Western North Pacific Monsoon Index Research Articles

Impact of Long‐Distance Interaction Indicator (Monsoon Indices) on Spatio‐Temporal Variability of Precipitation Over the Mahanadi River Basin

AbstractThe variability of the Indian Summer Monsoon Rainfall (ISMR), lasting from June to September, has a major impact on the Indian economy and ecosystem. An attempt to assess the temporal fluctuation of ISMR with the long‐distance interaction indicators has been presented here. This also corroborates the argued statement of a weakening relationship between ISMR and Indian Summer Monsoon Index (ISMI) due to significant temporal fluctuation. Monsoon indices: the ISMI, East Asian Summer Monsoon Index (EASMI), South Asian Summer Monsoon Index (SASMI), and Western‐North Pacific Monsoon Index (WNPMI) have been studied to create a correlation, coherency, and teleconnection with the ISMR over the Mahanadi River basin (MRB) during 1948–2017. High‐resolution gridded precipitation data were employed to assess the statistical properties of the monsoon and to address the correlation, coherency, and teleconnection with the ISMR. Spearman's rank‐based correlation test, a non‐parametric Mann‐Kendall trend test, and Sen's slope have been used to assess the statistical properties of observed precipitation and the monsoon indices. Furthermore, wavelet transform coherence is used to detect transient correlations between ISMR and monsoon indices that have previously been uncorrelated. SASMI (ρ = 0.440) dominates ISMI (ρ = 0.422) as the most significant and influential long‐distance indicator with ISMR. The findings were reconfirmed on a sub‐basin scale, with the middle Mahanadi River Basin (MMRB) being the most significant and influential, whereas the upper Mahanadi River Basin and lower Mahanadi River Basin were the second and third most influential, respectively.

Interannual Anomaly in Tropical Cyclone Frequency Around Korea by Western North Pacific Monsoon

ABSTRACT In this study, the correlation between the frequency of summer tropical cyclones (TC) affecting areas around Korea over the last 37 years and the western North Pacific monsoon index (WNPMI) was analyzed. A clear positive correlation existed between the two variables, and this high positive correlation remained unchanged even when excluding El Nino-Southern Oscillation (ENSO) years. To investigate the causes of the positive correlation between these two variables, ENSO years were excluded, after which the 8 years with the highest WNPMI (strong WNPM phase) and the 8 years with the lowest WNPMI (weak WNPM phase) were selected, and the average difference between the two phases was analyzed. In the strong WNPM phase, TCs usually occurred in the eastern waters of the tropical and subtropical western North Pacific, and tended to pass the East China Sea on their way north toward Korea and Japan. In the weak WNPM phase, TCs usually occurred in the western waters of the tropical and subtropical western North Pacific, and tended to pass the South China Sea on their way west toward the southeastern Chinese coast and the Indochina peninsula. Therefore, TC intensity was higher in the strong WNPM phase, during which TCs are able to gain sufficient energy from the sea while moving a long distance to areas nearby Korea. TCs also tended to occur more often in the strong WNPM phase. In the difference between the two phases regarding 850 hPa and 500 hPa streamline, anomalous cyclones were reinforced in the tropical and subtropical western North Pacific, while anomalous anticyclones were reinforced in mid-latitude East Asian areas. Due to these two anomalous pressure systems, anomalous southeasterlies developed in areas near Korea, with these anomalous southeasterlies playing the role of anomalous steering flows making the TCs head toward areas near Korea. Also, due to the anomalous cyclones developed in the tropical and subtropical western North Pacific, more TCs could occur in the strong WNPM phase.

Seasonality of Tides in Southeast Asian Waters

AbstractAn analysis of water level time series from 20 tide gauges in Southeast Asia finds that diurnal and semidiurnal astronomical tides exhibit strong seasonal variability of both amplitude and phase, which is not caused by known modulations of the astronomical tide-generating forces. Instead, it is found that the tidal properties are coherent with the western North Pacific monsoon index (WNPMI), indicating that monsoonal mechanisms are the likely cause. The study domain includes the Malacca Strait, Gulf of Thailand, the southern South China Sea, and Java Sea. The character of the geography and the tidal variability is different in each of these subregions. A new barotropic regional tide model is developed that incorporates the coupling between geostrophic currents, wind-driven (Ekman) currents, and tidal currents in the bottom boundary layer in order to examine the influence of these factors on tides. The dynamics thus preserve the frictional nonlinearities while neglecting advective nonlinearities and baroclinic tides, approximations that should be valid on the wide and shallow continental shelves in the study region. The model perturbation approach uses the climatological seasonal variability of wind stress and geostrophic currents, which are prescribed singly and in combination in the model, to explain the observed tidal variability. Results are most successful in the southern Gulf of Thailand and near Singapore, where it is found that the combined effect of geostrophic and Ekman currents shows increased skill in reproducing the tidal variability than individual models. Ambiguous results at other locations suggest more localized processes such as river runoff.

