Western North Pacific Circulation Research Articles

Unraveling the role of the western North Pacific circulation anomaly in modulating Indian summer monsoon rainfall variability beyond ENSO

The Indian summer monsoon (ISM) rainfall interannual variability is known to be strongly linked to the El-Niño–Southern Oscillation (ENSO). This linear relationship is the primary factor in controlling the interannual variation in ISM precipitation. However, there are many outlier cases, and such deviations pose significant challenges in seasonal prediction over this region. Here we show that such challenges can be attributed to anomalous atmospheric pressure patterns in the Western North Pacific (WNP) region. The anticyclonic circulation anomaly over WNP region causes the easterly wind toward the Indian subcontinent, leading to positive precipitation anomalies with stronger low-level moist convergence, while the cyclonic circulation decreases ISM precipitation. The linear baroclinic model simulation results further support that the WNP circulation pattern can serve as an independent factor for forecasting precipitation over India. The WNP circulation anomaly play the crucial role generating ISM precipitation particularly for July and September. Our study suggests that the role of the WNP circulation anomaly should be carefully considered as the secondary prevailing mechanism on the subseasonal timescale during the boreal summer in addition to the ENSO signal.

Abrupt sub-seasonal rainfall variability over India during summer monsoon 2021: Interaction between midlatitude and tropical circulation

The Indian subcontinent witnessed abnormal subseasonal rainfall variations during the summer monsoon season 2021 with abrupt change (54%) in rainfall from August to September. According to India Meteorological Department, in August Indian landmass received 81% rainfall of its long period average (LPA), which is lowest since 2002. On the other hand, anomalously high rainfall (+ 35% of its LPA) was received in September. The physical mechanisms responsible for such unusual changes in monthly/subseasonal rainfall are unravelled in this study. In August 2021, the southward displacement of Asian jet (SWDAJ) corroborated by westward and northwestward extension of the Western north Pacific (WNP) anticyclone induced moisture divergence over the monsoon trough region and northwestern parts of India are mainly responsible for the deficit rainfall. In the East Asian region, strong low-level westerly wind anomalies in the northern flank of WNP anticyclone couples with the upper-level westerlies associated with SWDAJ and the resulting interaction due to the barotropic–baroclinic coupling maintains WNP circulation. Apart from WNP anticyclone, anomalous Sea Surface Temperature warming over the southeast equatorial Indian Ocean also contributed to the suppressed rainfall over India. In contrast to August, during September the northward displacement of the Asian jet and associated anomalous upper-level anticyclone over northwest India linked to the mid-latitude wave train emanating from the northeast Atlantic contributed to enhanced rainfall over northwest India. Anomalous upper-level high over central Asia/northwest of India increased the moist baroclinic instability over northwest India, and enhanced rainfall there. The Climate Forecast System version 2 (CFSv2) model based on May initial conditions is some extent predicted rainfall change from August and September 2021 over India with much lesser magnitude. Inability of the model in capturing the changes in Asian jet, midlatitude wave train and associated teleconnections/interactions to Indian Summer Monsoon (ISM) rainfall limits its prediction skill. Further, the North American Multi Model Ensemble (NMME) models also could predict the correct sign of ISM rainfall anomaly in August and September, with May initial conditions, but failed to capture the spatial patterns, which needs further investigation.

Impact of Global Warming on the Western North Pacific Circulation Anomaly during Developing El Niño

AbstractEl Niño stimulates an anomalous cyclone over the North Pacific during its developing phase. Using 30 CGCMs and 11 AGCMs from CMIP5, we find a weakly strengthened anomalous North Pacific cyclone (NPC) in a warmer climate in CGCMs, and intermodel uncertainty exists. A similar change of the anomalous NPC is found in AGCMs with increased mean state SST but with a stronger amplitude of enhancement. Based on a simple Gill model, the diabatic heating anomaly, mean state static stability, and meridional gradient of relative vorticity are identified to be responsible for the change of the anomalous NPC. Analyses of the CMIP5 models suggest that the change of the anomalous NPC is largely determined by the competition between the enhanced diabatic heating anomaly and the enhanced mean state static stability. The amplitude of enhancement of the anomalous NPC is strongly modulated by the change of precipitation anomaly over the equatorial central-eastern Pacific, which depends on the changes of mean state SST and the El Niño–related SST anomaly. Compared with a uniform warming, an El Niño–like mean state SST warming favors a much stronger enhancement of the anomalous NPC, by enhancing the mean state precipitation and latent heating anomaly associated with the precipitation anomaly over the equatorial Pacific. However, the air–sea coupling acts to weaken the SST anomaly associated with El Niño in the CGCMs, which further reduces the enhancement of the anomalous NPC.

