The Major Modes Of The East Asian Winter Monsoon And Their Relationship With Sea-ice And Sea Surface Temperature

To investigate the spatial structure and circulation characteristics of East Asianwinter monsoon (EAWM) during the period of1948~2011, the NCEP/NCARreanalysis data and some statistic methods, such as empirical orthogonal function(EOF) analysis and regression analysis are used in this study. The EAWM can bedivided into winter monsoon (WM) high-latitude mode (H-mode) and low-latitudemode (L-mode), respectively. The two modes have notably different circulationstructures that together account for68.4%of the total variance. The H-mode,characterized by a westward shift of the East Asian (EA) trough and a northwestwardshift of EA westerly jet, corresponds to the negative phase of the North AtlanticOscillation (NAO), reflecting the strength variance of WM over Siberia, representinga significant cooling in the northern region of Eurasia. On the other hand, the L-modefeatures a deepening EA trough, a southward shift of EA westerly jet and enhancedsurface pressure over Mongolia, reflecting the extent of WM extending southwardfrom Mongolia, causing a wide range of cooling phenomenon resulting from cold-airintrusion from western Mongolia.By using harmonic analysis method, the inter-annual (IA) component andinter-decadal (ID) component of the two distinct modes can be filtered out. Thestructure of IA component of WM H-mode, while bearing an overall resemblancewith that of ID component of WM H-mode, shows a more obvious circulationanomalies and more northerly position. The structure associated with ID componentof WM L-mode is completely different from its IA counterpart, but it resembles thatof ID component of WM H-mode. For the WM H-mode, its IA component can bepreceded by pronounced large scale snow cover anomalies in the western Siberiaregion during the preceding autumn, while the ID component has a significant positive correlation with the Greenland–Barents sea-ice anomalies. For the WML-mode, its IA component has an anomalously positive correlation with thedevelopment of La Nina in autumn, its ID counterpart also has a positive correlationwith the Greenland–Barents sea-ice anomalies, but weaker than ID component ofWM H-mode. Without the impact of the long-term trend, the WM H-mode still has asignificant positive correlation with winter Greenland sea-ice anomalies, while theWM L-mode significantly negative correlates with Barents–Kara sea-ice anomalies.The intraseasonal evolution in winter associated with distinct modes of EAWMare analyzed with the application of the Season-reliant empirical orthogonal function(S-EOF) decomposition method. The Results show that the WM H-mode has aconsistent variation in early and late winter, but with a stronger variation in late winter,which may be affected by Greenland–Barents sea-ice anomalies. The WM L-mode,while bearing a consistent pattern in early and late winter, shows clear differencesbetween early and late winter. In early winter, the WM L-mode, on which ENSO has aremarkable influence, mainly reflects the extent of WM extending southward fromMongolia. On the other hand, the WM L-mode in late winter is a reflection of thestrength of the WM variance, being affected by the large scale snow cover anomaliesin the southern Mongolia region. In addition, there is a generally opposite variationmode of winter monsoon in early and late winter over northern East Asia. Thisout-of-phase mode relates closely to neither the Arctic sea-ice anomaly nor the ENSO,but it can be preceded by the excessive autumn snow cover anomalies in the westernSiberia–Lake Baikal.