Impact Of The Early Summer Tasman Sea-Southern Ocean Hybrid Teleconnection Pattern On Middle Summer Rainfall In East Asia

Origins from the weakening of the subtropical jet stream and the strengthening of the polar jet stream,a meridional dipolar atmospheric teleconnection between the Tasman Sea and the Southern Ocean,which describes a hybrid between the El Ni(?)o–Southern Oscillation,Indian Ocean Dipole,and Southern Annular Mode,was referred to as the Tasman Sea–Southern Ocean Hybrid Teleconnection(HT)pattern previously.Based on observation data and reanalysis data,we have investigated the remote connection of summer rainfall in East Asia with the HT and find the preceding May–June HT can be the precursor of the following July–August rainfall.The corresponding physical mechanism is examined based on analyses of observational datasets and numerical models.The main conclusions are summarized as follows:(1)With observational analysis,we investigate HT’s connection with East Asian summer rainfall.The results suggest that May–June HT activity leads to intensified summer(July–August)rainfall in the middle and lower reaches of the Yangtze River Basin and a reduction in rainfall in South and North China.Correspondingly,the 850 hPa wind field exhibits a large-scale anomalous anticyclone over the South China Sea and an anomalous cyclone centering over the East China Sea,while the East Asian uppertropospheric westerly jet stream shifts southward at 200 hPa.Furthermore,HT was found weakly correlated with the well-known climate modes including El Ni(?)o–Southern Oscillation,Indian Ocean Dipole,Southern Annular Mode,Pacific-South American,Zonal Wave 3 pattern,and Antarctic Dipole Pattern,and explains more than 25% of the total variance of the East Asian tripolar rainfall,thus can be treated as an independent predictor of East Asian summer rainfall.(2)A positive HT with positive sea level pressure anomalies over the Tasman Sea but negative over the Southern Ocean in the preceding boreal early summer contributes to generating cold sea surface temperature anomalies(SSTA)in the tropical Western Pacific through influencing surface turbulent heat flux and radiant heat flux.The cold SSTA in the tropical Western Pacific persists into the mid-summer via the thermal memory of seawater,and excites a Pacific–Japan–like atmospheric circulation anomaly,which manifests as an anticyclone over the subtropical northwestern Pacific along with a cyclone near the Sea of Japan.This pattern induces intensified convergence and vertical ascend motion,and subsequently intensifies rainfall in the Yangtze River Basin.The above observational results were validated well by the historical runs of coupled models attending phase 6 of the Coupled Model Intercomparison Project(CMIP6),which further confirmed the air-sea coupling in tropical Western Pacific as an important "bridge" in HT and following summer rainfall in East Asia.(3)In addition,a positive HT in the preceding boreal early summer also contributes to generating Atlantic–Ni(?)o–like SSTA in the tropical Atlantic through influencing surface heat exchange and driving the oceanic Ekman flow.The Ni(?)o SSTA in the tropical Atlantic develops into the mid-summer via the positive Bjerknes feedback processes.Then it induces Walker Cell anomalies in the tropics and excites the westwardpropagating Gill–type Rossby wave as well as the eastward-propagating extratropic atmospheric Rossby wave along the upper-tropospheric westerly jet stream.The westward-propagating tropical signals induce the anomalous lower-tropospheric anticyclone over the northwestern Pacific,while the eastward–propagating extratropic upper-tropospheric signal causes an anomalous upper cyclone over North China,which causes the East Asian upper-tropospheric westerly jet stream shift southward.The anomalous lower anticyclone intensifies warm and humid air flow toward the middlelower reaches of the Yangtze River Basin.The positive vorticity anomalies on the south flank of the westerly jet stream intensify the vertical convergence and ascending motion over Tibet Plateau to the Yangtze River Basin.They together induce intensified rainfall in mid-summer.The hypothesis has been supported by CAM4 simulations,which further confirmed the air-sea coupling in the tropical Atlantic as an important "bridge" in HT and following summer rainfall in East Asia.