Impacts Of ENSO And MJO On Interannual Variability Of Autumn Meteorological Drought Over Southern China

Southern China has suffered frequent autumn meteorological drought(autumn drought)in recent years,substantially limiting the sustainable development of economy and ecology.Autumn drought in southern China connects much more directly with the tropical oceanic–atmospheric system,in which the western North Pacific(WNP)anomalous cyclone serves as the major mediator linking the tropical climate system and precipitation anomalies over southern China.It's widely acknowledged that ENSO and MJO,known as the strongest variability of the tropical ocean and atmosphere respectively,are crucial potential predictability sources of autumn climate prediction in China.However,with the enhancement of ENSO spatio–temporal diversity and MJO activity,large uncertainties still remain in the autumn climate response to the tropical variability,leading to the great challenge for autumn climate prediction and the limited understanding of the causes of autumn drought over southern China to some extent.From the perspective of ENSO and MJO,this work explores the influence of ENSO and MJO on the interannual variability of autumn drought over southern China and key physical processes based on the dynamical statistical diagnosis of the reanalysis datasets.Main conclusions are summarized as follows:(1)The effects of the two types of ENSO events on autumn drought in southern China are unstable,especially for the CP Ni(?)a.During autumn of EP La Ni(?)a developing years,the robust response of WNP anomalous cyclone leads to prominent suppressed precipitation over southern China.During CP La Ni(?)a years,however,the precipitation response over southern China is asymmetric,featuring with half drought and half flooding years which results in the insignificant precipitation anomalies in southern China in total.Further analysis demonstrates that the unstable relationship between CP La Ni(?)a and southern China precipitation is possibly regulated by the SST anomalies in north Pacific.When the subtropical and central northern Pacific to the east of Japan are characterized by warm and cold sea surface temperature(SST)anomalies respectively,the simultaneously changing precipitable water related to the dipole SST anomalies triggers WNP anomalous cyclone through atmospheric diabatic heating.The anomalous cyclonic circulation,locating to the west of Philippines and shifting southward in comparison with that during EP La Ni(?)a events,is unfavorable for the water vapour transport and thus suppresses the convection over southern China.Henceforce,the negative precipitation anomalies over southern China tends to increase the probability of autumn drought occurrence.In contrast,the region of southern China is controlled by the southerly wind anomalies during the opposite dipole SST pattern in north Pacific,which is favorable for the increase of precipitation in coastal southeast China and areas along 30°N.(2)During those uncoupled El Ni(?)o(characterized by warm SST anomalies in the central and eastern tropical Pacific without conventional deep convection there)and non–ENSO years,MJO activity exerts significant effects on the occurrence of autumn drought over southern China.The probability of autumn drought over southern China,especially the probability of extreme autumn drought in the middle and lower reaches of the Yangtze River,is remarkably increased with more MJO active days in specific phases.When there are more MJO active days in Phases 7–8,MJO convection is more active in the northern side of equatorial central Pacific.MJO–related convection contributes to the anomalous cyclonic response in the WNP through atmospheric teleconnection.In that case,the northerly wind anomalies to the west of the cyclone is unfavorable for the transport of warm and moist air to southern China and could bring dry and cold air from middle and high latitudes,leading to the negative precipitation anomalies in southern China.Meanwhile,the Walker circulation anomalies triggered by MJO convection lead to strong sinking flow in the equatorial western Pacific and suppresses the corresponding convection in this region,accounting for the sea–air uncoupling in the western Pacific during El Ni(?)o events.For non–ENSO years,the reconstructed precipitation series by MJO active days index explains about 49% of the total variance of the observed precipitation series.Compared with the precipitation series over southern China reconstructed by ENSO–related SST index,the precipitation series reconstructed by MJO active days in key phases could capture the observed precipitation variation more accurately.In particular,for the extreme autumn drought in 1992,the reconstructed series can better reproduce the extremity of negative precipitation anomalies.(3)ENSO and MJO have a remarkable synergistic effect on autumn drought in southern China,and this synergistic effect weakens after 2000.In autumn,ENSO and MJO could individually affect the autumn drought in southern China via associated WNP anomalous cyclone,although there is no significant linear relationship between ENSO intensity and MJO active days in key phases during all years and ENSO years.For the averaged regional precipitation in southern China,considering the synergistic effect of ENSO and MJO will significantly improve the explained variance of the original precipitation series compared with the precipitation series reconstructed by ENSO–related SST index only.Besides,the synergistic effect of ENSO and MJO on autumn drought shows regional characterstics from the spatial distributions of the explained variance.The explained variance generally improves more than15% to the east of 110°E,mainly centered in the coastal southeast China and the area along30°N.In contrast,the improvement to the west of 110°E is weaker and less than 10% in general.Further analysis shows that the synergistic effect of ENSO and MJO on autumn drought in southern China weakens after 2000 at interdecadal timescale,which is possibly related to the fact that the linear relationship between ENSO and MJO turns to notablely negative after 2000.Before 2000,the explained variance of ENSO–MJO synergistic effect increases from 15% to31%,nearly twice as the one explained by ENSO individually for autumn drought in southern China.However,after 2000,the explained variance explained by ENSO–related SST index for autumn drought in southern China is already as high as 46.4%,and the improvement of the explained variance when considering the synergistic effect of ENSO and MJO was very limited.