Monsoon Winds Research Articles

Sedimentary organic matter signature hints at the phytoplankton-driven biological carbon pump in the central Arabian Sea

Abstract. The central Arabian Sea, a unique tropical basin, is profoundly impacted by monsoon wind reversal affecting its surface circulation and biogeochemistry. Phytoplankton blooms associated with high biological productivity and particle flux occur in the northern part of the central Arabian Sea due to summer-monsoon-induced open-ocean upwelling and winter convection. The core oxygen minimum zone (OMZ) at intermediate water depths is another important feature of the northern central Arabian Sea and fades southward. In this study, we attempt to interlink how these factors collectively impact phytodetrital export to the sediment. Short sediment core-top (1 cm) samples representing the recent particle flux signatures were analysed from five locations (21 to 11° N; 64° E) in the central Arabian Sea. Previously, we used core-top (0–0.5 cm) samples and observed a trend between diatom frustule abundance and diversity with bulk sedimentary parameters indicating a spatial variability in phytodetrital export to the sediment. To verify this observation further, lipid biomarkers of key phytoplankton groups and a sea surface temperature (SST) proxy have been analysed in addition to diatom frustules. The C37 alkenone-based SST proxy indicated cooler SST (27.6 ± 0.25 °C) in the north (21–15° N) mostly due to upwelling (summer) and convective mixing (winter). Warmer SSTs (+0.4 °C) are measured in the south, which usually remains nutrient-poor. This trend was consistent with satellite-derived average SST values (2017–2020). Lipid biomarker analysis suggests that dinoflagellates were likely to be the highest contributor, as indicated by dinosterol and its degradative product dinostanol, followed by brassicasterol and C37 alkenone, likely representing diatoms and coccolithophores, respectively. The north, which largely experiences periodic phytoplankton blooms and is influenced by the thick OMZ, revealed the highest contents of organic matter, diatom frustules (diversity and abundance), dominated by large, thickly silicified cells (e.g. Coscinodiscus and Rhizosolenia) and phytoplankton lipid biomarkers, as well as lower contents of zooplankton biomarkers (cholesterol and cholestanol). In contrast, relatively smaller chain-forming centric (e.g. Thalassiosira) and pennate (e.g. Pseudo-nitzschia, Nitzschia, Thalassionema) diatom frustules along with lower phytoplankton lipid biomarker contents were found in the south, where zooplankton biomarkers and silicious radiolarians were more abundant. The possible impacts of the OMZ on particle flux related to the phytoplankton community, including zooplankton grazing and other factors, have been discussed.

Climate-related risk to maize crops in China from Fall Armyworm, Spodoptera frugiperda

AbstractThe Fall Armyworm, Spodoptera frugiperda, invaded China late in 2018 and was responsible for substantial crop losses, especially to maize. This work focuses on the suitability of climate across China for Fall Armyworm survival and spread. It uses climate metrics derived with guidance from experts to enable assessment of the risks posed by Fall Armyworm on maize production in different regions of China. The locations and time of year when temperature conditions are within a viable range for Fall Armyworm survival (minimum temperature higher than 9.7$$^\circ$$ ∘ C and maximum temperature lower than 39.2$$^\circ$$ ∘ C) are used to estimate the spatial distribution of winter breeding and overwintering zones, which helps understand the regions and timing of Fall Armyworm migration risk into northern maize production regions. In addition, meridional wind conditions across the Yangtze River basin area are assessed, and a metric of migration potential from the winter breeding and overwintering regions in the south towards northern regions with maize production is established. Results show that temperature during the winter months currently limits Fall Armyworm winter breeding populations to the very southern regions of China (and bordering countries to the south). However, due to the consistent timing and direction of the East Asian Summer Monsoon winds, the Fall Armyworm could easily be directed northwards to the Yangtze River basin during summer months with a peak in July. For this reason, pest management actions against the Fall Armyworm on summer maize should be taken.

