Abstract

Ocean current is defined as the movement of sea water masses due to the dynamic relationship between the atmosphere and the oceans. One of the weather phenomena in the atmosphere which can affect the conditions of the ocean’s dynamics is the Madden Julian Oscillation (MJO). MJO is a movement of convection areas propagating eastward from the Indian Ocean to the Pacific Ocean over a 40 – 50 days period. Previous studies have shown that MJO activity can affect various parameters such as wind direction and speed, sea surface temperature, and ocean current patterns in the area in which it passes. This study aimed to determine changes in the pattern of ocean currents in the waters of the Sunda Strait when MJO is in an active and inactive phases. The dates and activities of MJO were determined using Real-Time Multivarate MJO (RMM) Index of Series 1 and Series 2 (RMM1 and RMM2) data. This research was conducted by utilizing MIKE 21 numerical model to describe the ocean currents when the MJO is in an active or inactive phase in the waters of the Sunda Strait. Data of bathymetry, coastline, wind, surface pressure, and tides were needed to run the model. Based on the analysis of the study, it was found that MJO activity influenced on the changes of ocean current patterns in the Sunda Strait waters. In addition, there was a significant influence on changes in the direction of ocean currents which occurred every 12 hours. The average current velocity also increased ranging from 0.5 – 1 m/s when the MJO was in the active phase.

1. Introduction

Indonesia is one of archipelago countries with quite complex weather dynamics due to the strong interaction between the atmosphere and the ocean. Weather dynamics in Indonesia can be influenced by various phenomena both on global, regional and local scales. Among these various phenomena, there is a movement of the convection area from the Indian Ocean to the Central Pacific Ocean known as the Madden Julian Oscillation (MJO). MJO is an intra-seasonal phenomenon which propagates eastward with a 40 - 50 days period ¹. The areas of MJO propagation are divided into 8 phases, i.e. phases 1 and 8 show the MJO in the West Hemisphere and Africa region, phases 2 and 3 are in the Indian Ocean region, phases 4 and 5 occur in the Indonesian Maritime Continent (local term: BMI- Benua Maritim Indonesia), and phases 6 and 7 are in the West Pacific region.

Haryanto et al’s study ² showed that MJO propagation can affect various parameters of the atmosphere and oceans such as wind speed and direction, sea surface temperature, and the patterns of ocean current. Ocean current can be defined as continuous movement of water masses both vertically and horizontally ³. In general, the movement of ocean currents on the surface can be influenced by wind and solar radiation received by the sea surface. However, there are other factors causing ocean currents like seabed topography, Coriolis force, and Ekman transport ⁴. This shows that ocean currents occur due to the dynamic relationship between the atmosphere and the oceans.

The influence of MJO activities can be seen from the Indian Ocean to the Pacific Ocean, including in the Sunda Strait. The waters of the Sunda Strait are the waters connecting the Indian Ocean to the Java Sea. The conditions in this area are very dynamic as a result of the confluence of two different water masses. In addition, the Sunda Strait also becomes an international shipping route and has been included in the Traffic Separation Scheme (TSS). Based on the study conducted by Marshall et al. ⁵, when MJO occurs, there is an increase in zonal wind speed reaching 4 m/s and there is a significant increase in waves reaching 50 cm. Moreover, the existence of a significant increase in wind speed and wave height can affect changes in the parameters of ocean current patterns. Therefore, this study was conducted to find out the correlation between the MJO and the ocean current patterns in the waters of the Sunda Strait.

2. Methods

This study was conducted in the waters of the Sunda strait located in the coordinate of 5,23 ^oSL – 6,89 ^oSL and 104,5 ^oEL – 106,14 ^oEL (Figure 1). The study was carried out from 10 – 13 January 2020 when MJO was active in phases 4 and 5 and from 22 – 25 January when the MJO was inactive in phase 7.

