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How the Impact of ENSO Events on the Indonesian Throughflow and Interannual Variability of Circulation in the Halmahera Sea
(2026-09-01)
Lubis, Muhammad Zainuddin
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Dwinovantyo, Angga
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Batara, B.
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Kausarian, Husnul
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Hidayat, Syarief
This study investigates the influence of El Niño–Southern Oscillation (ENSO) on the Indonesian Throughflow (ITF) in the Halmahera Sea (HS) during 2013–2023. Although ITF variability has been widely studied, the response of the HS to different ENSO phases remains insufficiently understood. Using a validated CROCO model, we quantified changes in HS throughflow and associated oceanographic conditions under El Niño and La Niña events. EOF analysis of sea surface height (SSH), meridional velocity, and salinity revealed dominant modes explaining more than 50% of the total variance and exhibiting clear ENSO-related variability. During El Niño events, ITF transport weakened substantially, with volume transport becoming less negative from − 4.0 ± 2.0 Sv to − 1.5 ± 1.5 Sv in the upper layer (0–100 m) and from − 5.0 ± 2.5 Sv to − 2.0 ± 2.0 Sv in the intermediate layer (0–400 m). These conditions were associated with a shallower thermocline, lower sea surface temperatures (25.5–26.0°C), and reduced surface salinity (34.74 PSU). In contrast, La Niña events strengthened the ITF, with transport reaching −7.0 ± 1.5 Sv in the upper layer and −7.0 ± 2.0 Sv in the intermediate layer, accompanied by a deeper thermocline and higher SSTs (29.8–30.42°C). The results suggest that ENSO-related variability in the HS is associated with coupled changes in thermocline structure, SSH gradients, and salinity, highlighting the importance of the HS in regulating ITF transport variability.
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Climate thresholds and yield elasticity of durian, mangosteen, and coffee under hydroclimatic variability in Thailand
(2026-08-01)
Ansari, Kutubuddin
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Tanır Kayıkçı, Emine
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Jamjareegulgarn, Punyawi
This study examines the empirical relationships between hydroclimatic variability and the yields of durian, mangosteen, and coffee across Thailand using meteorological observations from 56 stations and provincial level yield statistics for 2008 to 2024. An integrated statistical framework was applied, including exploratory correlation analysis, nonlinear quadratic additive modelling, lagged climate response analysis, elasticity estimation, model comparison, mangosteen regional sensitivity analysis, and a Climate Risk Index (CRI). The results show clear spatial gradients in temperature, humidity, and precipitation across the six agroclimatic regions of Thailand, broadly corresponding to regional crop productivity patterns. Annual anomaly analysis indicates that warm and humid years are generally associated with higher durian and mangosteen yields, whereas excessive rainfall and warming are associated with reduced coffee productivity. Pairwise linear correlations between annual climate variables and crop yields are generally weak, suggesting that simple linear models may not fully capture crop climate relationships. Nonlinear modelling indicates approximate empirical temperature turning points near 28.0°C for durian, 27.3°C for mangosteen, and 26.6°C for coffee, with humidity related turning points around 76–78%. Lagged climate response models suggest that multi-year hydroclimatic conditions may influence perennial crop productivity, particularly humidity for durian, precipitation for coffee, and temperature and precipitation for mangosteen. The mangosteen sensitivity analysis shows that humidity and precipitation thresholds are affected by regional composition, especially when marginal production regions are included. The CRI formulation under the equal weight, mangosteen shows the highest rainfall related climate risk signal, while coffee is more sensitive to temperature related risk and durian shows moderate vulnerability to prolonged humid and wet conditions.