Interdecadal modulation of the Indo-western Pacific Ocean Capacitor mode and its influence on Indian summer monsoon rainfall

The present study investigates the interdecadal modulations of interannual variability in the Indo-western Pacific Ocean Capacitor (IPOC) mode and its influence on the Indian summer monsoon (ISM) rainfall during 1886–2014. The IPOC mode is extracted using singular value decomposition (SVD) analysis of tropical Indian Ocean (TIO) sea surface temperature (SST) and western North Pacific (WNP) 850 hPa relative vorticity anomalies during the boreal summer season (June to August; JJA). Analysis suggests that the IPOC mode induces a southwest-northeast dipole pattern in the Indian subcontinent rainfall with significant positive anomalies over the southern peninsular India and negative anomalies over the eastern parts of the Indo-Gangetic plains during the positive phase of IPOC. The 21-year sliding correlation coefficients between rainfall dipole index and leading principal component of SVD analysis (SVD-PC1) suggest that IPOC and ISM rainfall relationship exhibits significant interdecadal variability during the study period. The entire period has been divided into three distinct epochs as 1895–1926 (epoch-1), 1932–1972 (epoch-2) and 1976–2007 (epoch-3) based on 21-year sliding correlation. It is found that the IPOC mode strongly affect ISM rainfall in epoch-1 and epoch-3, whereas this relationship between the IPOC mode and ISM rainfall is inadequate in epoch-2 pertaining to the absence of the dipole rainfall pattern. The mechanisms responsible for such epochal changes in relationship between IPOC mode and ISM rainfall are discussed. In epoch-1 and epoch-3, anomalous TIO SST and the Pacific–Japan (PJ) pattern/WNP circulation both are well organized as part of the IPOC mode. However, in epoch-2 vigorous warm SSTs over the equatorial central Pacific and cyclonic circulation over the north Pacific accompanied by SST cooling around 40°N are noticed. The north Pacific cyclonic circulation accelerates the southwesterlies emerging from north-western flank of WNP anticyclone. On the other hand, positive SST anomalies and associated low-level convergence in central equatorial Pacific limit the extension of northeasterlies from the southern edge of WNP anticyclone to TIO. As a result, WNP anticyclone is squeezed/twisted between them, thereby weakening and shifting it northeastward. This in turn weakens the PJ mode and the inter-basin interaction of WNP circulation and TIO SSTs, causing disorganized IPOC pattern in epoch-2.

Impact of El Niño Modoki on Indian summer monsoon rainfall: Role of western north Pacific circulation in observations and CMIP5 models

AbstractThe Indian summer monsoon (ISM) rainfall is often influenced by El Niño‐Southern Oscillation (ENSO) on the interannual time scale. Physical mechanisms that link the canonical El Niño and ISM rainfall have been well documented. Very few studies have discussed the pathways that link El Niño Modoki and ISM rainfall variations. In this study using long period (1901–2014) reanalysis and observed rainfall data, it is found that rainfall anomalies are negative over the southern peninsular India and positive over the central parts of India during El Niño Modoki years. Detailed analysis suggests that the El Niño Modoki modulates ISM rainfall by inducing changes in the western north Pacific (WNP) low‐level circulation. The weak positive rainfall over the monsoon trough region is due to weak moisture convergence associated with the westward extension of WNP cyclonic circulation corroborated by lows and depressions. Anomalous WNP cyclonic circulation is induced by the central Pacific Sea Surface Temperature (SST) warming associated with the El Niño Modoki. Further, strong moisture divergence over south peninsular India caused negative rainfall anomalies during the El Niño Modoki years. Thus, WNP circulation plays a critical role in determining the ISM rainfall patterns during the El Niño Modoki years as evidenced by significant correlation between WNP circulation pattern and EMI. These regional rainfall patterns over the Indian subcontinent corresponding to El Niño Modoki are not well explained in previous studies. It is found that many Coupled Model Intercomparison Project Phase 5 (CMIP5) models displayed poor skill in representing the relation between ISM rainfall and El Niño Modoki events due to unrealistic simulation of SST and circulation patterns over the Indo‐Pacific region. This suggests that CMIP5 models tend to have a large diversity in representing the El Niño Modoki teleconnections to ISM rainfall.

