Variability Of Summer Monsoon Rainfall Research Articles

Interdecadal Variability in Myanmar Rainfall in the Monsoon Season (May–October) Using Eigen Methods

In this study, we investigated the interdecadal variability in monsoon rainfall in the Myanmar region. The gauge-based gridded rainfall dataset of the Global Precipitation Climatology Centre (GPCC) and Climatic Research Unit version TS4.0 (CRU TS4.0) were used (1950–2019) to investigate the interdecadal variability in summer monsoon rainfall using empirical orthogonal function (EOF), singular value decomposition (SVD), and correlation approaches. The results reveal relatively negative rainfall anomalies during the 1980s, 1990s, and 2000s, whereas strong positive rainfall anomalies were identified for the 1970s and 2010s. The dominant spatial variability mode showed a dipole pattern with a total variance of 47%. The power spectra of the principal component (PC) from EOF revealed a significant peak during decadal timescales (20–30 years). The Myanmar summer monsoon rainfall positively correlated with Atlantic multidecadal oscillation (AMO) and negatively correlated with Pacific decadal oscillation (PDO). The results reveal that extreme monsoon rainfall (flood) events occurred during the negative phase of the PDO and below-average rainfall (drought) occurred during the positive phase of the PDO. The cold phase (warm phase) of AMO was generally associated with negative (positive) decadal monsoon rainfall. The first SVD mode indicated the Myanmar rainfall pattern associated with the cold and warm phase of the PDO and AMO, suggesting that enhanced rainfall for about 53% of the square covariance fraction was related to heavy rain over the study region except for the central and eastern parts. The second SVD mode demonstrated warm sea surface temperature (SST) in the eastern equatorial Pacific (El Niño pattern) and cold SST in the North Atlantic Ocean, implying a rainfall deficit of about 33% of the square covariance fraction, which could be associated with dry El Niño conditions (drought). The third SVD revealed that cold SSTs in the central and eastern equatorial Pacific (La Niña pattern) caused enhance rainfall with a 6.7% square covariance fraction related to flood conditions. Thus, the extra-subtropical phenomena may affect the average summer monsoon trends over Myanmar by enhancing the cross-equatorial moisture trajectories into the North Atlantic Ocean.

Performance of the RegCM-MITgcm Coupled Regional Model in Simulating the Indian Summer Monsoon Rainfall

An effort is made to evaluate the performance of an atmosphere–ocean coupled regional model in simulating the Indian summer monsoon rainfall during the years 1995–2015. The RegCM and MITgcm models are taken as the atmospheric and oceanic components of the coupled model, respectively. It is found that the coupling helps to improve the SST of the Arabian Sea and Bay of Bengal regions of the model domain. The air–sea coupling also improves the wind speed simulation. The RegCM-MITgcm coupled model correctly represents the intraseasonal variability of the Indian summer monsoon rainfall as compared to the RegCM stand-alone model. It is shown that the very high and extreme rainfall intensity events are correctly simulated by the RegCM-MITgcm coupled model, whereas the RegCM stand-alone model underestimates the precipitation intensities. It is argued that the atmosphere–ocean coupling helps to increase the predictability skill of the Indian monsoon rainfall. For example, the limit of predictability of the Indian monsoon rainfall obtained from the coupled model (22 days) is closer to that obtained from the IMD observation (24 days). The corresponding limit of predictability from the stand-alone model is 20 days.

Influence of subtropical circulation systems on the changing El Niño-Indian summer monsoon relationship

Remote factors have a significant influence on the variability of Indian summer monsoon rainfall. The study investigates atmospheric conditions influenced by the El Niño episodes on the variable summer monsoon over India. Four composites of years were selected based on the occurrence / non-occurrence of El Niño and extreme monsoons over India. This study examines the upper tropospheric and lower tropospheric wind patterns, tropospheric temperature, and velocity potential for 1958 to 2019 using Japanese Reanalysis 55 (JRA 55) data. The regional meridional circulation established by the north-south pressure gradient is more susceptible to the monsoon condition over the Indian Subcontinent than the external factors. The spatial variability and intensity of Australian High in the southern hemisphere play a prominent role in the Indian summer monsoon characteristics. Subtropical circulations in both hemispheres influence monsoon conditions over India irrespective of El Niño condition. The tropical zonal circulation has appeared to be modulated by the El Niño Southern Oscillation (ENSO) condition over the tropical Pacific Ocean, thereby modify the monsoon circulation.

