The Ras Al Hadd Jet: Stirring the Arabian Sea
SeaWiFS image of the Arabian Sea from November 4, 2002,
showing the Ras Al Hadd jet and related circulation features. (Click on the
image for a high-resolution version with labels referring to features discussed
in the text below.)
Every year, commencing in May and June, winds began to blow northward
from the Saudi Arabian peninsula across the Arabian Sea. As these winds
intensify, they foster the powerful Southwest Monsoon, which
brings torrential rains to the subcontinent of India. During the
height of the Southwest Monsoon in July and August, the thick layer
of clouds over the Arabian Sea makes visible remote-sensing of the sea
surface by sensors such as SeaWiFS and MODIS, virtually impossible.
But when the clouds clear in autumn, ocean color sensors reveal a
roiled, colorful sea surface. The prevailing oceanic currents
created by the winds of the Southwest Monsoon bring cold, nutrient-
rich water to the ocean surface, fostering intense phytoplankton
activity that is the base of the one of the world's last great
unspoiled marine ecosystems.
The most powerful current feature in the Arabian Sea during the Southwest
Monsoon is an extension of the northward flowing (from May-December) Somali
Current and Oman Coastal Current. At the easternmost point of Oman - Ras Al
Hadd - this current veers abruptly off the coast into the Arabian Sea,
forming the Ras Al Hadd jet (also termed the Ras Al Hadd front). As the
Southwest Monsoon peaks in intensity through the month of August, it
significantly enhances the strength of the northward-flowing coastal current
and the Ras al Hadd Jet extending into the Arabian Sea.
The largely uninhabited coastal region of Ras Al Hadd is famous for one
particular reason: its pristine beaches provide a safe haven for the nests of
green sea turtles that inhabit the Indian Ocean. The Ras al Hadd beaches are
the largest of three main green turtle nesting areas in the Indian Ocean. The
nesting season for the turtles is September to November.
The circulation pattern created by the Ras al Hadd Jet is quite obvious.
During the period that the jet exists off of Oman, it creates a twin, or
dipole, gyre system. The jet forms the northern boundary of a large
anticyclonic (clockwise) eddy. Though phytoplankton concentrations in the jet
are elevated, the concentrations in the anticyclonic eddy are low. North of the
Ras al Hadd jet, a cyclonic (counter-clockwise) eddy forms, and this eddy
contains higher concentrations of nutrients and phytoplankton. Late in the
monsoon season, a third smaller eddy forms near the coast, forming a "tripole"
circulation pattern.
The image at the top of the page actually captures a transitional
period during the monsoon season. In September, the monsoon winds
weaken, and this results in a corresponding decrease in the strength
of the wind-forced current. As the Ras Al Hadd Jet "collapses", which
can be visualized in the image as the jet curves around the anticyclonic eddy,
both the cyclonic and anticyclonic eddies drift to the south.
Further to the east, in the central Arabian Sea, convoluted circulation
patterns formed by the dynamic effects of the sustained monsoon winds and
interwoven ocean currents seem to marbelize the ocean surface.
The diagram below shows the main features of the monsoonal circulation
in this region.
Schematic diagram of oceanic circulation in
the Arabian Sea during the southwest monsoon. Courtesy of the
Rosenstiel School of Marine and Atmospheric Science, University
of Miami.
The image below, which is derived from SeaWiFS data acquired on November 6,
1999, provides another view of the Ras al Hadd jet and the associated eddy
circulation. The tripole circulation is particularly evident; the
arrows show the anticyclonic circulation to the south, and the cyclonic
circulation of the coastal eddy.
Image of the diffuse attenuation coefficient at 532
nanometers (nm), derived from SeaWiFS data acquired on November 6, 1999.
Although SeaWiFS does not have a 532 nm band, calculations using SeaWiFS data
at 555 nm allow derivation of this quantity for comparison to MODIS data
acquired at the 532 nm wavelength. Image courtesy of Naval Research
Laboratory, Stennis Space Center, Mississippi.
Sea surface temperature data allows further insight into the dynamics
of the circulation in this region. The main features in the
image below are the upwelling regions caused by the flow of the Oman
Coastal Current. The cold water creates a connected upwelling zone
near the coast, with jets and filaments of cold water shooting into the
warm Arabian Sea. The productivity of these jets is evident in the SeaWiFS image at the top of the page.
Image of sea surface temperature in the Arabian
Sea, August 9, 1994. The sea surface temperature data for this image
were acquired by the Advanced Very High Resolution Radiometer (AVHRR)
instrument which orbits on polar-orbiting satellites operated by the
National Oceanic and Atmospheric Administration (NOAA).
Though the Ras al Hadd jet seems like an easy-to-observe feature
in these images, the difficulty of conducting oceanographic research
in the Arabian Sea has made elucidating the details of this circulation
pattern a fairly recent development. From 1994 to 1996, the Joint
Global Ocean Flux Study (JGOFS) conducted the Arabian Sea Process
Study, a focused study of the physical and biological dynamics of
this region. This study vastly increased scientific understanding
of the biological processes of the Arabian Sea. The use of sea
surface temperature data during the study, and subsequent analysis
of ocean color data from SeaWiFS, facilitated a much-improved
understanding of how the swift-flowing oceanic currents are related
to the marine productivity of this remarkable region.
However, the effect of the Ras Al Hadd jet can also be seen in Coastal Zone
Color Scanner (CZCS) imagery. The CZCS Classic Scenes chapter on The Arabian Sea includes a
time-series of monthly CZCS scenes of this region. Go to the page and stop
the animation by clicking the box on the lower left. Then slide the circular
button with the mouse until the image for October 1981 is visible. In that
image, a clear view of the Ras Al Hadd jet and its associated eddies can be
seen.
So, even as ocean color remote-sensing progresses in the 21st century with
an armada of ocean color satellite instruments, there is still considerable
value in the historical data provided by the plucky instrument that started it
all.
Acknowledgements
Dr. Robert Arnone, Head of the Ocean Sciences Branch at the Naval
Research Laboratory located at the Stennis Space Center, Mississippi,
provided valuable editorial comments for this Science Focus! article.
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