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Interannual variability of LTT, SST, rainfall and OLR. a Interannual variability of anomalous sea surface tem-perature (SST) (°C, black) and lower tropospheric temperature (LTT: 1000 hPa to 700 hPa average) (°C, red) anomalies respectively over the Arabian sea (65-70 °E and 15-20 °N) and the western coast of India (73-77 °E and 8-20 °N) during JJAS. b Interannual variability of anomalous rainfall (mm.day −1 , Blue) and OLR(Wm −2 , Orange) anomalies, respectively, over the western coast of India (73-77 °E and 8-20 ° N) during JJAS. Fitted trends obtained by linear regression on year are indicated by dashed lines.
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A disastrous cloudburst and associated floods in Kerala during the 2019 monsoon season raise the hypothesis that rainfall over the west coast of India, much of which is stratiform, may be trending towards being more convective. As a first exploration, we sought statistically significant differences in monthly ERA-5 reanalysis data for the monsoon s...
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... of meridional oceanic heat transports, themselves related to the seasonal monsoon winds 38 . As convenient indices of this highly coupled system, here we considered SST anomalies over the Eastern Arabian Sea (65-70 °E, 5-20 °N) and lower tropospheric temperature (LTT) over the west coast of India (73-77 °E, 8-20 °N) for time series analysis (Fig. 1a). These show an in-phase relationship interannually and comparable positive 40-year trends perhaps indicative of forced climate change 39 , with a robust correlation (r = 0.9, significant at 95% confidence level). Recently, Jin and Wang (2017) 20 reported that there has also been an increase in the land-sea thermal gradient, with more ...
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... series of seasonal mean rainfall and outgoing long-wave radiation (OLR) from ERA-5 (see Data and methods) show a rising and falling trend respectively over the west coast (Fig. 1b). These variables are highly anti-correlated on interannual timescales (r = −0.82 overall), but the strength of their trends appears to differ distinctly. This is our first hint that cloud tops may be trending upward in altitude, above and beyond the fact that surface precipitation is rising in magnitude. A significant rainfall trend ...
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... time series replicates those prior findings of upward rainfall trends (blue curve in Fig. 1b), although this gentle trend is not significant at 95% confidence. We also find a downward trend of OLR (orange curve), which does exceed 95% statistical confidence tests, despite the similar year-to-year noise level in the two-time series. Figure 2 shows scatter and regression results for the data in panel 1b. Precipitation and OLR ...
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Citations
... In Western India and Pakistan, fluctuations in precipitation pose a considerable threat to communities, as these changes significantly influence various socioeconomic factors and the overall well-being of the population. Over the past few decades, Pakistan and northwestern India frequently experienced heavy precipitation and floods (Lyngwa and Nayak, 2021;Sreenath et al., 2022;Yadav, 2016). ...
The phenomenon of climate change has significantly impacted weather patterns across the globe, particularly in South Asia. This study indicates a notable westward shift in precipitation and heat wave frequency over India, Pakistan and their adjacent regions. This shift is characterized by altered monsoon patterns, increased frequency of extreme weather events, and changing temperature regimes, which collectively pose challenges to water resources, cryosphere, agriculture, and public health. To comprehensively understand the dynamic nature of heat wave frequency, here we choose average vertical pressure velocity (ω), geopotential height, meridional wind vector and average temperature anomalies and heat wave frequency number of days meeting the criteria of at least three consecutive days above the 90 th percentile. Our findings reveal a pronounced westward shift in heat wave frequency, particularly between
... The west coast of India is generally known to be associated with shallow warm clouds during the SWM season (see Grossman and Garcia 1990;Roca and Ramanathan 2000;Francis and Gadgil 2006;Maheshkumar et al. 2014). Observational studies also report that more deep convective cloud occurrences are emerging within the latitude belt of 8°N-14°N of the west coast of India in recent decades (Vijaykumar et al. 2021;Sreenath et al. 2022). A longitude-pressure view of Q H climatology (diabatic heating rate) in June shows a deeper heating structure on the windward side of southern sectors of WG with a primary and secondary maximum at lower and upper levels, respectively, while only the heating maximum is seen at lower levels in July in the same region. ...
