Total Sea Surface Current Circulation Pattern in the South China Sea Derived from Satellite Altimetry

Sea surface currents and circulation patterns can be estimated globally and in a continuous manner using satellite radar altimeters. This study is concerned with the surface current circulation pattern in the South China Sea. The main data utilized are the tidal amplitudes from co-tidal charts, the sea level anomaly and the wind speed from the Jason-1 satellite altimetry data. Algorithms used to estimate the surface current are the geostrophic current algorithms, the wind-driven current algorithms and the tidal current algorithms. The derived surface currents from these models were combined to produce the total surface current. Maps of surface current circulation pattern are produced for four periods, which are the northeast monsoon, the southwest monsoon and the inter monsoon periods of April and October in 2004 and in 2005. In order to validate the derived surface current, the correlation coefficient with field measurement data was determined. The field data are measured using Lagrangian buoys under the Data Buoy Cooperation Panel programme (DBCP) and also from the Japanese Oceanography Data Center (JODC). In this study, it was found that the correlation of total surface current speed and direction is reasonable with coefficient of 0.9 and 0.8, respectively. As for the total surface current circulation pattern, the currents in the South China Sea move southward during the northeast monsoon and reversed during the southwest monsoon period. In general, during the northeast and southwest monsoon, the water moved in anticlockwise and clockwise directions, respectively. 1.0 INTRODUCTION In the work described in this paper, Jason-1 satellite altimetry data have been used together with field data to study the surface currents in the South China Sea. The South China Sea is a large marginal sea situated at the western side of the tropical Pacific Ocean. It is a semi-closed ocean basin surrounded by South China, Philippines, Borneo Island, Indo-China Peninsula and Peninsular Malaysia. This water body connects with the East China Sea, the Pacific and the Indian Ocean through the Taiwan Straits, the Luzon Straits and the Straits of Malacca, respectively (Chung et al., 2000). The study area of this research is from 2 to 15 north and from 100 to 120 east. The area of the South China Sea is shown in Figure 1. Since the capability of radar altimetry to provide the surface current has been established (Challenor et al., 1996), this study is concerned with the surface current in the southern part of the South China Sea during four monsoon periods, which are the northeast monsoon (NE), the southwest monsoon (SW) and the inter monsoon periods of April and October in 2004 and in 2005. The main data utilized in this study includes the tidal amplitudes from co-tidal chart, the sea level anomaly and the wind speed data from satellite altimetry. Besides field measurement, data obtained from the Coriolis ftp site and from the Japanese Organization Data Center (JODC) were also used for validation. The data are available via the Coriolis ftp site; ftp://ftp.ifremer.fr/ifremer/coriolis/lagrangian_buoy and JODC site; http://jdoss1.jodc.go.jp/cgi-bin/1997/ocs. The algorithms used to estimate the surface current are the geostrophic current algorithms, the wind-driven current algorithms and the tidal current algorithms. 2.0 SEA SURFACE CURRENT ALGORITHMS Algorithms used to estimate the surface currents include the geostrophic current algorithms (Chung et al., 2000), the wind-driven current algorithms (Chung et al., 2000; Yanagi, 1999) and the tidal current algorithms (AlRabeh et al., 1990). 2.1 Geostrophic Current Algorithms Algorithms of geostrophic current are expressed in Equation (1) to Equation (6). ug = -g / f (δζ / δy) (1) vg = g / f (δζ / δx) (2) θg = tan (vg / ug) (3) f = 2Ω cos φ (4) x = R (λ – λ0) cos φ (5) y = Rφ (6) where ug and vg are the geostrophic current in zonal and meridional direction, respectively, θg is the geostrophic current direction, g is the gravitational acceleration, f is the Coriolis force, ζ is the sea surface height relative to the geoid, x and y is the local east coordinate and the local north coordinate, respectively, Ω is the Earth rotation rate = 7.27 x 10 s, φ is the latitude in radian, λ is the longitude in radian and R is the Earth radius = 6371 kilometers. In order to implement the equations, firstly, the Coriolis force, the local east coordinate and the local north coordinates are determined using Equation (4), Equation (5) and Equation (6). Then, derivatives of zonal and meridional direction of sea surface height relative to the geoid are calculated. Then, geostrophic currents in zonal and meridional directions are calculated using Equation (1) and Equation (2). In order to estimate the direction of the geostrophic current, Equation (3) is used. 2.2 Wind-Driven Current Algorithms Algorithms of wind-driven current are expressed in Equation (7) to Equation (10). ue = V0 exp(az) cos(π/4 + az) (7) ve = V0 exp(az) sin(π/4 + az) (8) V0 = τ / (√ ρ f Az) (9) a = √ (f / 2 Az) (10) where ue and ve are the wind-driven current in zonal and meridional direction, respectively, Az is the vertical eddy viscosity, f is the Coriolis force, ρ is the water density, τ is the wind stress = ρaCdW where ρa is the air density, W is the wind speed 10 m above the sea surface and Cd is the drag coefficient = 2.6 x 10. In order to implement the equations, firstly, the wind-driven current components in zonal and meridional directions for water