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Non-stationary Synchronization

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1 Non-stationary Synchronization of Equatorial QBO with SAO in Observation and Model Le Kuai1*, Run-Lie Shia1, Xun Jiang2, Ka-Kit Tung3, Yuk L. Yung1 1 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 2 Department of Earth and Atmospheric Sciences, University of Houston, TX 77204 3 Department of Applied Mathematics, University of Washington, Seattle, WA 98195 * To whom all correspondence should be addressed. E-mail: [email protected] Accepted by J. Atmos. Sci.2Abstract It has often been suggested that the period of the Quasi-Biennial Oscillation (QBO) has a tendency to synchronize with the Semi-Annual Oscillation (SAO). Apparently the synchronization is better the higher up the observation extends. Using 45 years of ERA-40 data of the equatorial stratosphere up to the stratopause, we confirm that this synchronization is not just a tendency but a robust phenomenon in the upper stratosphere. A QBO period starts when a westerly SAO (w-SAO) descends from the stratopause to 7 hPa and initiates the westerly phase of the QBO (w-QBO) below. It ends when another w-SAO, a few SAO periods later, descends again to 7 hPa to initiate the next w-QBO. That it is the westerly but not the easterly SAO (e-SAO) that initiates the QBO is also explained by the general easterly bias of the angular momentum in the equatorial stratosphere so that the e-SAO does not create a zero-wind line, unlike the w-SAO. The currently observed average QBO period of 28 months, which is not an integer multiple of SAO periods, is a result of intermittent jumps of the QBO period from 4-SAO periods to 5-SAO periods. The same behavior is also found in a model, the two-and-a-half dimensional THINAIR model. We find that the non-stationary behavior in both observation and model is not caused by the 11-year solar-cycle forcing, but is instead caused by the incompatibility of the QBO’s natural period determined by its wave forcing, and the “quantized” period determined by the SAO. The wave forcing parameter for the QBO period in our current climate probably lies between 4-SAO and 5-SAO periods. If the wave forcing for the QBO is tuned so that its natural period is compatible with the SAO period above, e.g. at 24 months or 30 months, non-stationary behavior disappears.31. Introduction The Quasi-Biennial Oscillation (QBO) is an internal oscillation of the equatorial zonal wind in the stratosphere involving wave-mean flow interactions (Holton and Lindzen, 1972; Lindzen and Holton, 1968; Dunkerton, 1997; Baldwin et al., 2001). There have been numerous observational studies of the QBO in the zonal wind, temperature, and ozone (e.g., Angell and Korshover, 1970; Oltmans and London, 1982; Hasebe, 1983; Zawodny and McCormick, 1991; Randel and Wu, 1996; Pawson and Fiorino, 1998). The equatorial QBO affects the polar stratosphere during winter, with the easterly phase of the QBO creating the condition for a more perturbed and warmer polar vortex (Holton and Tan, 1980, 1982; Baldwin and Dunkerton, 1999, Ruzmaikin et al., 2005). Therefore, the variation of the QBO period has additional significance, especially with respect to the timing of its phase relative to the Northern Hemisphere (NH) winter, a phenomenon called seasonal synchronization (Baldwin et al., 2001). The mean period of the QBO is around 28 months but is known to have inter-annual variations of several months about the average. When the QBO was first discovered (Reed et al., 1961; Ebdon and Veryard, 1961), it was found to have a period of 26 months, 13 months each of easterly and westerly phases at 50 hPa. Later it was reported (Tung and Yang, 1994a, b) to have a period of 30 months based on the satellite record of 1979-1992. For the period 1958-2002 spanned by the ERA-40 data (Uppala et al., 2005), the mean QBO period is slightly less than 28 months (see below). It becomes a little longer than 28 months in the longest data record (rocketsonde and rawinsonde) (1953-2007), which is available from Free University of Berlin (FUB) (Baldwin et al., 2001; Fischer and Tung, 2008). It is interesting to point out that the length of QBO periods is not constant but is quite variable. Individual QBO episodes do not have a mean period of around 28 months with a4normal distribution of variability around the mean. For example, the current estimate of 28 months as the mean QBO period is composed of a collection of individual periods of approximately 24 months and 30 months (and an occasional 36 months in the longest records). Thus, the period of a QBO event is a multiple of the 6 month period of the Semi-Annual Oscillation (SAO). Since the SAO is seasonally synchronized, with respect to Northern and Southern Hemisphere winters, the tendency of the QBO to synchronize with the SAO may be an important cause of its seasonal synchronization. As pointed out by previous authors (Lindzen and Holton, 1968; Gray and Pyle, 1989; Dunkerton and Delisi, 1997), the SAO’s alternating easterly and westerly shear zones near the stratopause level serve to “seed” the QBO below. In particular, the onset of the westerly phase of the QBO (w-QBO) is tied to the downward propagation of the westerly phase of the SAO (w-SAO). A QBO period starts when the zero-wind line associated with the westerly shear zone of the SAO descends into the QBO region below. A QBO period ends when the next such westerly descent occurs after a multiple of SAO periods later and replaces the easterly phase of the QBO (e-QBO) below. In this way the QBO period is “quantized” in units of the SAO period. Lindzen and Holton (1968) found that “the appearance of successive westerly regimes at 30 km tends to be a multiple of 6 months”. Since it is thought that there may be other factors that can affect the descent rate of the QBO from the upper to the lower stratosphere, in the lower stratosphere this property has been regarded more as a “tendency” than a strict synchronization in reality (Dunkerton and Delisi, 1997). This paper is divided into 5 sections. In section 2, we will show that in fact the QBO period is better synchronized with the SAO than previously thought, using the ERA-40 reanalysis data that5extends to the stratopause. We will also show that the decadal variation in the QBO period previously reported often takes the form of a discrete jump in integer multiples of SAO period. In section 3, we will use a model to


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