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The diurnal cycle over tropical coastal waters is characterized by offshore migration of precipitation area during nighttime. This study analyzes in situ observational data collected during the YMC-Sumatra 2017 field campaign around the western coast of Sumatra Island, Indonesia, to examine the offshore migration phenomenon during 5–31 December 2017, when the Research Vessel Mirai was deployed about 90 km off the coast to perform observation. The offshore migration is observed in only less than a half of the 27 days. A comparison of radiosonde data at the vessel between days with and without the offshore migration reveals that vertical wind shear in the lower troposphere is a key environmental condition. In late afternoon of the days with the offshore migration, offshore (northeasterly) wind shear with height with considerable magnitude is observed, which is due to weaker daily mean southwesterly wind in the lower free troposphere, stronger southwesterly wind in the boundary layer, and sea breeze. As this condition is considered favorable for regeneration of convective cells to the offshore side of old ones, these results support an idea that the regeneration process is critical for the offshore migration. The Madden–Julian oscillation and cold surges play some roles in the weakening of the free-tropospheric wind. The migration speed is estimated at 2–3 m s−1, which is lower than that observed in another field campaign conducted in 2015 (Pre-YMC 2015). This difference is partly due to the difference in the environmental wind in the lower to midtroposphere.

This study analyzes data obtained by intensive observation during a pilot field campaign of the Years of the Maritime Continent Project (Pre-YMC) to investigate the diurnal cycle of precipitation in the western coastal area of Sumatra Island. The diurnal cycle during the campaign period (November–December 2015) is found to have a number of similarities with statistical behavior of the diurnal cycle as revealed by previous studies, such as afternoon precipitation over land, nighttime offshore migration of the precipitation zone, and dependency on Madden–Julian oscillation (MJO) phase. Composite analyses of radiosonde soundings from the Research Vessel (R/V) Mirai , deployed about 50 km off the coast, demonstrate that the lower free troposphere starts cooling in late afternoon (a couple of hours earlier than the cooling in the boundary layer), making the lower troposphere more unstable just before precipitation starts to increase. As the nighttime offshore precipitation tends to be more vigorous on days when the cooling in the lower free troposphere is larger, it is possible that the destabilization due to the cooling contributes to the offshore migration of the precipitation zone via enhancement of convective activity. Comparison of potential temperature and water vapor mixing ratio tendencies suggests that this cooling is substantially due to vertical advection by an ascent motion, which is possibly a component of shallow gravity waves. These results support the idea that gravity waves emanating from convective systems over land play a significant role in the offshore migration of the precipitation zone.

There has been a dramatic increase in flood events over Greater Jakarta since 2019. However, little study has been found regarding physical characteristics of rainfall during flood events. This study investigates the rainfall drop size distribution during the three cases of severe flood events in the Jakarta area on 31 December 2019–1 January 2020 (Case 1), 15–16 July 2022 (Case 2), and 6–12 October 2022 (Case 3). The characteristic of rainfall drop size is analysed and categorised based on stratiform, convective, and mixed convective-stratiform rainfall. This research used the Automatic Weather Station, Laser Precipitation Monitor, and ERA5 reanalysis datasets from December 2019 to October 2022. Overall, results show that raindrops with particle diameters up to 4 mm exist during the three flood cases over the lowland and mountainous regions. The mountainous region has a higher number of larger diameter sizes compared to the lowland. In Cases 1 and 2, the occurrence of stratiform rainfall dominated the lowland areas, while the convective rainfall was also present in the mountain during Case 1. In contrast to Cases 1 and 2, over the lowland during Case 3, mixed stratiform and convective rainfall have a significant proportion. Meanwhile, the stratiform and convective rainfall over the mountainous region occurred equally. The most prominent feature in Case 1, the stratiform rainfall persisted from 14 to 13 LT on the following day, coinciding with the severe flood over the Jakarta region. The anomaly of high moisture convergence and substantial negative vertical velocity over the lowland during Case 1 might generate a long duration stratiform rainfall that likely contributed significantly to floods.

