Study of Driving Force and Surface Current Patterns in Labuan Bajo from High Frequency Radar data from October to December 2018

Surface ocean current data in Labuan Bajo from October to December 2018 for determine the characteristics of the surface current and identify the influence of wind and tides in generating surface current. The current data set is derived from measurements of high frequency (HF) radar, which operates on the electromagnetic wave spectrum at high frequency or short wave. Method of data analysis by processing surface current velocity and direction of the zonal and meridional into spatial and temporal graphs. The result showed the average current speed in October was 0.13 m/s, the average current speed in November was 0.12 m/s, and the average current speed in December was 0.09 m/s. The empirical orthogonal function (EOF) Mode 1 has a variance of 56.2% because of the tides, Mode 2 has a variance of 26.6% because of bi-weekly variability, and Mode 3 has a variance of 17.2% because of the intra-seasonal period. This shows that tides have a greater influence on the study region than any other driving force. These results are intended to be useful to the public, particularly in terms of sustainable coastal marine management in Labuan Bajo. Especially in terms of preventing boat accidents caused by ocean currents.


Introduction
Generally, ocean current can be classified based on their forming force, namely currents related to density distribution, currents generated by wind, currents generated by ocean waves, and tidal currents.Marine surface current information needed in coastal management, especially related to sedimentation and utilized by marine users such as fishermen, tourism, coastal communities.Ocean current monitoring, in particular, can be used to prevent boat accidents and rescues, which are common in Labuan Bajo.Monitoring sea surface currents can be done using data from high-frequency (HF) radar.HF radar is a type of radar that uses the electromagnetic wave spectrum by employing high frequencies or short waves [6].
The installation of HF radar on Karanga Beach, Labuan Bajo can be utilized in analyzing surface ocean current patterns in Labuan Bajo.HF radar is able to monitor the direction and speed of currents around Labuan Bajo waters [1] Current mapping using hf radar is done in real time with readings taken every hour, making it easy to analyze current conditions.Because it has a wide enough spatial resolution, HF radar can also help map dangerous areas in real time.Analysis of the results of the current processing can be used to improve shipping safety and reduce the number of ship accidents in Labuan Bajo waters.Furthermore, the HF radar can read waves and wind, allowing it to be used as an early warning system for tsunamis or other severe weather conditions [10].The purpose of this study was to map surface current patterns spatially and temporally and identify the influence of wind and tides in generating surface currents in the waters around Labuan Bajo from October to December 2018 using data from HF radar.

Description of the Study Site
The research location is in Labuan Bajo Waters, which is located at coordinates 119°32'35.37"E -119°53'47.92"E and 8°4'15.24"S -8°35'3" S. The study area can be seen in Figure 1.There are six review points with coordinates, which can be seen in Table 1.Points 1 and 2 are located in shallow waters at a depth of 50 meters; Points 3 and 4 are located offshore around the island of Komodo at a depth of 300 meters; and Points 5 and 6 are located offshore around Labuan Bajo at a depth of 750 meters.The random location of the points is caused by a lack of data at other locations, especially offshore.

Data
The current data used is measurement data from HF radar obtained from stations on the Labuan Bajo coast starting from 3 October to 19 December 2018.The data obtained consists of vector data for hourly current velocity each day, which consists of zonal currents (u) and meridional currents (v) in centimeters per second.In addition, there is also data on the total current velocity (V) in centimeters per second and the current direction in degrees.Flow data has a resolution of 0.0091° x 0.0090°.
The wind data used is sourced from Copernicus Europe Eyes on Earth with an area coverage of 119°E -120° E and 8° S -9° S. The data consists of wind speed from October to December 2018 with a 6-hour data interval in units of m/s consisting of a zonal wind component (u) and a meridional wind component (v).Wind data uses a spatial resolution of 0.025° x 0.025°.The water level data used are 2-minute intervals sourced from the Indonesia Geospatial Information Agency (BIG) from October to December 2018 at Sape Station with coordinates of 119°01'08.41"E and 08°34'04.82"S.

Method Data Processing 2.3.1 Spectrum Analysis
The characteristics and driving force of currents in the water, as well as the dominant period in a time series, are determined using spectrum analysis.The spectrum analysis method can be done by using filtering.Filtering is one of the methods used to remove unwanted frequencies from data and save the necessary data [5].In processing sea surface currents, a high pass filter will be used to separate the tidal currents from the non-tidal currents to obtain the non-tidal currents.Tide is a superposition of infinite harmonic waves with a large period and small frequency, so it needs to be eliminated.This aim to eliminate the influence of the tides on the direction and speed of the currents, assist in determining the driving factors of surface currents, and see the seasonal variations of surface currents.

