Jun 15, 1992 - Oceans and Ice Branch, NASA Goddard Space Flight Center, ... Oceanography, Dartmouth, Nova Scotia, Canada. ...... Findlater, J., A major low-level air current near the Indian Ocean .... 759-769, Geological Society, 1990.
JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 97, NO. C6, PAGES 9455-9465, JUNE 15, 1992
A Southwest Monsoon Hydrographic Climatology for the Northwestern Arabian
Sea
JOHN C. BROCK 1 Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder CHARLES R. MCCLAIN Oceans and Ice Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland W.
W.
HAY
Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder This paper provides a detailed hydrographic climatology for the shallow northwestern Arabian Sea prior to and during the southwest monsoon, presented as multiple-year composite vertical hydrographic sections based on National Oceanographic Data Center historical ocean station data. Temperatureand salinity measurementsare usedto infer the water massespresentin the upper 500 m.
Thehydrographic evolution depicted onbimonthly sections is inferred to resultfromwind-driven physical processes.In the northwesternArabian Sea the water mass in the upper 50 m is the Arabian Sea Surface Water. Waters from 50 to 500 m are formed by mixing of Arabian Sea Surface Water with Antarctic and Indonesianintermediatewaters. The inflow of PersianGulf Water doesnot significantly influencethe hydrographyof the northwesternArabian Sea along the Omani coast. Nitrate has a high inverse correlation with temperature and oxygen in the premonsoonthermocline in the depth interval 50-150 m. During the southwestmonsoon, coastal upwelling off Oman and adjacent offshore upward Ekman pumping alter the shallow hydrography.
INTRODUCTION
In the Arabian
Sea the eastward
flow of the Southwest
Monsoon Current is usually established by May in response to the seasonalreversal of the winds, and reaches its peak strength in July and August. Embedded in this summer circulation are regions of strong monsoonal upwelling off Somalia
and Arabia
that have been attributed
to different
mechanisms [Smith and Bottero, 1977' Bauer et al., 1991' Brock et al., 1991' Brock and McClain, 1992]. Smith and Bottero [1977] suggested that two distinct modes of winddriven upwelling affect the northwestern Arabian Sea: (1) coastal upwelling driven by the Ekman transport of surface water offshore, and (2) open-oceanupwelling due to Ekman pumping driven by strong positive wind stress curl to the northwest of the axis of the low-level atmosphericSomali Jet [Findlater, 1969]. Further, Brock et al. [1991] and Bauer et al. [1991] have attributed late summer open-sea phytoplank-
Sea, (2) to examine the evolution of shallow hydrography during the southwest monsoon, and (3) to relate summertime changes in near-surface hydrography to wind-driven physical processes that occur in the northwestern Arabian Sea [Bauer et al., 1991; Brock et al., 1991; Brock and McClain, 1992; Luther and O'Brien, 1985]. The role of surface cooling in producingconvectivemixing is not consideredin the present study, as Molinari et al. [1986] have shown that in summer the northwesternArabian Sea undergoesstrongheat gain. The basic hydrography of the Arabian Sea has been described by Rochford [1964], Wyrtki [1971, 1973], Qasim [1982], and Sastry et al. [1986]. North of a strong hydrochemical front at about 10øS the Arabian Sea has high salinities and nutrient
concentrations
and is low in dissolved
oxygen [Wyrtki, 1973]. The high-salinity water masses originate in the north central Arabian Sea, the Persian Gulf, and the Red Sea; deeper waters are derived from the southern Indian Ocean [Wyrtki, 1971; Pickard and Emery, 1982]. ton blooms oceanward of the Omani shelf to this upward Three distinct saline water masses, Arabian Sea Surface Ekman pumping. However, previousclimatologiesfail to proWater, Persian Gulf Water, and Red Sea Water, merge in the vide the detaileddescriptionof shallowhydrographynecessary central Arabian Sea within a thick, nearly isohaline layer to synthesizerelationsbetween coalescingwater masses,rapextending from 150 to 900 m known as North Indian Highidly evolving hydrographicstructure,and summertimecoastal Salinity Intermediate Water [Wyrtki, 1973]. The Arabian Sea and open-seaupwelling in the northwesternArabian Sea. Surface Water forms in the central and northern Arabian Sea The primary objectives of this paper are (1) to define the under conditions of an annual excess of evaporation over water masses present in the shallow northwestern Arabian precipitation of 1000 to 1500 mm. The temperature-salinity indices of this water mass are 26.8øC and 36.5%0; oxygen I Nowat Biological Oceanography Division,Bedford Instituteof content is low. With a o't of 23.9, the water mass sinks to Oceanography, Dartmouth, Nova Scotia, Canada. about 75 m, forming a shallow salinity maximum. During the Copyright 1992 by the American Geophysical Union. southwest monsoon, Arabian Sea Surface Water is advected into the region south of Sri Lanka [Wyrtki, 1973; Qasim, Paper number 92JC00813. 0148-0227/92/92JC-00813505.00
1982]. 9455
9456
BROCK ET AL.: ARABIAN SEA MONSOON HYDROGRAPHIC CLIMATOLOGY
TABLE
1.
