This study investigates the projected changes in the upper troposphere and lower stratosphere (UTLS) water vapor over the Asian summer monsoon (ASM) region based on satellite records, numerical ...simulations using variable-resolution global climate model focused over south Asia (HIST-natural and anthropogenic forcing in the historical period, and FUT-following RCP4.5 in future), and Coupled Model Intercomparison Project Phase 5 (CMIP5) datasets. The simulations generally reproduced the seasonal cycle in the UTLS water vapor and regional water vapor maximum. With progressive warming in future, excessive upper tropospheric moistening is noted over the ASM region in far-future (2070–2095) climate against the HIST climate (1980–2005) with water vapor mixing ratio increasing to ~ 7.5 ppmv relative to ~ 5 ppmv noted in the HIST. It is further noted that projected changes in water vapor are linked to anomalous warming (~ 1–4 K) in the upper tropospheric layers juxtaposed with zonally elongated ASM anticyclone and enhanced water vapor flux divergence by amplifications in rotational winds. Further, the simulations indicate robust increase in ASM upper tropospheric water vapor as compared to those at mid- and lower- troposphere in accordance with the Clausius–Clapeyron temperature dependence of moisture response to warming and amplified troposphere warming with altitude. A simple comparison between the ASM and the entire globe indicates that upper tropospheric water vapor-temperature relationship has a similar response, however, the projected variability in temperature and moisture is significantly larger (about twice) over the ASM region highlighting strong regional influence. Nonetheless, the projections indicate that ASM is a potential regional source in modulating UTLS water vapor budget in a warming climate.
Extreme precipitation and flood episodes in the Himalayas are oftentimes traced to synoptic situations involving connections between equatorward advancing upper level extratropical circulations and ...moisture-laden tropical monsoon circulation. While previous studies have documented precipitation characteristics in the Himalayan region during severe storm cases, a comprehensive understanding of circulation dynamics of extreme precipitation mechanisms is still warranted. In this study, a detailed analysis is performed using rainfall observations and reanalysis circulation products to understand the evolution of monsoon-extratropical circulation features and their interactions based on 34 extreme precipitation events which occurred in the Western Himalayas (WEH) during the period 1979–2013. Our results provide evidence for a common large-scale circulation pattern connecting the extratropics and the South Asian monsoon region, which is favorable for extreme precipitation occurrences in the WEH region. This background upper level large-scale circulation pattern consists of a deep southward penetrating midlatitude westerly trough, a blocking high over western Eurasia and an intensifying Tibetan anticyclone. It is further seen from our analysis that the key elements of monsoon-midlatitude interactions, responsible for extreme precipitation events over the WEH region, are: (1) midlatitude Rossby wave breaking, (2) west-northwest propagation of monsoon low-pressure system from the Bay of Bengal across the Indian subcontinent, (3) eddy shedding of the Tibetan anticyclone, (4) ageostrophic motions and transverse circulation across the Himalayas, and (5) strong moist convection over the Himalayan foothills. Furthermore, high-resolution numerical simulations indicate that diabatic heating and mesoscale ageostrophic effects can additionally amplify the convective motions and precipitation in the WEH region.
This study examines the role of Asian monsoons on transport and spatial variability of atmospheric CO2 over the Indian subcontinent, using transport modeling tools and available surface observations ...from two atmospheric CO2 monitoring sites Sinhagad (SNG) and Cape Rama (CRI) in the western part of peninsular India. The regional source contributions to these sites arise from the horizontal flow in conduits within the planetary boundary layer. Greater CO2 variability, greater than 15ppm, is observed during winter, while it is reduced nearly by half during summer. The SNG air sampling site is more susceptible to narrow regional terrestrial fluxes transported from the Indo-Gangetic Plains in January, and to wider upwind marine source regions from the Arabian Sea in July. The Western Ghats mountains appear to play a role in the seasonal variability at SNG by trapping polluted air masses associated with weak monsoonal winds. A Lagrangian back-trajectory analysis further suggests that the horizontal extent of regional sensitivity increases from north to south over the Indian subcontinent in January (Boreal winter).