Impact of the tropical Pacific SST biases on the simulation and prediction of Indian summer monsoon rainfall in CFSv2, ECMWF-System4, and NMME models

Present study analyses the role of tropical Pacific sea surface temperature (SST) biases in simulating the mean and inter-annual variability of Indian summer monsoon rainfall (ISMR) using the hindcasts from Monsoon Mission CFSv2 (MMCFS) and ECMWF-SYSTEM4 (ES4) prediction systems. ES4 simulates the mean and annual cycle of ISMR better than MMCFS when initialized with February initial conditions (3-month lead, Feb IC). At the same time, interannual variability (IAV) and skill (correlation between the ensemble mean hindcast and observations) are the highest (least) for MMCFS (ES4). May IC (0-month lead) hindcasts of both the models have similar mean, annual cycle, IAV and skill. Further analysis shows that ES4_Feb IC hindcasts exhibit a strong cold bias (~ 2 °C) in the equatorial central Pacific, which is close to zero for MMCFS_Feb IC. Meanwhile, the 0-month lead (May IC) hindcast has a similar cold bias (~ 1.5 °C) in the equatorial Pacific for both the model hindcasts. Thus, model hindcasts with a better mean ISMR are having a strong cold bias in the equatorial Pacific and that having very less SST bias has higher IAV and skill for ISMR simulations. Additional investigation using North American National Multi-Model Ensemble Project (NMME) models is carried out and the following conclusion on the different roles of Pacific mean state bias is drawn. Hindcasts with strong cold SST bias in the tropical Pacific (ES4 hindcast and May IC hindcasts of MMCFS) tend to mimic the teleconnections associated with La Nina conditions reducing the dry bias over India, resulting in mean ISMR closer to the observed value, still less than the observed mean. At the same time, due to the same strong cold bias, the El Nino-Southern Oscillation (ENSO) induced rainfall and circulation pattern in the Pacific are weak and extended further northwestward. This weakens the ENSO induced heat sources over the Indian Ocean and monsoon region, resulting in reduced ENSO-ISMR teleconnections. These factors result in a poor simulation of interannual variability and skill of ISMR. On the other hand, model hindcasts with less cold bias in the equatorial Pacific (MMCFS_Feb IC) suffer from strong dry bias over India; however, the skill of ISMR is higher as a result of strong ENSO-Monsoon teleconnections. Thus the study confirms the differential role of SST bias in the tropical Pacific in simulating mean and IAV of ISMR in seasonal prediction models. A close to observed mean SST in the tropical Pacific and proper ENSO-Monsoon teleconnection is essential for the better skill of ISMR in the present generation seasonal prediction models.

Summer Monsoon Rainfall Variations and its Association with Atmospheric Circulations Over Sudan

Summer Monsoon Rainfall Variations and its Association with Atmospheric Circulations Over Sudan

Atmospheric moisture as a proxy for the ISMR variability and associated extreme weather events

This study explores the potential of atmospheric moisture content, its transport and its divergence over the ocean and land as proxies for the variability of Indian summer monsoon rainfall (ISMR) for the period 1950–2019. The analyses using multiple linear regression reveal that the interannual and intraseasonal variability of ISMR and the mean ISMR is largely controlled by Arabian Sea moisture flux and Ganga river basin moisture content, and these parameters exhibit statistically significant high correlations in most regions. The regression model and the parameters are statistically significant and the model could explain rainfall variability of about 12%–50% in various regions. The model shows a false alarm rate (FAR) of 0.25–0.45 and a probability of detection (POD) of 0.43–0.50 for wet years in West Central, North West and North Central India. The FAR and POD are about 0.06–0.32 and 0.60–0.70, respectively for dry years in those regions. The model reproduces flood and drought years of about 32%–50% and 55%–70% in those regions. Also, the moisture indices could clearly identify the majority of wet and dry years that occurred during the period. The ISMR variability associated with moisture indices is unaffected by El Niño Southern Oscillation. Henceforth, this study demonstrates the significance of atmospheric moisture on regional rainfall distribution and suggests that these parameters can be used in both statistical and dynamical models to better predict monsoon and global precipitation.

