*4.2. Vertical Motion of Air over the Indian Ocean and the East Asian Climate in Winter* 4.2.1. Vertical Motion of Air and Precipitation in January

Figure 2b shows that the correlation between precipitation in China and the vertical motion of air over the Indian Ocean passed the 95% significance test in five places in China

in January: there was a negative correlation with precipitation in southern Xinjiang and four positive correlations in northern Xinjiang, northeastern China, northern China and Sichuan (the upper Yangtze river), indicating that the precipitation in southern Xinjiang decreased, whereas the precipitation in the other four regions increased as the vertical motion of air over the Indian Ocean increased, and vice versa. *Water* **2021**, *13*, x FOR PEER REVIEW 8 of 14

**Figure 2.** Correlation analysis between the surface meteorological variables and the vertical motion of air over the Indian Ocean. (**a**) Correlation between the precipitation and the vertical velocity in June. (**b**) Correlation between the precipitation and the vertical velocity in January. (**c**) Correlation between the surface pressure and the vertical velocity in June. (**d**) Correlation between the vertical velocity and the surface pressure in January. (**e**) Correlation between the vertical velocity and the surface temperature in June. (**f**) Correlation between the surface temperature and the vertical velocity in January. **Figure 2.** Correlation analysis between the surface meteorological variables and the vertical motion of air over the Indian Ocean. (**a**) Correlation between the precipitation and the vertical velocity in June. (**b**) Correlation between the precipitation and the vertical velocity in January. (**c**) Correlation between the surface pressure and the vertical velocity in June. (**d**) Correlation between the vertical velocity and the surface pressure in January. (**e**) Correlation between the vertical velocity and the surface temperature in June. (**f**) Correlation between the surface temperature and the vertical velocity in January.

4.2.2. Surface Air Temperature, Pressure and Vertical Motion of Air in January 4.2.2. Surface Air Temperature, Pressure and Vertical Motion of Air in January

The correlation between the surface air temperature in China and the vertical motion of air over the Indian Ocean in January is positive in eastern Xinjiang and negative in the northwestern Tibetan Plateau. The correlation with surface pressure is the same as that with temperature. This distribution of correlation indicates that as the vertical motion of air over the Indian Ocean increased (weakened), the surface temperature (pressure) in southern Xinjiang increased (weakened), whereas it decreased (increased) in the northwestern part Tibetan Plateau (Figure 2d,f). The correlation between the surface air temperature in China and the vertical motion of air over the Indian Ocean in January is positive in eastern Xinjiang and negative in the northwestern Tibetan Plateau. The correlation with surface pressure is the same as that with temperature. This distribution of correlation indicates that as the vertical motion of air over the Indian Ocean increased (weakened), the surface temperature (pressure) in southern Xinjiang increased (weakened), whereas it decreased (increased) in the northwestern part Tibetan Plateau (Figure 2d,f).

We have shown that the precipitation in the western Tibetan Plateau is negatively correlated with the vertical motion over the Indian Ocean in June (Figure 2a). The negative

**5. Discussion** 

**Cold Air from the High Latitude**

> Strengthen (+)

> > Weaken (−)

**tion over the Indian Ocean**

> Weaken (−)

Strengthen (+)

#### **5. Discussion**

#### *5.1. Relevance to Precipitation*

We have shown that the precipitation in the western Tibetan Plateau is negatively correlated with the vertical motion over the Indian Ocean in June (Figure 2a). The negative correlation indicates that the precipitation in the western Tibetan Plateau decreased (enhanced) as the vertical motion over the Indian Ocean strengthened (weakened).

The average meridian circulation along 90◦ E in June showed that there is vertical ascending motion over both the Indian Ocean and the Tibetan Plateau (Figure 3a). These vertical motions from the Indian Ocean have sunken branches on the north of plateau, and an ascending motion below the descending branch. When the vertical motion from the Indian Ocean strengthened, the sinking motion over the north of the Tibetan Plateau is also strengthened, which suppresses the ascent of the lower layer and weakens precipitation in this area, resulting in a negative correlation. *Water* **2021**, *13*, x FOR PEER REVIEW 10 of 14

**Figure 3.** Wind fields. (**a**) Mean meridional circulation at 90° E in June. (**b**) Mean meridional circulation at 90° E in January. (**c**) Wind field at 500 hPa in June. (**d**) Wind field at 500 hPa in January. The red rectangles (**a**,**b**) represent the intersection of the updraft and the downdraft. The grey area in part (**b**) represents the Tibetan Plateau area. **Figure 3.** Wind fields. (**a**) Mean meridional circulation at 90◦ E in June. (**b**) Mean meridional circulation at 90◦ E in January. (**c**) Wind field at 500 hPa in June. (**d**) Wind field at 500 hPa in January. The red rectangles (**a**,**b**) represent the intersection of the updraft and the downdraft. The grey area in part (**b**) represents the Tibetan Plateau area.

