Indian Summer Monsoon Variability Research Articles

Fidelity of Regional Climate Model v4.6 in capturing seasonal and subseasonal variability of Indian summer monsoon

Climate modeling studies in the context of Indian summer monsoon (ISM) variability have usually been performed on the seasonal and interannual timescales. The present study assesses the fidelity of the Regional Climate Model (RegCM v4.6) in capturing the subseasonal active and break spells along with the seasonal mean rainfall during the ISM season. The model fields are obtained from 24 years (1982–2005) of simulation and validated against the observations and latest reanalyzed ERA5 data products. Our analysis indicates that RegCM v4.6 fairly captures the large scale features of ISM and improvement in seasonal rainfall is noted as compared to its precedent RegCM v4.4. At subseasonal timescales, though the model captures the active and break spells of ISM, the length and frequency of these events seem inconsistent as compared to the observations. Occurrences of breaks and associated circulation features are mostly consistent but the active spells are significantly misconstrued in the model. The dry air intrusion from the western region and lack of monsoon low over the mainland and Bay of Bengal seem to suppress the precipitation in the model. This subseasonal bias might persist due to systematic errors linked to the lack of ocean coupling, inefficiency of land surface and cumulus parameterization schemes in the model. Overall, RegCM v4.6 offers improvements at seasonal timescale but needs further improvements to realistically represent the subseasonal variability of ISM.

Millennial-scale variability of Indian summer monsoon during the last 42 kyr: Evidence based on foraminiferal Mg/Ca and oxygen isotope records from the central Bay of Bengal

Studying marine sedimentary archives from the Indian Ocean have provided great opportunities to assess the forcing/response relationship between the Indian monsoons, millennial-scale climate variability as revealed in Greenland ice cores, and possible roles of the Atlantic meridional overturning circulation (AMOC) changes that are most likely responsible for driving both. Here we present new evidence of sea surface temperature (SST) estimates and δ18O of seawater (δ18Osw, presumably salinity-driven) from the paired data of Mg/Ca and δ18O measured from planktonic foraminifer Globigerinoides ruber (white) in core BoB-24 collected from the central Bay of Bengal (BoB), tropical Indian Ocean. Eleven accelerator mass spectrometry (AMS) radiocarbon dates on planktonic foraminifera Neogloboquedrina dutertrei from the core provide a reliable age model. Our results indicate a gradual surface warming with several short-lived fluctuations since ~42 kyr ago, and a ~3 °C cooling during the Last Glacial Maximum (LGM) and a ~3 °C warming from the last glacial to the Holocene. Our δ18Osw revealed an increased salinity during the Marine Isotope Stage (MIS) 2, which is likely attributable to a weakening of Indian Summer Monsoon (ISM) precipitation and associated river discharge in the BoB. Our record also indicates clearly a phase of strengthening ISM during the mid-MIS 3 (37.5–32 ka) as evidenced by low foraminiferal δ18Osw values of ~0.2‰. Lower than present δ18Osw values during the Bølling/Allerød (B/A) event (14.5–12.6 ka) and the early Holocene (10–5 ka) in core BoB-24 indicate lower salinity that in turn suggest enhanced ISM precipitation and increased freshwater output from the Ganges-Meghna-Brahmaputra-Irrawaddy (GMBI) rivers together with peninsular rivers and Irrawaddy-Salween rivers from Myanmar. This study brings data evidence that implies weakened ISM was linked with Northern Hemisphere cooling events, especially during the MIS 2. Furthermore, the millennial-scale hydroclimate changes in the BoB presented in this study support the dynamic role of AMOC in driving a teleconnection between the North Atlantic and Indian Ocean monsoon regions.

