Variability Of Monsoon Rainfall Research Articles

Consistent increase in Indian monsoon rainfall and its variability across CMIP-5 models

Abstract. The possibility of an impact of global warming on the Indian monsoon is of critical importance for the large population of this region. Future projections within the Coupled Model Intercomparison Project Phase 3 (CMIP-3) showed a wide range of trends with varying magnitude and sign across models. Here the Indian summer monsoon rainfall is evaluated in 20 CMIP-5 models for the period 1850 to 2100. In the new generation of climate models, a consistent increase in seasonal mean rainfall during the summer monsoon periods arises. All models simulate stronger seasonal mean rainfall in the future compared to the historic period under the strongest warming scenario RCP-8.5. Increase in seasonal mean rainfall is the largest for the RCP-8.5 scenario compared to other RCPs. Most of the models show a northward shift in monsoon circulation by the end of the 21st century compared to the historic period under the RCP-8.5 scenario. The interannual variability of the Indian monsoon rainfall also shows a consistent positive trend under unabated global warming. Since both the long-term increase in monsoon rainfall as well as the increase in interannual variability in the future is robust across a wide range of models, some confidence can be attributed to these projected trends.

Can West African monsoon rainfall be predicted on decadal time scales?

The economies in much of West Africa are based on agriculture, which is strongly affected by rainfall variability. Thus, predictions of variations in West African monsoon rainfall could be valuable to people in those regions.

Decadal scale oscillations and trend in the Indian monsoon rainfall

The emerging need for extended climate prediction requires a consideration of the relative roles of climate change and low-frequency natural variability on decadal scale. Addressing this issue, this study has shown that the variability of the Indian monsoon rainfall (IMR) consists of three decadal scale oscillations and a nonlinear trend during 1901–2004. The space–time structures of the decadal oscillations are described. The IMR decadal oscillations are shown to be associated with Atlantic Multidecadal Oscillation (AMO), Atlantic tripole oscillation and Pacific Decadal Oscillation (PDO). The sea surface temperatures (SSTs) of the North Pacific and North Atlantic Oceans are also resolved as nonlinear decadal oscillations. The SST AMO mode has high positive correlation with IMR while the SST tripole mode and SST PDO have negative correlation. The trend in IMR increases during the first half of the period and decreases during the second half. The IMR trend is modified when combined with the three decadal oscillations.

Documentary reconstruction of monsoon rainfall variability over western India, 1781–1860

Investigations into the climatic forcings that affect the long-term variability of the Indian summer monsoon are constrained by a lack of reliable rainfall data prior to the late nineteenth century. Extensive qualitative and quantitative meteorological information for the pre-instrumental period exists within historical documents, although these materials have been largely unexplored. This paper presents the first reconstruction of monsoon variability using documentary sources, focussing on western India for the period 1781–1860. Three separate reconstructions are generated, for (1) Mumbai, (2) Pune and (3) the area of Gujarat bordering the Gulf of Khambat. A composite chronology is then produced from the three reconstructions, termed the Western India Monsoon Rainfall reconstruction (WIMR). The WIMR exhibits four periods of generally deficient monsoon rainfall (1780–1785, 1799–1806, 1830–1838 and 1845–1857) and three of above-normal rainfall (1788–1794, 1813–1828 and 1839–1844). The WIMR shows good correspondence with a dendroclimatic drought reconstruction for Kerala, although agreement with the western Indian portion of the tree-ring derived Monsoon Asia Drought Atlas is less strong. The reconstruction is used to examine the long-term relationship between the El Nino-Southern Oscillation (ENSO) and monsoon rainfall over western India. This exhibits peaks and troughs in correlation over time, suggesting a regular long-term fluctuation. This may be an internal oscillation in the ENSO-monsoon system or may be related to volcanic aerosol forcings. Further reconstructions of monsoon rainfall are necessary to validate this. The study highlights uncertainties in existing published rainfall records for 1817–1846 for western India.

