Seasonal And Interannual Variations Of The Southern Hemisphere Circulation And Their Influences On East Asian Summer Monsoon

Based on the NCEP/NCAR reanalysis products, the sea ice concentration data and the observation rainfall data in China, the seasonal, intraseasonal and interannual variations of the atmospheric circulation in the Southern Hemisphere (SH) are systematically analyzed together with their influences on East Asian Summer Monsoon (EASM). The major conclusions are summarized as follows:(1) In the lower level of the troposphere, the semi-annual oscillation (SAO) is most active along 40 S and 65 S in the extratropics of the SH. Over these latitudes, the variance percentages of SAO exceed 70%, and the anti-phase variation of the sea level pressure (SLP) between the two latitudes is primarily caused by their SAO components. A more reasonable SAO index is therefore defined as the SAO variance percentage of the difference of the zonally averaged SLP between 40°S and 65°S. In addition, in the austral winter, the westerly jet splits into two branches over the South Pacific from 600 to 100hPa, with the subtropical branch located at 27.5 S and the polar counterpart at 60 S. The maximal speed of the former is at 200hPa while the speed of the latter increases with height. Furthermore, the zonal propagation of the intraseasonal oscillation (ISO) of the SH subtropical high is also documented. Results show that the Australia high (AH) is influenced by the Mascarene high (MH) through the eastward propagation of the quasi-biweekly oscillation.(2)In the lower level of the troposphere, the center of the cross-equatorial flows (CEFs) is located at 925hPa other than 850hPa while it is at 150hPa instead of 200hPa in the upper level. The CEFs in the Eastern Hemisphere (EH) show monsoonal features while they are characterized by the trade wind in the Western Hemisphere. The mass transport between the Northern Hemisphere (NH) and the SH is stronger in boreal summer especially in the EH. The Somali jet and the South China Sea (SCS) CEF in the lower level play an important role in the onset of the South China Sea summer monsoon (SCSSM). The Somali jet is rapidly enhanced two pentads prior to the onset of SCSSM, thus accelerating the eastward extension of the westerly over the Bay of Bengal and leading to the eastward retreat of the western Pacific subtropical high (WPSH). At the same time, the rapid enhancement of SCS CEF can also result in the northward march of WPSH and the onset of SCSSM. The earlier establishment and stronger southerlies correspond to the earlier onset time of SCSSM, and vice versa. Both composite and correlation analyses show that the 150hPa Asia-AustraliaCEF (AACEF) in boreal spring has important influences on EASM. When AACEF is weaker, the summer WPSH tends to be stronger with a southwestward extension, and the South Asia high (SAH) will be stronger too. This circulation pattern will lead to more rainfall in the Yangtze and Huaihe River valley and less rainfall outside of this region. On the contrary, the opposite pattern occurs when the AACEF is stronger.(3) On interannual timescale, the changes in the SH Ferrel cell lead to the anti-phase variation of the subtropical high and the circumpolar low, i.e., the Antarctic oscillation (AAO). For this reason, the AAO index (AAOI) is defined as the normalized zonal mean SLP difference between 30 S and 65 S. The result indicates that when AAO is stronger in boreal summer, both the Antarctic vortex (AV) and the polar westerly jet (PWJ) are stronger while the subtropical westerly jet (STWJ) is weaker, and vice versa. Besides, AAO has a leading influence on EASM. When AAO is stronger in April and May, the MH and AH together with the Somali jet are stronger in summer, the intensity of EASM tends to be weaker, and vice versa. Corresponding to the stronger AAO in boreal spring, especially in May, there is more rainfall with a longer period of mei-yu along the Yangtze and Huaihe River valley. In contrast, there is less rainfall with a shorter period when AAO is weaker. AAO is therefore an important interannual signal influencing summer rainfall especially the mei...