Using data from the Sumatran GPS Array in Indonesia–a hero network in tectonic and earthquake studies–we study the summer intra-seasonal variability of precipitable water vapor (PWV) over Sumatra in years without strong inter-annual variability. Unlike most other studies that used external meteorological data to derive PWV from Global Positioning System (GPS) signal delays, we use the zenith wet delay (ZWD) time series estimated from a regular geodetic-quality processing routine as a proxy for PWV variations without using auxiliary meteorological data. We decompose the ZWD space-time field into modes of variability using rotated Empirical Orthogonal Function (EOF) analysis and investigate the mechanisms behind the two most important modes using linear regression analysis both with and without lags. We show that the summer intra-seasonal variability of daily ZWD over Sumatra in 2008, 2016, and 2017 was dominated by the South Asian Summer Monsoon and further influenced by dry-air intrusions associated with Rossby waves propagating in the Southern Hemisphere midlatitudes. Both active South Asian monsoons and dry-air intrusions contribute to the dryness over Sumatra during northern summer. Our results indicate an intra-seasonal connection between the South Asian and western North Pacific Summer Monsoons: when the South Asian monsoon is strong, it pumps atmospheric water vapor over the eastern Indian Ocean to feed into the western North Pacific monsoon. We also show a tropical-extratropical teleconnection where PWV over the southern Maritime Continent can be modulated by the activity of eastwardtraveling Rossby waves in the southern midlatitudes. Our case study demonstrates the use of regional continuously operating GPS (cGPS) networks for investigating atmospheric processes that govern intra-seasonal variability in atmospheric water vapor.