Interannual Relationship between Summer North Atlantic Oscillation and Subsequent November Precipitation Anomalies over Yunnan in Southwest China


  • The summer North Atlantic Oscillation (SNAO) strongly affects the climate variability over Europe and downstream East Asia similar to its winter counterpart. This study thus investigates the interannual relationship between SNAO and the subsequent autumn precipitation anomalies over Yunnan, Southwest China and related physical mechanisms based on reanalysis data during 1958–2020. The results show that the interannual variations in SNAO exhibit a significant positive correlation with anomalies of Yunnan precipitation in November. Composite analyses demonstrate that for the positive SNAO phase, the positive sea surface temperature anomalies (SSTAs) in midlatitude North Atlantic as part of a tripole SSTA tend to weaken from summer to November through changes in surface heat fluxes. In turn, the predominately negative SSTA in tropical North Atlantic that persists into November induces an anomalous cyclone at midlatitudes, which triggers two middle–upper tropospheric wave trains propagating from midlatitude North Atlantic to Yunnan. The subtropical wave train propagates eastward along the subtropical westerly jet, and the mid–high latitude wave train follows the great circle path across Scandinavia and central Asia to the Tibetan Plateau. Both wave trains favor development of an anomalous cyclone over the southern Tibetan Plateau. The upper-tropospheric divergent condition on the southeastern side of the anomalous cyclone is dynamically conducive to locally ascending motion over Yunnan, thus producing above-normal precipitation. The opposite situation occurs in the negative SNAO phase. A coupled model reproduces well the wave train propagation and thereby confirms the positive relationship between SNAO and Yunnan precipitation in November.
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