The Northeast China Persistent Drought in Spring-Summer of 2017: Joint Roles of Teleconnection and Land-atmosphere Coupling

Northeast China (NEC) suffered its worst persistent drought event in recent decades from March to July of 2017 with devastating impacts on the environment and agriculture. Previous drought mechanism studies focused on the atmospheric remote response to Arctic sea ice and ENSO, while less attention was paid to synergistic effects of large-scale teleconnections and local land-atmosphere coupling. Here we show that a strong positive phase of Arctic Oscillation in March triggered the NEC drought, and a quasi-stationary Rossby wave train maintained the drought with an anticyclone located over the area south to Lake Baikal (ASLB) in April-July. By using a land-atmosphere coupling index based on the persistence of positive feedback between boundary layer and land surface, we find that the NEC and ASLB experienced a wet coupling in March while a persistently strengthened dry coupling in April-July. Over ASLB, the dry coupling and sinking motion increased surface sensible heat, decreased cloud cover, and weakened longwave absorption, resulting in a diabatic heating anomaly in the lower atmosphere and a diabatic cooling anomaly in the upper atmosphere. This anomalous vertical heating profile generated a negative anomaly of potential vorticity, indicating that the land-atmosphere coupling had a phase-lock effect on the Rossby wave train originating from upstream areas, and therefore maintained the NEC drought over downstream regions. Numerical simulations with and without surface sensible heating are being conducted to verify the influence of teleconnected land-atmosphere coupling, i.e., dry land conditions over ASLB in May can cause positive height anomaly over ASLB and NEC during June-July through heating the low level atmosphere. Our study suggests that upstream quasi-stationary wave pattern strengthened by land-atmosphere coupling should be considered in diagnosing persistent droughts especially over northern mid-latitudes.