Impact of the Indo-Western Pacific Ocean Capacitor mode on South Asian summer monsoon rainfall

The influence of Indo-Western Pacific Ocean Capacitor (IPOC) mode on South Asian summer monsoon rainfall variability is investigated in the present study. It is found from the observations that the IPOC mode induces a tripole pattern in precipitation anomalies over the South Asian region. Strong positive precipitation anomalies over some parts of western Ghats and Sundarbans and Bangladesh region separated by negative precipitation anomalies over the monsoon trough region are found to be the main characteristic features closely associated with the IPOC mode. It is noted that anomalous western north Pacific (WNP) anticyclone and tropical Indian Ocean (TIO) warming maintain a tripole precipitation pattern over the South Asian region. Anomalous low level convergence in the northern flank of WNP anticyclone and orographic lift cause enhanced rainfall over the Sundarbans and Bangladesh region. Meanwhile, westward propagating atmospheric cold Rossby wave as a response to suppressed convection over the WNP region induced moisture divergence and downward motion play important role in maintaining negative rainfall anomalies over the monsoon trough. At the same time, TIO SST warming supported by easterly wind anomalies associated with WNP anticyclone has contributed to the enhanced rainfall over parts of eastern Arabian Sea and western Ghats. Coupled General Circulation Model sensitivity experiments are performed to investigate individual and combined impact of the TIO warming and WNP circulation on south Asian summer monsoon rainfall variability associated with IPOC mode. The model experiments revealed that WNP anticyclone influences the rainfall over the monsoon trough region and Sundarbans-Bangladesh region. Further the moisture convergence induced by local SST warming is mainly helping to sustain enhanced Western Ghats rainfall. Sub-seasonal variability of ISM rainfall associated with the IPOC modes is also discussed. This study advocates the importance of IPOC mode in exerting strong impact on regional summer rainfall variability over the Indian/South Asian summer monsoon region on the interannual time scale. Thus, it is important to consider the possible roles of TIO warming and WNP circulation in predicting the South Asian summer monsoon rainfall.

The Eurasian Jet Streams as Conduits for East Asian Monsoon Variability

This article gives a brief review on how the jet streams over the Eurasian continent influence the East Asian monsoon on intraseasonal to interdecadal time scales and discusses the seasonal predictability and change. The wave train along the Eurasian jet streams is found to be crucial for East Asian monsoon variability. Interaction of the upper-level Rossby wave train with the Siberian High causes changes in winter monsoon climate over East Asia. In the case of summer, the Silk Road pattern, embedded in the Asian jet in association with western North Pacific circulation and the Pacific-Japan pattern, alters the strength and phase of the monsoon. Current coupled models showed limited skills in seasonal prediction of the Eurasian jet variations and their influences on the East Asian monsoon variability. The Eurasian jets as conduits for East Asian monsoon variability involve multiple feedbacks. Its interaction with low-level circulation mostly determines the degree of strength of variations in the monsoon climate. Global warming projections based on RCP 4.5 and 8.5 in the CMIP5 (the Coupled Model Intercomparison Project phase 5) models indicate that the mean Asian jet strengthens in future during winter, but no change is reported during summer.

Interdecadal changes in the asymmetric impacts of ENSO on wintertime rainfall over China and atmospheric circulations over western North Pacific

Based on data diagnoses, in this study the interdecadal changes in asymmetric impacts of ENSO on wintertime East Asian climate are investigated. It is found that the Pacific Decadal Oscillation (PDO) can significantly modulate the asymmetry in the responses of the East Asian climate to El Nino and La Nina events. In positive PDO phase the response is asymmetric with a strong anomalous anticyclone over western North Pacific (WNP) and significant positive rainfall anomalies over southern China during El Nino winters, but a weak anomalous cyclone over WNP and insignificant negative rainfall anomalies over southern China during La Nina winters. However, such asymmetric responses do not appear in negative PDO phase, with comparable amplitudes of anomalous circulations over WNP and rainfall over southern China in El Nino and La Nina winters. Further analyses reveal that the warm background of the tropical Pacific in positive PDO phase causes significant difference in the amplitudes of convection anomalies over tropical western Pacific, resulting in asymmetric responses of the WNP circulation and rainfall over southern China to El Nino and La Nina events. Nevertheless, the cold background in negative PDO phase reduces the amplitude difference between El Nino and La Nina winters. A comparable convection anomalies appear in tropical western Pacific in El Nino and La Nina winters and the asymmetric responses do not occur.

