春季青藏高原对流层低层暖中心变化对中亚-高原降水异常的影响

Impact of the Lower-Tropospheric Warm Core over the Qinghai-Xizang Plateau on Central Asian-Tibetan Plateau Precipitation in Spring

在青藏高原上空对流层频繁出现的暖中心活动与高原加热作用密切相关,是东亚独特的天气气候现象。本文利用1979-2018年ERA-interim分辨率为6 h的再分析资料,基于暖中心事件的统计及其与降水场的奇异值分解(SVD),探讨了春季青藏高原低层暖中心强度变动对中亚-高原偶极型降水异常分布的调制作用。从年际变化来看,3-4月高原低层暖中心偏强(弱)时通常对应同期帕米尔高原至伊朗高原东北部降水偏多(少)而高原东部至印度北部降水偏少(多)的偶极型降水异常分布。通过高原暖中心与温度、环流和水汽场对应关系的诊断分析发现,偏强的低层暖中心显著改变高原周围的经向和纬向温度梯度,并引发高原中高层正压反气旋式环流异常,其内部的下沉运动抑制了对流发展,反气旋东侧和南侧异常的偏北风和偏东风不利于水汽输送,从而造成高原东部至印度北部降水减少;而反气旋西北侧异常西南气流携带水汽进入中亚,给帕米尔至伊朗高原东北部带来更多降水。当高原低层暖中心偏弱时会引发中高层气旋式环流异常并导致相反的降水异常分布。

The frequent occurrence of tropospheric warm cores over the Qinghai-Xizang Plateau(QXP)is an unique climate phenomenon in East Asia and is closely associated with the QXP heating effect. By using the six-hourly ERA-interim reanalysis data from 1979 to 2018,the modulation effect of the springtime intensity of the low-level warm core(LLWC)on the dipole pattern of precipitation in Central Asia-Tibetan Plateau(CA-TP)is investigated on the basis of the warm core event statistics and singular variance decomposition(SVD). At the interannual time scale,significantly enhanced(reduced)rainfall in Central Asia(from Pamirs Plateau to northeastern Iranian Plateau)and suppressed(enhanced)rainfall over the eastern QXP and northern India are observed in strong(weak)LLWC years. The linkages between the LLWC and the associated lower-tropospheric temperature variation,the atmospheric circulation and the corresponding water vapor transport are revealed through diagnostic analyses. The results indicate that the anomalously strong LLWC significantly changes the meridional and zonal temperature gradients,which further trigger the barotropic anticyclonic circulation anomalies at the middle and upper tropospheric levels over the QXP. Strong sinking flows overwhelm the interior of the anticyclone and suppress the convective precipitation development. In addition,the anomalous northerly and easterly winds on the eastern and southern sides of the anticyclone restrain the water vapor transport. These conditions favor the decreased rainfall over the eastern QXP and northern India. Meanwhile,powerful southwesterly flow,prevailing in the northwestern side of the anticyclone,carries abundant moisture and converges in Central Asia,consequently leading to surplus rainfall from Pamir Plateau to northeastern Iranian Plateau. The anomalously weak LLWC triggers the cyclonic circulation anomaly at the middle and upper tropospheric levels and results in the opposite precipitation anomaly pattern.