Indian Summer Monsoon Circulation Research Articles

Moisture source anomalies connected to flood‐drought changes over the three‐rivers headwater region of Tibetan Plateau

AbstractThe Flood‐Drought condition over the Three‐River Headwater region (TRHR) of the Tibetan Plateau (TP) exerts vital impacts on regional climate and downstream livelihoods. However, the reasons behind precipitation variability over this region remain elusive. In this study, the multi‐year moisture sources of water vapour reaching the TRHR were identified to reveal their connection to the interannual variability of summer precipitation by utilizing a Lagrangian diagnosis analysis. The results show that the moisture sources could be tracked backward covering vast areas, with the peak values located mostly at the northern Indian continent and adjacent regions of TRH itself. The variability of integrated moisture sources' contribution agrees well with the summer precipitation on the interannual scale; However, the role of different regions varies in line with their geographical location. More specifically, summer precipitation is positively correlated with the moisture source over the adjacent areas of TRH and the remote terrestrial‐oceanic regions, in contrast to a negative correlation to the water vapour departing from the Northeastern Asian and western regions to the TP. The Indian summer monsoon circulation and local recycling process play a dominant role in modulating the summer precipitation on the interannual scale, whereas the role of the westerlies is almost negligible. The abnormal atmosphere circulations triggered by mid‐high latitude wave trains and anomalies in SST in the central and eastern Pacific Ocean and Indian Ocean could serve as possible drivers for the precipitation variability over TRHR.

Unusual recent prolonged low flow in the Moqu River, northeastern Tibetan Plateau, inferred from tree-ring width variations

The runoff of Moqu river in Zoige, one of most important water conservation ecological function areas in the upper reaches of the Yellow River, located in the north-east of the Tibetan Plateau, is highly sensitive to climate change. However, the current understanding of the long-term changes in the hydrology of this rives is still fragmentary. In this study, we created a regional tree-ring width chronology from two sample sites in Moqu River Basin and analysed the relationships between tree-ring width, climate, and runoff. We found significant positive correlations between tree-ring width and runoff in multiple months, the highest correlation coefficient occurred with annual runoff from previous July to current June (r = 0.617, 1981–2013). Based on this highly significant (p < 0.01) relationship, we reconstructed runoff variations of Moqu River for the period 1672 to 2019. The reconstructed runoff showed unusual prolonged and frequent low flow periods in recent decades. We found that the continuous low precipitation and the high temperature in the context of global warming are the main reasons for the abnormal runoff in recent decades. We assume that a weakening of the Indian summer monsoon (ISM) circulation system may be the key large-scale factor that caused the unusually prolonged low flow. With global warming, the weakening of ISM intensity leads to reduced water vapor transport from the tropical Indian Ocean to High Asia, resulting in less precipitation and low runoff events in Zoige. Our reconstruction improves the understanding of regional runoff variability over multiple timescales, and also provides a scientific basis for the protection, development, and utilization of water resources related to ongoing global warming.

Anthropogenically-forced weakening of the Indian summer monsoon and enhancement of the western North Pacific tropical cyclogenesis

This study addresses the role of human-induced climate change on the interactions of convective activities between the Indian summer monsoon (ISM) and western North Pacific (WNP) regions - an important scientific issue which has been hitherto overlooked. We have examined this problem using two numerical experiments of a high-resolution climate model, with and without anthropogenic forcing (i.e., HIST and HISTNAT) for the historical period 1951–2005, supplemented by innovative diagnostics like causal network analysis. Our findings suggest that an anthropogenically-forced weakening of the ISM circulation tends to significantly enhance the genesis potential index (GPI) of the WNP tropical cyclones by 13.5% and associated convective activities, by reorienting the large-scale flow over the tropical Indo-Pacific in a manner that is conducive for enhancement of the WNP tropical cyclogenesis. Additionally, it is found that the probability of extremely low sea-level pressure (SLP) (&amp;lt;995.5 hPa) around Taiwan and Chinese mainland is significantly higher by 10.3% in the anthropogenically-forced simulation as compared to the natural run. Using the model outputs from HIST and HISTNAT, we also performed a causal effect network (CEN) analysis to understand the causal connections among the three indices involved in the ISM-WNP interactions (a) Indian monsoon circulation index (IMI) which is the difference in the area-averaged 850 hPa zonal winds between the boxes (40oE-80oE, 5oN-15oN) and (70oE-90oE, 20oN-30oN) (b) WNP tropical cyclone activity expressed as the genesis potential index (GPI) averaged over the region (120oE-180oE, 5oN-30oN) and (c) Tropical Indo-Pacific wind index (IPWND) which is based on the 850 hPa zonal winds averaged over the domain (100oE-130oE, 5oN-20oN). Results from the CEN analysis indicate that an anthropogenically-forced weakening of the IMI can lead to possible strengthening of GPI and IPWND with time-lags of 5 and 7 days, respectively. In general, it is noted that the causal relationships among IMI, GPI and IPWND are associated with shorter time-lags (∼4–9 days) in HIST and longer time-lags (∼19–28 days) in HISTNAT.

