Hamza Varikoden

Indian Institute of Tropical Meteorology | IITM · Centre for Climate Change Research

Indian summer monsoon (ISM) is the prominent global monsoon system, which occur annually from June to September and impacts the lives of over a quarter of the world's population. Studies show that global warming is one of the key reasons for the extreme events such as droughts and floods, and that also alter regional dynamics of ISM. Consequently,...

The study addresses the elevated occurrence of lightning activity and associated incidents over Kerala, India, where the topography is complex. It aims to systematically investigate the spatiotemporal variations in lightning activity while elucidating the intricate relationships of lightning occurrences with dynamic and thermodynamic variables. Lig...

We assess the spatio-temporal changes in global atmospheric nitrogen dioxide (NO₂) in its hotspots and 3000 cities with respect to past and current vehicular policies for the period 2002–2019. The quantile-quantile (Q-Q) adjustment method is applied to merge the NO₂ measurements from different satellites to make the long-term data (2002–2019) for t...

Sulphur dioxide (SO2) is a hazardous air pollutant, which is mostly emitted from burning of fossil fuels, and has an adverse impact on the human health and ecosystem functioning. The COVID-19 natural anthropause (lockdown) provides a great opportunity to understand the changes in SO2 pollution across the globe, as there was a temporary standstill f...

The Earth’s surface temperatures have increased significantly since the beginning of industrialisation. The substantial emissions of greenhouse gases have played a role in global warming and the ongoing climate change, with projections indicating continued trends. This study explores the long-term surface temperature trends in India from 1980 to 20...

Deciphering the rainfall trends over the Western Ghats situated along the west coast of India has been the subject of several recent studies. However, less attention is rendered, particularly to understanding the spatial characteristics of atmospheric features associated with widespread and elevation-dependent extreme rainfall occurrences over this...

We examined the interrelationship between the Indian summer monsoon rainfall (ISMR), the west Pacific subtropical high (WPSH), and tropical cyclone (TC) activity over the western north Pacific (WNP) Ocean during the peak monsoon season in 1951–2019. When WNP TCs are inactive (active), there is a noticeable 10° westward (eastward) shift observed alo...

The impact of climatic change on the summer monsoon season is studied to understand the rainfall pattern towards the end of the century utilizing the Coupled Model Intercomparison Project, Phase 6 (CMIP6) released by the World Climate Research Programme (WCRP). The analysis of model simulations from CMIP6 was carried out using 64 years of the histo...

The Earth's temperature has witnessed a substantial rise since the start of industrialization. In this study, we delve into the patterns of surface temperature within the arid region of northwest (NW) India. The arid regions across India exhibit remarkable diversity encompassing aspects like climate, topography, soil, wildlife, and human endeavors....

Temperature measurements are essential to monitor the changes in global and regional weather and climate over a period. Continuous measurements of temperature are used for in-depth analysis of rising temperature in a region, increasing heatwaves or changes in the seasonal temperature distribution. The Indian Space Research Organisation (ISRO) has d...

The Earth’s surface temperatures have increased significantly since the beginning of industrialisation. The high emissions of greenhouse gases have contributed to global warming and climate change, which is expected to continue. Here, we investigate the long-term trends of surface temperature in India using surface, satellite and reanalysis data fo...

The present study explores the intraseasonal variability of the west Pacific subtropical high (WPSH) and its relation with Indian summer monsoon rainfall (ISMR) based on the IMD rainfall data and NCEP-NCAR reanalysis data sets for the 1950–2021 period. The longitudinal position of the western edge of WPSH around 20° N is about 139.3° E in June, whi...

In this study, we have analyzed the variabilities in the global monsoon rainfall using Climate Research Unit monthly data for the period 1901 to 2020. The global monsoon domains are selected based on the difference in rainfall between the summer and winter as mentioned in the study of Wang and Ding (2006). The regions with high rainfall in the nort...

In this study, we investigated the possible relation between the Indian summer monsoon and the combination of the different phases of Pacific Decadal Oscillation (PDO) and El Niño Southern Oscillation (ENSO) before and after the climate shift in 1976. This study is carried out using IMD’s rainfall dataset, HadISST v1.1 dataset and twentieth century...

The Western Ghats (WG) is one of the important biodiversity hotspots and rainfall intense regions in south Asia. In this paper, the long-term trend analysis is carried out during southwest monsoon rainfall over different latitudinal regions of the WG. Further, the gridded daily rainfall data (0.25 × 0.25 spatial grid resolution) is classified into...

