Novel insights into the Northeast Winter Monsoon

Some of the most productive agricultural regions in the world are in Southern and Eastern Asia. This high organic productivity is sustained by the seasonal reversal of the winds in the tropics: the summer and winter monsoons. Because monsoon winds bring much of the yearly rainfall, they are critical to the survival of billions of people. For example, more than 85% of the world's rice is grown in the Asian Monsoon region, which also hosts more than 50% of the world's population. Monsoonal circulation is also one of the main ways heat is transported across the globe, and therefore affects all parts of the climate system. Numerous studies have focused on the Asian Summer Monsoon, but our knowledge of the Asian Winter Monsoon has significant gaps which we aim to address here.

Studying the monsoon requires data that document how parameters such as rainfall and temperature change in space and time. However, instrumental data are only available in most cases for the last 100 years or so. This is not sufficient to record the full range of possible patterns and changes. To go beyond the instrumental record, we need to find natural recorders of climate change. The past and present variability of the Asian Summer Monsoon has been well characterized using numerous observational, palaeoclimatological and modelling studies. However, we know much less about the Asian Winter Monsoon which is poorly represented in the climate models used by the IPCC, and its past variability is only loosely constrained by palaeoclimate proxies. Therefore, there is a significant knowledge gap in our understanding of the winter monsoon which limits our ability to evaluate future trajectories of rainfall variability in places which are highly dependent on the winter monsoon rain, such as Vietnam, Sri Lanka, India or the Philippines.

Stalagmite-based reconstructions of past rainfall variability in the tropics have transformed our understanding of summer monsoon dynamics and its links to global climate. We propose to use the strengths of cave stalagmites as climate archives in Central Vietnam where ~75% of annual precipitation is controlled by the Asian Winter Monsoon. Our preliminary work has identified speleothems in this region which grew over the last 40,000 years. This will allow us to determine the drivers and magnitude of changes winter monsoon under conditions similar to modern and under a background climate state very different to today's.

We will use 2 independent methods to reconstruct changes in rainfall and temperature.
1. Carbon isotopes and Mg/Ca ratios which we demonstrated that they are directly related to the amount of water infiltrating into the cave from the surface.
2. A new biomarker proxy pioneered by co-I Bendle and his collaborators which can be used to deconvolve temperature and hydrology from individual stalagmites.

Using these methods, we will:
1. Provide the missing data needed to close the knowledge gap identified we identified here, and transforms our ability to test the winter monsoon representation in models used by IPCC to forecast future climate changes. This is one of the most robust ways to improve confidence in model forecasts of winter monsoon response to human-induced climate forcing.
2. Provide added value to decision and policy makers, who are responsible for navigating future changes in agricultural production and geohazards in the region. Our experienced local partners in Vietnam will make the link between our findings and central and regional authorities in Vietnam, permitting our next proposal to be co-produced with them and closely suited to their needs.
3. Enhancing the UK research footprint in a country that will to be one of the top 20 global economies by 2050. Our engagement with local scientists and authorities sets the foundations for long-term collaborative research which will benefit both the UK and Vietnam.