The Warming of the Arabian Sea Induced a Northward Summer Monsoon over the Tibetan Plateau

Abstract Summer monsoon precipitation over the Tibetan Plateau (TP) has significant impacts on the ecology, economy, and Asian climate, but has exhibited anomalous spatial distribution since the early twenty-first century. To explore its distribution and attribution, this study investigated the northernmost boundary of the summer monsoon over the TP (TPSM) and the related precipitation. The TPSM and related precipitation extended poleward in the post-2000 period in response to the rising Arabian Sea surface temperature (ARB_SST) in the late spring and early summer. A warming Arabian Sea leads to the westward movement of the Indian summer monsoon (ISM) circulation and moisture convergence with a higher moisture layer, which exerts a midlatitude wave train that exhibits a dipole mode (a west cyclone and an east anticyclone) over the TP. Consequently, the west trough and southerly are strengthened, which is conducive to transporting more poleward moisture mass from the moisture layer over the ISM zone through the midwestern TP. Furthermore, it contributes to the poleward TPSM and precipitation in the northern TP by coupling with the dipole mode. These findings help explain the wetting of the Asian dryland and its effects on the Northern Hemispheric climate.

Weakening of Indian Summer Monsoon Synoptic Activity in Response to Polar Sea Ice Melt Induced by Albedo Reduction in a Climate Model

AbstractThe effect of polar sea ice melt on low latitude climate is little known. To understand the response of the Indian summer monsoon (ISM) synoptic activity to the sea ice melt, we have run a suite of coupled and uncoupled climate model simulations. In one set of simulations, the albedo of sea ice is reduced so that it would melt due to increased absorption of solar radiation. The coupled model simulation with a reduced sea ice albedo resulted in an almost complete melting of the sea ice in summer in both hemispheres. A high‐resolution (50 km) atmospheric general circulation model (AGCM) is forced with the climatological annual cycles of sea surface temperature (SST) and sea ice concentrations (SIC) from the coupled model outputs to better resolve synoptic scale variability. In the high‐resolution AGCM simulations forced with SST and SIC from the sea ice melt experiments, the ISM circulation weakened substantially, and the monsoon low‐pressure systems (LPS) activity experienced an overall decline of 23%, with a widespread weakening in the south and a moderate strengthening over the north, in response to a decline of 78% (24%) in SIC over the Arctic (Antarctic) in the June–September season. The changes in the LPS activity in response to polar sea ice melt are found to be mostly driven by the changes in low‐level absolute vorticity and vertical shear over the Bay of Bengal.

Near Future Projection of Indian Summer Monsoon Circulation under 1.5 °C and 2.0 °C Warming

The Indian summer monsoon’s (ISM) response to low (1.5 °C) and medium (2.0 °C) warming scenarios are examined during the period 2021–2050 using 11 member ensembles of the state-of-the-art Community Earth System Model, version 1 (CESM1-CAM5) datasets. We find that in the near-term period an increase in warming by 0.5 °C will strengthen the monsoon circulation and precipitation over the Indian landmass and east Asia. Under the 1.5 °C warming scenario, the ISM circulation will weaken, and precipitation will exhibit a decreasing trend, while the ISM precipitation and circulation will strengthen under the 2.0 °C warming scenario. The strengthening of ISM under the 2.0 °C warming scenario is due to the strengthening of the South China Sea (SCS) anticyclone and a high-pressure center near the Philippine Island. This high-pressure center over the SCS will facilitate a cross-basin strengthening of the easterly wind from the western Pacific Ocean/SCS to the equatorial Indian Ocean, which will, in turn, strengthen the transport of the Pacific-origin moisture to the Indian subcontinent and East Asia. This increase in moisture content over the Indian subcontinent will strengthen the monsoonal circulation and precipitation under the 2.0 °C warming scenario.