Bimonthly Multiyear Composite Vertical Sections Number
Section
TMP
SAL
SIG
OXY
SIL
NIT
rily from Arabian Sea Surface Water, Red Sea Water, and Persian Gulf Water.
of Casts
METHODS
April-May A
X
X
B C
X X
X X
X X X
D
X
X
X
6
E F
X X
X X
X X
A B
X X
X X
X X
C D E
X X X
X X X
X X X
A B
X X
X X
X X
X X
C D
X X
X X
X X
X X
8 7
X
X
6 7
X X
X
X
8 11
X X
X X X
X X
X
June-July
10 9 14
August-September 12 6
X
6 9
X
TMP, temperature; SAL, salinity; SIG, %; OXY, dissolved oxygen; SIL, silicate; NIT, nitrate.
The average water depth of the Persian Gulf is only 25 m, and because evaporation exceeds precipitation [Premchand et al., 1986], warm (27øC) and saline (35.9%0) Persian Gulf Water forms and flows into the Gulf of Oman at depths of 25 to 70 m through the Strait of Hormuz. In the Gulf of Oman this water mass sinks to depths between 200 and 250 m to form a salinity maximum of about 38.0%0. Within the northern Arabian Sea, the Persian Gulf Water spreads southeastward at a depth of about 300 m and at a o-t of 26.6. Oxygen concentration is moderate, about 3 mL/L. Mixing reduces the salinity at the core of this water mass to 35.1%oin the southeast
Arabian
Sea.
North
of 20øN
the
Persian
Gulf
Water forms a distinct tongue of high-salinity water, with nearly zonal isohalines [Premchand et al., 1986; Wyrtki, 1973; Qasim, 1982; Rochford, 1964; Sharma, 1976]. In the Red Sea evaporation exceeds precipitation by 2000 mm/yr, and river runoff is virtually nonexistent. The resulting saline Red Sea Water flows over a 110 m sill at the Strait of Bab el Mandeb
Ocean station data for 1960 to present obtained from many
9
to enter the Gulf of Aden at 36%0 and 15øC
research
and Red
Sea inflows
to the Arabian
Sea total
Gulf Water
in the Arabian
in the northwestern
Arabian
Northwestern
Arabian
Sea Water
Masses
and Premonsoon Hydrography Both the Antarctic Intermediate Water with a salinity of 34.2%0and Indonesian Intermediate Water with a salinity of 34.6%0are of low enough salinity to account for the rapid dilution of Red Sea and Persian Gulf waters. An argument for significant inflow of the Antarctic Intermediate Water comes from the work of Rochford [1966], who examined Indian Ocean hydrography along meridional sections and concludedthat this water mass spreadsnorthward within the
o-t range 26.9-27.3. He suggestedthat the Antarctic Inter-
ARABIAN SEA SURFACE WATER (30 ø, 36.5%o)
25 _
PERSIAN GULF WATER (27 o, 39.5%o)
about
0.43 x 106 m3/s.The rapiddilutionof thehigh-salinity Red Sea and Persian
undertaken
RESULTS AND DISCUSSION
[Pickard and Emery, 1982] and with a moderate oxygen content of about 3.0 mL/L [Swallow, 1984]. Red Sea Water enters the Arabian Sea at a depth of about 800 m and forms a strong salinity maximum through much of the northern Indian Ocean extending south along the African coast to 25øS in the Mozambique Channel [Wyrtki, 1971, 1973]. Siedler [1968] and Koske [1972] showed that the Persian Gulf
cruises
Sea was acquired from the National Oceanographic Data Center (NODC) [1989]. SEAPAK, a comprehensive oceanographicdata analysis system developed and implemented at NASA Goddard Space Flight Center, was used in a preprocessingstage of the analysis [Darzi et al., 1989; Firestone et al., 1990; McClain et al., 1991]. Data for the study were binned into climatological time periods April-May, JuneJuly, and August-September. Gridding and contouring of temperature, salinity, %, dissolved oxygen, silicate, and nitrate produced multipleyear composite hydrographic sections depicting the April through September bimonthly distributions of these parameters in the upper 500 m of the northwestern Arabian Sea. Although data from various years are in some cases merged into a single section, this is not believed to have introduced biases, as the sections depict uniform trends in water properties that are generally in close correspondence at section intersections.Temperature, salinity, and o't distributionsare provided for all of the section locations; the distributions of oxygen, silicate, and nitrate are shown for the same sections depending on availability of cast data (Table 1). Hydrographic data from an ocean station at 16øN, 56øE were used to generate a temperature-salinity diagram (Figure 1).