Month-to-month variability of Indian summer monsoon rainfall in 2016: role of the Indo-Pacific climatic conditions

Indian summer monsoon (ISM) rainfall during 2016 exhibited a prominent month-to-month fluctuations over India, with below normal rainfall in June and August and above normal rainfall in July. The factors determining the month-to-month fluctuations in ISM rainfall during 2016 are investigated with main focus on the Indo-Pacific climatic anomalies. Warm sea surface temperature (SST) anomalies associated with super El Nino 2015 disappeared by early summer 2016 over the central and eastern Pacific. On the other hand, negative Indian Ocean dipole (IOD) like SST anomaly pattern over the equatorial Indian Ocean and anomalous anticyclonic circulation over the western North Pacific (WNP) are reported in summer 2016 concurrently with decaying El Nino/developing La Nina phase. Observations revealed that the low rainfall over central north India in June is due to moisture divergence caused by the westward extension of ridge corresponding to WNP anticyclone and subsidence induced by local Hadley cell partly related to negative IOD. Low level convergence of southeasterly wind from Bay of Bengal associated with weak WNP anticyclone and northwesterly wind corresponding to anticyclonic circulation over the northwest India remarkably contributed to positive rainfall in July over most of the Indian subcontinent. While reduced rainfall over the Indian subcontinent in August 2016 is associated with the anomalous moisture transport from ISM region to WNP region, in contrast to July, due to local cyclogenesis corroborated by number of tropical cyclones in the WNP. In addition to this, subsidence related to strong convection supported by cyclonic circulation over the WNP also resulted in low rainfall over the ISM region. Coupled General Circulation model sensitivity experiments confirmed that strong convective activities associated with cyclonic circulation over the WNP is primarily responsible for the observed negative ISM rainfall anomalies in August 2016. It is noted that the Indo-Western Pacific circulation anomalies in August 2016 are well predicted when the coupled model is initiated with initial conditions from end of July and beginning of August compared to May. This analysis suggests the importance of the WNP circulation in forcing strong sub-seasonal/month to month rainfall variations over India.

Promising prediction of the monsoon trough and its implication for tropical cyclone activity over the western North Pacific

The monsoon trough (MT) is generally recognized as a feeding ground for tropical cyclones (TCs) over the western North Pacific (WNP). In view of the many challenges that remain in current seasonal TC forecasting, it would be a profound benefit to understand the predictability of variations in the MT and the implications of this for the seasonal prediction of TC activity. This study reveals that high predictability of the MT is shown by the current atmosphere–ocean coupled forecasting system, with the correlation coefficient being 0.84 for the model-ensemble prediction with observations from 1960 to 2005. This high predictability arises mainly from the tropical dipole sea surface temperature over the Maritime Continent and tropical Pacific Ocean, which favors convection around the warm pool and further excites the vorticity anomalies over the WNP. It is further found that good knowledge of the MT could provide promising prediction of TC activity over the WNP, including the occurrence and energy of TCs. The findings of this study suggest that coupling between the WNP circulation and tropical ocean acts as an important source of seasonal predictability in the WNP, and highlight the importance of the MT for seasonal prediction of TCs over the WNP.

Potential for long‐lead prediction of the western North Pacific monsoon circulation beyond seasonal time scales

AbstractAlthough the western North Pacific (WNP) monsoon circulation significantly impacts the socioeconomic communities around Asia, its prediction is only limited to a few months. By examining the Coupled Model Intercomparison Project phase 5 decadal hindcast experiments, we explore a possibility of the extended prediction skill for the WNP monsoon circulation beyond seasonal time scales. It is found that the multimodel ensemble (MME) predictions, initialized in January, successfully predict the WNP circulation in spring and early summer. Somewhat surprisingly, a reliable prediction of the WNP circulation appears even in the following spring with a maximum lead time of 14 months. This unexpected prediction skill is likely caused by the improved El Niño–Southern Oscillation (ENSO) prediction and the exaggerated dynamical link between the ENSO and premonsoon circulation in the MME prediction. Although further studies are needed, this result may open up new opportunities for the multiseasonal prediction of the WNP monsoon circulation.

