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# Seasonal Prediction of the Record-Breaking Northward Shift of the Western Pacific Subtropical High in July 2021

• The unprecedented Zhengzhou heavy rainfall in July 2021 occurred under the background of a northward shift of the western Pacific subtropical high (WPSH). Although the occurrence of this extreme event could not be captured by seasonal predictions, a skillful prediction of the WPSH variation might have warned us of the increased probability of extreme weather events in Central and Northern China. However, the mechanism for the WPSH variation in July 2021 and its seasonal predictability are still unknown. Here, the observed northward shift of the WPSH in July 2021 is shown to correspond to a meridional dipole pattern of the 850-hPa geopotential height to the east of China, the amplitude of which became the strongest since 1979. The meridional dipole pattern is two nodes of the Pacific–Japan pattern. To investigate the predictability of the WPSH variation, a 21-member ensemble of seasonal predictions initiated from the end of June 2021 was conducted. The predictable and unpredictable components of the meridional dipole pattern were identified from the ensemble simulations. Its predictable component is driven by positive precipitation anomalies over the tropical western Pacific. The positive precipitation anomalies are caused by positive horizonal advection of the mean moist enthalpy by southwesterly anomalies to the northwestern flank of anticyclonic anomalies excited by the existing La Niña, which is skillfully predicted by the model. The leading mode of the unpredictable component is associated with the atmospheric internal intraseasonal oscillations, which are not initialized in the simulations. The relative contributions of the predictable and unpredictable components to the observed northward shift of the WPSH at 850 hPa are 28.0% and 72.0%, respectively.
摘要: 2021年7月，在西太副高异常北移的大尺度背景环流下，郑州发生了一次前所未有的强降水事件。虽然当前的季节预测系统尚无法预测“7.21”郑州极端暴雨事件，但对其背景环流的准确预测能够为我们应对未来此类极端天气事件的发生及其防灾减灾提供决策依据。观测分析表明，2021年7月西太副高异常北移呈现为我国东部沿海地区850 hPa经向偶极型位势高度异常，与东亚夏季PJ（Pacific-Japan）遥相关的低层节点相对应，其振幅为1979年以来最强。为了研究其季节尺度上的可预测性，基于中国科学院大气物理研究所季节至年代际气候预测系统（IAP-DecPreS），针对2021年7月西太副高的异常北移开展了集合预测试验。研究表明，7月西太副高异常北移既包含可预测信号分量，也包含不可预测噪音分量。可预测信号由月平均尺度上热带西太平洋正降水异常驱动，其来源是La Niña冷海温异常激发的副热带北太平洋异常反气旋西侧的异常正湿焓平流。不可预测噪音则主要源自大气内部的次季节变率。两者对2021年7月西太副高异常北移的相对贡献分别为28.0%和72.0%。
• Figure 1.  Left panel: Observed climatological July mean (a) horizontal winds at the 150-hPa level (vectors, units: m s–1) and (c) precipitation (shaded, units: mm d–1) and horizontal winds at the 850-hPa level (vectors, units: m s–1) for the period 1995–2014. The observation derives from ERA5. Right panel: (b, d) as in (a, c), but for the FGOALS-f3-L historical simulations.

Figure 2.  Spatial distributions of (a) geopotential height anomaly at 500 hPa (units: gpm), (b) geopotential height anomaly at 850 hPa (units: gpm), (c) SSTA (units: °C), and (d) precipitation anomaly (units: mm d–1) and 850-hPa horizontal wind anomaly (units: m s–1) in July 2021. Time series of (e) the July Z850 dipole index and the Niño-3.4 index (the Niño-3.4 index is multiplied by –1), and (f) the July Z850 dipole index and the area-averaged precipitation anomalies over the western tropical Pacific (WP-PR index) for the period 1979–2021. Correlation coefficients between the two indices (COR) are noted at the top left of (e, f), with the asterisk (*) representing significance at the 99% level. The Z850 dipole index is defined as the difference of geopotential height anomaly averaged over Box-A (15°–30°N, 110°–150°E) and Box-B (35°–50°N, 120°–160°E) in (b). The WP-PR index is defined as the precipitation anomalies averaged over Box-C (15°–30°N, 125°–155°E) in (d).

