Dhrubajyoti Samanta - Academia.edu (original) (raw)

Papers by Dhrubajyoti Samanta

Earth and Space Science

Southwest monsoon, from June to September (JJAS), is the primary rainy season for the Indian subc... more Southwest monsoon, from June to September (JJAS), is the primary rainy season for the Indian subcontinent, which contributes ∼78% of India's total annual rainfall (e.g., Rajeevan et al., 2013). Variations in Indian summer monsoon rainfall (ISMR) on different time scales significantly impact agriculture, potable water, energy sector, food production, and gross domestic product (Gadgil & Gadgil, 2006), and the livelihood of millions of people living in the country. Thus, prediction of ISMR variability using global and regional models at a long lead time (i.e., 3-4 months in advance) is of great socioeconomic importance. Toward achieving this goal, concerted efforts were undertaken by the monsoon community over the last decade to develop dynamical models and to improve the prediction skill of ISMR with dynamical models. These efforts were the outcome of the Indian Monsoon Mission (MM) Program (Rao et al., 2019). The high-resolution Climate Forecast System version 2 (CFSv2) was set up at the Indian Institute of Tropical Meteorology to provide experimental forecasts of the ISMR since 2009 and is now operational in India Meteorological Department since 2017. The model has a reasonably high skill in predicting ISMR (Ramu et al., 2016; Rao et al., 2019) and the homogenous regions of the Indian sub-continent (Ramu et al., 2017) at the longer lead time (i.e., February Initial conditions). Based on the MM phase I's success, the MM phase II was launched with an emphasis on developing applications based on the skillful seasonal forecasts of ISMR. Seasonal forecasting is built on the realms of ensemble forecasting and noise reduction techniques. Various techniques are used to filter out the climatic "noise" from the seasonal forecasts. Time and spatial mean

The response of the hydrological cycle to anthropogenic climate change, especially across the tro... more The response of the hydrological cycle to anthropogenic climate change, especially across the tropical oceans, remains poorly understood due to the scarcity of long instrumental temperature and hydrological records. Massive shallowwater corals are ideally suited to reconstructing past oceanic variability as they are widely distributed across the tropics, rapidly deposit calcium carbonate skeletons that continuously record ambient environmental conditions, and can be sampled at monthly to annual resolution. Most coral-based reconstructions utilize stable oxygen isotope composition (δ 18 O) that tracks the combined change in sea surface temperature (SST) and the oxygen isotopic composition of seawater (δ 18 Osw), a measure of hydrologic variability. Increasingly, coral δ 18 O time series are paired with time series of strontium-to-calcium ratios (Sr/Ca), a proxy for SST, from the same coral to quantify temperature and δ 18 Osw variability through time. To increase

Journal of Geophysical Research: Oceans

Seawater transport through the South China Sea (SCS) and the Maritime Continent (MC) is the only ... more Seawater transport through the South China Sea (SCS) and the Maritime Continent (MC) is the only tropical pathway for the exchange of water between the Pacific and the Indian Oceans, playing an important role in the global thermohaline circulation (Gordon, 2001). Waters from the Pacific Ocean are significantly freshened and cooled by mixing in the SCS and MC before exiting into the Indian Ocean (Sprintall et al., 2014). These water masses then spread across the Indian Ocean with a component reaching the South Atlantic Ocean through a branch of the Agulhas Current that travels around the southern rim of Africa (Godfrey, 1996; Gordon, 1986). Once in the Atlantic Ocean, these waters are transported northward until they are sufficiently cooled to sink and

Geophysical Research Letters, 2022

Anthropogenic climate change has caused global mean sea level (GMSL) to rise primarily through th... more Anthropogenic climate change has caused global mean sea level (GMSL) to rise primarily through thermal expansion of the oceans, which increases ocean volume (e.g., Frederikse et al., 2020), and the melting of landbased ice, which increases ocean mass (e.g.,

