Weakened Relationship between the Tibetan Plateau Heat Source and the Western North Pacific Anomalous Anticyclone in Recent Summers

Abstract

Abstract Numerous studies have indicated that the atmospheric heat source (AHS) over the Tibetan Plateau (TP) is highly correlated with the western North Pacific anomalous anticyclone (WNPAC) in summer. However, such an interannual relationship has been weakened since the late 1990s. The present work shows that the TP AHS was significantly and positively correlated with the WNPAC during the period 1979–99 (P1), while this relationship became insignificant hereafter [2000–20 (P2)]. From an atmospheric perspective, we identify that the long-term change in the upper-level atmospheric circulation over the TP is an important cause for weakening the relationship. An obvious upper-level anticyclonic trend occurred over the northeastern TP in the past four decades, with an easterly trend on the anticyclone’s southern flank, with anomalous westerlies during P1 but anomalous easterlies during P2 over the main portion of the TP. With the anomalous upper-level westerlies in P1, abnormal high pressure induced by the TP heating (i.e., AHS) extended downstream in the upper troposphere. Subsequently, anomalous descending motions formed over the northwestern Pacific due to the eastward-extended high pressure, together with the vertical transport of negative relative vorticity, favorable for the enhancement of the WNPAC. In P2, the TP heating-induced abnormal high pressure was confined over the southern TP due to the anomalous easterlies, suppressing its downstream influence and finally breaking the connection between the TP AHS and the WNPAC. Modeling results from both linear baroclinic model (LBM) sensitivity experiments and the CESM Large Ensemble dataset further confirm the important role of the change in background circulation in weakening the relationship. Significance Statement The atmospheric heat source (AHS) over the Tibetan Plateau (TP) is generally believed to closely connect with the western North Pacific anomalous anticyclone (WNPAC) during boreal summer. Previous studies have revealed that a significant interannual correlation exists between the TP AHS and the WNPAC; however, such a relationship was weakened recently, but the causes are unknown. This study highlights the important contribution from the change in background circulation to the weakened relationship. An upper-level easterly trend occurred over the TP in recent summers, under which the TP heating-induced abnormal atmospheric response was confined in the TP area, limiting the downstream influence of the TP heating and finally destroying the connection between the TP AHS and the downstream WNPAC.