The Influence of Climate Variability Effects on Groundwater Time Series in the Lower Central Plains of Thailand

This research studies the relationship between the climate index and the groundwater level of the lower Chao Phraya basin, in order to forecast the groundwater level in the studied area by using Autoregressive Integrated Moving Average with Explanatory (ARIMAX). The combination of 6 climate indices—Dipole Mode Index, Indian Summer Monsoon Index, Multivariate ENSO Index, Sea Surface Temperature NINO4, Southern Oscillation Index and the Western North Pacific Monsoon Index—were used, along with the groundwater level data from 14 stations during the period 1980–2011 to develop the forecast model and verify it with the data of 2012.The first step was correlation of the ARIMA model with Autocorrelation Function and Partial Autocorrelation Function. The possible model was then selected using BIC statistics. Diagnostic Checking was done to consider the white noise characteristic of estimated residuals by using the statistics of Box and Ljung (Q-statistic). If the selected models were found to be proper, then the Granger Causality Test of the leading parameters or the climate index would be performed as the next step. The results show that there is a relationship between the groundwater level and the climate index. The model could be used to forecast effectively the average RMSE value at 0.6. The last step was to develop the MODFLOW for a conceptual model and synthesize groundwater levels in the study area, which covers around 43,000 km2 and has 8 layers of groundwater, with Bangkok clay on the top. All other boundary values were set to be steady. The calibration was done using the data of 325 observed wells. The normalized RMS value was 9.705%. The results were verified by the data using ARIMAX over the same time periods. To conclude, the simulated results of the monthly groundwater level in 2012 of the wells have a confidence interval of around 95%, which is near the result from the ARIMAX model. The advantages of the ARIMAX model include high accuracy, no requirement for a large amount of data and inexpensive implementation. It is one of the effective tools for the groundwater prediction.

Analyzing the non-linear trend and multiscale teleconnections of regional monsoon indices using empirical mode decomposition

This paper first performs the analysis of linear and non-linear trends of four representative monsoon Indices namely Indian Monsoon Index (IMI) and Webster-Yang index (WYI), Western North Pacific Monsoon Index (WNPMI) and Australian Monsoon Index (AMI) for the period 1948–2015. The study evaluates the trend in the time series of different monsoon months and seasonal (JJA/JJAS/DJF) series, of all the indices using Mann–Kendall test, Sen’s slope method, linear fitting and the Empirical Mode Decomposition (EMD) method. The trend analysis showed significant reduction in all the series of WYI index and the August month series of IMI. The results of sequential version of MK test revealed that the WYI time series of different months and monsoon season showed statistically significant reduction in the past with differences in commencement year of statistical significance of MK value. The comparison of long term trend of monsoon indices and ElNino Southern Oscillation (ENSO) displayed an opposing trend for different indices except for WNPMI. Further for a detailed examination of association of different indices with ENSO in multiple time scales, the running correlations between the oscillatory modes of monsoon indices and ENSO are performed by employing the time dependent intrinsic correlation analysis. The association between ENSO and monsoon indices at different time scales is found to be strongly negative and consistent for WNPMI and AMI while for rest of the indices, the nature of association is found to be positive in inter annual and inter decadal time scales.

Possible relation of the western North Pacific monsoon to the tropical cyclone activity over western North Pacific

ABSTRACTThis study examined the relationship between the western North Pacific monsoon (WNPM) and the tropical cyclone (TC) activity over western North Pacific during the summer (June to August) in a period from 1977 to 2013. According to the western North Pacific monsoon index (WNPMI), 10 years with high indices (referred to as the positive WNPMI phase) and 10 years with low indices (referred to as the negative WNPMI phase) were chosen. TCs frequently formed in the southeast quadrant of the subtropical western North Pacific in the positive WNPMI phase, and in the northwest quadrant in the negative phase. With respect to the TC track, TCs tended to move northward from the distant sea off the east coast of the Philippines towards Korea and Japan past the East China Sea in the positive WNPMI phase, whereas the characteristic pattern of the negative phase was the TC movement from the Philippines to southern China and the Indochinese peninsula over the South China Sea. As for the TC intensity, the TCs were stronger in the positive WNPMI phase than in the negative WNPMI phase. The anomalous cyclonic circulations in the south of the 30°N signified strong convection, causing the high TC genesis frequency in the positive WNPMI phase. In addition, the East China Sea, Korea and Japan were shown to be influenced by the anomalous southeasterly, which contributed as the anomalous steering flows and was favourable for the TCs to move into these regions in the positive WNPMI phase. Meanwhile, the positive anomaly of 600 hPa relative humidity and sea surface temperature as well as the negative anomaly of vertical wind shear presented in most regions of the subtropical western North Pacific, creating an environment that facilitated the generation of TCs in the positive WNPMI phase.