•Impact of the Indian monsoon circulations on surface CO2 observations over India•Continental and marine tracer transport mechanism influence the Indian subcontinent•The lower CO2 events along the west coast of India are due to marine transport•CO2 variability is 8–10ppm during summer monsoon and >15ppm for rest of the year
Accumulation of water vapor in the upper troposphere/lower stratosphere (UT/LS) over the Asian continent is a recognized feature during the boreal summer monsoon. While there has been a debate on the ...role of monsoon convective intensities on the UT/LS water vapor accumulations, there are ambiguities with regard to the effects of organized monsoon convection on the spatial distribution of water vapor. We provide insights into this aspect using high precision balloon measurements of water vapor from a high-elevation site Nainital (29.4° N, 79.5° E), India, located in the Himalayan foothills and satellite retrievals of water vapor from the Microwave Limb Sounder (MLS). We also use precipitation estimates from the Tropical Rainfall Measuring Mission (TRMM) satellite (i.e., merged product 3B42 and precipitation radar 3A25 estimates of rain rate and rain type viz convective/stratiform), reanalysis circulation data, as well as numerical model simulations. We first evaluate the MLS estimates of water vapor mixing ratios with in situ high precision hygrometer balloon observations over Nainital. It is seen from our analyses of the MLS data that the LS water vapor distribution is closely linked to the organization of the South Asian monsoon convection and its influence on the UT/LS circulation. This link between LS water vapor distribution and organized monsoon convection is also captured in the in situ observations on 3 August 2016. It is evidenced that periods of organized summer monsoon convective activity over the Indian subcontinent and Bay of Bengal promote divergence of water vapor flux in the UT/LS; additionally the Tibetan anticyclonic circulation causes widespread distribution of the UT/LS water vapor. In addition to the effects of Asian monsoon convection, we also note that global climate drivers such as El Niño-Southern Oscillation (ENSO), Brewer–Dobson circulation (BDC), and Quasi-Biennial Oscillation (QBO) can contribute to nearly 38% of the UT/LS water vapor variability over the Asian monsoon region. The main result of our study indicates that widespread spatial distribution and accumulation of water vapor in the LS (about 80% of total accumulation between May and August months) tend to co-occur with organized monsoon convection, intensified divergence of water vapor flux in the UT/LS and intensified Tibetan anticyclone. On the other hand, the circulation response and LS water vapor distribution to pre-monsoon localized deep convection tend to have a limited spatial scale confined to Southeast Asia. Results from model experiments suggest that the UT/LS circulation pattern to organized monsoon convection has resemblance to stationary Rossby waves forced by organized latent heating, with the westward extending response larger by about 15° longitudes as compared to that of the pre-monsoon localized deep convection.
Boreal wintertime planetary-scale atmospheric circulations and their possible consequences to widespread fog occurrences over the Indo-Gangetic Plains (IGP) region of the Himalayan valley are ...investigated in this study. Among the different fog types, radiation fog type seen at night or early morning hours favored by large-scale subsidence aloft and strong near-surface inversion is focused in this study. A composite analysis reveals that upper air circulation associated with 105 fog days over the IGP region show a trail linked to circulation anomalies over the Eurasian continents and the Arctic Circle. The findings suggest that there is a footprint of the Arctic Oscillation (AO) and conventional Eurasian (EU) circulation patterns linked to anticyclonic circulation aloft over the IGP region. Although widespread IGP fog occurrences under the large-scale subsidence environment are seen to occur during both phases of AO, the negative AO phase (high pressure environment over the Arctic Circle) portends a greater likelihood for fog occurrences in the IGP region. A coupling of positive mid-tropospheric height anomalies over western Eurasia and the anticyclonic circulation anomalies over the IGP region is evident during the IGP fog periods concomitant with EU positive (height excess over Siberia) phase. Further, anomalous circulation over the IGP region during the fog periods appears to rely more on the strength of the AO negative phase than the circulation strengths over Eurasia. On the contrary, the Eurasian circulation largely appears to influence the subsidence aloft over the IGP region irrespective of the strength of the AO positive phase. It is also noted that upper-air circulation during non-foggy periods over the IGP region has conformity with positive AO phase and rapidly progressing EU pattern. These planetary-scale teleconnection pathways offer new dynamical insights into comprehending widespread IGP fog scenario, which have been hitherto perceived mostly from a regional context.
