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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.

Precipitation is an important component of the global water cycle, and the impacts of anthropogenic climate change on precipitation have significant implications on agricultural activities (Porter et al. 2014).

This study presents an analysis of daily field-scale soil-moisture (SM) variations, measured using the COsmic-ray Soil Moisture Observing System (COSMOS), over a tropical monsoon site (IITM, Pune) in India, for the period 2017–2020. Being located in the core zone of the Indian summer monsoon, the daily field-scale SM observations at COSMOS-IITM provide an unique opportunity to understand the SM response to monsoon precipitation variations on sub-seasonal, seasonal and interannual time-scales. In addition to the COSMOS-IITM observations, we also evaluated SM variations over this location using satellite, reanalysis and model products for the same period. An important result from our analysis reveals the presence of biweekly (time-scale ~ 10–20 days) and low-frequency intra-seasonal (time-scale ~ 30–60 days) variations in the field-scale SM, which are linked to the dominant modes of Indian summer monsoon subseasonal variability. In particular, we find a pronounced enhancement of the low-frequency signal of SM variations during the 2019 monsoon which was characterized by abnormally excess precipitation and prolongation of rains well beyond the summer monsoon season, in contrast to 2018 monsoon. Moreover, this study highlights a longer persistence of SM memory time-scale (about 60 days) during 2019 as compared to 2017, 2018 and 2020. The validation of coarser resolution data sets revealed that GLDAS and ERA5 reasonably capture a range of observed field-scale SM variabilities over COSMOS-IITM site.

The semi-permanent systems such as Seasonal Heat Low (HL), Monsoon Trough (MT), Tibetan Anticyclone (TA), Tropical Easterly Jet (TEJ) and Low Level Jet (LLJ) or Somali jet are observed over Indian region during Indian summer monsoon season (June through September). These systems play a vital role in defining the strength of the Indian summer monsoon rainfall as a whole. Here we evaluate the ability of Consortium for Small-Scale Modeling (COSMO) regional Climate Model (COSMO-CLM), a high resolution regional climate model within the Coordinated Regional Climate Downscaling Experiment for South Asia (CORDEX-SA) framework, to simulate these systems of Indian summer monsoon. The historical runs of the COSMO-CLM for the period 1951-2000 are analysed. Overall the COSMO-CLM is able to simulate these components reasonably well. Possible changes in the position and the strength of these systems and their role in changing rainfall pattern over India are examined to assess the impact of global warming, under the RCP 4.5 simulations towards the end of the century (2051-2100). The analysis shows that the semi permanent systems may not strengthen in the future as compared to the present climate. The summer monsoon rainfall does not show uniform changes over the region. It is likely to enhance over the southern parts of the country, south of 20?S while it is projected to decrease in the northern parts under the global warming scenario.

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.

The impact of global warming on the characteristics of mean and extremes of rainfall over India is investigated using a high resolution regional climate model PRECIS developed by Hadley Centre, UK. Five simulations of PRECIS made using the lateral boundary conditions from a suite of Perturbed Physics Ensembles (PPE) generated using Hadley Center Coupled Model (HadCM3) for Quantifying Uncertainty in Model Predictions (QUMP) project corresponding to IPCC A1B emission scenario have been analyzed here for this purpose. The projected changes depict seasonally dependent fine scale structure in response to the topographic forcing and changes in circulation, especially along the west coast and North East (NE) region of India towards the end of the 21st century i.e. 2080s (2071–2098). Analysis of the extreme precipitation indices indicates an increase in the intensity of rainfall on wet days towards 2080s under A1B scenario. Changes in extreme precipitation events and dry spells suggest not only shifts, but also a substantial increase in the spread of the precipitation distribution, with an increased probability of the occurrence of events conducive to both floods and droughts. The projected changes in various precipitation extremes show a large regional variability. Total rainfall on very heavy rainy days (R95p) is projected to increase by around 40–50% over the central parts of the country. The number of rainy days > 10 mm (R10) may increase by 10–20% over west coast, east central India and northeastern parts while over northwest and rain shadow region they may increase by 40–50%. The consecutive dry days (CDDs) may decrease by 10–20% over Indo-Gangetic plain, however over west coast there may not be any significant change. The CDDs are projected to rise by 10–20% over west central and peninsular India. The precipitation per wet day (SDII) may be more intense by 10–40% over the entire land mass, however there may not be any significant change over south peninsular India.

