The Amazon hydrological cycle: past, present and future

Lead Research Organisation: University of Leeds
Department Name: Sch of Geography

Abstract

As one of the major centers of convection, tropical South America is an important component of the tropical (Walker Circulation) and also the global atmospheric circulation (Hadley Cell), and hydrological cycle. This is for example illustrated by the Amazon river who discharges approximately 17 % of all freshwater to the oceans. Tropical South America also hosts one of the largest forested areas in the world which is a huge carbon store amenable to fast release to the atmosphere, e.g. by forest destruction or drought-induced feed-backs.
Existing records of the hydrological cycle, the Amazon river discharge at Obidos, integrating 77% of the Amazon catchment area, and precipitation climatologies, reveal that the Amazon basin hydrological cycle exhibits a substantial intensifying trend over approximately the last two decades. The increase occurs mainly during the rainy season leading to an increase in the seasonal amplitude of river discharge. There is an even stronger trend in the daily maxima in precipitation and a decreasing trend of minimum daily precipitation pointing to an intensification of rain and drought events as well. Tropical South America has indeed witnessed severe droughts in 2005 and 2010 as well as strong flooding, most recently in 2009 and 2012.
Both because of substantial damage to livelihood by droughts and floods as well as from the perspective of global climate change, understanding changes in the Amazon hydrological cycle is important. However our understanding of ongoing changes of the Amazon's hydrological cycle is poor. The main reason is that there are many controls, which are poorly constrained by data. They include external factors, like water vapor input via the main air stream from the tropical Atlantic or changes of the location of the inter-tropical convergence zone, as well as internal factors like changes in rainforest functioning specifically the recirculation of water back to the atmosphere via forests.
The purpose of this proposal is to combine novel and existing data, with complimentary modelling and attribution techniques to understand ongoing and past trends of the Amazon hydrological cycle in order to help predict what to expect in the future. Our proposed work builds on two recent results from our research. First we have discovered that the tree species Cedrela odorata exhibits very clear annual rings and that the oxygen isotope 18O in tree ring cellulose is closely linked to the large-scale hydrological cycle of the Amazon. Specifically there is a strong correlation between 18O recorded in eight trees at a Bolivian Amazon site and Amazon river discharge at Obidos. Secondly we have recently succeeded to use atmospheric air parcel trajectory and remote sensing data of vegetation type to estimate the contribution of vegetation to water vapor in the air and thus recirculation of precipitation. We therefore propose to complement the Bolivian 18O precipitation record to further five sites across the basin to produce a good spatio-temporal coverage of precipitation 18O and indirectly precipitation over the last two centuries. Secondly we propose a modelling analysis employing both a climate vegetation model with isotopes to examine a range of processes and their effect on precipitation and 18O in precipitation and their time trends, and in parallel a back-trajectory approach to link observed isotope signatures along air parcel trajectories to estimate changes in water recycling in the basin. With our approach we expect to be able to pinpoint the causes of the intensification of the Amazon hydrological cycle over the last two decades and to what extent they are due to changes in functioning of the land vegetation and therefore to permit predictions of what to expect over the next decades. We also expect to be able to pinpoint the causes of the century long trends in tree ring based precipitation 18O and what they tell us about what causes longer term changes of the system.

Planned Impact

Impact summary

The proposed work aims at a better understanding of the future of the Amazon hydrological cycle as a basis for permitting secure livelihoods regionally and to anticipate climate effects and its eventual effects on humans on a much larger scale. We envisage that the results of our project will be of interest to a broad community of scientists, industry stakeholders, the next generation of scientific researchers as well as the general public. Routine dissemination of research outputs will be through publications in the scientific literature and presentation of research results at national and international scientific meetings. Beyond this we aim to inform the public and governing bodies about our results and to foster collaboration with South American colleagues and especially with younger scientists and students for education. We plan specific outreach activities in five domains

1. Outreach to the general public,
2. Dedicated website,
3. Beneficiaries and specific users of our research,
4. Outreach activities to schools and students,
5. Industry forum,

with activities explained in detail in the separate Impacts Plan document.