**Table 1.** Correlation between climate in China and the vertical motion over the Indian Ocean in January. **Vertical Mo-Precipitation in Upward Motion of Air in Temperature/Pressure in Sinks in the Temperature/Pressure in**  Figure 2a also shows that there are two correlations between the vertical motion of air over the Indian Ocean and precipitation in the lower reaches of the Yangtze River in June. The correlation is bounded by 30◦ N; north of 30◦ N is negative correlation

> **the Eastern Xinjiang**

> > Decrease/decrease (−/−)

Increase/increase (+/+)

The areas with a positive correlation (northern Xinjiang, northeastern China, northern China and the upper as well as the middle reaches of the Yangtze River) are all basins or plains, and they are all in the path of the cold wave that is invading China [37]. Cold air from the high latitude crosses the mountains (plateaus) before reaching the basin or

**North of the Tibetan Plateau**

> Increase (+)

Decrease (+)

**the Tibetan Plateau**

Increase/increase (+/+)

Decrease/decrease (−/−)

**Eastern Xin-**

Increase (+)

Decrease (−)

**Southern/Northern Xinjiang**

Increase/decrease (+/−)

Increase/decrease (−/+)

and south of 30◦N is a positive correlation in June. Figure 3c shows that the airflow from the Indian Ocean is also bound by 30◦ N, with a southwesterly airflow (summer monsoon direction) south of 30◦ N and a northwesterly airflow north of 30◦ N (in the lower reaches of the Yangtze River). The airflow is strengthened with the increase in the Indian monsoon, transporting abundant amounts of water vapor to the south of 30◦ N, increasing precipitation. The negative correlation to the north of 30◦ N (the Yangtze River) and the positive correlation between the vertical motion of air over the Indian Ocean and the precipitation in the northeastern side of the Tibetan Plateau and require further study.

In addition, we have also shown that the vertical motion over the Indian Ocean in January is negatively correlated with precipitation in southern Xinjiang and positively correlated with precipitation in northern Xinjiang, northeastern China, northern China and the Sichuan in China (the upper and middle reaches of the Yangtze River) (Figure 2b).

The negative correlation between the vertical motion of the Indian Ocean and precipitation in southern Xinjiang in January suggests that the precipitation in southern Xinjiang decreased when the vertical motion of the Indian Ocean strengthens. Figure 3b shows that cold air from the north (the high latitude) sinks over southern Xinjiang and leads to an ascending motion of the lower layer (Figure 3b). When the amount of cold air from the north (the high latitude) increases, the ascending motion in the lower layer and precipitation both increase. Cold air from the high latitude meets the air from the Indian Ocean at about 30◦ N, producing a strong downdraft (Figure 3b). The strengthening cold air inhibits the movement of air from the Indian Ocean, and the vertical motion over the Indian Ocean decreases—that is, precipitation in southern Xinjiang increases with the decrease in vertical motion over the Indian Ocean, leading to a negative correlation (Table 1).

**Table 1.** Correlation between climate in China and the vertical motion over the Indian Ocean in January.


The areas with a positive correlation (northern Xinjiang, northeastern China, northern China and the upper as well as the middle reaches of the Yangtze River) are all basins or plains, and they are all in the path of the cold wave that is invading China [37]. Cold air from the high latitude crosses the mountains (plateaus) before reaching the basin or plain, and then produces a downward airflow in these four areas which decreases the precipitation. The amount of precipitation decreases when the downward airflow strengthens. When cold air from the high latitude strengthens, the vertical upward motion over the Indian Ocean weakens (Table 1)—that is, when the vertical upward motion over the Indian Ocean weakens, the vertical subsidence motion over these four regions increases and the amount of precipitation decreases, leading to a positive correlation. Here, we have only given some explanation for the phenomenon we found, and there may be other reasons to be further researched. We can only show cross-sections along 90◦ E (Figure 3b), due to the limitation of our drawing level. The above results will be more clearly shown if a flow profile can be drawn from the origin area of high-latitude cold air to these areas with a positive correlation.

