Regional Oceanography: The Arabian Sea

The Arabian Sea is the northwestern part of the Indian Ocean, covering a total area of about 4 000 000 square km and forming part of the principal sea route between Europe and India. It is bounded to the west by the Horn of Africa and the Arabian Peninsula, to the north by Iran and Pakistan, to the east by India, and to the south by the remainder of the Indian Ocean. To the north the Gulf of Oman connects the sea with the Persian Gulf via the Strait of Hormuz. To the west the Gulf of Aden connects it with the Red Sea via the Bab el-Mandeb (Bab al-Mandab) Strait.

Physical features • Most of the Arabian Sea has depths that exceed

2,990 metres, and there are no islands in the middle.

• Deep water reaches close to the bordering lands except in the northeast, off Pakistan and India.

• To the southeast the Lakhadeeve atolls form part of the submarine Maldive Ridge, which extends farther south into the Indian Ocean where it rises above the surface to form the

atolls of the Maldives.

Global topography at 30 minute resolution is shown via a pseudo-colored, radially deformed

surface.

Historical development

• To unravel the mysteries of the monsoon developing over the Indian Ocean, a multinational effort was made in the form of International Indian Ocean Expedition (IIOE) during the period 1963-65 during which aircraft soundings (drop-sondes) were launched for the first time.

• After the IIOE, the Indo-Soviet Monsoon Experiment (ISMEX-73) was conducted during the summer of 1973 by U.S.S.R. and India.

Six research vessels (four from the U.S.S.R. and two from India) obtained meteorological and oceanographic measurements over the Arabian Sea, the equatorial region and southern Indian Ocean.

• The Joint Global Ocean Flux Study (JGOFS) was an international and multi-disciplinary project with participants from more than 20 countries. Its aim was to understand the processes controlling the cycling of carbon in the oceans, its exchange with the atmosphere and sea floor, and the sensitivity of these processes to climate changes. JGOFS was launched in 1987 under the auspices of the Scientific Committee on Oceanic Research (SCOR). The CDROM contains CTD, XBT and SeaSoar data for the Arabian Sea area that were collected as part of the JGOFS Arabian Sea Process Study (1990-1997).

Netherlands Indian Ocean Programme (NIOP, 1990-1995) -studied the effect of the monsoon on the climate system in the northern Indian Ocean. The CD-ROM contains information on the research themes, scientists involved and instruments used.

Arabesque is the UK contribution to the JGOFS Arabian Sea Process Study. It was a study of upper ocean microbial biogeochemistry in the Arabian Sea in 1994. Its focus was carbon and nitrogen cycling processes linked to climate change.

The NASEER (North Arabian Sea Environment and Ecosystem Research) Program of Pakistan has started in 2001.

weak convective

activity

coolSST

P-E < 0

weak near-surface

stratification

strong winds (Findlater Jet)

strong mixing and upwelling

strong convective

activity

P-E > 0

warm SST

strong near-surface

stratification

weak mixing and upwelling

weak winds

ARABIAN SEA BAY OF BENGAL

Differences between western and eastern parts of the Indian Ocean

ITCZ: Intertropical Convergence Zone

Seasonal changes: The Indian Ocean Monsoons • The word “monsoon” is derived from an

Arabic word meaning winds that change with the seasons.

• During the hot summer months, India is overtaken by a humid flow from the southwest that brings extensive cloudiness and impressive amounts of precipitation.

• This southerly monsoon (i.e., winds from the south) is caused by the heating of the Asian land mass in summer. The land heats the air above it, causing the air to rise, and consequently drawing air inland

from the oceans to replace the rising air.

Goes et al, 2005

Chlorophyll a

LOW

HIGH

0.1 0.5 1.0 2.0 5.0 10.0

By October, the Sun is well into its southerly winter decline and the Asian land mass cools rapidly during the longer nights.

The air over the Asian continent cools and sinks, and the warm, moist southerly monsoon is replaced by a dryer and cooler flow from the north, descending from the slopes of the Himalayas.

Goes et al, 2005

Chlorophyll a

LOW

HIGH

0.1 0.5 1.0 2 5.0 10 20.0

Due to intensive switch in seasonal monsoons, there is no constant subtropical gyre in the northern Indian Ocean.

The thickness of the lines indicate the relative intensities of the flows. Major components of the circulation system are the NEC, ECC, and SEC.

The most spectacular seasonal change is the reversal of the Somali Current (see January versus July)

Ocean circulation duringSouthwest (summer) monsoon (June-Sept)

SC-somali current;

SWC-southwest monsoon current;

GW-great whirl;

SG-southern gyre

During the SW monsoon the westward flowing Southern Equatorial Current feeds the Somali Current (SC) which then flows northward with speeds up to 200 cm/s and turning offshore at 2 -3 deg. S.