Influence of Mini Warm Pool Extent on Phytoplankton Productivity and Export in the Arabian Sea

The Arabian Sea is a highly productive tropical ecosystem of the Indian Ocean that supports high fluxes of particulate organic carbon to the mesopelagic zone from two distinct periods of elevated biological productivity associated with the semiannual reversals of the monsoonal wind system. There are now strong indications that the Arabian Sea’s monsoonal wind patterns and hydrographic conditions are being impacted by long-term temperature increases, but the consequences of these changes on primary production and carbon export to the mesopelagic zone are unknown. This is especially true for the summer monsoon period when cloud cover obscures much of the Arabian Sea basin and therefore precludes remotely sensed ocean color measurements for estimating phytoplankton biomass and productivity. Here we overcome this limitation by using a database of bio-optical profiles from Biogeochemical Argo floats collected over the last decade to evaluate the impact of interannual temperature increases on Arabian Sea primary production and carbon export. We classify individual years of float observations based on the spatial extent of the Arabian Sea Mini Warm Pool that appears in the southeast Arabian Sea before the onset of the summer monsoon. This mini warm pool, which begins to build in winter and collapses with the onset of the summer monsoon in late spring, has gained considerable interest on account of its influence on the timing of the onset of the summer monsoon. We observed a 35 percent decrease in primary production during the summer monsoon phytoplankton bloom in strong warm pool years, and a 13 percent decrease in particle stocks in the upper mesopelagic zone following the peak of the bloom. Decreases in production and export were additionally accompanied by a decrease in average particle size, indicating a shift from larger cells like diatoms that appear from fertilization of the oligotrophic waters to smaller phytoplankton size classes in response to a deepening of the thermocline and increased stratification of the water column. These results suggest changes in phytoplankton community structure and further decreases in primary production and carbon export in the Arabian Sea in response to future warming.

Detection of Dominant Rainfall Patterns in Indonesian Regions Using Empirical Orthogonal Function (EOF) and Its Relation with ENSO and IOD Events

Several studies indicate that Indonesia’s position on the equator, where winds from the northern and southern hemispheres converge, leads to year-round rainfall in Indonesia. Previous studies have also shown the presence of monsoon winds over Indonesia, which causes rain in Indonesia to follow the Asian and Australian monsoon wind patterns. Recently, variations in rainfall patterns and intensity in Indonesia have been highly volatile and difficult to predict. This study uses gridded data analysis of ocean-atmospheric rainfall for 69 years (1948–2016) to detect monsoonal, equatorial, and local signals and relate them to ENSO and IOD events. Rainfall analysis reveals that the first mode accounts for 38.57%, the second 13.92%, and the third 8.93% of the total variance. These results show that these variants are very variable, both in space and time. Monsoonal patterns were detected in the southern region of Indonesia, equatorial patterns in northern Sumatra and western Kalimantan, while local patterns were detected in Maluku and the Maluku Islands. Furthermore, the monsoonal and local rainfall patterns are strongly correlated (> 0.85) with local SST, but the equatorial rainfall pattern is weakly correlated (< 0.5) with local SST. Note that the equatorial pattern is associated with the Inter Tropical Convergence Zone (ITCZ) phase. The varied vector correlation map illustrates the asymmetric fluctuations in sea surface temperature and the simultaneous existence of winds coming from the west from the Indian Ocean and winds coming from the east from the Pacific Ocean during the period of heavy precipitation.

Study of Natural Disaster in Manali Valley (Himachal Pradesh), India on 09 July 2023

Continuous heavy rainfall in July 2023, particularly from the 7th to the 10th, led to devastating flash floods, cloudbursts, and landslides in Himachal Pradesh, causing extensive damage to infrastructure, properties, and loss of lives. The most severely affected districts were Kullu, Mandi, Sirmaur, Shimla, Solan, and nearby areas. Manali sub-division, located in Himachal Pradesh, spans an altitude range of 1074 to 4017 meters above sea level. The region's climate is shaped by its mountainous terrain, with the Beas Valley's weather influenced by factors like relief, aspect, and altitude. The Pir Panjal Range's windward slopes create a barrier to monsoon winds, resulting in heavy rainfall and cloudbursts in the area. The report from the India Meteorological Department (IMD) in Shimla, Himachal Pradesh, on July 12, 2023, highlighted the active monsoon conditions in the state from July 7th to 10th, exacerbated by a Western Disturbance (WD). This combination led to intense and unprecedented rainfall, causing extensive damage to public and private properties, particularly in hilly regions. The report mentioned that similar disasters have occurred in the past due to heavy rainfall, cloudbursts, and landslides, possibly exacerbated by factors like unscientific construction, climate change, and increased human activities, including tourism. The report emphasized the need for accurate predictions and proactive planning to mitigate such disasters in the future.