A common mode of variability of African and Indian monsoon rainfall at decadal timescale

There are many factors and mechanisms capable of influencing and perturbing rainfall in both African and Indian monsoon regions. Using observed data and ensembles of Atmospheric General Circulation Model simulations, evidence is presented that an association between the two systems exists on decadal timescales and the mechanism responsible for this common mode is suggested. Decadal variability of rainfall in the two monsoon systems results from a large scale forcing induced by an interplay of different ocean basins. The emerging pattern is characterized by warmer (cooler) equatorial and cooler (warmer) extratropical regions, more visible in the northern hemisphere. This large scale forcing pattern leads to an upper-level pressure gradient between the equator and the monsoon regions which modifies also the Tropical Easterly Jet, thus providing a potential link between the African and Indian monsoon. The response is baroclinic, therefore at low levels, the pressure gradient reverses and leads to increased (reduced) pressure over the Saharan and Indian region, both being favourable for a weakening (strengthening) of the respective monsoons. Therefore, the predictability of the monsoon trends depends mainly on how well the sea surface temperature modes, which modulate the monsoons variability, can be predicted.

Intraseasonal variability of terrestrial biospheric CO2 fluxes over India during summer monsoons.

AbstractThe intraseasonal oscillations (ISOs) in terrestrial biospheric fluxes of carbondioxide (CO2) over the Indian subcontinent were investigated for the summer monsoon season from June to September. We utilized two optimized datasets of Net Ecosystem Exchange (NEE) fluxes of CO2 at a spatial resolution of 1° × 1° grid and at daily time scale for the years 2000–2009. Seasonally, over the whole of Indian subcontinent, terrestrial biospheric CO2 fluxes were found to be a net source (sink) during June and July (August and September). Intraseasonal variability of CO2 fluxes for the two distinct time scales, 30–60 days and 10–20 days, was extracted with a spectral harmonic filter. The dominant ISO mode in the CO2 flux over India is at a period of 60 days or longer during weak monsoons years but at 10–30 days for strong monsoon years. The ISOs of CO2 flux show coherent structures along with corresponding rainfall ISOs at a 2–3 day lag (CO2 lags rainfall) and nearly 3–4 day lag with ISOs in surface air‐temperature (CO2 lags air‐temperature). The ranges of these lags are consistent in the two data products examined here. The apparent lags between CO2 flux and rainfall ISOs are found to be induced by the temperature effects on net primary production (NPP) and ecosystem respiration (RE). The terrestrial biospheric fluxes over the subcontinent are coherent with the northward propagating summer monsoon ISOs albeit as a combination of rainfall, available radiation, and air‐temperature. The study offers a mechanistic understanding of variability of terrestrial biospheric sources and sinks of CO2 over the Indian subcontinent, in tandem with the intraseasonal variability of the summer monsoon rainfall.

Global Warming Shifts the Monsoon Circulation, Drying South Asia

Abstract Monsoon rainfall over South Asia has decreased during the last 5 to 6 decades according to several sets of observations. Although sea surface temperature (SST) has risen across the Indo-Pacific warm pool during this period, the expected accompanying increased rainfall has occurred only in the tropical western Pacific. The above changes noted in observations are also seen in a coupled climate model, but only when the model includes the recent increase in greenhouse gas concentration. The hypothesis that the robust rise in SST over the warm pool, perhaps anchored by an increase in greenhouse gas concentrations, is instrumental in the east–west shift in monsoon rainfall (enhanced rainfall over tropical western Pacific and decreased rainfall over South Asia) is proposed. A suite of controlled experiments with an atmospheric general circulation model has been performed to isolate the impact of regional SST warming trends on the dryness over South Asia. Model experiments support the hypothesis that the rising SST trend over the tropical western Pacific has changed the atmospheric circulation: over the Bay of Bengal more dry and cool air is advected from the northeast than previously. Moist static energy budget diagnostics on the model solutions identify the sources for this east–west shift. SST warming over the warm pool has accelerated in recent decades. Therefore, a close monitoring of that warming is important for long-term variations of monsoon rainfall. The inconsistency in the amplitude of drying over South Asia among the various land-based rainfall observations and lack of sustained rainfall observations over the open oceans, however, poses constraints in the results.