Figure 3.  The western Pacific subtropical high (WPSH) at (a) 850 hPa, (b) 700 hPa, (c) 600 hPa, and (d) 500 hPa in July. The WPSH at 850 hPa, 700 hPa, 600 hPa, and 500 hPa are denoted by the 1470-, 3130-, 4390- and 5880-gpm contour of geopotential height, respectively. The bold black lines represent the climatology for the period of 1979–2021. The red and light gray lines represent the cases in 2021 and other years, respectively.

Figure 4.  Spatial distributions of (a) the 500-hPa geopotential height anomalies (units: gpm) and (b) precipitation anomalies (units: mm d–1) associated with the first SVD mode between the July 500-hPa geopotential height field over East Asia (10°–70°N, 100°–160°E) and July precipitation over the Philippines (10°–30°N, 125°–155°E). The hatching denotes values exceeding the 95% confidence level. (c) The expansion coefficients of the first SVD mode. The black (red) line is for the 500-hPa geopotential height (precipitation).

Figure 5.  Relative vorticity anomalies (shaded, units: 10−6 s−1) for (a) the meridional sections averaged between 125°–155°E and (b) the zonal sections averaged between 40°–50°N. The 3D TN wave activity fluxes are shown by vectors (units: m2 s–2), where the vertical components have been multiplied by 100.

Figure 6.  Spatial distributions of the observed and predicted global SSTA (units: °C) in July 2021. The observation and ensemble mean forecast are shown on the left and the right of the first row. Forecasts from individual members are shown below the first row. PCC is the uncentered pattern correlation coefficient. The forecasts by the IAP-DecPreS are started from the end of June in 2021.

Figure 7.  As in Fig. 5, but for the 850-hPa geopotential height anomalies (units: gpm) over the Northern Hemisphere.

Figure 8.  The 21-member ensemble mean predicted (a) 500-hPa geopotential height anomalies (units: gpm), (b) 850-hPa geopotential height anomalies (units: gpm), (c) precipitation (shaded, units: mm d−1) and 850-hPa horizontal wind (vector, units: m s−1) anomalies, and (d) the meridional sections of relative vorticity (shaded, units: 10−6 s−1) and TN-flux (vector, units: m2 s−2, the vertical components have been multiplied by 100) averaged between 110°–130°E in July 2021.

Figure 9.  Budget analysis for (a) the moisture equation [Eq. (1), units: mm d–1] and (b) the MSE equation [Eqs. (4–5), units: W m–2] for the area 10°–25°N, 125°–150°E in July 2021 from the 21-member ensemble mean forecasts by IAP-DecPreS.

Figure 10.  Spatial distributions of (a) the cloud-related longwave radiative flux anomalies (${R}_{\mathrm{c}\mathrm{l}\mathrm{o}\mathrm{u}\mathrm{d}}'$), (b) the surface latent heat anomalies ($\mathrm{L}\mathrm{H}^{'}$), (c) the zonal advection of climatological enthalpy by wind anomalies ($- < {u}'{\partial }_{x}\overline{\left({C}_{p}T+{L}_{v}q\right)} >$), and (d) the meridianal advection of climatological enthalpy by wind anomalies ($- < {v}'{\partial }_{y}\overline{\left({C}_{p}T+{L}_{v}q\right)} >$). (e) the July 925-hPa climatological specific humidity (shading; units: kg kg–1) and 925-hPa horizontal wind anomalies in the 21-member ensemble mean predictions by IAP-DecPreS.