Indian summer monsoon variability in northeastern India during the last two millennia

High-resolution proxy records help to understand natural forcing of climate variability and improving our capability to predict climate variability on decadal to centennial time scales. Present study from the Mawmluh cave, northeastern India shows sudden shifts in speleothem oxygen isotope values, indicating several abrupt changes in the Indian summer monsoon (ISM) during ~212 BCE to 1986 CE. Moderate ISM conditions prevailed during ~212 BCE to 400 CE punctuated with weak intervals, strong ISM during 400 and 500 CE and from 640 to 1060 CE, whereas weak ISM conditions prevailed during 520–540 CE, 820–850 CE and 940–980 CE and after 1060 CE. The interval from 1060 to 1986 CE witnessed decreased precipitation than the previous millennium. The latter phase of the Medieval Climate Anomaly (MCA; 1060 to 1200 CE) was quite drier in contrast to the earlier intervals. The ISM was generally weak during the Little Ice Age (LIA; 1350 to 1850 CE) with short-term pulses of high precipitation when sun-spot activity was high. The data shows the weakest ISM condition during 1640–1740 CE (Maunder Minimum) of the last two millennia. Variations in extra-tropical northern hemisphere temperatures due to volcanic activity and solar insolation, and accompanying northward/southward shifting of the Inter-Tropical Convergence Zone played a pivotal role in modulating the strength of the ISM during the past two millennia.

Unstable relationship between tree-ring δ18O in the transitional zone of the Asian summer monsoon and the Indian summer monsoon

The Asian Summer Monsoon system (ASM) affects a large population in the monsoonal Asia. Previous studies have thoroughly discussed the temporally unstable response of tree-ring oxygen isotope (δ18O) to the regional climate variables and ocean–atmosphere interaction modes, however, the response to the ASM remains ambiguous. In this study, an annually resolved tree-ring δ18O chronology is developed from eastern Yunnan-Guizhou Plateau, in the transitional zone of the ASM. Based on the results from response analyses, the climatic controls of tree-ring δ18O is considered to be the moisture-related variables of the monsoon season. Spatial correlations suggest that in the study area, the water vapor is mainly originated from the Bay of Bengal and transported by the Indian Summer Monsoon (ISM). Furthermore, the temporal stability in relationships between the tree-ring δ18O and the ISM variability are explored. Our findings suggest that on interannual to decadal timescales, the tree-ring δ18O is significantly anti-correlated, but temporally unstable with respect to the ISM. The strength of interannual relationship is affected by the ISM intensity, which is modulated by the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) variability. Moreover, the strength of decadal relationship is associated with the phase shift of the Pacific Decadal Oscillation (PDO). Additionally, the decreasing trend of tree-ring δ18O after the late 1970 s that diverges from the simultaneous normal level of the ISM is caused by the isotopically-depleted trend in upstream precipitation of the moisture source. The opposite patterns of correlations between the tree-ring δ18O and the ENSO and IOD in recent decades demonstrate that the regional hydroclimate in the study region has been undergoing significant secular variations that are characterised by an increasing effect of the IOD and a decreasing effect of the ENSO. The findings from this study further improve our understanding of the tree-ring δ18O response to the ASM, at multi-time scales, and shed light on the impact of the ASM on regional hydroclimate variability, across the transitional zone of the ASM.

Orbital Forcing of Late Miocene–Early Pleistocene Environmental Change in the Zhada Basin, SW Tibetan Plateau