Corals Reveal an Unprecedented Decrease of Arabian Sea Upwelling During the Current Warming Era

AbstractUpwelling in the Arabian Sea driven by the Indian summer monsoon pumps deep, cold, and eutrophic seawater to the sea surface every summer. The Indian summer monsoon and the Arabian Sea upwelling were expected to intensify with global warming, following the hypothesis that the Eurasian landmass would warm faster than the Indian Ocean. Contrary to expectations, the northern Indian Ocean currently warms faster than the Indian subcontinent. A weakening of the Indian summer monsoon circulation is reported, which possibly weakens the Arabian Sea upwelling. However, a lack of observations limits understanding of current and historical changes of the Arabian Sea upwelling. Here, we reconstruct the Arabian Sea upwelling over the past millennium using modern and fossil corals. Our coral records show that the Arabian Sea upwelling intensity was very stable over the last millennium and unprecedentedly declines at present. Our finding implies anthropogenic forcing likely weakens the Arabian Sea upwelling.

Simulating the impact of extended desertification on Indian hydro climate using ICTP-RegCM4.4.5.10 model

Presented here is the numerical study of impact of extended desertification of the Thar desert (a data scarce arid region), on the Indian Summer Monsoon (ISM) meteorology using a robust state-of-the-art ICTP - regional climate model (RCM) RegCMv4.4.5.10 coupled with Biosphere – Atmosphere Transfer Scheme (BATS) land surface scheme. Firstly, the evolution of ISM precipitation, mean temperature and soil moisture during June to September 2001–2010 is verified using Emanuel over land and Grell over ocean with Arakawa and Schubert (1974) closure, in combination with the two planetary boundary layer (PBL) parameterization schemes, Holtslag and University of Washington - UW Turbulence closure model for PBL. Then the feedback due to extended desertification between land surface and PBL, during ISM regime is studied. The RCM simulations with Emanuel over land and Grell over ocean with Arakawa and Schubert (1974) closure in combination with UW PBL scheme, is able to properly simulate the rainfall, mean temperature JJAS climatology with bias in rainfall ~ 2 mm/day and bias in mean temperature is ~ 0.5–1 °C over the monsoon core region. Numerical experiments (control and design experiment) with the above mentioned combination of PBL schemes is performed for investigating the impact of extended desertification on the changes in the ISM precipitation, surface fluxes, circulation pattern, moisture fluxes and net radiation etc. In the extended desert design experiment, the land-use pattern in the model over Thar Desert (i.e. northwestern part of Indian sub-continent between Pakistan and northwestern India) is changed from “crop/mixed farming and semi-desert” to “desert/semi-desert” type of vegetation, mimicking extended desertification in the RegCMv4.4.5.10 model. Results show that due to increase in desertification over Thar deserts there is increase in albedo resulting from a decrease in plant cover, along with formation of anomalous anti-cyclonic circulation causes a decrease in JJAS rainfall (p < 0.01) over north, north-west India by −1 to −2 mm/day (~25% reduction). Anomalous anti-cyclonic circulation over the Indian land mass in the extended desertification experiment causes less convective heating and is responsible for the decrease in precipitation (with respect to control experiment). The anomalous anti-cyclonic circulation developed covers northern part of Bay of Bengal resulting in decrease in moisture advection of the order of −12 to −18 kgm-1sec-1 (~10% reduction). This drives away all moisture from the nearby regions. The effect of extended desertification induced feedbacks on variability and dominant modes of variability of Indian monsoon precipitation and soil wetness is also studied and reported. Finally, it is concluded that the relative magnitudes of the changes in albedo and vegetation (or vegetation roughness length) are important determinants of the size of climate change due to prescribed extended desert landscape over Thar desert region of Indian subcontinent. The results of this numerical study provides a basis and reference study for LULCC and mitigation measure human beings need to undertake for climate change adaptation.

