A Stratospheric Approach to Diagnose the North American Winter Dipole

iii PUBLIC ABSTRACT . v ACKNOWLEDGMENTS . vi CONTENTS . vii LIST OF FIGURES . viii ACRONYMS . x CHAPTER I. GENERAL INTRODUCTION . 1 II. STRATOSPHERIC FORCING OF THE NAWD . 6 Introduction . 6 Data and methods . 9 Results . 12 Vertical structures associated with the NAWD . 12 Stratosphere-troposphere relationship . 14 Downward migration of stratospheric anomalies . 28 Summary and discussion . 31 III. GENERAL CONCLUSIONS . 55 References . 57 viii LIST OF FIGURES Figure Page 1 Detrended vertical composition correlations between zonal-mean zonal wind for a November/December/January (NDJ) and b December/January/ February (DJF) seasonal averages with DJF Dipole Index (DI) from 19792018. DI is calculated by taking the area-averaged 250mb geopotential height at 225-230°E and 50-55°N and subtracting by the area-averaged 250mb geopotential height at 280-285°E and 55-60°N. Correlation values are shaded with contours for the 1979-2018 zonal-mean zonal wind climatology for a NDJ and b DJF. Hatching represents statistical significance at the 95% confidence level . 41 2 Lead/lag vertical composition correlations between the zonal-mean zonal wind at 65°N and Dipole Index (DI) from 1979-2018. In a the zonal-mean zonal wind at 65°N is varying with green dots representing October/November/December (OND) averages, orange dots NDJ averages, and blue dots DJF averages, while DI stays constant with DJF averages. In b DI is varying with green dots representing OND averages, orange dots NDJ averages, and blue dots DJF averages, while the zonal-mean zonal wind at 65°N stays constant with NDJ averages. Yellow shading represents statistical significance at p=0.1, orange at p=0.05, and red at p=0.02 . 42 3 Scatterplot with normalized 30mb NDJ zonal-mean zonal wind at 65°N (x-axis), and normalized 250mb DJF DI (y-axis). Red lines represent cutoff points of 0.5 standard deviations . 43 4 Horizontal composites of a, c, e, g 30mb NDJ and b, d, f, h 250mb DJF real wind (vectors) and geopotential height (shading) anomalies from the 1979-2018 climatology for a, b PNJ+DI+ c, d PNJ-DIe, f PNJ-DI+ and g, h PNJ+DIcomposite groups. Red x’s denote the regions used to calculate Dipole Index . 44 5 1000-10mb full column vertically integrated DJF meridional wind anomalies (shading) from the 1979-2018 climatology, for a PNJ-DI+ b PNJ-DIc PNJ+DI+ d PNJ+DIcomposite groups. Red x’s denote the regions used to calculate Dipole Index . 45 6 Vertical compositions of a NDJ and b DJF meridional wind anomalies for each composite group from the 1979-2018 climatology in the DI region (area-averaged 250mb geopotential height at 225-285°E and 50-60°N). ++ corresponds to PNJ+DI+, -with PNJ-DI-, -+ with PNJ-DI+, and +with PNJ+DI.46 ix 7 Same as Fig. 6 but with zonal wind . 47 8 Latitude-altitude composites of a, c, e, g NDJ and b, d, f, h DJF zonal-mean zonal wind anomalies (shading) from the 1979-2018 climatology (contours), for a, b PNJ+DI+ c, d PNJ-DIe, f PNJ-DI+ and g, h PNJ+DIcomposite groups . 48 9 Vertical compositions of zonal-mean zonal wind at 25-35°N (blue contours), and 50-60°N (red contours) during NDJ (dashed) and DJF (solid), for a PNJ-DI+ b PNJ-DIc PNJ+DI+ d PNJ+DIcomposite groups . 49 10 Horizontal compositions of a, c, e, g 30mb NDJ and b, d, f, h 250mb DJF zonal wind anomalies (shading) from the 1979-2018 climatology (contours), for a, b PNJ+DI+ c, d PNJ-DIe, f PNJ-DI+ and g, h PNJ+DIcomposite groups . 50 11 Same as Fig. 5 but with zonal wind . 51 12 Time-altitude October-March expansions of a, c, e, g 25-35°N and b, d, f, h 50-60°N zonal-mean zonal wind anomalies (shading) from the 1979-2018 October-March monthly climatology, for a, b PNJ+DI+ c, d PNJ-DIe, f PNJ-DI+ and g, h PNJ+DIcomposite groups . 52 13 Time-altitude October-March expansions of a, c, e, g zonal wind and b, d, f, h meridional wind anomalies (shading) from the 1979-2018 OctoberMarch monthly climatology in the DI Region, for a, b PNJ+DI+ c, d PNJ-DIe, f PNJ-DI+ and g, h PNJ+DIcomposite groups . 53 14 Same as Fig. 13 but with geopotential height. a, c, e, g is in the vicinity of the ridge (area-averaged geopotential height at 225-230°E and 50-55°N), and b, d, f, h is near the trough (area-averaged geopotential height at 280285°E and 55-60°N) . 54