Effects And Mechanism Of Tropical And Mid-latitude Intraseasonal Oscillations On Subseasonal Variability In Winter In South China

In this study,K-means cluster analysis is applied to the tropical intraseasonal OLR anomalies to identify the southern China(SC)precipitation events and the SC intraseasonal cold surges,respectively.The composite analysis is used to study the anomalous circulation in both the upper and low level and the tropical convection in each cluster.The perturbed thermodynamic equation is also used to derive the intraseasonal temperature diagnostics.Some conclusions are concluded as below:(1)About 53% of strong intraseasonal precipitation events are unrelated to the MJO.They are caused by southward propagation of a low-pressure anomaly in the lower troposphere from higher latitudes.Southerly anomalies associated with the low-pressure system transport high mean moisture from South China Sea,leading to moisture accumulation over SC.(2)47% of the events are accompanied by the MJO,and they can be further divided into two groups: one with enhanced MJO convection over the eastern Indian Ocean(termed as IO group),and the other over the Maritime Continent(termed as MC group).For the IO group,the SC precipitation is triggered by low-level southerly anomalies associated with an anomalous anticyclone over the western North Pacific(WNP)in association with suppressed MJO convection in situ,as well as the upper-tropospheric divergence related to a wave train excited from the MJO convection.For the MC group,both the upper-tropospheric wave train related to MJO and the southward propagation of low-pressure anomaly from higher latitudes in the lower troposphere contribute to trigger the SC precipitation.(3)About 32% of the SC intraseasonal cold surges are unrelated to the MJO.They are caused by the upper-tropospheric divergence triggerd by the southeastward propagation of lowpressure anomaly in Asia and the southward propagation of low-pressure anomaly over the Pacific,as well as the meridional advection associated with the southward expansion of the Siberian High.(4)68% of the events are accompanied by the MJO,and they can be further divided into two groups: one with enhanced MJO convection over the eastern Indian Ocean(termed as Cluster 1),and the other over the Maritime Continent(termed as Cluster 2).For Cluster 1,the zonal circulation induced by a cyclonic circulation over the northwestern Pacific and the anomalous northerly associated with the high-pressure anomaly contribute to the surface temperature variation.Besides,the upward motion over SC leads to the eastward propagation of the MJO convection.For Cluster 2,the persistent cold events are triggered by both the lowertropospheric high-pressure anomaly from higher latitudes and the local overturning circulation forced by the suppressed convection over the Maritime Continent.