## *5.2. Correlation between Vertical Motion over the Indian Ocean and Surface Temperature and Pressure in China in Winter*

Figure 2d,f has shown that there is a negative correlation between the vertical motion of air over the Indian Ocean and the surface temperature and pressure on the Tibetan Plateau in January, whereas there is a positive correlation between the vertical motion of

air over the Indian Ocean and the surface temperature and pressure in eastern Xinjiang in January. In January, deep, cold air from the high latitude moves south, and part of this package of air is overturned in the Altai Mountains and then accumulates in the northern Tibetan Plateau (eastern Xinjiang), resulting in an upward motion of air (Figure 3b). This cold air lowers the local surface temperature and pressure due to the ascent of the air. The forward movement of the cold air is then blocked by the Tibetan Plateau, and therefore sinks in the north of the plateau, increasing the surface pressure. When the sinking motion of air increases, the cloud amount decreases, the direct solar radiation increases, and the net radiation on the ground surface increases. Therefore, the radiation energy used to heat the atmosphere increases by sensible heat [38], which increases the surface temperature, and vice versa.

The cold air continues to move southward and meets the air from the Indian Ocean at about 30◦ N, producing a strong downdraft (Figure 3b). When the cold air from the north (high latitude) strengthens, the intersection of the two strands of air moves southward, inhibiting the vertical motion from the Indian Ocean, which then weakens. Corresponding to this weakened vertical motion over the Indian Ocean (the strengthening of cold air from the origin area of high latitude), the surface temperature and pressure decrease in eastern Xinjiang, leading to a positive correlation. As the sinking of air over northern Tibet Plateau increases, the surface pressure and temperature increase, leading to a negative correlation (Table 1).

## *5.3. Turning of the Axis of Vertical Motion Center*

The axis of vertical motion in the atmosphere over the Indian Ocean turns clockwise from winter to summer, but counterclockwise back to its original position from summer to winter (Figure 1a). We conducted an EOF analysis on the anomaly in the sea–air temperature difference (sea surface temperature minus the air temperature of two meters above the sea) over the Indian Ocean from January to December. The spatial distribution from the first pattern of the anomaly rotates clockwise from January to July (the explanatory variances are 15.9% in January and 16.1% in July) and counterclockwise from summer to winter (Figure 4a,c).

We know from the wind field over the Indian Ocean at 1000 hPa that the northeasterly wind (winter monsoon) from the high latitudes of the northern hemisphere in January crosses the equator and then turns to the northwest and meets the southeasterly wind from the southern hemisphere at about 10◦ S (Figure 4b). In July, the southeasterly winds from the high latitudes of the southern hemisphere flow to the equator and then become southwesterly near the equator. During this process, the wind speed in the western Indian Ocean is higher than that in the eastern Indian Ocean (Figure 4d). The wind-driven currents on the surface of the Indian Ocean are affected by these changes in wind direction and wind speed. The heating field of the ocean to the atmosphere also changes with the process, and the axis of vertical motion over the Indian Ocean changes to clockwise or counterclockwise.

This study only shows the correlation between the vertical movement of air over the Indian Ocean and China's climate. The mechanism behind this phenomenon will need further study. In addition, we only calculated the monthly and seasonal mean precipitation data for 30 years from 1981 to 2010; their homogeneity was not analyzed. There are some phenomena we cannot explain yet. These deficiencies will need to be addressed in future research.

By investigating the correlation between the vertical motion of air over the Indian Ocean and surface air temperature, atmospheric pressure and precipitation in China, we found that the vertical motion of air associated with the monsoonal circulation over the Indian Ocean plays a certain role in the formation of and change in the climate in China. The results of this research have certain significance and practical value for understanding and forecasting climate in East Asia.

