The APEC Climate Center (APCC) launched its 2026 Boreal Summer Intraseasonal Oscillation (BSISO) forecast service on its website in May, provided data through October 31, and concluded the service for this year on November 5, 2026.

The BSISO is a large-scale convective variability with a period of 15 to 60 days (mainly 30 to 60 days). It develops over the equatorial Indian Ocean and propagates eastward as well as northward (northwestward over the western North Pacific). It influences the onset of the Asian summer monsoon, its alternating active and break (wet and dry) spells, atmospheric circulation, and weather patterns. BSISO forecasts help trace the movement of large-scale rain bands and thus allow wet and dry spells across Asia to be anticipated in advance, serving as useful information for preparing for hydrometeorological extremes such as droughts and heavy rainfall.

Based on international cooperation and forecast data from leading institutions worldwide, APCC has provided BSISO forecast information daily on its website between May and October each year since July 2013.