Western Pacific Premoistening for Eastward-Propagating BSISO and Its ENSO Modulation

The boreal summer intraseasonal oscillation (BSISO) is a major source of subseasonal predictability of the East Asian summer monsoon. However, modeling and prediction of the BSISO remain major challenges partly due to an incomplete understanding of its eastward propagation. Our moisture budget analysis suggests that western Pacific (WPAC) premoistening leading the eastward-propagating (EP) BSISO is mainly attributed to the horizontal moisture advection with two centers in the lower and middle troposphere, respectively. The lower-tropospheric center is rooted in the linear moisture advection by flows from both the mean state and BSISO, while the middle-tropospheric center is induced by the nonlinear eddy moistening effect from the suppressed activity of synoptic tropical depression (TD) disturbances. The vertical profile of WPAC premoistening is significantly modulated by El Nino-Southern Oscillation (ENSO), with the premoistening being enhanced in the lower troposphere and weakened in the middle troposphere during an El Nino summer, and vice versa in a La Nina summer. During an El Nino summer, the nonlinear eddy moistening effect is weakened in the middle troposphere due to less southwest-northeast tilt of the TD, while the linear moisture advection is enhanced in the lower troposphere due to strengthened background cross-equatorial flows and moisture gradients. These results suggest an urgent need to improve the simulation fidelity of the BSISO's scale interactions with synoptic and interannual variabilities in climate models. Significance StatementIn this work, we use statistical analysis to explore multiscale interactions of BSISO with synoptic and interannual variabilities using observations and reanalysis data. Our key finding shows that the ENSO significantly modulates the premoistening process of the BSISO over the WPAC. In an El Nino summer, the WPAC nonlinear eddy moistening effect leading the BSISO is weakened in the midtroposphere due to smaller southwest-northeast tilt of the TD, while the linear moistening effect is enhanced in the lower troposphere due to enhanced background cross-equatorial flow and moisture gradient. These results offer new metrics for validating climate models and for projecting BSISO's future change under different global warming scenarios.