Possible Influence Of The Combination Of Phases Of The Leading Modes Of Tropical Indian Ocean On The Asian Monsoon In The Summer Half-year

Based on the sea surface temperature (SST) data from the Hadley climate prediction and research center, monthly mean reanalysis data from NCEP/NCAR, precipitation data from the Global Precipitation Climatology Centre (GPCC), the leading modes of the Indian Ocean SST anomalies (SSTAs) in the summer half year that mainly comprise the Indian Ocean Basin Mode (IOBM) and Indian Ocean Dipole (IOD), the leading modes of the Indian Ocean (IO) SST anomalies (SSTAs) in the summer half year, influences of whose different phases on the cross-equatorial flows(CEFs) and Indian precipitation are studied using the linear regression, composite analysis, etc. Furthermore, the combined in-phase impact of IOBM and the Central-Pacific SSTAs on the precipitation in East China are also analyzed. Some major conclusions are summarized as follows:(1) Three cross-equatorial flows are identified based on the areal average of the 925 hPa mean meridional winds over [5°S-5°N]; those are the SOM-CEF, BOB-CEF, SCS-CEF. The IOBM and IOD can induce different variations of the cross-equatorial flows. When both IOBM and IOD are in their positive (negative) phases, SOM-CEF weakens (strengthens), whereas SCS-CEF strengthens (weakens). For the warm (cold) phases of IOBM, SOM-CEF may change little, but BOB-CEF and SCS-CEF weaker (stronger). However, when IOBM and IOD are same signed (PPNN), SSTA-induced atmospheric circulation anomalies are displayed as a seesaw pattern in the eastern and western IO. Moreover, out-of-phase variation of SOM-CEF and SCS-CEF are caused, respectively. In contrast, when IOBM and IOD are out of phase against each other, the SSTAs in the tropical Indian Ocean may excite a Gill-type responses in atmosphere and consequently affect SOM-CEF, BOB-CEF and SCS-CEF.(2) For PPNN, more precipitation may take place in the entire India. However, for PNNP case, more precipitation can occur in the south and less in the north. For different phases of IOBM and IOD and their combination, their influences on the precipitation anomalies are different. Specifically, for PPNN events, large-scale circulation anomalies induced by SSTAs in the tropical southeastern IO are described the anomalous anticyclone over India Peninsula. Meanwhile, the single-cell anomalous vertical circulation may also facilitate the occurrence of increased precipitation. However for PNNP events, the Matsuno-Gill type atmospheric responses to the SSTAs in the southeastern IO inhibit the intensification of the southwesterly and contribute to the water vapor divergence. Also, the anomalous three meridional cell can boost increasing precipitation in the south India while decreasing precipitation in the north.(3) When the Central-Pacifc ENSO index(CPI) and IOBMI are same signed, a tri-pole SSTA pattern is observed with colder (warmer) than normal SST over the Maritime Continent(MC) region along with warmer (colder) than normal SST in both equatorial Pacific and Indian Ocean (referred to PPNN cases). When CPI and IOBMI are oppositely signed, a dipole pattern of SSTA is found in tropical Indo-Pacific sector with warmer (colder) than normal SST in Pacific while colder (warmer) than normal SST in Indian Ocean (referred to PNNP cases). For PPNN cases, the precipitation is found to be more than normal and moisture converges over Yangtze-Huaihe River basin. For PNNP cases, the rainfall decreases significantly with strong anomalous divergence of water vapor in region south of the Yangtze River. Different mechanisms behind different anomalous rainfall patterns in east China are different for PPNN and PNNP cases. For PPNN cases, moisture mainly can be transported from MC to the Yangtze-Huaihe River basin. The anomalous circulations are generated and maintained due to Gill-type response of atmosphere to the tropical SSTA, especially the cooling over MC region. The anomalous vertical circulation cells over region from MC northward to Yangtze-Huaihe River show some connections between anomalous thermal forcing over MC and rainfall anomalies over Yangtze-Huaihe River basin. However, for PNNP cases, no moisture is anomalously transported into Yangtze River and its southern flank areas from MC region. Due to the significant westward shift of locations of SSTA center in Pacific, the centers of anomalous circulations at 850hPa are also shifted westward apparently as a Gill-type response of atmosphere to central Pacific SSTA pattern. As a result, the water vapor is explicitly transported from east China into central Pacific region. However, Rossby wave energy can propagate into region south of Yangtze River from MC region, facilitating the maintenance of anomalous anti-cyclonic circulation over region south of Yangtze River. These results are beneficial to us for better understanding the cases of rainfall anomalies and the role of SSTA patterns in Indo-Pacific sector in inducing precipitation anomalies in east China.