The SC develops two anticyclonic gyres, the Great Whirl (GW) (centered at about 8 deg. N) and the Southern Gyre (SG) located in the area 0 - 5 deg. N, 53 deg. E.

Winter monsoon (Nov-Feb) ocean circulation.

SC-somali current;

NMC-northeast monsoon current;

SECC-south equatorial counter current.

During the NE monsoon there is a strong Somali Current (SC)off the northeast African coast which feeds the SECC

• The most notable signature of the Summer Monsoon in the sea surface temperature (SST) is strong upwelling along the Somali and Arabian coasts.

• The southwest winds associated with the summer monsoon drive an eastward Ekman Transport away from the Somali and Arabian coasts. This eastward movement of water leads to upwelling of cold, nutrient rich waters along the Somali and Arabian coasts.

http://www.ccpo.odu.edu/SEES/

In the southern part of the Indian Ocean, during the SW monsoon the Agulhas Current (AC) is even more powerful than the Somali Current.

AC is second only after Gulf Stream in its volume transport. Off the tip of South Africa the AC loops back on itself, forming a feature known as the “Agulhas retroflection”, which is the region of high packed eddies of high biological productivity.

Most of these eddies are injected into the Benguela Current in the Atlantic Ocean. Eddies are highly energetic and are thought to have life-spans of many years.

Salinity and water masses

Arabian Sea subsurface waters are composed of Red Sea and Gulf of Oman outflow waters, that spread slowly southward at a depth of ± 600 m and eventually mix with other water masses.

Bottom waters enter the Somali Basin from the south. Part of this water probably leaves the basin in the north, to become part of a south-westward flow.

In order to identify the water mass, a set of measurements of temperature and salinity for successive depths at a given location should be plotted.

This T-S%o diagram depicts the difference betweenthe mean offshore water mass (shown in the figure in grey) and coastally upwelled water mass (in black) during Southwest Monsoon.

Overall, three basic kinds of water masses can be distinguished in the Indian Ocean: those that are generated within the open ocean by subduction, those that are mixing products of other masses, and those that enter from outside.

Fisher et al, 2002

The most powerful current feature in the Arabian Sea during the Southwest Monsoon is an extension of the northward flowing Somali Current and Oman Coastal Current.

At Ras Al Hadd, this current veers 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.

During the period that the jet exists, it creates a twin, or dipole, gyre system. The jet forms a large anticyclonic eddy.

North of the Ras al Hadd jet, a cyclonic 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.

As the Ras Al Hadd Jet collapses, eddies drift to the south.

Oman Coastal Current:

Northeastward flow, up to Ras Al HaddExtension: up to 200km offshoreVelocity: 0.4 km/sWater mass transport:10 Svedrups Reversal from northeastward to southwestward direction (NE monsoon)

Shown schematically are: The southwest monsoon jet (SWMJ), Somali current (SC), Great Whirl (GW), and Southern Gyre (SG).

(Chereskin et al., 1997). 

One of the most intriguing phenomena of Arabian Sea is its cooling during the summertime southwest monsoon.

At a time when the net surface heat flux is at a maximum and is directed into the ocean, the Arabian Sea experiences a rapid heat loss due to the export of heat via the strong southeastward Ekman transport.

Since the Ekman transport is large, carries the warmest water, and reverses seasonally, it can potentially reverse the sign of the seasonal heat transport of the Arabian Sea.

Ship tracks and hydrographic stations

• The Gulf of Oman and Arabian Sea contain oxygen-poor water at depths of about 100 meters below the surface.

• This oxygen-poor layer is due to the fact that the whole northern Arabian Sea is highly productive (fertile for growth of plankton and other microorganisms).

• Although phytoplankton and other marine biota have relatively short life spans (roughly 1-3 days), under the right conditions they have the capacity to reproduce, or “bloom,” into exponentially large numbers in a matter of days.

• Over time, these biota die at the surface and begin to sink to the bottom as detritus. As this detritus sinks it decays, thereby using up oxygen in the water column.

• Consequently, the Arabian Sea has one of the thickest oxygen-depleted layers of ocean water found anywhere in the world.

• Sometimes, due to shifts in the overlying wind field, these deep oxygen-poor waters upwell to the surface. So, ironically, the very reason that Oman’s fish reserves are the largest in the world also indirectly leads to periodic mass fish kills.

Oceanographic cables in the Gulf of Oman

Since 2005, the system provides (in real time) every hour for 365 days – turbidity, oxygen, conductivity, temperature, density, current speed and direction, from the depth of 65 to 1100m.

