Nonstationary Synchronization of Equatorial QBO with SAO in Observations and a Model

Abstract

It has often been suggested that the period of the quasi-biennial oscillation (QBO) has a tendency to synchronize with the semiannual oscillation (SAO). Apparently the synchronization is better the higher up the observation extends. Using 45 yr of the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data of the equatorial stratosphere up to the stratopause, the authors 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. The fact 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 four SAO to five SAO periods. The same behavior is also found in the Two and a Half Dimensional Interactive Isentropic Research (THINAIR) model. It is found that the nonstationary behavior in both the observation and model is caused not by the 11-yr solar-cycle forcing but 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 the current climate probably lies between four SAO and five 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 or 30 months), nonstationary behavior disappears.

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

“…We follow Kuai et al (2009) in utilising data extracted from the ERA-40 global reanalyses obtained by the European Centre for Medium Range Weather Forecasts (ECMWF; Uppala et al, 2005), also supplemented by additional analyses since 2005 in the ERA-interim dataset (e.g. Dee and Uppala, 2009;Dee et al, 2011).…”

“…Further indications that the period of the QBO might adopt a bimodal distribution linked to periods commensurate with the annual cycle were apparent in the model simulations of Scaife et al (2002), though this was less evident in their analyses of observations. Recent research by Kuai et al (2009) has provided further evidence of intermittent, but frequently coherent, phase alignment between the QBO and stratospheric SAO, in which it was concluded that the westerly phase of the QBO (hereafter the w-QBO) only starts typically when the westerly phase of the SAO (w-SAO) has descended to approximately 7 hPa. To corroborate this apparently active synchronization of the two oscillations, Kuai et al determined the period between successive reversals of the zonal wind within each QBO cycle contained within the ERA-40 reanalysis dataset (Uppala et al, 2005) and found that the QBO did not exhibit a continuous range of periods, but occurred more or less at random with a period of either 24 or 30 months, i.e.…”

“…with exactly four or five times the period of the SAO. This would therefore appear to be a case of active nonlinear synchronization, although Kuai et al (2009) were not able to investigate this directly during the intervals between wind reversals.…”

Phase synchronization of baroclinic waves in a differentially heated rotating annulus experiment subject to periodic forcing with a variable duty cycle

“…Maraun and Kurths (2005) found evidence for phase synchronization between the El Niño-Southern Oscillation (ENSO) index and variations in the Indian Monsoon, while Feliks et al (2010) detected phase synchronization between various climate indicators in the middle East and the North Atlantic Oscillation (Hurrell et al, 2003) on timescales of 7-8 years. On annual/seasonal timescales, Gruzdev and Bezverkhny (2000), Kuai et al (2009), and Read and Castrejon-Pita (2012), showed that the stratospheric Quasi-Biennial Oscillation (QBO) was phasesynchronized with the annual cycle in that its period was found to be entrained to a rational ratio of the annual period, although this ratio evidently fluctuated chaotically on timescales of several years. Evidence of partial phase synchronization of the ENSO index to the annual cycle was also found by Stein et al (2011).…”