The Persian Gulf sits on top of the largest hydrocarbon reserve in the world, which makes this area extremely important for oil production. It is one of the most important strategic waterways in the world. The Persian Gulf is connected to the Gulf of Oman and the Indian Ocean through the Straits of Hormuz. The countries of United Arab Emirates, Qatar, Bahrain, Saudi Arabia, Kuwait and Iraq border, the Gulf along its Southern coastline and Iran is situated along the Gulfs Northern coastline (Fig. 1). The narrow Strait of Hormuz restricts water exchange between the Persian Gulf with the Northern Indian Ocean.
The Persian Gulf is about 990 km long and has average and maximum depths of
36 and 120 m, respectively. It is broadest (370 km) in its middle and narrowest
(56 km) across the Strait of Hormuz. The Gulf is a semi-enclosed marginal sea
in an arid sub-tropical region. Estimates of net evaporation rates vary between
98 mm year-1 to as large as 500 mm year-1 (Privett,
1959; Meshal and Hassan, 1986; Johns
et al., 2003). Strong evaporation makes the Gulf an inverse estuary
that is governed by outflow of saline Gulf water through the Strait and inflow
of low salinity surface water from the adjacent Gulf of Oman (Swift
and Bower, 2003). During summer a seasonal thermocline is evident with a
surface to bottom temperature difference of around 11°C in the Persian Gulf.
Knowledge of the Gulfs circulation is of great significance for management
of oil spill events, fisheries and shipping lines. On the basis of a comprehensive
suite of hydrographic data acquired a year after the Gulf War in 1991, Reynolds
(1993) proposed a sketch of the Gulfs general circulation (Fig.
1) that has been confirmed by most subsequent studies.
The most important episode of coastal upwelling, in terms of size, duration
and economic importance, occur due to longshore wind. The phenomenon of oceanic
upwelling in the coastal area is important from physical and ecological viewpoints.
||Sketch of the general circulation and region of upwelling
in the Persian Gulf. Modified from Reynolds (1993)
The occurrence of strong upwelling of cold water in a narrow coastal strip
contributes to increased productivity of the sea as well as to climate modification
of the adjacent land. The persistent Northwesterly wind stress, in the Northern
part of the Gulf, appears to set up coastal current regimes (upwelling) along
Iranian coast (Reynolds, 1993). Year-round Northwesterly
winds, known as the Shamal, are believed to create a Southeast ward coastal
upwelling jet along the Iranian coast between 28 29°N, but observational
evidence is sparse (Fig. 1). As far as the researcher is aware,
modeling studies of the Persian Gulfs coastal upwelling are lacking; that
is, previous studies were either on general circulation (Sadrinasab
and Kampf, 2004), or have focused exclusively on tides in the Persian Gulf
The principle objective of this study is to present a three-dimensional numerical
modeling of coastal upwelling in the Northern part of the Persian Gulf, where
Reynolds (1993) mentioned it.
MATERIALS AND METHODS
This study employs the three-dimensional hydrodynamic model COHERENS (Coupled
Hydrodynamic-Ecosystem Model for Regional Seas) (Luyten et
al., 1999) based on sigma coordinates. This model uses 10 sigma levels
on an ETOPO2 (two minute worldwide Bathymetry/Topography) bathymetry, being
interpolated and slightly smoothed onto a 4-minute grid. Maximum water depth
is set to 150 m, which only modifies the topography of the Gulf of Oman and
has no impact on the resultant circulation in the Persian Gulf. Cartesian lateral
grid spacing is 6.6 km (north-south) and 7.4 km (Eastwest). The model is forced
by climatologic monthly mean atmospheric forcing (wind speed, air temperature,
humidity, cloud cover and precipitation) derived from 54 years of National Oceanic
and Atmospheric Administration data. A reduced river discharge of 10 km3
year-1 as an estimate of flow rates after dam construction is used
by the model. Monthly mean vertical profiles of temperature and salinity, extracted
from previous hydrographic data (Alessi et al., 1999)
are prescribed at the Eastern open-ocean boundary. Tidal boundary forcing is
included using the four major constituents: M2, S2, O1
and K1. The tidal part of the model has been calibrated against previous
tidal studies (Najafi, 1997).
