Observational evidence of the downstream impact on tropical rainfall from stratospheric Kelvin waves

Many studies have shown that stratospheric anomalies can influence tropospheric circulation and convection (e.g. Gray et al. 1992; Ho et al. 2009; Hu et al. 2012; Garfinkel and Hartmann 2011; Huang et al. 2012), and much attention has been paid to the dominant mode of interannual variability of the equatorial stratosphere, i.e., quasi-biennial oscillation (QBO) (Yasunari 1989; Baldwin et al. 2001; Hamilton 2002; Garfinkel and Hartmann 2007; Taguchi 2010; Kawatani and Hamilton 2013; Yuan et al. 2014). In both observations and numerical simulations, it has been demonstrated that the QBO can affect tropical convection (Gray et al. 1992; Knaff 1993; Randel et al. 2000; Liess and Geller 2012; Nie and Sobel 2015). Two hypotheses of the QBO modulation of tropical convection have been put forward (e.g. Reid and Gage 1985; Collimore et al. 2003). One is through the influence of the stratospheric temperature anomalies on upper tropospheric static stability and tropopause height (Collimore et al. 1998), and the other is related to changes in the vertical wind shear in the lower stratosphere and upper troposphere associated with the QBO zonal wind anomalies (Gray et al. 1992). Collimore et al. (2003) compared the relative importance of the two mechanisms and pointed out that the QBO fluctuations of convection in the tropical region are primarily associated with tropopause height anomalies. These studies clearly show prominent impacts of stratospheric anomalies on tropical convection. Abstract Analysis of one continuous decade of daily, high-vertical resolution sounding data from five proximate islands in the western equatorial Pacific region reveals eastward and downward propagating Kelvin waves in the tropical stratosphere, with a zonal wave number one structure and a period of ~15 days. By defining an initiation index, we find that these waves are primarily generated over the western Pacific warm pool and South America–tropical Atlantic sector, consistent with regions of frequent deep convection. The zonal phase speed of the stratospheric Kelvin waves (SKWs) is relatively slow (~10 m s−1) over the initiation region due to coupling with deep convection, and becomes much faster (~30–40 m s−1) once decoupled from the downstream troposphere. SKWs have significant impacts on downstream tropical rainfall through modulation of tropopause height. The cold phase of SKWs at tropopause leads to higher tropopause heights and more convection in tropics—with opposite impacts associated with the warm phase. Downstream tropical precipitation anomalies associated with these SKWs also propagate eastward with the same speed and zonal scale as observed SKWs. Interannual variability of the amplitude of the SKWs is shown to be associated with the Quasi-Biennial oscillation (QBO); implications for predictability are discussed.