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过去千年中国东部年代际百年尺度干湿变化特征(4)
【作者】网站采编
【关键词】
【摘要】To check if the relationships between the hydroclimate change over eastern China and the climate indices are robust in the PHYDA, the regression patterns of PDSI anomalies against these three climate
To check if the relationships between the hydroclimate change over eastern China and the climate indices are robust in the PHYDA, the regression patterns of PDSI anomalies against these three climate indices on centennial (Fig. S1) and decadal (Fig. S2) time scales are to these regression patterns, the negative phase of the AMO could have caused the drier conditions over eastern China and the location of the ITCZ was not significantly associated with the hydroclimate change over eastern China both on centennial and decadal time , the regression patterns between the PDSI and the Ni?o3.4 SST index are different on these two different time scales. El Ni?o—like mean states could have caused the drier conditions over eastern China on decadal time scales, whereas the regression pattern on centennial timescales is characterized by an insignificant north—south dipole pattern. Thus, we conclude that the AMO variation may play an important role in the centennial hydroclimate change over eastern China and the negative phase of the AMO and the El Ni?o—like mean states may be the main contributors to the decadal droughts. To test the possible mechanisms for these connections between the hydroclimate over eastern China and these climate indices —for example, previous works have suggested the influence of the AMO on precipitation over eastern China acts chiefly via a Eurasian wave train emanating from the North Atlantic to China or the AMO—Northern Hemisphere teleconnection wave train ( Qian et al., 2014 ) —the related atmospheric circulation should be analyzed. Unfortunately, due to the lack of reconstructions of dynamic variables such as geopotential height and others, the specific processes and mechanisms in action during the AMO and ENSO phases remain to be revealed.
4. Discussion
The hydroclimatic variation on the centennial time scale and the decadal droughts identified in the PHYDA and other reconstructions show that the reconstructions depict a more homogeneous temporal pattern during the early part of the LIA. The decadal droughts that occurred during this period were linked to the El Ni?o—like mean states and the negative AMO state. These relationships resemble results based on instrumental data ( Zhou et al., 2009 ; Qian et al., 2014 ), indicating the relationships between the hydroclimatic change over eastern China and the AMO and ENSO are robust during the last millennium, at least during the early part of the LIA. However, the physical mechanism involved in this phenomenon remains unclear because of the lack of reconstructions of dynamic variables such as winds, sea level pressure, geopotential height etc., in the PHYDA. We note that the amplitudes of ENSO and the AMO are remarkably smaller during 1000—1500 AD than 1500—2000 AD. According to the general data assimilation equation as shown in the supporting information, the oラine data assimilation approach leads to a reconstruction that reverts to the prior ensemble, which is estimated by CESM—LME10 ( Tardif et al., 2019 ) in the limit of no proxy information. Therefore, it is possible that the larger amplitudes of ENSO and the AMO after 1500 AD can be attributed to improved skill over the model climatology because of the greater availability of proxy data during this period. The temporal evolution of influences from proxies should be further investigated in the future.
Differences also exist, particularly during the two periods of the MCA and the 18th—19th centuries. For the first period, due to the existence of many missing records in the DWI before 1470 AD, fewer proxy data being available in the early part of the last millennium in the PHYDA,and both drought events and the centennial hydroclimatic change over this period before the 1350s being inconsistent among reconstructions,there is large uncertainty in the hydroclimatic change in the MCA. For the period of the 18th—19th centuries, we hypothesize three reasons to explain the differences. Firstly, it is possible that the high temperatures during these two centuries may have played some role in the drying trend and drought events in the PHYDA and the tree-ring chronology reconstructions ( Fig. 1 (a)). Because the PDSI in the PHYDA and the treering chronology reconstructions are affected by a combination of temperature and precipitation, the PDSI may represent local combinations of dry/wet and cold/warm conditions, resulting from abnormally low precipitation and high temperature ( Yang and Shi, 2013 ). We also hypothesize that the droughts in the 18th—19th century over eastern China in the PHYDA may be influenced by the tree-ring records, because proxies over eastern China in the PHYDA include the tree-ring records. As discussed before, such influences should be further investigated. The third possibility is that the PHYDA overestimates the influence of the AMO and ENSO on the hydroclimate over eastern China during this period, as discussed before. It suggests that further improvements in the reconstructed hydroclimate over eastern China may be possible with a greater availability of proxy data and a careful selection of proxies over the region.
文章来源:《干旱区资源与环境》 网址: http://www.ghqzyyhjzz.cn/qikandaodu/2021/0714/590.html