Simulations cover a total period of 11 years including an initial spin-up period of 10 years. The model is run in a fully prognostic mode with 80 sec time step for all variables. Then model has been calibrated with available data and run for additional year to simulate upwelling. As the change of coastal water temperature is an indication of coastal upwelling and also with the knowledge that, thermocline occurs during summer, different wind speeds and durations were applied parallel to the coastline in the Northern part of the Gulf during summer (July).
RESULTS AND DISCUSSION
As mentioned formerly, this is the first study on upwelling in the Persian
Gulf, hence, there is no field data to compare the findings of the model with
them, but findings of simulated seasonal and spatial variations in temperature
and salinity in the Persian Gulf are in general agreement with field data (Alessi
et al., 1999). Figure 2 shows comparison of the
TS-diagrams computed by the model results with by Alessi et
al. (1999) which has been taken from historical data in the Northern
part of the Gulf during summer. Also circulation patterns, of the model compare
well with earlier investigations (Kaempf and Sadrinasab,
2006). However, a discussion of this would go beyond the scope of this study.
Figure 3 shows the annual surface and bottom temperature
difference in the Northern part of the Gulf for the last year of the simulation.
As can be seen from Fig. 3 the maximum temperature difference
is 12 degrees in summer (July) which is in agreement with.
As the change of coastal sea surface temperature is the best indicator of coastal
upwelling, summer time is chosen to blow upwelling-favorable wind over the domain
to simulate the upwelling in the Northern part of the Gulf. To do this, several
models have been run with different wind speeds and durations parallel to the
coastline in mid July.
||Calculated TS-diagram by the model (a) in the Northern part
of the Gulf and TS-diagram from historical data by Alessi
et al. (1999) and (b) during summer
||Computed annual surface and bottom temperature difference
in the Northern part of the Gulf by the model
||Sea surface temperature in the region of study (a) in normal
condition and (b) with resonant wind
A first experiment was preformed with wind speed of 4 m sec-1 and
duration of 7 days, but no indication of upwelling was observed. In the next
attempts, speed of the wind was increased by 1 m sec-1 intervals.
After 4 days of consecutive wind with a speed of 9 m sec-1, a longshore
baroclinic jet was observed which in turn led to active upwelling.
During July, in normal condition, sea surface temperature is nearly 32°C
everywhere in the domain except at the river mouth where it is slightly lower
(Fig. 4a, b). But in the condition when
upwelling-favorable wind exceeds 9 m sec-1 and blows parallel to
the Northern part of the coast, after a period of 4 days, a dynamic balance
is established between the wind-induced frictional force at the water surface
and the Coriolis force.
||Time series of SST at the location of upwelling
||Variation of SST with offshore distance at the location of
Due to this balance, a mass transport of water takes place in the surface layer,
which is directed to the right of the blowing wind and also due to a decrease
in water level along the coastline, mass conservation requirement causes vertical
motions of upwelling.
Figure 4b show, the coastal upwelling development which
substantially reduces coastal surface temperatures from 32°C to about 21°C
in the region of study.
Figure 5 shows the time series of sea surface temperature
by the model at the place marked red square in Fig. 1. As
shown in Fig. 5, a sudden drop of sea surface temperature
occurs in the model due to establishment of upwelling in the study area.
Figure 6 represents variations of sea surface temperature
with offshore distance at the mentioned location where upwelling occurred in
the model. This figure clearly exhibits the width of the upwelling strip which
is about 21 km at the same location. This is in close agreement with Reynolds
Figure 7 shows time series of the vertical profile of sea
surface temperature by the model along the coast at the location where upwelling
occurs in the model. As can be seen in Fig. 7, before blowing
of upwelling-favorable wind, thermocline is evident in the region of study,
but after upwelling-favorable wind, cold water from beneath moves towards surface
and diminishes the coastal sea surface temperature approximately by 11°C.
||Vertical profile of SST at the location of upwelling
A seasonal thermocline is evident with a surface to bottom temperature difference of around 12°C in summer. In the Northern part of the Gulf when the wind direction is parallel to the coast with a speed of greater than 9 m sec-1, upwelling can occur. Also, findings of the model suggested that 4 days continual wind parallel to the Northern coast is required to establish upwelling in this region. It is also found that the coastal sea surface temperature is a very good monitor of coastal upwelling at the study area.
This research was supported by postgraduate grant No. 1386/2 at Khorramshahr University of Marine Science and Technology, Iran.