Cable system extends 50km into the sea from the Shinas coast. Plus, three deepwater current meter moorings off Rass al Hadd.

Mesoscale eddies affect the ocean color and phytoplankton productivity.

The cyclonic eddy has intensive green color in the center. Green color is an indicator of the chlorophyll a concentration value.

Due to upwelling of nutrients in the center of cyclonic eddy,The chlorophyll concentration has maximal value in the center of this eddy.

The other (anticyclonic) eddy has a “gap” of chlorohyll in the center due to downwelling of waters in the central part of this eddy.

The high chlorophyll a concentration patch was about 100km in diameter.

The life time of this patch was about 3 weeks.

The cold eddy has a cyclonic circulation caused by the wind stress which inputs vorticity to the eddy region.

The development of the cold eddy in the Sea of

Oman

Eddies of the Somali coast

Formation of the Great Whirl and other eddies due to bifurcation of the Northern Equatorial Current.

(Sabrahmanyam & Robinson, 2000)

SeaWiFS captured this dust cloud blowing out over Oman

(in March)

The Arabian Sea blooms are supported by supplemental input of horizontally advected iron transported from the Arabian peninsula in the form of the mineral dust

Other factors affecting physical-chemical variability: The dust storms

This image shows the scale of the dust storms taking place in the Arabian Sea region

Gradual downward trend in the diversity of fish in the tropical ocean (Worm et al., “Science”, 2005).

Interannual changes of fishdiversity in the ocean

Trends of global change

in the ocean

Levitus et al, 2009

Trends of global warming in the World Ocean

85% of excess heat in climateis in the oceans!

~71% of the earth’s surface are oceans

In terms of heat balance, the Indian Ocean(in particular the Arabian Sea) –both exhibited minimalincrease in heat content during the past 40 years.

70 years of global warming: Photograph of the melting Pindari glacier in the Himalayas.

Provided by J.Goes

Interannual Changes

Goes et al, 2005

Time plot of the spatially averaged sea surface temperature (SST) over the Indian Ocean and the Pacific Ocean for the period 1951 through 1998. The dashed line denotes the linear trend.

Webster et al, 1998

SeaWiFS derived chlorophyll a during the peak southwest monsoon growth season of 1997 and 2003

Interannual Changes

0.0

0.2

0.4

0.6

0.8

1997 1998 1999 2000 2001 2002 2003 2004

Chlor

ophy

ll a

(mg m

-3)

-0.06

-0.01

0.04

0.09

0.14

Zona

l Wind

Stre

ss (N

m-2

)Chl aZonal Wind Stress

Interannual changes of satellite derived chlorophyll a and zonal wind stress in the offshore western Arabian

Sea.

YEAR

J. Goes

Maps of sea level anomalies averaged over summer monsoon months (June-September) for year 2008 and year 2005.

The comparison of surface winds between two years with different monsoon intensity (2006 and 2008) shows that the wind was stronger in 2008 compared to 2006.

Schematic diagram of SST anomalies. White patches indicate increased convective activity. Arrows indicate wind direction. Red shading- warming; blue shading –cooling.

Indian Ocean Dipole (IOD) is a coupled ocean-atmosphere phenomenon in the Indian Ocean. It is normally characterized by anomalous cooling of SST in the south eastern equatorial Indian Ocean and anomalous warming

of SST in the western equatorial Indian Ocean.

Associated with these changes the normal convection situated over the eastern Indian Ocean warm pool shifts to the west and brings heavy rainfall over the east Africa and severe droughts/forest fires over the Indonesian region.

Interannual variability: The Indian Ocean Dipole (IOD)

Climatic Changes

Global warming will reduce snow and ice cover on the Tibetan Plateau, the starting point of the monsoon system of this region and south Asia.

Less ice and snow leads to increased winds during the SW Monsoon, causing

● more vigorous upwelling,

● increased deposition on the seabed,

● changes of surface plankton that marine life feed on, including the food that commercially viable yellowfin tuna, billfish, and large squid like best.

(S.Smith)

Tropical cyclones (hurricanes).

The mechanism: Low atmospheric pressure (within the tropical cyclone) forms a broad dome of water -the sea surface anomaly (of about 1m). This dome will be driven by the storm to the coast, where the water rumps ashore, driven by wind.

GONU Tropical cyclon GONU (which madelandfall in the coast of Omanon June 5th, 2007) was documented as the strongest tropical cyclone ever recorded in the north Indian Ocean over the past 60 years.

About 13% of world cyclones occur in the North Indian Ocean.

The track of Gonu and sea surface temperature change

NASA MODIS database