Cross Correlation
Cross correlation is a method used to find a relationship between two time series data   and   , with   data having a relationship with the previous   data.In this study, the   data is wind and tide data, and the   data is ocean current data from HF-radar.The Equation 1 used to calculate the cross correlation for the normalized version.
Where   is cross correlation,   is cross covariance data x,   is cross covariance data y, and   is cross covariance between the two pieces of data.
According to Evans [2], the criteria for the value of the correlation coefficient are:

Empirical Orthogonal Functions
The empirical orthogonal function (EOF) aims to determine the dominant pattern both spatially and temporally.The EOF can search for a few independent variables that provide maximum information without being excessive.Calculating the EOF value can be done with the following Equation 2: Where   is the result of the transpose matrix,.. that shows the eigenvalues of EOF, Column U is the mode of the X matrix, k is component of matrix orthogonal, and column V is the PC (Principal Component) value of each mode [1].

Fast Fourier Transform
Fast Fourier Transform (FFT) is a mathematical method that decomposes a continuous function into simple waves [7].The FFT divides a signal as a function of time into frequency functions that have different amplitudes and frequencies quickly and efficiently.The FFT can be defined by Equation 3.

Tidal Data Processing Results
Tidal data processing is carried out to determine the occurrence of spring tides and neap tides in the study area and the types of tides.This processing is also used as supporting data in determining differences in speed and surface currents during spring tide and neap tide conditions.The tidal elevation in Labuan Bajo waters from October to December 2018 is shown in Figure 2, with an average elevation value for 3 months of 1.52 meters.Based on the results of tidal calculations using the least squares method, a Formzhal number value of 0.87 is obtained, meaning that the tides in the study area are generally semidiurnal mixed type tides.

The results of wind data processing.
Wind speed and direction can affect the pattern of surface currents in the study area.Wind data is processed to generate time series graphs of wind speed and windrose.The windrose chart showing variations in wind direction and speed from October to December in the study area can be seen in Figure 3.The transition from the east to west monsoon influences wind direction in October and November [12].The wind movement in October is predominantly from the north with a percentage of 55%, and the most dominant wind speed is at intervals of 5.7-7.1 m/s.Windrose shows wind movement dominated by moving north with a percentage of 40.13% and dominant wind speeds occurring at intervals of 2.5-3.8m/s in November.In October and November, the wind pattern is dominated by the west monsoon, which moves from the Asian continent to the Pacific Ocean.
The wind direction in December is influenced by the west monsoon [12].In December, windrose shows that the wind is predominantly moving from the northeast with a percentage of 41.52%, and the dominant wind speed is at intervals of 3.4-4.5 m/s.The maximum wind speed this month has the largest percentage compared to other months, caused by the influence of the west monsoon, which has been very strong and stable, and the influence of the east monsoon, has weakened.

Spectral Analysis
A spectral analysis was carried out using the high-pass filter method on current velocity and direction data in the study area for three months at six review points.Figure 4 shows high pass filter processing results from October to December 2018 at six points.The results show three time series graphs consisting of total current (top), tidal current (middle),and residual current (bottom).Each review point has a tidal current that is greater than the residual current that is affected by the wind.However, the residual current at Points 5 and 6 has almost the same speed as the tidal current.This shows that offshore, tidal currents have a decreasing effect with, the influence of residual currents starting to increase.So the effect of residual currents is greater in offshore waters than in shallow waters.

Cross correlation
To determine the magnitude of the relationship between tides, wind, surface currents, and the lag phase, the cross-correlation method is used.Cross correlation is a technique that compares two time series data sets and objectively determines their compatibility with each other.Lag determines the variable that occurs first and affects other variables.A negative lag value indicates that x occurs before y begins, and a positive lag value indicates that x occurs after y occurs.Residual current data is used, which is correlated with wind speed, to show the correlation between wind and residual current.The blue color shows the relationship between winds and surface currents, while the red color shows the relationship between tides and surface currents.Figure 5 shows correlation of wind and tides to surface currents in 2018 at six points.The greatest correlation between wind and surface currents is at Point 6 with a value of 0.72, and the largest correlation between tides and surface currents is at Point 2 with a value of 0.90.At each review point (shallow waters and offshore), it shows that tides have a greater correlation value than wind.However, the correlation of wind to surface currents has a greater value at points located offshore than at points located in shallow waters.This shows that the effect of wind is greater offshore than in shallow waters.The correlation of tides and winds to surface currents has a positive value and a large value, indicating that it moves in the direction of the surface currents.Wind always has a faster effect than tides on the formation of surface currents because wind affects surface currents in a short period of time and all at once, as opposed to tides, which have a period of 12 hours [3].