The synergistic impact of ENSO and IOD on Indian summer monsoon rainfall in observations and climate simulations – an information theory perspective

Abstract. The El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are two well-known temporal oscillations in sea surface temperature (SST), which are both thought to influence the interannual variability of Indian summer monsoon rainfall (ISMR). Until now, there has been no measure to assess the simultaneous information exchange (IE) from both ENSO and IOD to ISMR. This study explores the information exchange from two source variables (ENSO and IOD) to one target (ISMR). First, in order to illustrate the concepts and quantification of two-source IE to a target, we use idealized test cases consisting of linear and nonlinear dynamical systems. Our results show that these systems exhibit net synergy (i.e., the combined influence of two sources on a target is greater than the sum of their individual contributions), even with uncorrelated sources in both the linear and nonlinear systems. We test IE quantification with various estimators (linear, kernel, and Kraskov estimators) for robustness. Next, the two-source IE from ENSO and IOD to ISMR is investigated in observations, reanalysis, three global climate model (GCM) simulations, and three nested higher-resolution simulations using a regional climate model (RCM). This (1) quantifies IE from ENSO and IOD to ISMR in the natural system and (2) applies IE in the evaluation of the GCM and RCM simulations. The results show that both ENSO and IOD contribute to ISMR interannual variability. Interestingly, significant net synergy is noted in the central parts of the Indian subcontinent, which is India's monsoon core region. This indicates that both ENSO and IOD are synergistic predictors in the monsoon core region. But, they share significant net redundant information in the southern part of the Indian subcontinent. The IE patterns in the GCM simulations differ substantially from the patterns derived from observations and reanalyses. Only one nested RCM simulation IE pattern adds value to the corresponding GCM simulation pattern. Only in this case does the GCM simulation show realistic SST patterns and moisture transport during the various ENSO and IOD phases. This confirms, once again, the importance of the choice of GCM in driving a higher-resolution RCM. This study shows that two-source IE is a useful metric that helps in better understanding the climate system and in process-oriented climate model evaluation.

Influence of Madden–Julian Oscillation on the Intraseasonal Variability of Summer and Winter Monsoon Rainfall in the Philippines

AbstractThe present study investigates the influence of the Madden–Julian oscillation (MJO) on the intraseasonal variability of summer and winter monsoon rainfall in the Philippines. The monsoon wind–terrain interaction that gives rise to rainfall in the Philippines leads to a sharp contrast between west and east coast rainfall patterns during summer and winter monsoon seasons, respectively. During the summer monsoon season, the eastward-propagating MJO modulates tropical cyclone (TC) activity in the western North Pacific (WNP) on MJO phases 5–7. TC rainfall and remote TC precipitation due to Rossby wave response to TC heating induces strong southwesterly flow that enhances moisture transport and brings anomalous rainfall to the western region of the Philippines. On the other hand, during the winter season, an induced Matsuno–Gill-type pattern to the southeast of the Philippines influences the strength of the Asian winter monsoon. Active (inactive) convection in the tropical Maritime Continent and western Pacific during MJO phases 4–6 produces the anomalous Philippine Sea cyclonic (anticyclonic) circulation. The cyclonic (anticyclonic) anomaly leads to an enhanced (weakened) northeast monsoon flow and wet (dry) rainfall anomaly along the eastern Philippines.

Connection between the South and East Asian Monsoons: Comparing Summer Monsoon Rainfall of Pakistan and South Korea

This study investigates the tele-connection of the southeast Asian monsoon systems by comparing the summer monsoon (June to September) rainfall variability between Pakistan and south Korea. The daily data sets (19812014) of rainfall of Pakistan and south Korea are utilized to explore the possible link. The data products of the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) were also used for the understanding of the large-scale atmospheric environments. The patterns of summer monsoon rainfall on a daily basis between Pakistan and south Korea followed to each other throughout the year. Sub-seasonal differences of the summer monsoon revealed that July is the wettest month in both countries. The large-scale atmospheric environment of higher geopotential height revealed that the Tibetan high and the western north Pacific subtropical high are showing positive anomalies during positive phases over south Asia and east Asia, respectively. The anomalies of zonal wind are negative during positive phase and adverse in the negative phase between 20-40oN. The reduced westerly is interpreted as the seasonal variation and moving of jet streams from the east Asian route. The Tibetan high, northwestern Pacific subtropical high and the east Asian jet stream have reliable and sufficient linkage between the Pakistan and south Korea summer monsoon system.

