Surface Thermal Front Persistence in Malacca Strait

The Malacca strait is an essential seaway for international sea traffic and a provider of biological and non-biological resources. This strait has dynamic conditions resulting from the interaction between the Indian Ocean in the north and the Pacific Ocean in the south. The characteristic of the thermal front is the strait dynamics that have not been studied comprehensively. This research aims to map and identify the spatial and temporal pattern of the thermal front in Malacca Strait. The data used are sea surface temperature of AquaMODIS level 3 satellite images and bathymetry data of Malacca Strait, Ocean Nino Index (ONI), and Dipole Mode Index (DMI). The sea surface temperature data were processed from 2010 to 2020 using the Single Edge Image Detection (SIED) method. This research denotes the thermal front phenomenon found with several variations in each season. The highest (lowest) number of thermal fronts was discovered in the east season (first transitional season). The total number of thermal fronts each year happened to be maximum in 2015 and minimum in 2019. Annual variability (ENSO and IOD) impacts the number of thermal front events, but the observation period has to be explicitly adjusted in the analysis needed. Persistent thermal fronts in the Malacca Strait occurred in 1-5 repetitions at the exact location. Thermal fronts are commonly found in the northern region of the strait and areas with significant depth changes.Keywords: Malacca Strait; sea surface temperature; thermal front.AbstrakSelat Malaka terkenal sebagai perairan penting dalam lalu lintas laut internasional serta penyedia sumber daya hayati dan non hayati. Perairan ini juga memiliki kondisi yang dinamis sebagai hasil interaksi antara Samudera Hindia di bagian utara serta Samudera Pasifik di bagian selatan selat. Dinamika perairan yang belum dikaji secara komprehensif di perairan ini adalah karakteristik thermal front. Sehingga tujuan dari penelitian ini adalah memetakan secara spasial dan temporal kejadian thermal front di perairan Selat Malaka serta menganalisis karakteristik dari thermal front yang dipetakan tersebut. Data yang digunakan pada penelitian ini adalah data suhu permukaan laut citra satelit AquaMODIS level 3, data batimetri Selat Malaka, serta data Ocean Nino Index (ONI) dan Dipole Mode Index (DMI). Metode Single Edge Image Detection (SIED) digunakan untuk mengolah data suhu permukaan laut pada periode pengamatan tahun 2010 hingga 2020. Hasil penelitian ini menunjukkan bahwa di perairan Selat Malaka dapat ditemukan thermal front dengan jumlah variatif setiap musim. Jumlah thermal front tertinggi ditemukan pada musim timur dan terendah pada musim peralihan 1. Jumlah total thermal front setiap tahun ditemukan maksimum pada tahun 2015 dan minimum pada tahun 2019. Variabilitas annual (ENSO dan IOD) memberikan dampak terhadap jumlah kejadian thermal front, namun untuk kebutuhan analisis perlu disesuaikan terhadap periode pengamatan dari variabilitas tersebut. Thermal front persisten di Selat Malaka dapat terjadi pada rentang 1-5 kali pengulangan pada lokasi yang sama. Lokasi thermal front persisten lebih sering terjadi di wilayah utara selat dan/atau pada wilayah dengan perubahan kedalaman yang signifikan.Kata kunci: Selat Malaka, suhu permukaan laut, thermal front

Study on Seasonal Variation of Cohesive Suspended Sediment Transport in Estuary of Mahakam Delta by using A Numerical Model

Abstract. A coupled three-dimensional hydrodynamics and sediment transport model of HydroQual, Inc., (2002), ECOMSED, has been used to simulate variation of suspended cohesive sediment transport in Estuary of Mahakam Delta. The simulation results indicate that tides and seasonal variation of river discharges are the main causes of variations in the suspended sediment concentration in this area. A one-year simulation of suspended sediment distribution shows that the suspended cohesive sediment discharge to the Makassar Strait is mainly transported southward, namely through locations of Muara Jawa and Muara Pegah and seems to reach a maximum distance of distribution in January and a minimum one in October. The simulation results also show that river discharges less influence the suspended sediment concentration at Tanjung Bayur, which is located at the tip of the channel in the middle, compared to the other locations. Abstrak. Suatu model kopel tiga dimensi hidrodinamika dan transpor sedimen HydroQual, Inc., (2002), ECOMSED, diterapkan untuk mensimulasi variasi transpor sedimen kohesif melayang di Estuari Delta Mahakam. Hasil simulasi menunjukkan bahwa pasut dan variasi musiman dari debit sungai adalah penyebab utama dari variasi konsentrasi sedimen melayang di daerah ini. Simulasi satu tahun distribusi sedimen melayang memperlihatkan bahwa sedimen kohesif yang memasuki Selat Makassar terutama ditransporkan ke arah selatan melalui Muara Jawa dan Muara Pegah. Jangkauan maksimum dari aliran sedimen memasuki Selat Makassar terjadi pada bulan Januari dan minimum pada bulan Oktober. Hasil simulasi juga menunjukkan bahwa pengaruh debit sungai terhadap konsentrasi sedimen di Tanjung Bayur lebih kecil daripada di lokasi-lokasi yang lain

