The Important Role Of Intraseasonal Signals In Extreme Climate Events In The Context Of Tropical SSTs

The extreme climate events occur frequently in recent years,which brought huge impacts on public health,social economy and ecosystems.However,the physical mechanism driving the extreme climate events is complex and remains unclear.Since most of the extreme climate events exist on sub-seasonal and interannual time-scales,previous studies tended to investigate the mechanism of formation for these events from the perspective of interannual time-scale,especially focus the role of the tropical Pacific sea surface temperature(SST)signals(such as ENSO,El Ni(?)o-Southern Oscillation)on these events.Although ENSO and its related tropical SST anomalies can provide favorable large-scale thermal conditions for the extreme climate events,their extreme nature usually cannot be well explained.As the most significant signal on the subseasonal time-scale,the tropical intraseasnal oscilltation(ISO)has been recognized as one of the predictability sources of the global weather and climate.Previous studies have focused on the impacts of ISO on weather and climate anomalies,but the connection between it and climate extremes remain unclear.Based on three extreme climate events with high attention in recent years(2018 extreme Northeast Asia heatwave,2019 extreme positive Indian Ocean dipole(IOD)event and 2020 East Asia extreme plum rain),several high-resolution reanalysis datasets and CFSv2 forecast product,the physical mechanisms for these three climate extremes are deeply explored from subseasonal and interannual time-scales.Firstly,the modulation of the tropical SST signal on the large-scale thermal conditions for these climate extremes is analyzed.Then,the contribution of the abnormal ISO activities to these climate extremes is investigated.Finally,the universality of the synergistic effects of tropical SST and ISO events on the extreme events is further explored.Main conclusions are summarized as follows:(1)The intensity and persistence of the three extreme climate events rank among the top three in history.The number of 2018 extreme Northeast Asia heatwave days is the most since1980,the occurrence of this extreme climate event is closely associated with the maintenance of the northward shifted western Pacific subtropical high(WPSH).The related circulation anomalies are highly consisted with the Pacific-Japan pattern(P-J pattern)teleconnection,indicating this climate extreme is mainly caused by the tropical signals.The intensity of 2019 extreme positive IOD event is second only to 1997,it develops in May,peaks in boreal autumn and decays rapidly in December.The 2020 East Asia extreme plum rain rank first since 1980 in terms of intensity and persistence.The moisture transport associated with the abnormally strong western north Pacific anticyclone(WNPAC)is the direct cause of this extreme plum rain.Compared with the observations,the forecast results with different initial conditions from CFSv2 model grossly underestimated the intensity of three extreme climate events.(2)Tropical SST anomalies provide favorable large-scale thermal conditions for three extreme climate events.During late summer 2018,the negative SST anomalies in the north Indian Ocean(NIO)can modulate the P-J teleconnections and favor the northward movement of WPSH,which provides a favorable large-scale thermal condition for the positive surface air temperature in Northeast Asia.However,the intensity of the negative SST anomalies is relatively weak,which is insufficient to explain the extremely northward shift of WPSH.The2019 extreme positive IOD events is accompanied by the development of a CP El Ni(?)o event.During boreal summer,the convective anomalies induced by the tropical Pacific warming promote to the development of the positive IOD by adjusting the Walker circulation.While in boreal autumn,the atmospheric convection anomalies are decoupled with the underlying SST anomalies in the tropical Pacific,suggesting the CP warming cannot contribute to the further development of the positive IOD.During early summer 2020,the Indian Ocean Basin Warming(IOBW)provides very favorable large-scale thermal condition for the persistent heavy rainfall in the plum rain belt by enhancing the WNPAC,but it can only capture a small part of the local convective anomalies,and thus cannot well explain the 2020 East Asia extreme plum rain.(3)The tropical abnormal ISO activities contribute to the formation of three extreme climate events under the favorable large-scale thermal conditions.During late summer 2018,the convective activities associated with the ISO activities in WNP induce the maintenance of the northward shifted WPSH by triggering a P-J like teleconnection,contributing to the occurrence of 2018 extreme Northeast Asia heatwave.During September to October 2019,in the absence of the remote forcing related to the CP El Ni(?)o,the persistent low-level easterly anomalies related to the quasi-stationary active ISO over the central-western IO superimposed on the seasonal mean easterly wind anomalies,favoring the amplification of the western and eastern IO SST gradient,and further promoting the development of the 2019 positive IOD events.During early summer 2020,the convection anomalies associated with the extraordinary longlasting ISO activities in IO enhance the WNPAC and provide a continuous supply of tropical moisture for the long-lasting rainfall over the plum rain belt,suggesting the abnormal ISO activities is the key driver for 2020 East Asia extreme plum rain.(4)The combined effect of the tropical SST and ISO anomalies has universal significance for the formation of three types of extreme events.The negative SST anomalies in NIO and active ISO activities in WNP are favorable for the occurrence of the heatwaves in Northeast Asia on interannual and subseasonal time-scales,respectively.When these two work together,the probability of moderate and extreme heatwaves in Northeast Asia can significantly increase from 3.6% and 0.5% to 6% and 2.4%,the area can significantly expand by 15% and 7%,and the probability of large-scale heatwaves will also be greatly increased.Similarly,the IOBW and active ISO in IO,Maritime Continent or WNP are all favoring the occurrence of the extreme precipitation events over the plum rain belt on interannual and subseasonal time-scales,respectively.With the combined effect of the IOBW and ISO,the probability of moderate and extreme precipitation events in plum rain belt can significantly increase from 3% and 1% to5.5% and 2.5%,the area can significantly expand by 8% and 2.5%,and the probability of largescale extreme precipitation events will also be greatly increased.In addition,the 1994 extreme positive IOD event,which ranks third since 1980,is caused by CP El Ni(?)o event and abnormal ISO activities.While in 1997,the super El Ni(?)o event is the main driver for the concurrent strongest extreme positive IOD event through modulating the Walker Circulation.