# Prediction of the Western North Pacific Subtropical High in Summer without Strong ENSO Forcing

• Corresponding author: Chaofan LI, lichaofan@mail.iap.ac.cn
• Funds:

Supported by the National Key Research and Development Program of China (2018YFC1506005), National Natural Science Foundation of China (41775083), and the Second Tibetan Plateau Scientific Expedition and Research (STEP) Program (2019QZKK0102). Nick Dunstone was supported by the UK–China Research and Innovation Partnership Fund through the Met Office Climate Science for Service Partnership (CSSP) China as part of the Newton Fund

• doi: 10.1007/s13351-021-0113-3
• The western North Pacific subtropical high (WNPSH) is one of the deterministic predictors of the East Asian summer climate, and a better prediction of the WNPSH favors more reasonable forecast of the East Asian summer climate. This study focuses on seasonal prediction of the WNPSH during neutral summers without strong El Niño–Southern Oscillation (ENSO) forcing, and explores the associated predictable sources, using the one-month lead time retrospective forecasts from the Ensembles-Based Predictions of Climate Changes and Their Impacts (ENSEMBLES) project during 1960–2005. The results indicate that the ENSEMBLES atmosphere–ocean–land coupled models exhibit considerable prediction skill for the WNPSH during neutral summers, with successful reproduction of the WNPSH in the majority of neutral summers. The anomalous WNPSH in neutral summers, which corresponds to cyclonic/anticyclonic anomalies in the lower troposphere, is highly correlated with an east–west dipole local sea surface temperature (SST) distribution over the tropical WNP, suggesting an intimate local air–sea coupling. Further diagnosis of the local SST–rainfall relationship and surface heat flux indicates that the anomalous local SST plays an active role in modulating the variation of the WNPSH during neutral summers, rather than passively responding to the atmospheric change. The local SST anomalies and relevant air–sea coupling over the tropical WNP are reasonably well reproduced in the model predictions, and could act as primary predictable sources of the WNPSH in neutral summers. This could aid in forecasting of the East Asian rainband and associated disaster mitigation planning.

• Fig. 1.  Prediction skill (shaded; ${\rm P}\_{\rm Cor}$) of the (a) 850-hPa zonal wind, (b) precipitation, and (c) SST for ENSO-neutral summers. The ENSO-neutral summers include 1960, 1961, 1962, 1967, 1979, 1980, 1981, 1990, 1993, 2001, 2004, and 2005 (after 1979 for precipitation). The contours represent statistical significance of the prediction correlation at the 0.05 and 0.01 confidence levels. The blue boxes indicate the domains of the WNPSH index.

Fig. 2.  (a) The normalized WNPSH index for the observations (OBS; blue bar) and model predictions (MME; red bar) during all neutral summers and (b) associated observed evolution of the Niño 3.4 SST index. The positive (negative) years with an observed WNPSH index larger (less) than 0.8 (–0.8) are marked by grey dashed lines in (a) and green (orange) lines in (b) as the significant neutral summers.

Fig. 3.  Composites of the anomalies of (a, b) 850-hPa wind (vector; m s−1) superimposed with precipitation (shading; mm day−1) and (c, d) SST (°C) during the significant neutral summers for (a, c) observations and (b, d) MME predictions. The purple boxes indicate the domains of the dipole SST over the tropical WNP.

Fig. 4.  The normalized dipole SST index over the WNP for the observations (OBS; blue) and the MME predictions (Model; red) during the significant neutral summers. The index is defined by the east–west SST gradient over the WNP, with the domains indicated in Fig. 3c.

Fig. 5.  Seasonal evolution of the observed SST gradient (°C) over the WNP. The anomaly in 1993 (short dashed line) is multiplied by −1 for ease of comparison with the other positive years. Com. denotes composite.

Fig. 6.  Scatter diagrams of the standardized SST and precipitation anomalies over the (a) eastern and (b) western WNP for the significant neutral summers after 1979. The blue, red, and grey markers indicate the observations, the MME prediction, and the individual ensemble members, respectively. The domains for the eastern and western WNP are shown in Fig. 3c. The anomaly in 1993 is multiplied by –1. The correlation coefficients between the SST and precipitation anomalies for all these dots are 0.28 in (a) and 0.62 in (b).

Fig. 7.  As in Fig. 6, but for the standardized WNPSH index and precipitation anomalies over the (a) eastern and (b) western WNP. The correlation coefficients between the WHPSH index and precipitation anomalies for all these dots are 0.61 in (a) and –0.31 in (b).

Fig. 8.  Composites of the surface fluxes from the observations (left panels) and MME predictions (right panels) during the significant neutral summers, including (a, b) longwave (LW) radiation, (c, d) shortwave (SW) radiation, (e, f) latent heat (LH) flux, and (g, h) sensible heat (SH) flux. The units are W m−2 and positive values indicate downward flux.

Fig. 9.  As in Fig. 8, but for the total surface flux anomalies.

###### 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

## Prediction of the Western North Pacific Subtropical High in Summer without Strong ENSO Forcing

###### Corresponding author: Chaofan LI, lichaofan@mail.iap.ac.cn;
• 1. Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
• 2. State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
• 3. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
• 4. Met Office Hadley Centre, Exeter EX1 3PB, UK
Funds: Supported by the National Key Research and Development Program of China (2018YFC1506005), National Natural Science Foundation of China (41775083), and the Second Tibetan Plateau Scientific Expedition and Research (STEP) Program (2019QZKK0102). Nick Dunstone was supported by the UK–China Research and Innovation Partnership Fund through the Met Office Climate Science for Service Partnership (CSSP) China as part of the Newton Fund

Abstract:

The western North Pacific subtropical high (WNPSH) is one of the deterministic predictors of the East Asian summer climate, and a better prediction of the WNPSH favors more reasonable forecast of the East Asian summer climate. This study focuses on seasonal prediction of the WNPSH during neutral summers without strong El Niño–Southern Oscillation (ENSO) forcing, and explores the associated predictable sources, using the one-month lead time retrospective forecasts from the Ensembles-Based Predictions of Climate Changes and Their Impacts (ENSEMBLES) project during 1960–2005. The results indicate that the ENSEMBLES atmosphere–ocean–land coupled models exhibit considerable prediction skill for the WNPSH during neutral summers, with successful reproduction of the WNPSH in the majority of neutral summers. The anomalous WNPSH in neutral summers, which corresponds to cyclonic/anticyclonic anomalies in the lower troposphere, is highly correlated with an east–west dipole local sea surface temperature (SST) distribution over the tropical WNP, suggesting an intimate local air–sea coupling. Further diagnosis of the local SST–rainfall relationship and surface heat flux indicates that the anomalous local SST plays an active role in modulating the variation of the WNPSH during neutral summers, rather than passively responding to the atmospheric change. The local SST anomalies and relevant air–sea coupling over the tropical WNP are reasonably well reproduced in the model predictions, and could act as primary predictable sources of the WNPSH in neutral summers. This could aid in forecasting of the East Asian rainband and associated disaster mitigation planning.

Reference (39)

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