Impact of Climate Change on Past Indian Monsoon and Circulation: A Perspective Based on Radiogenic and Trace Metal Geochemistry

The Indian summer monsoon (ISM), one of the dramatic illustrations of seasonal hydrological variability in the climate system, affects billions of lives. The ISM dominantly controls the northern Indian Ocean sea-surface salinity, mostly in the Bay of Bengal and the Andaman Sea, by the Ganga-Brahmaputra-Meghna and Irrawaddy-Salween rivers outflow and direct rainfall. In the past decade, numerous studies have used radiogenic neodymium (εNd) isotopes of seawater to link Indian subcontinent erosion and the ensuing increase in discharge that results in changes in the north Indian Ocean sea surface. Here we synthesized the state of the ISM and ocean circulation using the neodymium and hafnium isotopes from north Indian Ocean deep-sea sediments. Our data suggest that the Bay of Bengal and north Indian Ocean sea-surface conditions were most likely modulated by changes in the ISM strength during the last glacial-interglacial cycle. These findings contrast to the hypothesis that suggests that the bottom water neodymium isotopes of the northern Indian Ocean were modulated by switching between two distant sources, namely North Atlantic Deep Water and Antarctic bottom water. Furthermore, the consistency between the neodymium and hafnium isotopes during the last glacial maximum and Holocene suggests a weak and dry ISM and strong and wet conditions, respectively. These data also indicate that the primary source of these isotopes was the Himalayas. Our results support the previously published paleo-proxy records, indicating weak and strong monsoons for the same periods. Moreover, our data further support the hypothesis that the northern Indian Ocean neodymium isotopes were decoupled from the global ocean neodymium budget due to the greater regional influence by the great Ganga-Brahmaputra-Meghna and Irrawaddy-Salween discharge draining the Indian subcontinent to the Bay of Bengal and the Andaman Sea.

Model Uncertainty in the Projected Indian Summer Monsoon Precipitation Change under Low-Emission Scenarios

The projected ISM precipitation changes under low-emission scenarios, Representative Concentration Pathway 2.6 (RCP2.6) and Shared Socioeconomic Pathway 1-2.6 (SSP1-2.6), are investigated by outputs from models participating in phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6). Based on the high-emission scenarios like RCP8.5, the Intergovernmental Panel on Climate Change Fifth Assessment Report suggests a wetter Indian summer monsoon (ISM) by the end of 21st century. Although the multi-model ensemble mean (MME) ISM precipitation under RCP2.6 and SSP1-2.6 is still projected to increase over 2050–2099 referenced to 1900–1949, the intermodel spread of the ISM precipitation change is tremendous in both CMIPs. Indeed, the signal-to-noise ratio (SNR) of ISM precipitation change, defined as the MME divided by its intermodel standard deviation, is even below 1 under the low-emission scenarios. This casts doubts on a future wetter ISM in a warmer climate. Moisture budget analyses further show that most of the model uncertainty in ISM precipitation change is caused by its dynamical component from the atmospheric circulation change. As expected, the interhemispheric surface warming contrast is essential in causing the intermodel differences in ISM circulation and precipitation changes under low-emission scenarios. In addition, the projected wetter ISM is prominently enhanced from CMIP5 to CMIP6, along with reduced model uncertainty. However, the resultant increased SNR in CMIP6 is still low in most ISM regions. The results imply that ISM precipitation change is highly uncertain under low-emission scenarios, which greatly challenges the decisions-making in adaptation policies for the densely populated South Asian countries.

Why Indian summer monsoon circulation indices? Fidelity in representing rainfall variability and teleconnections

