Journal
Journalof
ofCoastal
CoastalResearch
Research
SI 64
pg -- pg
1989
1997
ICS2011
ICS2011 (Proceedings)
Poland
ISSN 0749-0208
Currents on the Southern Continental Shelf of the Caspian Sea off
Babolsar, Mazandaran, Iran
N. H. Zaker†, P. Ghaffari‡, S. Jamshidi‡ and M. Nouranian‡
†Faculty of Environment
University of Tehran
Tehran, Iran
Iranian National Center for
Oceanography, Iran
[email protected]
‡ Iranian National Center for Oceanography, Iran
[email protected]
[email protected]
ABSTRACT
ZAKER, N.H., GHAFFARI, P., JAMSHIDI, S. and NOURANIAN, M., 2011. Currents on the Southern
Continental Shelf of the Caspian Sea off Babolsar, Mazandaran, Iran. Journal of Coastal Research, SI 64
(Proceedings of the 11th International Coastal Symposium), – . Szczecin, Poland, ISSN 0749-0208
This paper presents the results of the first in situ current velocity observation on the southern continental shelf of
the Caspian Sea adjacent to Iran that conducted for 254 days between August 2003 and April 2004. Time series
of current velocity and direction collected at two moorings, each with 3 current meters, across the Caspian shelf
off Babolsar, Mazandaran in Iran. The shelf in the study area is narrow and has a width of about 10 km.
Alongshore currents were uniform across the depth and were dominated by low frequencies less than 0.33 cpd
with peak energy at 7-9 days periods. Monthly eastward alongshore currents of up to 20 cms-1 were observed
indicating the effect of basin scale anticlockwise circulation in the Caspian Sea. There was no upwelling favorite
wind during the study and no upwelling event or Ekman transport was observed. A maximum of daily
alongshore current of 66 cms-1 was observed in summer and autumn. The results showed a low relationship
between low frequency dominated alongshore currents and local wind forcing and raised the hypotheses of the
existence of west to east traveling coastal trapped waves along the southern coast of the Caspian Sea.
ADITIONAL INDEX WORDS: Continental shelf circulation, Direct current velocity measurements, Mooring
station, Coastal trapped waves
INTRODUCTION
The Caspian Sea, as the largest inland water body on the Earth
has a unique marine environment and is of great importance for
the world and the lateral countries around it (Karpinsky et al,
2005; Zonn, 2005b; Dumont, 1998). However, the marine
environment of the Caspian Sea, due to extensive human
exploitation and discharge of large magnitudes of urban, industrial
and agricultural waste, is under extensive pressure (Zonn, 2005a;
Korshenko and Gul, 2005).
Although one of the basic requirements for environmental,
biological or any other marine related studies, the understandings
about circulation and characteristics of the currents on the
southern shelf of the Caspian Sea are very limited and before this
study, no in situ current measurements were performed in this
area. Therefore, there are several basic questions in this regard.
These questions include how variable are the shelf currents in
terms of diurnal, seasonal or annual scales and how do these
currents relate to local wind or other forcing mechanisms.
To answer these questions confidently, sufficient amounts of
direct current meter measurements are required. In this paper, we
discuss these questions using the results of current velocity
measurements conducted over a period of 254 days at two
mooring stations on the southern shelf of the Caspian Sea adjacent
to Iran; the first direct observations of currents in this region.
Study Area
The study area is located at approximately N36o, 41’, 54”
latitude and E52o, 33’, 23” longitude (Fig. 1). The shelf in the
study area has a width of about 10 km. The depth gradually
increases to about 45 m near the shelf break, after that the depth
sharply increases to 400 m at about 18 km from the coastline.
