Characteristics of Planetary Wave Activity during the Stratospheric Sudden Warming in the Winter of 2009

Based on MERRA reanalyzed data from December 2008 to April 2009, the major SSW event and the relevant Planetary Wave (PW) activities are investigated. Spectral analyses show that the stratosphere over Northern Hemispheric polar region is dominated by quasi-16-day PW during this SSW event. The corresponding amplitudes and phases of its four wave modes, i.e. W1, W2, E1, and E2, have been fitted by using the two-dimensional harmonic fitting method. The results show that all the modes display increasing amplitudes in varying degrees when the westerly wind weakens, and reach their peaks from 50°N to 80°N in the middle and upper stratosphere finally. Particularly, the W2 mode shows the largest increment and the strongest convergence among them, leading to the maximum westerly deceleration of background wind which is more than 4m·^-1·d^-1. The W2 mode of quasi-16-day PW exhibits the most important contribution to this SSW event. Furthermore, the results also demonstrates that the propagation of PWs is closely related to the movement of zero wind line. The W2 mode propagates vertically upward and shows longitudinally quasi-standing structure in mid-high latitudes, and then propagates to the pole and equator respectively. The mid-high latitudes are likely to be a source region of PWs.