Prediction of Diverse Boreal Summer Intraseasonal Oscillation

Boreal summer intraseasonal oscillation (BSISO), the dominant summertime intraseasonal mode in the Asian monsoon region, is a primary predictability source for summertime weather and climate on the subseasonal-to-seasonal (S2S) timescale. The prediction of BSISO is examined based on 20 years’ hindcast experiments (2000-2019) using the GFDL SPEAR S2S prediction system. The overall BSISO prediction skill, with all cases initialized from May to October, reaches out to 22 days measured by BSISO indices before the bivariate anomalous correlation coefficient (ACC) drops to 0.5. It is demonstrated that the northeastward propagating canonical BSISO (CB) has a higher prediction skill (28 days) than the northward propagating dipole-type BSISO (DB, 23 days). This is attributed to the more periodic feature with longer persistence for CB, while DB decays rapidly after reaching the maximum enhanced convection over the equatorial Indian Ocean.

Investigation of individual CB events shows a large inter-event spread in terms of their prediction skills. The CB events with weak and fluctuating amplitude have relatively lower prediction skills due to their weak convection-circulation coupling in the equatorial Indian Ocean, and vice versa. Intriguingly, the prediction skills of individual CB events tend to be generally lower and more scattered during early summer (May-July) than those in later summer (August-October), suggestive of the pronounced modulation of seasonality on its evolution and predictability.