The southern coast of the Caspian Sea has a subtropical climate
characterized by warm humid summers and mild, wet winters
(Kosarev, 2005; Kosarev and Yablonskaya, 1994; Rodionov,
1994). The air temperature is maximum in August and minimum
in January. Winds with northerly and southeasterly directions are
the most stable and dominate winds over the Caspian Sea during
the major part of the year (Kosarev, 2005). Throughout the year,
the field of the northerly winds is observed in 40% of the cases; in
the summertime they dominate (up to 50%) and almost half of the
winds are northwesterly. The southeasterly winds feature a
recurrence of about 36% and are more frequent (about 40%) in the
winter and spring. In the South Caspian, strong winds are rarely
observed and the recurrence rate of weak winds here reaches 90%.
In the southern part of the sea, the number of the days with storms
Journal of Coastal Research, Special Issue 64, 2011
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Currents on the Southern Continental Shelf of the Caspian Sea off Babolsar, Mazandaran, Iran
Figure 1. a) Southern Caspian Sea and position of the study area. b) Position of moorings (MR1, 26 m; MR2, 42 m) and weather station
(wind speed more than 15ms–1) is not greater than 20–30 per year
(Kosarev, 2005)
Owing to the isolation of the Caspian Sea from the World
Ocean, the formation of its thermohaline and circulation regime
proceeds only under the action of atmospheric processes over the
sea basin and its vast drainage area. The impact of the wind in the
form of the fluxes of momentum and relative vorticity generates
the three dimensional general circulation of the Caspian Sea
(Tuzhilkin and Kosarev, 2005).
Due to the lack of required instrumental current observations,
the Knowledge of the general water circulation in the Caspian Sea
has been based on diagnostic simulations by numerical
hydrodynamic models (Tuzhilkin and Kosarev, 2005). Based
on the diagnostic calculations, in the South Caspian a dipole
system, consisting of an anticyclonic gyre in its northwestern part
and a cyclonic gyre in its southeastern part, exists throughout the
year. In the wintertime the anticyclonic gyre in the South Caspian
is most intense, while in the summertime, in contrast, the cyclone
in the South Caspian has the greatest intensity. In the synoptic
range of the current variability in the Caspian Sea, oscillations
with periods from 2–3 days to 1–3 weeks prevail. They are
related to the synoptic variability of the direct wind impact and to
coastal trapped waves. In the higher frequency range, current
variability is dominated by inertial gravity waves and seiches
(Tuzhilkin and Kosarev, 2005).
The temperature structure in the study area is characterized by a
strong seasonal thermocline, located between 20m and 50m depths
with 15oC temperature difference across it, in summer. In
autumn, the thermocline gradually weakens and at the end of
winter it disappears before its re-formation in the early spring.
The temperature in the surface layer ranges between 25 and 30o C
in summer and gradually decreases to 15o C at the end of autumn
and remains mainly between 14-15o C in winter. Below
thermocline, the temperature ranges between 10.5o C and 7.5o C at
110m depth, with small seasonal variations (Zaker et al, 2005).
The salinity in the study area has small vertical and horizontal
variations with slight seasonal changes. Salinity mainly ranges
between 12.1 and 12.35 vertically in summer and autumn. In the
spring the lower salinities in the surface layer (11-12) are observed
due to an increase in the local river inflows. (Zaker et al, 2005)
DATA COLLECTION
Two mooring stations, MR1 and MR2, each with three current
meters (RCM 9-MK II) were deployed across the shelf (Figure 1)
and collected data for 254 days between August 4, 2003 and April
18, 2004. The moorings were located at 26 m (MR1) and 42 m
(MR2) depths, respectively.
The current meters at MR1 mooring station were positioned at
3.5, 13.5 and 23.5 m below surface water and the current meters at
MR2 mooring station were positioned at 3.5, 13.5 and 35.5 m
below surface water. Each current meter was equipped with a
temperature sensor. Time series of current velocity and direction
and temperature were collected at 10 minute intervals. The period
of the deployment and location of the current meters are listed in
table 1. Current data had an accuracy of ± 0.5 cms-1 in speed and
± 5o in direction. The period of the deployment and location of
the current meters are listed in Table 1.
