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Research focusing on seagrass ecosystems as carbon storage has been conducted in various Indonesian waters. However, an essential aspect that remains unexplored is the simultaneous analysis of blue carbon storage in seagrass alongside carbon dioxide (CO2) flux values, particularly within Karimunjawa waters. This study aims to assess the organic carbon stock and sea–air CO2 flux in seagrass ecosystems in Karimunjawa. Our hypothesis posits that although seagrass ecosystems release CO2 into the water, their role as blue carbon ecosystems enables them to absorb and accumulate organic carbon within seagrass biomass and sediments. This investigation took place in Karimunjawa waters, encompassing both vegetated (seagrass meadows) and unvegetated (non-seagrass meadows) areas during August 2019, 2020, and 2022. Over this period, the organic carbon stock in seagrass and sediment displayed an increase, rising from 28.90 to 35.70 gCorg m−2 in 2019 and from 37.80 to 45.25 gCorg m−2 in 2022. Notably, the expanse of seagrass meadows in Karimunjawa dwindled by 328.33 ha from 2019 to 2022, resulting in a total carbon stock reduction of the seagrass meadows of 452.39 tC to 218.78 tC. Sediment emerges as a pivotal element in the storage of blue carbon in seagrass, with sedimentary organic carbon outweighing seagrass biomass in storage capacity. The conditions in Karimunjawa, including a high A:B ratio, low dry bulk density, and elevated water content, foster a favorable environment for sediment carbon absorption and storage, facilitated by the waters' CO2 emission during the southeast monsoon season. Notably, our findings reveal that CO2 release within vegetated areas is lower compared to unvegetated areas. This outcome underscores how seagrass ecosystems can mitigate CO2 release through their adeptness at storing organic carbon within biomass and sediment. However, the presence of inorganic carbon in the form of calcium carbonate introduces a nuanced dynamic. This external source, stemming from allochthonous origins like mangroves, brown algae like Padina pavonica, and calcareous epiphytes, leads to an increase in sedimentary organic carbon stock of 53.2 ± 6.82 gCorg m−2. Moreover, it triggers the release of CO2 into the atmosphere, quantified at 83.4 ± 18.26 mmol CO2 m−2 d−1. [ABSTRACT FROM AUTHOR]

The distribution and variability of surface winds in the seas and straits in the Maritime Continent, or the Indonesian seas, are investigated using scatterometer wind measurements and reanalysis wind data. This study focuses on the detailed climatology of surface winds in the Indonesian seas and has significant implications for a better understanding of regional climate, meteorological disasters, and offshore wind resource. The monsoon winds predominantly blow over the seas along the routes formed by the coastlines of the large islands and the chains of the small islands. The wind speeds are persistently strong along these routes, and the major direction of the variability in winds is well aligned with these routes. The wind jets are formed in the central part of the routes. The interisland gaps and channels form wind jets of various scales. The Australian winter monsoon is stronger and more persistent than the East Asian winter monsoon in the Indonesian seas. The onset of each monsoon differs by approximately 40 days between the east and west of the Indonesian seas. The amplitudes and spatial extents of diurnally varying winds dominate along the northern coasts of Java Island and the small islands to the east, along the northwest and northeast coasts of Borneo, along the coast of southern Sulawesi Island, and to the southwest of Papua, and vanish along the centre lines of the monsoon routes. Sea areas with a large amplitude of diurnally varying wind are much the same throughout the year. The amplitude of diurnally varying wind has an annual maximum in September in the southern Indonesian seas, possibly in correspondence with the annual maximum of the temperature difference between land and sea. [ABSTRACT FROM AUTHOR]

The previous studies of upwelling within the Indonesian seas only focused on the Southeast Monsoon (SEM) season. The western coast of Sumatra Island; the southern coast of Java Island, Sunda Islands, and Sulawesi Island; the Banda Sea, the Maluku Sea, and the Arafura Sea are well-known SEM upwelling areas. However, the upwelling events during Northwest monsoon (NWM) have never been investigated. The investigation of NWM upwelling is challenging due to the broad cloud coverage, limiting the observation of infrared and visible sensors of the satellites. This study used the blended products of the satellite-based sea surface temperature (SST), chlorophyll-a (chl-a), sea level anomaly (SLA), surface wind, and rainfall to identify the NWM upwelling occurrences. Remote sensing reflectances 443 nm and 555 nm were also used to examine the influence of suspended sediment and organic matter which may bias the chl-a concentration. Along the northern coast of the Lesser Sunda Islands, the north coast of Papua Island and the Malacca Strait are the areas of NWM upwelling as denoted by the positive anomaly of chl-a, the negative anomaly of SST, and the negative anomaly of SLA. Focusing on the area along the northern coast of the Lesser Sunda Islands, the NWM upwelling is generated by offshore Ekman Mass Transport that reduces SST and increases chl-a. From 2010 onwards, El Niňo Southern Oscillation has a more consistent effect on the NWM upwelling than Indian Ocean Dipole. El Niňo (La Niňa) tends to weaken (strengthen) upwelling. The magnitude of NWM upwelling is weaker than SEM upwelling. [ABSTRACT FROM AUTHOR]