The rising number of extreme rainfall events over the Himalayan foothill states of India during the recent decades has become a serious issue with the growing concern of aerosol influences. This ...study intends to provide some insight into aerosol and gas chemistry responses to changes in monsoon circulation and precipitation, and also assess the impact of aerosols on two recent infamous heavy rainfall events using coupled meteorology–chemistry–aerosol (WRF-Chem) model simulations. The sensitivity of aerosols and chemistry on rainfall distribution and the amount is evaluated using the simulations with and without chemistry. Results from this study show that the magnitude and spatial distribution of precipitation are significantly influenced by including aerosol and gas chemistry in the model simulations. Realistic meteorological conditions as well as rainfall amount and distribution are reproduced when aerosols and gasses are taken into account in the simulation. There is an overall enhancement of total cumulative rainfall as high as 20% due to aerosols and gas chemistry over the western Himalayan Indian states. This study shows that cloud-microphysical properties and the resulting precipitation distribution depend critically on the aerosol types and their concentrations under similar thermodynamic conditions. This study highlights the role of aerosol and gas chemistry and recognizes the importance of atmospheric chemistry in the model simulation for the analysis of Himalayan extreme precipitation events, and its further associations with the Himalayan hydrology.
In this study, a suite of ten Weather Research and Forecasting (WRF) model simulations is conducted using various choices of local and non-local representations of vertical mixing adopted in the ...planetary boundary layer (PBL) parameterizations to assess the boreal summertime marine layer cloud environment in the southern Indian Ocean (SIO). The local, non-local, and hybrid type PBL schemes used in the simulations are generally able to produce the pertinent features of the marine layer embedded with stratocumulus clouds in the SIO region, viz., nocturnal coverage of low clouds with warm cloud tops, well-mixed marine layer, inversion capping, strong temperature and moisture jumps across the inversion layer, cloud water generation and its coupling with cloud-top radiative cooling, as conceptually envisaged in earlier investigations. However, there is a large spatial variability noticed in the simulated low clouds over the SIO region. While the satellite diagnosed cloud liquid water paths (LWPs) are noted as high as 0.4 mm in the region 60° E–90° E, 15° S–25° S, the simulated cloud LWPs show a large mesoscale variability exhibiting a range of values between 0.04 and 2.8 mm, corresponding to cloud depths as small high as 200 1700 m. It is further noted that LWPs reproduced from hybrid PBL schemes employed with eddy diffusivity mass flux (EDMF) formulation are closer to the observed estimates in the SIO region. While the non-local PBL mixing schemes tend to produce more organized sheeted low-cloud layers, the local closure schemes tend to produce more discreteness in the low-cloud decks and also exhibit substantial cloud depth variability. In addition, the coupling of in-cloud turbulence in association with cloud-top radiative cooling and sub-cloud layer turbulence are not uniquely reproduced by these schemes. The inferences from this study suggest that non-local/hybrid type of PBL parameterizations accounting for stratocumulus cloud-top driven mixing processes shows greater promise in the reproduction of low-clouds in the SIO region.