Possible influence of western North Pacific monsoon on TC activity in mid-latitudes of East Asia

This study analyzed the correlation between tropical cyclone (TC) frequency and the Western North Pacific monsoon index (WNPMI), which have both been influential in East Asia’s mid-latitude regions during the summer season over the past 37 years (1977–2013). A high positive correlation existed between these two variables, which was not reduced even if El Nino-Southern Oscillation (ENSO) years were excluded. To determine the cause of this positive correlation, the highest (positive WNPMI phase) and lowest WNPMIs (negative WNPMI phase) during a nine-year period were selected to analyze the mean difference between them, excluding ENSO years. In the positive WNPMI phase, TCs were mainly generated in the eastern seas of the tropical and subtropical western North Pacific, passing through the East China Sea and moving northward toward Korea and Japan. In the negative phase, TCs were mainly generated in the western seas of the tropical and subtropical western North Pacific, passing through the South China Sea and moving westward toward China’s southern regions. Therefore, TC intensity in the positive phase was stronger due to the acquisition of sufficient energy from the sea while moving a long distance up to East Asia’s mid-latitude. Additionally, TCs occurred more in the positive phase. Regarding the difference of the two phases between the 850 and 500-hPa streamlines, anomalous cyclones were strengthened in the tropical and subtropical western North Pacific, whereas anomalous anticyclones were strengthened in East Asia’s mid-latitude regions. Due to these two anomalous pressure systems, anomalous southeasterlies developed in East Asia’s mid-latitude regions, which played a role in the anomalous steering flows that moved TCs into these regions. Furthermore, due to the anomalous cyclones that developed in the tropical and subtropical western North Pacific, more TCs could be generated in the positive phase. Both the lower and upper tropospheric layers had warm anomalies in most regions of the Western North Pacific, while relative humidity in the middle tropospheric layer showed a positive anomaly in the tropical and subtropical western North Pacific, which provided a better environment to strengthen TC intensity in the positive WNPMI phase. Furthermore, a negative anomaly was manifested not only in the tropical and subtropical western North Pacific, but also in East Asia’s mid-latitude regions, with 200–850-hPa vertical wind shear, while a warm sea surface temperature anomaly was shown in East Asia’s mid-latitude seas, which further strengthened TC intensity in the positive phase. The analysis on the global-scale atmospheric circulations showed that converged air in the lower layer of the subtropical western Pacific during the positive phase diverged in the upper layer, which moved westward and converged in the upper layer of the equatorial Indian Ocean and then diverged in its lower layer.

북서태평양 몬순이 한국 영향태풍활동에 미치는 영향

In this study, the correlation between the frequency of summer tropical cyclones (TC) affecting areas around Korea over the last 37 years and the western North Pacific monsoon index (WNPMI) was analyzed. A clear positive correlation existed between the two variables, and this high positive correlation remained unchanged even when excluding El Nio-Southern Oscillation (ENSO) years. To investigate the causes of the positive correlation between these two variables, ENSO years were excluded, after which the 8 years with the highest WNPMI (positive WNPMI phase) and the 8 years with the lowest WNPMI (negative WNPMI phase) were selected, and the average difference between the two phases was analyzed. In the positive WNPMI phase, TCs usually occurred in the eastern waters of the tropical and subtropical western North Pacific, and tended to pass the East China Sea on their way north toward Korea and Japan. In the negative WNPMI phase, TCs usually occurred in the western waters of the tropical and subtropical western North Pacific, and tended to pass the South China Sea on their way west toward the southeastern Chinese coast and the Indochina peninsula. Therefore, TC intensity was higher in the positive WNPMI phase, during which TCs are able to gain sufficient energy from the sea while moving a long distance to areas nearby Korea. TCs also tended to occur more often in the positive WNPMI phase. In the difference between the two phases regarding 850 and 500 hPa streamline, anomalous cyclones were reinforced in the tropical and subtropical western North Pacific, while anomalous anticyclones were reinforced in mid-latitude East Asian areas. Due to these two anomalous pressure systems, anomalous southeasterlies developed in areas near Korea, with these anomalous southeasterlies playing the role of anomalous steering flows making the TCs head toward areas near Korea. Also, due to the anomalous cyclones developed in the tropical and subtropical western North Pacific, more TCs could occur in the positive WNPMI phase.