Sea involves
•
20
-
ß
RED SEA WATER
(20 o, 39.7 %o)
the _
entrainment of lower-salinity water at intermediate depths.
Theentrainment of 1.2 x 106 m3/sof waterof 35.1%o salinity could yield the observed salinity pattern, and Swallow [ 1984] suggestedthat this water is advected into the Arabian Sea at intermediate depths from the southern hemisphere Subtropical Gyre by way of the northern branch of the South Equatorial Current. This interpretation differs from that of Wyrtki [1973] and Colburn [1975], who suggestedthat the North Indian High-Salinity Intermediate Water forms prima-
H INDI
-
o,
.
ß INDONESIAN
N HIGH-SALINITY
INTERMEDIATE
WATER
øoo Y, o 14 ø, 35.4 %o)
INTERMEDIATE WATER (3.5 ø, 34.6 %o)
ßANTARCTIC INTERMEDIATE WATER (2.5ø, 34.2 %o) 0 34.0
I 35.0
I 36.0
I 37.0
I 38.0
I 39.0
I 40.0
41.0
SALINITY
Fig. 1. Temperature-salinity diagram based on an ocean station at about 16øN, 56øE on April-May section FF'.
BROCK ET AL.: ARABIAN
SEA MONSOON
HYDROGRAPHIC
CLIMATOLOGY
9457
26øN 24
20 18
16 14
12ON
•oo
52 ø 54
56 ø
.•,o
m • 300
58
m•
500
60 62
64 ø
•:
66
T - S plot station location
APRIL-MAY CONTOUR
TEMPERATURE INTERVAL
(øC)
2øC
68øE
Fig. 2.
April-May temperature sections.
mediate Water penetrates beneath Persian Gulf Water and
A salinity maximum of 36.2%ois evident above 80 m. There
above Red Sea Water
is a pycnocline in the upper 200 m (Figure 4), with o-t increasing from a surface value of 22.4 to about 26. Below 200 m there is a monotonic increase in o-t to 500 m. The dissolved oxygen section (Figure 5a) indicates oxygen con-
to 15øN off the Arabian
coast.
A temperature-salinity plot to 500 m (Figure 1) that is typical for ocean stations off Arabia in April-May (Figure 2) suggests that the major water masses are Arabian Sea Surface Water and an imported intermediate water, probably a mixture
of Antarctic
Intermediate
Water.
