SPARC SOLARIS & HEPPA Intercomparison Activities: Global aspects of the QBO modulation of the solar influence on the stratosphere WCRP Open Science Conference October 2011 Denver, CO, USA Session C7:Atmospheric Composition and Forcings, Poster M06A Motivation/Introduction K. Kodera 1,2 and K. Matthes 3, 4 (contact: 1 STEL Nagoya University, Nagoya, Japan, 2 Meteorological Research Institute, Tsukuba, Japan, 3 Helmholtz Centre Potsdam, GFZ German Centre for Geosciences, Potsdam, Germany, 4 Institut für Meteorologie, Freie Universität Berlin, Berlin, Germany Results Summary/Discussion Acknowledgments: The authors thank Y. Kuroda for discussions and comments. KM is supported within the Helmholtz-University Young Investigators Group NATHAN funded by the Helmholtz-Association through the President’s Initiative and Networking Fund, the GFZ Potsdam, and the Freie Universität Berlin. References: Kodera, K., Y. Kuroda, H. Schmidt, and K. Matthes (2011), Mechanisms for Solar/QBO Influences on the Atmosphere, J. Geophys. Res., in preparation; Labitzke, K. (2001), The global signal of the 11-year sunspot cycle in the stratosphere: Differences between solar maxima and minima, Meteorologische Zeitschrift, Vol. 10, No.2, The global signal of the 11-year sunspot cycle in the stratosphere: Differences between solar maxima and minima, Meteorologische Zeitschrift, Vol. 10, No.2, 83-90; Labitzke, K. (2004), On the Signal of the 11-Year Sunspot Cycle in the Stratosphere and its Modulation by the Quasi-Biennial Oscillation (QBO), J. Atmos. Solar-Terr. Phys., 66, ; Labitzke, K., M. Kunze, and S. Brönnimann (2006), Sunspots, the QBO, and the Stratosphere in the North Polar Region - 20 Years later, Meteorologische Zeitschrift, Vol. 15, No. 3, (9). Labitzke and van Loon found that the solar influence appears clearly only when the data are stratified according to the phase of the stratospheric Quasi-Biennial Oscillation (QBO) at 45hPa (Fig. 1a). This exceptionally good relationship is seen, however, limited in time and space: in the lower stratospheric north polar region in February. More recent work of K. Labitzke showed that there is also a solar signal in the tropics (Fig. 1b). She also showed a global aspects of the QBO modulation of the solar signal in the stratosphere throughout the year (Fig. 3). An apparent QBO modulated solar signal is not only seen during boreal winter but also during boreal summer, i.e. austral winter. In this study we try to characterize the essential features of the solar/QBO modulation to get insight into a possible mechanism. Figure 4 shows similar composite differences of 30hPa temperature than Fig. 3, but the data have been stratified according to the phase of the QBO. In both phases of the QBO, the tropical warming is followed by the polar warming. During QBO-west conditions, the tropical warming is smaller and the polar warming occurs earlier compared to the QBO-east case. A characteristic feature of the impact of the QBO on the solar signal may be described as a change in the timing of the transition between tropical and polar warming. The well know Labitzke-van Loon relationship in February (Fig. 1) can be understood as a difference in transition time from tropical to polar warming. The solar signal in the stratosphere is characterized by an early winter tropical warming and a late winter polar warming due to the interaction with planetary waves. The influence of the QBO on the solar signal originates from a modulation of planetary wave propagation in the subtropical upper stratosphere and stratopause region, which can be produced by a secondary circulation associated with the vertical shears over the equator. Superposition of anomalous westerly winds associated with the QBO and the solar cycle (Fig. 6 left) creates a modulation effect through interaction with planetary waves as schematically shown in Fig. 6 (right). Figure 5 shows meridional cross-sections of composite differences of the zonal wind and the E-P flux vector (left) as well as temperature and mean meridional circulation between Smax and Smin in the NH winter for QBO-East (left) and QBO-West (right). Tropical and polar warmings correspond to stronger subtropical jet and weaker polar night jet situations, respectively. The dynamical influence of the solar UV heating first appears in the subtropical upper stratosphere and stratopause region as a strengthening of the subtropical jet, which then shifts poleward and downward through the interaction with planetary waves. Therefore the modulation of the solar signal by the QBO can be characterized also as change in the speed of poleward and downward movements of anomalous zonal winds. Conceptual model for solar response Solar signal Dynamical aspects of the Solar/QBO modulation Possible Mechanism for the Solar/QBO modulation North poleGlobal There are two possible responses to solar UV heating variation in the winter middle atmosphere. (i) Solar UV heating produces tropical warming => stronger westerly jet deflects planetary waves => reduced mean meridional circulation further enhances tropical warming. (ii) Solar UV heating is balanced by a dynamical cooling due to increased planetary wave forcing => enhanced mean meridional circulation produces polar warming and westerly jet is further weakened. Solar/QBO modulation Fig. 1 (a)(b) Fig. 3 Fig. 4 Fig. 5 Fig. 6 First (i) and second (ii) type responses are produced when wave activity is small and large, respectively. This implies that the solar warming starts from the tropics in early winter and shifts to the polar region in late winter as planetary wave activity increases during winter. Figure 3 shows the seasonal and latitudinal distribution of the differences of stratospheric temperatures between Smax and Smin. Tropical warming is followed by a polar warming in both NH and SH winters. This transition timing occurs later in spring in the SH due to smaller wave activity in the SH. Labitzke (2001) Labitzke (2004) Labitzke-van Loon relationship Fig. 2 (i) (ii)