AbstractMechanisms controlling the long‐ and short‐term variability of the Indian Summer Monsoon (ISM) and high‐elevation environmental change have largely been examined using low‐elevation or marine records with less emphasis on high‐elevation non‐marine records. We address this using a high‐resolution, long‐term record from upper Miocene–lower Pleistocene (~9.0–2.2 Ma) fluvio‐lacustrine strata in the Zhada Basin, southwestern Tibetan Plateau. Long‐term changes include the onset of lacustrine deposition, a decrease in mean grain size, and an increase in δ18Ocarb and δ13Ccarb values at ~6.0 Ma in response to basin closure following regional extension. This was followed by a return to palustrine/fluvial deposition, an increase in mean grain size, and a decrease in δ18Ocarb and δ13Ccarb values at ~3.5 Ma in response to tectonically driven long‐term ISM weakening. Spectral analysis reveals that high‐frequency variations in the δ18Ocarb record are dominated by 100 and ~20 kyr cycles from ~6.0–2.2 Ma. Wavelet and spectral analysis of the most densely sampled interval (4.23–3.55 Ma), tuned to the record of daily insolation (21 June at 35°N) confirms and highlights 100 and 20 kyr cycles. The tuned Pliocene δ18Ocarb record is coherent with the record of Northern Hemisphere insolation at precession periods but not at obliquity or eccentricity periods. Additionally, the tuned δ18Ocarb record is anticorrelated to the insolation record, indicating that stronger Northern Hemisphere insolation correlates with a stronger ISM. These results suggest that variations in daily insolation drove late Miocene–early Pleistocene high‐frequency ISM variability and environmental changes in the high‐elevation southwestern Tibetan Plateau.

Holocene Indian Summer Monsoon variations inferred from end-member modeling of sediment grain size in the Andaman Sea

Whether the Indian Summer Monsoon (ISM) Maximum started in the early Holocene or middle Holocene remains a controversial issue. Grain-size measurements were conducted on 157-cm-long core ADM-C1 from the Andaman Sea; the core spanned the last 11.2 kyr BP. Four end-members (EMs) were unmixed using lognormal parametric end-member modeling. Genetic analyses of the grain-size EMs suggested the EM1 and EM2 may correlate with suspension in the upper layers and transportation in the benthic nepheloid layer, respectively, while the EM3 and EM4 were tied with deposition affected by monsoonal currents and sedimentation under extreme events, respectively. The EM3 was dominated by ISM intensity and transported by summer monsoon currents, with increase in the proportion of EM3 reflecting increasing ISM intensity, and vice versa. Variations in EM3 were used to define three stages of ISM evolution, as follows: (1) During 11.2–9.1 kyr BP, EM3 was relatively low, indicating a weak ISM; (2) during 9.1–4.5 kyr BP, EM3 increased substantially, indicating a strengthened ISM; (3) during 4.5-0 kyr BP, EM3 decreased gradually, suggesting a gradual weakening of the ISM. The Holocene ISM Maximum started at ~9.1 kyr BP rather than in the early Holocene, which may have been partly due to the slowdown of the Atlantic meridional overturning circulation (AMOC) during the early Holocene that resulted in a decreased land-sea thermal contrast between the landmass and the Indian Ocean. An additional cause may have been the remnant Northern Hemisphere ice sheets that impeded the northward shift of the Intertropical Convergence Zone (ITCZ). Seven relatively brief decreases in the ISM intensity (events 7, 6, 5, 4, 3, 2, 1) occurred at ca. 10.3, 9.7, 7.3, 5.7, 4.2, 2.4, and 0.8 kyr BP, respectively. They corresponded, within the age uncertainties, to the increased supplies of ice-rafted detritus to North Atlantic sediments, implying that changes in the ISM intensity on millennial scale were dominated by climatic processes in northern high latitudes.

Indian summer monsoon variability during the last millennium as recorded in stalagmite from Baratang Mahadev cave, Andaman Islands

This study presents a stable oxygen isotope record of Indian summer monsoon (ISM) variability using cave speleothem carbonate from Baratang Island (Andaman Islands) encompassing the last millennium (from 10th to 19th century CE). The studied interval encompasses two major climatic events; the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA) as well as the Current Warm Period (CWP). Our data suggest generally higher precipitation during the warm MCA when the ISM was relatively strong, and low precipitation during the cold LIA with a few exceptions when wet conditions prevailed. The Baratang cave record shows greater similarity with the records from the Sahiya and Klang caves, and opposite trends relative to those from the northeastern Himalaya owing to the dipolar nature of ISM precipitation. High-frequency sub-decadal to centennial scale pulses of weakened summer monsoons are observed during 16th to 19th century CE. These phases of weak ISM are correlated with severe droughts and famines in the Indian subcontinent owing to deficient monsoon rainfall, which had pronounced effect on South Asian societies. Spectral analysis suggests a strong periodicity of 190 yr (de Vries cycle) indicating strong influence of solar insolation.

