The role of transient eddies in the intraseasonal reversal of East Asian winter air temperature anomalies

Abstract

The intraseasonal reversal of wintertime surface air temperature (SAT) anomalies over East Asia (15°-55°N, 80°-130°E) is becoming a matter of concern. Thus, the predominant features of the reversal mode of SAT anomalies and its relevant dynamical mechanism associated with transient eddies (TEs) are studied by using the Multivariate Empirical Orthogonal Function (MV-EOF) analysis method and the geopotential tendency equation. Results show that the third MV-EOF mode well depicts a colder (warmer) DJ with a warmer (colder) February and is closely linked to an out-of-phase variation in both East Asian Polar Front Jet (EAPJ) and East Asian Subtropical Jet (EASJ). It is found that synoptic-scale (2.5-8d) and low-frequency (10-90d) TE activities over Eurasia play important roles in the intraseasonal reversal in SAT anomalies by adjusting time-mean flows. A cold DJ in East Asia dynamically corresponds to a strengthened EASJ and weakened EAPJ, while the TE forcing dominated by low-frequency TE vorticity forcing and synoptic-scale TE heating forcing tends to weaken the EASJ and reinforce the EAPJ, unfavorable to the maintenance of cold anomalies over East Asia. However, the characteristics of both synoptic-scale and low-frequency TE activities in February are significantly different from that in DJ. By comparison, the TE forcing is dominated by synoptic-scale TE vorticity forcing and low-frequency TE heating forcing, which makes great contribution to the weakening of EASJ and the enhancement and maintenance of EAPJ, conducive to a weaker East Asian winter monsoon and eventually to a warmer East Asia.