Current Velocity Time Series Plot
The temporal pattern shows the current velocity in each review to compare current velocity in shallow and open water areas.The data used is the total flow data that has not been filtered.The direction of surface currents in spring tide and neap tide conditions helps to show the effect of wind and tides on the current direction in shallow waters and open waters.Current speed and direction from October to December 2018 during spring tide and neap tide conditions at six point shown in Figure 6.The highest average current velocity value for three months is at Point 2, and the lowest average current velocity value for three months is at Point 1.This can be caused by the disturbance of the dominant current pattern in shallow waters caused by the presence of small islands.Points 3, 4, 5, and 6, which are located offshore, have a small average current velocity difference, indicating that the average current velocity offshore is almost uniform due to deep bathymetry.It also shows that the speed and direction at a point near Komodo Island and a point near Labuan Bajo have the same value.It can be seen that the closer the observation point is to Komodo Island, the higher the current speed every month, and the farther the observation point is to Komodo Island, the smaller the current speed.This is because the effect of tides tends to be greater around the coast or in shallow waters than on the high seas, which causes the current velocity around Komodo Island to have a greater speed than other review points [3].So that it can be concluded, the influence of the tides on the speed of the current at the review point is greater than that of the wind.
The current direction at points in shallow water (Points 1 and 2) follows the tidal pattern, namely moving north during high tide and south during low tide.Whereas at points located offshore (Points 3,  and 6), follow the direction of the wind, namely the current direction, which is predominantly moving to the west and southwest.

Spatial Pattern of Current Movement
Hourly unfiltered total current velocity data for three months was averaged to produce a spatial pattern of surface current velocity and direction in the study area shown in Figure 7.The highest current speed in October is in the east of Komodo Island, at 0.27 m/s, and the lowest current speed in October is in the west of Labuan Bajo, close to 0 m/s.The average current speed in October is 0.13 m/s.In November, the highest average speed of the current in one month reached 0.30 m/s, and the lowest average speed was 0.02 m/s.The average current speed in November is 0.12 m/s.
In shallow waters, especially between Komodo Island and the islands of East Nusa Tenggara, the currents move irregularly and are faster due to the influence of the topography, which is surrounded by small islands.These small islands act as a barrier that disrupts the dominant current pattern, especially in the eastern part of Komodo Island [11].Offshore, deep bathymetry causes currents to move more smoothly than in shallow waters [10].The difference in movement between the current directions in offshore and shallow waters is caused by the influence of dominance on the causal factors [10].On offshore, current movement is in accordance with the direction of wind movement, and there is no significant difference during high tide, low tide, either towards high tide or towards low tide.However, in shallow waters, the current moves with the tides, that is, away from Komodo Island during high tide and approaching Komodo Island during low tide.In December, the highest average current speed reached 0.31 m/s and the lowest average current speed reached 0.01 m/s.The average current speed in December is 0.09 m/s.The current movement pattern in December shows a pattern that is quite different from the current movement in October and November.Offshore, the current rotates counterclockwise with a smaller speed toward the center of rotation and a speed range of 0-0.15 m/s.This rotational motion is caused by the west monsoon winds meeting the southeast trade winds, which move in opposite directions at around 10°S, so that the west monsoon winds are deflected to the north.As a result, surface wind turbulence is formed in the waters around Labuan Bajo, which pushes surface water masses to follow a rotational pattern [4].Whereas in shallow waters, surface currents show a southward movement from Komodo Island with a speed range of 0-0.31 m/s.
The pattern of surface currents during high and low tide conditions during the spring tide and neap tide is shown in Figure 8.In shallow waters, the current velocity is smaller during high and low tides compared to high tide and low tide, which indicates that the highest sea level elevation will have a current velocity of zero or a small current velocity.The difference in speed values is clearly visible, especially in the waters around Komodo Island and Sebayur Island.In shallow waters, surface currents move away from Komodo Island during high tide and move closer to Komodo Island during low tide, which indicates that current movement is dominated by tides.However, on the offshore, the difference in the speed of the tides and ebb currents is quite small compared to those heading up and down with the same direction of movement, namely towards the west of the Flores Sea.The direction of the current, which always moves to the northwest of Flores both at high and low tide, indicates that surface currents offshore follow the direction of wind movement, but the current speed is still influenced by the tides.

