Relationship Between Structure Variation Of The Asia-Australian Monsoon And Indian Ocean Dipole SST Anomalies

By using the observational data, the reanalysis data and the output data of atmosphere general circulation model FrAM1.0, the influence of the Indian Ocean dipole (IOD) on Asian-Australian monsoon circulation structure and East Asian climate change in summer were studied from multi-angles. The main results were as follows:(1) The Indian Ocean dipole (IOD) significantly affected the Indian monsoon, the south Asia high and the atmospheric circulation in the upper troposphere over Northeast Asia during the summer (June-August). The IOD also influenced atmospheric circulation anomaly in the troposphere over Northeast Asia through multiple ways.The IOD event in1994can lead to the strengthening of the western South Asia High in June-August and the atmosphere disturbance spread eastward along the Asian jet, which affected the formation and variation of the anomalous anticyclonic circulation over Northeast Asia([120°E-145°E,35°N-45°N]). The variation of this anomalous anticyclonic circulation with the equivalent barotropic structure was intrinsically related to the accumulation and the eastern-spread of the lower potential vorticity and the abnormal accumulation of air quality. In addition, the three cells circulation decomposition method was proposed, by which the local meridional and zonal circulation of1994were calculated. The results showed that the IOD event in1994can cause significant zonal and meridional circulation anomalies.Analysis of observational data and numerical simulation studies of AGCM demonstrated that the East Asian summer monsoon circulation and climate during July-August were affected by the IOD.In the years of positive Indian Ocean dipole event, parts of East Asia including the eastern and northeastern parts of China, Japan and Korea may suffer from abnormally hot and dry summer. The positive IOD resulted in the anomalous anticyclone over Japan and the eastern and northeastern part of China, causing abnormally dry summer in these regions. The anomalous cyclonic circulation over the southern part of China and the western Pacific induced by the IOD weakened the monsoonal southward wind from the Bay of Bengal, the South China Sea, and the tropical Western Pacific, preventing the subtropical East Asia from receiving the normal water vapor from the tropical regions. However, the cyclonic circulation anomalies in the positive IOD year can cause more precipitation over South China than normal during the summer monsoon.During the positive IOD events, the anomalous dynamic heating was responsible for the abnormally hot summer Over East Asia. Over Japan and the Sea of Okhotsk, the dominant anomalous positive horizontal advection of temperature balanced the abnormal non-adiabatic cooling and the abnormal cooling caused by weak upward motion, causing and maintaining these unusually high temperatures in summer. Over the eastern part of China, the dominant sinking motion caused constant power warming to offset the non-adiabatic cooling and the anomalous negative horizontal advection of temperature due to the reduced condensation.During the positive IOD events, the anomalous relative vorticity advection balanced the anomalous planetary vorticity advection in various isobaric surfaces of the troposphere, leading to the formation of anomalous anticyclonic circulation with the equivalent barotropic structure over the Sea of Okhotsk and the region from northwestern part of Pacific Ocean to northeastern part of Japan.The IOD SSTA caused abnormal changes of circulation in the Mediterranean-Sahara and the Caspian Sea region. The divergent irrotational anomalous wind field from the tropical western Indian Ocean played a key role in the relationship between the IOD and circulation variation over the Mediterranean-Sahara and the Caspian Sea regions. The temperate circulation change was affected by the divergent flow outward from West polar region of the Indian Ocean Dipole and the abnormal divergent flow due to the IOD over the Tibetan Plateau.The abnormal change of circulation in East Asia during July-August was affected by the disturbance over the Mediterranean-Sahara region, the western part of India and the Qinghai-Tibet Plateau. The eastern spread of Rossby wave energy played an important role in connecting Europe and East Asia along the westerly jet. Through the westerly flow in the lower troposphere coming from Somalia, the Bay of Bengal and the southern part of China, until to the Japanese island, the disturbance wave energy during the positive IOD event can spread over to East Asia and led to the abnormal monsoon circulation over East Asia. Thus, East Asia can be seen as a unique area. In a positive IOD event, the IOD connected Eurasia and East Asia by westerly flow in the higher troposphere and connected Indian Ocean and Bay of Bengal and East Asia by westerly flow in lower troposphere. So the IOD affected the activities of the monsoon and the summer climate anomaly in East Asia through the "triangle" teleconnection.