Asynchronous 500-year summer monsoon rainfall cycles between Northeast and Central China during the Holocene

Although the summer monsoon is the dominant source of precipitation in the East Asian monsoon region (EAMR), there is long-standing uncertainty about the spatial and temporal variability of precipitation on the centennial time scale. In this study we analyzed two well-dated, high-resolution, published paleoclimatic records in order to determine the variability of precipitation in the EAMR during the Holocene. One record is a decadal-resolution pollen record from an annually-laminated Maar lake in Northeast (NE) China, and the other is a U Th dated, multi-decadal-resolution speleothem record from Central China. Advanced time-series analysis of the two records provides unambiguous evidence for asynchronous summer monsoon rainfall variability on centennial to millennial time scales in the EAMR. The pollen and speleothem records indicate long-term and ~ 500-yr quasi-periodic changes in humid/dry climatic shifts against the background of an insolation-forced climatic trend throughout the past 8580 years. Phases of wetter climate (during ~7000–5500 cal yr BP) and drier climate (during ~1400–300 cal yr BP) in NE China are closely related to different climatic phases in Central China. However, the ~500-yr cyclic rainfall changes in Central China lag those in NE China by several decades to almost two centuries. During the ~500-yr periodic changes in the monsoon, a climatic pattern encapsulated as “Northern drought, southern flooding” prevailed during an El Niño-like phase, whereas the opposite pattern “Southern drought, northern flooding” occurred during a La Niña-like phase. The asynchronous pattern of ~500-yr climatic cyclicity between the northern and southern EAMR may have been caused by the El Niño-Southern Oscillation, linked to low-latitude oceanic and atmospheric interactions. • Holocene rainfall changes between Northeast and Central China are asynchronous. • 500-year cyclic rainfall changes in Central China lag those in Northeast China. • The lag of the 500-year rainfall cycles is several decades to almost two centuries. • Rainfall changes in Northeast and Central China are anti-phased on the millennial scale.

Four-dimensional structure and sub-seasonal regulation of the Indian summer monsoon multi-decadal mode

A better understanding of the drivers and teleconnection mechanisms responsible for the multi decadal mode (MDM) of variability of the Indian summer monsoon rainfall (ISMR) with major socio-economic impacts in the region through clustering of large-scale floods or droughts is key to improving the poor simulation of ISMR MDM by most climate models. Here, using the longest instrumental record of ISMR available (1813–2006) and longest atmospheric and oceanic re-analyses, the global four dimensional (space–time) structures of atmospheric and oceanic fields of the multi-decadal mode of ISMR and sub-seasonal evolution of the teleconnection mechanism are brought out, essential for understanding underlying drivers but lacking so far. The relationships between the spatial structure of winds, Sea Surface Temperature (SST) and thermocline depth with the ISMR MDM indicate that the tropical ocean over the Indo-Pacific domain is passive responding primarily to the surface winds associated with the mode. A close association between the Atlantic Meridional Overturning Circulation (AMOC), north Atlantic (NA) SST, NA sea surface salinity (SSS) and the ISMR MDM indicate a slow oceanic pathway linking NA SST and the ISMR. In addition to strong correlation (~ 0.9) between global spatial patterns of JJAS SST associated with the MDMs of ISMR, NA SST and AMOC, strong temporal coherence (correlations ~ 0.9) between them is suggestive of regulation of the ISMR MDM (T ~ 65-years) by the NA SST associated with the Atlantic Multidecadal Oscillation (AMO) through a ‘fast’ atmospheric bridge. On a seasonal time scale, the atmospheric bridge manifests in the form of a stationary Rossby wave train generated by an anticyclonic (cyclonic) barotropic vorticity located above positive (negative) SST anomaly over NA in two phases of the AMO. That the AMO SST is the driver of the ISMR MDM is further supported when we unravel the sub-seasonal face of the teleconnection between the two. We show that phase locking of active (break) spells with annual cycle during positive (negative) phases of the ISMR MDM are forced by a similar phase locking of barotropic anticyclonic (cyclonic) vorticity over the NA SST with the annual cycle through the generation of a quasi-stationary Rossby wave train with an anticyclonic (cyclonic) vorticity at upper level over the Indian region with the NA columnar vorticity leading Indian monsoon rainfall by about a week. Our findings provide a basis for enhanced predictability of tropical climate through slow modulation by extra-tropical SST.