Southeast Asia is especially vulnerable to the impacts of sea-level rise due to the pres... more Southeast Asia is especially vulnerable to the impacts of sea-level rise due to the presence of many low-lying small islands and highly populated coastal cities. However, our current understanding of sea-level projections and changes in upper-ocean dynamics over this region currently rely on relatively coarse resolution (~100 km) global climate model (GCM) simulations and is therefore limited over the coastal regions. Here using GCM simulations from the High-Resolution Model Intercomparison Project (HighResMIP) of the Coupled Model Intercomparison Project Phase 6 (CMIP6) to (1) examine the improvement of mean-state biases in the tropical Pacific and dynamic sea-level (DSL) over Southeast Asia; (2) generate projection on DSL over Southeast Asia under shared socioeconomic pathways phase-5 (SSP5-585); and (3) diagnose the role of changes in regional ocean dynamics under SSP5-585. We select HighResMIP models that included a historical period and shared socioeconomic pathways (SSP) 5-8.5 future scenario for the same ensemble and estimate the projected changes relative to the 1993-2014 period. Drift corrected DSL time series is estimated before examining the projected changes. Due to improved simulation of heat, salt, and mass distribution in the ocean, HighResMIP models not only reduce mean state biases in the tropical Pacific (such as cold-tongue sea surface temperature bias), but also near Southeast Asia including DSL. Despite intermodel diversity, there is an overall agreement of increasing sea-level over Southeast Asia. The multimodel ensemble of HighResMIP models suggests a rise of 0.2 m sea level in dynamic sea level (combined with thermosteric component) over Southeast Asia by 2070. Sea-level rises further up to 0.5 m by the end of the 21st century. Further, we found regional heat and mass transport changes have a major role in the projected sea-level pattern over Southeast Asia. For example, heat convergence to the east of Vietnam can account for most of the sea-level rise in the region. Our study can provide better insight into the contribution of regional ocean dynamics to DSL projections and useful to suggest for further ocean modelling studies.

Bulletin of the American Physical Society, 2016

Journal of Geophysical Research: Atmospheres, 2021

Monthly precipitation samples from Singapore were collected between 2013 and 2019 for stable isot... more Monthly precipitation samples from Singapore were collected between 2013 and 2019 for stable isotope analysis to further our understanding of the drivers of tropical precipitation isotopes, in particular, 17O‐excess. δ18O ranges from –11.34‰ to –2.34‰, with a low correlation to rainfall (r = –0.31, p = 0.014), suggesting a weak amount effect. d‐excess is relatively consistent and has an average value of 10.89‰ ± 3.45‰. Compared to high‐latitude regions, 17O‐excess in our samples generally falls in a narrower range from 2 to 47 per meg with an average of 21 ± 11 per meg. Moreover, 17O‐excess shows strong periodic variability; spectral analysis reveals 3‐month, 6‐month, and 2.7‐year periodicities, likely corresponding to intraseasonal oscillations, monsoons, and the El Niño–Southern Oscillation (ENSO), respectively. In contrast, d‐excess shows no clear periodicities. Although spectral analysis only identifies 6‐month periodicity in the δ18O time series, δ18O tracks the Nino3.4 sea surface temperature variability; the average δ18O value (–5.2‰) is higher during El Niño years than ENSO neutral years (–7.6‰). Therefore, regional convection associated with monsoons and ENSO has different impacts on δ18O, d‐excess, and 17O‐excess. 17O‐excess and d‐excess are anticorrelated and do not relate to the relative humidity in moisture source regions. Extremely low humidity and drought conditions in moisture source regions would be required to account for high 17O‐excess. Processes during transport and precipitation likely modify these two parameters, especially 17O‐excess, which no longer record humidity conditions of moisture source regions. Our findings will be useful for further modeling studies to identify physical processes during convection that alter d‐excess and 17O‐excess.

Paleoceanography and Paleoclimatology, 2021

Limited instrumental records of sea surface temperature (SST) have hindered our ability to fully ... more Limited instrumental records of sea surface temperature (SST) have hindered our ability to fully understand the natural long-term climate variability driven by surface ocean-atmosphere interactions, particularly in Southeast and East Asia where approximately 30% of the world's population resides (Morton & Blackmore, 2001; United Nations, 2019). Recent modeling studies have begun to document the uncertainty in the centennial-scale behavior of SST in the Western Pacific Ocean and surrounding seas, further emphasizing our need to reconstruct ocean hydrography at a high temporal resolution over longer periods (Karnauskas et al., 2012; Samanta et al., 2018). Complicating our understanding of climate in the marginal seas of the Maritime Continent are the changing interactions between climate drivers of surrounding areas. Climate drivers in Southeast Asia and the marginal seas, including the South China Sea (SCS), are complex and operate at multiple frequencies. For example, the East Asian Monsoon (EAM) varies primarily on a seasonal timescale, whereas the El Niño Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO) exert influence on interannual to interdecadal timescales, respectively. The EAM is defined by seasonal shifts in winds and moisture delivery to East Asia from the Indian and Pacific Oceans. In the summer, winds blow from the Indian Ocean over the SCS onto the Asian continent, driving precipitation. In the winter, winds reverse, delivering cold, dry air from the Pacific Ocean across the East Asian continent to the SCS. These winds have a significant impact, driving seasonal changes in ocean circulation, temperature and precipitation (Lau & Li, 1984; B.