Publications

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Gloor E (2018) Tropical land carbon cycle responses to 2015/16 El Niño as recorded by atmospheric greenhouse gas and remote sensing data. in Philosophical transactions of the Royal Society of London. Series B, Biological sciences

 
Description Amazonia experiences an increase in severe floods and droughts. We try to understand the reasons and whether this will be a lasting state of the system or just decadal variability. Steps in this direction have included to make scientists in the field aware that drought is not the main signal but rather an increase in extremes including very severe flooding events. From the literature it seems this has led to a shift in this field. Secondly we can attribute the changes in the Amazon hydrological cycle to the combination of cooling of the Eastern Pacific and strong warming of the tropical Atlantic over the last two to three decades. The Atlantic warming in turn seems to be related to increased 'Alguhas leakage' as a result of southwards shifts of zonal winds bands at high southern latitudes. These shifts in turn are due to the ozone hole and man-made global warming.

We have also analysed the 2013/14 Sao Paulo water shortage. We find that similar climate conditions have occurred several in the record of existing meteorological data for the region. We can trace the climatic anomaly to sea surface temperature anomalies alone. Deforestation is very unlikely a reason for the shortage. A pattern analysis applied to climate simulations for the future suggests similar climate situations will occur again in the future but not with increased frequency.
Exploitation Route We generate several isotope datasets which will be of interest for characterising the tropical hydrological cycle. Besides tree ring isotope records this includes also precipitation and river water isotope measurements (by now approximately 3 years length). The records will be stored at IAEA. We will use these to compare with records from the 1970s. We have also performed a series of isotope enabled climate model simulations which are available and help us understanding ongoing changes and underlying mechanisms. We are currently in the process of publishing these studies. A important diagnostic modelling tool we developed is the joint use of water isotopes and water recirculation fraction.
Sectors Agriculture, Food and Drink,Environment

 
Description We have analysed changes of the Amazon hydrological cycle including the recent water shortage periods in the Sa Paulo region. Our results - particularly on the Sao Paulo water shortage period (2013/14) - reveals key information how to avoid similar situations in the future. We have also analysed why the Amazon hydrologic cycle has become more variable with more frequent severe floods and drier than usual conditions. We have been able to demonstrate a key role played by the recent rapid warming of the tropical Atlantic and as such are able to make predictions whether the phase of increased occurrence of severe floods will last.
Sector Energy,Environment
Impact Types Societal,Economic,Policy & public services

 
Description ISOAM - Testing novel isotope approaches to reconstruct past precipitation regimes in the Amazon 
Organisation University of Sao Paulo
Country Brazil 
Sector Academic/University 
PI Contribution This collaboration has just started. We (Roel Brienen and myself) collaborate with Chico Cruz and Gregorio Ceccantini, both Professors at Universidade de Sao Paulo, Brazil to understand better Oxygen the origin of isotope signatures in tree ring cellulose. Specifically we are interested in separating the source water signal from the leaf enrichment signal. It involves several approaches to try to separate these signals - in essence by separately measuring oxygen isotopes at different positions in the cellulose molecule. If we succeed it will permit to study changes of leaf enrichment - or i.e. water use efficiency of trees - over time separately from source water signals. The trees we will study are located at Peruacu caves where our colleagues measure oxygen in precipitation, soil water, tree sap and in stalagmites. They use the stalagmite oxygen isotopes to reconstruct changes of the Amazon hydrological cycle over ten thousands to hundred thousands of years. They are equally interested in understanding in more detail what the stalagmite oxygen signals really tell them. For our component of the project we sample tree rings for a range of species at Peruacu and then perform the chemical analyses to identify
Collaborator Contribution The contribution of our partners is to measure precipitation and soil water oxygen 18 content - and to relate them to stalagmite oxygen 18. They provide also the infrastructure for working at these caves.
Impact The project has just started thus there are no outcomes yet.
Start Year 2018