Latitudinal precipitation characteristics and trends in Pakistan

Precipitation variability has direct and serious impacts on society. In this study, the latitudinal precipitation characteristics and trends in Pakistan have been examined to identify vulnerable areas. Nonparametric statistical method was used to detect trends in observational data collected over a 60-year period from 48 rain gauges. The results reveal increasing precipitation trends over high latitudes, and no significant trends over lower latitudes. The spatial display of the observed trends is also shown to understand the significant variation and shift of monsoon rainfall in the specific areas. This study not only provides important information on precipitation variability in Pakistan, but is also helpful for policy makers seeking to adopt measures to mitigate the effects of rapidly changing climate in the vulnerable areas.

Future change of the global monsoon revealed from 19 CMIP5 models

The variability of global monsoon area (GMA), global monsoon precipitation (GMP), and global monsoon intensity (GMI) in the present climate (1979–2003) and the future warmer climate (2075–2099) under Representative Concentration Pathways 4.5 (RCP4.5) scenario was examined based on 19 Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations. In the present‐day simulations, the ensemble mean precipitation reproduces the observed GMA, GMP, and GMI, although the spread of individual models is large. In the RCP4.5 simulations, most (17 of 19) of the CMIP5 models project enhanced global monsoon activity, with the increases of GMA, GMP, and GMI by 1.9%, 3.2%, and 1.3%, respectively, per 1 K of surface warming. The diagnosis of a column‐integrated moisture budget indicates that the increase in GMP is primarily attributed to the increases of moisture convergence and surface evaporation, whereas horizontal moisture advection has little effect. A further separation of dynamic and thermodynamic factors shows that increase of the moisture convergence comes mainly from the increase of water vapor, but is partly offset by the convergence effect. The increase of the surface evaporation is caused by the increase of sea‐air specific humidity difference, while the change in surface wind speed plays a minor role. The GMP experiences a great year‐to‐year variation, and it is significantly negatively correlated with the Niño3.4 index averaged over a typical monsoon year (defined from May to the following April) in the pre‐industrial control and present‐day simulations, similar to observations. Under the RCP4.5 warming, such rainfall variability is intensified, and the relationship between monsoon and El Niño strengthens. A large proportion of intensification in the year‐to‐year monsoon rainfall variability arises from the land monsoon region.

Seasonal and intraseasonal variations of East Asian summer monsoon precipitation simulated by a regional air-sea coupled model

The performance of a regional air-sea coupled model, comprising the Regional Integrated Environment Model System (RIEMS) and the Princeton Ocean Model (POM), in simulating the seasonal and intraseasonal variations of East Asian summer monsoon (EASM) rainfall was investigated. Through comparisons of the model results among the coupled model, the uncoupled RIEMS, and observations, the impact of air-sea coupling on simulating the EASM was also evaluated. Results showed that the regional air-sea coupled climate model performed better in simulating the spatial pattern of the precipitation climatology and produced more realistic variations of the EASM rainfall in terms of its amplitude and principal EOF modes. The coupled model also showed greater skill than the uncoupled RIEMS in reproducing the principal features of climatological intraseasonal oscillation (CISO) of EASM rainfall, including its dominant period, intensity, and northward propagation. Further analysis indicated that the improvements in the simulation of the EASM rainfall climatology and its seasonal variation in the coupled model were due to better simulation of the western North Pacific Subtropical High, while the improvements of CISO simulation were owing to the realistic phase relationship between the intraseasonal convection and the underlying SST resulting from the air-sea coupling.