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Ionospheric gradients in multi-constellation global navigation satellite system signals onboard UAV using GIM and Klobuchar model over Thailand region
(2026-07-01)
Ansari, Kutubuddin
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Panda, Sampad Kumar
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Venkatesh, Kavutarapu
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Jamjareegulgarn, Punyawi
The effects of the ionosphere on Global Navigation Satellite System (GNSS) signals have been a focal point of research nowadays. During adverse ionospheric conditions, ionospheric gradients become more pronounced and disruptive compared to quiet days, potentially leading to increased positioning errors or loss of satellite signal lock. We introduce an ionospheric spatial gradient estimation method to detect the anomalous gradients from multi-constellation GNSS signals (i.e., GPS, GLONASS, and Galileo) signals recorded by the onboard sensor of flying real-time kinematic unmanned aerial vehicle (RTK UAV) over the Thailand region. We employ the Klobuchar model and global ionospheric maps (GIMs) for estimating the slant total electron contents (STECs) and the corresponding ionospheric spatial gradients between base station and rover (RTK UAV) receivers among the studied multi-constellation systems. The results show that the STEC values estimated from IGS-GIM are larger than those computed by Klobuchar model. Such kind of gradient variation cannot show a perfect correlation due to limited accuracy of Klobuchar model parameters. As for our analysis, the ionospheric spatial gradients estimated from GPS satellites are higher than those calculated from GLONASS and Galileo satellites due to the smallest differences between the two successive positions of flying rover estimated from GPS satellites. The outcomes from this study complement the multi-GNSS cooperative strategy for monitoring ionospheric gradients, thereby mitigating the adverse effects in dynamic positioning and navigation solutions over low-latitude regions.
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Annual analysis of surface temperature, land cover, and precipitation impacts on durian harvests in Thailand
(2026-01-01)
Lubis, Muhammad Zainuddin
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Dwi Wulandari, Pratiwi
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Simanjuntak, Andrean V.H.
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Batara, B.
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Kausarian, Husnul
This study analyzes the impact of land surface temperature (LST), NDVI, land cover, precipitation, and climatic factors on durian productivity in Thailand (2019–2025). Results show a significant positive correlation between LST and durian yields, particularly in the southern regions, where LST increased by 0.2–0.3 °C. Statistical models indicate that the southern sector will contribute over 50% of the national durian harvest by 2025, with an average yield of 617,990 tons. The precipitation and NDVI trends also correlate with regional variations in yield. The results underscore the complex interactions of temperature, land cover, and precipitation, emphasizing the need for targeted adaptation strategies and integrated planning to sustain agricultural productivity. These findings highlight the critical role of climate variables in shaping durian productivity and stress the importance of adaptive strategies to ensure sustainable agricultural output.
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Decadal and seasonal oceanographic trends influenced by climate changes in the Gulf of Thailand
(2025-06-01)
Lubis, Muhammad Zainuddin
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Ghazali, Muhammad
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Simanjuntak, Andrean V.H.
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Riama, Nelly F.
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Pasma, Gumilang R.
Our study investigates the decadal and seasonal variability of sea surface height (SSH) and sea surface temperature (SST) in the Gulf of Thailand (GoT) using data from CMEMS from 1993 to 2021. We employed statistical analyses utilizing GLM and GAM to assess the variables comprehensively. The reveals a significant upward trend in SSH, increasing from ∼0.79 m in 1993–1998 to ∼0.89 m in 2017–2021, highlighting the impacts of climate change. SST analysis revealed fluctuations, with a maximum reaching ∼30.6 °C in 2019–2020, correlating with climatic events such as El Niño. Our study results at station 1 (near Bangkok) showed that the average SSH in 1998 during strong El Niño years was equal to 0.82 m, while the maximum SST was equal to 29.89 °C. Seasonal patterns indicated SSH peaks in DJF and SON at ∼0.92 m, while SST peaked in spring MAM and summer JJA at ∼30.7 °C. Volume transport analysis showed significant variability, with 0.3634 Sv (0–55 m) at longitude 99°E-107° E and latitude 6° N, indicating complex circulation patterns influenced by bathymetry and wind. Time series analysis revealed an average SSH increase of 0.0038 m/year, with a high pseudo-R-squared of 0.99. Our findings underscore the critical influence of climate variability on oceanographic conditions in the GoT, emphasizing the need for ongoing monitoring to address the implications of rising sea levels and temperature fluctuations. In conjunction with increased SSH, the rising SST heightens the risk of flooding in low-lying areas, exacerbating vulnerabilities for local populations and necessitating adaptive management strategies to mitigate these impacts.
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Numerical simulation of hydrodynamics and measurement in the Northwest Banda Sea
(2025-03-01)
Lubis, Muhammad Zainuddin
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Simanjuntak, Andrean V.H.
