monsoon – Cabot Institute for the Environment blog

Sub-tropical rainforest in China. Image credit: UMBRELLA project

The East Asian monsoon covers much of the largest continent on Earth leading to rain in the summer in Japan, the Koreas and lots of China. Ultimately, more than 1.5 billion people depend on the water it provides for agriculture, industry and hydroelectric power.

Understanding the monsoon is essential. That is why colleagues and I recently reconstructed its behaviour throughout its 145m-year history, in order to better understand how it acts in response to changes in geography or the wider climate in the very long term, and what that might mean for the future.

Our study, published in the journal Science Advances indicates that the East Asian monsoon is much older and more varied than previously thought. Until quite recently the general consensus was that the monsoon came into being around 23m years ago, some time after the Tibetan Plateau was formed.

However, we show that it has been ever present for at least the past 145m years (except during the Late Cretaceous: the era of T. Rex), regardless of whether there was a Tibetan Plateau or how much CO₂ was in the atmosphere.

What is a monsoon?

At its most simple level a monsoon is a highly seasonal distribution in precipitation leading to a distinct “wet” and “dry” seasons – the word even derives from the Arabic “mausim”, translated as “season”.

The East Asian monsoon is a “sea breeze monsoon”, the most common type. They form because land and sea heat up at different rates, so high pressure forms over the sea and low pressure over land which results in wind blowing onshore in the summer.

It’s the world’s largest, highest plateau.
Rashevskyi Viacheslav / shutterstock

Although The Tibetan Plateau is not strictly needed to form the East Asian monsoon it can serve to enhance it. At 5km or more above sea level, the plateau simply sits much higher in the atmosphere and thus the air above it is heated much more than the same air would be at a lower elevation (consider the ground temperature in Tibet compared to the freezing air 5km above your head). As that Tibetan air is warmer than the surrounding cold air it rises and acts as a heat “pump”, sucking more air in to replace it and enhancing the monsoon circulation.

Changes over the (millions of) years

We found the intensity of the monsoon has varied significantly over the past 145m years. At first, it was around 30% weaker than today. Then, during the Late Cretaceous 100-66m years ago, a huge inland sea covered much of North America and weakened the Pacific trade winds. This caused East Asia to become very arid due to the monsoon disappearing.

However, rainfall patterns changed substantially after the Indian tectonic plate collided into the Asian continent around 50m years ago, forming the Himalayas and the Tibetan Plateau. As the land rose up, so did the strength of the monsoon. Our results suggest that 5-10m years ago there were “super-monsoons” with rainfall 30% stronger than today.

But how can we be sure that such changes were caused by geography, and not elevated carbon dioxide concentrations? To test this, we again modelled the climate for all different time periods (roughly every 4m years) and increased or reduced the amount of CO₂ in the atmosphere to see what effect this had on the monsoon. In general, irrespective of time period chosen, the monsoon showed little sensitivity (-1% to +13%) to changes in CO₂ compared to the impact of changes in regional geography.

Climate models are working

The monsoon in East Asia is mainly a result of its favourable geographic position and regional topography – though our work shows that CO₂ concentrations do have an impact, they are secondary to tectonics.

The past can help us better understand how the monsoon will behave as the climate changes – but its not a perfect analogue. Although rainfall increased almost every time CO₂ doubled in the past, each of these periods was unique and dependent on the specific geography at the time.

The reassuring thing is that climate models are showing agreement with geological data through the past. That means we have greater confidence that climate models are able to accurately predict how the monsoon will respond over the next century as humans continue to emit more CO₂ into the atmosphere.

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This blog was written by Cabot Institute member Dr Alex Farnsworth, Postdoctoral Research Associate in meteorology at the University of Bristol. This article is republished from The Conversation under a Creative Commons license. Read the original article.

Alex Farnsworth

NERC’s BAe-146 research aircraft at the Facility for Airborne Atmospheric Measurements (FAAM). Image credit: FAAM

This summer, researchers across the UK and India are teaming up to study the Indian monsoon as part of a £8 million observational campaign using the NERC research aircraftBAe-146.

India receives 80% of its annual rainfall in three months – between June and September. There are large year-to-year differences in the strength of the monsoon, which is heavily impacted by drivers such as aerosols and large-scale weather patterns, and this has significant impact on the livelihoods of over a billion people. For example, due to the strong El Nino last year, the 2015 monsoon experienced a 14% lower precipitation than average with some regions of India facing up to 50% shortfall. Forecasting the timing and strength of the monsoon is critical for the region and particularly for India’s farmers, who must manage water resources to avoid failing crops.

Roadside mural of the BAe-146 in Bangalore, India. Original artist unknown. Image credit: Guy Gratton

The observational campaign, which is part of NERC’s Drivers of Variability in the South Asian Monsoon programme, is led jointly by UK researchers: Professor Hugh Coe (University of Manchester), Dr Andy Turner (University of Reading) and Dr Adrian Matthews (University of East Anglia) and Indian scientists from the Indian Space Research Organization and Indian Institute of Science.

Bristol PhD student Dan Say installing sample containers on the BAe- 146. Image credit: Angelina Wenger

To complement this project to study physical and chemical drivers of the monsoon, I am measuring greenhouse gas from the aircraft with PhD student Dan Say (School of Chemistry, University of Bristol). Dan is gaining valuable field experience by operating several instruments aboard the BAe-146 through the intense heat and rain of the Indian monsoon.

Two of the greenhouse gases that we are studying, methane and nitrous oxide, are primarily produced during the monsoon season from India’s intensive agriculture. Methane is emitted from rice paddies, in which flooded soils create prime conditions for “anaerobic” methane production. Nitrous oxide is also emitted from these flooded soils due the large quantity of fertilizers that are applied, again through anaerobic pathways.

Rice fields near Bangalore, India. Image credit: Guy Gratton.

Our previous understanding of the large-scale emissions of these greenhouse gases from India’s agricultural soils has been limited and we aim to further our knowledge of what controls their production. In addition to the methane concentrations measured on the aircraft, with collaborators at the Royal Holloway, University of London’s isotope facility, we are also measuring the main isotope of methane (the 13-carbon isotope), which will provide us with a valuable tool for differentiating between agricultural and other sources of methane in the region. By combining this information with other measurements from the aircraft (for example, of moisture and of other atmospheric pollutants), we aim to gain new insights on how we may reduce these emissions in the future.

In addition, many synthetic “man-made” greenhouse gases are being measured for the first time in South Asia, giving us the first look at emissions from this region of some of the most potent warming agents. These include the suite of halocarbons such as hydrofluorocarbons (HFCs) and their predecessors the hydrochlorofluorocarbons (HCFCs) and chlorofluorocarbons (CFCs). These gases will be measured on the University of Bristol School of Chemistry’s ‘Medusa’ gaschromatography-mass spectrometer (GC-MS) facility run by Professor Simon O’Doherty.

Sample canisters for collecting air that will be measured on the School of Chemistry’s ‘Medusa’ GC-MS facility. Image credit: Angelina Wenger

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This blog is written by University of Bristol Cabot Institute member Dr Anita Ganesan, a NERC Research Fellow, School of Geographical Sciences, who looks at greenhouse gas emissions estimation.

Anita Ganesan

Tag: monsoon

The East Asian monsoon is many millions of years older than we thought

What is a monsoon?

Changes over the (millions of) years

Climate models are working

Measuring greenhouse gases during India’s monsoon