Intermediate Arabian
Sea
Water Surface
and Indonesian Water
extends
down to 50 m. Below this to 500 m, the water is transitional, exhibiting the temperature-salinity characteristics of North Indian High-Salinity Intermediate Water. The salinity maxima
of
the
Red
Sea
and
Persian
Gulf
Waters
are
not
apparent. Since North Indian High-Salinity Intermediate Water originates from poorly understood mixing in the interior of the Arabian Sea rather than at the surface, it is not a true water mass "end-member." Overall, from the surface to 500 m the northwestern Arabian Sea waters grade gradually from Arabian Sea Surface Water to Antarctic and Indonesian intermediate waters. Although these inferences are here described in reference to the temperature-salinity plot of one station, quite similar temperature-salinity curves were observed for other premonsoon stations. April-May cast data depict the shallow hydrographic structure of the northwestern Arabian Sea prior to the onset of the southwest monsoon (Figures 2-4). During this period the isopleths for all hydrographic parameters are flat. Figure 2 showsthe thermocline in the upper 200 m; the temperature drops from above 30øCat the surface to 18øC.No significant mixed layer of uniform temperature is apparent on the April-May temperature sections, in agreement with the analyses of Sastry and D'Souza [1970], Wyrtki [1971], and Colburn [1975]. Consistent with Premchand et al. [1986], the vertical salinity variation is weak (Figure 3), with salinity decreasing slightly from about 36.0%0at the surface to 35.7%0at 500 m.
centrations
below
0.4 mL/L
from
about
190 to 500 m and
depicts an oxygen maximum exceeding 4 mL/L from 40 to 60 m. Silicate is depleted in the upper 50 m (Figure 5b) but increasesfrom 2 to 23/xM within a nutracline from 60 to 180 m. Nitrate has a minimum of less than 0.4/xM in the upper 40 m and increaseswith depth to 27/xM at the profile base at 500 m (Figure 5c). Oxygen and Nutrients
Many researchers have noted the oxygen-depleted nature of the intermediate waters of the Arabian Sea [Ryther and Menzel, 1965; Rochford, 1966; Rao and Jayaraman, 1970; Wyrtki, 1971, 1973; Sen Gupta et al., 1976; Swallow, 1984; Shimmield et al., 1990; Olson et al., 1992]. More specifically, several researchers have investigated the relationships between oxygen content and nutrient concentrations in the Arabian Sea [Ryther and Menzel, 1965; Ryther et al., 1966; Sen Gupta et al., 1976; Naqvi and Sen Gupta, 1985]. The results of the present study are consistent with previous suggestionsof decomposition of organic matter at shallow depths in the northwestern Arabian Sea, causing a sharp drop in dissolved oxygen content from 50 to 150 m. Predictably, this is mirrored by an abrupt increase in dissolved silica, and an even stronger increase in nitrate with depth. A linear regression of oxygen and nitrate in the 50- to 150-m depth range for April-May section FF' shows a strong
inversecorrelationwith an r 2 of 0.98 (Figure6). Nitrate in this depth range within the premonsoon northwestern Arabian Sea thermocline has a strong inverse correlation with
9458
BROCK ET AL.: ARABIAN SEA MONSOON HYDROGRAPHIC CLIMATOLOGY
26ON
22 2O 18
16
12ON 52 ø
•
o
o loo
2oo
54
300 4o0
56 ø
500 3OO m 40O
58
m
500
6O 62 64 66
APRIL-MAY
CONTOUR
68øE
Fig. 3.
SALINITY
INTERVAL 0.2%o
April-May salinity sections.
temperature,with an r 2 value of 0.94 (Figure7). This Evolution of Shallow Hydrography During suggeststhat a simple empirical algorithm may be used to estimate nitrate in the thermocline based on temperature measurements'
the Southwest
Monsoon
On all of the June-July sections normal to the coast, the
isoplethsfor temperature, salinity, and o-t shoaladjacentto N = -3.03'(T)
+ 81.94
where N is nitrate concentration (micromolar) and T is temperature in degrees Celsius.
the Arabian coast (Figures 8, 9, and 10). This rise of the contoursstarts 150-300 km offshoreand extends to depths of 200 to 400 m directly off the Oman continental shelf. Nearshore the surface water is cooler, fresher, and denser than
26ON 24
2O 18
16 14
12ON 52
t o loo
20O 2o0
54
300 300 400
400
50O
56 ø
300
500 30O
m
4OO
40O
58
m
500
6O 62 64
66 68OE
Fig. 4.
April-May •rt sections.