Subsurface Marine Heatwaves in the South China Sea

AbstractMarine heatwaves (MHWs), extreme warm ocean temperature events, greatly impact marine ecosystems. While most MHW studies in the South China Sea (SCS) focus on the sea surface, subsurface characteristics remain less explored. This study uses high‐resolution (1/12°) ocean reanalysis data sets to assess MHW properties with depth in the SCS from 1993 to 2022. We find that MHW intensities are typically stronger at subsurface levels (30–150 m) than at the surface, with significant spatial variations. The vertical structures of MHW maximum and cumulative intensities peak between 70 and 100 m with 2.7°C and 63°C days, respectively; MHW annual days and duration increase gradually from 10 to 2,000 m with duration peaks at 2,000 m (∼60 days), four times longer than that at the surface. The SCS experienced two major periods of extensive El Niño‐related subsurface MHWs from 1997 to 2002 and 2008 to 2014, with MHWs reaching depths of 150 m and covering 70% of the upper 40 m. Strong El Niño events strengthen the western Pacific subtropical high, reducing summer monsoon winds, weakening coastal upwelling east of Vietnam, and increasing water temperatures. Regions of elevated eddy kinetic energy in the SCS suggest that significant mesoscale eddy activities are situated east of Vietnam. In addition, notable eddy heat transport was observed in the above region and west of the Luzon Strait in the upper 1,000 m. This may help to explain the high maximum and cumulative intensities of MHWs exceeding 100 m in the northern SCS.

Dynamic of upwelling variability in southern Indonesia region revealed from satellite data: Role of ENSO and IOD

The Southern Indonesian (SI) region is known for its high-intensity coastal upwelling caused by monsoonal wind. Interannual phenomena such as El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) also influence upwelling activity in this region. This study analyzed the relationship between upwelling intensity (UIsst) and those variables and their impact on oceanographic features such as Sea Surface Temperature (SST) and chlorophyll-a concentration. We used satellite imagery data, including SST from the National Oceanic and Atmospheric Administration (NOAA) and chlorophyll-a from MODIS, to analyze the aforementioned issue. To identify the impact of wind patterns on coastal upwelling, we analyzed using zonal wind stress from ERA-5 Data. Quantification of UIsst is defined as the SST gradient between the coastal and open ocean waters. Linear and partial correlation analysis between UIsst with the Ocean Niño Index (ONI) and Dipole Mode Index (DMI) were conducted to see the influence of ENSO and IOD phenomena. Anomaly analysis was also conducted on SST, chlorophyll-a concentration, zonal windstress and UIsst to see how large the values were during the years of the ENSO and IOD events. Upwelling in the SI region typically occurs during southeast monsoon (SEM) periods, starting earlier in the East side (Nusa Tenggara Islands) and moving towards the West side (South Coast of Java). The correlation analysis (both linear and partial) indicates that the IOD has a stronger influence on UIsst in the SI region compared to ENSO, especially during June to October (SEM periods). This finding is confirmed by anomaly analysis, which reveals significant changes in SST, chlorophyll-a concentration, zonal windstress, and UIsst during ENSO and IOD events. The magnitude of the anomalies is generally stronger during IOD events than those observed under ENSO conditions.

Near-Inertial Response of a Salinity-Stratified Ocean

Abstract We study the near-inertial response of the salinity-stratified north Bay of Bengal to monsoonal wind forcing using 6 years of hourly observations from four moorings. The mean annual energy input from surface winds to near-inertial mixed layer currents is 10–20 kJ m−2, occurring mainly in distinct synoptic “events” from April–September. A total of fifteen events are analyzed: Seven when the ocean is capped by a thin layer of low-salinity river water (fresh) and eight when it is not (salty). The average near-inertial energy input from winds is 40% higher in the fresh cases than in the salty cases. During the fresh events, 1) mixed layer near-inertial motions decay about two times faster and 2) near-inertial kinetic energy below the mixed layer is reduced by at least a factor of three relative to the salty cases. The near-inertial horizontal wavelength was measured for one fresh and one salty event; the fresh was about three times shorter initially. A linear model of near-inertial wave propagation tuned to these data reproduces 2); the thin (10 m) mixed layers during the fresh events excite high modes, which propagate more slowly than the low modes excited by the thicker (40 m) mixed layers in the salty events. The model does not reproduce 1); the rapid decay of the mixed layer inertial motions in the fresh events is not explained by the linear wave propagation at the resolved scales; a different and currently unknown set of processes is likely responsible.

