Phases Of Indian Summer Monsoon Research Articles

Southern Indian Ocean SST as a modulator for the progression of Indian summer monsoon

This study explores the possibility of southern Indian Ocean (SIO) sea surface temperature (SST) as a modulator for the early phase of Indian summer monsoon and its possible physical mechanism. A dipole-like structure is obtained from the empirical orthogonal function (EOF) analysis which is similar to an Indian Ocean subtropical dipole (IOSD) found earlier. A subtropical dipole index (SDI) is defined based on the SST anomaly over the positive and negative poles. The regression map of rainfall over India in the month of June corresponding to the SDI during 1983–2013 shows negative patterns along the Western Ghats and Central India. However, the regression pattern is insignificant during 1952–1982. The multiple linear regression models and partial correlation analysis also indicate that the SDI acts as a dominant factor to influence the rainfall over India in the month of June during 1983–2013. The similar result is also obtained with the help of composite rainfall over the land points of India in the month of June for positive (negative) SDI events. It is also observed that the positive (negative) SDI delays (early) the onset dates of Indian monsoon over Kerala during the time domain of our study. The study is further extended to identify the physical mechanism of this impact, and it is found that the heating (cooling) in the region covering SDI changes the circulation pattern in the SIO and hence impacts the progression of monsoon in India.

The impact of revised simplified Arakawa‐Schubert scheme on the simulation of mean and diurnal variability associated with active and break phases of Indian summer monsoon using CFSv2

AbstractThe impact of revised simplified Arakawa‐Schubert (RSAS) convective parameterization scheme in Climate Forecast System (CFS) version 2 (CFSv2) on the simulation of active and break phases of Indian summer monsoon (ISM) has been investigated. The results revealed that RSAS showed better fidelity in simulating monsoon features from diurnal to daily scales during active and break periods as compared to SAS simulation. Prominent improvement can be noted in simulating diurnal phase of precipitation in RSAS over central India (CI) and equatorial Indian Ocean (EIO) region during active periods. The spatial distribution of precipitation largely improved in RSAS simulation during active and break episodes. CFSv2 with SAS simulation has noticeable dry bias over CI and wet bias over EIO region which appeared to be largely reduced in RSAS simulation during both phases of the intraseasonal oscillation (ISO). During active periods, RSAS simulates more realistic probability distribution function (PDF) in good agreement with the observation. The relative improvement has been identified in outgoing longwave radiation, monsoon circulations, and vertical velocities in RSAS over SAS simulation. The improvement of rainfall distribution appears to be contributed by proper simulation of convective rainfall in RSAS. CFSv2 with RSAS simulation is able to simulate observed diurnal cycle of rainfall over CI. It correctly reproduces the time of maximum rainfall over CI. It is found that the improved feedback between moisture and convective processes in RSAS may be attributed to its improved simulation. Besides improvement, RSAS could not reproduce proper tropospheric temperature, cloud hydrometeors over ISM domain which shows the scope for future development.

Post-glacial landform evolution in the middle Satluj River valley, India: Implications towards understanding the climate tectonic interactions

Late Quaternary landform evolution in monsoon-dominated middle Satluj valley is reconstructed using the fragmentary records of fluvial terraces, alluvial fans, debris flows, paleo-flood deposits, and epigenetic gorges. Based on detailed field mapping, alluvial stratigraphy, sedimentology and optical chronology, two phases of fluvial aggradations are identified. The older aggradation event dated between ∼13 and 11 ka (early-Holocene), occurred in the pre-existing topography carved by multiple events of erosion and incision. Climatically, the event corresponds to the post-glacial strengthened Indian summer monsoon (ISM). The younger aggradation event dated between ∼5 and 0.4 ka (mid- to late-Holocene), was during the declining phase of ISM. The terrain witnessed high magnitude floods during transitional climate (∼6.5–7 ka). The fluvial sedimentation was punctuated by short-lived debris flows and alluvial fans during the LGM (weak ISM), early to mid-Holocene transition climate and mid- to late-Holocene declining ISM. Based on the terrace morphology, an event of relatively enhanced surface uplift is inferred after late Holocene. The present study suggests that post-glacial landforms in the middle Satluj valley owe their genesis to the interplay between the climate variability and local/regional tectonic interactions.

