Intraseasonal mode of East Asian trough anomalies in boreal winter and specific possible mechanisms

Different evolutionary patterns of the East Asian trough (EAT) anomaly could cause different climate patterns over East Asia. In this study, the leading mode of month-to-month variation in winter (December-January-February) EAT during 1979/80-2018/19 is investigated using the extended empirical orthogonal function (EEOF). The first EEOF mode (EEOF1) shows a persistent anomalous EAT pattern throughout the three winter months, which results in persistent air temperature anomalies over East Asia. Mechanism analyses indicate that the El Nino-Southern Oscillation (ENSO) negatively relates to the EEOF1, but its significant influence on the EAT is only in December. In contrast, a dipole pattern of sea surface temperature (SST) anomalies over the Eastern North Atlantic-Eastern Mediterranean Sea closely links with the EAT EEOF1. Further analyses indicate that the connection of the dipole SST pattern with the EAT EEOF1 depends on its persistence. When there is positive feedback between the local atmosphere and sea, the dipole SST pattern is well-sustained in the three winter months and further leads to a persistent EAT anomaly by exciting the Rossby wave train over mid-latitude Eurasia. Numerical simulations by the linear baroclinic model further validate the role of dipole SST pattern-related diabatic and vorticity forcing in air-sea interactions. In contrast, in the SST poorly-sustained years, the dipole SST pattern has a weak connection with the EAT EEOF1. Based on the aforementioned analysis, we propose a combined index using the dipole SST pattern and anomalous Northeast Atlantic cyclone in December, which could provide valuable information to predict the dipole SST pattern persistence and EAT EEOF1 in winter.