Figure 11.  Spatial distributions of the 850-hPa geopotential height anomalies (units: gpm) associated with (a) the first EOF mode (EOF1) and (c) the second EOF mode (EOF2) of the ensemble spread of the forecasts. Variance contributions of the two EOF modes are noted in the parentheses. Relationships between predicted Z850 dipole index and (b) the normalized principal component (PC) corresponding to the first EOF mode (PC1) and (d) the normalized PC corresponding to the second EOF mode (PC2). The correlation coefficients between PC1 (PC2) and the predicted Z850 dipole index are shown in the legends. The black line in (b) is the linear fitting equation between PC1 and the predicted Z850 dipole index. The blue triangle represents the observation, for which the PC value is estimated through the linear fitting equation in the model world. The hatching in (a, c) denotes values exceeding the 95% confidence level.

Figure 12.  Regressions of the ensemble spread of (a) relative vorticity (shaded; units: m2 s–2) and TN-flux (vector, units: m2 s–2) anomalies at 200 hPa, (b) SSTA (units: °C), (c) relative vorticity (shaded; units: m2 s–2) and TN-flux (vector, units: m2 s–2) anomalies at 850 hPa, (d) precipitation anomalies (units: mm d–1), (e) the meridional sections of relative vorticity (shaded, units: 10–6 s–1) and TN-flux (vector, units: m2 s–2, the vertical components have been multiplied by 100) averaged between 110°–130°E, and (f) 850-hPa eddy kinetic energy (units: m2 s–2) at a time scale of 10–30 days onto the normalized principal component (PC) corresponding to the first EOF mode (PC1) of the ensemble spread of the forecasts in July 2021. The hatching denotes values exceeding the 95% confidence level. The triangles in (a, c, d, f) represent the center of precipitation anomaly over the western tropical Pacific.

Figure 13.  Regressions of the ensemble spread of moisture budgets onto the normalized principal component (PC) corresponding to the first EOF mode (PC1) of the ensemble spread of the forecasts in July 2021 at time scales of (a–d) monthly, (e–h) 10–30 days, and (i–l) less than 10 days. (a, e, i) precipitation anomalies and (b, f, j) the dynamic components, (c, g, k) the thermodynamic components, and (d, h, l) the nonlinear components of the anomalous moisture advection. The hatching denotes values exceeding the 95% confidence level.

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## Manuscript History

Manuscript revised: 26 September 2022
Manuscript accepted: 17 October 2022
###### 通讯作者: 陈斌, bchen63@163.com
• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

## Seasonal Prediction of the Record-Breaking Northward Shift of the Western Pacific Subtropical High in July 2021

###### Corresponding author: Tianjun ZHOU, zhoutj@lasg.iap.ac.cn;
• 1. State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
• 2. University of the Chinese Academy of Sciences, Beijing 100049, China

Abstract: The unprecedented Zhengzhou heavy rainfall in July 2021 occurred under the background of a northward shift of the western Pacific subtropical high (WPSH). Although the occurrence of this extreme event could not be captured by seasonal predictions, a skillful prediction of the WPSH variation might have warned us of the increased probability of extreme weather events in Central and Northern China. However, the mechanism for the WPSH variation in July 2021 and its seasonal predictability are still unknown. Here, the observed northward shift of the WPSH in July 2021 is shown to correspond to a meridional dipole pattern of the 850-hPa geopotential height to the east of China, the amplitude of which became the strongest since 1979. The meridional dipole pattern is two nodes of the Pacific–Japan pattern. To investigate the predictability of the WPSH variation, a 21-member ensemble of seasonal predictions initiated from the end of June 2021 was conducted. The predictable and unpredictable components of the meridional dipole pattern were identified from the ensemble simulations. Its predictable component is driven by positive precipitation anomalies over the tropical western Pacific. The positive precipitation anomalies are caused by positive horizonal advection of the mean moist enthalpy by southwesterly anomalies to the northwestern flank of anticyclonic anomalies excited by the existing La Niña, which is skillfully predicted by the model. The leading mode of the unpredictable component is associated with the atmospheric internal intraseasonal oscillations, which are not initialized in the simulations. The relative contributions of the predictable and unpredictable components to the observed northward shift of the WPSH at 850 hPa are 28.0% and 72.0%, respectively.

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