AbstractWith the recognition that the circulation is strongly coupled with the Indian summer monsoon rainfall (ISMR), a number of large‐scale circulation indices have been proposed to represent the Indian summer monsoon to aid diagnosis of teleconnections with ISMR and evaluation of simulations by climate models. The circulation indices, however, correlate poorly with ISMR raising doubts on the coupling between the two. In this study, we identify reasons behind the poor correlations and estimate a potential limit on this correlation between circulation indices and ISMR. One mechanistic reason for poor correlations between circulation indices and ISMR is that the period and variance explained by the oscillatory modes of circulation indices differ significantly from those of ISMR leading to their phases also differing between modes of circulation indices and corresponding modes of ISMR. Further, the wind anomaly patterns associated with wind indices project strongly on the climatological monsoon circulation unlike that associated with the ISMR, another reason for their poor correlation with ISMR. As a result, all circulation indices underestimate the negative correlation between June–September sea‐surface temperature (JJAS SST) over the Niño3.4 and ISMR and fail to simulate the lead–lag relationship between the two. A conservative estimate of the interannual variance explained by coupling of the JJAS rainfall over continental India and winds in the neighbourhood is strong, explaining about 55% variance distributed in three leading coupled modes. Therefore, the effectiveness of any circulation indices in explaining the ISMR variance is potentially limited to that associated with the dominant mode (∼30%). The local response of winds to deep convection over the waters of the Indian Ocean and western Pacific warm pool leads to a strong increasing trend in some of the circulation indices and adds “noise” modes contributing to the de‐correlation between circulation indices and ISMR.

Investigating the relative responses of regional monsoon dynamics to snow darkening and direct radiative effects of dust and carbonaceous aerosols over the Indian subcontinent

A large uncertainty exists today in quantifying the absorbing aerosol snow darkening effect (SnoDE) and its relative role compared to the direct radiative effect (DRE), especially at the regional scale. Using a regional climate model over an enlarged South Asia Coordinated Regional Climate Downscaling Experiment (CORDEX) domain, here we isolate these effects and examine their impacts on the dynamics of the Indian pre-monsoon and summer monsoon (ISM) circulation. Our results clearly indicate that the regional dynamics associated with the pre-monsoon and ISM are highly sensitive to both aerosol SnoDE and DRE, with the SnoDE dominating during the pre-monsoon, and the DRE during the ISM. During the pre-monsoon, the deposition of aerosols on the Himalayas and Tibetan Plateau (HTP) reduces the surface albedo, leading to an increase in absorption of solar radiation and accelerated snow melting. The associated increase in surface and tropospheric temperature draws dry air from central Asia towards northern India and along the Himalayan foothills, creating a low-level anti-cyclonic anomaly over eastern India and the Bay of Bengal. This limits the northward propagation of southwest winds and leads to a decrease in precipitation over the central and eastern parts of India during the pre-monsoon and across northern India in the following ISM. Conversely, an increase in precipitation during the ISM is due to the strengthening of southwest winds caused by large-scale convergence over the Arabian Peninsula. The shift of winds from central Asia towards northern India in the SnoDE and towards the Arabian Peninsula in DRE leads to contrasting effects and can possibly modulate the monsoon onset dates. The SnoDE, with a prevailing contribution from dust, leads to an increase in precipitation over the HTP, which is amplified during the ISM when DRE is also considered. Our study provides evidence that both these aerosol effects have substantial impacts on the pre-monsoon and ISM dynamics and hence need to be considered in climate model simulations over the region.

Meridionally Extending Anomalous Wave Train over Asia During Breaks in the Indian Summer Monsoon

Anomalous interactions between the Indian summer monsoon (ISM) circulation and subtropical westerlies are known to trigger breaks in the ISM on subseasonal time-scales, characterised by a pattern of suppressed rainfall over central-north India, and enhanced rainfall over the foothills of the central–eastern Himalayas (CEH). An intriguing feature during ISM breaks is the formation of a mid-tropospheric cyclonic circulation anomaly extending over the subtropical and mid-latitude areas of the Asian continent. This study investigates the mechanism of the aforesaid Asian continental mid-tropospheric cyclonic circulation (ACMCC) anomaly using observations and simplified model experiments. The results of our study indicate that the ACMCC during ISM breaks is part of a larger meridional wave train comprising of alternating anticyclonic and cyclonic anomalies that extend poleward from the monsoon region to the Arctic. A lead–lag analysis of mid-tropospheric circulation anomalies suggests that the meridional wave-train generation is linked to latent heating (LH) anomalies over the CEH foothills, Indo-China, and the Indian landmass during ISM breaks. By conducting sensitivity experiments using a simplified global atmospheric general circulation model forced with satellite-derived three-dimensional LH, it is demonstrated that the combined effects of the enhanced LH over the CEH foothills and Indo-China and decreased LH over the Indian landmass during ISM breaks are pivotal for generating the poleward extending meridional wave train and the ACMCC anomaly. At the same time, the spatial extent of the mid-latitude cyclonic anomaly over Far-East Asia is also influenced by the anomalous LH over central–eastern China. While the present findings provide interesting insights into the role of LH anomalies during ISM breaks on the poleward extending meridional wave train, the ACMCC anomaly is found to have important ramifications on the daily rainfall extremes over the Indo-China region. It is revealed from the present analysis that the frequency of extreme rainfall occurrences over Indo-China shows a twofold increase during ISM break periods as compared to active ISM conditions.