The day-to-day behavior of Indian summer monsoon rainfall (IMR) is associated with a hierarchy of quasi-periods, namely 3–7, 10–20 and the 30–60 days. These two periods, the 10–20 days and the 30–60 days have been related with the active and break cycles of the monsoon rainfall over the Indian sub-continent. The seasonal strength of Indian summer monsoon rainfall may depend on the frequency and duration of spells of break and active periods associated with the fluctuations of the above intra-seasonal oscillations (ISOs). Thus the predictability of the seasonal (June through September) mean Indian monsoon depends on the extent to which the intra-seasonal oscillations could be predicted. The primary objective of this study is to bring out the dynamic circulation features during the pre-monsoon/monsoon season associated with the extreme phases of these oscillations The intense (weak) phase of the 10–20 (30–60) days oscillation is associated with anti-cyclonic circulation over the Indian Ocean, easterly flow over the equatorial Pacific Ocean resembling the normal or cold phase (La Nina) of El Nino Southern Oscillation (ENSO) phenomenon, and weakening of the north Pacific Sub-tropical High. On the other hand the weak phase of 10–20 days mode and the intense phase of 30–60 days mode shows remarkable opposite flow patterns. The circulation features during pre-monsoon months show that there is a tendency for the flow patterns observed in pre-monsoon months to persist during the monsoon months. Hence some indications of the behavior of these modes during the monsoon season could be foreshadowed from the spring season patterns. The relationship between the intensity of these modes and some of the long-range forecasting parameters used operationally by the India Meteorological Department has also been examined.

South Asian summer monsoon (June through September) rainfall simulation and its potential future changes are evaluated in a multi-model ensemble of global coupled climate models outputs under World Climate Research Program Coupled Model Intercomparison Project (WCRP CMIP3) dataset. The response of South Asian summer monsoon to a transient increase in future anthropogenic radiative forcing is investigated for two time slices, middle (2031–2050) and end of the twenty-first century (2081–2100), in the non-mitigated Special Report on Emission Scenarios B1, A1B and A2 .There is large inter-model variability in the simulation of spatial characteristics of seasonal monsoon precipitation. Ten out of the 25 models are able to simulate space–time characteristics of the South Asian monsoon precipitation reasonably well. The response of these selected ten models has been examined for projected changes in seasonal monsoon rainfall. The multi-model ensemble of these ten models projects a significant increase in monsoon precipitation with global warming. The substantial increase in precipitation is observed over western equatorial Indian Ocean and southern parts of India. However, the monsoon circulation weakens significantly under all the three climate change experiments. Possible mechanisms for the projected increase in precipitation and for precipitation–wind paradox have been discussed. The surface temperature over Asian landmass increases in pre-monsoon months due to global warming and heat low over northwest India intensifies. The dipole snow configuration over Eurasian continent strengthens in warmer atmosphere, which is conducive for the enhancement in precipitation over Indian landmass. No notable changes have been projected in the El Nino–Monsoon relationship, which is useful for predicting interannual variations of the monsoon.

South Asian summer monsoon precipitation and its variability are examined from the outputs of the coupled climate models assessed as part of the Intergovernmental Panel on Climate Change Fourth Assessment. Out of the 22 models examined, 19 are able to capture the maximum rainfall during the summer monsoon period (June through September) with varying amplitude. While two models are unable to reproduce the annual cycle well, one model is unable to simulate the summer monsoon season. The simulated inter-annual variability from the 19 models is examined with respect to the mean precipitation, coefficient of variation, long-term trends and the biennial tendency. The model simulated mean precipitation varies from 500 mm to 900 mm and coefficient of variation from 3 to 13%. While seven models exhibit long-term trends, eight are able to simulate the biennial nature of the monsoon rainfall. Six models, which generate the most realistic 20th century monsoon climate over south Asia, are selected to examine future projections under the doubling CO2 scenario.

The relationship of summer monsoon over India with the Indian Ocean Dipole Mode has been investigated applying simple statistical techniques. While a long time series of 132 years based on 1871–2002 for both summer monsoon rainfall as well as dipole mode index has been used in this study, the NCEP–NCAR reanalysis data (1948–2002) are used to examine the circulation features associated with the extreme dipole and monsoon phases. These flow patterns bring out the dynamics of the dipole – monsoon relationship.

