A new study by scientists from the Institute of Atmospheric Physics and Nanjing University of Information Science and Technology investigates the trends in the mean state and the day-to-day variability (DDV) of the surface weather conditions over northern and northeastern China (NNEC) using CN05.1 observational data.
During 1961-2014, the surface temperature (wind speed) increased (decreased) over NNEC and the DDV of the surface temperatures and wind speeds decreased, indicating a trend towards a stable, warm and windless state of the surface weather conditions over NNEC. This finding implies a trend towards more persistent hot and windless episodes, which threaten human health and aggravate environmental problems. The trends were also examined in reanalysis data. Both the ERA-40 and NCEP data showed an increasing (decreasing) trend in the mean state of the surface temperatures (wind speeds). However, the reanalysis data only showed a consistent decreasing trend in the DDV of the surface weather conditions in spring.
The underlying reason for the decreased DDV of the surface weather conditions was further analyzed, focusing on the spring season. "Essentially, the decreased DDV of the surface weather conditions can be attributed to a decrease in synoptic-scale wave activity, which is quantified using the 2-7-day bandpass filtered daily SLP [sea level pressure] in this study," explains Dr. SUN Bo, first author of the study.
The authors found that the decreased synoptic-scale wave activity was caused by a decrease in the baroclinic instability. There was a contrasting change in the baroclinic instability over East Asia, showing a decreasing (increasing) trend north (south) of 40°N. This contrasting change in the baroclinic instability was primarily caused by a tropospheric cooling zone over East Asia at approximately 40°N, which led to a decreased (increased) meridional temperature gradient over the regions to the north (south) of 40°N.