Physical control of primary productivity on a seasonal scale in central and eastern Arabian Sea

Abstract

Usingin situ data collected during 1992–1997, under the Indian programme of Joint Global Ocean Flux Study (JGOFS), we show that the biological
productivity of the Arabian Sea is tightly coupled to the physical forcing mediated through nutrient availability. The Arabian
Sea becomes productive in summer not only along the coastal regions of Somalia, Arabia and southern parts of the west coast
of India due to coastal upwelling but also in the open waters of the central region. The open waters in the north are fertilized
by a combination of divergence driven by cyclonic wind stress curl to the north of the Findlater Jet and lateral advection
of nutrient-rich upwelled waters from Arabia. Productivity in the southern part of the central Arabian Sea, on the other hand,
is driven by advection from the Somalia upwelling. Surface cooling and convection resulting from reduced solar radiation and
increased evaporation make the northern region productive in winter. During both spring and fall inter-monsoons, this sea
remains warm and stratified with low production as surface waters are oligotrophic. Inter-annual variability in physical forcing
during winter resulted in one-and-a-half times higher production in 1997 than in 1995.

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Physical control of primary productivity on a seasonal scale in
central and eastern Arabian Sea
S PRASANNA KUMAR, M MADHUPRATAP, M DILEEP KUMAR, M GAUNS,
P M MURALEEDHARAN, V V S S SARMA and S N De SOUZA
National Institute of Oceanography, Dona Paula, Goa 403 004, India.
Using in situ data collected during 1992--1997, under the Indian programme of Joint Global Ocean Flux
Study (JGOFS), we show that the biological productivity of the Arabian Sea is tightly coupled to the
physical forcing mediated through nutrient availability. The Arabian Sea becomes productive in summer
not only along the coastal regions of Somalia, Arabia and southern parts of the west coast of India due to
coastal upwelling but also in the open waters of the central region. The open waters in the north are
fertilized by a combination of divergence driven by cyclonic wind stress curl to the north of the Findlater
Jet and lateral advection of nutrient-rich upwelled waters from Arabia. Productivity in the southern part
of the central Arabian Sea, on the other hand, is driven by advection from the Somalia upwelling. Surface
cooling and convection resulting from reduced solar radiation and increased evaporation make the
northern region productive in winter. During both spring and fall inter-monsoons, this sea remains warm
and stratified with low production as surface waters are oligotrophic. Inter-annual variability in physical
forcing during winter resulted in one-and-a-half times higher production in 1997 than in 1995.
1. Introduction
The Arabian Sea experiences extremes in atmospheric
forcing which leads to one of the largest intra-annual
variability compared to other ocean basins of the
world. The semi-annual reversals in atmospheric
and ocean circulation, the inflow of warm high saline
waters from the Persian Gulf and the Red Sea, and
the zone of oxygen deficient waters (?150--1000 m)
make the Arabian Sea a unique tropical basin. Plan-
kton blooms occur during both summer and winter
monsoons and we are only beginning to understand
the processes that regulate biological production in
the area. In this paper we attempt to focus on the
overall physical forcing that controls productivity in
the central and eastern Arabian Sea on a seasonal
scale, based on the ship-board measurements during
1992--1997 (see figure 1 for location and table 1 for
cruise details). An important question is, how well the
relationship between physical forcing and biological
production on a seasonal scale can be established with
data collected over 6 years? The rationale behind it is
based on the fact that though the semi-annual switch-
ing of the winds may have inter-annual variability, on
the whole, it is highly regular on a seasonal basis
(Fieux and Stommel 1977). Apart from this, the
amplitude of the seasonal signal is much higher than
the inter-annual signal (Unnikrishnan et al 1997;
Prasanna Kumar et al 1998).
The classification of seasons considered in this study
is
? Spring Inter-monsoon
? Summer monsoon
? Fall Inter-monsoon
? Winter monsoon
--
--
--
--
March -- May
June -- August
September -- October
November -- February