The data used in this study included Real-Time Multivarate MJO (RMM) Index of Series 1 and Series 2 (RMM1 and RMM2) during 2020 to calculate the date and phenomena of strong MJO occurrences which were active in phases 4 and 5 and inactive MJO in phase 7. Furthermore, it was found out that strong MJO with > 3 amplitude values occurred from 10 – 13 January 2020 and inactive MJO occurred from 22 – 25 January 2020. This result was obtained from Bureau of Meteorology (BOM) in their website https://www.bom.gov.au/climate/mjo/graphics/rmm.74toRealtime.txt.

Some data are required to run hydrodynamic modelling, such as bathymetry data (points of sea floor depth), coastline data, wind data, surface pressure data, and tidal data. The bathymetry data used as input for the model employed 0.27 arcsecond resolution with the .xyz format which can be obtained from Geospatial Information Agency (local term: BIG- Badan Informasi Geospasial). In addition to bathymetry data, coastline data were also used in modelling to differentiate coastal and marine areas.

Wind data and surface pressure data used to run the modelling can be obtained from the ECMWF ERA-5 Reanalysis via their website https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5. The data had 0,125^o x 0,125^o grid resolution with a one hour temporal resolution. Moreover, the wind data used were wind component data U and V 10 m. The data of wind and surface pressure used were in the form of wind and surface pressure data obtained on 10 – 13 January, 2020 when the MJO was active and on 22 – 25 January, 2020 when the MJO was inactive. Meanwhile, tides data can be obtained from the Global Tide Model with 0,25^o x 0,25^o grid resolution. Furthermore, tides data used were the tidal average from 10 – 13 January 2020 during active MJO and from 22 – 25 January 2020 during inactive MJO.

This study used descriptive analysis method. This method is a method intended to describe the existing phenomena, happening at the mean time or in the past by regularly reviewing, prioritizing objectivity, and being carefully conducted (Furchan, 2004).

The modelling used in this study was MIKE 21 software with Flow Model FM module to determine the current movement patterns based on the data used. In addition, this study also employed ArcGIS 10.3, Ocean Data View (ODV), Microsoft Office Excel 2016, and notepad. ArcGIS 10.3 software was used to digitize the depth and the coastline to obtain bathymetry data (.xyz) and coastline data (.xy). Meanwhile, ODV was used for Microsoft Office Excel 2016 to convert the bathymetry data (.xyz) and the coastline data (.xy).

The currents were modelled by Mike 21 Flow Model FM (Unstructured Flexible Mesh) using a 2D hydrodynamic model approach. The current patterns will be drawn when MJO is in active phase and inactive phases. The implementation of the model was carried out in 3 stages, i.e. the pre-processing model, the processing model, and the post-processing model. The pre-processing stage of this model included the preparation of bathymetric and coastline data to create a mesh boundary model. The processing stage of the model was the set-up model by entering the coefficient values of the parameters used to run the model and to calculate the time interval, the length of the model, and so on. The last stage was the post processing model, i.e. visually displaying the simulation results of the model.

The analysis of model simulation results was done visually by investigating the changes in the current pattern when the MJO was active and when the MJO was inactive. The results of model simulation will be displayed every six hours, at 00.00, 06.00, 12.00, and 18.00 UTC according to the requirements set by WMO in modelling ocean currents ⁶.

3. Results and Discussion

The output of current patterns resulted from the Flow Model FM in the waters of the Sunda Strait when the MJO was in an inactive state in the Indonesian territory (phase 7) showed a change in current direction every 12 hours. Then, the current conditions at 00.00 UTC and 06.00 UTC showed the current movement from the northeast to the southwest towards the Indian Ocean, while at 12.00 UTC and 18.00 UTC the current was seen moving from the southwest to the northeast towards the Java Sea (Figure 2 , Figure 3, Figure 4, and Figure 5).