Signatures of Tibetan Plateau heating on Indian summer monsoon rainfall variability

Despite recent challenges, conventional wisdom has held that heating over the Tibetan Plateau leads to increased Indian summer monsoon rainfall via enhancement of cross‐equatorial circulation aloft, and a concurrent strengthening of both the Somali Jet and westerly winds that bring moisture to southern India. We show that such heating, quantified by monthly estimates of moist static energy in the atmosphere just above the surface, correlates with summer monsoon rainfall, but only in the early (20 May to 15 June) and late (September 1 to 15 October) monsoon season. Correlations during the main monsoon season (15 June to 31 August) are small and insignificant. The positive correlations with early and late monsoon season, however, allow for heating over Tibet to modulate as much as ~30% of the total rainfall. Furthermore, we demonstrate that heating over Tibet is independent of the El Niño Southern Oscillation, so that together they explain a substantial portion of variability in the early and late season rainfall, providing potential predictability. These links may also explain the wet conditions over India during early Holocene time and provide a quantitative link between a rise of Tibet and stronger Somali Jet.

Emerging role of Indian ocean on Indian northeast monsoon

This study examines the emerging role of Indian Ocean sea surface temperature (SST) on the inter-annual variability (IAV) of Indian north-east monsoon rainfall (NEMR). The IAV of NEMR is associated with the warm SST anomaly over east Bay-of-Bengal (BoB) (88.5oE–98.5oE; 8.5oN–15.5oN) and cool SST anomaly over east equatorial Indian Ocean (80.5oE–103.5oE; 6.5oS–3.5oN). The gradient of SST between these boxes (i.e. northern box minus southern box) shows strong and robust association with the Indian NEMR variability in the recent decades. For establishing the teleconnections, SST, mean sea level pressure, North Indian Ocean tropical storm track, and circulation data have been used. The study reveals that during the positive SST gradient years, the inter-tropical convergence zone (ITCZ) shifts northwards over the East Indian Ocean. The tropical depressions, storms and cyclones formed in the North Indian Ocean moves more zonally and strike the southern peninsular India and hence excess NEMR. While, during the negative SST gradient years, the ITCZ shifts southwards over the Indian Ocean. The tropical depressions, storms and cyclones formed in the North Indian Ocean moves more northwestward direction and after crossing 15oN latitude re-curve to north-east direction towards head BoB and misses southern peninsular India and hence, deficient NEMR.

Interannual variability of rainfall over the Sahel based on multiple regional climate models simulations

We analyse the interannual variability of the averaged summer monsoon rainfall over the Sahel from multiple regional climate models driven by the ERA-interim reanalysis and seek to provide effective information for future modelling work. We find that the majority of the models are able to reproduce the rainfall variability with correlation coefficient exceeding 0.5 compared with observations. This is due to a good representation of the dynamics of the main monsoon features of the West African climate such as the monsoon flux, African Easterly Jet (AEJ) and Tropical Easterly Jet (TEJ). Among the models, only HIRHAM fails to reproduce the rainfall variability exhibiting hence a correlation coefficient of −0.2. This deficiency originates from the fact that HIRHAM does not properly capture the variability of monsoon flow and the relationship between rainfall and the AEJ dynamic. We conclude that a good performance of a regional climate model in simulating the monsoon dynamical features variability is of primary importance for a better representation of the interannual variability of rainfall over the Sahel.

Effects of intraseasonal variation of summer monsoon rainfall on stable isotope and growth rate of a stalagmite from northwestern Thailand

Effects of intraseasonal variation of summer monsoon rainfall on stable isotope and growth rate of a stalagmite from northwestern Thailand

Diurnal and spatial variation of Indian summer monsoon rainfall using tropical rainfall measuring mission rain rate

