厄尔尼诺-南方涛动（El Ni?o - Southern Oscillation, ENSO）是重要的年际海气相互作用信号，对全球农业生产和生态环境有巨大影响。在ENSO循环中，大气热机作为将大气内能转为动能的系统，可反映大气温度场异常对环流状况的影响，故能揭示大气状况改变的物理机制，对ENSO循环的物理内涵作出进一步解释。本文针对ENSO循环中Walker环流强度异常变化问题，筛选并构建了东西大气热机模型，定义了东西大气热机指数以解释赤道太平洋Walker环流变化的原因，并依照前人的垂直大气热机模型定义了垂直大气热机指数，以解释西太平洋Walker环流上升支变化的原因。最后用格点资料结合统计方法予以验证，其结果符合预期，通过了显著性检验。主要结论如下：

[1] 筛选了东西大气热机模型，定义了东西大气热机指数，并阐明了二者的物理意义。

东西大气热机是以气团为工质、以同纬度对流层低层气温最低和最高区域作为冷热源的机械模型，该热机将存在差异的大气内能转化为纬向环流动能，环流在对流层低层由冷源指向热源方向，在高层方向相反，在冷、热源区域形成下沉支和上升支，由此构成闭合环流回路。我们定义东西大气热机指数为东西大气热机的Carnot效率乘以相对位置系数并除以尺度参数，它与斜压性矢量等效。根据相对环流定理，若惯性项和摩擦项不变，在确定环流正方向后，东西大气热机指数为正时，环流将正向增强或反向减弱。一般来说，该热机效率越高，热机机械功输出率增加，纬向环流动能增大；同时，热机效率越高，环流定理的力管项越大，会造成纬向环流变强。因此，东西大气热机模型可从热机做功效率和相对环流定理力管项这两个方面，揭示纬向环流变化的机制，并有潜在的预报价值。

[2] 揭示了东西大气热机在ENSO位相发展中发生变化的原因和对环流的影响，确定了其在Bjerknes正反馈中的作用。

将东西大气热机模型应用在ENSO现象发生的赤道太平洋地区，从理论上可知赤道太平洋低层气温场纬向梯度改变，使该热机冷热源条件发生变化，热机效率改变，进而影响Walker环流的强度，因此东西大气热机参与了Bjerknes正反馈。分析观测资料可知，El Ni?o发展时，赤道中东太平洋海表面温度（Sea Surface Temperature, SST）异常升高，使其上空对流层低层大气接收更多的感热通量，原本赤道太平洋对流层低层气温西高东低的格局被打破，造成东西大气热机效率异常下降，大气斜压性下降，使Walker环流强度减弱，产生较弱海表风应力，使得Ni?o 3.4区SST继续升高，加剧El Ni?o现象。拉尼娜（La Ni?a）的加强过程亦然。因此东西大气热机被增加（减少）的Ni?o 3.4区海表感热通量影响后，冷源温度提高（降低），运行效率下降（上升），减弱（增强）了Walker环流，成为ENSO位相发展期SST作用于大气风场的桥梁。

[3] 定义了垂直大气热机指数，揭示了西太平洋垂直大气热机在ENSO循环负反馈中发生变化的原因及影响，阐明了其对ENSO位相转换的作用。

垂直大气热机是以气团为工质、并以同一经纬度范围内对流层中高层和低层大气作为冷、热源的机械模型，它将不同海拔间存在差异的大气内能转化为对流抬升动能。本文定义垂直大气热机指数为其Carnot效率。理论上，在ENSO循环中增加（减少）的西太平洋垂直大气热机效率，对西太平洋的Walker环流上升支对流抬升起到增强（减弱）作用，而更强（弱）的对流抬升为该热机冷源带来更多（少）相变潜热，使该热机效率下降（上升）。经数据资料分析，ENSO冷暖位相转变过程符合这一规律。所以，西太平洋垂直大气热机效率随Ni?o 3.4区SST升高而逐渐升高，进而增强Walker环流，促进Ni?o 3.4区SST下降，使ENSO位相转变。因此，西太平洋区域垂直大气热机是ENSO循环负反馈中海气相互作用的桥梁。

  El Ni?o - Southern Oscillation (ENSO) is an important interannual air-sea interaction signal, which has a huge impact on global agricultural production and ecological environment. In the ENSO cycle, the atmospheric heat engine, as a system that converts the internal energy of the atmosphere into kinetic energy, can reflect the influence of anomalies in the atmospheric temperature field on the circulation conditions, so it can reveal the physical mechanism of changes in atmospheric conditions and further explain the physical connotation of the ENSO cycle. Aiming at the anomaly of the Walker circulation intensity in the ENSO cycle, we screened the zonal atmospheric heat engine model, defined the zonal atmospheric heat engine index to explain the reasons for the variation of the Walker circulation in the equatorial Pacific, and defined the vertical atmospheric heat engine index according to the previous vertical atmospheric heat engine model to explain the changes in the ascending branch of the Western Pacific Walker Circulation. Finally, the grid data and statistical methods were used to verify the results. The results were in line with expectations and passed the significance test. The main conclusions are as follows:

[1] The zonal atmospheric heat engine model is selected, whose index is also defined, and the physical meanings of them are clarified.