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Riama, Nelly F.
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Pasma, Gumilang R.
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Dwinovantyo, Angga
This study provides critical insights into hydrodynamic and measurement techniques in the NW Banda Sea, enhancing our understanding of oceanic processes. It focuses on the physical oceanography conditions in Eastern Indonesia, demonstrating the connection between a 2D hydrodynamic model simulation and oceanographic phenomena, including surface current patterns, waves, and tides. The model analyzes the east monsoon during August, September, and October 2023, encompassing spring and neap tides. The correlation results between the model and observed tidal data over 15 days showed a strong correlation value of 0.96 and an RMSD of 0.29. The tides are predominantly mixed and semi-diurnal, reflecting the complex interplay between lunar and solar gravitational forces. Spring tides exhibit higher elevations than neap tides. The highest ocean surface current speed was observed in August at 2.23 m/s (southeast to northwest), while the lowest was noted in September at a value of 0.95 m/s (northwest to southeast). Wave patterns indicate a monthly cycle increase in wave height from September to January, with notable wave heights (Hs) and peak periods (Ts) recorded during the east monsoon from August to October. We found the interplay between surface currents and tidal patterns significantly influences wave height and current dynamics.
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Indonesian Throughflow, spatial–temporal variability, and its relationship to ENSO events in the Lombok Strait
(2025-03-01)
Lubis, Muhammad Zainuddin
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Situmorang, Edriyan
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Simanjuntak, Andrean V.H.
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Riama, Nelly F.
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Pasma, Gumilang R.
Understanding the interactions between ocean currents and climate variability is crucial for predicting future oceanic and climatic shifts in the Lombok Strait and beyond. Our study examines the Lombok Strait from 2013 to 2018, focusing on the impact of interannual and seasonal variability driven by climatic events such as the El Niño-Southern Oscillation. Our findings reveal significant fluctuations in oceanographic parameters, including temperature, salinity, and ocean currents. The lowest recorded temperature was ∼27.2 °C, and the minimum salinity was ∼32.3 psu, linked to the strong El Niño event in 2015, which altered local rainfall and evaporation patterns. Analysis of meridional and zonal velocities showed enhanced water flow, with maximum meridional velocities reaching around ∼0.04 m/s and zonal velocities peaking at ∼0.18 m/s in early 2018. Seasonal variations in sea surface height show higher values during warmer months, reflecting the influence of ENSO phenomena. The vertical distribution of salinity indicated stratification, with values ranging from ∼27.5 psu to ∼34.0 psu across different depths, corresponding to water density ranging from 22.50 σ<inf>θ</inf> to 24.0 σ<inf>θ</inf>, highlighting the interaction between Pacific and Indian Ocean waters. Water mass transport averages at 100 m and 200 m show values of −1.21 ± 0.97 Sv and −1.350 ± 1.069 Sv, with the lowest transport occurring during the summer at −1.97 Sv with a depth range of 0–100 m, suggesting robust outflow. Our findings underscore the critical role of the ITF in shaping regional oceanographic conditions and their implications for global climate dynamics.
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Thin-Plate Spline Interpolation Spatial Modelling of GNSS Retrieved Water Vapour Over Northeast Japan
(2025-01-01)
Ansari, Kutubuddin
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Walo, Janusz
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Wezka, Kinga
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Marjanska, Dorota
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Jamjareegulgarn, Punyawi
The study investigated precipitable water vapour (PWV) variation over Northeast Japan by using more than two decades (2000 to 2022) of measurements. These data have been extracted from 36 Global Navigation Satellite System (GNSS) networks and interpolated with thin-plate spline interpolation to understand its spatial variation over the region. PWV annual and seasonal variation is studied and correlated with ERA5 reanalysis observations. These PWV variation shows that the northern part of Northeast Japan which is located at higher latitudes showed a smaller magnitude compared to PWV magnitude in the southern part of low latitudes. This is possible because the cold air is drier and the warm air holds more moisture in the lower southern part. The correlation coefficient variation based on geographical area is visible which quantifies the consistency between GNSS PWV and ERA5 and visualises the effects in sampling. The southern part, where the correlation coefficients are very high (more than 0.80), indicates good agreements between GNSS PWV and ERA5 values and can be considered well-sampled observations. Moreover, the correlation coefficients drop the values to approximately 0.40–0.60 for poorly sampled observations on the northern coast, pointing out the interannual inconsistency of PWV measurements, which are loosely related to ERA5 observations. Finally, the correlation coefficient between PWV with surface temperature and atmospheric pressure over Northeast Japan is investigated. This is clear from the results that the correlation coefficient between PWV and temperature is around 0.98 for each site, indicating that PWV has a very high dependency on surface temperature. The correlation coefficient between PWV and pressure is highly negative around −0.80, indicating their inverse relationship. We believe the outcomes from this study will contribute to a better understanding of PWV over Japan and the future refinements of atmospheric models over the East Asian region.