Abrupt mid-Holocene decline in the Indian Summer Monsoon caused by tropical Indian Ocean cooling

The evolution of the Indian Summer Monsoon (ISM) has been widely investigated and traditionally interpreted as consequences of orbital forcing change and the migration of the Intertropical Convergent Zone (ITCZ). The potential influence of sea-surface temperature (SST) on ISM variation at millennial time scales has been progressively proposed in recent studies. Towards a better understanding of Holocene evolution of ISM, especially the potential response to variations in SST, a 643 cm continuous sediment sequence covering the past 11.1 ka was recovered from Beihai Wetland in western Yunnan Plateau. Fossil pollen analysis revealed that evergreen broadleaved forest dominated the regional vegetation in the early Holocene, which represented a warm and humid period associated with strong ISM. Expansions of deciduous broadleaved forest and grassland from 5.2 cal. ka BP to the present, together with the appearance of abundant plant remains, indicate the formation of the wetland system. Ordination analysis of the pollen assemblages further confirmed the abrupt vegetation turnover at ca. 5.2 cal. ka BP, which is consistent with the abrupt decline in reconstructed mean annual precipitation (Pann, from ca. 1400 to 1100 mm), and indicates an abrupt decline in the summer monsoon. We performed a climate regime shift test on various records from the ISM domain and eastern Africa and found this phenomenon as well, which could not be explained by changes in solar insolation or the migration of the ITCZ. A mid-Holocene cooling was detected from the Indian Ocean, especially a decreased SST in the tropical Indian Ocean ca. 5.5 cal. ka BP. Cooling of the tropical Indian Ocean possibly caused the abrupt retreat of the summer monsoon by restraining the duration of the monsoonal season and the convergent process, which was supported by an abrupt monsoon decline based on results from a minimal concept model.

Geochemical records of Qionghai Lake sediments in southwestern China linked to late Quaternary climate changes

Knowledge of the chemical weathering history under the subtropical monsoonal climate will help to resolve the discrepancies regarding how climate conditions constrain chemical weathering. Major element and rare earth element (REE) compositions of the Qionghai Lake sediments in Sichuan Province, southwestern China were analyzed, aiming to provide information on sediment source discrimination and history of chemical weathering intensities and its responses to variations of Indian summer monsoon (ISM) and paleoclimate conditions. As revealed by the A-CN-K diagram and other provenance proxies including TiO2/Zr ratio, Th/Sc vs. Zr/Sc and La/Th vs. Hf diagrams, as well as the REE characteristics, the source of Qionghai Lake sediments was felsic rocks. Additionally, these sediments were subjected to small changes in provenance, sedimentary sorting and recycling, and weak diagenesis after deposition and/or metasomatism, indicating original signals were maintained on the intensity of chemical weathering of source area. The chemical weathering intensities inferred from CIA (chemical index of alteration) values have significantly fluctuated over the past 28 kyr, reflecting three stages of paleoclimate evolution, which is consistent with grain size characteristics, carbonate contents, low-frequency magnetic susceptibilities, TOC and biomarker proxy (Paq). The CIA values changed significantly during the typical warm and cold climate events including the Last Glacial Maximum, Heinrich 1 cold event, Younger Dryas and Holocene Climatic Optimum, and generally exhibited an increasing trend in amplitude correspondingly, in concert with the climate shifting from cold and dry to warm and humid conditions. Generally warm and humid climate conditions suggest enhanced chemical weathering intensity, whereas weak chemical weathering denotes cold and dry climate in the ISM region. In addition, sequential variations of the CIA values during the past 28 kyr may reflect changing chemical weathering processes, in responses to the varying intensities of Indian summer monsoon and the climate conditions forced by the Northern Hemisphere solar insolation.