Journal of Geophysical Research: Oceans, 2021

Geological Society of America Abstracts with Programs, 2020

Palaeogeography, Palaeoclimatology, Palaeoecology, 2021

Constraining past variability in ocean conditions in the Western Pacific Warm Pool (WPWP) and exa... more Constraining past variability in ocean conditions in the Western Pacific Warm Pool (WPWP) and examining how it has been influenced by the El-Niño Southern Oscillation (ENSO) is critical to predicting how these systems may change in the future. To characterize the spatiotemporal variability of the WPWP and ENSO during the past three decades, we analyzed climate proxies using coral cores sampled from Porites spp. from Kosrae Island (KOS) and Woleai Atoll (WOL) in the Federated States of Micronesia. Coral skeleton samples drilled along the major growth axis were analyzed for oxygen isotopes (δ 18 O c) and trace element ratios (Sr/Ca), used to reconstruct sea surface salinity and temperature (SSS and SST). Pseudocoral δ 18 O time series (δ 18 O pseudo) were calculated from gridded instrumental observations and compared to δ 18 O c , followed by fine-tuning using coral Sr/Ca and gridded SST, to produce age models for each coral. The thermal component of δ 18 O c was removed using Sr/Ca for SST, to derive δ 18 O of seawater (δ 18 O sw), a proxy for SSS. The Sr/Ca, and δ 18 O sw records were compared to instrumental SST and SSS to test their fidelity as regional climate recorders. We found both sites display significant Sr/Ca-SST calibrations at monthly and interannual (dry season, wet season, mean annual) timescales. At each site, δ 18 O sw also exhibited significant calibrations to SSS across the same timescales. The difference between normalized dry season SST (Sr/Ca) anomalies from KOS and WOL generates a zonal SST gradient (KOSWOL SST), capturing the east-west WPWP migration observed during ENSO events. Similarly, the average of normalized dry season δ 18 O sw anomalies from both sites produces an SSS index (KOSWOL SSS) reflecting the regional hydrological changes. Both proxy indices, KOSWOL SST and KOSWOL SSS , are significantly correlated to regional ENSO indices. These calibration results highlight the potential for extending the climate record, revealing spatial hydrological gradients within the WPWP and ENSO variability back to the end of the Little Ice Age.

Geophysical Research Letters, 2020

La Niña years tend to provide increased Indian summer monsoon (ISM) rainfall. However, observatio... more La Niña years tend to provide increased Indian summer monsoon (ISM) rainfall. However, observations show 6-8% reduction in ISM rainfall during post-1980 La Niñas relative to pre-1980. Using a suite of atmospheric general circulation model experiments, we replicate this observed phenomenon and attribute it to a combination of weakening La Niña events themselves plus strongly warming tropical Indian Ocean. We demonstrate that half of the ISM rainfall reduction during post-1980 La Niñas can be attributed to changes in the spatial pattern and intensity of La Niña within the tropical Pacific Ocean. Warmer eastern-equatorial Pacific Ocean temperatures during post-1980 La Niñas weaken the Walker circulation, resulting in large-scale anomalous subsidence over the Indian subcontinent, thereby inhibiting the deep convection that drives ISM rainfall. Further, we demonstrate the declining central ISM rainfall during La Niña years with increasing tropical Indian Ocean warming, which has several serious concerns for regional water resources and stability. Plain Language Summary Historically, wetter conditions of Indian summer monsoon (ISM) during June to September of La Niña years are important for water resources in particular groundwater recharge. Observations from recent decades, however, show a reduction of 6-8% in ISM rainfall during post-1980 La Niñas relative to pre-1980 La Niñas, which is a serious concern for regional water resources and stability particularly if the trend continues. Using a suite of atmospheric model experiments, we replicate this observed phenomenon and attribute it to weakening La Niña events combined with a strongly warming tropical Indian Ocean. Model simulations indicate that 50% reduction in ISM rainfall during post-1980 La Niñas can be attributed to changes in the spatial pattern and intensity within the tropical Pacific Ocean. The tropical Pacific east-west atmospheric circulation pattern (Walker circulation) is crucial for deep convection over the South Asian region. We show that warmer eastern equatorial Pacific Ocean temperatures during post-1980 La Niñas weaken the Walker circulation, resulting in inhibition of deep convection over the Indian subcontinent, thereby reducing ISM rainfall. Furthermore, we demonstrate that ISM rainfall over central India during La Niña years is likely to decline with increasing tropical Indian Ocean warming, which has several important implications for agriculture and economy of the Indian subcontinent.