Wind and meteorological data were obtained by a weather
station (AWS2700, AANDERAA) that was set up for the same
period of current data collection at 10 m height in the land,
adjacent to the mooring stations (Figure 1). Time series of wind
and meteorological data were also collected at 10 minute intervals.
Wind stress was computed using a quadratic stress equation
(Large and Pond, 1981).
The 10 minute interval time series of currents and wind stress
were changed to hourly average data before being used in the
analysis.
The mean coastline direction near the mooring stations is 15
degrees non-clockwise relative to the east; therefore we used a
system whose y-axis was rotated non-clockwise by 15 degrees
with respect to the west–east direction. The principal coordinate
system, direction with the maximum and minimum standard
deviation of velocity, coincided with the coastline associated
coordinate system. Therefore, we decomposed both the velocity
and the wind stress into the along-shore and cross-shore
components. The positive sign of the along-shore velocity
component indicates eastward motion along the coastline, and the
positive sign of the cross-shore velocity corresponds to a motion
directed from the coast towards the open sea.
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Zaker et al
Table 1 Current meter stations
Station
(depth)
Position
Latitude
Longitude
Current meters location
below water surface
Period of deployment
Sampling
MR1
(26 m)
-3.5 m, -13.5 m, -23.5 m
36, 44.793
52, 32.967
4 Aug. 2003 –
18 Apr. 2004
10 minutes
MR2
(42 m)
-3.5 m, -13.5 m, -35.5 m
36, 46.637
52, 32.816
4 Aug. 2003 –
18 Apr. 2004
10 minutes
RESULTS AND DISCUSSION
Figures 2 and 3 show hourly time series of the along-shore and
cross-shore components of current velocity collected at mooring
stations MR1 and MR2. Figures 4 and 5 present corresponding
wind stress and current velocity, low-pass filtered using
MATLAB Butterworth IIR filter with cut off period of 36 hours.
Spectral density of the surface current velocity and wind
stress are presented in Figure 6. Basic statistics of the wind
stress and current velocity data are presented in Tables 2-4.
The observed data were dominated by low frequency fluctuating
velocities (Figs 2 to 5). There was also a “residual” mean current
flowing eastward along the shore (Tables 3 and 4). The eastward
mean flow varied in magnitude during the year. Its maximum of
20 cms-1 occurred in November and its minimum of a few cms-1
observed in February. The average alongshore current velocities
in summer (August-September) were 5.2 cms-1 and 10.7 cms-1 at
MR1 and MR2 stations, respectively. In winter (January-March)
they reduced to 4.1 cms-1 and 4.3 cms-1, respectively, indicating a
stronger summer mean current at 42 m depth (Tables 3 and 4).
The mean alongshore wind stress during the study was near zero
(Table 2). Therefore the observed mean currents were not wind
driven and were caused by larger scale circulations in the Caspian
Sea.
Along shore currents (Figs 2 and 4) were dominated by strong
low frequency fluctuations with frequencies less than 0.33 cycles
per day and had peak energies at 7-9 days periods (Figure 6, Table
2). They were generally uniform across the depth for the whole
period of the recording and also were strongly correlated across
the shelf (r=0.86 for low frequency data at surface water).
However, currents at 42 m depth were generally larger than those
at 26 m depth (Tables 3 and 4). Alongshore currents showed
seasonal variations with lower magnitudes in winter and had a
maximum daily velocity of 66cms-1 in summer (Tables 3 and 4).
Local low frequency alongshore wind stress was mostly very
weak during the study (Figs 4 and 5, Table 2). There were
frequent events when wind stress was near zero in the area, while
strong low frequency alongshore current fluctuations were
observed (Fig 4). Also In some events wind and current were in
opposite directions (Fig 4). In general, the correlation between
alongshore current and wind stress, except during strong wind
events, was low (r=0.48 in summer and r=0.43 in autumn and
winter using low pass filtered data). In addition, current and wind
stress spectra were not similar and did not contain similar isolated
peaks (Figure 6).
The weak relationship between the low frequency alongshore
currents and wind stress indicates that these currents were not
mainly driven by local winds and other forcing mechanisms
should be brought into consideration.