Variability in AIRS CO2 during active and break phases of Indian summer monsoon

Due to human activities, the atmospheric concentration of Carbon Dioxide (CO2) has been rising extensively since the Industrial Revolution. Indian summer monsoon (ISM) has a dominant westerly component from ocean to land with a strong tendency to ascend and hence may have role in CO2 distribution in lower and middle troposphere over Indian sub-continent. A substantial component of ISM variability arises from the fluctuations on the intra-seasonal scale between active and break phases which correspond to strong and weak monsoon circulation. In view of the above, an attempt is made in this study to examine the AIRS/AQUA satellite retrieved CO2 distribution in response to atmospheric circulation with focus on active and break phase. Correlation analysis indicates the increase in AIRS CO2 linked with strong monsoon circulation. Study also reveals that anomalous circulation pattern during active and break phase show resemblance with high and low values of AIRS CO2. Homogeneous monsoon regions of India show substantial increase in CO2 levels during active phase. Hilly regions of India show strong contrast in CO2 and vertical velocity during active and break phases.

Cloud–aerosol coupled index in estimating the break phase of Indian summer monsoon

The Indian summer monsoon (ISM) is largely influenced by intra-seasonal variability like break and active phases of monsoon. In the present study, different cloud and aerosol parameters are considered and analyzed to formulate a cloud–aerosol coupled index (CACI) that can aid in forecasting the break phase of ISM. The method of principal component analysis is implemented to identify the significant cloud and aerosol parameters during break and active phases of ISM. The threshold ranges of each parameter are evaluated by using the normal probability density function. The result reveals that for break phase, the significant parameters are cloud water path (CWP), cloud optical depth, aerosol index, zonal wind (ZW), and meridional wind (MW) at 850 hPa pressure level whereas for active phase, the parameters found to be important are aerosol optical depth, CWP, ZW, and MW at 850 hPa pressure level. The significantly correlated (p < 0.05) parameters are taken for formulating CACI. The results show that no such significant correlation is possible with the parameters during active phase of ISM. The CACI is thus formulated to forecast the break phase of ISM. The result shows that the CACI is capable of forecasting the break phase of Indian summer monsoon over central India with 88 % certainty and 10 days lead time. The result is validated with the observations.

Moist dynamics of active/break cycle of Indian summer monsoon rainfall from NCEPR2 and MERRA reanalysis

ABSTRACTThe vertically integrated moisture and moist static energy (MSE) budgets during the active and break phases of Indian summer monsoon are studied using National Centers for Environmental Prediction Reanalysis2 (NCEPR2) and Modern Era Retrospective Analysis for Research and Applications (MERRA) data. The different parameterization procedures adopted by the reanalysis models will impose inevitable differences between the reanalysis products, which will make an impact on their intraseasonal oscillation (ISO) also. At the same time, the agreement of the moisture budget terms of the reanalysis products has enabled us to come up with a common moisture mechanism responsible for the ISO. Estimates of the moisture budget indicates that the rainfall anomalies over the Indian monsoon region in both active and break phases are mainly (70–80%) balanced by local moisture divergence. The remaining 20–25% is accounted by the horizontal moisture advection. The moisture advection is found to lead the rainfall by 8–9 d, while there is no lead‐lag relationship is observed between the moisture divergence and rainfall. The anomalous moisture advection from the northwest pacific initiates the active phase of Indian monsoon and the dry air advection from the northwest of India after the onset of active convection sets off the break conditions. MSE budget over India is balanced by MSE divergence and moisture advection, indicating that the variation in MSE is mainly governed by moisture in the intraseasonal timescales. A positive covariance between radiative and surface fluxes and precipitation anomalies indicates that these flux terms slowdown the variation of MSE during active/break phase and may help to extend the ISO. The anomalous moisture advection has a major role in northward propagation of ISO also. A large residual of MSE budget during onset of ISO is the main caveat of the reanalysis products and it is the major constrain in the present analysis also.

Predictability during active break phases of Indian summer monsoon in an ensemble prediction system using climate forecast system

This study examines the phase dependant temporal and spatial error evolution and prediction of active break spells of Indian summer monsoon rainfall in an ensemble prediction system (EPS) on a pentad time scale using climate forecast system (CFS). The EPS system shows systematic wet bias (overestimation) over west coast over the Arabian Sea and Myanmar coast and dry bias (underestimation) over Indian land mass even at pentad 1 lead and these biases consistently increase up to 4 pentad lead and saturate thereafter. Irrespective of the phases of the monsoon, the lower bound of predictability is 2 pentads, while upper bound of predictability for initial conditions starting from active phase saturates at 3 pentads and for break and transition phases predictability error saturates at a later stage at about 5 pentad. Initial conditions started from transition phase shows higher potential predictability followed by break phase and then active phase.