Earlier studies have shown a declining precipitation trend over the Indian state of Kerala during the southwest monsoon season since the early twentieth century. However, a notable change in trend is witnessed in this region during the 1990–2020 period relative to the previous decade. This study provides some insights using observations and ERA5 reanalysis. Seasonal rainfall amounts for the state of Kerala during the study period (1990–2020) show significant zonal variations, with rising trends over the midland (elevations 30–200 m asl) and highland (exceeding 400 m asl) regions and a falling trend over the lowland. In monthly data, near-compensating precipitation trends are noted from the first and second halves of the monsoon season (i.e., a negative precipitation trend during June and July, followed by a positive trend in August and September.). This pattern signals a slowing down of the overall declining trend in Kerala’s monsoon rainfall since the 1990s. Circulation trends appear to support this pattern, with June and July showing deficient moisture convergence over the Kerala region, while August and September exhibit enhanced moisture convergence. Additionally, increased moisture transport in August and September favors stronger ascent and deeper convective cloud development in the region. Observations also indicate a shift in Kerala’s rainfall patterns in recent decades relative to the 1960–1989 period. We investigated the climate change signals by decomposing precipitation changes into dynamic and thermodynamic contributions using a water vapor vertical advection budget framework. Our analysis demonstrates that the precipitation changes during 1990–2020 are predominantly driven by alterations in large-scale atmospheric circulation rather than by variations in moisture availability when compared with the earlier period. Contrary to earlier assessments in the scientific literature regarding highland rainfall in the Western Ghats, recent decades have shown greater rainfall variability and more frequent intense rain events in the highlands of Kerala. This also motivates the critical need for a larger observation network, particularly in mid- and highland regions of Kerala, to further understand the impact of complex mesoscale terrain on zonal variations in rainfall as the climate changes.
... However, previous assessments 64- 66 have solely focused on mean and extreme precipitation events and their intra-seasonal variability, and have not considered the interactions between humid heat and extreme rainfall. ...
Hot-wet compound events, the sequential occurrence of humid hot days followed by extreme rainfall, can cause catastrophic consequences, often exceeding the impacts of the isolated occurrence of each event. The urban-coastal microclimate is confounded by complex interactions of land-sea breeze circulations, urban effects of convection and rainfall, and horizontal advection of moisture, which can favor the hot-wet compound occurrence. We present the first observational assessment (1951-2022) of summertime hot-wet compound events across global coastal megacities. We find a significant (P < 0.001) increase in the frequency of hot-wet compound events in both hemispheres: on average,~3 events in the 1950s to 43 events in the 2020s. Cities with upward trends in the frequency of hot-wet compound events are situated < 30 km from coasts, with cities in the southern hemisphere showing faster hot-to-wet transition times (<3 days) than cities in the northern hemisphere. Further, 26 out of 29 sites show increased extreme precipitation, reaching 153%, when humid heat amplitude rises from the 50th to 90th percentiles. Understanding hot-wet compound interactions over the world's coasts is highly relevant for climate change impact assessment and informing climate adaptation. Most megacities are located in the coastal zone, with about 40% of the world's population residing within 100 km of the coast 1. Globally, coastal areas are at increased risk of flooding due to relative sea level rise, land subsidence, and altered storm intensity and frequency 2-4. In addition, the world coastlines are also hotspots of humid heat stress 5. Increased urbanization can alter the wind direction in coastal areas due to changes in the density and height of buildings, which reduces the sea breeze in the fall, whereas increased surface temperature at night in urban areas often leads to decreased land breeze 6. Observations showed a latitudinal pattern of heatwaves over the coasts with a robust increase in severity in the past decades due to increasing air temperature and reduced wind speed, often leading to slower-moving heatwave events, elevating the risk of ecosystem productivity reduction, rising energy consumption, and capacity needs 7. Further, coastal heatwaves are often accompanied by persistent high sea surface temperature , resulting in exposure to high temperature and humidity in cities close to coasts 8. Moreover, globally, coastal precipitation peaks in the boreal summer 9. The superposition of heat stress, humidity, and precipitation may lead to hot-wet compound events-the sequential occurrence of humid hot days and extreme rainfall. Such events can pose a significant threat to coastal communities and cause greater damage than the isolated occurrence of either of these extremes. For instance, a heatwave could massively increase the number of people who need medical assistance and trigger power blackouts 10. In such a vulnerable period, extreme rainfall and flooding could place additional stress on the critical infrastructure 2,11 , for instance, by interrupting traffic and water provision. During summer, temperature and precipitation are generally antic-orrelated over the interior part of the continent but positively correlated over oceans and near the coasts 12,13. Coasts are transition areas where local characteristics also affect the interplay between temperature, humidity, and precipitation. Extreme humid heatwaves often lead to high atmospheric instability and moisture convection, increasing the likelihood of precipitation extremes 14. High atmospheric instability, moisture, and frontal systems jointly mediate rainfall extremes that follow heatwaves 15. There is growing evidence of the co-occurrence of humid heatwaves and extreme precipitation in several regions from observations, such as in China 16-21 , India 14 , Australia 22 , and the USA 23 , and global climate projection scenarios 24-26. These assessments, however, are limited to either smaller spatial domain focusing on particular country 19-21 or coarse-resolution gridded observations [e.g., 0.5°spatial resolution in CRU grid-based observations in Europe 27 and China 21,28 , and 2.5°observational records from the India
... This means that low OLR values, indicating strong convection, are associated with high monsoon intensity. Sreenath et al. (2022) found an inverse relationship between precipitation and OLR in the west coast region of India. In their results, there is a weak trend of decreasing OLR and increasing precipitation in this region. ...
