62
D. A. MCGILL:
aa) Surface Observations (Fig. 6)
Observations near Cape Guardafui and Socotra correspond with the peak of the
upwelling induced by the monsoon, as indicated by surface temperatures less than 18° C
found along the African coast. Profiles from offshore into the coast of Arabia taken only
a month or two earlier by RRS "Discovery" showed a marked shoreward uptilting of
isotherms and phosphate isopleths in that area, with the 18 ° C isotherm in one case
rising from 200 m at the offshore end of the section to the surface at the shoreward end
(Royal Society, 1963). P04 3 - - P in the region of the Socotra upwelling reaches values
at the surface greater than 1 ,Ltg-at I-I. For the SW Indian Ocean, ORREN (1963) has
reported values of 0.25 to 0.54 ,Ltg-at I-I, with the higher values being found southward.
Surface phosphate was high near the South African coast but then steadily decreased
seaward.
The highest Oz concentration in Fig. 6 is found in the Gulf of Aden, but other values
in excess of 5 mll- 1 are also seen in the western equatorial region and south of Madagascar. The water masses of the northern Arabian Sea have been analysed by RAMAM,
KURUP and MURTHY (1968). Low salinities associated with Indus water are not apparent
in Fig. 6 but could be expected to be more prominent in the post-monsoon interval.
f3f3) Observations at 200 m (Fig. 7)
Most indications of the surface upwelling have been lost by the 200 m depth (Fig. 7),
but the circulation along the African coast clearly indicates the pattern given to the
Equatorial Undercurrents as they are drawn into the complex Somali Current system.
An excessive Oz depletion (to the level of 0.25 mll- 1 and a minimum concentration of
0.05 ml I-I) is seen in the Arabian Sea, but no HzS concentrations were observed by
"Atlantis II", contrary to reports of some Russian surveys (IVANENKOV and ROZSNOV,
1961). The very low Oz concentrations distinguish this Arabian Sea subsurface water
mass of MOKIEVSKAYA (1961) from the Central Indian Ocean subsurface water mass to
the south.
REDDY and SANKARANARAYANA (1968a) have noted considerable variation in the
concentration and thickness of the layers of phosphate and silicate in the upper 200 m
of the central western North Indian Ocean. They distinguish three regions of convergence-from latitudes 15° 30' N to 16° 30' N along 68° E longitude, around 8° Nand
71 ° 30' E and around 5° Nand 75° E in the Maldive Islands. These areas distinguish the
North Equatorial Indian Ocean from the geographically similar regions of the Pacific
and Atlantic Ocean. RAO and ]AYARAMAN (1968a) have considered the same area during
the NE monsoon period.
R ITHER and MENZEL (1965) conclude that the general levels of nutrients in the
western Arabian Sea are appreciably higher than those of the North Atlantic. "Phosphate
concentrations within the upper 200 m of the Arabian Sea exceed those found at any
depth in the Sargasso Sea. Phosphate is used here merely as an index of nutrients in
general. Nitrate and silicate, which were measured, and undoubtedly other non-conservative nutrient elements, are distributed much the same as is inorganic phosphate ...
Roughly speaking, it could be said that nutrient concentrations in the Arabian Sea are
about twice those in the North Atlantic." (RITHER and MENZEL, 1965, p. 202).
yy) Observations at 600 m (Fig. 8)
The depth of 600 m (Fig. 8) represents the beginning of the zone of the phosphate
maximum. Values are found in the Arabian Sea as high as 2.70 ,ug-at I-I. This phosphate
D. A. MCGILL:
aa) Surface Observations (Fig. 6)
Observations near Cape Guardafui and Socotra correspond with the peak of the
upwelling induced by the monsoon, as indicated by surface temperatures less than 18° C
found along the African coast. Profiles from offshore into the coast of Arabia taken only
a month or two earlier by RRS "Discovery" showed a marked shoreward uptilting of
isotherms and phosphate isopleths in that area, with the 18 ° C isotherm in one case
rising from 200 m at the offshore end of the section to the surface at the shoreward end
(Royal Society, 1963). P04 3 - - P in the region of the Socotra upwelling reaches values
at the surface greater than 1 ,Ltg-at I-I. For the SW Indian Ocean, ORREN (1963) has
reported values of 0.25 to 0.54 ,Ltg-at I-I, with the higher values being found southward.
Surface phosphate was high near the South African coast but then steadily decreased
seaward.
The highest Oz concentration in Fig. 6 is found in the Gulf of Aden, but other values
in excess of 5 mll- 1 are also seen in the western equatorial region and south of Madagascar. The water masses of the northern Arabian Sea have been analysed by RAMAM,
KURUP and MURTHY (1968). Low salinities associated with Indus water are not apparent
in Fig. 6 but could be expected to be more prominent in the post-monsoon interval.
f3f3) Observations at 200 m (Fig. 7)
Most indications of the surface upwelling have been lost by the 200 m depth (Fig. 7),
but the circulation along the African coast clearly indicates the pattern given to the
Equatorial Undercurrents as they are drawn into the complex Somali Current system.
An excessive Oz depletion (to the level of 0.25 mll- 1 and a minimum concentration of
0.05 ml I-I) is seen in the Arabian Sea, but no HzS concentrations were observed by
"Atlantis II", contrary to reports of some Russian surveys (IVANENKOV and ROZSNOV,
1961). The very low Oz concentrations distinguish this Arabian Sea subsurface water
mass of MOKIEVSKAYA (1961) from the Central Indian Ocean subsurface water mass to
the south.
REDDY and SANKARANARAYANA (1968a) have noted considerable variation in the
concentration and thickness of the layers of phosphate and silicate in the upper 200 m
of the central western North Indian Ocean. They distinguish three regions of convergence-from latitudes 15° 30' N to 16° 30' N along 68° E longitude, around 8° Nand
71 ° 30' E and around 5° Nand 75° E in the Maldive Islands. These areas distinguish the
North Equatorial Indian Ocean from the geographically similar regions of the Pacific
and Atlantic Ocean. RAO and ]AYARAMAN (1968a) have considered the same area during
the NE monsoon period.
R ITHER and MENZEL (1965) conclude that the general levels of nutrients in the
western Arabian Sea are appreciably higher than those of the North Atlantic. "Phosphate
concentrations within the upper 200 m of the Arabian Sea exceed those found at any
depth in the Sargasso Sea. Phosphate is used here merely as an index of nutrients in
general. Nitrate and silicate, which were measured, and undoubtedly other non-conservative nutrient elements, are distributed much the same as is inorganic phosphate ...
Roughly speaking, it could be said that nutrient concentrations in the Arabian Sea are
about twice those in the North Atlantic." (RITHER and MENZEL, 1965, p. 202).
yy) Observations at 600 m (Fig. 8)
The depth of 600 m (Fig. 8) represents the beginning of the zone of the phosphate
maximum. Values are found in the Arabian Sea as high as 2.70 ,ug-at I-I. This phosphate