Marine Heatwaves in the Indonesian Fisheries Management Areas

Ocean temperatures increased during the 20th century and are predicted to continue to rise during the 21st century. Simultaneously, the extreme phenomena of shorter time ocean warming, known as Marine Heatwaves (MHWs), are also taking place. The present study used the Daily Optimum Interpolation Sea Surface Temperature (DOISST) v2.1 with a spatial resolution of 0.25˚. The time period of the DOISST data used in this study was from January 1, 1982 to December 31, 2020, and the region was 90° E–150° E and 16° S–16° N, which is divided into 11 Fishing Management Areas (FMAs). MHWs have a set of metrics derived from the SST data to describe the statistical characteristics of each event. To examine and quantify the influence of the Pacific Ocean and the Indian Ocean, we used the Niño 3.4 SST index and the Dipole Mode Index (DMI), respectively. Based on the data analysis, there has been an increase in the duration and frequency of the occurrence of MHWs in the study area, with the highest increase occurring in FMA 573, FMA 716, and FMA 711. Based on the severity, MHWs in Indonesia are dominated by category I, which is dominantly located in FMA 716, category II in FMA 573, category III with the center of events in FMA 771, and category IV with irregular spatial patterns

Numerical Modeling of Tidal Current Patterns Using 3-Dimensional MOHID in Balikpapan Bay, Indonesia

HighlightsThe results of the model and observation showed the similarity of the amplitude and phase formedAt the highest tide was in the position above the Sea Mean level and the current velocity was lower than when heading to the tideThere was a difference in the direction of the current when heading towards high tideStratification of the water column in both seasons tended to be classified in well-mixed watersAbstractBalikpapan Bay is significant as a link between the cities within and outside of East Kalimantan by becoming the primary path used for local transportation and distribution of produced goods. The various anthropogenic activities increased liquid wastes and debris, which flowed through channels and rivers along the bay. This study aimed to determine tidal current patterns in Balikpapan Bay and its influence on salinity and temperature distributions. This study applied a baroclinic three-dimensional (3D) hydrodynamic model, employing wind, tides, and density variations, resulting from the differences of temperature and salinity, as the model input. To simulate the tidal current flow, we applied MOHID Water Modeling System, which the tidal current patterns depicted current directions and speeds at the different tidal conditions. During the displacement toward the high tidal condition, the water mass moves northwestward entering the river body, while at the displacement toward the low tidal condition, the water mass moves southeastward, which flows toward the coast and without the bay. The current speed varies at certain tidal conditions. At the highest tidal condition, the surface elevation ranged 1.3 - 1.5 m above mean sea level; the current rate is lower compared to the displacement toward high tidal condition, which ranged from 0.01 - 0.15 m/s. At the lowest tidal condition, the surface elevation reached 1 - 1.2 m below mean sea level, and the weaker flow velocity took place (less than 0.15 m/s). The results also showed that the water mass temperature tends to be higher in the inner part of Balikpapan Bay, the Balikpapan Bay waters profile that is increasingly shallow towards the bay head also causes this area to tend to have a higher temperature. Also, areas located on the inside of the bay tend to get more freshwater input from rivers, so this area has lower salinity while the area located at the mouth of the bay tends to be of higher salinity because it gets a lot of mass input of seawater from the Makassar Strait

Exposure of coastal ecosystems to river plume spreading across a near-equatorial continental shelf