APRIL-MAY CONTOUR
SlGMA-T INTERVAL
1.0
BROCK ET AL.' ARABIAN SEA MONSOON HYDROGRAPHIC CLIMATOLOGY
April -
a)
b)
May Oxygen Section .-,
•
E -200
•
April -
F
0
9459
May Silicate Section F F'
o
-100
•
-200
•-•
E
o
-100
-100
-200
-200
t- -300
-300
c- -300
-300
• -400
-400
• -400
-400
-5oo 'l ' •' 0
1
100
•- I
r
200
•
500
•
400
•' -500
-500
-500 o
100
April o
400
F'
August -
d)
May Nitrate Section F
F
September Silicate Section C
oc
o
-100
•-•
-lOO
-lOO
•'•-200
-200
v
-200
-200
• -300
- 300
r- -3o0
-300
l• -400
-400
• -400
-400
•,,
- 100
-500 100
200
o
300
August-
D
0
•
•-100
1oo
200
Distonce (kin)
Distance (km) e)
-500
-500
- 500 o
•
.300
Distance (kin)
Distance (kin)
c)
200
September Nitrate Section D
D'
t
ß
ß
_]
15•.10•
-200
-500
0
100
200
300
400
500
600
700
800
900
1000
1100
Distance (kin) Fig. 5.
Hydrographic sections not included or poorly depicted on fence diagrams' (a) oxygen, (b) silicate, and (c) nitrate sectionsfor April-May and (d) silicate and (e) nitrate sectionsfor August-September.
3O
3O
25
25
2O
2O
I.U
uJ
cc 15 z
lO
10-
5
o o.o
I
i
1.0
2.0
I
3.0
O 16.O
4.0
OXYGEN
Fig. 6.
Nitrate-oxygen regression plot for April-May section FF' in the depth range 50 to 150 m.
I
18.0
I
20.0
I
22.0
I
24.0
26.0
TEMPERATURE
Fig. 7.
Nitrate-temperature regressionplot for April-May section FF' in the depth range 50 to 150 m.
9460
BROCK ET AL.: ARABIAN SEA MONSOON HYDROGRAPHICCLIMATOLOGY
........ =============================================================== • 28 0 18
;;::•::•::•E•::•?:•::•::•::•::•::•::•i•::•::•::•::•?:•i•i•?:•::•::•::•::•:•::•:•:•:• .............. ' •oom
300m
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: • ,,,
400
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 3o0m
. • o"'•o•F ""'""'"•""'. '-.--F
...... ,•
200m
'14ø1-
22
•m
52øE
56 ø
-
I
•00;------1--
J
58 ø
TEMPERATURE INTERVAL 2øC
60 ø
_
II__• --•l
L•
!
I •,-'•"•"• --
/
62 ø
-2øøm
64 ø
66øE
(øC)
Fig. 8.
o I•'""'"'""'"'•:':':'•:'•:.•i•••iii::i::iii::iiii!i - - -
•
m•l "';o•'
•8
,.
54 ø
JUNE-JULY CONTOUR
.. !'
--
m
June-July temperature sections.
.......
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::i:i: :i:: :i:i: :i:i: :i:i:i:: :i:: :i:: :i:i:i:i'"" ":• "=+" ''' m •
::::::: ::::::::::::::::::::::::::::::::::::::::: ::: ::::::::: :::::: ::::::::: :-:-:::: :-::::::::.::: :.::::-:.' • m ß • .• 0 o :;=;:::: =============================================================== ::: ::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
om
-
====================================================================================================================== ::: :::.::-:.o m 3•.2 00 m
.:--•.. ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::: ::::: :::::.:-:.:.:+:.: ................•
•-•
o ':-::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: o• ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: • • '•
16o::3
,•
_
m['1- •6
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400 LI
--Km TM
I
mI I
/
---35•
0
I
I
I
I
54 ø
D[
I
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I
I
I
I I -1300 m
I J.,,o•o m
12 øN - øo• 1;oKm 52øE
1
/t
I
h
35.8 N•104 .n•35
I I
5OOm
-
o
I
I
I
I
I
•
I
I
I
I
5oo•, _ P•Et • E'-
! --3.2
100 m __ 8-----'-['• '• 200 m I
I
'•,
I
'•. I
I
I
-'1400 m __
I•o.
'•5•m 56 ø
JUNE-JULY
SALINITY
CONTOUR
INTERVAL
58 ø
-60 ø
62 ø
64 ø
66øE
0.2 %o Fig. 9.
June-July salinity sections.
