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Monsoons are familiar elements of tropical life, bringing rain to the
crops that feed millions. The effects do not stop there; in the Arabian
Sea, the twice-yearly monsoon winds enrich the fishing grounds, as well
as setting up currents that are constant enough to be used for navigation.
But the monsoon also acts as a sign of the climatic times. Although its
timing is remarkably regular, the intensity of its effects varies considerably
from year to year. On top of natural variations in the strength of the monsoons,
there are also fluctuations arising from human activities. Most scientists
believe that large-scale deforestation and burning of fossil fuels will
alter global climatic patterns significantly. For the sake of those people
whose lives and livelihoods depend on this regular cycle, it is essential
that researchers understand how the monsoon will respond to the expected
change in global temperatures. To this end, scientists from the National
Institute of Oceanography at Goa, and the Institute of Biogeochemistry and
Marine Chemistry at the University of Hamburg are looking at the effects
of the monsoon in the Arabian Sea.

The monsoon cycle has existed for some 12 million years, and it is one
of the aspects of the Earth’s climatic history that is recorded by the
sediments that accumulate on the ocean floor. Alterations in the rate at
which particles settle, their composition and the proportion of dust particles
originating on the continents through the years paint an accurate picture
of the Earth’s past climate. But to quantify the changes that happened in
the past we have to find out what is happening in these deep-sea sediments
now, as the monsoon waxes and wanes.

The Indo-German project, involving 10 researchers over a period of eight
years, began in 1986. One of its most valuable contributions to the climate
debate will be data that link the monsoon record with the climate worldwide
through the sediment record in the area. The data will give scientists
the chance to compare the course of the monsoon each year with the patterns
of sediment that build up on the sea floor. This information in turn can
be used to find out about the monsoon climate of the past, from the record
held in older sediments.

The word ‘monsoon’ is derived from the Arabic word mausim, meaning season.
Around the Arabian Sea, the summer monsoon winds blow from the southwest
from June to September, and from the northeast in winter, from December
to February. The monsoon has a powerful impact on local people’s lives through
its effect on the fishing grounds, especially rich in the Arabian Sea.
Here the fertility of the sea – its primary productivity – is twice the
world average, thanks to the monsoon.

The characteristic weather of the monsoon was familiar to the ancient
Greeks and Romans, who used the seasonal winds for navigation. Alexander
of Macedonia used the trade winds to send his general Nearchus with some
of his troops from the mouth of the Indus to the Persian Gulf in 329 BC.
From the 8th century onwards, Arab sailors used the monsoon winds to travel
between Arabia and India. Pressure to discover a new trade route from Europe
to India via the Cape of Good Hope, between 1497 and 1499 AD, focused attention
on navigation skill; Vasco da Gama, leader of the first successful voyage,
used a pilot who understood the monsoons to cross the Arabian Sea.

The weather of the monsoon is unusually regular and reliable, arising
from a combination of topography and seasonal variations in sunlight. The
summer sun heats the Asian landmass, forming a low-pressure area above it
in the atmosphere. This in turn sets up winds which blow from the northern
Indian Ocean onto the land. Because the Earth and its atmosphere are rotating,
winds in the Northern Hemisphere are deflected to the right (the Coriolis
effect). This happens in the summer monsoon, sending the winds over India.
The wind picks up water vapour from the ocean as it blows towards India.
Above land, they form clouds and the moisture condenses to fall as monsoon
rain. Evaporation from the tropical Indian Ocean adds moisture and latent
heat to the winds, intensifying the monsoon.

The monsoon cycle started during the Miocene age about 12 million years
ago, after the uplift of the Himalayas. These mountains play a critical
part in the development of the monsoon – because they block the cold northerly
winds that would otherwise cool the Indian subcontinent and prevent it heating
up so much in summer. Since the monsoons began they have arrived in India
every year with amazing regularity. But the intensity of the monsoon has
not been as constant.

The Arabian Sea research project has shown how closely fluctuations
in the flow of particles to the ocean floor match the intensity of the monsoon.
And variations in the type of particles, as well as changes in the ratios
of isotopes of elements such as carbon and oxygen, relate this regional
variation to the fluctuations of the climate of the world as a whole. The
research team achieved success with unusually detailed measurements.