The Makassar Strait (MS) is characterized by water mass from the Pacific Ocean and is one of the ITF (Indonesia Throughflow) branches. It carries warm water masses from the Pacific Ocean to the Indian Ocean. This research aims to analyze the relationship between CPUE of Eastern Little Tuna (Euthynnus affinis) and oceanographic variables, likewise predict the fishing area using the Generalized Additive Model (GAM). The research method used is spatial and temporal analysis. The data was used from 2015 to 2020. The data processed were sea surface temperature, chlorophyll-a, salinity, currents, sea level as predictor variables, and Eastern Little Tuna production as a response. Eastern Little Tuna catch data were normalized into Catch per Unit Effort, while the oceanographic data were extracted using ArcGIS. Based on the results of the GAM model, it was found that the model with five variables is the most suitable predictive model, with 16.4% CDE. Salinity is the most influential parameter on the catch of Eastern Little Tuna with a significance value of <2.00 × 10−16 ***. The optimum value for SST is 30–31 °C, chlorophyll-a is 1–2 mg/m3, salinity is 29–30 ppt, current velocity is 0.3–0.5 m/s and sea level is between 0.6–0.7 m. Based on the GAM prediction results, a high CPUE value will be obtained in the southwest monsoon (March to May). Fishing activity carried out in the best season will implement the adoption of harvest control measures.

We investigated ocean variability at intermediate depths (300–750 m) in the western equatorial Pacific (WEP) using data from underwater sensors of the Triangle Trans-Ocean Buoy Network (TRITON) buoys. The data were specially processed because high accuracy is necessary for such an application, and we confirmed that sufficient accuracy was achieved. Variability of the temperature and salinity was large in the western boundary regions of the WEP. Seasonal variability is generally dominant at intermediate depths in the WEP; however, intraseasonal variability with period of 30–80 days is not negligible, particularly at 2°N, 138°E, where the New Guinea Coastal Undercurrent retroflects to the east as the Equatorial Undercurrent and/or Northern Subsurface Countercurrent. High correlation is seen between 12-month running means of the Niño 3.4 index anomaly and temperature at 300 and 750 dbar. In other words, the influence of El Niño/Southern Oscillation phenomena reaches to at least a depth of 750 dbar. We found a decreasing trend in salinity at intermediate depths in the WEP: − 0.0032 Practical Salinity Unit (PSU) year −1 at 300 dbar and − 0.00053 PSU year −1 at 750 dbar. This trend is likely associated with the freshening of the North Pacific Intermediate Water and Antarctic Intermediate Water.

Longtail tuna catch in Indonesia, especially in Makassar Strait, is increasing, but habitat information remains scarce, posing challenges for management and targeted harvesting. Here, we aimed to examine the species-environment responses and the potential habitat of longtail tuna in the Makassar Strait. To do this, we developed a habitat suitability model based on the maximum entropy (MaxEnt) algorithm using fishery-based abundance and environmental data (sea surface temperature, chlorophyll-a, salinity, and sea surface height) between 2015 and 2021. Environmental predictors were evaluated for multicollinearity prior to model development. Our results suggest that the MaxEnt model performed well in predicting the potential longtail tuna habitat. Monthly averaged fishing ground estimates revealed seasonal variations, with the widest in December (37,68 km2) and the narrowest in June (6,62 km2) for locations above a 0.6 HSI threshold. The model-derived environmental variable importance revealed the significant contributions of SSH (74.5%), and chlorophyll-a (22.1%) to longtail tuna habitat distribution. The model also identified the optimal ranges for SSH (0.58–0.59 m), chlorophyll-a (0–1 mg·m⁻3) and salinity (32.5–33 ppt). Findings from this work could offer valuable insights for fishermen engaged in fishing operations and stakeholders involved in the sustainable management of longtail tuna in the Makassar Strait.

