- Tracing oxygen and hydrogen isotope distributions in rain can improve the estimation of monsoon withdrawal date, find scientists.
- While the conventional method of monsoon study relies exclusively on meteorological observations and model simulation, precipitation isotopes provide a means to understand the thermodynamical characteristics of monsoons.
- Isotopic values are sensitive to specific environmental parameters, such as, temperature and humidity.
- The precipitation isotope technique established in the study can improve monsoon and climate studies as well as monsoon prediction in a situation where forecasts are getting difficult with the changing climate.
Every year, the monsoon’s onset is accompanied by sharp changes in wind speed, which makes monsoon forecasts easier for experts. During the monsoon’s withdrawal, however, wind speed and other meteorological parameters witness slow change, making the prediction of its date, a challenge.
In recent research, scientists experimenting with better methods of studying the Indian monsoon have found that the tracing of oxygen and hydrogen isotope distributions in rain can improve monsoon studies and estimations of monsoon withdrawal date. Isotopes are atoms with the same number of protons but different numbers of neutrons. They have almost the same chemical properties but differ in mass.
The new research could aid and improve climate studies based on previous monsoon data and also increase long-term prediction skills in future.
The conventional method of monsoon study relies exclusively on meteorological observations and model simulation. Various parameters such as, temperature, rainfall, humidity, wind speed and direction, sea-level pressure, and several other variables are measured at the surface and at different levels up till about the tropopause – the atmospheric boundary between the troposphere and the mesosphere – to study the monsoon process.
In contrast, precipitation isotopes provide a means to understand the thermodynamical characteristics of monsoons rather than the dynamical characteristics. Isotopic values are sensitive to specific environmental parameters, such as, temperature and humidity.
This is particularly helpful for economies in India and Southeast Asia which are largely dependent on the summer monsoon. While these regions rely on the monsoon to fulfil rain-fed agricultural practices, many industries in these areas are powered by hydroelectricity that needs both summer and winter monsoons for its continued sustenance.
The uncertainty of the summer monsoon can therefore make agricultural as well as industrial decisions difficult, thereby adversely affecting the economy. Scientists at institutes across India are studying different aspects of the Indian monsoon to make more robust estimates and forecasts.
Analysing isotopic records to derive environmental information
In the collaborative study, scientists from multiple institutions across India have suggested that tracing oxygen and hydrogen isotopes in rainfall records, through a method called the isotopic technique (tracing stable isotopes from rainfall records), better records these monsoon processes. By analysing the isotopic records of natural samples like corals in a marine environment and speleothems, which are geological formations by mineral deposits in calcareous caves, environmental information can be retrieved.
Published in August this year in the journal Water Resources Research of the American Geophysical Union, the paper shows that rainfall and temperature signals recorded in the natural archives through the precipitation isotopes would provide a better means to reconstruct and interpret monsoon variability and estimate monsoon withdrawal date.
The scientists have proposed a technique that can address the limitations of current methods of monsoon reconstruction. They have found that precipitation isotopes respond to moisture dynamics, which, in turn, significantly change during the transition phase of the southwest to the northeast monsoon. Hence, they can be used in characterising the thermodynamical properties of the atmosphere, studying the monsoon process and estimating the summer monsoon withdrawal phase.
Read more: How does the Indian monsoon develop?
Supriyo Chakraborty, a scientist at Indian Institute of Tropical Meteorology (IITM) and one of the co-authors of the study, said, “The isotopic technique does not offer an alternative means to study to conventional monsoon, but one that can supplement it.”
The institutions that collaborated in the study include Anna University, Chennai; Indian Institute of Technology, Kanpur; Central University of Kerala, Periye; Indian Institute of Tropical Meteorology (IITM), Pune; Savitribai Phule Pune University, Pune; India Meteorological Department, Pune; Indian Institute of Science Education and Research, Bhopal; Physical Research Laboratory (PRL), Ahmedabad and Pondicherry University, Port Blair.
Isotopes for respite in uncertain season
Meteorological parameters such as rainfall and wind speed change rather slowly around late September to early October making the estimation of the summer monsoon withdrawal date considerably uncertain. The isotopic technique proposed by the multi-institutional study can reduce this uncertainty particularly in southern India.