The concentrations of particles in a water column are normally measured
by collecting bottles of sea water at various depths, then filtering out
and analysing the sediment – a method that is tedious, time-consuming and
expensive. The technique is also misleading, because it measures the ‘standing
crop’ of particles, not the flow of particles to the ocean floor. You could
compare it to trying to measure rainfall by collecting samples of air and
measuring the amount of water in it – all that would tell you is the humidity
of the atmosphere.

The Indo-German team solved the problem by using time-series sediment
traps, instruments that measure the rates at which particles settle through
the water. The instruments take samples of water at chosen time intervals
and record the volume of particles in each. Each trap can automatically
collect 13 samples at intervals of between 10 and 30 days. The traps are
moored and left unattended for a year at a time. So far, teams on the Indian
research vessels Sagar Kanya and Nand Rachit and the German research ship
Sonne have set up six time-series traps at three sites in the Arabian Sea,
about 4 kilometres deep.

MATCHING PATTERNS

Their six years of work have brought exciting results. Far more particles
are trapped during the monsoons than at other times, so recent sediments
clearly signal the season in which they formed. The team also found a strong
correlation between the speed of the monsoon winds and the flux of particles
– related to the numbers of particles settling. This link arises from the
changes that the monsoon brings to surface water of the sea.

Most of the particles that fall to the ocean floor are the remains
of microscopic plants and animals that live in the surface layers of the
ocean. These creatures can multiply quickly, and soon use up all the nutrients
present; their abundance is at least partly controlled by the availability
of the nutrients in their surrounding water. During the months before the
monsoon, the surface waters of the ocean are warm and settle into layers,
with little vertical mixing. The upper layers contain very few nutrients,
but enough light for the organisms to grow. When the monsoon begins, wind
speeds increase threefold and the resulting extra turbulence breaks down
the stratification. Water carrying nutrients from the layer below mixes
with the surface waters. As a result the plankton multiply in a burst of
productivity. More plankton means more debris and more particles falling
through the sea and reaching the sediment traps.

A similar burst of productivity happens where cold water from deep in
the oceans wells up to the surface at the edges of continents such as Peru.
But the monsoon effect, arising from higher winds, can make the open ocean
temporarily more fertile. Although generally good for fish, because it
provides more food, upwelling triggered by the monsoon can occasionally
have the opposite effect. Below the upper layers of water where light penetrates,
the water of the Arabian Sea contains very little oxygen. If the monsoon
winds bring this anoxic water to the surface, fish can die by the million.
In 1957, for example, more than 200 000 square kilometres of the Arabian
Sea were affected and about 20 million tonnes of fish suffocated.

The Indo-German project has given new insights into a more general problem
facing climatologists: how carbon dioxide from the atmosphere becomes locked
away in the deep layers of the ocean. The open ocean is generally considered
to be a desert, because its surface waters contain almost no nutrients.
Strong winds can make these deserts bloom in two ways. They can introduce
nutrient-rich waters into the surface layers and they can bring dust from
the continents, which supplies trace metals such as iron, essential for
the growth of plankton. When life blooms in the open ocean, the microscopic
plants take in carbon dioxide from the atmosphere and the settling particles
transport it to the interior of the ocean.

In times such as recent glacial periods, both atmospheric temperatures
and carbon dioxide concentrations in the atmosphere were much lower than
today. During these periods, the winds tended to be much stronger than now
and the continents were much more arid. Stronger winds could have mixed
the surface waters of the oceans more thoroughly and blown even more dust
with its trace metals into the oceans. More plankton growing in these fertile
waters could have absorbed more carbon dioxide from the atmosphere.

The project is not confined to analysing how natural climate change
has affected the Arabian Sea. The results have already highlighted the effects
of large-scale deforestation. As the forests shrink, soil erosion increases
and more minerals are carried to the sea.

