Eos, Vol. 93, No. 33, 14 August 2012

PAGE 324
Atmospheric moisture residence time affects rainwater isotope ratios
Droughts threaten
Bornean rainforests

T. Kumagai

At 130 million years old, the rainforests ofSoutheast Asia are the oldest in the worldand home to thousands of species of plants
and animals, some endemic to these forests.
The rainforests also play important roles
in modulating regional rainfall and in theglobal carbon cycle.
However, since the 1960s, increased warming in the Indian Ocean and frequent El Niño
events have reduced rainfall in the region byapproximately 1.0% per decade. Furthermore,
the Intergovernmental Panel for Climate
Change predicts that over the 21st century,
Southeast Asia will experience higher land
temperatures, more droughts, and increased
seasonality; that is, wet seasons during thefall will get wetter, and dry seasons during
the spring will get drier. However, few studies in the past have investigated how trees inthe southeastern Asian rainforests respond todroughts and climate change.
In a new study, Kumagai and Porporato
combine extensive field observations, historical records, and global climate models
to investigate the potential impact of rainfall
shifts and droughts on tree mortality in theBornean rainforests of Southeast Asia. They
found that as El Niño events become more
frequent in the future in response to warming in the tropical oceans, even the speciesof trees that can adapt to drought conditions
will be at increased risk of dying off. The
small number of species that cannot adapt
well to drought conditions will be at evengreater risk of dying off.
Their study has implications for predictions of ecological changes, regional rainfall
patterns, and global climate as well as direct
applications for policies aimed at reducing additional human impacts on these ecosystems, which not only are vulnerable toclimate change but also have the highest
rates of deforestation in the world. (Journal

Tropical rainforest in Lambir Hills NationalPark, Sarawak, Malaysia.

The water cycle is expected to respondto rising air temperature, which redistributes moisture in the lowest part of theatmosphere, known as the boundary layer.
To understand how boundary layer processes redistribute rainfall across different
climate settings, climatologists use stable
isotope ratios of oxygen and hydrogen inrainwater. A process known as Rayleighfractionation—­which depends on air temperature and amount of rainfall among
other factors—­controls the stable isotope
makeup of rainwater. For example, inthe tropics the isotope ratio depends on
the amount of rainfall, whereas in high-­latitude continents it depends on the airtemperature.
The ratio, however, does not always follow the Rayleigh model, particularly inregions receiving moderate to heavy rainfall, suggesting that scientists still do not
fully understand the complex atmosphericprocesses that distribute moisture in theboundary layer. Poor understanding ofboundary layer processes also makes it

difficult to interpret oxygen isotope ratios
in paleorecords, such as ice cores andcave deposits, as proxies of past responseof water cycle to climate change.
In a new study, Aggarwal et al. present
a model linking oxygen isotope ratios inrainwater to a single atmospheric parameter, the residence time of moisture in
the atmosphere, which is the ratio of theamount of moisture in a column of theatmosphere (available from satellite observations) to the amount of precipitation in
the region underlying that column.
Their model holds across all climate settings and explains why stable isotope ratios in rainwater shouldreflect regional dependences on different parameters. The new model not
only helps explain boundary layer processes and past responses of the water
cycle but also could help societies tobetter prepare for the future as rainfall
patterns respond to rising air temperatures. (Geophysical Research Letters,
doi:10.1029/2012GL051937, 2012) —ABof Geophysical Research-Biogeosciences,doi:10.1029/2011JG001835, 2012) —AB

about-­face causes the Agulhas current toshed counterclockwise rotating warm water
eddies. These eddies travel westward along
the African coast and break down in theSouth Atlantic.
The authors used simulated floats totrack three-­dimensional ocean circulation,
finding that the eastward deep water current traveled directly beneath the Agulhas
rings. Further, they found that the Agulhas
rings actually drive the eastward flow. Theynote that their finding implies a transitory
nature for the eastward branch of AMOC
deep water transport, as a break in Agulhas ring occurrence would stymie the flow.
(Journal of Geophysical Research-­Oceans,
doi:10.1029/2011JC007684, 2012) —CS

Section of thermohaline circulation
driven by transient current rings
The southward branch of the Atlantic
Meridional Overturning Circulation (AMOC),
the stretch that carries deep, cold water fromthe North Atlantic to the Southern Ocean,predominantly flows along the eastern shoreline of the Americas. This deep water transport, known as the Deep Western Boundary Current, hits a fork in the flow at 25°S, off
the coast of Brazil. Here the current splits intwo, with the majority continuing its southbound journey and a smaller ribbon veeringto the east. This eastbound water travels at
depth across the South Atlantic, eventually
passing around Africa’s southern coast. In its
path from Brazil to Africa the water crosses
lines of consistent vorticity—essentially traveling uphill. Though researchers have knownof this current for decades, little is known ofwhat drives it.
Using an ocean circulation model fed
with data recorded from 1980 to 2006, vanSebille et al. found that the eastward flow isdriven by Agulhas rings, a transient feature
off the South African shoreline. The Agulhas current flows southbound along Africa’s eastern shore. Where the Agulhas current meets the Southern Ocean most of thewarm Agulhas water retroflects and turns
back into the Indian Ocean. This sharp© 2012. American Geophysical Union. All Rights Reserved.

Remote sensingof hydrological connectivity
The subsurface flow of water withina watershed is traditionally thought todepend solely on topographic or physical influences, such as slope, soil permeability, and channel density. Though often
overlooked, some researchers proposethat the role of vegetation and its feedback on subsurface flow could be just as
important in some cases. Plants tend toincrease subsurface flow by increasing soil
conductivity and preferential flow paths.
They can also cause it to decline, as plants
use subsurface water and nutrients to fueltheir growth, drawing it out of the systemEos, Vol. 93, No. 33, 14 August 2012
through evapotranspiration. Both sets offeedbacks affect the downslope availability of water. To capitalize on this idea,
Hwang et al. devised a system called thehydrological vegetation gradient (HVG).
HVG represents how vegetation density
patterns develop along hydrological flow
paths within a watershed. The authors’system draws on two values: the normalized difference vegetation index and thetopographic wetness index, which can bederived through remote sensing and digital terrain analysis. The authors suggestthat their HVG system could be used tounderstand the role of subsurface lateralwater transfer within a watershed without
needing to conduct intensive field work.
To test their system, the authors useddata collected in the Coweeta Hydrological Laboratory, a 5400-acre series ofheadwater catchments in North Carolina. They used rain gauge measurements,
topographic information, and a distributed ecohydrological model to chart how
water flows through each of the eight
different watersheds. They compared© 2012. American Geophysical Union. All Rights Reserved.

their HVG values with established hydrologic, geomorphic, and topoclimate variables to assess how well HVG serves as
a proxy for subsurface flow and hydrological connectivity. The authors suggest that the work provides evidence forthe coevolution of forest ecosystem patterns with hydrologic and geomorphicsystems. (Water Resources Research,doi:10.1029/2011WR011301, 2012) —CS—Atreyee Bhattacharya and Colin Schultz,
Writers