IAP-25-002

Quantifying the frequency and causes of extreme wet and dry periods in the Himalayas

Snowfall occurring in mountainous regions during winter provides a crucial source of freshwater for rivers during spring and summer when it is released by melting, securing essential water resources for large populations, which are also vital for food security, economic livelihoods, and healthy ecosystems. Snowfall is also the principal source of mass for all the world’s mountain glaciers, another critical natural reservoir.

Despite its importance, the amount of falling and accumulated snow in high mountain regions remains one of the least observed, least understood, and most uncertain components of the terrestrial water budget. This knowledge gap also limits our ability to predict the impacts of climate change on the amount and seasonality of meltwater, river flow, and stream flow – with snow amounts projected to decline in almost all mountainous regions throughout the twenty-first century.

One of the biggest knowledge gaps related to mountain water resources are the occurrence of extreme (multi-year) wet and dry periods. For example, severe droughts lasting 2-3 years are a particular threat to societies in Himalayan river basins, where populations are large and growing, glaciers are shrinking, and substantial water resources are shared by more than one country. However, because such hydrological extremes are extremely rare and therefore poorly represented by the relatively short ~100 year instrumental weather records in this region, their frequency and magnitude are poorly known.

To overcome this limitation, sediment cores taken from the bottom of lakes in mountain areas can be used to construct a multi-century long, potentially annually-resolved, record of extreme wet and dry periods. Multi-proxy datasets from lake records provide a unique record of extreme wet and dry periods in mountainous regions, and drier signals at the lake sites could be representative of drought events over a much wider geographical area.

As part of the BAS-led Big Thaw project (https://www.bas.ac.uk/project/the-big-thaw/), sediment cores from a number of high-elevation lakes in the north-western and central parts of Nepal have recently been collected.

This PhD project will focus on analysis of these cores, which span a strong precipitation gradient across Nepal, to develop an unprecedented understanding of the frequency and magnitude of extreme wet and dry periods in the Nepal Himalayan region over the last few centuries.

High-resolution sediment core datasets will be generated and used to assess the representation of these events in simulations using coupled atmosphere-ocean climate models that cover the period 1000 to 2100 CE. The overall aim is to see if climate models are able to accurately reproduce past hydrological extremes. Model simulations will be generated by the student and used to increase our understanding of extreme wet and dry periods even further back in time and into the future, allowing the underlying climate factors that cause them to be better understood.

In the first year, the PhD student will perform laboratory and data analysis of the lake sediment cores retrieved from Nepal during The Big Thaw project, with a particular focus on using state-of-the-art high-resolution SPECIM hyperspectral scanning.

Access will be provided to other data that has already been collected from the same cores as part of the Big Thaw project, including XRF core scan data, CT scan imagery, as well as supporting chronological and biological proxy data. Combined, these datasets will be used to reconstruct records of extreme wet and dry periods, extending the historical instrumental record by centuries.

Data from sediment records provides a larger sample of the frequency distribution of these extreme hydrological events, allowing the PhD student to produce robust statistical models, underpinned by well-established time series, spectral, cyclicity and recurrence methods. For extreme wet periods, we expect this to reveal the frequency and trends that apply locally at each core site, while extreme dry periods might apply  to at a larger regional, Himalayan scale.

During the 2nd and 3rd years, the PhD student will analyse outputs from climate model simulations covering the period 1000 to 2100. These simulations are part of the Coupled Model Intercomparison Project 6 (CMIP6) archive and are well suited to capture strong inter-annual precipitation variability.

Extreme wet and dry periods based on outputs from an ensemble of these models will be compared to the multi-century lake sediment records. Analysis of the climate model simulations will enable understanding of the underlying climatic drivers that cause these hydrological extremes, e.g., variability in regional and global atmospheric circulation patterns, like the Indian Monsoon and ENSO (El-Nino Southern Oscillation).

The climate model simulations extend further in time than the sediment records and will therefore be used to reveal how extreme wet and dry periods have changed in the industrial 20th Century under anthropogenic forcing compared with the pre-industrial period. Moreover, the models also extend till the end of the 21st century and therefore facilitate examination of how hydrological extremes in Nepal/Himalayas are likely to evolve in the future.

Throughout, there will be significant flexibility in the science undertaken in this PhD, allowing the science direction to being led and driven by the PhD student.

Year 1

• Attend BAS and IAPETUS induction and training activities.
• Take training courses in computing, analysing large datasets, and analytical skills.
• Produce a review of Himalayan climate and paleoclimate literature.
• Analyse lake sediment cores using SPECIM hyperspectral scanning at the University of Bern, focussing on generating records of extreme wet and dry periods, which are potentially annually-resolved.
• Obtain and analyse modern-day precipitation gauge datasets for the lake core sites.
• Compare lake sediment records with gauge datasets to extend the chronology of extreme wet and dry periods over the full multi-century sedimentary record.
• Identify robust signals of extreme wet and dry periods over the full multi-century sedimentary record and determine how frequently these strike the Nepal/Himalayan region.
• Begin work on a chapter (paper) based on Year 1 results.

Year 2

• Undertake IAPETUS training activities and attend/present work at BAS & Iapetus student conferences.
• Analyse the occurrence of extreme wet and dry periods in Nepal/Himalayas from an ensemble (~5) of climate model simulations covering the period 1000 to 2100 CE.
• Compare simulations with signals over the full multi-century sedimentary record and use the model data to increase our understanding of extreme wet and dry periods further back in time.
• Use the model data to determine what climate factors cause these hydrological extremes in Nepal/Himalayas.
• Use the model data to determine how extreme wet and dry periods in Nepal/Himalayas have changed throughout the 20^th century, and how they will likely evolve over the 21^st
• Begin work on a chapter (paper) based on Year 2 results.

Year 3

• Continue to write chapters (papers) based on Year 1 and 2 results.
• Prepare chapters for final thesis.
• Present key findings at major international conference(s) and at BAS & Iapetus student conferences.

Year 3.5

• Completion of thesis.
• Completion of papers.

• High-resolution analysis of sediment cores
• Bespoke data science training and courses for analysing sediment core and climate data.
• Iapetus and BAS training courses (e.g., R, writing research papers, science communication).
• Climate modelling
• Analytical skills