It may be noted that though June to September is
generally considered as summer monsoon, we chose to
include September into fall inter-monsoon based on
the property distribution in the central Arabian Sea
along 64?E.
Keywords. Primary production; upwelling; winter cooling; Ekman-pumping, nutrient transport; Arabian Sea.
Proc. Indian Acad. Sci. (Earth Planet. Sci.), 109, No. 4, December 2000, pp. 433--441
# Printed in India
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2. Data and method
Under the Indian programme of the Joint Global
Ocean Flux Study (JGOFS) 7 cruises were undertaken
on-board ORV Sagar Kanya : two minor (SK-77 along
65?E and SK-87 along 66?E) during 1992--1993 and
five major during 1994--1997 (table 1). A more or less
common transect along 64?E (figure 1) was followed
in all the major cruises except during winter 1997
(SK-121) when a time series station was occupied at
21?N and 64?E.
Measurements taken at one degree interval and up
to 1000 m depth included temperature and salinity
profiles using a Sea-Bird Electronics CTD (Conducti-
vity-Temperature-Depth) (see Prasanna Kumar and
Prasad1996),and nutrients(nitrite,nitrate,phosphate
and silicate) analysed with a SKALAR auto-analyser
(see de Sousa et al 1996).14C primary production (PP)
was estimated by in situ incubation and clean tech-
niques, and chlorophyll a by using a Turner Design
Flurometer (see Bhattathiri et al 1996) at two degree
intervals. CTD salinities were calibrated against those
in water samples collected simultaneously and ana-
lysed with ship-board Guildline 8400 Autosal. Water
samples used for nutrients as well as biological
measurements were collected by a rosette fitted with
10/30 litre Go Flo bottles and attached to the CTD
system. Surface meteorological observations were
carried out along the track at 6 hourly intervals and
at stations.
3. Results
3:1 Spring inter-monsoon
Spring inter-monsoon was characterised by light (< 4
m s?1) and variable winds predominantly from the
west (figure 2). Being the primary heating season of
the year, as the incoming solar radiation peaks to
about 260 W m?2in April-May (Hastenrath and
Lamb 1979), the air temperature increased to more
than 28?C along 64?E and at times reached 30?C (not
presented). Consequently, the sea surface temperature
(SST) rose to about 28?C. The mixed layer depth
(MLD) remained very shallow, varying from about
35 m in the south to 10 m in the north. Increased SST,
due to increased insolation, combined with weak
winds during this season led to the formation of the
observed shallow and (highly stratified upper layer
with) uniform mixed layer.
Nitrate in the surface layers was below detection
limit and 2 ?M occurred at depths greater than 50m
when nutracline was situated in the upper thermocline
(figure 3a). This was in general applicable to the
entire Arabian Sea except during summer and winter
monsoons, as we would see later, when prevailing
physical forcing supplies nutrients from subsurface to
nutrient depleted surface layer. Consistent with oligo-
trophic nutrient distribution, the surface as well as
Figure 1. Cruise track of ORV Sagar Kanya in the eastern
Arabian Sea under Indian JGOFS programme. Dark circles
indicate the station location and data from these were used in
the study. The section along 64?E was sampled in all the
cruises during 1994--1997, except in winter 1997 (SK-121) when
time series station was occupied at 21?N, 64?E. Temperature,
salinity and nutrients were sampled at every degree (dark
circles) while biological measurements were made at every two
degrees (open circles). During 1992 stations were occupied
along 65?E, while in 1993 it was along 66?E (not shown). See
table 1 for details of individual cruises.
Table 1.
1992-1997. See figure 1 for station locations.
Cruise details of the Indian Joint Global Ocean Flux Study (JGOFS) programme in the eastern Arabian Sea during
Serial
number
Cruise
numberPeriod Region of sampling Representative season
1
2
3
4
5
6
7
SK-77
SK-87
SK-91
SK-99
SK-104
SK-115
SK-121
20th September--7th October, 1992
11th--20th September, 1993
12th April--12th May, 1994
3rd February--5th March, 1995
20th July--12th August, 1995
3rd--25th August, 1996
5th--26th February, 1997
Along 65?E from 3?N to 15?N
Along 66?E from 16?N to 21?N
Along 64?E from 11?N to 21?N
Along 64?E from 11?N to 21?N
Along 64?E from 11?N to 18?N
Along 64?E from 13?N to 19?N
Time series at 12?N, 64?E
Fall inter-monsoon
Fall inter-monsoon
Spring inter-monsoon
Winter
Summer
Summer
Winter
434S Prasanna Kumar et al

