Thermodynamic Characteristics of the Steady Warming Process over North Asia Before the Summer Onset

Thermodynamic characteristics of the warming process before the summer onset (from pentad 19 to pentad 28) over North Asia are studied based on the thermodynamic equation and the daily reanalysis data of the Japan Meteorological Agency (JRA55) from 1958 to 2019. Results show that the temperature advection and diabatic heating successively dominate the temperature tendency before the summer onset over North Asia. Interestingly, these two phenomenon show an opposite trend to each other. Before pentad 22, the adiabatic advection of the climatological mean temperature dominates the warming process over North Asia. However, after that, the diabatic heating enhances gradually, while the adiabatic advection process weakens gradually, making the diabatic heating dominant. Based on the linear baroclinic model, the numerical results reveal that the diabatic heating over mid- and high-latitude Eurasia can heat the local air masses aloft as well as force a cyclonic circulation and upward motion. Circulation responses are favorable for the southeastward movement of the low-pressure system, which is originally located around Novaya Zemlya toward North Asia, leading to the formation of cold advection in both horizontal and vertical directions. The cold advection partially counteracts the local diabatic heating, forming the steady warming process over North Asia.