The Berau Continental Shelf is located close to the Equator in the Indonesian Archipelago, hosting a complex of coral reefs along its oceanic edge. The Berau coral reefs have a very high biodiversity, but the area is under serious risk due to river‐derived nutrients and sediments. The region is characterized by weak winds, moderate tides, and almost absent Coriolis forcing. Existing knowledge about river plume behavior in tropical environments is limited. The aim of this paper is to investigate the influence of the subtle physical forcing on the dynamics of the Berau river plume. A three‐dimensional model (ECOMSED) was calibrated with observational data. The model was forced by freshwater input from the Berau river distributaries, tides at the open boundaries, and measured hourly wind. The model reproduces the freshwater dynamics on the shelf adequately and highlights that the river plume spreads symmetrically for river forcing only. Tides cause vertical mixing and suppress the cross‐shelf spreading of the river plume. However, the spreading of the river plume over the shelf is mainly controlled by the weak monsoonal winds, resulting in a seasonal development. During the Southeast Monsoon, the southerly winds push the plume northeastward and cause a stratified water column in the northern part of the continental shelf. Northerly winds during the Northwest Monsoon disperse the plume to the south, promoting a vertically well‐mixed water column. The results can be used to predict the possible impact of land‐use changes in the steadily developing Berau region on coral reef health.

We conducted an investigation into the characteristics of marine heatwaves (MHWs) in Indonesian waters, aiming to understand the underlying mechanisms responsible for their formation, particularly the ones generated by net surface heat flux. To accomplish this, we utilized remote sensing data from the National Oceanic and Atmospheric Administration and the European Centre for Medium-Range Weather Forecasts. The dataset covered a 40-year period (1982–2021) encompassing both warm (1982–2007) and cold (2008–2021) phases of the Pacific Decadal Oscillation (PDO). Statistical analysis methods were employed to process the data. Our study reveals significant findings regarding MHWs in Indonesian waters. We observed the highest average frequency and maximum intensity of MHWs, occurring approximately 2–3 times a year and exceeding 1.5 °C, respectively, during the warm phase of PDO (with El Niño events occurring more frequently than La Niña). Conversely, the longest durations of MHWs in Indonesian waters were recorded during the cold phase of PDO (with La Niña events occurring more frequently than El Niño), spanning approximately 7–15 days. We identified local forcing in the form of net surface heat flux as the primary driver of MHW frequency and maximum intensity. During the warm phase of PDO, the net surface heat flux was notably higher, primarily due to increased shortwave radiation (heat gain) and reduced latent heat flux (heat loss) through the evaporation process. These factors collectively contributed to maintaining warmer ocean temperatures. Moreover, our study provides valuable insights into the interannual variability of MHWs through the application of composite calculations. We discovered a strong correlation between the occurrence of El Niño and positive Indian Ocean Dipole (IOD) events during the warm PDO phase and the highest frequency and maximum intensity of MHWs, with approximately 2.52 events and 1.54 °C, respectively. In contrast, we found that MHWs with the longest durations were closely linked to La Niña and negative IOD events during the cold PDO phase, lasting approximately 10.90 days. These findings highlight the complex interplay between climate phenomena and MHW characteristics, further deepening our understanding of their dynamics.

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.

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.