Upper tropospheric water vapor (UTWV) is a key greenhouse gas which exerts a major influence on the energy balance of the earth-atmosphere system. The Asian monsoon region is an important channel by which UTWV enters into the stratosphere, and accordingly plays an important role in exchange between the upper troposphere and the lower stratosphere (UTLS) as well as distribution and variability of stratospheric water vapor. This study investigates intraseasonal oscillations (ISOs) of UTWV in the Asian monsoon region in boreal summer by using the Atmospheric Infrared Sounder (AIRS) satellite instrument data. Besides, Radiosonde measurements over the Tibetan Plateau (TP) are used to validate measurements of water vapor column mixing ratio from the AIRS. In general, AIRS reproduces the observations of water vapor with good fidelity. The all-year and summer AIRS data have good confidence, while the winter AIRS, especially in the upper troposphere, has a less confidence, but is still usable when high spatial and temporal resolution dataset is not available.

There are robust intraseasonal cycles with periods of 30–60 days and 10–20 days in the UTWV field over both South Asia and East Asia. The 30–60-day oscillation accounts for more than 60 percent of the total variance. For the 30–60-day mode, the source and propagating signature of the UTWV disturbances are distinct in two monsoon sub-systems. Two patterns in the 30–60 day oscillation are evident: a South Asian pattern that originates on the western side of the Arabian Sea and moves eastward, and an East Asian pattern that develops over West Pacific and moves westward. The 10–20-day mode exhibits a uniform westward propagating signature from West Pacific to the Arabian Sea. Furthermore, the relationship between monsoon activity and the 30–60-day oscillation of UTWV is studied. The result shows that the upper troposphere is moistened following intense monsoon convection with lags about 5–10 days. An examination of the low level circulation reveals that wet and dry periods in UTWV are closely related to active and break (inactive) periods in monsoon convection, suggesting that the Asian summer monsoon plays an important role in the intraseasonal variations of UTWV.

The empirical orthogonal function (EOF) analysis shows that the summer UTWV over the TP has three modes in spatial distribution as follows: monopole with same anomalous signs in the whole TP, west-east dipole with reverse signs in western and eastern TP, and south-north dipole with reverse signs in southern and northern TP. The monopole exhibits seasonal variations, while the dipoles oscillate on intraseasonal scale. Moreover, it is found that the intraseasonal cycles of summer UTWV over the TP have periods of 10-20 days and 30-60 days. The former moves eastwards and arrives over the Yangtze River-Huaihe River valleys. The latter propagates southwards but oscillates just inside the TP. The result indicates that the ISOs of summer UTWV over the TP are closely related to heating in the TP, activities of the South Asian high, and coupling convection.