An important process related to shelf dynamics is the
distribution of the sea level and accompanied currents along the
coast that is known as the coastal trapped waves. Coastal trapped
waves have significant effect on the currents over the continental
shelf in many areas and are a major characteristic in many shallow
continental shelves. The waves spread and leave the area of
periodic wind forcing as periodic water movement and are
observed in a place that wind fluctuations are not related to them.
The direction of coastal trapped waves is in a way that coastline
stands to the right side of spreading in the northern hemisphere.
The coastal trapped waves have periods of several days to 2-3
weeks and their maximum relative sea surface oscillation occurs at
the shelf edge. They are barotropic waves and alongshore current
velocities across the depth are somehow uniform and the currents
are in geostrophic balance with the pressure gradient
perpendicular to the coastline (Csanady, 1997; Allen, 1980;
Kundu and Allen, 1976; Smith, 1974).
Considering the characteristics of the observed alongshore
currents, the peaks of energy at 7-9 days periods, the uniformity
across the depth and weak correlation with the local wind stress,
raise the hypothesis of the presence of the eastward coastal
trapped waves and their related currents along the southern coast
of the Caspian Sea. The relationship between low frequency
currents in the Caspian Sea and coastal trapped waves has already
been discussed in the literature and northward movement of the
area of water upwelling along the eastern coast of the Caspian Sea
had been attributed to the northward propagation of coastal
trapped waves (Tuzhilkin and Kosarev, 2005)
Table 2 Monthly mean, variance and maximum daily wind stress (2003-2004)
Month
Mean
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Variance
−3
N.m × 10
Along
Crossshore
shore
8.6
-7.8
8.7
-6.7
-0.6
-4.3
2.5
-3.6
2.8
-3.2
-0.9
1.1
2.4
-4.6
11.0
-10.9
-2
Daily Variance
−3
(N.m ) × 10
Along
Crossshore
shore
0.76
0.39
0.89
0.37
0.70
0.60
1.80
1.10
1.10
0.40
0.24
0.11
0.78
0.54
1.50
0.60
-2 2
−3
(N m ) × 10
Along
Crossshore
shore
0.35
0.11
0.36
0.12
0.22
0.08
0.71
0.52
0.54
0.24
0.06
0.03
0.29
0.13
1.10
0.30
-2 2
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Maximum Daily
N m-2
Along
shore
0.09
0.08
-0.04
0.10
0.12
0.03
0.05
0.14
Crossshore
-0.06
-0.05
-0.03
-0.09
-0.08
-0.02
-0.04
-0.09
Currents on the Southern Continental Shelf of the Caspian Sea off Babolsar, Mazandaran, Iran
Figure 2. Series of hourly along-shore component (positive 75o N) of current velocity at MR1 (26 m) and MR2 (42 m) stations. August
2003 to April 2004.
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Zaker et al
Figure 3. Series of hourly cross-shore component (positive 345o N) of current velocity at MR1 (26 m) and MR2 (42 m) stations. August
2003 to April 2004.
.
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Currents on the Southern Continental Shelf of the Caspian Sea off Babolsar, Mazandaran, Iran
Figure 4. Series of low pass filtered along-shore component of current velocity at MR1 (thick line 3.5 m, thin line 13.5 m, dashed line
23.5 m below surface water), MR2 ( thick line 3.5 m, thin line 13.5 m, dashed line 35.5 m below surface water ) and wind stress.
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Figure 5. Series of low pass filtered cross-shore component of current velocity at MR1 (thick line 3.5 m, thin line 13.5 m, dashed line
23.5 m below surface water), MR2 ( thick line 3.5 m, thin line 13.5 m, dashed line 35.5 m below surface water ) and wind stress.