Empirical Orthogonal Function (EOF)
To determine the driving force of surface currents both in shallow waters and offshore, the EOF method is used.The EOF method displays spatially and temporally the dominant variables of the total unfiltered current in three main modes.The EOF spatial structure of surface currents with Mode 1 for three months is shown in EOF Mode 1 with a variance of 56.2%, with the dominant period of Mode 1 located at 12 hours and 24 hours, which indicate periods of highs and lows.The EOF spatial structure of surface currents with Mode 1 for three months is shown in Figure 9. period of 566 hours, or about 24 days, which is a monthly period.However, Mode three cannot be further identified due to the short data range.

Conclusion
The average current speed in October was 0.13 m/s, the average current speed in November was 0.12 m/s, and the average current speed in December was 0.09 m/s.The highest average speed for three months is at Point 2 of 0.70 m/s, and the lowest average speed for three months is at Point 1 of 0.23 m/s.The direction of surface currents in shallow waters indicates the current moves northward during high tide and moves southward during low tide, while the direction of surface currents in offshore waters moves westward and southwest, which is influenced by the wind.This is consistent with Rizki's research [8], which found that the ocean current in Labuan Bajo flow into north at high tide and south at low tide.
The largest correlation between wind and surface currents is offshore (Point 4), at 0.83, and the largest correlation between tides and surface currents is in shallow waters (Point 2), at 0.89.Each review point has a tidal current velocity that is greater than the residual current.Points located offshore (Points a dominant period of 10 days, which is included in the biweekly period, and Mode 3 has a variance of 17.2% with a predominant period of 24 days, which is an intra-seasonal period.
In this study, a three-month time was utilized due to a lack of surface current data from the HF radar, which was recently installed in 2018 at the same time as Covid-19, so the researchers could not come to review directly.It is intended that in the future, this research will be extended over a longer time to examine seasonal factors as well as the effects of El Nino and La Nina.So that the research findings might be expanded, particularly in terms of assisting coastal management in Labuan Bajo and preventing ship mishaps caused by ocean currents.

Figure 1 .
Figure 1.Research Area in Labuan Bajo, Indonesia and Coordinates point review.Blue circle is the HF-radar location

Figure 2 .
Figure 2. Tidal elevation from October to December 2018.

Figure 4 .
Figure 4. High pass filter processing results for October-December 2018 at A) Point 1, B) Point 2, C) Point 3, D) Point 4, E) Point 5, and F) Point 6.Each review point has a tidal current that is greater than the residual current that is affected by the wind.However, the residual current at Points 5 and 6 has almost the same speed as the tidal current.This shows that offshore, tidal currents have a decreasing effect with, the influence of residual currents starting to increase.So the effect of residual currents is greater in offshore waters than in shallow waters.

Figure 5 .
Figure 5. Correlation of wind and tides to surface currents in 2018 at A) Point 1, B) Point 2, C) Point 3, D) Point 4, E) Point 5, and F) Point 6.

Figure 6 .
Figure 6.Current speed and direction in October-December 2018 during spring tide and neap tide conditions at A) Point 1, B) Point 2, C) Point 3, D) Point 4, E) Point 5, and F) Point 6

Figure 7
Current direction and surface current velocity (m/s) are shown by spatial color shading in A) October, B) November, and C) December

Figure 8
Figure 8 Surface current pattern for spring tide and neap tide conditions in October.A) at high tide, B) towards the high tide, C) at low tide, and D) towards the low tide.

Figure 9 .
Figure 9. Spatial structure of EOF Mode 1 of meridional currents and zonal currents October-December 2018.The EOF spatial structure of surface currents with Mode 2 for three months is shown in Figure10EOF Mode 2, with a variance of 26.6% has the largest amplitude of 0.06 and is located in the high seas.The dominant period in Mode 2 is 238 hours, or about 10 days, which is classified as a biweekly period.The Mf (lunar fortnightly) tidal component is included in a 2-week period.Based on Stewart[9], the period of Mf is 327.850hours, or about 13 days.So it can be concluded that the dominant influence on Mode 2 is the component of the fortnightly tide.The EOF spatial structure of surface currents with Mode 3 for three months is shown in Figure11.EOF Mode 3 with a variance of 17.2% has a dominant

Figure 10 .
Figure 10.Spatial structure of EOF and FFT Mode 2 of meridional and zonal currents October-December 2018.

Figure 11 .
Figure 11.Spatial structure of EOF Mode 3 of meridional currents and zonal currents October-December 2018.
5 and 6)  show a greater residual current than other points.EOF Mode 1 has a variance of 56.2%, with a dominant period of 12 hours, which is a semidiurnal tide period, Mode 2 has a variance of 26.6% with Speed (