Sensitivity of convective and land surface parameterization in the simulation of contrasting monsoons over CORDEX-South Asia domain using RegCM-4.4.5.5

An effort for testing the capability and suitability of different cumulus and land surface parameterization schemes in simulating the Indian summer monsoon (ISM) and associated features during contrasting monsoon years using a regional climate model framework is presented. The latest Regional Climate Model system (hereafter RegCM, Vn4.4.5.5) developed and managed by the International Centre for Theoretical Physics (ICTP) has been used to downscale the ERA-Interim reanalysis dataset for three different ISM years, namely, normal (1990), deficit (1987), and excess (1988). Three different convection schemes, namely, Grell (GFC), Emanuel (MIT), and mixed type (i.e., Grell over land and Emanuel over ocean (MIX)), have been used for the simulation of ISM and its associated features. Moreover, three different land surface schemes, namely, the Biosphere–Atmosphere Transfer Scheme or BATS (control), subgrid disaggregation of BATS (SUB-BATS), and Community Land Model 4.5 (CLM4.5), have also been tested for their performance. With these combinations, 27 different suites of experiments were simulated at a horizontal resolution of 50 km over the COordinated Regional Climate Downscaling Experiment (CORDEX)-South Asia domain. The validation of experiments was performed against the India Meteorological Department (IMD) and Climatic Research Unit (CRU) daily dataset for precipitation and near surface air temperature, respectively. In light of different performance metrics, it is found that the near surface air temperature and precipitation in the seasonal-scale simulations are well represented in space for most of the experiments with inherent biases. Among all the experiments, the SUB-BATS scheme in association with GFC cumulus scheme is found to perform consistently among all the years in simulating the ISM precipitation. In most of the cases for the simulation of precipitation, the SUB-BATS seems to outperform the control and CLM4.5 experiments. The domination of dry bias in CLM4.5 experiments is attributed to the improper representation of mean sea level pressure, weaker ISM circulation, and higher atmospheric stability, thus inhibiting the convective activity. Compared to other convection schemes, the MIT scheme is more suitable in representing the features of ISM simulation while using CLM4.5 as a land surface scheme. However, the GFC scheme was found suitable for the simulation of different precipitation years while using SUB-BATS as a land surface model. Such simulations portray reduced biases and improved spatial patterns compared to other schemes. Additionally, CLM4.5 experiments display more utility for the simulation of near surface air temperature. A more comprehensive process-based approach is envisaged to investigate the performance of CLM4.5 in the simulation of ISM for climate-scale simulations.

On the connection between remote dust aerosol and Indian summer monsoon

Understanding of interaction between remote dust aerosol and Indian summer monsoon (ISM) remains dubious in literature because of wide disagreement among previous studies. This problem is revisited using version-2 of Modern-Era Retrospective Analysis for Research and Applications (MERRA-2) datasets. ISM variability at intraseasonal timescale is not constrained by the changes in dust aerosols over the most parts of West Asia and North Africa. A new hypothesis is proposed to explain covariability of dust over the Arabian Sea and ISM circulation. Large-scale forcing modulates monsoon intraseasonal oscillations (ISOs) and creates active-like condition over the Indian landmass extending up to the Arabian Sea and the Arabian Peninsula. Organized convection induces anomalous southwesterly and northwesterly circulation as a response to enhanced latent heating. Enhanced northwesterly winds strengthen transport of dust over the Arabian Sea. Augmented dust forcing and associated warming over the Arabian Sea is unlikely to create a positive feedback because of its limited spatial extent. Findings of the present study provide a new insight into remote dust-ISM connection problem and the hypothesis proposed differs considerably from the previously proposed hypotheses.