... This is particularly noticeable in June, when the CI is highest, resulting in widespread precipitation. Additionally, the analysis of the Outgoing Longwave Radiation (OLR) in the region between latitudes 10°-20° N, which includes the Bay of Bengal and the Arabian Sea, reveals an increase in cloud cover, rainfall, and convective activity over India and its neighbouring regions (Sreenath et al. 2022;Pattanaik 2021). This corresponds with a significant drop in sea-level pressure in the Bay of Bengal and the Indian Peninsula, which facilitates the westward transport of moistureladen air from nearby water bodies. ...
In the summer of 2022, Iran experienced severe flooding that caused extensive damage to infrastructure, agriculture, and residential areas. This event prompted an investigation into whether the frequency and extent of severe summer flooding in Iran have increased due to global warming. This study investigates long‐term trends in Widespread Rainy Days (WRDs) in Iran during the summer season (June to September) and analyses their correlation with Indian summer monsoon indices, specifically the Convective Index (CI) and U‐wind Index (UI). Reanalysis data, including precipitation, Sea Level Pressure (SLP), and Geopotential Height (GPH) from 1836 to 2015, were used for the analysis. WRDs in Iran were calculated for each year using the connected components labelling method. The results indicate a positive correlation between precipitation and WRDs with the CI, implying that an increase in Outgoing Longwave Radiation (OLR) in the Bay of Bengal leads to higher rainfall and more WRDs in Iran. Amongst all observed correlations, the highest correlation (r = 0.58) was found in southeastern Iran. On days with the highest CI values, negative anomalies in sea level pressure (SLP) and 850‐hPa geopotential height (GPH) are observed. These anomalies, along with increased humidity advection, contribute to the occurrence of WRDs in Iran. Conversely, Iran's summer rainfall shows a negative correlation with the UI. On days with the highest UI values, no negative anomalies were observed in SLP and 850‐hPa GPH, and reduced humidity advection resulted in the absence of WRDs in Iran. The study also reveals that the majority of WRDs occur in June, with the highest density observed in the southeast region of Iran. The Mann–Kendall test indicates a decrease in WRDs in Iran, particularly in June (R² = 17%) and September (R² = 24%). Whilst July and August also exhibit a decline, the trend is relatively weaker (R² = 2%). These findings demonstrate a statistically significant decline in WRDs over the 180‐year period, contradicting the hypothesis of increased monsoon rains in Iran due to global warming.
... Inland regions such as northwest and northeast India exhibit significant trends, with northeast India showing the highest increase in extreme rainfall. A study of intense precipitation over the west coast regions attributes the increased convective activity to the warming of eastern Arabian Sea (Sreenath et al., 2022). ...
Plain Language Summary
India, like many other regions worldwide, is experiencing extreme weather. Traditionally dry areas are now receiving more rain, while typically rainy areas are seeing less. Additionally, intense rainfall events are increasing, leaving people uncertain on how to adapt. Reports like those from the IPCC, project up to a 20% increase in extreme rainfall events, but there is still much uncertainty about what to expect and where these changes will occur. Global warming leads to stronger convective activity, forming deep convective clouds that cause heavier rainfall. Cloud top height is crucial for understanding this activity and can be measured using satellite data. Higher clouds appear colder in satellite images, with the coldest pixels indicating the tallest clouds, especially deep convective ones. This study examines the changes in deep convective clouds from 2000 to 2020 using climate quality satellite data. The results suggest that deep convective cloud tops have risen by about 1 km over the study period. This increase in cloud height is linked to more extreme rainfall during India's monsoon season. Given the ongoing trend in deep convective cloud top temperatures, these changes are poised to continue influencing extreme weather patterns across India in the future.