Holocene East Asian Summer Monsoon Rainfall Variability in Taiwan

Characterizing the spatiotemporal variability of the East Asian summer monsoon (EASM) advances our understanding of its rhythm, dynamics, and future impacts. EASM variations during the Holocene have been reconstructed from a variety of geological archives and proxies. However, the spatiotemporal heterogeneity of EASM rainfall during the Holocene remains controversial. Taiwan is geographically suitable for studying the EASM history through its geological archives. Herein, we synthesize the reported lake and peat sedimentary records of the entire Holocene, in addition to the records of mass-wasting and on-land deposition from cores collected from Taiwan to illustrate the EASM induced hydroclimate changes in Taiwan throughout the Holocene. Records from Taiwan indicate that the EASM rainfall maximum occurred during the early Holocene, concurring with other EASM records from monsoon region of southern China. We suggest that the early Holocene EASM rainfall maximum in southern China was mainly forced by the higher Northern Hemisphere summer insolation and sea surface temperatures in the Western Pacific Warm Pool (WPWP). A synthesis of EASM rainfall records from across China shows that the timing of Holocene EASM rainfall maximum occurred progressively later from southern to northern China. This time-transgressive EASM rainfall maximum may be due to the latitudinal shift of the westerlies and Western Pacific subtropical high (WPSH) that induced by changes of interhemispheric temperature gradients (ΔTN‐S) and northern high latitude ice volume. Moreover, records from Taiwan suggest a significant collapse of the EASM in Taiwan at ~4–2 ka BP. Based on the records from Taiwan, coastal East Asian and Tropical Pacific, we propose that the sea surface temperatures (SSTs) of the WPWP and/or El Niño -Southern Oscillation (ENSO) activity may have exerted a strong influence on the EASM rainfall changes during the late Holocene. Moreover, increased EASM rainfall in southern China during the last 2 ka was likely caused by a southward shift of WPSH which was associated with gradual decreases in ΔTN‐S during the late Holocene.

Changes in ENSO-monsoon relations from early to recent decades during onset, peak and withdrawal phases of Indian summer monsoon

The El Nino Southern Oscillation (ENSO) is an important coupled ocean–atmosphere phenomenon in the tropical Pacific and an important modulator of the spatio-temporal variability of Indian summer monsoon rainfall (ISMR). Here, we explore the impact of ENSO during onset (June), peak (July–August) and withdrawal (September) phases of ISMR for the period 1951–2015, by studying the changes in the ENSO-monsoon relationship from early decades (1951–1980) to recent decades (1986–2015). We observe noticeable changes in the ENSO-monsoon relationship from the early to recent decades during all the three phases. During El Nino events, rainfall over most of the Indian regions is significantly increased in recent decades during onset phase, but a significant decrease in rainfall is observed during peak and withdrawal phases. On the other hand, the rainfall during La Nina events is significantly decreased over the monsoon core zone of India during all the three phases. A significant increase (decrease) in sea surface temperature (SST) is observed over the central equatorial Pacific and Indian Ocean during El Nino (La Nina) events in recent decades. These changes differ from one phase to another and related to the observed rainfall patterns over the Indian region. Apart from these, the changes in the anomalies of low level circulation and Walker circulation over the Indo-Pacific domain are also linked to the changes in rainfall during the ENSO events, with most significant relationship during the onset phase.

Characteristics of southwest summer monsoon rainfall events over East India

The variability in Indian summer monsoon rainfall (ISMR) has an immense socioeconomic impact on India, especially over the monsoon core region. In this study, a thorough analysis of the ISMR (June to September—JJAS) has been performed for East India (EI), a part of the monsoon core zone, and for its four subdivisions (Odisha, Chhattisgarh, Jharkhand, and Bihar). The long-term climatology, variability, and trends of various categorical rainfall events as defined by India Meteorological Department (IMD) have been analyzed by using IMD high-resolution (0.25° × 0.25°) gridded dataset for 116 years (1901–2016). Also, the percentage contribution of various rainfall events to the total seasonal rainfall and wet days is estimated. To get the changes in the frequency of various dry spells, rainfall events, and associated seasonal rainfall, study period is divided into two halves, earlier period (P1: 1901–1958) and recent period (P2: 1959–2016). The results suggest that the percentage contribution of medium-intensity rainfall events (7.6–64.4 mm/day) to the total ISMR is higher (~ 72%) followed by high-intensity rainfall events (~ 15%) and low-intensity rainfall events (12%), while most percentage contribution to the total number of wet days is from low-intensity rainfall events (~ 54%) followed by medium-intensity rainfall events (~ 43%) and high-intensity rainfall events (~ 3%). In the recent period (P2), the ISMR significantly decreased over the central part of EI because of the significant decrease in the frequency of low and medium-intensity rainfall events and wet days. The remarkable decrease of wet days leads to an increase in the short (1 week) and long-duration (> 2 week) dry spells in P2. And at the same time, high-intensity rainfall events significantly increased over EI in the recent period (P2) and led to an increase in the flood situation over this region. The profound relation of deficit rainfall of EI with ENSO is getting weak in the P2. This study is certainly useful in determining the effects on various sectors because of the variability and changes in various categorical extreme rainfall events, short and long-duration dry spells during summer monsoon time over EI. Also, this study may assist the risk management sectors in acclimating advanced technologies for a sustainable future in the changing climate of the present global warming era.

