DATA AND METHODS

A. Data Acquisition and Processing

The satellite altimetry data were extracted and

processed by using the Radar Altimeter

Database System (RADS) within a 25 year period from January 1993 to 2017 based on sea level anomalies (SLAs). RADS is a processing software that processes altimeter data and conveniently enable users to define suitable corrections to be applied to their data. In this study, the altimeter data extracted ranged between 0ºN ≤ Lat ≤ 14ºN and 95ºE ≤ Long ≤ 125ºE, which covers the Malaysian seas.

B. Sea Level Anomaly (SLA) Derivation and Extraction

Ten (10) satellite altimeter missions were selected in this study namely TOPEX/Poseidon, ERS-1, ERS-2, Jason-1, Jason- 2, Jason-3, ENVISAT, Saral, Cryosatand Sentinel for the SLA extraction purpose. The period of this satellite altimetry data was within 25 years. However, a few corrections were compulsory to be applied to the sea level derived from satellite altimetry in order to produce accurate and reliable results. The corrections for satellite altimetry data were orbital altitude, sea state bias, altimeter range correction for instrument,

dry and wet tropospheric corrections,

ionospheric delay, ocean tide loading, solid earth and ocean tides, inverse barometer corrections and pole tide electromagnetic bias. The bias can be reduced by applying these models for each satellite altimeter mission in RADS. Crossover adjustments were performed to integrate data processing for multi-mission satellites after the extraction of sea level data

from RADS. In order to obtain “standard

surface” for sea surface height (SSH), crossover adjustment was performed due to the instance of orbit error factors and the inconsistency of satellite orbit frame. The minimization was

92 accomplished by adjusting ESA satellites simultaneously with the orbit of the NASA satellites, which were held fixed. The accuracy of satellites launched by NASA (NASA-class)

outperformed the orbit accuracy and

measurements of the satellites launched by ESA (ESA-class) (Hamid et al., 2016; Hamid et al., 2018).

III. RESULTS AND DISCUSSION

A. Sea Level Trend and Magnitude

The sea level magnitude in Malaysian seas from 1993 to 2017 is shown in Figure 1. The mean SLA differences between 2017 and 1993 indicated a rise in sea level during this 25 year period. Sea level magnitude in the Malacca Straits ranged between 0.02m to 0.08m, while in the South China Sea the magnitude was a little bit higher than in the Malacca Straits, being up to 0.14m. However, in the Sulu Sea and the Celebes Sea, the sea level was much higher compared to the other two Malaysian seas, being up to 0.2m. The sea level rise over Malaysian seas is presented in Figure 3 and it was clearly shown to be different for each sea. The Malacca Straits average sea level rise was 3.64mm/year +- 0.13mm, which was the second lowest for the four seas.

Figure 1. Sea level magnitude and sea level rise rate from 1993 to 2017

The rate was influenced by the depth and the shape of the Malacca Straits. In addition, the annual cycle was disturbed by many higher harmonics (Din et al., 2014, Khairuddin et al., 2019). The time series in the Malacca Straits as shown in Figure 3(a) illustrated an irregular long term tidal anomaly pattern. The South

China Sea’s mean sea level rise was the lowest

among the four seas being 3.55mm/year +- 0.06mm. It consisted of deep basin, shallow shelf and periodic pattern of monthly sea level following the characteristics of mixed tide rather than the diurnal characteristic (Hamid et al., 2016). The Celebes Sea was the highest being 4.24mm/year +- 0.10mm followed by the Sulu Sea which was 3.82mm/year +- 0.10mm. The Sulu Sea and the Celebes Sea are enclosed seas, isolated from the surrounding by a chain of islands such as Borneo and the Philippines Archipelago while the Celebes Sea surrounds

Borneo, the Philippines Archipelago and

Sulawesi Island. (Wang et al., 2006; Din et al., 2015; Hamid et al., 2016; Hamid et al., 2018). This might explain the significant high sea level rise rates around the Sulu and the Celebes Seas, compared to the other locations. The value of the mean sea level rise for each sea from 1993 to 2017 using robust-fit regression analysis is shown in Table 1.