level, z = 1 m and z = 15 m are determined. Then, the mean of these two velocities is computed and the vector sums of the components are calculated to obtain the current speeds. This speed is called the wind-driven surface current. In this study, wind-driven current velocities at the water level of 15 m from the sea surface are important because the current movement in this level varies significantly. For example, if wind speed blowing in the water surface is 400 cm s, the wind-driven current speed at the water level of 1 m, 15 m and 30 m are 8.5 cm s, 2.2 cm s and 1.5 cm s, respectively. This shows that the difference of wind-driven velocities in 15 m from surface water is 6.3 cm s which is significant. However, below the 15 m of water level from the sea surface, the current velocity does not change significantly whereby the difference is less than 1 cm s. The equations above indicate that the speed of the wind-driven surface current is proportional to wind stress. According to Ekman spiral theory, the surface water movement is deflected by π/4 to the wind direction. In the northern hemisphere, the wind-driven surface current is deflected in a clockwise direction. This is because of the force balance between the wind drag and the water drag (Yanagi, 1999). Therefore, in this study, the direction of wind-driven current (θe) is estimated by adding 45 clockwise to the direction of the surface wind. 2.3 Tidal Current Algorithms Algorithms of tidal current are expressed in Equation (11) to Equation (13). δ ut/ δt –(δ/ δz(Av δu/ δz)) – fv = -g δη/ δx – (1/ρ(δP/ δx)) (11) δ vt /δt –(δ/ δz(Av δv/ δz)) + fu = -g δη/ δy – (1/ρ(δP/ δy)) (12) θt = tan (vt / ut) (13) where ut and vt are the zonal and meridional tidal currents, respectively, f is the Coriolis force, g is the gravitational acceleration, η is the tidal amplitudes, Av is the vertical eddy viscosity, x and y is the local east coordinate and the local north coordinate, respectively, t is the time, P is the atmospheric pressure, ρ is the water density, z is the water level and θt is the tidal current direction. In order to implement the algorithms of tidal current, firstly, tidal current in zonal and meridional direction is determined using Equation (11) and Equation (12). Then, the direction of tidal current is determined using Equation (13). 2.4 Total Sea Surface Current Algorithms In the work described in this paper, total surface current is used to present the combination of geostrophic current, wind-driven current and tidal current. In order to get the total surface current velocities, the vector sum of geostrophic velocities, wind-driven velocities and tidal velocities are computed. As far as the total surface current directions are concerned, means of the geostrophic current, the wind-driven current and the tidal current are estimated. Equation (14) to Equation (16) is the formulae to compute the total surface current velocity and Equation (17) is the formula to compute the total surface current direction. utotal = ug + ue + ut (14) vtotal = vg + ve + vt (15) Vtotal = √ (utotal + vtotal) (16) θtotal =( θg + θe + θt) / 3 (17) where utotal and vtotal are the total surface current in zonal and meridional direction, respectively, Vtotal is the vector sum of total surface current and θtotal is the total surface current direction. 3.0 RESULTS The sea surface circulation patterns over the four monsoon periods in 2004 and in 2005 are shown in Figure 2. These are presented in vector plots where the arrows represent the directions of the total surface current and the length of arrows represent the magnitudes of the total surface current in cm s. The big arrows present the general circulation pattern of the sea surface current. Generally, during the inter monsoon periods of April in 2004 (Figure 2 (a)), water moves in a clockwise circulation over the sea. On the coast of Peninsular Malaysia, water moves towards the north and then deflects to the right on the northern part of the sea. On the coast of Borneo Island, water moves along the coast towards the south. In the Gulf of Thailand, the surface current moves in an anticlockwise direction. In addition, there are two clockwise eddies near the Natuna Islands. As for the surface current speed, the mean surface current is 52.9 cm s. The minimum and the maximum speed over the period are 2.6 cm s and 286.9 cm s, respectively. Strong current movements are found on the Gulf of Thailand, on the south coast of Indochina and on the east coast of Peninsular Malaysia. As for the surface circulation pattern during the inter monsoon periods of April in 2005 (Figure 2 (b)), similar circulation pattern is found with minor changes over the period. There are two eddies near the Natuna Islands that move in anticlockwise directions. As for the surface current speed, the mean speed over the period is 41.7 cm s. In addition, the minimum and the maximum speed are 1.8 cm s and 220.4 cm s, respectively. Strong current movements are found on the Gulf of Thailand and on the southern part of the sea. Means during the two-year period indicates that surface current during the inter monsoon periods of April in 2004 is stronger than in 2005. As for the surface circulation pattern during inter monsoon period of October, there was no significant difference between inter monsoon period of October in 2004 and in 2005 (Figure (c) and Figure (d)). Generally, surface water moved in anti-clockwise direction towards the south over the period. Water in the coast of Indochina moved southwards along the coast and then entered the Gulf of Thailand. In the Gulf of Thailand, the