An intriguing feature associated with ‘breaks’ in the Indian summer monsoon is the occurrence of intense/flood-producing precipitation confined to central-eastern parts of the Himalayan (CEH) ...foothills and north-eastern parts of India. Past studies have documented various large-scale circulation aspects associated with monsoon-breaks, however the dynamical mechanisms responsible for anomalous precipitation enhancement over CEH foothills remain unclear. This problem is investigated using diagnostic analyses of observed and reanalysis products and high-resolution model simulations. The present findings show that the anomalous precipitation enhancement over the CEH foothills during monsoon-breaks emerges as a consequence of interactions between southward intruding mid-latitude westerly troughs and the South Asian monsoon circulation in its weak phase. These interactions facilitate intensification of mid-tropospheric cyclonic vorticity and strong ascending motion over the CEH foothills, so as to promote deep convection and concentrated rainfall activity over the region during monsoon-breaks. Mesoscale orographic effects additionally tend to amplify the vertical motions and precipitation over the CEH foothills as evidenced from the high-resolution model simulations. It is further noted from the model simulations that the coupling between precipitation and circulation during monsoon-breaks can produce nearly a threefold increase of total precipitation over the CEH foothills and neighborhood as opposed to active-monsoon conditions.
Using available satellite observations and Modern‐Era Retrospective analysis for Research and Applications Version‐2 data sets for the period 1980–2016, this study reveals a summertime dust ...amplification over the Arabian Peninsula (AP) during about past two decades. Our results demonstrate a significant positive trend in summertime dust loading over the AP since the year 2002, with a maximum increase of 21% over the southern Red Sea. The increased summertime dust over the southern AP is attributed to the intensification of the remote dust transport from the Sahara Desert through Sudan by a strengthened Tokar Gap westerly jet and a general increased gustiness in the AP. Furthermore, increased both air and soil temperature and reduction in the soil moisture along with increased sensible heat flux led to increased local dryness in the AP and played a significant role in enhancing wind‐induced localized dust emissions therein. The associated changes in the AP include an enhancement of net radiative fluxes, particularly the long wave radiative flux.
Plain Language Summary
Our results show a significant positive trend in summertime dust loading over the Arabian Peninsula in the recent decade. This increasing summertime dust over the southern Arabian Peninsula is attributed to intensification of westerly winds. Air and soil temperatures play a pivotal role in enhancing wind induced localized dust through a positive feedback. Enhanced dust may also modulate the regional surface net radiative fluxes.
Key Points
We find a significant positive trend in summertime dust loading over the Arabian Peninsula (AP) in recent decades
The increased summertime dust, particularly over the southern AP, is attributed to intensification of westerly winds
Air and soil temperatures play a pivotal role in enhancing the wind‐induced localized dust through a positive feedback
This study investigates the sub-synoptic scale circulation aspects associated with the extreme rainfall event occurred over the North Indian state of Uttarakhand located in the western Himalayas (WH) ...during the 15–18 June 2013 period. A diagnosis based on hourly ERA5 reanalyzed circulation products archived on finer grids reveals that sustenance of heavy rains during the event period is supported by a propensity of cyclonic vorticity sources channeled toward the WH region through a narrow quasi-steady conduit in the lower troposphere from the ISM circulation. The equatorward segregating mesoscale potential vorticity (PV) structures from the quasi-stationary upper level PV anomaly (trough) during the event administered two pathways for vorticity sources. The first pathway is from the base of the trough culminating into longer horizontal conduit path from the western Arabian Sea, lending perpetual cyclonic vorticity support to the ISM environment. The second pathway is from the right flank of the trough, which promotes sustained environment of deeper mesoscale convergence zone, potentially unstable atmosphere and strong ascent over the Uttarakhand region. The convergence zone is potentially viewed as a region for strong monsoon and extratropical circulation interactions to occur on finer horizontal scales of motion, where significant vertical synchronization of positive PV advection is realized during the 16–17 June 2013 period. In addition to orographic precipitation enhancements, deeper advective synchronization noticed at sub-synoptic time periods is accredited to the nearly doubling 24-h rainfall amounts in the foothill region of Uttarakhand during the event period. The ERA5 diagnosed diabatic heating additionally indicates that precipitating systems at higher (foothill) elevations contribute to upper (lower) tropospheric heat sources.
PDF