24 ø
,oo. 22 o 18 ø
2,•5
16 ø
••"•-••• ?•!!! E'•
i
52OE
i
54 ø
JUNE-JULY CONTOUR
56 ø
SIGMA-T INTERVAL
58 ø
60 ø
1.0
Fig. 10. June-July crt sections.
62 ø
i
64 ø
I
66OE
BROCK ET AL.: ARABIAN SEA MONSOON HYDROGRAPHIC CLIMATOLOGY
o
o
•
9461
....... i:iii!iiii!iiiiiiiiiii ................................. ':-'" --'•'"'"'"'"'•"••-"••'••
.......:.::i!•!iiiiiii!ii!iiiii!iiiiiii:i:i:i:i:i:iii:iii:i:ii•:i:i::-...
_
Om
100 m
16 ø
...................... oo:
200
m
300
m
4OOm
,•E • Om
100
100 m
200
14 ø -
3O0
:300 m
m
4.00 m
1(o 500 m ,,[....
12 oN
t
I
52øE
500
i
54 ø
I
I
56 ø
I
m
m
I
I,
58 ø
I
I
60 ø
I
62 ø
m
I
64 ø
66OE
JUNE-JULY OXYGEN (ML/L) CONTOUR INTERVAL 1.0 ML/L Fig. 11. June-July oxygen sections.
that shown on the April-May sections. During June-July the Oman coastal surface waters are 19øto 22øCwith a salinity of 35.8%øand a o-t of 25.0. June-July section EE', running east-west at 16øN, terminates about 150 km east of the Arabian coast; the isopleths for temperature and o-t are essentially horizontal everywhere along this offshore section. Overall, the evolution of surface temperature and salinity from the premonsoon into the early phase of the southwest monsoon shown on the April-May and June-July sections corresponds to previous individual measurements and climatologies [Hastenrath and Lamb, 1979; Wooster et al., 1967; Wyrtki, 1973; Bruce, 1974]. At distances greater than 300 km from the Arabian coast, the June-July sections for oxygen, silicate, and nitrate (Figures 11, 12, and 13) closely resemble those constructed for April-May. Nearshore the oxygen and nutrient distributions mimic those for
the conservative water properties, with contours generally rising within the 150- to 300-km-wide coastal region. The coastal surface waters are depleted several milliliters per liter in dissolved oxygen relative to the premonsoon; concentrations drop to 2-3 mL/L. June-July surface silicate concentrations at the coast exceed 15 •M, and nitrate exceeds 20/zM. These results are consistent with previous studies that have attributed high summer surface concentrations of inorganic phosphate, nitrate, and silicate to seasonal upwelling [Wyrtki, 1973; Ryther et al., 1966; Aruga, 1973; Currie et al., 1973; Krey, 1973; McGill, 1973; Swallow, 1984].
The August-September sections show a region of domed isopleths centered 250 to 600 km offshore (Figures 14-17). On the temperature sections, this broad uplift of isotherms extends to water deeper than 250 m and colder than 16øC
© ø'
Lol
12 oN i I
52 øE
.
m
o
•
• •
•oo Km I
_
m •..• 500 m
•,*• •
54 o
I
56 o
JUNE-JULY CONTOUR
•
5•m
I
•
58 o
_ I
I
60 o
I
•
62 o
SILICATE (MICRoGRAM-ATOMS/LITER) INTERVAL
5 MICRoGRAM-ATOMS/LITER
Fig. 12. June-July silicate sections.
I
64 o
•
66 øE
9462
BROCK ET AL.' ARABIAN SEA MONSOON HYDROGRAPHIC CLIMATOLOGY
2 4 o ............................................. •:•:•i•::•!ii::•i::iiii::?:iiiii::::i::.:ii!•::!::i•ii::ii::i::::i::•ii:.!i::!i:::•iii•:• ...................
300
16ø
;•
-
m
•m 5OOm
ß
300
14 o
m
4•.
12 ON
o
,ooK.
I
I
[
I
54 ø
52øE
I
56 ø
JUNE-JULY CONTOUR
I
•1 •'•
58 ø
I
[
I
60 ø
I
62 ø
NITRATE
(MICROGRAM-ATOMS/LITER)
INTERVAL
5 MICROGRAM-ATOMS/LITER
I 64 ø
66OE
Fig. 13. June-July nitrate sections.