... Studies have indicated that rainfall over the west coast of India is becoming more convective, which may be attributed to the SST increase over the southeastern Arabian Sea (Sreenath et al. 2022). Arora (2021) has shown that rainfall over the Western Ghats is positively correlated with SST anomalies over the Arabian Sea during the monsoon season. ...
The west coast of India has recently been experiencing torrential monsoon rains, a trend that studies indicate is likely to continue under future warming scenarios. This study investigates the link between moisture flux and extreme rainfall over the west coast, using observational and reanalysis datasets for the monsoon seasons (June to September) from 1990 to 2023. The analysis shows that, over the Indian subcontinent, rainfall along the west coast is primarily influenced by large‐scale moisture flux from the Arabian Sea. By decomposing the vertically integrated moisture flux into dynamic and thermodynamic components, this study observes that the thermodynamic component of moisture flux exhibits an increasing trend over the southwest coast, while this increasing trend is more prominent for the dynamic component over the northwest coast. Extreme rainfall over the southwest coast is increasing at a rate of 0.23 mm per season, attributed primarily to the increase in the thermodynamic component of moisture flux. It is observed that the rate of sea surface temperature (SST) increase over the Arabian Sea is faster than over the Bay of Bengal, with the average SST over the southeast Arabian Sea exceeding 28°C in recent years. Observations indicate that warming over the southeast Arabian Sea is strongly coupled with moisture accumulation observed over the southwest coast. This study provides strong evidence of a link between moisture transport, extreme rainfall and SST, identifying the southwest coast as a region vulnerable to climate change. Over the northwest coast, the incidence of extreme rainfall is associated with the strengthening of dynamic processes, and the mean monsoon rainfall in this region is increasing in alignment with the rising dynamic component of moisture flux.
... India comprises the tropical monsoon zone bearing high spatial variation in rainfall (Mohapatra et al., 2021). Country receive most of the rainfall through the south west monsoon which strikes the Western Ghats in the month of June and progress throughout all the region (Sreenath et al., 2022). The state of Gujarat is located in the extreme western part of the country with the longest sea coast. ...
Climate change, recognized as the long-term change in atmospheric conditions is one of the most evident feature of 21st century. Limited studies on Saurashtra's rainfall variability underscore the need to explore temporal and spatial patterns, vital for sustainable water resource management and agricultural planning in the region. This study examines seasonal and sub-seasonal rainfall trends (1981–2020) using high-resolution IMD data, applying Z-score, Mann-Kendall test, Sen's slope estimator, and Rainfall Anomaly Index for analysis. The region has experienced increased variability in rainfall, particularly at the upper extremes. Statistical analysis shows a significant increase in mean rainfall during 2001–2020, with distinct trends compared to earlier decades. Notably, September exhibits the largest upward trend in rainfall at +1.53 mm/year. Seasonal RAI highlights a shift from predominantly negative rainfall anomalies (1981–2000) to positive anomalies in the later period (2001−2020). These findings emphasize studying atmospheric dynamics to understand future rainfall changes.
... Since Karnataka state is geographically diverse (Kumar, 2019), its different geographic regions show different trends in crop production (Satnami & Surendra, 2019) as well as various indices. The western coastal region of the state with abundant rainfall (Sreenath et al., 2022) shows higher values of indices exhibited by SPI (Fig. 3) and represents healthy vegetation conditions in the form of higher values of NDVI and VCI (Fig. 4) in the region. Although a high percentage of forest and tree cover leaves little land for crop cultivation. ...