It has been widely known that the coastal upwelling along the southern coast of Java is generated by southeasterly wind which induces offshore Ekman Mass Transport (EMT) during southeast monsoon. However, the variability of EMT has not been fully described in previous studies. The present study investigated the variability of Ekman dynamics which consist of EMT and Ekman Pumping Velocity (EPV) along the southern coast of Java, based on remotely sensed data. We demonstrated the incongruity between the distribution of southeasterly wind speed and sea surface temperature (SST) during southeast monsoon which is related to the distribution of Ekman dynamics. Offshore EMT at the western region of the southern coast of Java is stronger than offshore EMT at the eastern region. However, stronger offshore EMT at the western part is inhibited by downwelling EPV while weaker offshore EMT at the eastern part is accelerated by upwelling EPV. Consequently, SSTs at the eastern parts are lower than those at the western parts. Thus, the changes of offshore EMT intensity from eastern to western parts are balanced by their EPV distributions which explain the incongruity between the distribution of wind speed and SST during southeast monsoon. On an interannual timescale, the combination of La Niña and negative Indian Ocean Dipole (IOD) events tends to weaken offshore EMT and EPV which reduce the intensity of Chl-a bloom and SST cooling during southeast monsoon season. Furthermore, ENSO has a less significant impact on the Ekman dynamics variability than IOD. [ABSTRACT FROM AUTHOR]

Novel and long-term satellite data (2003–2017) are analyzed to investigate the variability of ocean surface chlorophyll-a (Chl-a) concentration off the Western Lesser Sunda Islands (WLSI) under the influence of the Indonesian Australian monsoon, the El Niño-Southern Oscillation, and the Indian Ocean Dipole (IOD). In this article, we first analyzed the seasonal variability of Chl-a, and then describe the relationship among the sea surface Chl-a, sea surface temperature (SST), and sea surface wind stress in the region. Our results demonstrate that prevailing southeasterly wind stress plays a pivotal role in generating the Chl-a maxima off the WLSI. Particularly on seasonal time scale, the strengthening of southeasterly wind stress (up to ∼0.01 N·m−2) during the southeast monsoon season produces enhanced Chl-a concentrations (0.59 mg·m−3) associated with sea surface cooling (∼28.8 °C) in the area of study. In contrast, the Chl-a maxima completely vanished during the northwest monsoon season. On interannual time scale, the largest positive Chl-a and wind stress anomalies and the coolest SST anomaly are observed in 2006 when El Niño and positive IOD events occur at the same time. Meanwhile, the greatest negative Chl-a anomaly is prevailed during the 2016 negative IOD event. This article demonstrates that wind variability is the essential factor in determining the magnitude of the Chl-a maxima off the WLSI. [ABSTRACT FROM AUTHOR]

The Sulawesi Sea and the northern part of the Maluku Sea belong to the western equatorial Pacific that is influenced by the Mindanao Current. During the southeast (SE) monsoon, contrasting sea surface temperature (SST) distributions occur between these two seas. The purpose of this research is to investigate the mechanisms of contrasting SST distribution in the Sulawesi Sea and the northern part of the Maluku Sea under the condition of strong southerly wind during the SE monsoon season. Our analysis was based on satellite observations. Warmer (colder) SSTs are identified in the Sulawesi Sea (northern part of the Maluku Sea), which correspond to the weaker (stronger) southerly wind speed. Strong southerly winds in the northern part of the Maluku Sea generate eastward Ekman mass transport (EMT) that carries the water mass away from the coast, inducing upwelling. In contrast, the southerly winds from the Maluku Sea are blocked by the Gorontalo and North Sulawesi Mountains, creating low-wind areas in the Sulawesi Sea that are located on the lee side of those mountains. Furthermore, the existence of a valley between those mountains acts as a wind gap that channels the southerly wind to the Sulawesi Sea. Since the seaward EMT is weak and the latent heat release is not dominant, SSTs in front of the wind gap are only slightly reduced, possibly through the mechanisms of oceanic mixing. [ABSTRACT FROM AUTHOR]

Chlorophyll-a is the predominant phytoplankton pigment responsible for determining primary productivity. In the present study, we used satellite-based data of chlorophyll-a, surface wind, and precipitation from 2003 to 2019 to investigate the variability of chlorophyll-a in the northern Maluku Sea and the Sulu Sill and examine its generating mechanism. We found that the chlorophyll-a bloom in the northern Maluku Sea occurs during the southeast monsoon season, while in the Sulu Sill, the chlorophyll-a concentration is higher than that in the northern Maluku Sea and occurs throughout the year. In the northern Maluku Sea, the chlorophyll-a bloom is generated by coastal upwelling. The maximum southerly wind during the southeast monsoon generates the strongest offshore Ekman Mass Transport (EMT) in the northern Maluku Sea triggering coastal upwelling. However, the power spectra analysis of satellite-derived chlorophyll-a shows strong peaks and amplitudes at both fortnightly (MSf) and monthly (Mm) frequencies, indicating that tidal mixing is an important generating mechanism for chlorophyll-a blooms in the Sulu Sill. Shallow bathymetry in the Sulu Sill may aid tidal mixing in effectively transporting nutrients from the near bottom to the sea surface, increasing chlorophyll-a concentration. • Chlorophyll-a bloom in the northern Maluku Sea (Sulu Sill) occurs during the southeast monsoon season (throughout year). • In the northern Maluku Sea, the chlorophyll-a bloom is generated by coastal upwelling due to strongoffshore Ekman Transport. • In the Sulu Sill, tidal mixing is an important mechanism for chlorophyll-a blooms which has strong MSf and Mm signals. [ABSTRACT FROM AUTHOR]