The Indian Ocean
277
equator-crossing Southwest Monsoon of boreal summer reverses this situation, spinning
up anticyclonic gyres in the Arabian Sea and Bay of Bengal, and establishing eastward
flow across the ocean north of the equator. Most importantly, it forces a jet current, deep
and fast, northwest along the Somali and Arabian coasts. Though the response of the
ocean is swift, often occurring within 1 month, there is some lag so that the extremes of
current speed occur in February and August.
Because the equatorial near-zonal winds are relatively weak, compared with the Atlantic
and Pacific Oceans, and are not symmetric about the equator, there is no significant
upwelling at the equator in the Indian Ocean. Wind stress along the equator is greatest
during the intermonsoon periods when westerly winds prevail and force a convergent
eastward flow in the ocean—the fast Indian Equatorial Jet (600 km wide). Shallow overturning cells in the equatorial circulation from 5
S to 10
S are driven by wind curl in
the zonal wind field and, although isopycnals rise toward the surface here, no actual
upwelling appears to occur at the surface; however, some chlorophyll enhancement is
observed and will be discussed later (Miyama et al., 2003). Note in satellite images how
the Maldive Island chain, which lies at right angles across the equator, perturbs the
chlorophyll pattern there.
The present scheme of biogeochemical provinces of the Indian Ocean owes much
to the analysis of Colborn (1975), who partitioned the Indian Ocean by reference to
the monthly evolution of the mixed layer, and the gradient in the thermocline. He
thus obtained 274 “sub areas” which were aggregated into 40 “primary areas having
distinct thermal characteristics” and these again were gathered into a smaller number
of “major geographic provinces.” Banse (1987) and Brock et al. (1991) both interpreted
their studies of thermal structure and algal blooms in terms of Colborn’s classification
of the northwest Arabian Sea. For convenience of comparison, the equivalence of the
biogeochemical provinces proposed here with Colborn’s areas is noted in each section.
A monthly thermocline topography for the Indian Ocean that was compiled by Rao
et al. (1989) followed Colborn in defining mixed-layer depth solely by thermal criteria.
Where heavy rainfall occurs at sea (as in some parts of the monsoon provinces) or where
river runoff lowers the salinity of the surface layers, a purely thermal definition of mixedlayer depth is not satisfactory, as was pointed out by Banse (1987). Global mixed-layer
topography based on density criteria has been computed by Levitus (1982), but these
have only a limited information content for the Arabian Sea. Despite all these difficulties,
changes in the topography of the thermocline and in sea surface elevation do reflect
the major seasonal changes in circulation during northeast and Southwest Monsoons
(Hastenrath, 1989).
When discussing the thermal structure of the Arabian Sea, as of other areas, we should
note that the presence of phytoplankton modifies the heating-cooling cycle of the upper
part of the water column. The extremely clear water, and strong DCM, of the Arabian
Sea has enabled a calculation to be made of an inverse effect of biology on physics: the
relative heating of seawater by differential absorption of short-wavelength solar radiation
by pigmented phytoplankton cells in the DCM (Sathyendranath et al., 1991). A maximum
rate of 4
C per month (August) was calculated, which is not insignificant compared with
cooling due to upwelling of about 25
C per month (July); thus, phytoplankton pigment
enhances the rate of heating during the period of warming surface water and reduces
the rate of cooling during upwelling periods. This effect should be watched for in other
regions, perhaps especially where stratification is initiated in spring-bloom situations.
The northern Arabian Sea is more difficult to partition rationally than the remainder
of the Indian Ocean. Several different schemes could be proposed following the principles
outlined in Chapter 7, and, although none is entirely objective, the reasons for the choices
made are as follows. The continental shelf and borderland along the west coast of the
Indian continent has special characteristics that make it sensible and simple to recognize
277
equator-crossing Southwest Monsoon of boreal summer reverses this situation, spinning
up anticyclonic gyres in the Arabian Sea and Bay of Bengal, and establishing eastward
flow across the ocean north of the equator. Most importantly, it forces a jet current, deep
and fast, northwest along the Somali and Arabian coasts. Though the response of the
ocean is swift, often occurring within 1 month, there is some lag so that the extremes of
current speed occur in February and August.
Because the equatorial near-zonal winds are relatively weak, compared with the Atlantic
and Pacific Oceans, and are not symmetric about the equator, there is no significant
upwelling at the equator in the Indian Ocean. Wind stress along the equator is greatest
during the intermonsoon periods when westerly winds prevail and force a convergent
eastward flow in the ocean—the fast Indian Equatorial Jet (600 km wide). Shallow overturning cells in the equatorial circulation from 5
S to 10
S are driven by wind curl in
the zonal wind field and, although isopycnals rise toward the surface here, no actual
upwelling appears to occur at the surface; however, some chlorophyll enhancement is
observed and will be discussed later (Miyama et al., 2003). Note in satellite images how
the Maldive Island chain, which lies at right angles across the equator, perturbs the
chlorophyll pattern there.
The present scheme of biogeochemical provinces of the Indian Ocean owes much
to the analysis of Colborn (1975), who partitioned the Indian Ocean by reference to
the monthly evolution of the mixed layer, and the gradient in the thermocline. He
thus obtained 274 “sub areas” which were aggregated into 40 “primary areas having
distinct thermal characteristics” and these again were gathered into a smaller number
of “major geographic provinces.” Banse (1987) and Brock et al. (1991) both interpreted
their studies of thermal structure and algal blooms in terms of Colborn’s classification
of the northwest Arabian Sea. For convenience of comparison, the equivalence of the
biogeochemical provinces proposed here with Colborn’s areas is noted in each section.
A monthly thermocline topography for the Indian Ocean that was compiled by Rao
et al. (1989) followed Colborn in defining mixed-layer depth solely by thermal criteria.
Where heavy rainfall occurs at sea (as in some parts of the monsoon provinces) or where
river runoff lowers the salinity of the surface layers, a purely thermal definition of mixedlayer depth is not satisfactory, as was pointed out by Banse (1987). Global mixed-layer
topography based on density criteria has been computed by Levitus (1982), but these
have only a limited information content for the Arabian Sea. Despite all these difficulties,
changes in the topography of the thermocline and in sea surface elevation do reflect
the major seasonal changes in circulation during northeast and Southwest Monsoons
(Hastenrath, 1989).
When discussing the thermal structure of the Arabian Sea, as of other areas, we should
note that the presence of phytoplankton modifies the heating-cooling cycle of the upper
part of the water column. The extremely clear water, and strong DCM, of the Arabian
Sea has enabled a calculation to be made of an inverse effect of biology on physics: the
relative heating of seawater by differential absorption of short-wavelength solar radiation
by pigmented phytoplankton cells in the DCM (Sathyendranath et al., 1991). A maximum
rate of 4
C per month (August) was calculated, which is not insignificant compared with
cooling due to upwelling of about 25
C per month (July); thus, phytoplankton pigment
enhances the rate of heating during the period of warming surface water and reduces
the rate of cooling during upwelling periods. This effect should be watched for in other
regions, perhaps especially where stratification is initiated in spring-bloom situations.
The northern Arabian Sea is more difficult to partition rationally than the remainder
of the Indian Ocean. Several different schemes could be proposed following the principles
outlined in Chapter 7, and, although none is entirely objective, the reasons for the choices
made are as follows. The continental shelf and borderland along the west coast of the
Indian continent has special characteristics that make it sensible and simple to recognize