Interaction of the quasi‐biennial oscillation and stratopause semiannual oscillation

Abstract

Abstract. Analysis of rawinsonde and rocketsonde data at Ascension Island (7.6øS, 14.4øW) and Kwajalein (8.7øN, 167øE) in 1962-1991 suggests that the quasi-biennial oscillation (QBO) in the middle stratosphere is synchronized with the seasonal cycle and that descending westerly phases of the stratopause semiannual oscillation (SAO) are strongly influenced by the underlying QBO. The effect of the seasonal cycle on the QBO in the middle stratosphere is revealed in two, perhaps unrelated, observations: first, a tendency for deseasonalized QBO westerly maxima to occur in local winter (or to avoid local summer); second, a smooth, uninterrupted connection between descending SAO westerly shear zones and the formation of a new QBO westerly shear zone aloft. The timing of deseasonalized QBO westerly maxima in the middle stratosphere allows a simple composite of 2-and 3-year cycles to be constructed from the data, illustrating the effect of the QBO on descending westerly phases of the stratopause SAO. IntroductionThe quasi-biennial oscillation (QBO) of the equatorial stratosphere is so named because the average period of the oscillation is slightly longer than 2 years. Almost 20 cycles have been observed since rawinsonde data first became available in the early 1950s; the average period is now 28.4 months. Individual cycles range from ---22 to 36 months, i.e., from about 2 to 3 years.The prefix "quasi" would apply equally well to an oscillation with continuously variable period, or an oscillation consisting of discrete periods (e.g., 24 months, with a few 30-or 36-month cycles) strung together in some regular or irregular fashion. The period of the QBO is customarily measured at mandatory rawinsonde pressure levels, e.g., 50, 30, or 10 mbar [Dunkerton and Delisi, 1985;Angell, 1986;Naujokat, 1986;Dunkerton, 1990]. This approach, based on one-dimensional time series, leads to the conclusion that the QBO period is variable, tending to form a continuous distribution as time progresses. Exact synchronization with the seasonal cycle is not observed, although as noted by Dunkerton [1990], the deseasonalized mean flow acceleration, distribution of phase onsets, and period of the QBO evidently depend on the time of year. This dependence on the seasonal cycle is due, in part, to a pronounced tendency for descending QBO easterlies to "stall" near 30 mbar between July and February [Naujokat, 1986]. Modulation of the QBO by the seasonal cycle can be distinguished from exact synchronization, which would produce one or more delta functions in the distribution of onset times. Both types of behavior were simulated in a QBO model [Dunkerton, 1990] Data AnalysisData from the historical rocketsonde network were obtained from a variety of sources and processed to form monthly means 26,107

Climatology and Forcing of the Quasi-Biennial Oscillation in the MAECHAM5 Model

“…The QBO exerts filtering on the vertically propagating wave spectra, such that a QBO dependence of the SAO must be expected, and is indeed found (Burrage et al 1996). At the same time it is observed that the QBO westerly phases often start in SAO westerly phases (Dunkerton and Delisi 1997), so that the SAO and QBO are dynamically coupled to some degree. Also in CL90, the stratospheric SAO varies with the QBO, as seen in the deeper propagation of westerlies when the QBO westerly phase descends to the lower stratosphere, and the QBO westerly phases start from an SAO westerly phase (Fig.…”

“…These waves are horizontally well resolved in most GCMs so that it was expected that middleatmosphere GCMs could simulate the QBO, provided that the vertical resolution is improved such that the vertical structures of these waves are well resolved. Only later it became clear that large-scale waves are not sufficient to explain the QBO (Takahashi and Boville 1992) and that gravity waves must play an important role (Dunkerton 1997). Currently, it is assumed that a broad spectrum must be considered.…”

Driving of the SAO by gravity waves as observed from satellite

“…It has also been found that the QBO and the SAO interact with each other. For example, the QBO and SAO periods are often synchronized (e.g., Dunkerton and Delisi, 1997;Krismer et al, 2013), and it has been suggested that the eastward phase of the SAO can initiate an eastward phase of the QBO (e.g., Kuai et al, 2009).…”

Phase synchronization between stratospheric and tropospheric quasi‐biennial and semi‐annual oscillations

“…It is widely understood to arise from the interactions of upward propagating waves, initially excited in the troposphere and including both eastward and westward phase speeds (such as equatorially trapped Kelvin and Rossby-gravity waves, and a range of smaller-scale gravity waves). These waves respectively encounter critical layers where they break and dissipate, depositing zonal momentum into the mean flow in the stratosphere (Dunkerton and Delisi, 1997;Baldwin et al, 2001). Other examples in the stratosphere include at least two different types of semi-annual oscillation (SAO), which tend to be present at higher altitudes in the stratosphere Copyright c 2012 Royal Meteorological Society (Garcia et al, 1997), either close to the stratopause around 1 hPa or higher still in the mesosphere, and have a mean period of precisely 6 months.…”

“…The QBO and the seasonal cycle and/or SAO have long been regarded as somewhat 'coupled' (Dunkerton and Delisi, 1997;Garcia et al, 1997;Baldwin et al, 2001), in such a way that the highly periodic rhythms of the seasonal cycle and SAO help to trigger, at least partially, the start of the downward phase of the QBO. Hampson and Haynes (2003) used a 3D mechanistic model to explore the possible phase alignment between the QBO and SAO, in which a partial locking was found between the QBO and certain phases of the annual cycle.…”