Geophysical Research Letters, 2019

 Equatorial Pacific sea surface temperature bias contributes to double ITCZ bias in climate mode... more  Equatorial Pacific sea surface temperature bias contributes to double ITCZ bias in climate models  Interpretation of future tropical rainfall projections requires care as double ITCZ bias affects precipitation trends  Improvement of bias in equatorial Pacific sea surface temperature may reduce uncertainty in rainfall projections

Hydrological Processes, 2018

To investigate stable isotopic variability of precipitation in Singapore, we continuously analyze... more To investigate stable isotopic variability of precipitation in Singapore, we continuously analyzed the -value of individual rain events from November 2014 to August 2017 using an online system composed of a diffusion sampler coupled to Cavity-Ring Down Spectrometry. Over this period, the average value ( 18 O Avg), the lowest value ( 18 O Low) and the initial value ( 18 O Init) varied significantly, ranging from-0.45 to-15.54‰,-0.9 to-17.65‰, and 0 to-13.13‰, respectively. All three values share similar variability, and events with low 18 O Low and  18 O Avg values have low  18 O Init value. Individual events have This article is protected by copyright. All rights reserved. limited intra-event variability in -value (Δδ) with the majority having a Δδ below 4‰. Correlation of  18 O Low and  18 O Avg with  18 O Init is much higher than that with Δδ, suggesting convective activities prior to events have more control over -value than on-site convective activities. The d-excess of events also varies considerably in response to the seasonal variation in moisture sources. A 2-month running mean analysis of  18 O reveals clear seasonal and interannual variability. Seasonal variability is associated with the meridional movement of the Intertropical Convergence Zone (ITCZ) and evolution of the Asian monsoon. El Niño-Southern Oscillation (ENSO) is a likely driver of interannual variability. During 2015-16, the strongest El Niño year in recorded history, the majority of events have a  18 O value higher than the weighted average  18 O of daily precipitation.  18 O shows a positive correlation with Outgoing Longwave Radiation (OLR) in the western Pacific and the Asian monsoon region, and also with Oceanic Niño Index. During El Niño, the convection center shifts eastward to the central/eastern Pacific, weakening convective activities in Southeast Asia. Our study shows that precipitation -value contains information about ENSO and the ITCZ, which has a significant implication for the interpretation of water isotope data and understanding of hydrological processes in tropical regions.

