热带各大洋热力异常协同调制南北半球冬季平流层环流的数值模拟研究

The thermal condition over tropical oceans is one of the key factors that control the interannual variations of the stratospheric circulation, especially of the stratospheric polar vortex. It was widely reported that, in Northern Hemisphere, the tropical diabatic heating related to the convection activities during ENSO winter could induce a Pacific–northern American (PNA) pattern in the extratropical troposphere, which modulates the amplitudes of the climatological planetary waves in the northern winter. As a result, the upward propagation of the planetary waves is enhanced, and more eddy heat flux is transported into the stratospheric polar region. In such situation, the stratospheric polar vortex is usually warmer, and the circumpolar westerly jet is weaker. However, the thermal anomalies over different tropical oceans are usually highly correlated, and are seldom independent. For example, during El Niño, warm sea surface temperature (SST) anomalies do not only appear in the tropical middle and eastern Pacific, but also in the tropical Indian and Atlantic Oceans. It is not quite sure whether the thermal changes over the tropical Indian and Atlantic Oceans can also modulate the extratropical planetary waves, especially in the stratosphere. The planetary wave responses to diabatic heating over different tropical oceans may be in phase to enhance the climatological waves, or be out of phase to weaken the climatological waves, which together strengthen or cancel the ENSO’s impacts on the northern winter stratosphere. Our recent study has indicated that the diabatic heating over the tropical Pacific and Indian Oceans is just contrary when they modulated the PNA pattern and the upward-propagating waves, respectively. In this way, the tropical Indian Ocean can cancel part of the ENSO signal in the extratropics. However, the role played by the tropical Atlantic Ocean during ENSO in modulating the stratospheric circulation is still unknown. It is not certain whether warm SST anomalies in the tropical Atlantic individually have induced warm and weak or cold and strong stratospheric polar vortex. In addition, it have also been reported that ENSO shows no significant effect on the stratospheric circulation in the Southern Hemispheric extratropics. Is this low correlation between ENSO and the Southern Hemispheric circulation is also partially caused by the cancelling effects of the tropical Indian Ocean SST anomalies induced by the “atmospheric bridge” during ENSO? This proposal will attempt to solve the above questions mentioned, and to improve our understanding of contribution of the thermal conditions over different tropical oceans to the interannual variations of the stratospheric circulation in both hemispheres. A detailed comparison is required to make between the extratropical stratosphere responses in both hemispheres under the same thermal configuration. Furthermore, it is favorable to explore the stratospheric sources from the stratosphere-troposphere coupling processes for the climate variations and extreme weather patterns. Finally, we hope to reveal the physical linkage between the configuration of the thermal conditions over different tropical oceans and the climate anomalies in the mid-to-high latitudes.

以ENSO为代表的热带海洋热力异常是影响南北半球冬季平流层环流的重要因子。ENSO可以在北半球激发出异常的太平洋-北美型，调制热带外行星波的强度，引发平流层极涡的冷暖和强弱变异。热带印度洋和大西洋的热力异常对南北半球行星波的调制作用有待深入研究。三个热带海洋的热力强迫所激发的热带外行星波响应既可能同位相叠加，也可能反位相抵消。假设三个热带海洋热力异常调节南北半球平流层环流的效应是可以叠加的，数值敏感试验已表明暖ENSO期间，热带印度洋热力强迫可以抵消太平洋强迫对北半球平流层的影响。热带大西洋影响南北半球平流层的行为和热带印度洋对南半球平流层的调制作用还不清楚。有必要进行多组数值试验检验三个热带海洋热力强迫协同调制南北半球环流的特征与差异，揭示平流层环流年际变化与热带热力强迫之间的关系。从平流层-对流层耦合角度探索气候变化的高层信号源，探寻热带热力配置与中高纬度气候异常之间的物理联系与机制。

本项目系统地比较了南北半球冬季热带外环流对ENSO期间不同热带海盆尺度热力异常的响应。热带太平洋暖海温强迫可以在北半球激发出正位相的太平洋-北美型（PNA）异常响应，增强热带外行星波振幅，从而引起平流层极涡的增暖和减弱响应。伴随ENSO发展的热带印度洋海温强迫部分抵消了热带太平洋的海温强迫效应，激发出负位相的PNA响应，削弱热带外行星波振幅，促使北半球平流层极涡变冷和增强。因此，热带印度洋海温异常对平流层的ENSO信号具有削弱作用，使平流层ENSO信号不会过强。热带大西洋的海温异常对北半球冬季平均的平流层ENSO信号调制较弱，与大气对热带大西洋海温强迫的响应在不同月份反复反号有关。数值试验和观测资料都表明，在早冬-隆冬-初春，热带外对流层对相同的热带大西洋海温强迫表现出不同的响应符号，对应行星波先加强-再减弱-再加强，平流层极涡先减弱-再加强-再减弱。与北半球冬季类似，南半球冬季平均的平流层极涡对热带大西洋海温异常的响应较弱，对热带太平洋和印度洋强迫的响应强度相当但符号相反。热带太平洋对南半球极涡的强迫效应可以完全被热带印度洋强迫所抵消。热带太平洋暖海温异常激发出正位相的太平洋-南美型（PSA）响应，热带印度洋激发出负位相的PSA响应，因此在观测中无法合成出显著的南半球平流层ENSO信号。研究结果提升了我们对南北半球平流层环流与热带热力强迫关系的认识，但是对热带热力强迫与热带平流层环流变异的关系仍有待进一步研究。