Characteristics And Mechanisms Of The Intraseasonal-to-Interannual Variations Of Northwest Pacific Anomalous Anticyclone During Boreal Summer

The Northwest Pacific anomalous anticyclone(NWP-AAC)shows great impact on regional atmospheric and oceanic variability across different timescales during boreal summer.In this dissertation,multiple reanalysis data,the HadISST dataset,observational precipitation data and the output of ECHAM5 atmospheric general climate model,as well as the statistical analyses and dynamic diagnoses,are utilized to systematically study the intraseasonal to interannual features of the NWP-AAC.The main results show as follows,(1)The NWP-AAC is a local atmospheric base mode across timescales from intraseasonal to interannual.By using the EOF analysis of normalized tropospheric upper and low-level wind anomalies,the leading modes are the Intraseasonal Oscillation(ISO)modes over the summer Indo-western Pacific region.Based on the modes ISO1 and IS02,we reconstruct the monthly anomalies and conduct a further EOF to get the leading interannual modes EOF1,2rec,which represent the interannual NWP-AAC mode(EOFi1rec)and the strengthened Indian summer monsoon mode(EOF2rec).The cross spectra and power spectra analyses suggest that the EOF1,2rec can be treated as the base function of summer ISO.On the interannual timescale,the EOFi1rec exhibits quasi-biennial tendency,which is related to the phase transition of ENSO.While the power spectrum of EOF2rec is flat and white,suggesting that the EOF2rec is an atmospheric internal mode.The underlying physical mechanisms of leading modes are further investigated by analyzing the energetics.The form location of NWP-AAC is related to the distribution of low-level mean state.The barotropic energy conversion and convective feedback help maintain the NWP-AAC over the low-level confluence zone of monsoonal westerly and easterly trade winds across different timescales.These suggest that the NWP-AAC is an atmospheric intrinsic mode over the Indo-western Pacific region.(2)The formation of NWP-AAC is closely related to different phases of ENSO.Based upon the ENSO index,the regression and correlation analyses suggest that the lower tropospheric anticyclonic anomaly is generated in El Nino decaying and La Nina developing summer over the northwestern Pacific.The anticyclonic anomaly is excited over the northwestern Pacific.Month to month variations of precipitation over East Chia are directly associated with the anticyclone induced temperature advection.Besides,the combining effect of the heat source of Tibetan Plateau,mid-latitude westerly jet and the Northwest Pacific subtropical high also helps induce the month-to-month variations of precipitation in East China.(3)The interannual variability of NWP-AAC is not only related to ENSO,but also independent with ENSO,which is generated by the atmospheric internal process.Using the August anomalies as an example,further study suggests that the observational internal mode resembles the leading EOF mode of ensemble spread of ECHAM5 model.The spatial pattern of ensemble spread EOF1 shows an anomalous anticyclone in the northwestern Pacific.These results prove that the atmospheric internal process induces the NWP-AAC.In addition,the atmospheric internal variability associated with the NWP-AAC arises from the tropical ISO.(4)As a regional atmospheric intrinsic mode,the NWP-AAC has great implications for climate monitoring over the Asian summer monsoon region.In August of 2016,the atmospheric internal cyclonic anomaly which is generated by the ISO,overwhelms the preceding ENSO effect,resulting in the rainfall increasing in the northwestern Pacific and decreasing in the middle and lower reaches of the Yangtze River basin.On the other hand,the Meiyu rainfall shows significant intraseasonal variability during summer 2020.Further investigation suggests that the interannual and intraseasonal rainfall anomalies in summer 2020 are dominated by the NWP-AAC.On the interannual timescale,both the North Indian Ocean warming and concurrent La Nina help maintain the NWP-AAC.While on the intraseasonal timescale,the ISO is the key reason causing the rainfall increasing over the Yangtze River basin.Since the NWP-AAC plays a dominant role for local air-sea coupling and climate impact across timescales,it is necessary to propose an index for real-time monitoring of the NWP-AAC and its orthogonal mode.Thus,the RTI1 and RTI2 is proposed to represent the real-time NWP-AAC and the real-time EOF2rec,respectively.On the intraseasonal timescale,the combination of RTI1 and RTI2 could be used for the real-time monitoring of tropical ISO over the Asian summer monsoon region.(5)The column-integrate moisture budget of EOFi1,2rec is utilized to diagnose the propagating features of summer ISO.During summer,the column-integrate moisture tendency leads the column-integrate moisture,resulting in the ISO propagation.The decomposition of column-integrate moisture budget suggests the northward propagation of summer ISO is mainly contributed to the horizontal moisture advection.Besides,the "column process" is also benefit to the specific propagation direction of summer ISO.These results will have great implications for a deep understanding of the cross-timescale characteristics and physical mechanisms of the NWP-AAC,as well as its impact on the Asian summer monsoon variability.Moreover,these results might provide some clues for further studies of the multi-scale interactions and climate prediction and monitoring of the Asian summer monsoon.