Amplification of synoptic to annual variability of West African summer monsoon rainfall under global warming

Increased knowledge of future changes in rainfall variability is needed to reduce vulnerability to potential impacts of global warming, especially in highly vulnerable regions like West Africa. While changes in mean and extreme rainfall have been studied extensively, rainfall variability has received less attention, despite its importance. In this study, future changes in West African summer monsoon (WASM) rainfall variability were investigated using data from two regional climate models that participated in the Coordinated Regional Climate Downscaling Experiment (CORDEX). The daily rainfall data were band-pass filtered to isolate variability at a wide range of timescales. Under global warming, WASM rainfall variability is projected to increase by about 10–28% over the entire region and is remarkably robust over a wide range of timescales. We found that changes in mean rainfall significantly explain the majority of intermodel spread in projected WASM rainfall variability. The role of increased atmospheric moisture is examined by estimating the change due to an idealized local thermodynamic enhancement. Analysis reveals that increased atmospheric moisture with respect to warming following the Clausius–Clapeyron relationship can explain the majority of the projected changes in rainfall variability at all timescales.

Decadal variability of tropical Indian Ocean sea surface temperature and its impact on the Indian summer monsoon

Decadal variability of climate over the Pacific and Atlantic Ocean is well studied. However, the decadal climate variability over the Indian Ocean and its possible impact on the summer monsoon received relatively less attention. The present study aims to explore the decadal variability of the Tropical Indian Ocean (TIO) sea surface temperature (SST) and its associations with the Indian summer monsoon rainfall (ISMR) variability. More than a hundred years of observed monthly SST data from Extended Reconstructed sea surface temperature and rainfall data from India Meteorological Department are used for the analysis. In addition to these, century reanalysis fields of winds, moisture, vertical velocity, tropospheric temperature, and sea level pressure are used for diagnosing different processes. Time series and wavelet analysis confirmed the presence of decadal variability (~ 9 to 30 years) in the TIO SST. The decadal variance of TIO SST is maximum in the eastern equatorial Indian Ocean, followed by the north Arabian Sea. Decadal EOF of TIO SST shows a dominant basin-wide mode explaining about 50% of total variance and has robust decadal variability during 1940 to 1952; wavelet analysis supported this robust signal statistically. Similar analysis for the ISMR reveals that the decadal variance of rainfall has significant strength over monsoon core zone and Western Ghats. The EOF analysis further confirms this spatial pattern of rainfall decadal variability over India. Correlation between of decadal TIO SST and over monsoon core zone (MCZ) rainfall is significant with 2 years lag. To understand how the decadal variability of TIO SST influences the ISMR, monsoon features during strong warm and cold phase are studied. During the warm phase, MCZ and Western Ghats receive more rain than normal and vice versa for cold phase. Which is consistent with strong southwesterly winds, strong pressure gradient, and strong convergence over the MCZ for the warm phase. Also, during the warm phase, positive anomaly of mid-troposphere temperature, vertical velocity, and moist static energy are found to be associated with excess convective activity. Apart from this, larger scale zonal (Walker) and meridional (Hadley) circulation fields are also in phase with the TIO SST and rainfall variability. Our study advocates that decadal variability in TIO SST influences the monsoon dynamics and moist thermodynamics leading to near in-phase changes in the rainfall over the MCZ and Western Ghats region.

