**4. Causes of the Precipitation Anomalies**

## *4.1. The 1953–1954 El Niño–Southern Oscillation*

The El Niño–Southern Oscillation (ENSO) is a coupled climate phenomenon. El Niño refers to the unusual warming of surface waters on a large scale in the tropical Pacific Ocean; whereas, southern oscillation refers to the negatively correlated "seesaw" changes in tropical sea level pressure in the tropical region between the West and East Pacific Ocean, as well as the resulting phenomenon of strengthening or weakening of the easterly wind in the tropical Pacific. These two situations actually represent different phases of the same phenomenon, namely ENSO [30]. ENSO events have a great impact on the precipitation over the Yangtze River valley in China [11–13]. Some studies show that the floods in the Yangtze River valley are closely related to ENSO events [31–33].

Figure 2 displays the SSTA distribution over the Pacific Ocean in 1953. The SST rise extended from the equatorial Pacific region to areas near the coasts of Mexico in North America, and Peru in South America. Additionally, Southeast Asia, the eastern coast of China, Japan, and the mid-latitude Pacific region to the east of Japan witnessed an SST rise. Near the high-latitude Sea of Okhotsk, the SST level was low.

**Figure 2.** Pacific sea surface temperature anomalies in 1953 based on the NOAA Extended Reconstructed SST V5 dataset [26].

The intensity of El Niño is represented using the SSTA index and is monitored in various key regions, of which the NINO3 region has the closest relation to the precipitation in China [34]. Figure 3a reveals the SSTAs in the NINO3 region from 1950 to 2000. In 1953, the NINO3 index (Figure 3b) measured a maximum SSTA value of 1.5 ◦C, suggesting that 1953's El Niño was less intense compared with those of other years. Nevertheless, Figure 3a reveals a rising trend of El Niño intensity since the 1960s [35]. The El Niño event of 1951–1953 is no less intense than its counterparts that occurred a decade beforehand and a decade later.

**Figure 3.** Sea surface temperature anomalies (SSTAs) in the NINO3 region: (**a**) SSTAs from 1950 to 2000, the shadows indicate the flood disasters that may relate to the ENSO events, grayscale represents the disaster degree; (**b**) SSTAs between 1951 and 1954.

The 1951–1954 SSTA (higher than average conditions) continued for a long time. SSTs became higher than average starting in April 1951, peaked before temporarily dropping to the short-term slight negative anomaly, and quickly returned to increasing afterwards such a double-peaked anomaly continued for 31 months. The SSTs in 1953 were relatively high, with a double-summit SSTA pattern (i.e., the later anomaly occurred immediately after the first), causing an extended period of El Niño-induced SSTAs.

An ENSO-induced SSTA often leads to anomalies of the West Pacific subtropical high (WPSH) [14–16]. Subtropical high anomalies played a critical role in 1954. In the 500-hPa geopotential height graph in the summer of 1954 (Figure 4a), the location of the 588-dagpm contour is that of the WPSH. The location of the subtropical high in 1954 was clearly further south than during average conditions, and the subtropical-high ridge line was stably located in the 20N–25N region. In 1954, the ridge line first moved north in May, which was earlier than in previous years. The second subtropical high northward jump did not appear until August, which was delayed by 20 days compared with previous data. The WPSH area index was relatively small in 1954 and even smaller than the multiyear average, yet the westernmost ridge point was further west than ever during that year. It is clearer in Figure 4b that the SSTAs in the equatorial Pacific induced positive geopotential height anomalies in the South China Sea, indicating anomalous anticyclonic circulation over this region. These characteristics were conducive to the transport of moist and warm airflows from the Indian Ocean in the southwest toward the mid- and low-Yangtze areas [22,24].

**Figure 4.** (**a**) The 500 hPa geopotential height in summer 1954; (**b**) the geopotential height (contours) and wind anomalies (vectors) in summer 1954. The data used in the figures include geopotential height data, meridional wind data, and zonal wind data in NCEP/NCAR reanalysis dataset [27].

Between May and July, the WPSH stably moved between 20N and 25N, causing wet and warm air to move toward the Yangtze River valley and precipitation to be maintained around the mid- and low-Yangtze areas along the river.