The fidelity of a regional coupled model in capturing the relationship between intraseasonal variability and the onset/demise of the Indian summer monsoon

Onset and demise of the Indian summer monsoon (ISM) and intraseasonal variability (ISV) embedded within the ISM are dominant climatological phenomena observed over the Indian region. In this study, a quantitative relationship between these two phenomena is assessed and the performance of a regional coupled model in simulating the relationship in the ISM is examined. An objective definition of the local onset and demise of the ISM is used based on more than a century-long India Meteorological Department rain-gauge observation and 10-year-long model simulated precipitation. Two distinct modes of intraseasonal oscillations (ISOs), northwestward propagating 10–20-day periodic high-frequency ISO and northward moving 20–70-day periodic low-frequency ISOs are extracted using Multichannel Singular Spectrum Analysis (MSSA). It is found that nearly 60% of the local onset of the ISM occurs during the positive developing phases of the ISO; whereas, a similar fraction of local demise occurs during positive decaying phases of the ISO across almost entire India. Essentially, the phase-locking of two ISOs creates the most conducive environment for onset or demise of the ISM to occur. In this study we found that the model not only captures the overall structure of the ISO modes, but also simulates the observed distribution of the onset and demise dates of ISM in different ISO phases. The diagnostics described here, with the ability of the model to produce the observed phenomena, provide an essential tool to improve our understanding of the monsoon system and its predictability.

Pollen records of vegetation dynamics, climate change and ISM variability since the LGM from Chhattisgarh State, central India

Pollen records of a lacustrine sedimentary profile from the Chhattisgarh State, central India revealed that between ~22,200 and 18,658 cal yr BP, open vegetation occupied the landscape under a cool and dry climate, probably indicating a reduced monsoon precipitation. This cool and dry climate, influenced by weakening of the Indian Summer Monsoon (ISM), globally matches with the Last Glacial Maximum (LGM). Between ~18,658 and 7340 cal yr BP, under the influence of a warm and moderately humid climate with increased monsoon precipitation, the open vegetation was succeeded by open, mixed tropical deciduous forest. This open, mixed tropical deciduous forest continued to grow in the region, with an increase in the frequency of some of the tree taxa especially, between ~7340 and 1961cal yr BP under a warm and relatively more humid climate with increased monsoon precipitation, correlatable with the Holocene Climatic Optimum (HCO)/Holocene Thermal Maximum (HTM). Between ~1961 cal yr BP and the Present, dense, mixed tropical deciduous forest came into existence under a warm and relatively more humid climate with further increase in monsoon precipitation. The present study provides an unprecedented record of the vegetation dynamics, climate change and the ISM variability since the LGM from one of the poorly understood areas of the tropics where rainfall is essentially controlled by the monsoon (ISM) variability.

Prolonged warming over the last ca. 11,700 cal years from the central Indian Core Monsoon Zone: Pollen evidence and a synoptic overview

Paleopalynological records of 1.1 m deep lacustrine sediment profile from the central Indian Core Monsoon Zone (CMZ) have provided a prolonged warming trend over the last ca. 11,700 years. Palynological evidence suggests that between ca. 11,700 and 8500 cal yr BP, a mixed tropical deciduous forest occurred in the region under a warm and humid climate with good monsoon precipitation. The forest expanded into a dense mixed tropical deciduous forest since ca. 8500 cal yr BP to the present, under warm and relatively more humid climatic conditions with further increase in monsoon precipitation. This warm climatic phase represents the Holocene Climate Optimum (HCO) or Holocene Thermal Maximum (HTM). Prior to the initiation of the long warm period, an open vegetation occurred in the region between ca.14,100 and 11,700 cal yr BP, under a cool and dry climate, probably indicating reduced monsoon precipitation, and partially matching with the Older Dryas stadial. The present study provides insights into the vegetation dynamics and associated climatic changes in response to the Indian Summer Monsoon (ISM) variability over the last ~14.1 ka from a poorly understood area of the tropics in South Asia, where rainfall is essentially controlled by the ISM. In addition, an account of the asynchronous warming during the Holocene from the central Indian CMZ, as well as other tropical regions of the Indian sub-continent, has been presented here. Global contextualization of the Holocene warming has also been summarized in this communication.