Swapping of the Pacific and Atlantic Niño influences on north central India summer monsoon

The highly populated north central India receives 90% of annual rainfall during June to September. The interannual variation of summer monsoon rainfall is less studied compared to central and western India, due to its weak signal with the El-Nino-Southern Oscillation (ENSO). Previous studies have reported a marked decadal variation in the ENSO influences on north India rainfall, but the teleconnections of this variation are not satisfactorily understood. A pathway of the changing ENSO influences on north central India rainfall is revealed from observational data analysis and numerical experiments. While La Nina-like conditions produce anomalous northeasterly wind over India and reduce the tropospheric wind shear, the emergence of the Atlantic Nino appears to overtake this ENSO influence. The Atlantic Nino intensifies the meridional stationary wave affecting pressure anomaly over northwest Europe. This excites the Eurasian Rossby wave train along the mid-latitude producing upper-troposphere high pressure anomaly, subsequently affecting north India. Future work should examine the extent to which these teleconnections are represented in climate forecast models to aid the seasonal prediction of north central India rainfall.

Understanding the Variability of West African Summer Monsoon Rainfall: Contrasting Tropospheric Features and Monsoon Index

West African Summer Monsoon (WASM) rainfall exhibits large variability at interannual and decadal timescales, causing droughts and floods in many years. Therefore it is important to investigate the major tropospheric features controlling the WASM rainfall and explore its potential to develop an objective monsoon index. In this study, monthly mean reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and monthly rainfall data from three gridded observations during the 65-year period of 1950–2014 were employed. Dry and wet rainfall years were identified using a standardized precipitation index. In a composite analysis of wet and dry years, the dynamical features controlling the WASM exhibit an obvious contrast between these years, and a weaker (stronger) African Easterly Jet (Tropical Easterly Jet) is observed during the wet years. Also, a well-developed and deep low-level westerly flow at about 850 hPa is evident in wet years while an obvious reversal is observed in dry years. Considering this, the main regions of the two easterly jet streams and low-level westerly wind are proposed for objectively defining an effective WASM index (WASMI). The results indicate that the WASMI defined herein can reflect variations in June–September rainfall over West Africa. The index exhibits most of the variabilities observed in the rainfall series, with high (low) index values occurring in the 1950–1960s (1970–1980s), suggesting that the WASMI is skilled in capturing the respective wet and dry rainfall episodes over the region. Also, the WASMI is significantly correlated (r = 0.8) with summer monsoon rainfall, which further affirms that it can indicate not only variability but also the intensity of WASM rainfall.

Intraseasonal variability of summer monsoon rainfall over the lower reaches of the Yangtze River basin

This work investigates the boreal-summer intraseasonal variability (ISV) of the precipitation over the lower reaches of the Yangtze River basin (LYRB) during 1979–2016, based on daily Climate Predi...

Impact of vertical structure of the atmosphere on the variability in summer monsoon rainfall over Gangetic West Bengal, India

The influence of temperature (T), potential temperature (⊖), vertical velocity (ω), specific humidity (q), geopotential height (hgt), zonal wind (ZW), total cloud cover (TCC), momentum flux (mf), latent heat flux (lhf), tropopause height, and kinetic energy of wind (KE) at various pressure levels from 1000 to 10 hPa on the variability in summer monsoon rainfall (SMR) over Gangetic West Bengal (GWB) (21.5° N–24.5° N; 86° E–89° E), India is explored in this research with NCEP reanalysis data set during the period from 1986 to 2016. The said period has been divided into three categories: excess rainfall years (ERY), normal rainfall years (NRY), and deficit rainfall years (DRY) based on the percentage rainfall anomaly over GWB. The principal component analysis (PCA) has been implemented with the evaluation of eigenvectors and correlation coefficients to extract the significant parameters from the original data set. The result shows slight decrease in rainfall and its anomaly over GWB during the summer monsoon (SM) season (JJAS) from 1986 to 2016, which is statistically insignificant. However, the rainfall is observed to be maximum for all the categories in the month of July with decreasing trend until September, whereas the rainfall is minimum in the month of June. The study shows slight cooling in tropospheric temperature than the climatology during ERY compared with NRY and DRY. It is observed that the availability of tropospheric moisture is more during ERY and is much less during DRY. The descending motion during DRY is found to lead subsidence and thus increase in surface temperature. The weak upper level wind during DRY leads to the reduction in moisture advection. The geopotential height is observed to descend during DRY making the level colder, whereas it goes up for NRY and ERY generating warmth in the upper level. The cold anomaly at upper level pulls down the tropopause height during DRY, whereas the warm anomaly during ERY and NRY pushes it up. The total cloud cover anomaly is observed to be positive during ERY and NRY with higher value for ERY and negative anomaly during DRY.