Formation of the northern East Asian low: role of diabatic heating

Abstract

The northern East Asian low (NEAL), characterized by a mid-latitude closed low (or trough) in the lower troposphere, is an important component of the East Asian summer monsoon system. This study investigates formation mechanism of the summer NEAL, with emphasis on the roles of diabatic heating and transient eddy forcing, using a linear baroclinic model (LBM) with prescribed forcing derived from the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) dataset during the 1979–2010 period. The LBM simulations show that the formation of NEAL is induced by the diabatic heating, and the effect of transient eddy forcing is negligible. Further investigation indicates that the diabatic heating over northern East Asia (NEA) is the key factor for the NEAL’s formation, in which the latent heating and radiative cooling in concert to play a major role and the vertical diffusion sensible heating plays a secondary role. In response to the NEA diabatic heating (latent and sensible heating), a local northerly is induced that advects cold air to partially balance the diabatic heat source and forms a low to the east. On the other hand, the radiative cooling induces a southerly anomaly over NEA, shaping the position and intensity of the low induced by the latent and sensible heating. The total effect of the diabatic heating and radiative cooling causes a closed NEAL. Results are similar for the NEAL’s formation in June, July, and August, respectively. Moreover, we further investigate sub-seasonal change of the NEAL in summer. The weakened intensity of the NEAL in August, relative to that in June and July, is contributed to mainly by the enhanced offset effect of the radiative cooling over NEA and partially by the weakened impact of the latent and diffusion sensible heating.