Interactions Between East Asian And Australian Monsoons In Association With Climate Anomalies In The Maritime Continent

The Australian winter monsoon and East Asian summer monsoon anomalies are closely related to climate variations in the Maritime Continent(MC).In this dissertation,in order to systematically explore the role of the MC in the boreal summer(June-August)in the interaction between the East Asian summer monsoon and the Australian winter monsoon,the NCEP/NCAR reanalysis product,the Had ISST dataset,CMAP precipitation data and the ERA-interim monthly reanalysis product are applied for discussed their connection mechanism and possible links with regional climate anomalies and interannual signals of tropical oceans.Besides,based on Regional Climate Model(Reg CM4.6),the influence of sea-land topographic distribution in the MC on Asian and Australian summer monsoon activities is simulated.It is found that,there may be a pattern of simultaneous variations in the East Asian summer monsoon and the Australian winter monsoon systems under influence of climate variations and topography in the MC.Results suggest as follow:(1)A joint monsoon index I_AAM is defined for capturing variation features of East Asian summer monsoon and Australian winter monsoon as an entity.The East Asian-Australian monsoons(EAAM)are found to be stronger after 1993 whereas weaker before this year.When EAAM is anomalously strong,cold sea surface temperature anomalies(SSTA)appear around north coast of Australia,which is partly associated with the upwelling of cold sub-surface water induced by strong equatorial easterly anomalies.The equatorial easterly anomalies also intensify the Australian high and northwestern subtropical anticyclone in the lower troposphere.The divergent flows emanating from SPCZ region and tropical north Pacific respectively force Australian High and northwestern Pacific subtropical anti-cyclone to reinforce.The anomalous convergence around Kalimantan Island links East Asian monsoon to Australian monsoon via the divergent flows.During strong EAAM years,rainfall is surplus over south coast of China and southern Japan whereas deficient over northwestern tropical Pacific and southeastern tropical Indian Ocean.(2)After the ENSO and IOD signals are filtered out,the Australian winter monsoon and the East Asian summer monsoon are still closely associated with each other.When the EAAMs are anomalously strong,cold sea surface temperature anomaly(SSTA)appears in regions near northern and northeastern coasts of Australia whereas the warmer SSTA appears in the northwestern tropical Pacific and South China Sea.The colder SSTA is associated with the upwelling of cold water from below induced by the equatorial easterly anomalies,inducing an abnormal anticyclonic circulation to be generated over Australia through the Matsuno/Gill-type response whereas warm SSTA appears in the northwestern tropical Pacific and South China Sea as a result of oceanic response to the intensified northwest subtropical anticyclonic circulation.The East Asian summer monsoon couples with Australian winter monsoon via the anomalous easterlies near equator in the Maritime Continent(MC)region and the slanted vertical anomalous circulations.In the years with strong EAAMs,precipitation decreases in northern Australia and over the area from the western Pacific to Bohai Sea and Yellow Sea of China.Meanwhile,the western MC and the southeastern China experience more than normal precipitation.(3)The coherent variations of the East Asian summer monsoon and the Australian winter monsoon are highly correlated with equatorial zonal wind anomalies over the key Maritime Continent(KMC)area.For one thing,when warmer SST anomalies are observed over the equatorial Indian Ocean with high-pressure system and descending motion and colder over the equatorial western Pacific,anomalous thermal forcing above the tropical Indian Ocean induces anomalous low-pressure system in the lower troposphere and triggers eastward propagating Kelvin waves,which motivates easterly wind anomalies around the KMC.For the other thing,colder SSTA in the equatorial western Pacific triggers anticyclonic anomalies in the lower troposphere of tropical northwestern Pacific and SPCZ region via the Gill-type response.Thus,equatorial easterly wind anomalies along equator in the KMC region are reinforced by lateral coupling of these two anomalous anticyclonic circulations.The reinforce of equatorial easterlies in the KMC leads to intensification of both the negative and positive vorticity anomalies located to the north and south of the equator,respectively.As a result,anticyclonic circulation over the Australian continent and that over the western North Pacific both intensify,further,the Australian winter monsoon and the East Asian summer monsoon simultaneously strengthen.The anticyclonic perturbation around the Philippines can affect the East Asian summer monsoon through the P-J/EAP teleconnection,and lead to regional climate anomalies in China and Japan.Meanwhile,the anticyclonic circulation anomaly in Australia affects winter climate in northern Australia.(4)Kalimantan Island is an important region in MC,where anomalies of atmospheric divergence have significant impacts on anomalous EAAM.When negative SST anomalies appear in the western Pacific on both sides of the equator,the atmospheric cooling over the northwestern Pacific and from the South Pacific Convergence Zone(SPCZ)to Australia is favorable for the maintenance of anticyclonic anomalies over the areas from the western Pacific to the South China Sea and from the south of the equator to the northeast of Australia.These anomalous anticyclonic circulations can induce equatorial easterly anomalies,which subsequently facilitate the maintenance of anomalous convergence around Kalimantan Island.In turn,the anomalous convergence in Kalimantan Island is favorable for the maintenance of anomalous anticyclonic over the northwestern Pacific and SPCZ,and thereby intensifies both the western Pacific subtropical high and the Australian high,inducing concurrent intensifications of the East Asian summer monsoon and Australian winter monsoon.Related to the anomalous convergence over Kalimantan Island,precipitation and temperature decrease from the western Pacific to SPCZ but increase from the Indian Ocean to western MC and Australia,while temperature is lower than normal in Kalimantan.(5)Variations in the circulation of the Australian winter monsoon and East Asian summer monsoon are possibly related to topography and land-sea distribution in MC.The Regional Climate Model Version 4.6(Reg CM4.6)is applied for the simulation of summertime climate over the Asian-Australian monsoon region.Results indicate that Regc M4.6 can well reproduce precipitation,temperature and low-level and upper-level circulation patterns over the Asian-Australian monsoons region.A sensitivity experiment with zero topographic height in the MC region shows that the western Pacific subtropical high and the high pressure over Australia both weaken simultaneously,while the cross-equatorial flows decline and the East Asian-Australian monsoons become weak.When the land areas in the MC region are absent,four anomalous circulation systems symmetric about the equator develop in the lower troposphere.These four systems consist of a pair of anomalous anticyclones located over the southern and northern Pacific and a pair of cyclones located in the southern and northern Indian Ocean,respectively.These two pairs of anomalous circulation systems couple with each other in the equatorial region,leading to the development of anomalous westerlies over the Indian Ocean and anomalous equatorial easterlies over the Pacific,which subsequently intensify the East Asian summer monsoon and Australian winter monsoon.These results will be helpful for a deep understanding of the role of climate anomalies over the MC in the variations of East Asian summer monsoon and the Australian winter monsoon,and provide some clues for further studies of the regularity and formation mechanism of the Asian-Australian monsoon activity anomalies.