Although annual rainfall over most of Australia has followed a continuously decreasing trend over the past 6 decades (i.e., 1948–2007), summer rainfall in northwest Australia (NWA) has followed a pronounced increasing trend, with a significant linear trend of 2.65 mm/yr. This study investigates the causes of increasing rainfall over NWA and focuses on influences originating in the middle to high latitudes, i.e., southern Indian Ocean climate variability. Our results indicate that variations in summer rainfall over NWA are related to the concurrent strength of the Mascarene high. In years with a strong Mascarene high in the lower troposphere, the cyclonic vorticity over NWA is enhanced, and more moisture is transported into NWA. Simultaneously, an anomalous vertical closed circulation forms, with an ascending branch over NWA and subsidence over the region occupied by the Mascarene high. As for the upper troposphere, the Mascarene high is related to a teleconnection wave train from the southern Indian Ocean to NWA, which in turn influences the circulation over NWA. This setting is favorable for enhanced rainfall over NWA. Analysis of the cause of variations in the Mascarene high indicates that underlying external forcing of the high by, for example, sea surface temperature (SST) has intensified, possibly due to the influence of the southern annular mode (SAM) during the preceding spring. This mechanism was further examined using numerical experiments. We believe that the strengthening of the Mascarene high, which is caused by the enhanced SST gradient within the southern Indian Ocean associated with the preceding (upward) SAM, has had a significant influence on the increase in summer rainfall over NWA.