# Decadal Change of Meiyu Onset over Yangtze River and Its Causes

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## Abstract

**:**

## 1. Introduction

## 2. Data and Methods

## 3. Results

#### 3.1. Decadal Changes in Meiyu Onset and Associated Circulation Modulation

#### 3.2. Possible Cause of the Epochal Changes in CISO

## 4. Discussion and Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## Abbreviations

CISO | climatological intraseasonal oscillation |

EASM | East Asian summer monsoon |

ISO | intraseasonal oscillation |

OLR | outgoing longwave radiation |

SST | sea surface temperature |

WNP | western North Pacific |

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**Figure 1.**Decadal change of Meiyu onset over Yangtze River Basin in China. (

**a**) Time series of Yangtze River Meiyu onset date during 1960–2014 relative to the climatological mean onset date of 14 June. Blue and red bars mark the early and delayed onset years, respectively. Black lines denote the epochal averages of Meiyu onset date of 6 June for 1989–2001 and of 19 June for 2002–2014. The posterior probability mass function of the change point as a function of time (year) is displayed at the bottom. (

**b**) Frequency of onset occurrence in each pentad for E1 (1989–2001; blue bars) and E2 (2002–2014; red bars).

**Figure 2.**(

**a**) Same as in Figure 1 but for the results detected by STARS method. The RSI values of the change point as a function of time (year) is displayed at the bottom. (

**b**) Heat map of different RSI values when different cut-off lengths were used during 1985–2016.

**Figure 3.**Composites of outgoing long-wave radiation (OLR; shading; units: W m

^{−2}), 850-hPa moisture flux (vector; units: m s

^{−1}) and 700-hPa stream function with the value of 0 m

^{2}s

^{−1}(green curve) in (

**a**) pentad 30, (

**b**) pentad 31, (

**c**) pentad 32, and (

**d**) pentad 33 during E1 (1989–2001). (

**e**–

**h**) and (

**i**–

**l**) as in (

**a**–

**d**), but for the composites during E2 (2002–2014) and the epochal differences between the two periods (E1 minus E2). Shaded and bold vectors represent the epochal changes (between E1 and E2) in OLR and moisture flux and are statistically significant at the 95% confidence level according to two-tailed t-test. Solid and dashed green contours delineate 0-value stream function at 700 hPa during E1 and E2, respectively. Letter “A” in the right panels marks the center of anticyclonic anomaly.

**Figure 4.**Composites of 200-hPa zonal wind (shading; units: m s

^{−1}) and geopotential height (contour; units: m) in (

**a**) pentad 30, (

**b**) pentad 31, (

**c**) pentad 32, and (

**d**) pentad 33 during E1 (1989–2001). (

**e**–

**h**) and (

**i**–

**l**) as in (

**a**–

**d**), but for the composites during E2 (2002–2014) and the epochal differences between the two periods (E1 minus E2). Thick contours represent the epochal change (between E1 and E2) in 200-hPa geopotential height exceeding the 95% significance level according to two-tailed t-test. Letter “A” denotes the center of high pressure/anticyclonic circulation system.

**Figure 5.**Composites of 20–90-day filtered OLR (shading; units: W m

^{−2}) and 850-hPa wind (vector; units: m s

^{−1}) in (

**a**) pentad 30, (

**b**) pentad 31, (

**c**) pentad 32, and (

**d**) pentad 33 during E1 (1989–2001). (

**e**–

**h**) and (

**i**–

**l**) as in (

**a**–

**d**), but for the composites during E2 (2002–2014) and the epochal differences between the two periods (E1 minus E2). Only the epochal changes (between E1 and E2) with statistical significance at the 95% confidence level, based on two-tailed t-test, are shown in (

**i**–

**l**).

**Figure 6.**Amplitude of CISO-related convection (20–90-day filtered OLR; units: W m

^{−2}) during pentads 30–33 of (

**a**) E1, (

**b**) E2 and (

**c**) E1 minus E2. Stippling in (

**c**) marks the region with statistically significant change at the 95% confidence level. (

**d**–

**f**) As in (

**a**–

**c**), but for the CISO during pentads 34–37.

**Figure 7.**Similar to Figure 3, but for the composites of surface–700-hPa integrated moisture (shading; units: kg kg

^{−1}), 850-hPa wind field (vector; units: m s

^{−1}) and vertical wind shear (defined as 200-hPa zonal wind minus 850-hPa zonal wind; contour; units: m s

