ECMWF Newsletter #173

ECMWF contributes to APEC Climate Center’s BSISO prediction system

A-Young Lim, Hyung-Jin Kim, Bong-Geun Song (all APEC Climate Center, Republic of Korea)


The Boreal Summer Intraseasonal Oscillation (BSISO) is one of the dominant sources of subseasonal convective variability in the tropics. It exhibits northward/northeastward propagation over the Indian summer monsoon region, and northward/ northwestward propagation over the western North Pacific and East Asia region. As a consequence, BSISO strongly influences the onset of the Asian summer monsoon, the active and break phase of the monsoon, and middle latitude teleconnections. BSISO predictions thus help monsoon-affected countries to prepare for hazards like heavy rainfall, droughts, and heatwaves. ECMWF has contributed to APCC (APEC Climate Center) BSISO products since they started in 2013 by providing real-time extended-range weather forecast data.

Collaboration on BSISO predictions

APCC, the only institution that regularly releases BSISO information from various sources, has provided BSISO monitoring and forecast products every day from May to October since 2013. With the goal of improving our ability to understand and predict BSISO, APCC’s BSISO activity was initiated in close cooperation with the Madden Julian Oscillation Task Force of the Working Group on Numerical Experimentation under the World Climate Research Programme.

BSISO forecasts are produced using five subseasonal-to-seasonal (S2S) prediction models from four operational centres worldwide. These are Australia’s Bureau of Meteorology (BOM), the US National Centers for Environmental Prediction (NCEP – CFS and GFS models), ECMWF, and Chinese Taipei’s (Taiwan’s) Central Weather Bureau (CWB). Continuous support and the remarkable skill of ECMWF’s model play an important role in making BSISO predictions more informative.

BSISO impact anomaly using ECMWF’s S2S prediction model.
BSISO impact anomaly using ECMWF’s S2S prediction model. The BSISO impact anomaly shows the expected local impacts of BSISO activity for up to the next three weeks over the Asian monsoon region. These are reconstructed fields based on the composite of the historical BSISO index (1991–2020) in combination with ECMWF model predictions.

Products available

APCC’s BSISO products include phase diagrams, index-based information, anomalies for monitoring and prediction, and simple verifications. In addition to these conventional services, APCC has recently started to provide BSISO impact anomalies for ten important variables up to three weeks ahead, including temperature, precipitation, wind, and other relevant variables. These impact anomalies are reconstructed based on the composite of the historical BSISO index in combination with model predictions. They are expected to capture the local impacts of BSISO activity over the Asian monsoon region.

In 2022, APCC improved BSISO information by reflecting recent climate characteristics and their influence. To do this, the climatological reference period has been updated from 1981–2010 to 1991–2020 for all BSISO information, following the World Meteorological Organization’s recommendation to use the most recent 30‑year period as the climatological standard. Graphical representations on the BSISO website were also improved to increase readability and visibility. We expect that this improvement will help enhance user experience.

User support

BSISO monitoring, forecasts, and fundamental verification are posted on the APCC website ( Users are also able to download digitised data, such as most recent and historical monitoring and forecast indices, through APCC’s Climate Information toolKit (

Participating S2S prediction models.
Participating S2S prediction models. A brief summary of five S2S prediction models participating in the APCC BSISO prediction system.