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Currents on the Southern Continental Shelf of the Caspian Sea off Babolsar, Mazandaran, Iran
Table 3 Monthly mean, variance and maximum daily surface currents at MR1 (2003-2004)
Month
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Mean
(cm s-1)
Along
Crossshore
shore
4.64
1.37
5.74
0.89
3.93
0.06
7.49
1.66
2.96
2.30
3.78
1.65
2.28
0.75
6.18
0.56
Alongshore
228.97
453.90
263.30
382.70
180.90
95.82
120.31
194.57
Variance
(cm s-1)2
Crossshore
28.8
21.4
25.2
18.6
33.6
12.6
14.5
20.3
Cross-shore currents were much smaller than along-shore
currents (Figures 2-5) and the energy of cross-shore currents was
an order smaller than that of the along-shore currents (Figure 6,
Tables 3 and 4). The maximum observed daily cross-shore
velocities were 9 cms-1 and 17 cms-1 at MR1 and MR2 stations,
respectively.
Surface cross-shore currents were correlated across the shelf
(r=0.71 with zero lag for August to November for low-pass
filtered data) and also were similar in magnitude. Across the
depth the cross-shore currents were frequently similar in direction
and magnitude and positively correlated (Figure 5). No
relationship was found between cross-shore and along-shore
currents and also there was no correlation between cross-shore
currents and along-shore or cross-shore wind stresses. There was
no upwelling favorite wind during the study, and no upwelling
event or Ekman transport was observed (Figure 5).
CONCLUSIONS
Characteristics of the currents on the southern continental shelf
of the Caspian Sea, adjacent to Iran, off Babolsar in Iranian
province of Mazandaran, were studied using time series of current
and wind data collected over the period of August 2003 to April
2004. The results showed uniform alongshore currents across the
shelf both horizontally and vertically. Cross shore currents were
generally correlated positively and Ekman transport was not
observed. Wind stress was generally weak and mainly in west to
east direction and no upwelling favorite condition or occurrence
was observed. During the whole period of measurements, the
results showed considerable monthly currents up to 20 cms-1 in
Daily Variance
(cm s-1)2
Along
Crossshore
shore
172.4
5.2
368.3
8.7
207.5
8.8
308.4
8.9
136.5
14.3
84.8
5.1
100.5
3.9
184.1
5.3
Maximum Daily
(cm s-1)
Along
Crossshore
shore
38.4
5.4
46.0
7.0
-28.7
8.1
53.1
7.6
27.7
9.2
22.3
6.9
22.0
5.6
40.6
4.8
west to east direction indicating the effect of basin scale
anticlockwise currents in the Caspian Sea. The results showed
low relationship between low frequency dominated alongshore
currents in the study area and wind forcing and raised the
hypotheses of the existence of west to east traveling coastal
trapped waved along the southern coast of the Caspian Sea.
ACKNOWLEDGEMENTS
The data used in this study were obtained during the “Khazar
Physical Oceanographic Project Phases 1and 2” funded and
supported by Iranian National Center for Oceanography (INCO)
and conducted by the first author of this paper. We thank all the
colleagues at INCO who assisted the project or contributed in field
measurements.
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Table 4 Monthly mean, variance and maximum daily surface currents at MR2 (2003-2004)
Month
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Mean
(cms-1)
Along
Crossshore
shore
8.14
2.66
13.10
1.35
4.41
1.43
20.76
4.01
8.30
2.90
4.13
1.33
0.52
0.28
8.03
-0.16
Variance
(cms-1)2
Along
Crossshore
shore
278
63
728.5
61.3
439.1
46.9
431.9
38.5
145
39.9
99.6
18.1
98.7
17.2
281.6
35.6
Daily Variance
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Along
Crossshore
shore
234.2
22.4
650.6
29.0
400.0
21.9
393
20.9
94.1
13.4
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5.5
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shore
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64.0
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37.0
12.6
65.8
15.8
30.5
11.4
22.6
7.4
22.5
6.0
46.7
-10.0
Zaker et al
Figure 6. Frequency spectra of the along-shore (thick line) and cross-shore (thin line) components of hourly current velocity (a , b)
and wind stress (c , d). Left column August to November 2003, right column December 2003 to April 2004.
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