Geophysical Research Letters, 2018

Coupled general circulation model (GCM) biases in the tropical Pacific are substantial, including... more Coupled general circulation model (GCM) biases in the tropical Pacific are substantial, including a westward extended cold sea surface temperature (SST) bias linked to El Niño-Southern Oscillation (ENSO). Investigation of internal climate variability at centennial timescales using multicentury control integrations of 27 GCMs suggests that a Pacific Centennial Oscillation emerges in GCMs with too strong ENSO variability in the equatorial Pacific, including westward extended SST variability. Using a stochastic model of climate variability (Hasselmann type), we diagnose such centennial SST variance in the western equatorial Pacific. The consistency of a simple stochastic model with complex GCMs suggests that a previously defined Pacific Centennial Oscillation may be driven by biases in high-frequency ENSO forcing in the western equatorial Pacific. A cautious evaluation of long-term trends in the tropical Pacific from GCMs is necessary because significant trends in historical and future simulations are possible consequences of biases in simulated internal variability alone. Plain Language Summary The tropical Pacific Ocean exhibits natural climate variability on a wide range of timescales, some of which are similar to the length of instrumental records (~100 years). Characterizing natural cycles with periods of~100 years is therefore important for detecting and attributing human forced changes. Analysis of climate model simulations shows that a previously defined 100-year cycle in tropical Pacific sea surface temperatures is the result of mismatches between the models and the real world and therefore may not exist in reality. Our results show that a 100-year periodicity in the tropical Pacific Ocean is a robust feature of models with an erroneously strong El Niño pattern in the western Pacific Ocean including wind fluctuations. We reveal the causes of 100-year cycles in the western Pacific, which relies on the ocean's large thermal capacity to smooth out frequent short-term wind events into slower cycles. Our study highlights the need for cautious interpretations of trends in the tropical Pacific Ocean from climate models due to these possibly spurious 100-year cycles, especially for attributing historical changes and predicting future climate. If these model mismatches can be corrected, it may allow more accurate predictions of El Niño and long-term trends over 21st century.

Scientific Reports, 2018

A dry bias in climatological Central Indian rainfall plagues Indian summer monsoon (ISM) simulati... more A dry bias in climatological Central Indian rainfall plagues Indian summer monsoon (ISM) simulations in multiple generations of climate models. Here, using observations and regional climate modeling, we focus on a warm coastal Bay of Bengal sea surface temperature (SST) front and its impact on Central Indian rainfall. The SST front, featuring sharp gradients as large as 0.5 °C/100 km, is colocated with a mixed layer depth (MLD) front, in a region where salinity variations are known to control MLD. Regional climate simulations coupling a regional atmospheric model with an ocean mixed layer model are performed. A simulation with observed MLD climatology reproduces SST, rainfall, and atmospheric circulation associated with ISM reasonably well; it also eliminates the dry bias over Central India significantly. Perturbing MLD structure in the simulations, we isolate the SST front’s impact on the simulated ISM climate state. This experiment offers insights into ISM climatological biases in...

Climate Dynamics, 2015

Abstract Interactions between midlatitude disturbances and the monsoonal circulation are signific... more Abstract Interactions between midlatitude disturbances and the monsoonal circulation are significant for the Indian summer monsoon (ISM) rainfall. This paper presents examples of monsoon–midlatitude linkage through anticyclonic Rossby wave breaking (RWB) over West Asia during June, July and August of the years 1998–2010. RWB events over West Asia are identified by the inversion of the potential vorticity air mass at three different isentropic levels (340, 350, and 360 K) using daily NCEP–NCAR reanalysis. It is observed that RWB took place over West Asia before/during breaks in the ISM. Further, these events occur on the anticyclonic shear side of the subtropical jet, where the gradient of the zonal wind is found to be high. RWB is responsible for the southward movement of high potential vorticity air from the westerly jet, leading to the formation of a blocking high over the Arabian region. In turn, this blocking high advects and causes the descent of upper tropospheric cold and dry air towards Central India. Such an air mass with low moist static energy inhibits deep monsoonal convection and thereby leads to a dry spell. In fact, we find that RWB induced blocking over West Asia to be one of the major causes of dry spell/break episodes in ISM. Additionally, the presence of cold air over Central India reduces the north–south thermal contrast over the monsoon region thereby modifying the local Hadley circulation over the region.

Natural Hazards, 2012

The circulation patterns over the Indian Ocean and the surrounding continents have been studied d... more The circulation patterns over the Indian Ocean and the surrounding continents have been studied during June 2009 and July 2002 to explain the failure of Indian summer monsoon (ISM) rainfall. This study presents evidences that the failure of the ISM during these 2 months was probably due to the development of cyclonic circulation anomaly over the Western Asia and anticyclonic circulation anomalies downstream of Eastern Asia. These circulation anomalies were associated with the equatorward advection of cold air up to 10°N. This may be due to the equatorward intrusion of midlatitude Rossby waves. We hypothesize that the intrusion of midlatitude Rossby wave is responsible for breaking the east-west circulation cell over the Indian region into two cells and weakening it. The weak east-west cell reduces the strength of the easterly wind field usually present over the monsoonal region, thus reducing the cross-equatorial moisture transport into the Indian subcontinent and decreasing monsoon rainfall.