The increasing population and its associated amenities demand innovative devices, infrastructure, methods, plans and policies. Regional climate has a great role in deciding the air quality and energy demand, and therefore, weather and climate have an indisputable role in its consumption and storage. Here, we present the changes in trace gases and a...

In the present study, we have examined extreme rainfall events during the southwest monsoon and their relationship with vertically integrated atmospheric moisture transport. For the analysis, we have used daily rainfall data from IMD, wind and humidity data from NCEP from 1951 to 2020. Extreme rainfall events are identified when the daily rainfall...

In this study, characteristics of summer monsoon rainfall (SMR) are investigated for two regions, West Coast (WC) and Central India (CI), in the Indian subcontinent during 1901–2020. We classified rain events into different intensity bins, viz. dry, low, moderate, high, very high and extreme. SMR in CI exhibits a significant decreasing trend of −5....

The Indian subcontinent is home to 1.39 billion people, mainly dependent on the agrarian-based economy. The Indian summer monsoon rainfall from June to September plays a vital role in providing water for agricultural and other lively needs. This rainfall is highly varying in the spatiotemporal domain and thus, leads to localized and organized extre...

In this study, we investigated the possible relation between the Indian summer monsoon and the combination of the different phases of Pacific Decadal Oscillation (PDO) and El Niño Southern Oscillation (ENSO) before and after the climate shift in 1976. This study is carried out using IMD’s rainfall dataset, HadISST v1.1 dataset and twentieth century...

Indian summer monsoon provides rainfall over a large area during 01 June to 30 September and it plays vital role for the water needs of the population of India. It is intense because strong differential heating prevails over the region due to geographical features of India. Further, it can be viewed as a synoptic scale ocean-atmosphere interactive...

Changes in rainfall patterns can have a profound impact on water availability, and therefore, examining the variability of rainfall is critical in addressing water resources and regional climate change. Tripura state of northeast India is known to have received a high amount of rainfall in the past few decades, but currently the region suffers from...

An out‐of‐phase pattern of fishery between anchovy and sardine exists in upwelling systems of different geographical locations. We examined the inverse relationship of anchovy (Stolephorus sp., Engraulidae) and sardine (Sardinella longiceps, Clupeidae) fishery along the coast of the south‐eastern Arabian Sea (SEAS) by analysing the annual landing d...

Structure of the atmospheric boundary layer (ABL) over the west coast of India has been investigated using conserved variable analysis (CVA). Daily radiosonde data of Thiruvananthapuram (TVM), Mangalore (MNG) and Mumbai (MUM) at 00 UTC during the monsoon season of 2018 procured from India Meteorological Department have been primarily utilised for t...

In the present paper, the variabilities and long-term trends of summer monsoon rainfall for different intensity bins (dry, low, moderate, high, very high, and extreme) are studied for five homogeneous regions, namely Northeast India (NEI), Northcentral India (NCI), Northwest India (NWI), Westcentral India (WCI), and Peninsular India (PI) for 118 ye...

We examine the projected changes in Indian summer monsoon rainfall (ISMR) and El Niño Southern Oscillation (ENSO) along with their changing relationships by the end of 21st century using Coupled Model Inter Comparison Project Phase5 (CMIP5) models. The historical (1951–2005) and future (2050–2099) projections of ISMR were assessed in addition to th...

We investigate the characteristics and causes of extreme rainfall event occurred in Kerala (south western regions of the peninsular India) in August 2018. The changes in large-scale circulations over the Indo-Pacific domain and their association with regional circulation features, which made the extreme rainfall in Kerala are analysed. During this...

The study explores the spatial variability and related characteristics of south Asian summer monsoon rainfall and extreme rainfall events based on APHRODITE rainfall products. An attempt was also made to investigate their association with global climate indices. Most of the South Asian regions are vulnerable to extreme rainfall events and many of t...

The study investigates thermodynamic structure of the atmospheric boundary layer (ABL) over the west coast of India during the southwest monsoon season. Radiosonde profiles at 00 UTC (0530 IST) are utilized for three stations, namely Thiruvananthapuram (TVM), Mangalore (MNG) and Mumbai (MUM) to represent south, central and north parts of the west c...

The present study explored the influence of different convective parameters such as lower tropospheric stability (LTS), low and medium cloud covers (LCC and MCC), and convective available potential energy (CAPE) on the regional variability of Indian summer monsoon rainfall (ISMR). The variabilities and trends of summer monsoon rainfall over the wes...

