5:15 PM - 6:45 PM
[AAS10-P09] Zonal asymmetry of the QBO
Keywords:Quasi-Biennial Oscillation, QBO
The Quasi-Biennial Oscillation (QBO) is a global and zonal wind circulation. Previous studies have mostly considered it a zonally symmetric structure, however, Hamilton et al. (2004) and Sakazaki and Hamilton (2022) have demonstrated zonal asymmetry in the QBO near 10hPa due to the penetration of mid-latitude Rossby waves into the equatorial region. On the other hand, there are a few studies on its zonal asymmetry in the lower stratosphere. Recent studies have revealed new teleconnections between the QBO and the equatorial troposphere, such as the QBO’s modulation of the Madden-Julian Oscillation (MJO). Subsequently, the significance of studying the QBO zonal asymmetry has been recognized. In this study, zonal asymmetry of the QBO amplitude depending on longitudes, latitudes, seasons and the QBO phase were investigated by using JRA-55 reanalysis monthly mean data from 1958 to 2018.
First, the QBO amplitude was calculated using the method of Dunkerton and Delish (1985). Horizontal maps of the QBO amplitude showed zonal asymmetry in the equatorial stratosphere at different altitudes. At 10 hPa, the amplitude is weaker around 120W than at other longitudes, showing a zonal wave-number 1 structure, which is consistent with previous studies. On the other hand, the horizontal map at 70 hPa also displayed zonally different structures with a strong amplitude at 180W showing wave-numbers greater than 1. Analysis of QBO amplitude during each season illustrated nearly the same characteristics of zonal asymmetry.
Next, 26 QBO cycles were extracted from 60 years of data and were composited to observe the QBO phase dependence of its zonal asymmetry. The zonal asymmetry at 10hPa and 70hPa was observed in both easterly and westerly phases. At 70hPa, large longitudinal differences were observed during easterly and/or westerly phases descending into the lowermost stratosphere. These results indicate that the QBO descending near the tropopause varies in longitudes.
Finally, the QBO amplitude was also calculated by extracting 18-36 month components using Fast Fourier Transform. The fundamental structures of the QBO amplitudes were confirmed to be equivalent to those obtained by the previous method. In the mid-to-high latitudes, the QBO components of the zonal wind showed large longitudinal differences near the edges of the polar vortex and the upper parts of the subtropical jets. This indicates that the Holton-Tan effect has also zonal dependence.
First, the QBO amplitude was calculated using the method of Dunkerton and Delish (1985). Horizontal maps of the QBO amplitude showed zonal asymmetry in the equatorial stratosphere at different altitudes. At 10 hPa, the amplitude is weaker around 120W than at other longitudes, showing a zonal wave-number 1 structure, which is consistent with previous studies. On the other hand, the horizontal map at 70 hPa also displayed zonally different structures with a strong amplitude at 180W showing wave-numbers greater than 1. Analysis of QBO amplitude during each season illustrated nearly the same characteristics of zonal asymmetry.
Next, 26 QBO cycles were extracted from 60 years of data and were composited to observe the QBO phase dependence of its zonal asymmetry. The zonal asymmetry at 10hPa and 70hPa was observed in both easterly and westerly phases. At 70hPa, large longitudinal differences were observed during easterly and/or westerly phases descending into the lowermost stratosphere. These results indicate that the QBO descending near the tropopause varies in longitudes.
Finally, the QBO amplitude was also calculated by extracting 18-36 month components using Fast Fourier Transform. The fundamental structures of the QBO amplitudes were confirmed to be equivalent to those obtained by the previous method. In the mid-to-high latitudes, the QBO components of the zonal wind showed large longitudinal differences near the edges of the polar vortex and the upper parts of the subtropical jets. This indicates that the Holton-Tan effect has also zonal dependence.