This was deduced from oxygen isotopic records of rainwater samples collected by researchers at Minicoy Island in Lakshadweep, Port Blair in the Andaman and Nicobar Islands and Trivandrum in Kerala and analysed by IITM and PRL for isotopic seasonality. These records were correlated with temperature as well as other monsoon variables.
Scientists discovered that during the summer monsoon, isotopic records depended on the surface and tropospheric temperature and correlated with monsoon circulation, i.e. low-level circulation that carries oceanic moistures.
A detailed analysis, however, revealed that the thermodynamical characteristics rather than dynamical characteristics of the atmosphere play an essential role in determining precipitation isotopes. In other words, the summer monsoon significantly warms the troposphere over India relative to the winter monsoon season.
This is where precipitation isotopes come in, as they captured this seasonal difference in tropospheric temperature. This helped estimate the summer monsoon withdrawal date with greater certainty.
Isotopic values of rainfall in the southern parts of India exhibited stronger seasonality compared to other regions in India, mainly due to the seasonal reversal of the monsoon winds. “During the summer monsoon season, moisture is primarily transported from the Arabian Sea, while during the winter it is sourced mainly from the Bay of Bengal. Additionally, atmospheric processes contribute to these differences,” the study explained.
The isotopic history
Stable water isotopes have been useful tracers in the hydrological cycle due to a process called isotopic fractionation. When water vapour condenses, heavier isotopes condense more because of their greater binding energies and lower velocities of diffusion. These have been in use since the 1950s for studying past rainfall or climate.
Explaining the phenomenon, M. Rajeevan, former secretary of the Ministry of Earth Sciences, who made major contributions to India’s Monsoon Mission, said, “Many natural processes cause variations of the isotopic composition of natural waters like evaporation and condensation. This makes isotopic techniques useful for identification of the origin of monsoons and providing information which sometimes cannot be obtained by other techniques.”
Though considered powerful indicators of paleoclimate, quantitative interpretation of isotope variations in terms of climate changes is hampered by a limited understanding of physical processes controlling the global isotope behaviour. Analyses of monthly precipitation and surface air temperature data available through the International Atomic Energy Agency-World Meteorological Organisation (IAEA-WMO) global network in research published in the journal Science in 2003 showed that long-term changes of isotopic composition of precipitation over mid- and high-latitude regions over three decades closely followed long-term changes of surface air temperature.
Read more: Reconstructing 90-year-long rainfall history in northeast India using tea garden records
Recently, satellite remote sensing techniques have dramatically increased the number of tropospheric water vapour isotope observations which hold more direct information of atmospheric processes than precipitation isotopes and made it possible to observe detailed water isotope patterns in the atmosphere. However the method is yet to be incorporated in operational weather forecasting.
A separate study published last year in Geophysical Research Letter shows that water isotopes can substantially improve weather forecasts. This could facilitate further modelling developments in isotopic processes and improve operational weather forecasts. According to scientists, precipitation isotopes could have potential in Indian summer monsoon forecasts. However, for that sufficient data from the past as well as the present is needed.
Meanwhile, the recent multi-institutional study further pointed out that isotopic analysis of rainwater provides valuable information on the atmospheric water cycle. “Isotopic characterisation of monsoon withdrawal could complement our understanding of the large-scale circulation-based definition. However, unlike the meteorological parameters, routine observations of the water isotopes on a country scale are still lacking. Adopting such an effort, especially by the meteorological departments, is likely to add value to our understanding of the monsoon processes.”
Indian Summer Monsoon rainfall accounts for over 80% of the annual rainfall over in India and the dependence of agriculture, drinking water and energy production on this rainfall makes it the lifeline for a large fraction of the world’s population.
“While at present, the isotopic values are not used for prediction purposes due to lack of isotope data on an extensive scale, vapour isotopes have the potential to improve the prediction. For short-term predictions, we need high-quality isotopic data on the modern scale and for climate-scale prediction (decades to century) we need good quality proxy rainfall data (derived from the isotopic studies),” Chakraborty told Mongabay-India.
The precipitation isotope technique established in the study can improve monsoon and climate studies as well as monsoon prediction in a scenario where changing climate is making forecasts increasingly difficult, even with improved technologies.
Read more: New approach of Indian monsoon estimation to account for rainfall variations
Banner image: Clouds over a harbour in West Bengal. Photo by Rajarshi MITRA/Wikimedia Commons.
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