Summary This paper deals with the diurnal and spatial variation of summer monsoon (JJAS) rainfall, extremes and its frequency for a period of 12 years using the TRMM (Tropical Rainfall Measuring Mission) rain rate data set. Rainfall and frequency of rain events are more observed over the western coastal belts, northeastern regions, foothills of Himalayas and Bay of Bengal regions. Average seasonal rainfall is about 400 mm from 150 rainy hours. The rainfall amount and frequency of rain events show a peak at 1730 IST over Central India, 0230 IST over the foothills of Himalayas and 0830 IST over the north Bay of Bengal. Rainfall over the west coast is more evenly distributed throughout the day. Diurnal pattern of rain intensity is different than that of the rainfall, it is more over the Gujarat regions and peak is observed at 1730 IST and 0230 IST. The relative contribution of rainfall by each octet towards the seasonal total is different. Contribution of rainfall towards its seasonal is maximum at 1730 IST over the main land and is different over other regions. In addition to this, the diurnal pattern of high intensity (90th percentile and above) and low intensity (10th percentile and below) rainfall during the season were also analyzed to find out their spatial distribution. We found that the diurnal pattern is more prominent for the high intensity rainfall than that for the low rainfall intensity. Maximum rainfall hours show systematic southward propagation from early morning to late night.

Climate change and the South Asian summer monsoon

More than one billion people live in regions affected by the South Asian summer monsoon. This Review provides an overview of our understanding of summer monsoon rainfall variability and its causes, and considers how the monsoon will change as a consequence of global warming.

The influence of Indian Ocean sea surface temperature on the variability of monsoon rainfall over India

This study focuses on the spatial and temporal variability of southwest monsoon (SWM) and northeast monsoon (NEM) rainfall over India, after the 1976–1977 regime shift. Through wavelet analysis the dominant mode of variability in the SWM and NEM rainfall is found to be in the 2–8-year scale (ENSO band). Therefore, scale-averaged wavelet power (SAP) of SWM (SAPSWM) and NEM (SAPNEM) rainfall obtained from a high-resolution dataset for the period 1977–2001 was subjected to empirical orthogonal function analysis, in order to understand the homogenous regions of rainfall variability. As the study utilizes SAP, the decadal variability is observed in the spatial and temporal patterns of rainfall. The maximum variability of SAPSWM rainfall is found to be in the 2–4-year period, whereas, for the SAPNEM rainfall, the biennial mode explains the variability over the core region of NEM rainfall. Significant power is also observed in the 4-year period. The evolution of the principal components (PCs) is consistent with the above/below normal rainfall received over the region. Similar analysis has been carried out also for the SAP of monthly sea surface temperature (SAPSST) over Indian Ocean for the months from January to September. Compared with any other month, the decadal changes of the PCs of SAPSWM and SAPNEM rainfall are highly correlated with the decadal variations of PC1 of SAPSST for the month of March. Therefore, the relation between the rainfall and SST was explained using SAPSST for the month of March. Significant biennial power is also observed in March SST over southeast Indian Ocean (85–110°E and 5–40°S) after the climate shift of 1976. In order to identify any change in the relation between NEM rainfall and SAPSST (March) before and after 1976, a correlation between NEM rainfall and PC1 of SAPSST (March) was found for the period 1951–1975 and 1977–2001. Results show that correlation has increased after 1976. Therefore, this PC can be made use of in the prediction of NEM rainfall. Copyright © 2012 Royal Meteorological Society

