Relationships Of Intraseasonal Oscillations Of Summertime Rainfall In Regions Between Southeastern China And Sichuan-Chongqing-Shanxi-Gansu

Using the NCEP/NCAR daily reanalyses and753station data of daily rainfall from1979through to2010, the relationship of intraseasonal oscillation of summertime rainfall in the Southeastern China against Regions of Sichuan-Chongqing-Shanxi-Gansu is studied in the present paper. The results are as follows.The mean climatological intraseasonal oscillations (30-60d)(CISO hereafter) in rainfall have been investigated firstly. An interesting phenomenon has been revealed, which is characterized by an out-of-phase relationship between the intraseasonal oscillations(ISO hereafter) of rainfall in the southeastern China(SEC hereafter) and the Sichuan-Chongqing-Shanxi-Gansu Regions (CYSG hereafter). When the rainfall in SEC is getting stronger, its in CYSG is getting weaker. The low-frequency oscillation of summertime rainfall in CYSG changes apparently in local whereas it does in SEC with obvious propagations in meridional. No zonal propagations of CISO of rainfall in these two areas are found. The CISO rainfall in both SEC and CYSG is found to be related to the migratory low-frequency circulations that migrate into and out the southeastern part of China. However, the CISO of rainfall in the CYSG is in association with replacement of low-frequency circulations between an anti-cyclonic ridge and a cyclonic trough northeast of Tibetan Plateau. Vertical motion also facilitate the out-of-phase variations of CISO of rainfall in between SEC and CYSG. Furthermore, the examinations of the Rossby wave propagations using a Rossby wave theory proposed by Hoskins et al.(1983) show that the energy of low-frequency eddies propagates intrinsically westward to CYSG from the middle-low reaches of Yangtze River (MLYR hereafter), indicating a dynamic relation of CISO rainfall among SEC,MLYR and CYSG. This relation can partly explain the out-of-phase relationship between SEC and CYSG. This out-of-phase relationship is also in association with both the temporal variation of the Western Pacific Subtropical High and the ridges around Lake Baikal in the geopotential height at500hPa. When the SEC is mainly affected by the southwesterly flow to the northwest of WPSH and facilitates more rainfall, the CYSG is mainly affected by the northwesterly flow of the ridges around Lake Baikal and less rainfall. However, when the WPSH controls the SEC and generates less rainfall, the ridges around Lake Baikal weakened obviously, the CYSG turns to be influenced by the southwesterly flow to the northwest of WPSH and more rainfall.Because of the phase locking of ISO in East Asian monsoon activities and the innegligible contributions of CISO rainfall to total rainfall, the out-of-phase relation of CISO of rainfall in regions between SEC and CYSG is of great importance for our better understanding the local characteristics of ISO and for mechanism of ISO rainfall.There are6years which out-of-phase relationships of ISO rainfall are found in regions between SEC and CYSG. The typical features look similar to these of CISO.However, a special phenomenon has been revealed in2002, which is characterized by an in-phase relationship between the ISO rainfall in the SEC and in the CYSG This in-phase relationship is in association with the ISO circulation. In the positive(negative) phase of the ISO rainfall, the CYSG and the SEC are on the both sides of the shear line of anticyclonic(cyclonic) type respectively. This in-phase relationship is found to be related to the ISO divergence both in the lower and higher troposphere. In the positive(negative) phase of the ISO rainfall, the low-frequency circulation showed convergence(divergence) in the lower troposphere and divergence (convergence) in the higher troposphere both in the SEC and CYSG. When the energy of the low-frequency eddies propagate intrinsically southwestward to the SEC and CYSG, because the SEC and CYSG are in the path of the downstream direction, the ISO rainfall in these two areas will be strengthened and weakened simultaneously. Furthermore, this in-phase relationship is also in association with the stronger subtropical high and westerly trough. When the WPSH is stronger with its west edge of the ridge more southern and the westerly though is stronger, the SEC and CYSG are both influenced by the southwesterly flow to the northwest of WPSH, facilitating moisture transport and more rainfall. When the Subtropical High controls the SEC and CYSG, ISO of rainfall are to be in its less phase.