Indian Ocean Coastal Biome
299
end of the Omani coastline, which is visible as a cold plume in the surface temperature
field throughout the Southwest Monsoon season from May to October (Böhm, 1999;
Manghnani et al., 1998). The thermal contrast between this and the adjacent Gulf of
Oman waters increases as the monsoon season develops. Off Somalia, upwelled water
may be as cool as 13
C, indicating source depths of at least 200 m, although upwelling
occurs only in a few relatively restricted locations; along the Omani coast, upwelling has
a relatively shallower origin but is more widespread.
On the Omani coast, upwelling plumes are not located primarily in relation to features
of coastal topography, as they are off Somalia, but rather in relation to the form of
the offshore sea-level anomaly field, being advected preferentially through areas of low
sea level. Generally, the offshore extent of upwelled water along this coastline increases
during the Southwest Monsoon, so that the cold front may come to lie as much as
120 km offshore. It is the Murray Ridge, lying across the mouth of the Gulf of Oman,
that induces the persistent offshore field of mesoscale eddies which intensifies during
the Southwest Monsoon. Filaments of cool upwelled water from the Oman coast pass
along the flanks of the ridge, and the eddy field is more prominent to the north, where
the surface chlorophyll field suggests that a significant anticyclonic eddy persists in the
central Gulf of Oman (Weaks, 1984).
The thermocline of the northwest Arabian Sea is shallower at the coast than offshore
but shoals again above the Murray Ridge. In the mouth of the Gulf of Oman the
thermocline is extremely strong and shallow (a 4
C change occurs over about a 5 m depth
range below a 10 m deep surface mixed layer). Farther into the Gulf of Oman the mixed
layer deepens progressively, responding to the outflow of highly saline surface water from
the Arabian Gulf (Owens et al., 1993). The regional mixed layer deepens in response
to both monsoons so that depth maxima occur in August and February, respectively,
in response to the southwest and northeast monsoons. The JGOFS studies of the 1990s
found that the effects of convective mixing induced in the Arabian Sea by the onset of
the Northeast Monsoon were more significant than had previously been thought (Marra
and Barber, 2005).
Seasonal nutrient availability in this province is dominated, of course, by the upwelling
cycle and by changes in the depth of the offshore mixed layer, as discussed recently by
Woodward et al. (1999). Nitrate concentration of water upwelled close to the coast is of
order 18 mol liter
−1 , considerably higher than in water upwelled offshore and beyond
the shelf (< 125 mol liter
−1 ); these values may be compared with average ambient
concentrations at, say, 1500 km offshore of 0035 mol liter
−1 . N/P ratios suggest nitrate
limitation almost everywhere. A linear relationship (r
2
= 089) exists between surface
temperature and nitrate content for surface temperatures of 14–27
C (Smith, 1984);
average daily warming rates for surface water and the indicated loss rate for nitrate suggest
algal primary production of about 98 mgC m
−3 d
−1 , which is approximately the same as
that observed in experimental data. Rates of 25 gC m
−2 d
−1 are usual in the upwelling
area off the coast of Oman compared with < 03 gC m
−2 d
−1 in the oligotrophic central
gyre of the Arabian Sea (Mantoura et al., 1993; Owens et al., 1993).
The monsoon regime also induces major seasonal shifts in surface irradiance levels;
the cloud cover during the Southwest Monsoon of boreal summer causes a shoaling of
the euphotic layer from about 90 m to about 50 m between April and August; it is not
until February that irradiance from clearing skies again begins to penetrate significantly
deeper.
Regional Response of the Pelagic Ecosystem
The seasonal reversal of monsoon winds clearly has major biological consequences over
the entire northwest Arabian Sea; during the Southwest Monsoon a large area of high
299
end of the Omani coastline, which is visible as a cold plume in the surface temperature
field throughout the Southwest Monsoon season from May to October (Böhm, 1999;
Manghnani et al., 1998). The thermal contrast between this and the adjacent Gulf of
Oman waters increases as the monsoon season develops. Off Somalia, upwelled water
may be as cool as 13
C, indicating source depths of at least 200 m, although upwelling
occurs only in a few relatively restricted locations; along the Omani coast, upwelling has
a relatively shallower origin but is more widespread.
On the Omani coast, upwelling plumes are not located primarily in relation to features
of coastal topography, as they are off Somalia, but rather in relation to the form of
the offshore sea-level anomaly field, being advected preferentially through areas of low
sea level. Generally, the offshore extent of upwelled water along this coastline increases
during the Southwest Monsoon, so that the cold front may come to lie as much as
120 km offshore. It is the Murray Ridge, lying across the mouth of the Gulf of Oman,
that induces the persistent offshore field of mesoscale eddies which intensifies during
the Southwest Monsoon. Filaments of cool upwelled water from the Oman coast pass
along the flanks of the ridge, and the eddy field is more prominent to the north, where
the surface chlorophyll field suggests that a significant anticyclonic eddy persists in the
central Gulf of Oman (Weaks, 1984).
The thermocline of the northwest Arabian Sea is shallower at the coast than offshore
but shoals again above the Murray Ridge. In the mouth of the Gulf of Oman the
thermocline is extremely strong and shallow (a 4
C change occurs over about a 5 m depth
range below a 10 m deep surface mixed layer). Farther into the Gulf of Oman the mixed
layer deepens progressively, responding to the outflow of highly saline surface water from
the Arabian Gulf (Owens et al., 1993). The regional mixed layer deepens in response
to both monsoons so that depth maxima occur in August and February, respectively,
in response to the southwest and northeast monsoons. The JGOFS studies of the 1990s
found that the effects of convective mixing induced in the Arabian Sea by the onset of
the Northeast Monsoon were more significant than had previously been thought (Marra
and Barber, 2005).
Seasonal nutrient availability in this province is dominated, of course, by the upwelling
cycle and by changes in the depth of the offshore mixed layer, as discussed recently by
Woodward et al. (1999). Nitrate concentration of water upwelled close to the coast is of
order 18 mol liter
−1 , considerably higher than in water upwelled offshore and beyond
the shelf (< 125 mol liter
−1 ); these values may be compared with average ambient
concentrations at, say, 1500 km offshore of 0035 mol liter
−1 . N/P ratios suggest nitrate
limitation almost everywhere. A linear relationship (r
2
= 089) exists between surface
temperature and nitrate content for surface temperatures of 14–27
C (Smith, 1984);
average daily warming rates for surface water and the indicated loss rate for nitrate suggest
algal primary production of about 98 mgC m
−3 d
−1 , which is approximately the same as
that observed in experimental data. Rates of 25 gC m
−2 d
−1 are usual in the upwelling
area off the coast of Oman compared with < 03 gC m
−2 d
−1 in the oligotrophic central
gyre of the Arabian Sea (Mantoura et al., 1993; Owens et al., 1993).
The monsoon regime also induces major seasonal shifts in surface irradiance levels;
the cloud cover during the Southwest Monsoon of boreal summer causes a shoaling of
the euphotic layer from about 90 m to about 50 m between April and August; it is not
until February that irradiance from clearing skies again begins to penetrate significantly
deeper.
Regional Response of the Pelagic Ecosystem
The seasonal reversal of monsoon winds clearly has major biological consequences over
the entire northwest Arabian Sea; during the Southwest Monsoon a large area of high