Quasi-biennial oscillation modulation of stratospheric water vapour in the Asian monsoon

The Asian monsoon (AM) plays a key role in the transport of water vapour to the lower stratosphere and contributes significantly to the wet phase of the annual global stratospheric water vapour cycle. Although it is known that the quasi-biennial oscillation (QBO) is one of the main drivers of the interannual variability in the AM water vapour, the physical mechanisms responsible for this variability remain unclear. Here we have used daily microwave limb sounder data for the period 2005-2020 to characterize the QBO signature on the lower stratosphere AM water vapour during the boreal summer. We show that the QBO has the strongest impact during August, when QBO westerly minus QBO easterly differences may reach 1 ppmv at 100 hPa, although a significant signature is also observed during July. We find that the region whose temperature controls the QBO signal on water vapour over the AM differs between July and August. In July, when the key region is over the tropical Indian Ocean, the QBO modulation of the AM water vapour occurs in phase with the signal over the Equator, whereas in August, when the key region is at the subtropics, over the southern edge of the monsoon, the signal over the AM is opposite to that over the Equator. Our results reveal that the QBO signal on the temperature on the south side of the AM anticyclone, which ultimately has an impact on AM water vapour, is, in turn, modulated by the QBO impact on tropical clouds. Thus, we find that the QBO signature on clouds over the eastern Indian Ocean gives rise to Rossby wave trains that produce variations in the circulation over the southern side of the AM anticyclone such that weaker anticyclone over this region generates an increase in water vapour, and vice versa.