The Berau Continental Shelf (BCS) in East Kalimantan, Indonesia, harbours various tropical marine ecosystems, including mangroves, seagrass meadows and coral reefs. These ecosystem are located partly within reach of the Berau River plume, which may affect ecosystem health through exposure to land-derived sediments, nutrients and pollutants carried by the plume. This study aims (1) to assess the exposure risk of the BCS coastal ecosystems to river plume water, measured as exposure time to three different salinity levels, (2) to identify the relationships between these salinity levels and the abundance and diversity of coral and seagrass ecosystems, and (3) to determine a suitable indicator for the impacts of salinity on coral reef and seagrass health. We analysed hydrodynamic models, classified salinity levels, and quantified the correlations between the salinity model parameters and ecological metrics for the BCS systems. An Empirical Orthogonal Functions (EOF) analysis revealed three modes of river plume dispersal patterns, which strongly reflect monsoon seasonality. The first mode, explaining 39% of the variability, was associated with the southward movement of the plume due to northerly winds, while the second and third modes (explaining 29% and 26% of the variability, respectively) were associated with the northeastward migration of the plume related to southwesterly and southerly winds. Exposure to low salinity showed higher correlations with biological indicators than mean salinity, indicating that low salinity is a more suitable indicator for coastal ecosystem health. Significant correlations (R2) were found between exposure time to low salinity (days with salinity values below 25 PSU) with coral cover, coral species richness, seagrass cover, the number of seagrass species, seagrass leaf phosphorus, nitrogen, C:N ratio and iron content. By comparing the correlation coefficients and the slopes of the regression lines, our study suggests that coral reefs are more susceptible to low salinity levels exposure than seagrass meadows. Regarding the risk of corals being exposed to low salinity, nearshore and northern barrier reefs were classified as “high risk”, the middle barrier reef as “medium to high risk” and southern barrier reefs as “medium risk”. Further offshore, the oceanic reefs were classified as “low risk”. Regarding the seagrass meadows, the nearshore region was categorized as “high risk”, the barrier reef as “medium to low risk” and oceanic reefs as “low risk”. This study contributes to assessing the potential impacts of salinity on the BCS ecosystems, and further provides a knowledge base for marine conservation planning

Exposure of coastal ecosystems to river plume spreading across a near-equatorial continental shelf

The Berau Continental Shelf (BCS) in East Kalimantan, Indonesia, harbours various tropical marine ecosystems, including mangroves, seagrass meadows and coral reefs. These ecosystem are located partly within reach of the Berau River plume, which may affect ecosystem health through exposure to land-derived sediments, nutrients and pollutants carried by the plume. This study aims (1) to assess the exposure risk of the BCS coastal ecosystems to river plume water, measured as exposure time to three different salinity levels, (2) to identify the relationships between these salinity levels and the abundance and diversity of coral and seagrass ecosystems, and (3) to determine a suitable indicator for the impacts of salinity on coral reef and seagrass health. We analysed hydrodynamic models, classified salinity levels, and quantified the correlations between the salinity model parameters and ecological metrics for the BCS systems. An Empirical Orthogonal Functions (EOF) analysis revealed three modes of river plume dispersal patterns, which strongly reflect monsoon seasonality. The first mode, explaining 39% of the variability, was associated with the southward movement of the plume due to northerly winds, while the second and third modes (explaining 29% and 26% of the variability, respectively) were associated with the northeastward migration of the plume related to southwesterly and southerly winds. Exposure to low salinity showed higher correlations with biological indicators than mean salinity, indicating that low salinity is a more suitable indicator for coastal ecosystem health. Significant correlations (R2) were found between exposure time to low salinity (days with salinity values below 25 PSU) with coral cover, coral species richness, seagrass cover, the number of seagrass species, seagrass leaf phosphorus, nitrogen, C:N ratio and iron content. By comparing the correlation coefficients and the slopes of the regression lines, our study suggests that coral reefs are more susceptible to low salinity levels exposure than seagrass meadows. Regarding the risk of corals being exposed to low salinity, nearshore and northern barrier reefs were classified as “high risk”, the middle barrier reef as “medium to high risk” and southern barrier reefs as “medium risk”. Further offshore, the oceanic reefs were classified as “low risk”. Regarding the seagrass meadows, the nearshore region was categorized as “high risk”, the barrier reef as “medium to low risk” and oceanic reefs as “low risk”. This study contributes to assessing the potential impacts of salinity on the BCS ecosystems, and further provides a knowledge base for marine conservation planning