The Climatology and the Midwinter Suppression of the Cold-Season North Pacific Storm Track in CMIP6 Models

The reproducibility of climatology and the midwinter suppression of the cold-season North Pacific storm track (NPST) in historical runs of 18 CMIP6 models is evaluated against the NCEP reanalysis data. The results show that the position of the climatological peak area of 850-hPa meridional eddy heat flux (v'T'(850)) is well captured by these models. The spatial patterns of climatological v'T'(850) are basically consistent with the NCEP reanalysis. Generally, NorESM2-LM and CESM2-WACCM present a relatively strong capability to reproduce the climatological amplitude of v'T'(850) with lower RMSE than the other models. Compared with CMIP5 models, the intermodel spread of v'T'(850) climatology among the CMIP6 models is smaller, and their multimodel ensemble is closer to the NCEP reanalysis. The geographical distribution in more than half of the selected models is farther south and east. For the subseasonal variability of v'T'(850), nearly half of the models exhibit a double-peak structure. In contrast, the apparent midwinter suppression in the NPST represented by the 250-hPa filtered meridional wind variance (v'v'(250)) is reproduced by all the selected models. In addition, the present study investigates the possible reasons for simulation biases regarding climatological NPST amplitude. It is found that a higher model horizontal resolution significantly intensifies the climatological v'v'(250). There is a significant in-phase relationship between climatological v'v'(250) and the intensity of the East Asian winter monsoon (EAWM). However, the climatological v'T'(850) is not sensitive to the model grid spacing. Additionally, the climatological low-tropospheric atmospheric baroclinicity is uncorrelated with climatological v'v'(250). The stronger climatological baroclinic energy conversion is associated with the stronger climatological v'T'(850).

Automated summary

The majority of CMIP6 models perform well in capturing the spatial distribution characteristics of climatological eddy heat flux. NorESM2-LM and CESM2-WACCM demonstrate relatively strong capability in reproducing the climatological amplitude of eddy heat flux. Compared with CMIP5 models, the intermodel spread of eddy heat flux climatology is smaller in CMIP6 models, and their multimodel ensemble is closer to the NCEP reanalysis.