Northeast monsoon over India: variability and prediction

AbstractSouth peninsular India experiences a large portion of the annual rainfall during the northeast monsoon season (October to December). In this study, the facets of diurnal, intra‐seasonal and inter‐annual variability of the northeast monsoon rainfall (the NEMR) over India have been examined. The analysis of satellite derived hourly rainfall reveals that there are distinct features of diurnal variation over the land and oceans during the season. Over the land, rainfall peaks during the late afternoon/evening, while over the oceans an early morning peak is observed. The harmonic analysis of hourly data reveals that the amplitude and variance are the largest over south peninsular India. The NEMR also exhibits significant intra‐seasonal variability on a 20–40 day time scale. Analysis also shows significant northward propagation of the maximum cloud zone from south of equator to the south peninsula during the season. The NEMR exhibits large inter‐annual variability with the co‐efficient of variation (CV) of 25%. The positive phases of ENSO and the Indian Ocean Dipole (IOD) are conducive for normal to above normal rainfall activity during the northeast monsoon. There are multi‐decadal variations in the statistical relationship between ENSO and the NEMR. During the period 2001–2010 the statistical relationship between ENSO and the NEMR has significantly weakened. The analysis of seasonal rainfall hindcasts for the period 1960–2005 produced by the state‐of‐the‐art coupled climate models, ENSEMBLES, reveals that the coupled models have very poor skill in predicting the inter‐annual variability of the NEMR. This is mainly due to the inability of the ENSEMBLES models to simulate the positive relationship between ENSO and the NEMR correctly. Copyright © 2012 Royal Meteorological Society

ENSO connection with monsoon rainfall over Bangladesh

This paper investigates the variability and trends of summer monsoon rainfall over Bangladesh and its relation with ENSO (El Nino Southern Oscillation). The data of rainfall for 30 selected rain gauge stations of Bangladesh have been used over a period of 47 years (1961–2007) collected from Bangladesh Meteorological Department (BMD) and ENSO data have been downloaded from the website of Golden Gate Weather Services, Canada. The data are analyzed using different statistical parameters. The major findings of the study are that the mean monthly rainfall of Bangladesh shows a unimodal pattern throughout the year with highest in July (20%) whereas lowest in January (0.36%). Most of the rainfall occurs in the monsoon season (70.69%). However, the pre-monsoon and post-monsoon rainfall accounts 18.94% and 8.94% respectively. Moreover, the winter is very dry and has 1.53% of the annual rainfall. The coefficient of variation is high in the winter and low in the monsoon season. All Bangladesh mean summer monsoon rainfall is 1616 mm (70.6%) where annual country average rainfall is 2286 mm, and standard deviation and coefficient of variance is 208.9mm and 11.76% respectively. Summer monsoon rainfall widely varies over the geographical areas with lowest in central-western part conversely highest in southeastern part with moderate in northeastern part of the country. The short term variability of the scale 2–4 years is highly dominant. The plots using 5-year moving average data smooth out the high frequency oscillations and the variability of longer time scale are better discerned. The decadal scale variation with time period around 10–11 years and 14–16 years are found to be prominent in the filtered series. The higher time scales of around 20 years are also evident in the pre-monsoon and monsoon rainfall. The trend analysis results show that winter has the trend value of 0.56 mm/year against an annual rainfall of 34.9 mm. The relatively high trends of rainfall are obtained from pre-monsoon and post monsoon (2.14 and 2.12 mm/year). In case of ENSO connection, it is found that there have strong impact of ENSO on the monsoon system of subcontinent scale, though the relationship is weak in case of Bangladesh.

Reduced river discharge intensifies phytoplankton bloom in Godavari estuary, India

Changes in river discharge alter material load and the consequent estuarine and coastal biogeochemical process. Evidence for biogeochemical response to variable rainfall over catchment area or changes in river discharges due to dam regulations is sparse. Inter-annual variability in monsoon rainfall and the consequent river discharges, either dam regulated or otherwise, in India are the best suited to test the influence of altered discharges on estuarine biogeochemistry. Our experiments in Godavari river estuary over three years revealed that a decrease in precipitation over the Indian subcontinent from 2007 to 2009 resulted in the lowering of mean annual discharge from 748.63m3s-1 in 2007 to 218.40m3s-1 in 2009. The reduced water discharge, during the peak discharge period, slowed the flushing of the estuary from 1.2days to 6.3days, respectively. The consequent increase in stability of water column and reduced suspended material load gave rise to intense phytoplankton blooms (Chl a 18μgl-1 in 2007 to 28μgl-1 in 2009). Resilience towards the unwanted phytoplankton bloom and overall health of the Indian estuaries is thus tuned by the variability in monsoon rainfall and dam regulated discharge.