water was deflected and moved in an anti-clockwise direction. Along the coast of Peninsular Malaysia and the coast of Borneo Island, the surface current moved parallel to the land towards the south. In 2004 and 2005, an anti-clockwise eddy was found in the middle of the sea. As for the current speed, surface current speed in 2004 is greater than in 2005 whereby the mean speed was 52.8 cm s and 40.1 cm s, respectively. In 2004, the minimum and maximum current speed was 1.5 cm s and 264 cm s, respectively. Strong current movements were found in the south coast of Indochina and the west coast of Borneo Island. In 2005, the minimum and maximum speed was 2.1 cm s and 238.9 cm s, respectively. However, strong current movements were found in the east coast of Peninsular Malaysia, the Natuna Islands, the south coast of Indochina and the east coast of Indochina. As for the surface current circulation pattern during the SW monsoon in 2004 (Figure 2 (e)), the surface circulation pattern shows not much difference from the inter monsoon periods of April in 2004 whereby surface water moves in a clockwise circulation over the periods. Similarly, water on the east coast of Peninsular Malaysia and on the east coast of Indochina moves along the coast towards the north. However, the direction deflects to the right on the northern part of the sea over the period. Besides, surface water on the coast of Borneo Island moves along the coast towards the south. Over the period, strong surface current movements are found on the Gulf of Thailand, near the Natuna Islands and on the south coast of Indochina. Mean, minimum and maximum surface current speeds are 55.2 cm s, 3.1 cm s and 280.4 cm s, respectively. In 2005 (Figure (f)), a clockwise circulation was found in the South China Sea over the period. In the coast of Peninsular Malaysia, Indochina and Borneo Island, the water moved northwards and in the northern part of the sea, water deflected to the right. Water circulation pattern in the Gulf of Thailand in 2005 is similar to 2004 where the surface current moved in anti-clockwise direction over the period. As for the surface current speed, the current speed in 2005 is weaker than in 2004 where the mean, minimum and maximum speed was 43.4 cm s, 3 cm s and 233.7 cm s, respectively. Strong surface current was found in the east coast of Indochina and the southern part of the sea. Generally, surface circulation pattern during the NE monsoon is opposite to the SW monsoon. In 2004 (Figure 2 (g)), it is found that surface water moves in anticlockwise direction during the NE monsoon. In the coast of Borneo Island, the water moved along the coast towards the south. In addition, surface water in the middle of the sea deflected to the left and then moved southwards along the coast of Indochina. As for the surface circulation pattern in the Gulf of Thailand, surface water moved in a clockwise direction and then moved to the south along the coast of Peninsular Malaysia. In the middle of the sea, a gyre move in anticlockwise direction is found. In 2005 (Figure 2 (h)), there is a minor change on the surface circulation pattern whereby a clockwise eddy appears in the middle of the sea. As for the surface current speed, it is found that the current movement in 2004 is stronger than in 2005 whereby the means of current speed are 53.2 cm s and 49.2 cm s, respectively. As for the speed in 2004, the minimum and the maximum speeds are 1 cm s and 299.7 cm s, respectively. Strong current movements are found in the Gulf of Thailand and on the east coast of Peninsular Malaysia. In 2005, the minimum and the maximum speeds are 1.9 cm s and 265.9 cm s, respectively. Strong current movements are found in the Gulf of Thailand, near the Natuna Islands, along the east coast of Peninsular Malaysia and along the south coast of Indochina. 4.0 ANALYSIS OF RESULTS In order to assess the accuracy of the total surface current derived from the equations, the correlation coefficient between these satellite derived values and field measurement values was computed. It was found that the correlation of total surface current speed and direction is reasonable with correlation coefficients of 0.9 and 0.8, respectively. Figure 3 and Figure 4 represent the graph of relationship between derived value and field measurement for speed and direction, respectively. In addition, the mean of the derived total surface current is not significantly different from the mean of the field measurement values. The standard deviations of derived current speed and direction are improved significantly to 5 cm s and 76 compared to the standard deviation of the field measurement which are 10 cm s and 88. Table 1 illustrates the mean and the standard deviation of derived wind-driven current and field measurement current values. 5.0 CONCLUSIONS Total surface current can be estimated using the sea surface height data and wind data derived from satellite altimeter in conjunction with field data. It was found that the derived total surface current is well correlated with the field data whereby the correlation coefficient for the speed and the direction are 0.9 and 0.8, respectively. The means of derived total surface current are not significantly different from the field measurement values and the standard deviations of derived total surface current are improved significantly compared to the field measurement values. This leads to the conclusion that the surface current in the South China Sea can be estimated using the combination of geostrophic current algorithms, wind-driven current algorithms and tidal current algorithms.