Improving the Prediction of the East Asian Summer Monsoon: New Approaches

Abstract East Asian summer monsoon (EASM) prediction is difficult because of the summer monsoon’s weak and unstable linkage with El Niño–Southern Oscillation (ENSO) interdecadal variability and its complicated association with high-latitude processes. Two statistical prediction schemes were developed to include the interannual increment approach to improve the seasonal prediction of the EASM’s strength. The schemes were applied to three models [i.e., the Centre National de Recherches Météorologiques (CNRM), the Met Office (UKMO), and the European Centre for Medium-Range Weather Forecasts (ECMWF)] and the Multimodel Ensemble (MME) from the Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) results for 1961–2001. The inability of the three dynamical models to reproduce the weakened East Asian monsoon at the end of the 1970s leads to low prediction ability for the interannual variability of the EASM. Therefore, the interannual increment prediction approach was applied to overcome this issue. Scheme I contained the EASM in the form of year-to-year increments as a predictor that is derived from the direct outputs of the models. Scheme II contained two predictors: both the EASM and also the western North Pacific circulation in the form of year-to-year increments. Both the cross-validation test and the independent hindcast experiments showed that the two prediction schemes have a much better prediction ability for the EASM than does the original scheme. This study provides an efficient approach for predicting the EASM.

The Philippines–Taiwan Oscillation: Monsoonlike Interannual Oscillation of the Subtropical–Tropical Western North Pacific Wind System and Its Impact on the Ocean

Tide gauge and satellite data reveal an interannual oscillation of the ocean’s thermoclines east of the Philippines and Taiwan, forced by a corresponding oscillation in the wind stress curl. This so-called Philippines–Taiwan Oscillation (PTO) is shown to control the interannual variability of the circulation of the subtropical and tropical western North Pacific. The PTO shares some characteristics of known Pacific indices, for example, Niño-3.4. However, unlike PTO, these other indices explain only portions of the western North Pacific circulation. The reason is because of the nonlinear nature of the forcing in which mesoscale (ocean) eddies play a crucial role. In years of positive PTO, the thermocline east of the Philippines rises while east of Taiwan it deepens. This results in a northward shift of the North Equatorial Current (NEC), increased vertical shear of the Subtropical Countercurrent (STCC)/NEC system, increased eddy activity dominated by warm eddies in the STCC, increased Kuroshio transport off the northeastern coast of Taiwan into the East China Sea, increased westward inflow through Luzon Strait into the South China Sea, and cyclonic circulation and low sea surface height anomalies in the South China Sea. The reverse applies in years of negative PTO.

Relative Contributions of the Indian Ocean and Local SST Anomalies to the Maintenance of the Western North Pacific Anomalous Anticyclone during the El Niño Decaying Summer*

Abstract To investigate the relative role of the cold SST anomaly (SSTA) in the western North Pacific (WNP) or Indian Ocean basin mode (IOBM) in maintaining an anomalous anticyclone over the western North Pacific (WNPAC) during the El Niño decaying summer, a suite of numerical experiments is performed using an atmospheric general circulation model, ECHAM4. In sensitive experiments, the El Niño composite SSTA is specified in either the WNP or the tropical Indian Ocean, while the climatological SST is specified elsewhere. The results indicate that the WNPAC is maintained by the combined effects of the local forcing of the negative SSTA in the WNP and the remote forcing from the IOBM. The former (latter) contribution gradually weakens (enhances) from June to August. The negative SSTA in the WNP is crucial for the maintenance of the WNPAC in early summer. However, because of a negative air–sea feedback, the negative SSTA gradually decays, as does the local forcing effect. Enhanced local convection associated with the IOBM stimulates atmospheric Kelvin waves over the equatorial western Pacific. The impact of the Kelvin waves on the WNP circulation depends on the formation of the climatological WNP monsoon trough, which does not fully establish until late summer. Therefore, the IOBM plays a crucial role in late summer via the Kelvin wave induced anticyclonic shear and boundary layer divergence.

On the low‐level circulation over the western north Pacific in relation with the duration of El Niño

We investigate how the low‐level circulation centered at 10°N over the western north Pacific differs between the shorter (SHORT‐E) and longer (LONG‐E) El Niño events based on their duration time. One major difference is the intensity of this circulation. This difference is due to the greater contribution of the 2–3 year quasi‐biennial (QB) signal for the SHORT‐E; strength of the 3–6 year interannual signal is almost the same in a mean sense between the two event types. Further, the amplitude of the QB signal is in phase with that of the interannual signal during the SHORT‐E so that the circulation intensifies and evolves rapidly. The enhanced anticyclonic circulation during the mature stage of SHORT‐E results in shallower thermocline in the equatorial western Pacific, and exerts influence on the duration of the event. During the LONG‐E, the mentioned feature diminishes because of the weak QB. Our analysis further indicates a significant correlation between the zonal SSTA gradient over the Indian Ocean and the intensity of the western north Pacific circulation. A complete understanding of the related mechanism needs further study.