^{−1}) for pentads 30–33 during (

**a**) E1 and (

**b**) E2, respectively, and (

**c**) their differences (E1 minus E2).

**Figure 8.**Epochal differences (E1 minus E2) in SST (units: K) during pentads 30–33. Only the change with statistical significance at the 95% confidence level, according to two-tailed t-test, is shown.

**Figure 9.**(

**a**) Observed distributions of climatological intraseasonal (20–90-day) OLR variance (shading; units: W

^{2}m

^{−4}) and vertical wind shear (contour; units: m s

^{−1}) during pentads 30–33 of 1989–2014. (

**b**) Same as (

**a**), but for the results derived from 30-year simulation of EXP_CTRL.

**Figure 10.**Effects of western Pacific, Indian Ocean, and North Atlantic SST anomalies on variability of May–June ISO convection variability and background conditions. (

**a**) Difference in variability of CISO convection (i.e., standard deviation of 20–90-day OLR; units: W m

^{−2}) between EXP_WP and CTRL. (

**b**,

**c**) Same as (

**a**), but for differences in variability of CISO convection simulated by EXP_WP and EXP_NA relative to CTRL, respectively. (

**d**–

**f**) Same as (

**a**–

**c**), but for the changes in 1000–700-hPa averaged specific humidity (shading; units: kg kg

^{−1}) and vertical wind shear (contour; units: m s

^{−1}).

**Table 1.**(Top to bottom) Meiyu onset date, retreat, precipitation amount, and intensity averaged over E1 (1989–2001) and E2 (2002–2014), and their difference (E1 minus E2).

Meiyu Features | E1 (1989–2001) | E2 (2002–2014) | Epochal Diff. (E1-E2) |
---|---|---|---|

Onset date | 6 June | 19 June | −13 * |

Retreat date | 11 July | 17 July | −6 |

Meiyu precipitation amount | 324.6 | 259.8 | 64.8 |

Precipitation intensity | 7.78 | 7.11 | 0.67 |

^{−1}). Asterisk indicates that the epochal difference between E1 and E2 is statistically significant at the 95% confidence level.

Experiment | Low Boundary Conditions | Integration Length | Purposes |
---|---|---|---|

EXP_CTRL | Climatological monthly SST | 30 years | Assessing the simulation skill |

EXP_WP | Epochal change (E1 minus E2) in SST over the western Pacific (100° E–180°, 0°–25° N) superimposed on climatological SST | 20 years | Clarifying the regional SST effects on WNP background states and CISO intensity |

EXP_IO | Epochal change (E1 minus E2) in SST over the Indian Ocean (40°–100° E, −20°–20° N) superimposed on climatological SST | ||

EXP_NA | Epochal change (E1 minus E2) in SST over the North Atlantic (90° W–0°, 0°–80° N) superimposed on climatological SST |

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**MDPI and ACS Style**

Qian, Y.; Hsu, P.; Fu, Z.; Liu, Y.; Li, Q.
Decadal Change of Meiyu Onset over Yangtze River and Its Causes. *Sustainability* **2022**, *14*, 5085.
https://doi.org/10.3390/su14095085

**AMA Style**

Qian Y, Hsu P, Fu Z, Liu Y, Li Q.
Decadal Change of Meiyu Onset over Yangtze River and Its Causes. *Sustainability*. 2022; 14(9):5085.
https://doi.org/10.3390/su14095085

**Chicago/Turabian Style**

Qian, Yong, Pangchi Hsu, Zhen Fu, Yunyun Liu, and Qiaoping Li.
2022. "Decadal Change of Meiyu Onset over Yangtze River and Its Causes" *Sustainability* 14, no. 9: 5085.
https://doi.org/10.3390/su14095085