#### *4.2. SSTAs of the Sea of Okhotsk*

In addition to the southward shift of the WPSH ridge line, Figure 4b exhibits another characteristic: a blocking high hovering over the east of Siberia, Russia, and the Sea of Okhotsk. Describing the mechanism behind the formation of this blocking high, Lu (1954) confirmed that when SSTs of the region between the Sea of Okhotsk and Bering Sea were low with abundant sea surface ice [23], a blocking high can easily develop and sustain over the Sea of Okhotsk. Conducting potential vorticity inversion, Hisashi et al. (2004) proved that the cold Okhotsk Sea surface is necessary for highs to develop in this region [36]. The difference between the high temperature land surface and low SST in the Okhotsk region can cause cold advection with east wind anomalies, thus inducing the development of blocking highs. In the fall and winter of 1953, positive SSTAs of the central and eastern equatorial Pacific region were a typical phenomenon of El Niño. In 1954, the relatively low SSTAs in the central and eastern equatorial Pacific Ocean, as well as the high SSTAs in the Philippine Sea area, were both conducive to the formation and maintenance of highs over the Sea of Okhotsk [37].

Another study clarified that when a high is formed over the Sea of Okhotsk and becomes stable, the precipitation throughout the Yangtze River valley tends to be higher than average during the East Asian rainy season [38]. Wang's research work [39] also specified that following the development of an Okhotsk Sea high a wave train is generated, which moves from the Sea of Okhotsk to subtropics throughout the east of Japan. The dissemination of this wave train then forms a cyclonic circulation centered on the sea surface to the east of Japan. This circulation is a crucial factor in weakening the northward shift of the WPSH, causing the subtropical high to move southward and remain there for nearly 3 months.

Figure 2 reveals that the SSTs in the Sea of Okhotsk region were unusually low in 1953, whereas the SSTs in regions of the central and eastern equatorial Pacific were relatively high. Figure 5 shows the SSTAs in the Sea of Okhotsk region from 1953 to 1954. SSTAs were continually present in the region between January 1953 and May 1954, with the annual SSTs averaging −0.61 ◦C in 1953 (the lowest SST is −1.3 ◦C). As shown in Figure 2, the usually low SSTs continued a necessary condition of blocking high formation. In 1954, El Niño turned into La Niña, during which time the central and eastern equatorial Pacific SSTAs switched from positive to negative, fostering the development and maintenance of a blocking high in the Sea of Okhotsk.

**Figure 5.** Sea surface temperature anomalies of the Sea of Okhotsk.

The standardized anomalies of average monthly 500-hPa geopotential heights within the region of 120E–150E, 50N–60N were defined as the Okhotsk high index (OKHI). The index represents the activity level of a blocking high. An OKHI ≥ 1.0 indicates that the geopotential height anomaly exceeds the mean by 1 standard deviation, suggesting that the blocking high in question is active. Figure 6 presents the time series of OKHIs. In 1954, the Okhotsk high was of substantially high intensity and peaked in June and July. This trend was consistent with the corresponding peak precipitation values. Intense Okhotsk highs brought the cold air branches in the mid-latitude westerlies southward to the Yangtze River valley, causing extended precipitation in the area.

**Figure 6.** Okhotsk high index (OKHI) in summertime from 1950 to 2000, based on the geopotential height data in NCEP/NCAR reanalysis dataset [27], the shadow indicates years that OKHI ≥ 1.0.

#### **5. Precipitation Anomaly Causal Model**

Numerous studies have proven that Pacific SSTAs are closely associated with the formation of AC anomalies [14–16,36,37]. Notably, SSTAs occur earlier than AC anomalies; therefore, SSTAs have been widely recognized as an indicator of unusual AC [40–42]. For example, Pacific SSTAs have become a crucial indicator for researchers seeking to predict summertime AC anomalies and precipitation in the Yangtze River valley [17,18].

On the basis of SSTAs and previous data analysis results, we traced the unusual precipitation process back to 1954, and proposed a causal model of SSTAs affecting precipitation. Figure 7 presents the schematic diagrams showing the circulation anomalies associated with SST anomalies. Between the fall of 1953 and the spring of 1954, the El Niño recession generated abnormal convection activities across the Philippines, resulting in anticyclones at the bottom of the troposphere in the region and southward shifts of WPSHs.

**Figure 7.** Schematic diagram showing the circulation and precipitation anomalies associated with SST anomalies. WPSH—western Pacific subtropical high.

An extended period of cold SSTAs was detected near the mid and high latitudes of the Sea of Okhotsk in contrast to the high temperature of the land surface, which led to the formation of a blocking high. Moreover, the lowered central and eastern Pacific SSTAs strengthened the potential energy of the Okhotsk blocking high. The formation of such a blocking high also weakened the northward shift of the WPSH, causing it to continue retreating southward. Southward shifts of summertime subtropical highs and the blocking high over the Sea of Okhotsk jointly and continually brought warm and moist airflows from over the sea, as well as high-latitude cold air into the drainage basin of the Yangtze. Consequently, an unusually high volume of precipitation occurred during the summer in said drainage basin in 1954.

