Intraseasonal oscillation intensity over the western North Pacific: Projected changes under global warming

The 30-60-day intraseasonal oscillation (ISO) and 10-20-day ISO are two dominant oscillation modes over the western North Pacific during boreal summer. With daily data derived from eight CMIP5 models, changes of the ISO intensities are projected under the 1.5 and 2.0 degrees C global warming levels under the Representative Concentration Pathway (RCP) 4.5 and RCP8.5 scenarios. Most of the models agree that the ISO intensities increase along a belt region from the south Indochina Peninsula (ICP) to the east to the Philippines. The variation pattern shows little difference between different warming levels or scenarios. Results indicate that the spatial distribution of ISO anomalies is related with the variation of background fields. Enriched lower-level humidity and moist static energy favor the intensity increases of ISOs, which are projected to be larger over the whole western North Pacific, with the most conspicuous changes located over the east to the Philippines for humidity but over the south of the ICP for moist static energy. In contrast, the ISOs over the west to Indonesia and northeast to the Philippines decrease, which is consistent with the local descending motions.