Indo-China monsoon indices.

Myanmar and Thailand often experience severe droughts and floods that cause irreparable damage to the socio-economy condition of both countries. In this study, the Southeastern Asian Summer Monsoon variation is found to be the main element of interannual precipitation variation of the region, more than the El Niño/Southern Oscillation (ENSO). The ENSO influence is evident only during the boreal spring season. Although the monsoon is the major factor, the existing Indian Monsoon Index (IMI) and Western North Pacific Monsoon Index (WNPMI) do not correlate well with the precipitation variation in the study regions of Southern Myanmar and Thailand. Therefore, a new set of indices is developed based on the regional monsoon variations and presented here for the first time. Precipitation variations in Southern Myanmar and Thailand differ as well as the elements affecting the precipitation variations in different seasons. So, separate indices are proposed for each season for Southern Myanmar and Thailand. Four new monsoon indices based on wind anomalies are formulated and are named as the Indochina Monsoon Indices. These new indices correlate better with the precipitation variations of the study region as compared to the existing IMI and WNPMI.

CMIP5/AMIP GCM simulations of East Asian summer monsoon

The East Asian summer monsoon (EASM) is a distinctive component of the Asian climate system and critically influences the economy and society of the region. To understand the ability of AGCMs in capturing the major features of EASM, 10 models that participated in Coupled Model Intercomparison Project/Atmospheric Model Intercomparison Project (CMIP5/AMIP), which used observational SST and sea ice to drive AGCMs during the period 1979–2008, were evaluated by comparing with observations and AMIP II simulations. The results indicated that the multi-model ensemble (MME) of CMIP5/AMIP captures the main characteristics of precipitation and monsoon circulation, and shows the best skill in EASM simulation, better than the AMIP II MME. As for the Meiyu/Changma/Baiyu rainbelt, the intensity of rainfall is underestimated in all the models. The biases are caused by a weak western Pacific subtropical high (WPSH) and accompanying eastward southwesterly winds in group I models, and by a too strong and west-extended WPSH as well as westerly winds in group II models. Considerable systematic errors exist in the simulated seasonal migration of rainfall, and the notable northward jumps and rainfall persistence remain a challenge for all the models. However, the CMIP5/AMIP MME is skillful in simulating the western North Pacific monsoon index (WNPMI).

북서태평양 아열대고기압 지수를 이용한 북동아시아 여름철 강수의 진단

동아시아 여름몬순의 강도와 북서태평양 여름몬순의 강도는 음의 상관을 갖는 것으로 알려져 왔다. 여기서 우리는 이 관계를 이용하여 북동아시아 여름철 강수의 잠재예측성을 조사하였다. 북서태평양 아열대 고기압은 북서태평양 여름몬순을 적절히 나타내며, 북서태평양-동아시아 지역 여름철 기후편차에 주된 성분이다. 그리고 북서태평양 아열대고기압 변동성을 이용한 북동아시아 여름철 강수 편차의 추정값은 북서태평양 여름몬순지수를 이용하는 것보다 더 낫다. The intensity of the East Asian summer monsoon has a negative correlation with that of the western North Pacific summer monsoon. Based on the relationship, we suggest the potential predictability of Northeast Asian summer precipitation by using the relationship. The western North Pacific subtropical high (WNPSH) properly represents the intensity of the western North Pacific summer monsoon. It also dominates climate anomalies in the western North Pacific-East Asian region in summertime. The estimates of the Northeast Asian summer rainfall anomalies using WNPSH variability have a greater benefit than those using the western North Pacific monsoon index.