Figure 3a shows that many flood disasters occurred during the recession of ENSO events, and when the Okhotsk high was also active in that year (Figure 6), the superposition effect of two anomalies may have intensified the precipitation and generated massive flood disasters. There are three years that the blocking high in Okhotsk is active: 1954, 1988, and 1998. Severe flood disasters also occurred in 1954 and 1998; both floods caused large casualties and economic losses [43]. Similar to the 1954 Yangtze floods, studies have reported that the 1998 precipitation anomalies in the Yangtze River valley were accompanied by the El Niño event and active Okhotsk highs (Figure 3a, Figure 6) [44–46]. Although the 1998 El Niño was more intense and rapid than in 1954, the circulation anomalies caused by it are similar [24]. There was an El Niño event in 1988, and its intensity was similar to that of 1954, which was relatively weak; however, it did not cause strong anomalies of the WPSH. Therefore, the precipitation in 1988 was relatively normal in spite of the existence of the Okhotsk blocking high. We speculate that the Okhotsk blocking high will enormously intensify the precipitation anomalies caused by El Niño events, resulting in extreme precipitation events.

The common meteorological background of 1954 and 1998 demonstrated that the causal model shown in Figure 7 is not particular, but a general pattern of anomalies prone to generate severe flood disasters; therefore, such a causal model can be used as a forecast tool for future severe flood disasters in the Yangtze River valley.

#### **6. Conclusions**

The 1954 precipitation anomalies were characterized by high total rainfall, an extended period of rainfall and numerous cloudbursts, with the rain mostly occurring during June and July. The total rainfall along the mid- and low-Yangtze areas of the Yangtze exceeded 1500 mm between April and July, which was roughly double the volume under average conditions in previous years; furthermore, heavy rainbands were extremely widespread.

The analysis revealed that between 1951 and early 1954, the SSTs near the eastern equatorial Pacific were unusually high, indicating the presence of El Niño. The El Niño event then led to an anomalous anticyclonic circulation in the summer of 1954, affecting the WSPH and the precipitation throughout the Yangtze River valley. Furthermore, the continued low SST of the Sea of Okhotsk between 1953 and 1954 generated a blocking high over the sea during the flood season. This blocking high prompted cold air at high latitudes to move southward continually, where it met moist and warm airflows over the sea, finally triggering continuous precipitation. The superposition effect of the above two anomalies intensified the precipitation and generated a severe flood disaster in the Yangtze River valley.

This study proposed a causal model of extreme summertime precipitation in the Yangtze River valley in 1954. The unusual changes in SSTs first resulted in AC anomalies, which caused the unusually heavy rainfall that year. This model indicated a pattern of anomalies prone to generate severe flood disasters in the Yangtze River valley, and thus can be used as a forecast tool for future severe flood disasters in this region.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/rs14030555/s1, Figure S1: (a) Precipitable water across the Yangtze river valley during April–July of 1954 based on NCEP/NCAR reanalysis. (b) Precipitation across the Yangtze river valley during April–July of 1954 based on ground station dataset. Figure S2: (a) Precipitable water anomalies across the Yangtze river valley during April–July of 1954 based on NCEP/NCAR reanalysis. (**b**) Precipitation anomalies across the Yangtze river valley during April–July of 1954 based on ground station dataset.

**Author Contributions:** Conceptualization, J.C. and S.Z.; methodology, M.L.; software, Y.Z. and Y.W.; validation, J.C. and Y.Z.; formal analysis, J.C. and S.Z.; investigation, J.C.; resources, S.Z.; data curation, P.X.; writing—original draft preparation, J.C.; writing—review and editing, S.Z. and Y.Z.; visualization, J.C. and M.L.; supervision, S.Z.; project administration, S.Z.; funding acquisition, S.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Key R&D Program of China (grant number 2020YFC1512401) and the National Natural Science Foundation of China (grant number 42074176, 41874169, U1939204).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Publicly available datasets were analyzed in this study. The ground station precipitation data presented in this study can be found here: [data.cma.cn]. The NOAA Extended Reconstructed SST V5 dataset and the NCEP/NCAR dataset can be found here: https: //psl.noaa.gov/data/gridded/ accessed on 1 October 2021.

**Acknowledgments:** The authors would like to thank the editors and anonymous reviewers for their constructive suggestions.

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

#### **References**

