Changes in the Boreal Summer Intraseasonal Oscillation under Global Warming in CMIP6 Models

Changes in the activities of the Boreal Summer Intraseasonal Oscillation (BSISO) at the end of 21st century under the SSP5-8.5 scenario are assessed by adopting 17 CMIP6 models and the weak-temperature-gradient assumption. Results show that the intraseasonal variations become more structured. The BSISO-related precipitation anomaly shows a larger zonal scale and propagates further northward. However, there is no broad agreement among models on the changes in the eastward and northward propagation speeds and the frequency of individual phases.

In the western North Pacific (WNP), the BSISO precipitation variance is significantly increased, at 4.62% K⁻¹, due to the significantly increased efficiency of vertical moisture transport per unit of BSISO apparent heating. The vertical velocity variance is significantly decreased, at −3.51% K⁻¹, in the middle troposphere, due to the significantly increased mean-state static stability. Changes in the lower-level zonal wind variance are relatively complex, with a significant increase stretching from the northwestern to southeastern WNP, but the opposite in other regions. This is probably due to the combined impacts of the northeastward shift of the BSISO signals and the reduced BSISO vertical velocity variance under global warming.

Changes in strong and normal BSISO events in the WNP are also compared. They show same-signed changes in precipitation and large-scale circulation anomalies but opposite changes in the vertical velocity anomalies. This is probably because the precipitation anomaly of strong (normal) events changes at a rate much larger (smaller) than that of the mean-state static stability, causing enhanced (reduced) vertical motion.