The Interannual Variability Of Co-variation Mode In Boreal Summer Monsoon Rainfall And Its Possible Mechanism

The boreal summer monsoon(BSM),which includes the monsoons over the North America(NAM),North Africa(NAF),India(IND),western North Pacific(WNP)and East Asia(EA),interconnect with each sub-components through atmospheric bridge and air-sea interaction effects.The BSM rainfall accounts for 1/2 of the world total precipitation,modulates the global transport of atmospheric energy and water vapor,causes the droughts and flood-related extreme climate events and influences 2/3 of world population's daily lives over monsoon areas.Thus,the interannual variability(IAV)of the summer monsoon is always the focus of monsoon studies and monsoon prediction is the most challenging problem in climate science.The internal linkages among regional monsoons in the Northern Hemisphere spring up after early 1990 s,which provides new clues in enhancing the prediction skill over BSM regions.This study used statistical analysis,data diagnosis and model simulations to study the interannual co-variation mode in BSM IAVs and investigate its maintaining mechanisms from both the internal dynamics and external forcings aspects,especially the ENSO events and its associated tropical sea surface temperature(SST)anomaly forcings.Furthermore,the distinct characteristics of the co-variation mode and its possible mechanisms before-and after early 1990 s were investigated,the simulation skill and seasonal prediction skill among climate system models were evaluated.This study aimed to enhance the seasonal prediction on the BSM rainfall from the BSM covariation mode aspect.Main conclusions are summarized as follows.1.Reveal the key contributions of WNP and NAM monsoon rainfall anomalies to the interannual dominant co-variation mode of BSM rainfallAn EOF analysis is used on the IAVs of BSM rainfall,and the EOF1 mode is treated as the BSM dominant co-variation mode during 1979–2014.This mode is featured by the out-of-phase rainfall IAV over the WNP to the other BSM members.The BSM co-variation mode is closely associated with the upper-and lower-level coupled circulations,which are characterized by two anomalous zonal circulations over the tropical oceans coupled near the dateline and an abnormal meridional cell over the WNP and EA regions,respectively.Statistical analysis and a series of sensitivity experiments in the linear baroclinic model(LBM)approve that regional monsoon variations have distinct contributions to the BSM co-variation mode.The WNP and NAM monsoons are key components in the BSM co-variation mode.2.Validate the effects of ENSO evolution on the strength and phase of the BSM co-variation modeThe strength and phase of the BSM dominant co-variation mode are closely connected with key monsoon variations,which are modulated by the ENSO events and its related tropical SSTAs evolution over the tropical Indian ocean(TIO)and tropical North Atlantic(TNA)from previous winter to current autumn.The weaker mode is accompanied by the in-phase rainfall IAV between the WNP and NAM monsoon regions,along with the persisting ENSO events from previous winter to summer.While in the years with fast-decaying ENSO events,the out-of-phase rainfall IAV between the WNP and NAM regions takes place to enhance this mode.The strong positive(SP)phase and the strong negative(SN)phase of the BSM co-variation mode have prominent asymmetric pattern in relative importance of key monsoon contributions and seasonal evolutions of the El Ni?o event.The SP-phase is determined by the suppressed WNP rainfall in summer following a decaying El Ni?o event,when a tripole SSTA pattern can be observed over the TIO–eastern equatorial Pacific(EEP)–TNA.However,the SN-phase in summer with a developing El Ni?o event is associated with the anomalous NAM rainfall,when a dipole SSTA pattern appears over the EEP–TNA.Historical run in FGOALS-f2 model reproduced the linkages between the ENSO evolution and the asymmetric precipitation pattern over key monsoon regions,which further modulate the strength and phase of the BSM co-variation mode.3.Clarify that changes of ENSO property result in the BSM co-variation mode modulation before and after 1990sThe dominant co-variation mode of BSM rainfall shifts during 1979–2014,the former stage P1(1979–1993)is primarily characterized by an EOF2 pattern,while the latter stage P2(1994–2014)is mainly featured by an EOF1 pattern.Fewer internal linkages among regional monsoons and the dominant co-variation mode is featured by variations between tropical monsoons and the equatorial ITCZ region in P1;however,more interrelationships can be observed among BSM sub-components in P2,which is characterized by rainfall variations between the WNP and NAM monsoon regions.In comparison to the P1,ENSO events experience shorter cycle and westward shift of maximum SSTAs over the equatorial Pacific in P2.The more frequently central Pacific(CP)type ENSO events,the speeding ENSO cycle and the Atlantic Multi-decadal Oscillation(AMO)changes from cold phase to warm phase after the 1990 s possibly induced the BSM co-variation mode modulation before and after 1990 s.4.Evaluate the simulation skills in CMIP5 models and point out the potential seasonal prediction skill in FGOALS-s2 model on the BSM co-variation modeThe 40 coupled models in the Coupled Model Intercomparison Project Phase 5(CMIP5)have distinct simulation skill on the BSM co-variation mode.The best 7 models(CESM1-CAM5,CESM1-WACCM,CNRM-CM5,FGOALS-g2,FIO-ESM,GFDL-CM3,HadGEM2-CC)are selected and its multi-model ensemble(MME)mean notably improve the simulation skill on the BSM co-variation mode.The historical run of the coupled CAMS-CSM model reproduces the BSM co-variation mode and its external linkages to ENSO evolution,while the atmospheric run of this model can not.The importance of ENSO related ocean–atmosphere interaction is examined.The subseasonal-to-seasonal(S2S)forecast system in the FGOALS-s2 model well predicts the WNP and NAM summer monsoon rainfall variations 5–6 months in advance,showing potential seasonal prediction skill on the BSM co-variation mode.