Septembersectionsis interpreted as the result of upward Ekman pumpingactive within an southwest-northeast elongateregionhavinga breadthof 250 km and centered375 km trends southwest-northeast at an oblique angle to the Arabian coast. On section DD' at 16øN, the region of higher off Ras al Hadd. The region influenced by upward Ekman temperatureand greaterdensityis centered600 km offshore, pumpingbroadensto the southto a breadthof morethan 300 while further to the north on section BB' the domed contours km on section CC' at 16øN. August-September sectionsfor silicate and nitrate (Figures 5d and 5e) suggest that an show peak uplift at 375 km from the coast. The August-September sections (Figures 14-17) suggest offshoreupward nutrient flux is associatedwith this openthat during the later portion of the southwest monsoon, ocean upwelling, in addition to that over the continental upwellingcontinuesto alter the coastalhydrographywithin shelf forced by coastal upwelling. These offshore upward a 200-km-wide band along the coast of Oman. During the nutrient fluxes are believed to drive the summerphytoplank(Figure 14). Inspection of August-SeptembersectionsBB', CC', and DD' reveals that the core of the domed region
second half of the southwest monsoon, coastal upwelling
ton bloom in the northwestern
effects extend to depths of 200-300 m. Offshore uplift of isopleths for all hydrographic parameters on the August-
documentedby ship-basedchlorophyll concentrationmeasurements[Banse, 1987; Bauer et al., 1991] and by Brock et
Arabian
Sea that has been
26ON i11
24 ø
i11
i11
i11
22 ø
m
RABIA 2O !00 m
18 2OO
16 o
D
"
OOm
'
)Om
-
3m
D'
•oo
400
14 ø
12øN 52øE
54 o
56 o
58 o
60 o
62 ø
AUGUSTSEPTEMBER
TEMPERATURE
CONTOUR
2øC
INTERVAL
Fig. 14. August-Septembertemperaturesections.
(øC)
64 ø
66øE
BROCK ET AL.' ARABIAN SEA MONSOON HYDROGRAPHIC CLIMATOLOGY
9463
26ON
2
::::::::::::::::::::::::::::::::::::::::::::::::::::::
18 .•i•i•i•i•i•i•i•i•i•i•::•i•i•i•i•!•i•i•i•i•::•?•i•!•i•:::::; .................... " ,oo. ':i:i:i:i:i:i:i:i:i:i:i:i:!:i:!:!:!:i:!:i:!:i:i:ii!iiiiiiii:iiiiii!iiiiiiiii: toom '.:iiiiiii•i•iiiiiiiii:::::':':: ........
16o':i::!::::" D _ m•
,oom
,oo., 400 m
• ß
12øN •, 52 øE
•
•. • 54 o
•-.-3•.s
•
•
•
56 o
58 o
60 o
AUGUST-SEPTEMBER
62 o
64 o
66 øE
SALINITY
CONTOUR INTERVAL 0.2%o Fig. 15. August-September salinity sections.
al. [1991] and Brock and McClain [1992] with satellite ocean color data. Overall, upward Ekman pumping affects upper ocean hydrography later in the monsoon season than the
composite vertical sections showing the changes in hydrography in the upper 500 m over 2-month time intervals prior to and during the southwest monsoon support the following
coastal upwelling and affects a much more extensive region.
conclusions'
1.
Along the Arabian coast, Arabian Sea Surface Water
extends down to 50 m. Waters
CONCLUSIONS
from 50 to 500 m are formed
by mixing of Arabian Sea Surface Water with Antarctic and
This paper presents the regional premonsoon and southwest monsoon hydrography of the shallow northwestern Arabian Sea in more detail than previous studies. Multiyear
Indonesian
intermediate
waters.
The
Persian
Gulf
Water
salinity maximum noted by previous researchers at about 300-m depth in the northeast Arabian Sea is not apparent; it
26ON m
24 ø
m
m
22
m
•ARAB 20 • 18 ø 20O
16 ø
t
30O
D
m
14 ø m m
m
12ON 52OE
54 ø
56 ø
58 ø
60 ø
AUGUST-SEPTEMBER CONTOUR INTERVAL Fig. 16. August-Septembero't sections.
