Interannual Variability of Late-spring Circulation and Diabatic Heating over the Tibetan Plateau Associated with Indian Ocean Forcing

Abstract

The thermal forcing of the Tibetan Plateau (TP) during boreal spring, which involves surface sensible heating, latent heating released by convection and radiation flux heat, is critical for the seasonal and subseasonal variation of the East Asian summer monsoon. Distinct from the situation in March and April when the TP thermal forcing is modulated by the sea surface temperature anomaly (SSTA) in the North Atlantic, the present study shows that it is altered mainly by the SSTA in the Indian Ocean Basin Mode (IOBM) in May, according to in-situ observations over the TP and MERRA reanalysis data. In the positive phase of the IOBM, a local Hadley circulation is enhanced, with its ascending branch over the southwestern Indian Ocean and a descending one over the southeastern TP, leading to suppressed precipitation and weaker latent heat over the eastern TP. Meanwhile, stronger westerly flow and surface sensible heating emerges over much of the TP, along with slight variations in local net radiation flux due to cancellation between its components. The opposite trends occur in the negative phase of the IOBM. Moreover, the main associated physical processes can be validated by a series of sensitivity experiments based on an atmospheric general circulation model, FAMIL. Therefore, rather than influenced by the remote SSTAs of the northern Atlantic in the early spring, the thermal forcing of the TP is altered by the Indian Ocean SSTA in the late spring on an interannual timescale.

摘要

青藏高原(以下简称高原)热力强迫作用, 包括地表感热, 大气凝结潜热以及气柱净辐射通量, 对东亚夏季风的季节以及次季节变率有着重要影响.本文基于台站观测资料以及MERRA再分析数据集, 发现北大西洋海温异常是3-4月高原非绝热加热年际变率的主控因子, 而晚春(5月)高原大气热源则主要受印度洋海盆一致模(IOBM)的影响.在IOBM正位相年, 伴随着西南印度洋的异常上升以及高原东南部的异常下沉运动, 印度洋上空的局地Hadley环流将会加强, 对应于高原近地层西风与地表感热的显著正异常, 而高原降水和凝结潜热总体受到抑制.与此同时, 高原上的辐射通量由于各分量之间的相互抵消适应仅有微弱变化;IOBM负位相年则相反.此外, 基于大气环流模式的一系列敏感性实验还进一步揭示了主要物理过程.