The influence of the Indian Ocean Zonal Mode on the extreme summer monsoon rainfall over East Asia (China, Korea, Japan) has been investigated applying simple statistical techniques of correlation and composite analysis. While the observed rainfall data are used as a measure of rainfall activity, the NCEP-NCAR Reanalysis data are used to examine the circulation features associated with the extreme monsoon phases and the dynamics of the zonal mode – monsoon variability connections. The data used covers the period 1960 to 2000.

The Intergovernmental Panel on Climate Change (IPCC) constituted by the World Meteorological Organisation provides expert guidance regarding scientific and technical aspects of the climate problem. Since 1990 IPCC has, at five-yearlyintervals, assessedand reported on the current state of knowledge and understanding of the climate issue. These reports have projected the behaviour of the Asian monsoon in the warming world. While the IPCC Second Assessment Report (IPCC, 1996) on climate model projections of Asian/Indian monsoon stated ``Most climate models produce more rainfall over South Asia in a warmer climate with increasing CO2'', the recent IPCC (2001) Third Assessment Report states ``It is likely that the warming associated with increasing greenhouse gas concentrations will cause an increase in Asian summer monsoon variability and changes in monsoon strength.'' Climate model projections(IPCC, 2001) also suggest more El Nino – like events in the tropical Pacific, increase in surface temperatures and decrease in the northern hemisphere snow cover. The Indian Monsoon is an important component of the Asian monsoon and its links with the El Nino Southern Oscillation (ENSO) phenomenon, northern hemisphere surface temperature and Eurasian snow are well documented. In the light of the IPCC globalwarming projections on the Asian monsoon, the interannual and decadal variability in summer monsoon rainfall over India and its teleconnections have been examined by using observed data for the 131-year (1871–2001) period. While the interannual variations showyear-to-year random fluctuations, thedecadal variations reveal distinct alternate epochs of above and below normal rainfall. The epochs tend to last for about three decades. There is no clear evidence to suggest that the strength and variability of the Indian Monsoon Rainfall (IMR) nor the epochal changes are affected by the global warming. Though the 1990s have been the warmest decade of the millennium(IPCC, 2001), the IMR variability has decreased drastically. Connections between the ENSO phenomenon, Northern Hemisphere surface temperature and the Eurasian snow with IMR reveal that the correlations are not only weak but have changed signs in the early 1990s suggesting that the IMR has delinked not only with the Pacific but with the Northern Hemisphere/Eurasian continent also. The fact that temperature/snow relationships with IMR are weak further suggests that global warming need not be a cause for the recent ENSO-Monsoon weakening. Observed snow depth over theEurasian continent has been increasing, which could be a result of enhanced precipitation due to the global warming.

¶This study presents the monthly climatology and variability of the INSAT (Indian National Satellite) derived snow cover estimates over the western Himalayan region. The winter/spring snow estimates over the region are related to the subsequent summer monsoon rainfall over India. The NCEP/NCAR data are used to understand the physical mechanism of the snow-monsoon links. 15 years (1986–2000) of recent data are utilized to investigate these features in the present global warming environment.

The satellite derived outgoing longwave radiation (OLR) over the Indian Ocean (30°N–30°S and 40°E–100°E) from 1974 to 1996 has been analysed for the relationship with the Indian summer monsoon total (June–September) rainfall. The OLR of two regions appears to be related to summer monsoon rainfall. One of the regions is located over the Head Bay of Bengal (near 22.5°N and 92.5°E) during May and the other one over the south Indian Ocean (near 30°S and 97.5°E) during April. The average OLR (index) for these two regions shows a strong and stable relationship with the Indian summer monsoon rainfall and they are found to be independent. A multiple linear regression equation is developed to predict the Indian summer monsoon rainfall using these indexes and the empirical relations are verified on independent data. Good results were obtained in forecasting the summer monsoon rainfall for the whole of India. The forecast of summer monsoon rainfall for west-central India and all-India rainfall for July also appears to be encouraging. The indexes, thus, seem to be useful in long-range forecasting of the Indian summer monsoon rainfall. Copyright © 2000 Royal Meteorological Society