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Table 1. Mean Sea Level Rise

Seas Mean Sea Level Rise

Stand Error

Malacca Straits 3.64 mm/year +- 0.13mm

South China Sea 3.55 mm/year +- 0.06mm

Sulu Sea 3.82 mm/year +- 0.10mm

Celebes Sea 4.24 mm/year +- 0.10mm

B. Sea Level Trend and Magnitude

In the Malaysian region, the main factor that clearly affects sea level variation is the monsoon seasons i.e.Northeast Monsoon and Southwest Monsoon. During these different monsoons, there is a distinct effect of wind and ocean circulation that brings different SLA patterns.

1. Northeast Monsoon and Southwest monsoon

Northeast monsoon causes heavy rainfall especially in the east coast of Peninsular Malaysia and western Sarawak. Based on Figure 2, only the Malacca Straits sea level variations remained stable with (0-0.04 m).

Figure 2. SLA during the Northeast Monsoon

This was because, during that time, winds blew from the Sumatra coast towards the west coast of Peninsular Malaysia with the current flow to the Thailand coastal and the Andaman region (Ku Mansor et al., 2017; Khairuddin et al., 2019). However, for the South China Sea, Sulu and Celebes seas, there were significant changes in variations of sea level. In the South China Sea, some parts decreased in sea level, while others increased. During this time,

surface water moved in the anticlockwise direction. Water moved from Vietnam towards the Gulf of Thailand. In the Gulf of Thailand, the surface water flowed in a clockwise direction and after that moved to the south along the coast of Peninsular Malaysia. Along the coast of Borneo, surface water moved along the coast in a southerly direction. This indicated the sea levels along the coast of Peninsular Malaysia were higher than those in the other areas. In addition, surface water deflected to the left in the middle of the sea, and then moved southwards along the coast of Indochina. In the middle of the sea, the sea surface circulation pattern with a gyre moved in an anticlockwise direction. For the Sulu Sea, the sea levels increased by 0.04 to 0.06m, while the Celebes Sea slightly decreased by 0 to 0.02m. The sea level in the Sulu Sea was influenced by the circulation of the South China Sea and the Celebes Sea was influenced by the inflow and outflow of sea water from the Sulu Sea. According to Han et al., 2009, during this time, water flowed inwards to the Sulu Sea at the Mindoro Straits and combined with the Balabac Straits current which flowed eastwards and then outflowed into the Celebes Sea at the Sibutu Passage. The slightly decreasing sea level in the Celebes Sea was because the surface outflow and inflow at the Sibutu Passage towards the Sulu Sea were balanced.

The Southwest Monsoon (May – August) originates from the deserts of Australia and signifies a drier season or weather with minimum rainfall throughout the country except for Sabah in east Malaysia. From Figure 3, there was a significant variation in the sea level around the Malaysian seas. For the

94 Malacca Straits, the sea level slightly increased by 0.04-0.06m. The current from the Andaman Sea entered the Straits of Malacca then flowed to the Thailand coastal water (Ku Mansor et al., 2017). Both the Sulu and the Celebes Seas slightly decreased with range of 0 to -0.02m. The southwestward currents that brought water from the South China Sea were pushed to the central Sulu basin. However, the outflow current at the Sibutu Passage was basically balanced by the surface inflows into the Sulu Sea from the Mindoro, Balabac, Tablas and Dipolog Straits (Han W. et al., 2008). The Southwest monsoon had a slight effect on the sea level changes. For the South China Sea, the sea level predominantly decreased with range from 0 to -0.04m. According to Pa'suya et al., (2013), during the southwest monsoon, surface water flowed northwestwards at the southern part of the east coast of Peninsular Malaysia towards the coastal area. This was because of the eddy system that influenced the sea surface current and it continued flowing along the east coast of Peninsular Malaysia. After that, the current changed direction about 6ºN-7°N to north-eastwards and continued moving along the southeast of the Vietnam coastal area. Then, the current moved southwards with a part of the current turning around 3°–6°N eastwards and reached the Natuna Island and the west of Borneo Island. After reaching Borneo Island, the current turned in a northwest direction.

