Intensified impacts of central Pacific ENSO on the reversal of December and January surface air temperature anomaly over China since 1997
AbstractThe reversal of surface air temperature anomalies (SATA) in winter brings a great challenge for short-term climate prediction and the mechanisms are not well understood. This study found that the reversal of SATA between December and January over China could be demonstrated by the second leading mode of multivariate empirical orthogonal function analysis on the December-January SATA. It further reveals that the central Pacific El Niño-Southern Oscillation (CP ENSO) has contributed more influences on such a reversal of SATA since 1997. CP ENSO shows positive but weak correlations with SATA over China in both December and January during pre-1996, whereas it shows significantly negative and positive correlations with the SATA in December and January, respectively, during post-1997. The CP ENSO-related circulations suggest that the change of the Siberian high plays an essential role in the reversal of SATA since 1997. Sea surface temperature anomalies pattern associated with the CP ENSO leads to a westward-replaced Walker circulation which alters the local meridional circulation and further impacts the Siberian high and SATA over China since 1997. Moreover, the seasonal northward-march of convergence zone from December to January causes northward-replaced west branch of the Walker circulation in January compared with that in December. The west branch of Walker circulation in December and January directly modulates local Hadley and Ferrel circulations, and then causes contrasting Siberian high anomalies through inducing opposite vertical motions anomalies over Siberia. The reversal of SATA between December and January is therefore more frequently observed over China since 1997. The above-mentioned mechanisms are validated by the analysis at pentad time scale and confirmed by numerical simulations.