctic stratosphere are intermittently disrupted by planetary-scale waves propagating up from the surface in events known as stratospheric sudden warmings. It is shown here that following roughly half of these sud- den warmings, the winds take as long as three months to recover, during which time the polarstratosphereevolvesinarobustandpredictablefashion. Theseextendedrecoveries, termed here Polar-night Jet Oscillation (PJO) events, are relevant to understanding the response of the extratropical troposphere to forcings such as solar variability and climate change. They also represent a possible source of improvement in our ability to predict weather regimes at seasonal timescales. Four projects are reported on here. In the first, the approximation of stratospheric radiative cooling by a linear relaxation is tested and found to hold well enough to diag- nose effective damping rates. In the polar night, the rates found are weaker than those typically assumed by simplified modelling studies of the extratropical stratosphere and troposphere. In the second, PJO events are identified and characterized in observations, reanalyses, and a comprehensive chemistry-climate model. Their observed behaviour is reproduced well in the model. Their duration correlates with the depth in the strato- sphere to which the disruption descends, and is associated with the strong suppression of further planetary wave propagation into the vortex. In the third, the response of the zonal mean winds and temperatures to the eddy-driven torques that occur during PJO events is studied. The collapse of planetary waves following the initial warming permits ii radiative processes to dominate. The weak radiative damping rates diagnosed in the first project are required to capture the redistribution of angular momentum responsible for the circulation anomalies. In the final project, these damping rates are imposed in a simplified model of the coupled stratosphere and troposphere. The weaker damping is found to change the warmings generated by the model to be more PJO-like in character. Planetary waves in this case collapse following the warmings, confirming the dual role of the suppression of wave driving and extended radiative timescales in determining the behaviour of PJO events. iii For Lucy Pickard, my Grandma Africa. iv Acknowledgements Drink. It is what you will have to remember: rain’s vowelless syntax, how mathematics was an elegy, the slenderness of trees. Jan Zwicky – K. 219, Adagio To my supervisor, Ted Shepherd, for the depth of his insight, for the intellectual freedom he afforded me, for the strong sense of community he has cultivated in his group, and for all of his support. To Charles McLandress, Isla Simpson, Tiffany Shaw, James Anstey, Martin Keller, and Karen Smith for all of their answers to my questions, and for the many, many discussions. To Shigeo Yoden for his hospitality and the remarkable generosity he showed in shar- inghistimewithmeduringmyvisitstoKyoto. ToGloriaManneyforherencouragement and interest in my work, and for the colour schemes. To Charles McLandress, Michael Neish, Isla Simpson, and Michael Sigmond for tech- nical support here in Toronto; to John Scinocca and Norm McFarlane for sharing the depth of their familiarity with CMAM and climate modeling in general; and to Shunsuke Noguchi and Masakazu Taguchi for their collaboration and support in working with the mechanistic model. To my committee members, Paul Kushner and Kaley Walker, for their guidance throughout my time in Toronto. To my external examiner, Alan Plumb, for sharing his insight and his perspective on the relevance of these results. To Krystyna Biel and Ana Sousa for their administrative support. To NSERC, CFCAS, the Walter Sumner Foundation, CGCS, and JSPS for financial support. To Patricia for all the rolled eyes, and to Heather for never touching her nose. To Bel Helen for the poetry, and to Brad and the Chorus for the songs that needed patience to fall in love with. Finally, to my parents for their love, and to my father in particular for showing me just how much fun science can be. v Contents 1 Introduction 1 1.1 Dynamical ingredients . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.1 Eliassen adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.2 Radiative heating . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1.3 Wave, mean-flow interaction . . . . . . . . . . . . . . . . . . . . . 6 1.2 Variability of the Arctic Polar-night Jet . . . . . . . . . . . . . . . . . . . 10 1.2.1 Stratospheric sudden warmings . . . . . . . . . . . . . . . . . . . 14 1.2.2 The Polar-night Jet Oscillation . . . . . . . . . . . . . . . . . . . 15 1.3 Outline and contributions . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2 Data 22 2.1 Model simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.1.1 Canadian Middle Atmosphere Model . . . . . . . . . . . . . . . . 22 2.1.2 Mechanistic Circulation Model . . . . . . . . . . . . . . . . . . . . 24 2.2 Reanalyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.2.1 ERA40 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.2.2 MERRA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.2.3 ERA Interim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.3 Satellite observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.3.1 Microwave Limb Sounder . . . . . . . . . . . . . . . . . . . . . . . 26 3 Radiative Damping 27 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 3.2 Longwave relaxation rates . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.2.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.2.2 Linear regression estimates . . . . . . . . . . . . . . . . . . . . . . 