Importance of seasonally resolved oceanic emissions for bromoform delivery from the tropical Indian Ocean and west Pacific to the stratosphere

Abstract. Oceanic very short-lived substances (VSLSs), such as bromoform (CHBr3), contribute to stratospheric halogen loading and, thus, to ozone depletion. However, the amount, timing, and region of bromine delivery to the stratosphere through one of the main entrance gates, the Indian summer monsoon circulation, are still uncertain. In this study, we created two bromoform emission inventories with monthly resolution for the tropical Indian Ocean and west Pacific based on new in situ bromoform measurements and novel ocean biogeochemistry modeling. The mass transport and atmospheric mixing ratios of bromoform were modeled for the year 2014 with the particle dispersion model FLEXPART driven by ERA-Interim reanalysis. We compare results between two emission scenarios: (1) monthly averaged and (2) annually averaged emissions. Both simulations reproduce the atmospheric distribution of bromoform from ship- and aircraft-based observations in the boundary layer and upper troposphere above the Indian Ocean reasonably well. Using monthly resolved emissions, the main oceanic source regions for the stratosphere include the Arabian Sea and Bay of Bengal in boreal summer and the tropical west Pacific Ocean in boreal winter. The main stratospheric injection in boreal summer occurs over the southern tip of India associated with the high local oceanic sources and strong convection of the summer monsoon. In boreal winter more bromoform is entrained over the west Pacific than over the Indian Ocean. The annually averaged stratospheric injection of bromoform is in the same range whether using monthly averaged or annually averaged emissions in our Lagrangian calculations. However, monthly averaged emissions result in the highest mixing ratios within the Asian monsoon anticyclone in boreal summer and above the central Indian Ocean in boreal winter, while annually averaged emissions display a maximum above the west Indian Ocean in boreal spring. In the Asian summer monsoon anticyclone bromoform atmospheric mixing ratios vary by up to 50 % between using monthly averaged and annually averaged oceanic emissions. Our results underline that the seasonal and regional stratospheric bromine injection from the tropical Indian Ocean and west Pacific critically depend on the seasonality and spatial distribution of the VSLS emissions.

Different Asian Monsoon Rainfall Responses to Idealized Orography Sensitivity Experiments in the HadGEM3-GA6 and FGOALS-FAMIL Global Climate Models

Recent work has shown the dominance of the Himalaya in supporting the Indian summer monsoon (ISM), perhaps by surface sensible heating along its southern slope and by mechanical blocking acting to separate moist tropical flow from drier midlatitude air. Previous studies have also shown that Indian summer rainfall is largely unaffected in sensitivity experiments that remove only the Tibetan Plateau. However, given the large biases in simulating the monsoon in CMIP5 models, such results may be model dependent. This study investigates the impact of orographic forcing from the Tibetan Plateau, Himalaya and Iranian Plateau on the ISM and East Asian summer monsoon (EASM) in the UK Met Office’s HadGEM3-GA6 and China’s Institute of Atmospheric Physics FGOALS-FAMIL global climate models. The models chosen feature oppositesigned biases in their simulation of the ISM rainfall and circulation climatology.The changes to ISM and EASM circulation across the sensitivity experiments are similar in both models and consistent with previous studies. However, considerable differences exist in the rainfall responses over India and China, and in the detailed aspects such as onset and retreat dates. In particular, the models show opposing changes in Indian monsoon rainfall when the Himalaya and Tibetan Plateau orography are removed. Our results show that a multi-model approach, as suggested in the forthcoming Global Monsoon Model Intercomparison Project (GMMIP) associated with CMIP6, is needed to clarify the impact of orographic forcing on the Asian monsoon and to fully understand the implications of model systematic error.