ENSO-related droughts and ISM variations during the last millennium in tropical southwest China

Severe El Nino-Southern Oscillation (ENSO) droughts that occurred in southwest China during the last millennium have exerted important impacts on natural ecosystems and human society. Fossil phytoliths in high-resolution (~ 7 year) fluvial sediments in the monsoon rainforest are studied to reveal the influence of ENSO activity on tropical southwest China. The phytolith records together with multiproxy analysis indicate that three phases of distinctively warm-dry and cool-wet climate pattern have exerted profound influences on the sedimentary environment and tropical monsoon rainforest, which is sensitive to the occurrences of severe droughts and fires. Integrated comparisons demonstrate that climatic variations rebuilt by Ic (temperature proxy) are consistent with the temperature change in the Northern Hemisphere, Indian Summer Monsoon (ISM) variations in the northern Indian Ocean and southwest China, and aridity index (Iph) is linked to ENSO activity in the equatorial Pacific Ocean. At decadal timescale, there is an internal connection between ISM variability and ENSO activity in the tropical southwest China. Due to the modulation of ENSO activity, there is an antiphase relationship between temperature and ISM intensity, with a remarkably dry Medieval Warm Period (MWP), Modern Warming (MW) and a mostly wet Little Ice Age (LIA) during the last millennium. Spectral analysis of the climatic indices (Ic and Iph) indicated that the dominant cycle (~ 88 year) is a significant component of periodic solar activity variations. Our results confirmed that the possible forcing mechanism of such a “warm-dry and cool-wet” climate pattern is related to the coupling of solar periodic variations and ENSO variance spanning the tropical Indian Ocean and the “Indian subcontinent-southwest China”. This is significant of understanding the relationship among droughts, ENSO variance and ISM variation in the tropical southwest China under the background of global warming.

Correction to: Effects of winter and summer-time irrigation over Gangetic Plain on the mean and intra-seasonal variability of Indian summer monsoon

Correction to: Effects of winter and summer-time irrigation over Gangetic Plain on the mean and intra-seasonal variability of Indian summer monsoon

Depositional History and Indian Summer Monsoon Controls on the Silicate Weathering of Sediment Transported to the Eastern Arabian Sea: Geochemical Records From IODP Site U1456 Since 3.8 Ma

AbstractSystematic variation in elemental and Sr‐Nd isotopic compositions of the clay‐sized (<2 μm) siliciclastic fraction from the International Ocean Discovery Program (IODP) Site U1456 (Laxmi Basin) provides valuable insights into dynamic changes in continental weathering intensity in the western Himalayas and sediment inputs from the Indus River and Deccan Traps to the eastern Arabian Sea. Long‐term, high‐resolution proxy records from the sediment core reveal that the main sources (i.e., Indus River and Deccan Trap basalts) of the fine‐grained detrital sediments have significantly changed with Indian summer monsoon variation over the studied time interval. During two depositional periods (3.8–3.3 and 2.7–1.2 Ma) corresponding to a weak Indian summer monsoon, the Indus River contributed fine‐grained sediments with high K/Al, high 87Sr/86Sr, and low εNd to the study site. Between 3.3 and 2.7 Ma and between 1.2 and 0 Ma, deposition in the eastern Arabian Sea was governed by large inputs of Deccan Trap‐derived basaltic sediments, characterized by high Mg/Al and Fe/Al, low 87Sr/86Sr and chemical index of alteration values, and high εNd values, associated with a change to a stronger Indian summer monsoon driving stronger chemical weathering. Synchronous changes in the geochemical proxies since 3.8 Ma highlight that variations in the inputs of siliciclastic sediments and Indian summer monsoon development are closely coupled.