Figure 3. SLA during the Southwest Monsoon

2. El-Nino & La-Nina Events

In this research, the El Nino and La Nina year

events were based on the record of the Oceanic Nino Index (ONI) (ONI, 2017). The value of ONI was calculated by averaging the sea surface temperature anomalies in an area of the east- central equatorial Pacific Ocean region (5S to 5N; 120W to 170W).

i. Time Series Analysis of Sea Levels in Malaysian Seas

The effects of El Nino & La Nina could also clearly be seen on small or semi-enclosed areas such as the Malacca Straits. Based on Figure 3, the Malacca Straits is a semi-enclosed area in which the effects of the events could be clearly seen. Therefore, the 1997-1998 and 2002-2003 El Nino effect and the 2010-2011 La Nina effect could be clearly seen in the Malacca Straits. The South China Sea is a vast open sea and theoretically the effects of El Nino & La Nina events were barely seen. The time series indicated a stable time series and there was no sudden drop and sudden rise in SLA. The Sulu Sea is a semi-enclosed area surrounding east Borneo and the Philippines Archipelago and the effects of El Nino and La Nina could clearly be seen. For example, from Figure 3(c), during 1997-1998, the water level falling below 0 was more frequent, indicating strong El-Nino event affecting the sea level. The Celebes Sea is a semi-enclosed area surrounding Borneo and Sulawesi. The effects of these events could also be clearly seen for a particular period recorded by the ONI. The water level dropped during 1997-1998 and during 2015-2016, which clearly indicated strong El Nino event during those times. During 2010-2011, the water level jumped upwards, indicating a moderate La Nina event. The sea level drop also indicated a moderate El Nino during the 2009-2010 period. Therefore, there were certain periods of El Nino and La Nina events recorded by ONI which affected the Malaysian Seas. A summary of the El-Nino & La-Nina events that could be clearly seen are presented in Table 2.

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Figure 3. (a) Time Series of SLA at the Malacca Straits (4°N 100°E); (b) South China Sea (6°N 108°E); (c) Sulu Sea (6°N 119°E); and (d) Celebes Sea (4°N 120°E)

Table 2. Selected El-Nino & La-Nina events (ONI, 2017)

El-Nino La-Nina Very

Strong Moderate Moderate

1997-1998 2015-2016 2002-2003 2009-2010 2007-2008 2010-2011

3. Very Strong El-Nino (1997 to 1998 and 2015 to 2016)

Based on Figure 4, the results showed that during the very strong El Nino (1997-1998) the

sea level trend around Malaysian seas

significantly decreased from 0 to -0.12m. The minimum residual SLA during El-Nino (1997- 1998) was -0.12m, which indicated the sea level fell below the benchmark from 0 to 0.12m. Also, the map was in total contrast with the normal sea level trends as mentioned in Figure 6 with the dominant scale color changing from green to red orange during the El Nino event. However, during the very strong El Nino (2015- 2016), the results were the opposite; the sea level trends did not drop but remained positive with significant increases in sea levels, especially at the Malacca Strait with a range of 0.06 to 0.08m. There were several factors that induced this result. First, the selected the El

Nino event from ONI was based on the sea surface temperature from the east central

Pacific Ocean. Second, it might have

relationship with the ocean circulation in and out from the Andaman Sea.

Figure 4. Mean SLA from 1997 to 1998 (top) and 2015 to 2016 (bottom) using Altimetry Data

during the Very Strong El Nino event

4. Moderate El-Nino (2002 to 2003 and 2009 to 2010)

The sea level trend during 2002 to 2003 showed that the areas around the Malacca Straits, Sulu and Celebes Seas were the most

96 affected with drops in sea level of between 0 to 0.025 m (see Figure 5). However, the trend of the sea level around the South China Sea remained normal with 0 to 0.02 m.