The changes in current direction occurring at 12.00 UTC and 18.00 UTC were related to the West Monsoon Wind in Indonesia. The movement of ocean currents in Indonesia is greatly affected by the monsoon winds. This confirmed Wyrkti ⁷ which states that the surface currents in Indonesia during the western monsoon will move from west to east and it will move from east to west during the east monsoon. Therefore, the active West Monsoon Winds in Indonesia result in strong and persistent wind gusts from the Asian continent to the Australian continent. These wind gusts will make the water mass transport greater to the east so that the water level in the western Java Sea will decrease and create a significant difference in elevation between the Indian Ocean and the Java Sea ⁷. Accordingly, the current was seen moving from the direction of the Indian Ocean towards the Java Sea as described in Figure 2 (a), 2 (b), Figure 3 (a), 3 (b), Figure 4 (a), 4 (b), and Figure 5 (a), 5 (b).

Meanwhile, the current movement from the east to the Indian Ocean at 00.00 UTC and 06.00 UTC was related to differences in sea water density and sea level slope. Density will increase as salinity increases or temperature decreases. This result was in line with the study done by Azis ³ stating that waters with low density have a higher sea level than waters with high density so that currents will flow from low density to high density. Putri ⁹ states that the Java Sea has a higher temperature than the Indian Ocean, but the salinity is lower so that the current tends to flow from the Java Sea to the Indian Ocean (Figure 2 (c), 2 (d), Figure 3 (c), 3 (d), Figure 4 (c), 4 (d), and Figure 5 (c), 5 (d)).

The average current velocity in the waters of the Sunda Strait ranged from 0.001 – 0.8 m/s. In general, current velocity tended to be stronger at 06.00 UTC around Sangiang Island, a small island in the north of the Sunda Strait between Java and Sumatra islands. The amplification of the current was probably due to the narrowing of the flow width. This result confirmed the theory presented by Hagerman ⁸ which states that the wider and deeper a channel, the smaller the speed/velocity. Conversely, the narrower and shallower a channel is, the speed increases more. The maximum current velocity reached 2.22 m/s in the northern part of the strait around Sangiang Island (Figure 4 (b)). Meanwhile, the minimum speed was seen in the southern part of the strait bordering with the Indian Ocean with 0.001 m/s velocity.

Based on the results of the ocean current pattern model simulation when the MJO was active (phases 4 and 5), it was seen that the propagation of the MJO over Indonesian territory will affect the current pattern which occurs in the waters of the Sunda Strait. When compared to the current pattern when the MJO was not active, the current pattern when the MJO was active showed a significant change in the direction and the speed of the current. Current conditions at 00.00 UTC and 06.00 UTC indicated a change in the direction of the current from the northeast to the southwest towards the Indian Ocean. Meanwhile, the conditions of current at 12.00 UTC and 18.00 UTC show a change in the direction of the current from the southwest to the northeast towards the Java Sea (Figure 6, Figure 7, Figure 8, and Figure 9).

It has been previously stated in some studies that wind is one of the factors which influence the movement of currents in an area. This shows that MJO affects wind movement in an area. Thus, it can change the pattern of ocean currents. This result confirmed Jin et al. ¹⁰ stating that MJO will affect surface winds so that it will eventually affect several parameters in the sea including the currents.

The average current velocity in the waters of the Sunda Strait during active MJO ranged from 0.001 – 1 m/s. The increase in current velocity was also seen significantly at 18.00 UTC in the northern part of the strait with an increase in the average current velocity ranging from 0.5 – 1 m/s (Figure 6 (d), Figure 7 (d), Figure 8 (d), and Figure 9 (d)). This indicated an increase in current velocity compared to that when the MJO was not active. MJO activity will indirectly affect the increase in the speed of sea surface current. The maximum current velocity reached 2.34 m/s in the northern part of the strait around Sangiang Island (Figure 4 (b)). Moreover, the minimum speed was shown in the southern part of the strait bordering with the Indian Ocean with 0.001 m/s velocity.

4. Conclusion

Based on the study, it can be concluded that there was a change in the patters of ocean current caused by MJO activities. In addition, MJO activities brought a significant effect on the changes of current direction and the increase of sea surface current speed. The changes of current direction in the waters of the Sunda strait occurred every 12 hours. Moreover, the significant increase of current speed was seen at 18.00 UTC with the average current speed ranging from 0.5 – 1 m/s.

References