33 vi 3.2.3 Effective longwave damping rates . . . . . . . . . . . . . . . . . . 36 3.3 Shortwave relaxation rates . . . . . . . . . . . . . . . . . . . . . . . . . . 42 3.4 Radiative-photochemical equilibrium temperature . . . . . . . . . . . . . 45 3.5 Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 46 4 Statistical characterization 49 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 4.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 4.2.1 Sudden warmings . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 4.2.2 Annular modes and weak vortex events . . . . . . . . . . . . . . . 57 4.2.3 Polar-night Jet Oscillation events . . . . . . . . . . . . . . . . . . 57 4.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 4.3.1 Relationship to sudden warmings . . . . . . . . . . . . . . . . . . 71 4.3.2 Relationship to weak vortex events . . . . . . . . . . . . . . . . . 76 4.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 5 Zonal mean dynamics 84 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 5.2 Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 5.3 Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 5.4 Residual circulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 5.5 Temperature anomalies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 5.5.1 Radiative relaxation . . . . . . . . . . . . . . . . . . . . . . . . . 100 5.5.2 Radiative relaxation with Eliassen adjustment . . . . . . . . . . . 102 5.5.3 Transient adjustment . . . . . . . . . . . . . . . . . . . . . . . . . 104 5.6 Stratopause descent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 5.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 5.A Zonal-mean quasi-geostrophy on the sphere . . . . . . . . . . . . . . . . . 111 6 Mechanistic circulation modelling 114 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 6.2 Model setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 6.3 Time-mean response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 6.3.1 Stratospheric changes . . . . . . . . . . . . . . . . . . . . . . . . . 118 vii 6.3.2 Tropospheric changes . . . . . . . . . . . . . . . . . . . . . . . . . 123 6.4 Variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 6.4.1 Abacus plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 6.4.2 Stratospheric composites . . . . . . . . . . . . . . . . . . . . . . . 130 6.4.3 Tropospheric response . . . . . . . . . . . . . . . . . . . . . . . . 132 6.4.4 Summary of transient response . . . . . . . . . . . . . . . . . . . 136 6.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 6.A Uncertainty estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 6.A.1 Time-averaged quantities . . . . . . . . . . . . . . . . . . . . . . . 139 6.A.2 Annular mode timescales . . . . . . . . . . . . . . . . . . . . . . . 139 7 Conclusion 141 7.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 7.1.1 Statistics of the PJO . . . . . . . . . . . . . . . . . . . . . . . . . 141 7.1.2 Dynamics of the PJO . . . . . . . . . . . . . . . . . . . . . . . . . 142 7.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 7.2.1 Detecting and attributing changes in the Arctic vortex . . . . . . 146 7.2.2 Seasonal forecasting . . . . . . . . . . . . . . . . . . . . . . . . . . 147 7.2.3 Surface influence of stratospheric forcings . . . . . . . . . . . . . . 148 7.3 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Bibliography 151 viii List of Tables 4.1 SSW classification in MERRA . . . . . . . . . . . . . . . . . . . . . . . . 55 4.2 Percentage of variance explained by EOFs of polar-cap averaged temper- atures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 4.3 Probability of PJO occurrence following sudden warmings . . . . . . . . 73 4.4 Probability of PJO occurrence following weak vortex events . . . . . . . . 77 ix List of Figures 1.1 EOFs of zonal mean zonal wind in the Arctic stratosphere during the win- tershowingthepolewardanddownwardmigrationofanomaliesassociated with the PJO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 1.2 TemperatureanomaliesobservedbytheMLSinstrumentduringthewinter of 2008-2009, showing a clear example of a PJO event. . . . . . . . . . . 17 3.1 Linear regression of longwave heating rates against local temperature an- omalies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 3.2 Regression of longwave heating rates including a term quadratic in the local temperature anomalies . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.3 Vertical profile of effective longwave damping rates by latitude band, com- pared against parameterization of Fels (1982) . . . . . . . . . . . . . . . 37 3.4 Effective longwave damping rates in the meridional plane by season . . . 38 3.5 Effective longwave damping rates in the tropics as a function of zonal wave number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 3.6 Effective longwave damping rates above the poles; failure of regression in lower Antarctic stratosphere during austral spring . . . . . . . . . . . . . 40 3.7 Influence of non-local temperature anomalies on radiative heating rates during the breakdown of the Antarctic polar vortex . . . . . . . . . . . . 41 3.8 Linear regressions of shortwave heating rates against local temperature anomalies as a function of local time . . . . . . . . . . . . . . . . . . . . 42 3.9 Effective shortwave damping rates in the meridional plane for December and June . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.10 Effective total radiative damping rates in the meridional plane as a func- tion of season . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 x