Figure 5.Mean SLA from 2002 to 2003 (top) and 2009 to 2010 (bottom) using Altimetry Data during the Very Strong El Nino event.

This was because the South China Sea is an open sea, while the Malacca Straits, Sulu and Celebes Seas are closed or semi-closed seas with close exposure to the surroundings which caused more effects during this event. However, during the moderate El Nino from 2009 to 2010, there was insignificant rise in sea level around all Malaysian seas with 0.04 to 0.10 m as indicated in the blue to magenta color scale. This result indicated that the moderate El Nino from 2009 to 2010 insignificantly influenced sea level variation in the Malaysian seas.

5. Moderate La-Nina (2007 to 2008 and 2010 to 2011)

Based on the ONI record, strong La Nina event

occurred before 1993. However, Satellite Altimeter was not fully operational until 1993, which was why there was no altimetry data and analysis on the strong La Nina. Based on the results in Figure 6, during the moderate La Nina event in 2007 to 2008, the sea level trends were constant (normal) at the Malacca Straits and the South China Sea with 0 to 0.04 m. However, at the Sulu and Celebes Seas there were drastic increases in sea levels from 0.04 to 0.10 m. This was because the Sulu and Celebes seas are enclosed basins, which could be highly affected by temperature changes as compared to the open seas like the South China Sea. Next, during the la Nina event from 2010 to 2011, the sea level trends around Malaysian seas were significantly rising with the range from 0.04 to 0.11 m. The sea levels around the Malaysian seas during the 2010-2011 La Nina were slightly increased which might be due to an on-going sea level rise around the world. A summary of the analysis on the El Nino and La Nina events is shown in Table 3.

Figure 6. Mean SLA from 2007 to 2008 (top) and 2010 to 2011 (bottom) using Altimetry Data

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Table 3.Summary of Sea Level Range during the El-Nino & La-Nina Events

Event/Malays ia Seas

El-Nino

Very Strong Moderate

1997-1998 2015-2016 2002-2003 2009-2010

Malacca Straits -3 to -4 cm 4 to 8 cm -1 to -4 cm 2 to 8 cm

South China Sea 0 to -4 cm 0 to 4 cm -1 to 2 cm 2 to 6 cm

Sulu Sea 2 to -4 cm -2 to 2 cm 1 to -4 cm 4 to 8 cm Celebes Sea 1 to -4 cm -2 to 0.04m 1 to -4 cm 6 to 10 cm Event/Malaysia Seas La-Nina Moderate 2007-2008 2010-2011 Malacca Straits 1 to 3 cm 2 to 8 cm

South China Sea 1 to 3 cm 2 to 9 cm

Sulu Sea 4 to 7 cm 6 to 10 cm

Celebes Sea 2 to 12 cm 4 to 12 cm

IV. CONCLUSION

After an overview on the results of the SLA processing and assessment, information had been gathered to conclude this study. By using satellite altimetery, all the sea level data could be gathered on Malaysian seas over a 25 year period. The monsoon seasons in the Malaysian region have had a high impact on sea level changes especially during the Northeast monsoon and the Southwest monsoon. The sea level increased by up to 0.12 m at the South China Sea, which was influenced by the wind and ocean surface circulation. The size of the basin also brought a higher impact especially for the El-Nino and La-Nina events, in which the water level dropped significantly during El- Nino at the small basin seas like the Celebes Sea and the Sulu Sea. Malaysian seas also rose due to sea level rise occurring globally, with the highest value, 3.82 mm/year, in the Sulu Sea and the lowest value in the South China Sea with 3.55 mm/year. The results from this study provide reliable, multi-mission satellite

altimeter data for interpreting sea level changes in Malaysian seas and the results of the magnitude and pattern of the sea levels during the monsoon seasons in Malaysia. Moreover, these results are specifically beneficial in the determination of the sea levels in Malaysian seas for policy making, environmental planning,

marine engineering, economic activity,

agriculture, coastal development and coastal defense by the responsible agencies and professionals. They can also be used for studying environmental issues such as the flood issue, global warming, climate change and marine issues, especially in the Malaysian region.