Rainfall regime in Three Gorges area in China and the control factors

AbstractA set of daily precipitation data from 1958 to 2007 was analysed for the area in and around the Three Gorges Reservoir Region. Annual and monthly precipitation, number of rainy days (NRDs), precipitation intensity (INT), and seven indices of extreme precipitation and drought were examined. Correlation between circulation indices and climate parameters was analysed. Significant decreases were detected in spring, fall, winter and annual NRDs, while significant increases were found in precipitation INT. Inter‐decadal changes are obvious, it was wettest in the early 1980s, and dry in the first few years and after 1990. No significant changes are found in extreme precipitation/drought events, but a closer examination suggested they might become more frequent after 1980. Precipitation totals and days of extreme precipitation in a specific period were found to be negatively correlated with western North Pacific Monsoon Index (WNPMI); positively correlated with summer average of index of the ridge of western Pacific subtropical high; and positively correlated with summer average of index of INT of western Pacific subtropical high and the winter–summer difference of Tibetan Plateau (TP) index. These links were significant but not strong, which might manifest the characteristics of Three Gorges Area as a transition zone, where precipitation is influenced by multiple systems. Regional management should be more careful to adjust to these changes. Copyright © 2009 Royal Meteorological Society

Statistical downscaling for multi-model ensemble prediction of summer monsoon rainfall in the Asia-Pacific region using geopotential height field

The 21-yr ensemble predictions of model precipitation and circulation in the East Asian and western North Pacific (Asia-Pacific) summer monsoon region (0°–50°N, 100°–150°E) were evaluated in nine different AGCM, used in the Asia-Pacific Economic Cooperation Climate Center (APCC) multi-model ensemble seasonal prediction system. The analysis indicates that the precipitation anomaly patterns of model ensemble predictions are substantially different from the observed counterparts in this region, but the summer monsoon circulations are reasonably predicted. For example, all models can well produce the interannual variability of the western North Pacific monsoon index (WNPMI) defined by 850 hPa winds, but they failed to predict the relationship between WNPMI and precipitation anomalies. The interannual variability of the 500 hPa geopotential height (GPH) can be well predicted by the models in contrast to precipitation anomalies. On the basis of such model performances and the relationship between the interannual variations of 500 hPa GPH and precipitation anomalies, we developed a statistical scheme used to downscale the summer monsoon precipitation anomaly on the basis of EOF and singular value decomposition (SVD). In this scheme, the three leading EOF modes of 500 hPa GPH anomaly fields predicted by the models are firstly corrected by the linear regression between the principal components in each model and observation, respectively. Then, the corrected model GPH is chosen as the predictor to downscale the precipitation anomaly field, which is assembled by the forecasted expansion coefficients of model 500 hPa GPH and the three leading SVD modes of observed precipitation anomaly corresponding to the prediction of model 500 hPa GPH during a 19-year training period. The cross-validated forecasts suggest that this downscaling scheme may have a potential to improve the forecast skill of the precipitation anomaly in the South China Sea, western North Pacific and the East Asia Pacific regions, where the anomaly correlation coefficient (ACC) has been improved by 0.14, corresponding to the reduced RMSE of 10.4% in the conventional multi-model ensemble (MME) forecast.

Year-in-advance Prediction of Lightning Activity for the Pacific Northwest

This note outlines the development of a multivariate linear regression model to predict peak lightning activity a year in advance. Using cloud-to-ground lightning strike data from the inception of the National Lightning Detection Network in 1985, a predictive model is proposed for the peak monthly strike total for the Pacific Northwest. Six climate indices were evaluated as potential predictors of the lightning. From these, a multivariate linear model was developed, based on the year-in-advance values of two of the indices—The El Nino-Southern Oscillation Index, and the Western North Pacific Monsoon Index. The 2007 prediction of 56,618 strikes was tested against the measured value of 38,591 strikes. The values are in agreement within the expected accuracy of the model. The model prediction represents an improvement relative to the expectation based on the past statistical history of the data and should be most useful in predicting extreme events. The model parameters have been updated to include the 2007 ...

Decadal change in relationship between east Asian and WNP summer monsoons

It has been recognized that the intensity of the east Asian (EA) summer monsoon has a negative correlation with that of the western North Pacific (WNP) summer monsoon. Here we show that this relationship is much stronger in the recent decade (1994–2004) than in the epoch before 1994 (1979–1993). The first two leading modes of summer‐mean precipitation over the large region of the WNP and EA region are shown to be associated with two factors: the ENSO development and the WNP summer monsoon fluctuation. The leading mode has changed from an ENSO‐related mode in 1979–1993 to a WNP summer monsoon‐related mode in the recent decade (1994–2004). The summer‐mean mid‐tropospheric geopotential heights that are correlated with the WNP monsoon index also show a marked change in the teleconnection (wave‐train) pattern between 1994–2004 and 1979–1993. All together this evidence suggests that the relationship between the EA and the WNP summer monsoons has experienced a significant decadal change around 1993–1994.