Abrupt changes in Indian summer monsoon strength during the last ~900 years and their linkages to socio-economic conditions in the Indian subcontinent

Abrupt shifts in Indian summer monsoon (ISM) precipitation largely affected regional hydrometeorology, accelerating socio-economic and cultural adaptations of agrarian economies in South Asia in the last millennium. Linking ISM variability to human history in the Indian subcontinent is still limited by the scarcity of high-resolution paleoclimatic data for the period encompassing the last 1000 years or earlier. We present a new high resolution oxygen isotope proxy record from the Wah Shikar cave, northeastern India and identified abrupt shifts in the ISM that had profound impact on human societies during the last ~900 yrs. The ISM was strong during the Medieval Climate Anomaly (MCA) and the Current Warm Period (CWP), and show multiple shifts during the Little Ice Age (LIA). The abrupt shifts in the ISM were more frequent during warm intervals, showing a secular trend with few exceptions during the cold periods. Intervals of prolonged summer monsoon weakening are correlated to the decline of major dynasties in India, possibly due to decrease in agriculture productivity, declining economy and societal crisis. Changes in atmospheric temperature due to volcanic eruptions and sun spot activity have played major role in the onset of the LIA and other prolonged weak intervals of the ISM during the last ~900 yrs.

Impact of convective parameterization on the seasonal prediction skill of Indian summer monsoon

The sensitivity of seasonal predictions of the Indian summer monsoon (ISM) to convection parameterization schemes (CPS) is studied using 37 years of hindcast experiments. The predictions are quite sensitive to changes in these schemes and improve the skill by 18–28%. Though the mean state circulation and rainfall over India improves, the sea surface temperature (SST) biases increase in the sensitivity experiments compared to the control run. The ability of the model to realistically capture the teleconnections associated with monsoon such as the El-Nino Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) also appears to change with different CPS. It is found that the suitability of a CPS for ISM in the Climate Forecast System version 2 (CFSv2) stems from its ability to capture cloud fractions realistically and keep the SST biases to a minimum. The revised Simplified-Arakawa–Schubert (SAS2, Han and Pan in Weather Forecast 26:520–533. https://doi.org/10.1175/waf-d-10-05038.1 , 2011) scheme gives better prediction skill for ISM compared to the skill score obtained from SAS2 with shallow convection (SAS2sc) primarily because it simulates realistic clouds, without aggravating the SST biases, particularly in the tropical Pacific Ocean, and captures the Indian Ocean teleconnections realistically. SAS2sc significantly under-estimates the low-level clouds over global equatorial region, despite simulating better mid and high-level clouds, higher Nino 3.4 skill, and better inter-annual variability of ISM. The cold SST bias in the tropical basins is large in SAS2sc. Therefore, to exploit the merits of SAS2sc, unrealistic suppression of low clouds needs to be addressed, and the cold SST biases need to be minimized.

Assessment of Indian summer monsoon variability in a regional climate model coupled to a slab ocean model