During the Monsoon Asian Hydro−Atmosphere Scientific Research and Prediction Initiative (MAHASRI; 2006–16), we carried out two projects over the Indonesian maritime continent (IMC), constructing the Hydrometeorological Array for Intraseasonal Variation−Monsoon Automonitoring (HARIMAU; 2005–10) radar network and setting up a prototype institute for climate studies, the Maritime Continent Center of Excellence (MCCOE; 2009–14). Here, we review the climatological features of the world’s largest “regional” rainfall over the IMC studied in these projects. The fundamental mode of atmospheric variability over the IMC is the diurnal cycle generated along coastlines by land−sea temperature contrast: afternoon land becomes hotter than sea by clear-sky insolation before noon, with the opposite contrast before sunrise caused by evening rainfall-induced “sprinkler”-like land cooling (different from the extratropical infrared cooling on clear nights). Thus, unlike the extratropics, the diurnal cycle over the IMC is more important in the rainy season. The intraseasonal, seasonal to annual, and interannual climate variabilities appear as amplitude modulations of the diurnal cycle. For example, in Jawa and Bali the rainy season is the southern hemispheric summer, because land heating in the clear morning and water vapor transport by afternoon sea breeze is strongest in the season of maximum insolation. During El Niño, cooler sea water surrounding the IMC makes morning maritime convection and rainfall weaker than normal. Because the diurnal cycle is almost the only mechanism generating convective clouds systematically near the equator with little cyclone activity, the local annual rainfall amount in the tropics is a steeply decreasing function of coastal distance (e-folding scale 100–300 km), and regional annual rainfall is an increasing function of “coastline density” (coastal length/land area) measured at a horizontal resolution of 100 km. The coastline density effect explains why rainfall and latent heating over the IMC are twice the global mean for an area that makes up only 4% of the Earth’s surface. The diurnal cycles appearing almost synchronously over the whole IMC generate teleconnections between the IMC convection and the global climate. Thus, high-resolution (<< 100 km; << 1 day) observations and models over the IMC are essential to improve both local disaster prevention and global climate prediction.

Spatial and temporal information on oceanic flow is fundamental to oceanography and crucial for marine-related social activities. This study attempts to describe the short-term surface flow variation in the area south of the Lombok Strait in the northern summer using the hourly Himawari-8 sea surface temperature (SST). Although the uncertainty of this temperature is relatively high (about 0.6 ∘ C), it could be used to discuss the flow variation with high spatial resolution because sufficient SST differences are found between the areas north and south of the strait. The maximum cross-correlation (MCC) method is used to estimate the surface velocity. The Himawari-8 SST clearly shows Flores Sea water intruding into the Indian Ocean with the high-SST water forming a warm thermal plume on a tidal cycle. This thermal plume flows southward at a speed of about 2 m / s . The Himawari-8 SST indicates a southward flow from the Lombok Strait to the Indian Ocean, which blocks the South Java Current flowing eastward along the southern coast of Nusa Tenggara. Although the satellite data is limited to the surface, we found it useful for understanding the spatial and temporal variations in the surface flow field.

Two onboard observation campaigns were carried out in the western boundary region of the Philippine Sea in December 2006 and January 2008 during the 2006/07 El Niño and the 2007/08 La Niña to observe the North Equatorial Current (NEC), Mindanao Current (MC), and Kuroshio current system. The NEC and MC measured in late 2006 under El Niño conditions were stronger than those measured during early 2008 under La Niña conditions. The opposite was true for the current speed of the Kuroshio, which was stronger in early 2008 than in late 2006. The increase in dynamic height around 8°N, 130°E from December 2006 to January 2008 resulted in a weakening of the NEC and MC. Local wind variability in this region did not appear to contribute to changes in the current system.

This paper presents an overview of the HARIMAU2010 campaign focusing on convective activity with the diurnal rainfall meridional march (DRMM) over Jakarta, which is located on the northern coast of Jawa Island of the Indonesian maritime continent (IMC), based on 1-month intensive observations by a C-band Doppler radar and multi-point atmospheric sounding array conducted during 16 January–14 February 2010. The campaign period corresponded to a phase after large-scale Madden–Julian oscillation (MJO) active convections passed over Jakarta (MJO inactive phase). The cross-equatorial northerly surge (CENS) intruded into the Jawa Sea with a cold tongue (CT) of sea surface temperature (SST) in the beginning of the period (CENS active period: 16–26 January), and then, it started to retreat (transition period: 27 January–05 February); afterward, only a few signs of it were apparent (CENS inactive period: 06–14 February). The observational results showed that (1) rainfall over Jakarta has the nature of DRMM during the MJO inactive phase at least, (2) the DRMM is likely driven primarily by “land-breeze”-like local meridional circulation, and (3) the meridional spatiotemporal variation of rainfall over Jakarta is thus controlled by activities of both the CENS and CT over the Jawa Sea.