62 ø
SIGMA-T 1.0
64 ø
66øE
9464
BROCK ET AL.: ARABIAN SEA MONSOON HYDROGRAPHIC CLIMATOLOGY
26ON 2
18 ø"•:i:::::: i:•":•:••••••:'"'""'"'""%;•'••••"• ............................... '" • t:: • 12øN
52 øE
54 o
56 o
58 o
AUGUST-SEPTEMBER CONTOUR INTERVAL
60 o
62 o
64 o
66 øE
OXYGEN (ML/L) 1.0 ML/L
Fig. 17. August-September oxygen sections.
does not significantly influence the shallow hydrography of
plankton biomass distribution in the Arabian Sea, Deep Sea Res.,
the northwestern
38, 531-553, 1991.
2.
Increases
Arabian
in nitrate
Sea.
and silicate within
the thermocline
are mirrored by a drop in dissolvedoxygen in the same50- to 150-m depth interval. During the premonsoonperiod, nitrate has a strong inverse correlation with both temperature and oxygen.
3. In agreement with previous studies that describe coastal upwelling along the Arabian shore throughout the southwest monsoon [Smith and Bottero,
1977; Swallow,
1984; Brock et al., 1991], hydrographic effects of coastal upwelling are observed to penetrate to depths near 400 m at the Oman shelf break and alter the shallow hydrography oceanward
to 150 km offshore.
4. Consistent with the results of recent studies [Bauer et al., 1991; Brock et al., 1991; Brock and McClain, 1992],
Brock, J. C., and C. R. McClain, Interannual variability in phytoplankton blooms observed in the northwestern Arabian Sea during the southwest monsoon, J. Geophys. Res., 97(C1), 733750, 1992.
Brock, J. C., C. R. McClain, M. E. Luther, and W. W. Hay, The phytoplankton bloom in the northwestern Arabian Sea during the southwest monsoon of 1979, J. Geophys. Res., 96(C11), 20,61320,622, 1991.
Bruce, J. G., Some details of upwelling off the Somali and Arabian coasts, J. Mar. Res., 32,419-423,
1974.
Colburn, J. G., The Thermal Structure of the Indian Ocean, 173 pp., University of Hawaii Press, Honolulu, 1975. Currie, R. I., A. E. Fisher, and P.M. Hargreaves, Arabian Sea upwelling, in The Biology of the Indian Ocean, edited by B. Zeitzschel, pp. 37-52, Springer-Verlag, New York, 1973. Darzi, M., J. Chen, J. Firestone, and C. McClain, SEAPAK: A satellite image analysis system for oceanographicresearch, paper presented at Fifth Conference on Interactive and Information ProcessingSystems for Meteorology, Hydrography, and Oceanography, Am. Meteorol. Soc., Anaheim, Calif., Jan. 29 to Feb. 3,
offshore upward Ekman pumping is seen to influence the shallow hydrography to depths greater than 250 m in August 1989. • and September within a 300-km-wide southwest-northeast Findlater, J., A major low-level air current near the Indian Ocean elongate region in the northwestern Arabian Sea. during the northern summer, Q. J. R. Meteorol. Soc., 95, 362Acknowledgments. This research was conducted within the Oceans and Ice Branch at NASA Goddard Space Flight Center. J. B. gratefully acknowledges support from the NASA Graduate Student Researchers Program. Funding for C.R.M. was provided by NASA RTOPS 579-11-01-20, 579-11-02-20, and 579-11-03-20. The authors thank J. Firestone of General Sciences Corporation for assistancein the processingof hydrographic data.
380, 1969. Firestone, J. K., G. Fu, M. Darzi, and C. R. McClain, NASA's SEAPAK software for oceanographic data analysis, in Sixth International Conference on Interactive and Information Processing Systemsfor Meteorology, Oceanography, and Hydrology, pp. 260-267, American Meteorological Society, Boston, Mass., 1990. Hastenrath, S., and P. Lamb, Climatic Atlas of the Indian Ocean,
Part I, Surface Circulation and Climate, 109 pp., University of Wisconsin Press, 1979.
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J. C. Brock, Biological Oceanography Division, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada B2Y 4A2. W. W. Hay, Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309. C. R. McClain, Oceans and Ice Branch, NASA Goddard Space Flight Center, Greenbelt, MD 20771. (Received April 17, 1991; revised December 11, 1991; accepted March 11, 1992.)