A suite of Regional Climate Model (RegCM) experiments are performed over south Asia to examine the skill of RegCM to simulate the seasonal and sub-seasonal mean features of Indian summer monsoon (ISM). Because of coupled nature of ISM, three model experiments with RegCM are conducted to examine the skill of coupled and uncoupled configurations of RegCM by forcing it with observed SST (Exp1), coupling it to a simple slab ocean model (SOM) with constant mixed layer depth (MLD; Exp2) and with climatology of MLD (Exp3). The coupled experiments show an overall improvement in several aspects of ISM variability at seasonal and intraseasonal time-scales, despite bias in simulated SSTs. Between coupled experiments; Exp3 reduces biases in SST distribution over the region to the north of Arabian Peninsula, eastern Arabian Sea (AS), and broadly over north Indian Ocean (NIO). Noteworthy is the improved precipitation over central India (CI), head Bay of Bengal (BoB), as well as the representation of easterly wind shear in coupled experiments. At intraseasonal time scales, Exp3 produces spectral peaks above red noise at 25–50-day and 15–20-day periods closely representing the northward propagating intraseasonal mode and quasi-biweekly oscillating mode as in observed precipitation. The improved representation of spatial distribution of intraseasonal activity over NIO as well as the SST and precipitation relationships over head BoB and eastern AS is attributed to better representation of air-sea interaction in Exp3. In brief, the coupling improves the model skill for the true representation of mean ISM variability during boreal summer, and the thorough evaluation of model for longer periods is required to employ it as a downscaling tool for regional climate change studies.

Understanding the interannual variations of the zonal mean Indian summer monsoon seasonal rainfall

In this study, we seek to understand the variations of the zonal mean Indian summer monsoon (ISM). It is observed that the zonal mean precipitation anomalies explain about 20–30% of the total seasonal precipitation of the ISM. Additionally, we also show that the interannual anomalies of the ISM, at least for some of the most extreme seasons display significant zonal symmetry. Therefore, understanding the interannual variations of the zonal mean precipitation is quite relevant. Furthermore, the reduced and simplified dimensionality of the zonally symmetric framework is an additional attraction. Our study shows that the zonal mean precipitation anomalies of the ISM are significantly correlated with the corresponding anomalies of the vertically integrated moist static energy (H) at interannual scales. The forcing terms of the tendency of the zonal mean H of the ISM is dominated by the meridional flux of H by the Hadley cell followed by comparably smaller, yet, significant forcing terms of transverse boundary and vertical fluxes of H. We find that the individual correlations of the zonal mean precipitation anomalies of the ISM with each of these dominant terms of H are comparably weak in the core latitudes of the ISM. But some of the relatively weaker forcing terms of H like the radiative heating and the diffusion of the enthalpy fluxes display a very strong relationship with the corresponding zonal mean precipitation anomalies. These results suggest that although the description of the regional Hadley cell offers a heuristic model to describe the ISM variation, it belies the intricacies of the forcing terms that maintain it. Our study suggests that the challenge of the ISM seasonal rainfall anomaly prediction even in a reduced dimensional space of a zonally symmetric framework requires all the forcing terms of H with reasonable fidelity, irrespective of their contribution to the variations of H.

Mid–late Holocene fluvial aggradational landforms and morphometric investigations in the southern front of the Shillong plateau, NE India

The present study is an attempt to understand the antiquity of the preserved fluvial landforms and its response to the climate–tectonics nexus through geomorphological investigations along the Jadukata, Umpung, Umngot and Umtongoi rivers in the southern front of the Shillong plateau (SP), NE India. Sedimentological characteristics, chronological analyses and morphotectonic parameters were used to describe the spatial and temporal variability in the patterns of aggradation, landform evolution and neotectonic influences in the study. Our results indicate that valley aggradation processes occurred around the transitional zone in the southern front of the SP during the mid–late Holocene era along with a hiatus in sediment deposition after 4.3 ka. Sediment generation and aggradation is modulated by precipitation anomalies associated with the Indian Summer Monsoon (ISM) variability whereas morphometric analysis suggests that activity along the Dauki–Dapsi fault has been contributing to the uplift-related deformation. Sedimentological observation supported by optically stimulated luminescence chronology obtained on palaeoflood deposits, valley-fill fluvial terraces and alluvial fans indicate their deposition during three pluvial phases: (i) 5.3–4.3 ka, (ii) 2.4–1.0 ka and (ii) 0.7–0.3 ka. Our data indicate that valley aggradation and geomorphic processes in the southern part of SP responded to short-term changes in the ISM variability with contributions from the morphotectonic activities associated with the Dauki–Dapsi fault during the late Holocene period.