  The zonal atmospheric heat engine is a mechanical model with air mass as the working substance, and the higher and lower troposphere with the lowest and highest temperature are the cold and heat source. This heat engine converts the different atmospheric internal energy into zonal circulation kinetic energy. The cold source points in the direction of the heat source, and in the opposite direction at the high level, the sinking branch and the ascending branch are formed in the cold and heat source areas, thus forming a closed circulation loop. In order to know the reasons for the circulation variability, we define the zonal atmospheric heat engine index as the Carnot efficiency of the zonal atmospheric heat engine multiplied by the relative position coefficient and divided by the scale parameter, which is equivalent to the baroclinic vector. According to the relative circulation theorem, if the inertial term and frictional term remain unchanged, after determining the positive direction of the circulation, when the heat engine index of the zonal atmosphere is positive, the circulation will strengthen in the positive direction or weaken in the opposite direction; when it is negative, the circulation will strengthen in the opposite direction, or positive weakening. Generally speaking, the higher the efficiency of the heat engine, the higher the mechanical power output rate of the heat engine, and the greater the zonal circulation kinetic energy. At the same time, the higher the heat engine efficiency, the larger the solenold term of the circulation theorem, which will cause the zonal circulation to become stronger. Conversely, the lower the efficiency of the heat engine, the weaker the zonal circulation. Therefore, the zonal atmospheric heat engine model can reveal the mechanism of the zonal circulation change from the two aspects of the heat engine work efficiency and the relative circulation theorem, and has potential predictive value.

[2] The reason for the change of the zonal atmospheric heat engine in the phase development of ENSO and the influence of the engine on the circulation are revealed, and the role of the engine in the Bjerknes positive feedback is determined.

  Applying the zonal atmospheric heat engine model to the equatorial Pacific region where the ENSO phenomenon occurs, it is theoretically known that the zonal gradient of the temperature field in the lower layer of the equatorial Pacific changes, which changes the conditions of the heat engine’s cold and heat sources, and changes the heat engine efficiency, which in turn affects the strength of the Walker circulation. Therefore, the zonal atmospheric heat engine participates in the Bjerknes positive feedback. Analysis of the observational data shows that when El Ni?o develops, the Sea Surface Temperature (SST) in the equatorial central and eastern Pacific increases abnormally, so that the lower troposphere above the troposphere receives more sensible heat flux. The lower temperature in the east gradually increases and approaches or even surpasses the high temperature in the west, resulting in an abnormal decrease in the efficiency of the zonal atmospheric heat engine and a decrease in the baroclinicity of the atmosphere, which weakens the strength of the Walker circulation and generates weaker sea surface wind stress, which makes the SST in the Ni?o 3.4 area continue increasing, thus exacerbating the El Ni?o phenomenon. The same goes for the strengthening process of La Ni?a. Therefore, after the zonal atmospheric heat engine was affected by the increased (decreased) sea surface sensible heat flux in Ni?o 3.4 area, the temperature of the cold source increases (decreases), the operating efficiency decreases (increases), and the Walker circulation will be weakened (enhanced), so this heat engine acts as bridges for atmospheric wind fields and SST during the ENSO phase development period.

[3] The vertical atmospheric heat engine index is defined, the reasons and effects of the change of the vertical atmospheric heat engine in the western Pacific Ocean in the negative feedback of ENSO cycle are revealed, and effects of the engine on the phase transition of ENSO is clarified.

  The vertical atmospheric heat engine is a mechanical model that uses air mass as the working substance, and in the same place, the middle and upper, the lower atmosphere in as the cold and heat sources. Theoretically, the increased (decreased) efficiency of the vertical atmospheric heat engine in the western Pacific Ocean in the ENSO cycle enhances (weakens) the convective uplift of the ascending branch of the Walker circulation in the western Pacific Ocean, and the stronger (weaker) convective uplift contributes to the cooling (heating) of the heat engine cold source. The source brings more (less) latent heat of the phase transition, making the heat engine less efficient (increased). According to the data analysis, the conversion from one phase to the opposite phase of ENSO conform this rule. Therefore, in the western Pacific the vertical atmospheric heat engine efficiency increases gradually with the increase of SST in the Ni?o 3.4 area, which in turn enhances the Walker circulation, promotes the decline of the SST in the Ni?o 3.4 area, and changes the ENSO phase. Therefore, the vertical atmospheric heat engine in the western Pacific is a bridge for the air-sea interaction in the negative feedback of the ENSO cycle.