A study on the characteristics of temperature inversions in active and break phases of Indian summer monsoon

The thermal inversion characteristics during active and break cycles of two consecutive and contrasting monsoon years were studied using GPS radiosonde profiles in Goa (15°46′ N; 73°08′ E), located on the west coast of India. The lower tropospheric inversions were associated with the active phase and it dissipated during the break phase due to weakening of the low level jet. A strong secondary inversion also appeared at 5–6km in the break period. The genesis of both the inversions is linked with advection of warm and dry air from the Arabian and Persian high lands. During the active phase, convection overrides the stability shield provided by the advection induced low level inversion. Associated with the active phase, the temperature difference within the inversion invariably reaches above a threshold of 1°C, whereas it is low in break conditions.

Signature of a southern hemisphere extratropical influence on the summer monsoon over India

The weakening relationship of El Nino with Indian summer monsoon reported in recent years is a major issue to be addressed. The altered relationships of Indian monsoon with various parameters excite to search for other dominant modes of variability that can influence the precipitation pattern. Since the Indian summer monsoon circulation originates in the oceanic region of the southern hemisphere, the present study investigates the association of southern extratropical influence on Indian summer monsoon using rainfall and reanalysis parameters. The effect of Southern Annular Mode (SAM) index during the month of June associated with the onset phase of Indian summer monsoon and that during July–August linked with the active phase of the monsoon were analysed separately for a period from 1951 to 2008. The extra-tropical influence over the monsoon is illustrated by using rainfall, specific humidity, vertical velocity, circulation and moisture transport. The June high SAM index enhances the lower level wind flow during the onset phase of monsoon over Indian sub-continent. The area of significant positive correlation between precipitation and SAM in June also shows enhancement in both ascending motion and specific humidity during the strong phase of June SAM. On the other hand, the June high SAM index adversely affects July–August monsoon over Indian subcontinent. The lower level wind flow weakens due to the high SAM. Enhancement of divergence and reduction in moisture transport results in the Indian monsoon region due to the activity of this high southern annular mode. The effect is more pronounced over the southwest region where the precipitation spell has high activity during the period. Significant correlation exists between SAM and ISMR, even after removing the effect of El Nino. It indicates that the signals of Indian summer monsoon characteristics can be envisaged to a certain extend using the June SAM index.

On factors responsible for recent secular trend in the onset phase of monsoon intraseasonal oscillations

AbstractThe timing of the dramatic ‘onset’ and subsequent northward progression of the Indian summer monsoon (ISM) are unique features of the monsoon annual cycle and are strongly linked to planning of agricultural activity and water management. Here, we show that the northward propagation of the first episode of intraseasonal oscillation (ISO) associated with the onset phase of ISM has slowed down significantly during the past decade compared with prior decades. The slowdown is due to systematic weakening of two regional climatic features responsible for northward progression of the tropical convergence zone (TCZ) namely, the vertical easterly wind shear and the north–south gradient of the mean low‐level humidity around the equator. Eastward shift of the Walker circulation associated with increasing strength of El Ninos appears to be responsible for weakening of the vertical wind shear over Indian monsoon region. The slow northward propagation during recent decades leads to a climatological sub‐seasonal oscillation representing a predictable component of the onset phase of the ISM. Copyright © 2009 Royal Meteorological Society

Meteorological conditions associated with very sluggish advance of Indian summer monsoon in 2002

In the year 2002, the monsoon covered entire India on 15th August, one month later than the normal date. An attempt has been done in this study to analyse the meteorological conditions underlying this abnormally sluggish advance of the monsoon in 2002, using the NCEP-NCAR Reanalysis data sets. It has been observed that, there had been an anomalously high influence of systems in the westrelies during the advance phase of Indian Summer Monsoon in 2002. This has proved to be a factor unfavourable for the advance of the monsoon. Similar cases of hiatuses in the advance of monsoon during other recent years of delayed advance, viz., 2004 and 2006, have also been discussed.

Atmospheric circulation during active and break phases of Indian summer monsoon: A study using MST radar at Gadanki (13.5°N, 79.2°E)

The unique facility of measuring vertical winds using Indian mesosphere, stratosphere, and troposphere (MST) radar along with zonal and meridional winds enables the study of atmospheric circulation over Gadanki (13.5°N, 79.2°E) during the Indian summer monsoon season. The mean meridional circulations during winter and monsoon seasons represent part of two different Hadley circulations. The winter Hadley cell is observed to be stable whereas the monsoon Hadley cell seems to vary and depends on the monsoon activity. During active phase of the monsoon, the Hadley cell extends to the north, and during weak phase, it extends to the south of the study region. The observed features are compared with the winds obtained from National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data. The present study emphasizes that the atmospheric circulation during monsoon season is to be studied separately for active and break phases.