Background of Atmospheric Circulation and SST Evolution Features of Typical Midsummer Rainfall Patterns in the Northern China Monsoon Region

Based on the precipitation observation data of 46 stations in the northern China monsoon region (NCMR) and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis datasets, the midsummer rainfall patterns in the NCMR during 1961–2019 are objectively classified into four typical categories: pattern A, pattern B, pattern C, and pattern D. Pattern A (B) corresponds to the positive (negative) anomaly midsummer precipitation in the whole area, while pattern C (D) corresponds to positive (negative) anomaly precipitation in North China (Northeast China). The characteristics of the four rainfall patterns, atmospheric circulation, sea surface temperature (SST), and precursors are investigated with EOF and composite analysis along with other methods. The major circulation characteristics and the evolution of SST, resulting in the four rainfall patterns, are different. Pattern A: The East Asia subtropical westerly jet stream (EAJS) shows a northward trend relative to its normal position. The ridgeline of the western Pacific subtropical high (WPSH) is located to the north of its normal position, accompanying stronger southerly winds. The zonal circulation is dominant in the middle and high latitudes of Eurasia. There are significant convergences (divergences) at the low (high) level in the troposphere and cold and warm air convergence in the NCMR. The negative phase of the North Atlantic SST tripole and the eastern pattern of La Niña develop from the previous winter to summer. Meanwhile, the characteristics of atmospheric circulation and SST evolution of pattern B are basically opposite to those of pattern A. Pattern C (D): EAJS shows an anomalous northward (slight southward) trend relative to its normal position. The longitudinal position of WPSH is located westward (eastward), and their ridgelines both tend to be northward of the normal position. North China is controlled by stronger southwest (northwest) winds, while northeast (southeast) winds prevail over northeast China. The lower troposphere converges (diverges) and the upper troposphere diverges (converges) in North China, while the lower troposphere diverges (converges) and the upper troposphere converges (diverges) in northeast China, accompanying a weak (active) northeast cold vortex. Meanwhile, the warm SST transforms to cold SST (El Niño pattern gradually forms) in the middle eastern tropical Pacific.