Delayed Impacts of ENSO on the Frequency of Summer Extreme Hot Days in the Asian Monsoon Region. Part I: Observation, Historical Simulation, and Future Projection in CMIP6 Models

Abstract

Significant anomalies in frequency of summer extreme hot day (SEHD) are broadly observed in the Asian monsoon region (AMR) in the post-ENSO summers. The delayed ENSO impacts are mainly conveyed by provoking the Indo-western Pacific Ocean capacitor (IPOC) effect that maintains the anomalous anticyclone in the western North Pacific. The related diabatic heating anomaly can trigger the westward propagating Rossby wave to the Indian subcontinent, which increases the geopotential heights, reduces the cloud cover, and thus increases the seasonal surface temperature and SEHD frequency in the southern AMR. Besides, the reduced atmospheric moisture in the western North Pacific hinders the northward propagation of intraseasonal oscillation (ISO) and modulates the occurrence frequency of individual ISO phases, contributing to the significantly increased/decreased SEHDs in eastern China/Hokkaido of Japan in the post-El Niño summers. The 25-model-ensemble mean of CMIP6 historical runs can reproduce well the observed SEHD anomalies in the southern AMR in the post-ENSO summers mainly due to the realistic simulation of ENSO impacts on the seasonal surface temperature, although a large inter-model spread exists due to different strength of IPOC effect in each model owing to model biases in the mean state of eastern tropical Pacific, the ENSO variance and teleconnection to the Indian Ocean. Furthermore, future projections under SSP5-8.5 scenario show that the delayed ENSO impacts on the southern AMR remains stable under global warming via the similar mechanism as in the observations and historical runs.

Warming Tropical Indian Ocean Wets the Tibetan Plateau

“…With the presence of vertical mean-state wind shear, the baroclinic Rossby wave can be converted to the barotropic mode and then propagate northeastward to the midhigh latitudes (Kasahara & Dias, 1986;Yuan et al, 2012), as indicated by the northeastward propagation of wave activity fluxes (Figure 2a). Further, the diabatic heating can trigger the eastward propagating Kelvin waves, leading to the enhancement and westward extension of the WNPSH (Xie et al, 2009(Xie et al, , 2016Ye et al, 2023;Yuan et al, 2019).…”

Evaluation and projection of circulation conditions tied to summertime compound drought and heatwave frequency over the Yangtze River Delta, China, for the carbon neutrality period based on CMIP6 GCMs

“…Following the approaches in existing studies (B. Wang et al, 2020; Zha et al, 2023), the major evaluation metrics consisted of the pattern correlation coefficient (PCC) and normalized root‐mean‐square error (NRMSE) for climatology comparison between observations and CMIP6 model simulations. Moreover, in light of Chen et al (2019) and Ye et al (2023), the regression coefficients that reflected the interannual temporal changes in the compound DHD YRD ‐associated meteorological anomalies were evaluated against the observations of the CMIP6 historical experiments during 1979–2014. The monthly mean PET was evaluated using the Thornthwaite method (Thornthwaite, 1948).…”