Relative role of sea surface temperature and snow on Indian summer monsoon seasonal simulation using a GCM

Indian summer monsoon is a global scale phenomenon controlled by different land, ocean, and atmospheric parameters. Sea surface temperature (SST) and snow are two of the major parameters, which may alter the spatial and temporal patterns of circulation and rainfall during Indian summer monsoon. In the current paper, we study the monsoon variability using long integrations (20 years) of the Indian Institute of Technology Delhi (IITD) Spectral model at T80L18 resolution with observed and climatological SST and snow. Study shows response of IITD GCM in simulating the Indian summer monsoon rainfall and circulation relative to the snow and SST as boundary conditions. The model’s response to SST and snow is examined by conducting four types of experiments by varying observed and climatological values of snow and SST. This paper discusses the seasonal total rainfall for country as a whole and 850 and 200 hPa wind for the period of 20 years starting from 1985 to 2004. The model has been integrated in the ensemble mode with five different initial conditions from the last week of April and first week of May. The model is able to capture the climatological patterns of seasonal total rainfall and averaged wind at lower and upper levels. Observed snow in the presence of climatological SST as a boundary condition shows much impact on rainfall and circulation than observed SST in the presence of climatological snow. Model performance is good in simulating the normal and excess monsoon conditions; it shows poor skill in capturing deficit monsoon years.

Indian Summer Monsoon Sub-seasonal Low-Level Circulation Predictability and its Association with Rainfall in a Coupled Model

This study investigates predictability of the sub-seasonal Indian summer monsoon (ISM) circulation and its relation with rainfall variations in the coupled model National Centers for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2). Hindcasts based on CFSv2 for the period of 1982–2009 are used for detailed analysis. Though the model is capable of predicting the seasonal ISM rainfall at long lead months, the predication skill of the model for sub-seasonal rainfall in general is poor for short and long lead except for September. Rainfall over the ISM region/Indian Subcontinent is highly correlated with the low-level jet (LLJ) or Somali jet both in the observations and the model. The model displays improved skill in predicting LLJ as compared to precipitation in seasonal mean and September, whereas the model skill is poor for June and August. Detailed analysis reveals that the model LLJ variations throughout the season are overdependent on the El Nino-Southern Oscillation (ENSO) unlike in the observations. This is mainly responsible for the model’s low skill in predicting LLJ especially in July and August, which is the primary cause for the poor rainfall skill. Though LLJ is weak in September, the model skill is reasonably good because of its ENSO dependency both in model and the observations and which is contributed to the seasonal mean skill. Thus, to improve the skill of seasonal mean monsoon forecast, it is essential to improve the skill of individual months/sub-seasonal circulation and rainfall skill.

Response of the tropical Indian Ocean SST to decay phase of La Niña and associated processes

Delayed impact of El Niño on Tropical Indian Ocean (TIO) Sea Surface Temperature (SST) variations and associated physical mechanisms are well documented by several studies. However, TIO SST evolution during the decay phase of La Niña and related processes are not adequately addressed before. Strong cooling associated with La Niña decay over the TIO could influence climate over the Indian Oceanic rim including Indian summer monsoon circulation and remotely northwest Pacific circulation. Thus understanding the TIO basin-wide cooling and related physical mechanisms during decaying La Niña years is important. Composite analyses revealed that negative SST anomalies allied to La Niña gradually dissipate from its mature phase (winter) till subsequent summer in central and eastern Pacific. In contrast, magnitude of negative SST anomalies in TIO, induced by La Niña, starts increasing from winter and attains their peak values in early summer. It is found that variations in heat flux play an important role in SST cooling over the central and eastern equatorial Indian Ocean, Bay of Bengal and part of Arabian Sea from late winter to early summer during the decay phase of La Niña. Ocean dynamical processes are mainly responsible for the evolution of southern TIO SST cooling. Strong signals of westward propagating upwelling Rossby waves between 10°S to 20°S are noted throughout (the decaying phase of La Niña) spring and summer. Anomalous cyclonic wind stress curl to the south of the equator is responsible for triggering upwelling Rossby waves over the southeastern TIO. Further, upwelling Rossby waves are also apparent in the Arabian Sea from spring to summer and partly contributing to the SST cooling. Heat budget analysis reveals that negative SST/MLT (mixed layer temperature) anomalies over the Arabian Sea are mostly controlled by heat flux from winter to spring and vertical advection plays an important role during early summer. Vertical and horizontal advection terms primarily contribute to the SST cooling anomalies over southern TIO and the Bay of Bengal cooling is primarily dominated by heat flux. Further we have discussed influence of TIO cooling on local rainfall variations.