The variability of West Pacific subtropical high (WPSH) during the peak monsoon (July–August) season for the period 1951–2015 and its association with Indian summer monsoon rainfall (ISMR) is investigated based on various observational data sets. In the present study, we explored the long term variability of WPSH based on location index (zonal disp...

The Indian summer monsoon rainfall (ISMR) during June to September contributes most of the annual rainfall over India and plays an important role in Indian agriculture and thus the economy. It exhibits high spatio-temporal variabilities forced from both internal and external factors, which are important for better understanding and prediction of IS...

The sub-daily variability of latent (LHF) and sensible heat flux (SHF) in the Bay of Bengal (BoB) during the summer (May–September) is examined using moored buoys data at 8° N (2008 and 2011), 12° N (2010, 2011, 2012, 2013, 2014, and 2015), and 15° N (2009, 2013, 2014, and 2015) along 90° E. In the weak wind regime (< 6 ms⁻¹), LHF loss from the oce...

Changes in rainfall affect drinking water, river and surface runoff, soil moisture, groundwater reserve, electricity generation, agriculture production and ultimately the economy of a country. Trends in rainfall, therefore, are important for examining the impact of climate change on water resources for its planning and management. Here, as analysed...

This study explores the potential of atmospheric moisture content, its transport and its divergence over the ocean and land as proxies for the variability of Indian summer monsoon rainfall (ISMR) for the period 1950–2019. The analyses using multiple linear regression reveal that the interannual and intraseasonal variability of ISMR and the mean ISM...

During 2015, the southwest monsoon (SWM) rainfall over the country remained deficient with seasonal rainfall of about 86% of the long period average (Table 1.1). Last year, the seasonal rainfall deficiency over the country as a whole was 12% (www.imd.gov.in). Thus, this is a fourth episode of two consecutive years, with deficient monsoon, similar t...

The El Niño Southern Oscillation (ENSO) is an important coupled ocean–atmosphere phenomenon in the tropical Pacific and an important modulator of the spatio-temporal variability of Indian summer monsoon rainfall (ISMR). Here, we explore the impact of ENSO during onset (June), peak (July–August) and withdrawal (September) phases of ISMR for the peri...

In this study, we explored the changes in the Indian Ocean Dipole‐Indian summer monsoon rainfall (IOD‐ISMR) relationship from early (1951–1980) to recent (1986–2015) multidecades during onset (June), peak (July–August) and withdrawal (September) phases of Indian summer monsoon. The analyses indicate that the IOD‐ISMR relationship is strengthening (...

Atmospheric ammonia (NH3) is an alkaline gas and a prominent constituent of the nitrogen cycle that adversely affects ecosystems at higher concentrations. It is a pollutant, which influences all three spheres such as haze formation in the atmosphere, soil acidification in the lithosphere, and eutrophication in water bodies. Atmospheric NH3 reacts w...

Varikoden, H.; Roja, C.; Revadekar, J.V., and Milind, M., 2020. Rainfall variation in major river basins in India and the association with the Indo-Pacific Oceans. In: Sheela Nair, L.; Prakash, T.N.; Padmalal, D., and Kumar Seelam, J. (eds.), Oceanic and Coastal Processes of the Indian Seas. Journal of Coastal Research, Special Issue No. 89, pp. 1-...

Trends and features of Atmospheric Boundary Layer (ABL) over southern part of west coast of India has been studied using MERRA datasets for 1980–2018 during different seasons incorporating the marine and continental influences. A significant decrease in the ABL height (ABLH) has been observed in the westerly regimes of the west coast. The shallow A...

A regional tree ring-width index chronology prepared from various tree core samples of the western Himalaya has been analyzed in relation to climate fluctuations. The correlation analysis of tree ring chronology shows significant positive correlations with regional rainfall and standardized precipitation evapotranspiration index (SPEI) and negative...

This study examines the response of surface layer parameters and ABL height (ABLH) to the change in cloudiness over Kochi. Outgoing long wave radiation (OLR) is considered as a proxy for the cloudiness. Temperature, horizontal wind speed, vertical velocity variance (VVV), sensible heat flux, momentum flux, turbulent kinetic energy (TKE), drag coeff...

In the present study, seven-year-long observations of rain microphysical properties are presented using a ground-based disdrometer located at Braemore; a site on the windward slope of the Western Ghats (WG) over the Indian Peninsula. The annual cycle of rainfall shows a bimodal distribution with a primary peak during summer monsoon and secondary pe...