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column integrated (up to 120 m) chlorophyll and PP
showed low values (table 2). A pronounced subsurface
chlorophyll maximum (SCM) associated with the
nitracline (?60 m ) was also found during this time of
the year (figure 4a). Productivity in central Arabian
Sea was relatively lower than that in coastal waters
(table 2).
3:2 Summer monsoon
During summer 1995 south-westerly winds persisted
with speeds increasing from 9 m s?1at 11?N to 17 m
s?1at around 16.5?N, and declined beyond this
latitude (figure 2). The trend remained similar during
1996 but for the speeds, which were, lower (6 m s?1) in
the south and the highest at 17?N (13 m s?1). The
maximum wind speed indicates the position of the
axis of the Findlater Jet (Findlater 1969) which
becomes active during summer monsoon. Based on
the wind distribution for both the years, the axis of
the Findlater Jet along 64?E was found to occur
between 16 and 17?N. SST was about 27.5?C in the
south and dropped by 1?C towards the axis of the
Findlater Jet in 1995. However, SST did not show any
significant north-south variation in 1996 with values
around 26.5?C. Most significant changes occurred in
the MLD in both years, which shoaled from 100 m
depth in the south to about 45 m near the axis of the
Findlater Jet and remained shallow north of it.
Nitrate was undetectable or at very low levels in the
upper 50 m in the south (< 15?N), in both the years.
A lens of high nitrate (2--4 ?M) was observed within
15--17?N in 1995 (figure 3b). This lens of water was
0.5?C colder and 0.1 PSU fresher than the ambient
water. During 1996, however, this lens was absent,
but in the north the nitrate in the upper 50 m was
about 1 ?M (figure 3c). Vertical distribution during
both 1995 and 1996 showed a shoaling of nitrate
isopleths towards the north, especially north of the
Findlalet Jet axis. Contrary to earlier general obser-
vations in the open central Arabian Sea during this
period, we found higher column chlorophyll a values
up to 60 mg m?2and PP up to 1780 mg C m?2d?1
along 64?E which were comparable with values in near
shore waters (table 3). Interestingly, high productiv-
ity was not confined to the north where both
thermocline and nutracline shoaled, but occurred in
the southern areas as well. The chlorophyll a con-
centrations and production rates were comparable to
those in the active upwelling fields off Somalia coast
(Smith and Codispoti 1980; Hitchcock and Olson
1992; Veldhuis et al 1997). Incidentally, high chlor-
ophyll a concentrations were also reported by Mara
et al (1998) in the central Arabian Sea (15.5?N,
61.5?E) during summer, based on moored sensors.
Vertical profiles of chlorophyll a and PP (figure 4b)
showed that about 80% of the pigment concentrations
and 85% of the production in summer in the southern
areas occurred in apparently nutrient-poor waters in
the upper 60 m along 64?E. However, nutrients must
have been available in the upper water column in
order to attain such levels of chlorophyll and PP. We
speculate that the low/undetectable levels of nitrate
in the south (figure 3b and c) were presumably due to
rapid biological uptake.
3:3 Fall inter-monsoon
Winds during 1993 fall inter-monsoon were predomi-
nantly westerlies, varying from west-southwesterlies
to west-northwesterlies with speeds between 4 and 7 m
s?1(figure 2). SST during both 1992 and 1993 was on
an average 28?C, and increased to 28.5?C in 1993 at
around 21?N, which was similar to that in spring inter-
monsoon. Signatures of Findlater Jet disappeared
along 64?E but MLDs still remained deep in the south
and shoaled to 40 m at 21?N.
Figure 2. Latitudinal variation of (a) wind speed, (b) sea
surface temperature, and (c) mixed layer depth along 64?E.
Physical control of productivity435

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Figure 3. (Continued)
436 S Prasanna Kumar et al

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Nitrate in the surface layer was very low/undetectable
with nitrate isopleths showing a shoaling trend
towards north (figure 3d), similar to that of thermal
structure. Consistent with the generally oligotrophic
surface layers, the chlorophyll a also showed low values
(table 4) which indicated a tapering of biological pro-
ductivity during this time of the year.
3:4 Winter monsoon
During 1995 winter, winds along 64?E were predomi-
nantly north/north-easterly with average speed of
about 4 m s?1(figure 2). As the incoming solar radia-
tion decreases from its fall inter-monsoon value of 200
W m?2to 160 W m?2in peak winter (Hastenrath and
Lamb 1979), air temperature also decreased by an
average of 5?C north of 15?N -- being 23?C in the
north and increased to 26.5?C at 11?N. Consistent
with this, the SST was ? 25?C in the north, a decrease
of 3?C from the fall inter-monsoon value, and
increased to 27.5?C towards south (figure 2). MLD
showed the greatest variability, which was about
120m north of 15?N but thinned steadily towards the
south (to 65 m).
Figure 3. Vertical structure of nitrate ð?MÞ in the upper 300 m water column along 64?E during (a) spring inter-monsoon 1994,
summer monsoon (b) 1995 and (c) 1996, (d) fall inter-monsoon 1993 and (e) winter monsoon 1995. Stippled area shows the
position of upper thermocline, bounded below by 20?C isotherm.
Table 2.
eastern Arabian Sea during spring inter-monsoon. Surface Chl a and PP are in mg m?3and mg C m?3d?1while column integrated
values are in mg m?2and mg C m?2d?1respectively.
Surface and column integrated (up to 120 m) chlorophyll a (Chl a) and primary production (PP) in the central and
Latitude NLongitude E Surface Chl a Surface PPColumn Chl a Column PPDay
21
15
11
10
67
64
64
75
0.05
0.05
0.04
0.05
8.8
0.8
0.7
3.3
10.7
16.6
9.2
10.5
310
168
163
199
9th May 94
15th April 94
23rd April 94
27th April 94
Physical control of productivity437