Dynamic and Thermodynamic Characteristics of the Two Dominant Boreal Summer Intraseasonal Oscillation Modes

Abstract:

The two dominant modes of boreal summer intraseasonal oscillation (BSISO) that influence onset and retreat of the Asian summer monsoon system and strongly modulate extreme rainfall events over Asia have been identified. One is the canonical northward propagating mode (BSISO1) with quasi-oscillating periods of 30-60 days and strong variance throughout the May to October period often in conjunction with the eastward Madden-Julian Oscillation (MJO). The other is the northwestward propagating mode (BSISO2) with periods of 10-30 days during primarily the pre-monsoon and monsoon-onset season. This study investigates how the two modes are different from each other dynamically and thermodynamically. The BSISO1 circulation cells are more Rossby wave like with a northwest to southeast slope, whereas the circulation associated with the BSISO2 is more elongated and front-like with a southwest to northeast slope. The associated rainfall anomalies over the Indian monsoon and Western North Pacific region are out-of-phase in the BSISO1 but rather in-phase in the BSISO2. For both modes, moisture convergence plays more important role in development and propagation of BSISO than moisture advection but with different energetics. In the BSISO1, convective instability characterized by the low-level warm advection of moisture-laden air and upper-level cold advection of dry air is dominant but the baroclinic instability with warm-air rising and cold-air sinking tends to control the BSISO2. Phase-latitude composite analysis over the western North Pacific-East Asian region further indicates differences in intensification and propagation mechanism between the two modes.