The Unique Situation in the Environment of the Indian Ocean
5
GRASSHOFF (in DIETRICH et al., 1966) showed that the N03 - - N is reduced to N02 - - N,
but there was no H 2S in the open water.
These extreme O2 conditions are of special biological interest. Two facts are worth
noting: first, the poor O2 layer extends onto the shelf of India, as shown in Fig. 2. This
situation was found during the NE monsoon. In the SE monsoon season conditions are
different because the semi-annual reversal of the surface circulation has a great influence
on the O2 distribution and this affects the bottom fauna as well as the commercial fishes.
The second surprising fact concerns the deep echo-scattering layer. We followed the daily
ups and downs of the scattering layers entering the layer of extreme O2 depletion and
concluded that organisms, causing these deep scattering layers of sound, can exist for at
least 12 hours without O2•
The first result of the asymmetric configuration of this ocean is this Orpoor layer due
to the weak deep circulation of the northern Indian Ocean. The other one is the reversal of
the surface circulation with the monsoon, which is significant for the Indian Ocean.
RAMAGE (1969) has already published his results relating to the atmospheric circulation.
The ocean north of 100 S is a classical example of monsoons, and there is no area more
suitable for studying the response of the ocean circulation to a changing wind system.
The total reversal of winds and currents was well known, but the mechanism of this
reversal was unknown before the HOE. Theoretical considerations of some scientists
and the evaluation by DOING (1970) of the quasi-synoptic observations made in summer
1963 prove the breakdown of the circulation in a series of large gyres in the Arabian Sea
(Fig. 3). Theoretical, mathematical calculations of models of the Arabian Sea indicated
that one monsoon period of approximately 5 months is not long enough to establish a
stationary current system. The surveys had therefore to be intensified to analyze the
changes over time. This can be done by the concentration of several research vessels
during a monsoon period and by recording the infrared radiation of the sea surface by
satellites. Fig. 4 shows a map of the surface temperature with values obtained by the
weather satellite "Nimbus 2" in summer 1966 an(ievaluated by SZEKIELDA (1970a, b). The
absolute temperature shown in this tigure may not be exact, but the relative temperature
is, hence the gradients in the cloudless regions are correct. The upwelling along the Somali
coast is obvious from the low surface temperature, but this upwelling is not constant
along the whole coast, as shown by the patches of low temperature. The upwelling
is a process which varies with time and place in the region of the shelf edge in form of
large eddies. These cold eddies drift with the Somalia Current and turn with this current
to the east south of Socotra.
The reversing wind system also influences the equatorial current system. The
Equatorial Undercurrent (first observed in 1954) is a strong ocean current in the Pacific
and in the Atlantic Ocean. Measurements indicate that in the Indian Ocean the Equatorial
Undercurrent is only occasionally present. The rules are not known, but various observations independent of each other showed that even in winter during the NE monsoon, i. e.
when the atmospheric circulation resembles the trade wind circulation in the other oceans,
the undercurrent does not exist. This is what "Meteor" found early in 1965.
To summarize the characteristics of the Indian Ocean as compared with other oceans,
four special facts make it different from the others:
a) Semi-annual reversal of the surface circulation north of 100 S
b) Break-up of this circulation into large eddies
5
GRASSHOFF (in DIETRICH et al., 1966) showed that the N03 - - N is reduced to N02 - - N,
but there was no H 2S in the open water.
These extreme O2 conditions are of special biological interest. Two facts are worth
noting: first, the poor O2 layer extends onto the shelf of India, as shown in Fig. 2. This
situation was found during the NE monsoon. In the SE monsoon season conditions are
different because the semi-annual reversal of the surface circulation has a great influence
on the O2 distribution and this affects the bottom fauna as well as the commercial fishes.
The second surprising fact concerns the deep echo-scattering layer. We followed the daily
ups and downs of the scattering layers entering the layer of extreme O2 depletion and
concluded that organisms, causing these deep scattering layers of sound, can exist for at
least 12 hours without O2•
The first result of the asymmetric configuration of this ocean is this Orpoor layer due
to the weak deep circulation of the northern Indian Ocean. The other one is the reversal of
the surface circulation with the monsoon, which is significant for the Indian Ocean.
RAMAGE (1969) has already published his results relating to the atmospheric circulation.
The ocean north of 100 S is a classical example of monsoons, and there is no area more
suitable for studying the response of the ocean circulation to a changing wind system.
The total reversal of winds and currents was well known, but the mechanism of this
reversal was unknown before the HOE. Theoretical considerations of some scientists
and the evaluation by DOING (1970) of the quasi-synoptic observations made in summer
1963 prove the breakdown of the circulation in a series of large gyres in the Arabian Sea
(Fig. 3). Theoretical, mathematical calculations of models of the Arabian Sea indicated
that one monsoon period of approximately 5 months is not long enough to establish a
stationary current system. The surveys had therefore to be intensified to analyze the
changes over time. This can be done by the concentration of several research vessels
during a monsoon period and by recording the infrared radiation of the sea surface by
satellites. Fig. 4 shows a map of the surface temperature with values obtained by the
weather satellite "Nimbus 2" in summer 1966 an(ievaluated by SZEKIELDA (1970a, b). The
absolute temperature shown in this tigure may not be exact, but the relative temperature
is, hence the gradients in the cloudless regions are correct. The upwelling along the Somali
coast is obvious from the low surface temperature, but this upwelling is not constant
along the whole coast, as shown by the patches of low temperature. The upwelling
is a process which varies with time and place in the region of the shelf edge in form of
large eddies. These cold eddies drift with the Somalia Current and turn with this current
to the east south of Socotra.
The reversing wind system also influences the equatorial current system. The
Equatorial Undercurrent (first observed in 1954) is a strong ocean current in the Pacific
and in the Atlantic Ocean. Measurements indicate that in the Indian Ocean the Equatorial
Undercurrent is only occasionally present. The rules are not known, but various observations independent of each other showed that even in winter during the NE monsoon, i. e.
when the atmospheric circulation resembles the trade wind circulation in the other oceans,
the undercurrent does not exist. This is what "Meteor" found early in 1965.
To summarize the characteristics of the Indian Ocean as compared with other oceans,
four special facts make it different from the others:
a) Semi-annual reversal of the surface circulation north of 100 S
b) Break-up of this circulation into large eddies