The project team found that one effective way to monitor this process
was to identify the minerals in the trap samples. But the experiment threw
up a surprising result. The amount of clay minerals caught in the Arabian
Sea traps was far less than expected. With huge rivers such as the Indus
flowing into the area, the researchers had expected to find significant
traces of the sediments eroded from the vast Himalayan hinterland. The anomaly
can be traced to the dams and other hydroelectric projects that now pepper
the Indus and rivers like it. Dams tend to trap the sediment so that these
rivers carry as little as one tenth of the minerals and nutrients that they
once did. The effect is more coastal erosion, less life in the seas and
disruption of fragile coastal ecosystems.

The sediment-trap studies have also clarified what happens to particles
as they sink through the sea. The bulk of the particles originate from microscopic
plants and animals at the surface and sink to the sea floor, as ‘marine
snow’. On its way down, the sticky marine snow picks up other, smaller particles,
including pollutants that may normally stay in the water column for hundreds
of years. So these particles are carried to the ocean bottom much more quickly
than they would sink of their own accord. Mineral matter sticking to the
marine snow increases its density so that the organic matter travels to
the ocean floor more speedily.

DOWN TO THE DEPTHS

Also, some types of zooplankton feed on all the particles they can find
when they are in the surface layer and excrete them when they move down
to the deeper layers of the ocean. This process moves particles deep into
the oceans very quickly; during the monsoons most of the particles are transported
from the surface of the sea to the bottom in less than two weeks. Pollutants
discharged in the monsoons, when the productivity is so much higher, have
a better chance of being removed from the surface of the ocean than at other
times. This was illustrated by the fate of radioisotopes released in the
Chernobyl accident on 26 April 1986. They were detected at depths of more
than a kilometre in the Mediterranean Sea, Black Sea and North Sea within
the surprisingly short time of a few weeks, because the accident coincided
with the spring bloom in the seas.

Sediment trap studies in the Bay of Bengal have emphasised the role
that freshwater and sediment input from rivers plays in the global carbon
cycle. The greatest particle fluxes in the northern Bay of Bengal coincided
with the times when the discharge of the Ganges and Brahmaputra rivers
was also at its peak. The river water, rich in nutrients and minerals, spread
into plumes where they entered the sea and dispersed. The result was a plankton
bloom offshore in the bay. And the mineral matter delivered to the oceans
by rivers and winds was incorporated into organic aggregates, making them
denser. They settled faster, speeding the flow of atmospheric carbon into
the deep sea and its sediments.

Perhaps the most important role of river discharge is the way in which
it changes salinity and the availability of nutrients in the oceans. Marine
microorganisms are extremely sensitive to salinity changes; a drop by even
a few parts per thousand means that a different species can dominate. In
the northern Bay of Bengal salinity values drop from a norm of 35 parts
per thousand to less than 27 parts per thousand during the southwest monsoon.
When salinity values in the Bay of Bengal are high, the dominant microorganisms
include cocoliths and foraminifers, which have shells composed of calcium
carbonate. During the southwest monsoon, when salinity values drop and
nutrients are more freely available, foraminifers and cocoliths are replaced
by organisms such as diatoms which have shells composed of silica. Such
a change could be seen in the sea sediment record, but it also has an important
bearing on absorption of atmospheric carbon dioxide into the deep sea.
This is because, when carbonate-bearing organisms sink, they change the
alkalinity of the sea’s surface; this releases carbon dioxide from the oceans
into the atmosphere.

Monitoring plankton blooms at the surface of the ocean from satellites,
combined with sampling the particles using sediment traps, will enable scientists
to quantify better the way in which atmospheric carbon dioxide is removed
by processes in the oceans. International collaborative research using techniques
similar to those in this project is now underway in all the major seas and
oceans in a global effort to better quantify these processes. By understanding
how the monsoons have responded to varying temperatures in the past, researchers
hope to establish more accurately how they may behave in the near future,
for the sake of all those who depend on the monsoon.

V. Ramaswamy is a scientific researcher in the Geological Oceanography
Division of the National Institute of Oceanography, Goa, India. Ravindernath
Nair is the head of the Geological Oceanography Division of the National
Institute of Oceanography, Goa, India.