Drought indices are imperative for determining the occurrence and impact of drought on crop production and society. Selecting appropriate indices to comprehensively assess the stress conditions is critical to analyze their effects on crops. The current study focuses on a comparative analysis of various indicators including the Standardized Precipitation Index (SPI), Standardized Water level Index (SWI), Standardized Reservoir Level Index (SRLI), Moderate Resolution Imaging Spectroradiometer (MODIS) Normalized Difference Vegetation Index (NDVI), NDVI Anomaly, Vegetation Condition Index (VCI), and Evaporative Stress Index (ESI). The spatio-temporal pattern of these indices and their relationship with detrended Kharif foodgrain production anomaly (DPA) were examined for the Kharif crop growing season from 2001 to 2019. The results revealed that the region experienced drought conditions 15 times, with SPI-6 and ESI showing the strongest correlations with DPA — correlation coefficients of 0.76 and 0.74, respectively. To assess the percentage area under drought (AUD) values, the total pixels classified as drought based on threshold values for each index were calculated and the results were compared with figures declared by the state government. SPI-3, SPI-6, VCI, and ESI significantly captured the extent of drought yielding correlation coefficients of 0.75, 0.82, 0.62, and 0.74 respectively. Observations from this study may be used to determine the most critical indicators for drought monitoring.
... Therefore, it is likely that the lateritic soils are diluted by higher proportions of weathering products from Fe-Mn ores resulting in MREE-and HREEenriched REE patterns. This could be due to quick washing and transfer of ore material because of intense and heavy rainfall during the SW monsoon (Sreenath et al. 2022). ...
The sediments of 90 rivers from five states along the west coast of India were analysed for their chemistry. The major element ratios of sediments suggest lateritic soils in Kerala, lateritic soils admixed with particulates weathered from Fe–Mn ores in Karnataka and Goa and, non-lateritic, chemically weathered soils in Maharashtra and Gujarat. The sediments from the Archean-Proterozoic (A-P) terrain are strongly weathered, while those from Deccan Traps (DT) terrain are intermediate to strongly weathered. The mean total rare earth elements content (∑REE) of sediments from the rivers of Kerala is much higher than in other states. ∑REE shows strong positive correlation with oxides of Fe, Mn and P from A-P terrain and Al, Fe and Ti from DT terrain and, strong positive correlation with heavy metals (Zr, U, Hf and Th) from both terrains. The low Sm/Nd and Y/Ho ratios corresponding to high chemical index of alteration (CIA) and, high ratios corresponding to high and low CIA are typical. The Post-Archean average Australian shale (PAAS)-normalized REE shows LREE- and MREE-enriched patterns in the sediments of Kerala and MREE- and HREE-enriched patterns in other sediments. Distinct positive Ce anomaly occurs in the sediments of Karnataka, Goa and south Maharashtra and, weak positive to weak negative Ce anomaly in the sediments of other states. The Eu anomaly is negative in the silts of south Kerala and positive in all other sediments. The primary sources for REEs are adsorbed REEs onto secondary mineral phases and clay minerals and, heavy minerals. The change in REE patterns is primarily related to the source rock composition. Fractionation of REEs is related to the intensity of chemical weathering, supply of REE and transport processes. The average REE composition of river sediments from peninsular India is more mafic than in UCC and World Rivers Average Clay.
... Regional changes have also been observed in intermonsoon rainfall, independent of summer or winter monsoons. Intensification of intermonsoon rainfall in western India and Pakistan 5,25,26 , weakening of ISM rainfall, and long-term declining rainfall in some areas in Sri Lanka have been reported 27,28 . Hence, understanding the past behavior of both ISM and intermonsoon precipitation is critical to predicting future changes in response to anthropogenic global warming and minimizing impacts on agriculture and communities. ...
Precipitation variations in the tropical Indian Ocean region result from changes in the Indian Summer Monsoon (ISM), as well as convective and cyclonic rainfall. The relative roles of such forcing can be elucidated by constructing robust paleoclimate records, which help to better predict future variability in precipitation due to rising greenhouse gases. This study was carried out using a suite of paleoclimate proxies on a combination of marine and terrestrial sediment cores from Sri Lanka. The sites receive differing amounts of precipitation from the ISM versus convective and cyclone-driven intermonsoon rainfall and allow differentiation of precipitation sources through time. The constructed 80000-year-long precipitation record for the ISM-dominant regions of Sri Lanka shows strong sensitivity to orbital scale insolation variations as well as to millennial-scale events in the North Atlantic region. This precipitation response to external and internal forcings is a result of the combined effects of ISM and Intermonsoon intensity either of which dominated periodically. The strong positive correlation between different periods in the precipitation record and different monthly insolation curves shows that changes in solar insolation due to precession decide the dominant rainfall mechanism in the region, suggesting the ISM dominated after 13 ka (MIS1) while the first intermonsoon dominated between 29 and 13 ka (MIS2) and the second intermonsoon dominated period between 77 and 40 ka.