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Towards mitigating the effect of plasma bubbles on GPS positioning accuracy through wavelet transformation over Southeast Asian region
(2024-04-01)
Ansari, Kutubuddin
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Kumar Panda, Sampad
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Kavutarapu, Venkatesh
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Jamjareegulgarn, Punyawi
Ionospheric irregularities remain as a challenging concern among space scientists and engineers as it can pose severe threats to satellite-based services by introducing amplitude and phase scintillations in the transionospheric radio signals. The scintillations may cause degradation of the positioning, navigation, and timing (PNT) services or even complete system failure under extreme conditions. The present work focuses on understanding the ionospheric irregularities during the space weather event on May 12, 2021, witnessing the first strong geomagnetic storm in the 25th solar cycle. This particular event commenced around 13.00 UT, apparently altering the regular development of equatorial plasma bubbles around the local post-sunset terminator. We selected three global positioning system (GPS) stations, one at the near-magnetic equator (CUSV) and two at far low-latitude locations with a longitudinal separation (THKK and HWKS) over the Southeast Asian longitudes to investigate the characteristics of ionospheric plasma irregularities on the storm day. The results showed that the amplitude of ROTI was highest at the low-latitude CUSV, followed by the THKK and HKWS stations. The increased ROTI at CUSV is due to its latitudinal location close to the geomagnetic equator in the equatorial ionization anomaly (EIA) region which often witnesses more fluctuations than the inter-mediate latitudes. The strong TEC fluctuation occurrences were comparatively minimal at HWKS station on the storm day (May 12, 2021), whereas both intermediate and relatively higher TEC fluctuation occurrences were found at THKK and CUSV stations, respectively. Moreover, the occurrence rate of moderate irregularities was the highest at CUSV, followed by THKK and HKWS, while that of weak irregularities was maximum at HKWS, followed by THKK and CUSV. This happens because the systematic longitudinal time delay observed at the onset of ionospheric scintillations corresponds to the rise of PBs velocity at the magnetic equator that strongly depended upon the F-region dynamo electric fields in the east–west direction. Furthermore, the three-dimensional (3-D) positioning errors were estimated at a 30-sec interval to understand the impact of storm-induced irregularity on the positioning solution. An excellent agreement is realized between the ROTI pattern and positioning errors at all stations, irrespective of latitudinal extent from the magnetic equator. In order to model the amplitude of the scintillation index against the SNR measurements, the linear, quadratic, and cubic regression curve fittings are utilized that indicate the cubic polynomial manifesting the best fitted SNR measurements with the weak as well as moderate scintillation index. The continuous wavelet transformation (CWT) method was implemented to mitigate the ionospheric scintillation effects on positioning accuracy. The CWT estimation reveals the hidden information in positioning error due to the scintillation index, which makes it easy to understand the ionospheric scintillation effects on positioning. The attempt in this work is aligned with the efforts towards reliable PNT operations over equatorial and low latitudes.
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Publisher Correction: Quasi zenith satellite system-reflectometry for sea-level measurement and implication of machine learning methodology (Scientific Reports, (2022), 12, 1, (21445), 10.1038/s41598-022-25994-6)
(2023-12-01)
Ansari, Kutubuddin
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Seok, Hong Woo
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Jamjareegulgarn, Punyawi
In the original version of this Article a previous rendition of Figure 6 was published. The original Figure 6 and accompanying legend appear below. The original Article has been corrected.

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