北大西洋涛动与青藏高原东部夏季降水的不稳定关系
The nonstationary relationship between NAO and summer precipitation in the eastern Tibetan Plateau
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摘要: 青藏高原降水因其在区域水循环中的重要作用备受关注。基于96个气象站数据,分析了1961-2017年青藏高原东部夏季(6-8月)降水主模态与北大西洋涛动(North Atlantic Oscillation, NAO)的关系,解释了两者关系的演变特征及可能机制。结果表明,NAO主导着青藏高原东部夏季降水主模态,但两者的关系存在年代际变化,前期(1961-1993)NAO导致青藏高原东部夏季降水主模态呈南北偶极型,后期(2002-2017年)则为区域一致型。NAO能够在青藏高原高层激发具有准垂直正压结构的位势高度异常,在其低层形成相应的环流异常,进而影响该区域水汽异常。前(后)期,NAO引起该环流异常的西边缘位于青藏高原的东北部(西北部),有利于形成青藏高原东部夏季降水的南北偶极型(区域一致型)。青藏高原东部夏季降水主模态的转变,导致了对于整个研究时段青藏高原东部夏季降水主模态时间系数与NAO相关关系在1998年出现突变。鉴于NAO与青藏高原东部夏季降水的不稳定关系,在研究NAO对区域气候影响时,需要考虑年代际背景信号,以提高预测水平及模式模拟能力。Abstract: Precipitation on the Tibetan Plateau (TP) have received attention because of its importance for regional water cycle. Based on data of 96 meteorological stations from 1961 to 2017 on the eastern TP, the relationship between the lead mode of summer (JJA) precipitation variability and the North Atlantic Oscillation (NAO) is analyzed, and the possible mechanisms for the unstable relationship are explained. The results show that the NAO dominates the leading mode of summer precipitation on the eastern TP, but there are interdecadal variations in their relationship, with NAO leading to a north-south dipole pattern in the early period (1961-1993), and a monopole pattern in the later period (2002-2017). The NAO is able to stimulate potential height anomalies with a quasi-barotropic structure in the upper layers over the TP, and form corresponding circulation anomalies in the lower layers, which in turn affect the water vapor anomalies in the region. During the early (later) period, the west edge of the NAO-related circulation anomalies is located in the northeastern (northwest) TP, which is conducive to the formation of precipitation anomalies of north-south dipole pattern (monopole pattern) in the eastern TP. The shift in the leading modes of summer precipitation on the eastern TP resulted in an abrupt change in the NAO-precipitation relationship in 1998 for the entire study period. Given the nonstationary relationship between NAO and summer precipitation on the eastern TP, the interdecadal background signal needs to be considered in the study of NAO’s impact on regional climate in order to improve the prediction level and model simulation capability.