Classic CZCS Scenes Chapter 4:The Arabian Sea
(seasonal changes in upwelling and productivity)
In the previous chapters, CZCS images have illustrated
interactions of ocean currents and ocean biology, and how the process of
upwelling induces areas of high primary productivity. In this
chapter, composite seasonal images of one of the most dramatic
changes in the entire ocean will demostrate how different
conditions during different seasons of the year can affect the
productivity in a large oceanic region.
The area of the images is the Arabian Sea, on the northwest
side of the Indian Ocean. This area is roughly bounded to the east by
India, to the north by Pakistan, Iran, and Saudi Arabia, and to the
east by Oman and Somalia. This is the area of the monsoon. For
half the year (July-December), the winds in this region are from the
southwest, inducing a great deal of evaporation from the warm
waters of the Arabian Sea, and heavy rainfall along the coast of
India. In the other half of the year, the winds blow in the opposite
direction, and not as strongly as the southwest monsoon.
CZCS composite image of the Arabian Sea for July-September 1979, showing the large area
of high productivity waters that develops under the influence of the
southwest monsoon.
During the southwest monsoon, a low-level atmospheric feature called
the Findlater Jet forms over the ocean, near the coasts of Somalia,
Yemen, and Oman. The northeastward flow of the surface current induced by
the Findlater Jet causes strong upwelling near the coast, inducing a
period of high productivity that is easily observable in composite images
of the Arabian Sea for these months.
As the winds diminish and switch direction at the end of the
southwest monsoon season, the Findlater Jet ceases to flow. The
weaker southward flowing currents do not induce upwelling near the
coast, so that during the months of January to June, the Arabian Sea
is an area of low productivity. This marked change in conditions is
one of the most distinct seasonal alterations observed anywhere in
the world.
CZCS composite image of the Arabian Sea
for April-June 1979, the latter half of the annual period of low productivity
preceding the development of the southwest monsoon. Outflow features from
the Persian Gulf are also visible.
The animation below shows monthly composites of the Arabian
Sea area for the entire operational life of the CZCS instrument - November
1978 to June 1986. The peak of the monsoon season in the Arabian
Sea occurs in July, resulting in heavy cloud cover that obscured
most of the area from view. Also, because of the experimental nature of CZCS, it was not
continuously operational, leaving noticeable gaps in the coverage.
This distinct change in conditions made the Arabian Sea the
subject of a recent process study in the Joint Global Ocean Flux Study
(JGOFS) program. JGOFS seeks to determine the rates of
primary productivity in various ocean regions, to better understand
how carbon is cycled through the atmosphere and the oceans. One
reason that the Arabian Sea is of particular interest is that some
models of the Earth's future climate, as global warming influences
the oceans and atmosphere, indicate that many patterns of productivity
in the ocean will become more "monsoonal"; that is, more like the seasonal
patterns observed in the Arabian Sea. This possibility makes the Arabian Sea
an important region for intense study. (see References and related sites)
Chapter 5: Orinoco River plume
Chapter 3: The Galapagos plume
Index: Classic CZCS scenes
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Atmos Composition
