Low frequency characteristics of northeast China cold vortex and its background circulation pattern

In this study, the spatial-temporal distribution of the northeast China cold vortex (NCCV) in summer and the concurrent background teleconnection pattern have been investigated using a long-term data (1965-2007). The spatial extent of the NCCV activity is extended southward to the south of 40°N in June, while it is constrained to the north of 45°N in May. From July to August, it is gradually shrunk back to the north. Starting from May 1, the number of the NCCV days gradually increases as the East Asian Jet gradually weakens and moves northwards, and it reaches the peak about June 6. However, it begins to decrease when the meiyu period begins, and further decreases after the period. The strong NCCV events tend to happen in the pre meiyu period, while the weak and moderate events occur in the meiyu and post meiyu period. The NCCV activities show a quasi-biennial oscillation feature in the pre-meiyu and post-meiyu period. During years of 1965-2007, an increasing trend is found for the number of the NCCV days in summer, particularly during the meiyu period. Based on the position of the upstream ridge, the NCCV-related circulations can be classified into four types, namely, the Yenisei-type, the Baikal-type, the Ural/Yakutsk-type and the Okhotsk/Arctic-type. The western Pacific (WP) pattern is the primary background teleconnection pattern for the NCCV activity, with its negative phase being favorable for the NCCV formation. In addition, the NCCV activity is associated with the Pacific/North American (PNA) pattern in May and June and with the North Atlantic Oscillation (NAO) in August.