**Figure 4.** Difference in temperature between the sea and air and the wind field over the Indian Ocean. (**a**) The first pattern of the anomaly in the sea - air temperature difference (sea surface temperature minus the air temperature of two meters above the sea) in January. (**b**) Wind fields at 1000 hPa in January. (**c**) The first pattern of the anomaly in the sea-air temperature difference in July. (**d**) Wind fields at 1000 hPa in July. **Figure 4.** Difference in temperature between the sea and air and the wind field over the Indian Ocean. (**a**) The first pattern of the anomaly in the sea - air temperature difference (sea surface temperature minus the air temperature of two meters above the sea) in January. (**b**) Wind fields at 1000 hPa in January. (**c**) The first pattern of the anomaly in the sea-air temperature difference in July. (**d**) Wind fields at 1000 hPa in July.

Ocean is higher than that in the eastern Indian Ocean (Figure 4d). The wind-driven currents on the surface of the Indian Ocean are affected by these changes in wind direction and wind speed. The heating field of the ocean to the atmosphere also changes with the process, and the axis of vertical motion over the Indian Ocean changes to clockwise or

#### This study only shows the correlation between the vertical movement of air over the **6. Conclusions**

counterclockwise.

Indian Ocean and China's climate. The mechanism behind this phenomenon will need further study. In addition, we only calculated the monthly and seasonal mean precipitation data for 30 years from 1981 to 2010; their homogeneity was not analyzed. There are some phenomena we cannot explain yet. These deficiencies will need to be addressed in The vertical motion of the atmosphere over the Indian Ocean is closely related to the climate in some particular regions of China. Climate diagnosis and statistical analysis were used to study the vertical motion of air over the Indian Ocean and its relationship with the climate in China. The following conclusions can be drawn from these results:

future research. By investigating the correlation between the vertical motion of air over the Indian Ocean and surface air temperature, atmospheric pressure and precipitation in China, we found that the vertical motion of air associated with the monsoonal circulation over the Indian Ocean plays a certain role in the formation of and change in the climate in China. The results of this research have certain significance and practical value for understanding (1) The vertical motion of air is negatively correlated with precipitation in the Tibetan Plateau during summer and positively correlated with precipitation in northern Xingjiang, northeast China, northern China and the Sichuan province (the upper and middle reaches of the Yangtze River) during winter. This can be explained by the interaction between the vertical motion of air over the Indian Ocean and cold air from the high latitude of the Northern Hemisphere.

and forecasting climate in East Asia. **6. Conclusions** (2) The vertical motion over the Indian Ocean was weakened from 1981 to 2010, except at a height of 500 hPa in winter. The vertical motion of air over the Indian Ocean had a period of 7–9 years in summer and 2–3 in addition to 9–12 years in winter.

The vertical motion of the atmosphere over the Indian Ocean is closely related to the climate in some particular regions of China. Climate diagnosis and statistical analysis were used to study the vertical motion of air over the Indian Ocean and its relationship with the climate in China. The following conclusions can be drawn from these results: (1) The vertical motion of air is negatively correlated with precipitation in the Tibetan (3) The ascending motion of air over the Indian Ocean is dominant throughout the year. The center of ascending air moves and rotates as the seasons change, and the central axis rotates clockwise from winter to summer and counterclockwise from summer to winter. This is because the heating of the atmosphere over the Indian Ocean changes from winter to summer with the East Asian monsoon, and vice versa.

Plateau during summer and positively correlated with precipitation in northern Xingjiang, northeast China, northern China and the Sichuan province (the upper and middle reaches of the Yangtze River) during winter. This can be explained by the interaction **Author Contributions:** R.T. conceived the idea and wrote the manuscript. Y.M. and W.M. revised the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Second Tibetan Plateau Scientific Expedition and Research Program (STEP), grant no. 2019QZKK0103, and the National Natural Science Foundation of China (grant no. 41775142).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data and methods used in the research have been presented in sufficient detail in the paper.

**Acknowledgments:** This work was supported by the Second Tibetan Plateau Scientific Expedition and Research Program (STEP), grant no. 2019QZKK0103 and the National Natural Science Foundation of China (grant no. 41775142). We acknowledge the use of meteorological data collected from the National Centers for Environmental Prediction/National Center for Atmospheric Research, the China Meteorological Administration and the Scientific Data Center for the Cold and Arid Regions of China. Duo Zha, Xinfang Zhang and Yiwei Ye completed the drawing of this paper. All data in the research can be obtained by contacting the corresponding author, Rongxiang Tian (trx@zju.edu.cn).

**Conflicts of Interest:** The authors declare no conflict of interest.

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