Evaluating the subseasonal impacts of the MJO and BSISO in the East Asian extended summer

In the summer of East Asia, the dominant mode of climate variability on intraseasonal timescales is the boreal summer intraseasonal oscillation (BSISO). The BSISO is known to have significant impacts on East Asian summer climate, including monsoonal rainfall, but it is not clear how well the BSISO index serves as guidance for subseasonal forecasts. This study examines the lagged impacts of three different BSISO/MJO (Madden-Julian Oscillation) indices on extended summer (May - October) climate over East Asia for lags of one to six weeks. The three indices are the MJO (Wheeler and Hendon, 2004), Bimodal ISO (Kikuchi et al., 2012), and BSISO (Lee et al., 2012) indices, which are chosen because they are available in real time. A statistical forecast model based on these indices is used to quantify the subseasonal forecast skill of temperature, precipitation, and moisture over the region. Particular focus is placed on the indices, phases, and locations that offer "forecasts of opportunity," which provide unusually elevated forecast skill at lead times when skill is often absent. These results have the potential to provide forecast guidance related to the expected impacts of BSISO over lead times of up to six weeks and to assist in the development of a seamless subseasonal-to-seasonal prediction system over the region.