We observe significant changes in the ENSO-Indian summer monsoon rainfall (ISMR) relationship in past three multidecadal epochs (early epoch: 1931-1960, middle epoch: 1961-1990, and recent epoch: 1991-2015) based on consistent correlation. The rainfall during early epoch was above normal, however, in other two epochs it was relatively dry. The ENSO...

The extreme rainfall events over India during 2017 southwest monsoon and associated mechanisms are investigated using daily rainfall, winds and SST data sets. Extreme events occurred over mainly four locations according to the designed criteria, which are westcentral India (WC), central India (CI), northeast (NE) India and northwest coastal region...

Hydrological disasters are recurrent over northeast Indian region (NEI) due to heavy downpour within a short span of time. Therefore, we discuss characteristics of rainfall extremes during the southwest monsoon over NEI, their evolution and dissipation features using rainfall data from TRMM 3B42 V7 and CPC for 16 years from 2000 to 2015. Circulatio...

Westward propagation and the dynamics of intensification of Lakshadweep low (LL) have been investigated in detail. In this study, it is observed that LL is not propagated westwards in a single stretch to the western boundary of the southern Arabian Sea but takes place in multiple phases of propagation as well as through multiple eddy formations. LL...

This study presents the contrasting trends of rainfall in the northern and southern Western Ghats (WG) and examines possible reasons for the phenomenon. The WG is one of the important mountain ranges that run parallel to the west coast of India. The mountain chain lies almost perpendicular to the low level jet stream (LLJ) and hence, receives about...

This study explores the features of atmospheric surface layer over Kochi during active and weak phases of the southwest monsoon season. The classification of active and weak phases of monsoon is made on the basis of monsoon organized convection over the region. When the monsoon organized convection is over (away from) Kochi, considered it as an act...

An attempt has been made to classify stratiform and convective rain using an impact type disdrometer at a tropical coastal site based on (Testud et al., 2001) method. Using this classification scheme, all rain events were analysed and computed their duration (minutes) and accumulated water (mm) in convective, stratiform, transition and mixed rain t...

India is home for more than a billion people and its economy is largely based on agrarian society. Therefore, rainfall received not only decides its livelihood, but also influences its water security and economy. This situation warrants continuous surveillance and analysis of Indian rainfall. These kinds of studies would also help forecasters to be...

Soil moisture is a key parameter in the prediction of southwest monsoon rainfall, hydrological modelling, and many other environmental studies. The studies on relationship between the soil moisture and rainfall in the Indian subcontinent are very limited; hence, the present study focuses the association between rainfall and soil moisture during dif...

This study undertakes a comprehensive assessment of dynamical downscaling of summer monsoon (June–September; JJAS) rainfall over heterogeneous regions namely the Western Ghats (WG), Central India (CI) and North-Eastern Region (NER) for long term mean, excess and deficit episodes for the historical period from 1951 to 2005. This downscaling assessme...

The effect of global climate change on Indian summer monsoon has been analysed with special emphasis on active-break cycle. The changes in intensity and duration of active and break monsoon conditions towards the end of the century are studied by using 850 hPa zonal circulations. The analysis is carried out using twenty year climatology of historic...

The Western Ghats (WG) of India are basically north-south oriented mountains having narrow zonal width with a steep rising western face. The summer monsoon winds during June to September passing over the Arabian Sea are obstructed by the WG and thus orographically uplift to produce moderate-to-heavy precipitation over the region. However, it is see...

The eastern Pacific Ocean received a record highest number of sub-tropical convective activities during boreal summer (June–September) of 2015, since last four decades. The associated rainfall distribution was also atypical with anomalously enhanced rainfall extending from equator to sub-tropical central-eastern Pacific. The present analysis reveal...

Drop size distribution (DSD) over the tropical region exhibit pronounced variations during different monsoon seasons. Measurements from an impact type Joss–Waldovgel disdrometer is used for characterization of drop size distribution and its integral parameters over a tropical coastal station (Thiruvananthapuram, 8.31°N, 76.54°E, 20 m asl). Rain eve...

The Sultanate of Oman is located in the south-eastern part of the Arabian Peninsula and covers the larger part of the southern coasts of the Arabian Peninsula in both arid and semi-arid environments except for the southern part which is swept by the monsoon affecting the Arabian Sea during the period from June to September. The summer rainfall over...

The oceans and the atmosphere are tightly linked and they together form the most dynamic component of the climate system. Topography and proximity to the surrounding seas of the region determine the temperature of the area. West Coast (WC) of India is a high elevated region surrounded by large oceanic area, therefore, an attempt is made in this stu...