(2) The results of data experiments showed that both the barotropic and baroclinic structures of the monsoon circulation were significantly affected by the IOD. The stronger vertical shear of the zonal wind moved to the warming pole of the dipole.Monsoon is caused by land-sea thermal contrast and the monsoon circulation is characterized by the significant baroclinicity. The response of the monsoon circulation to the tropical Indian Ocean Dipole was studied by using the Atmospheric General Circulation Model (AGCM) T42L28and the data experiments. Both the barotropic and baroclinic structures of the monsoon circulation were significantly affected by the Indian Ocean dipole (IOD) sea surface temperature anomalies (SSTA). The IOD was able to induce anomalous changes in surface fluxes of both the sensible heat and latent heat causing the anomalous divergence in the troposphere, and hence forth resulting in the anomalous rainfall in Indonesia, the African continent and the Indian subcontinent. The IOD SST anomalies forced the atmosphere to change its temperature spatial distributions, which induced the modifications of the monsoon circulation baroclinicity. Both the barotropic and baroclinic structures of the monsoon circulation were hence to change. The stronger vertical shear of the zonal wind moved to the warming pole of the dipole.Further more, the IOD SSTA was able to change the circulations not only in the tropical region but also in the extra tropics. Whether positive or negative, the dipole mode forcing brought about an unusual atmospheric circulation over the warm pool of the West Pacific and the East-Asian region, and excited an anomalous wave train over the middle latitudes. However, the anomalous types were not obviously reversed each other.(3) The study showed that the frequent occurrence of the IOD can significantly affect the interannual connection between ENSO and IOV, resulting in significant decadal variation of ENSO-IOV relationship.The research using observational data and reanalysis data showed that the significant decadal variation was found in interannual connection between ENSO and IOV. The relationship between the tropical Indian Ocean variables and the ENSO indices had significant decadal variation in the study period from1871to1999. The relationship between ENSO and10V was closer in the1950s and1970s, but weaker in the1960s and after1980s. The EOF1of tropical Indian Ocean SSTA was unipolar mode with basin scale, interpreting more than60%of the population variance in1950s and1970s. In these two periods, EOF1and the ENSO indices showed high correlation.When the IOV-ENSO relationship was weak, the Indian Ocean Dipole events occurred more frequently. EOF analysis showed that, EOF2with a dipole structure only explained10%of the total variance of OSSTA during these periods. However, the variance explained by EOF1decreased significantly in the periods of1960-1969and1980-1997. The correlation between the time coefficients of EOF1and ENSO indices significantly reduced in these two periods.In1950s and1970s, the abnormal wind field near the sea surface showed circulation around Australia when the IOV-ENSO relationship was closer. While in the1960s and the period of1980-1997, this circulation broken over the southern Indian Ocean when the importance of EOF2modal increased and the correlation between EOF1and ENSO indices was weak. The circulation around Australia was considered to be the atmospheric bridge across the Pacific and the Indian Ocean in the horizontal plane, which was different from the Walker circulation atmospheric bridge in the zonal vertical plane.The EOF analysis and others partially confirmed that the zonal wind anomaly in the Alafula Sea region was an important indicating parameter of ENSO and IOD phenomenon. During the El Nino period, the positive zonal wind anomaly was prevailing. While during or before the strong positive IOD events, the negative or weak zonal wind anomaly was prevailing. This was consistent with the distribution of the anomalous sea level pressure over southeast Indian Ocean and Indonesia when the El Nino or IOD event happened.