Mechanisms And Impacts Of The First Northward-propagating Intra-Seasonal Oscillation Over The Bay Of Bengal

The summer monsoon firstly occurs at the southeastern Bay of Bengal(BoB), which is closely related with the first northward-propagating intra-seasonal oscillation(FNISO). FNISO is the critical event that marks the transition between the eastward-propagating wintertime Madden–Julian Oscillation(MJO) and the northward-propagating boreal summer intra-seasonal oscillation(BSISO). FNISO not only plays as important trigger of BoB summer monsoon, but also significantly modulates the tropical cyclone(TC) genesis and the subsequent monsoon onset over the South China Sea(SCS).Based on the data from reanalysis, remote sensing and in-situ observation, composite analysis is conducted to reveal the evolutions, mechanisms, and climate impacts of the FNISO:(1) Lifecycle and structures of the FNISO. It is found that the FNISO convection originates from the southwestern Indian Ocean(IO) and propagates eastward. After reaching the eastern IO, the major convective branch moves northward toward the northern BoB. Differed with the northward-propagating BSISOs, the FNISO happened in the transitional season bears some different structures especially in the vorticity and divergence fields.(2) Mechanisms of the FNISO. Differed with the northward-propagating BSISOs, the FNISO is closely related to the pre-monsoon sea surface warming in the central BoB. Sea surface warming heats the surface air through sensible heating, which raises the water vapor capacity. Under the condition that the relative humidity almost keeps constant, specific humidity increases due to the sea surface evaporation. In this way, the atmospheric convective instability rapidly increases in the BoB, which eventually establishes the meridional asymmetry of the in the eastern IO. The pre-established greater convective instability preconditions the FNISO through destabilization of Rossby waves and therefore induces the major convection shift to the northern Rossby gyre center from the equator. After that, the convective system is guided further northward by a distinct meridional asymmetry of humidity anomaly in the planetary boundary layer, that is, the maximum moisture is located at the north of the FNISO convection center. Consequently, the summer monsoon is triggered in the BoB region by the FNISO.(3) Climate impacts of the FNISO. Except for the seasonal modulation by monsoon, the BoB TC genesis is also significantly modulated by the FNISO. It indicates that most springtime TCs generate in the convective phases of the FNISO. The individual contributions from the four large-scale environmental fields associated with the intra-seasonal variations to the TC genesis are diagnosed with the aid of the genesis potential index(GPI). The significant moistening of mid-level atmosphere, which is embedded in the FNISO convection-enhanced phase, is shown to be the primary factor leading to the TC genesis. Further analysis indicates the mid-level moisture is mainly brought by the FNISO ascending flow from the low-level atmosphere. Besides the BoB, the FNISO exerts strong influence on the subsequent monsoon onset over the SCS. When the FNISO arrives at the eastern IO, the major convection veers northward into the BoB, while an eastward branch continues move to the equatorial western Pacific. After triggering the BoB summer monsoon, the BoB convection extends eastward to the SCS. At the same time, the equatorial eastward branch gets strengthened in the western Pacific and propagates northwestward to the SCS. The convections from both sides are important triggers for the SCS summer monsoon.The present study focuses on the evolutions, mechanisms and climate impacts of the FNISO, and the results would help improve the capabilities of monitoring and predicting of Asian summer monsoon onset.