Coastal water quality dynamics of the Red Sea, southeast coast of Egypt using GeoAI and ChatGPT

The Red Sea coastal environment of Halayeb and Shalateen area is renowned for its abundant marine flora and fauna. It also holds significant economic and cultural importance for local communities. However, this region is currently confronted with various challenges, including climate change and habitat destruction. To effectively address and mitigate these issues, advanced technologies that offer a holistic understanding of the area's environmental conditions are required. This paper applies the integration of Geospatial Artificial Intelligence (GeoAI) and ChatGPT to study the Red Sea Coastal water quality dynamics of Halayeb and Shalateen Area. Landsat imagery and Copernicus Marine Service were used to retrieve area boundaries and monitor the physicochemical characteristics of the coastal water respectively. ChatGPT was utilized to generate Python code that facilitates the creation of optimal distribution maps for each physical and chemical property criterion. The Python codes were incorporated into the Python program within the ArcGIS 10.7.1 and executed to generate the desired maps representing the dynamics of physical and chemical properties. It was found an observed fluctuation in chemical properties next to the coastline around the mouth of two main wadies; Wadi Hudain, and Wadi Da'eb. The degree of stability increased away from the coast toward the deep water. That proved the effect of the runoff on the seawater, as the runoff plays an essential role in the water state, especially in such semi-closed water bodies like the Red Sea where the flashfloods are the main source that can enrich water with sediment and nutrients. The state of seawater in terms of physical properties was not characterized by a specific pattern. The distribution of physical parameters in the Red Sea is influenced by factors such as regional climate variations, monsoonal winds, and local topography. This paper serves as a stepping stone for future research endeavors, exploring the full potential of this integrated approach. It can be concluded that the fusion of GeoAI and ChatGPT technologies has the potential to revolutionize our approach to studying and managing the coastal environment.

Increased Summer Monsoon Rainfall Over Northwest India Caused by Hadley Cell Expansion and Indian Ocean Warming

AbstractFrom 1979 to 2022, the summer monsoon precipitation has increased by a substantial 40% over Northwest India compared to the 1980s. This wetting trend aligns with the future projections of the Coupled Model Intercomparison Project 6 (CMIP6). The observationally constrained reanalysis data indicates that significant sea surface warming in the western equatorial Indian Ocean and the Arabian Sea is likely driving this increase in rainfall by enhancing the cross‐equatorial monsoonal flow and associated evaporation. We demonstrate that the strengthening of the cross‐equatorial monsoon winds is due to the rapid warming of the Indian Ocean and the enhanced Pacific Ocean trade winds, which result from the poleward shift and expansion of the Hadley cell. These strengthened winds boost the latent heat flux (evaporation), leading to increased moisture transport to Northwest India.

Two Characteristic Patterns of the Summer Indian Ocean Dipole

Abstract This study identifies two distinct patterns of the summer Indian Ocean dipole (IOD)—the coastal IOD and the offshore IOD—named based on the proximity of their eastern pole to Sumatra. Their spatial characteristics, evolutionary mechanisms, relationships with ENSO, impacts on precipitation, and the factors controlling the simulation performances of climate models are discussed. The coastal IOD has the same eastern pole as the conventional IOD, which is located off the coast of Sumatra, but its western pole is situated in the central region of the tropical Indian Ocean (TIO). In contrast, the offshore IOD shares the same western pole as the conventional IOD, which is located off the coast of Somalia, whereas its eastern pole is positioned in the central TIO. Regarding their evolutions, while they initially develop similarly, their later evolutions differ due to their distinct pole locations: The offshore IOD peaks in summer, while the coastal IOD can be sustained into autumn. The coastal IOD correlates to preceding and late ENSO states, but the offshore IOD does not, making it an independent internal mode of TIO. The two IODs affect climate differently, with only the coastal IOD affecting Australian rainfall. Climate models exhibit varied levels of performance in simulating the two IODs. Specifically, a stronger link between spring TIO rainfall and ENSO, as well as stronger southeasterly monsoon winds in the southern TIO, can enhance the coastal IOD modeling, while a stronger summer Somali jet benefits the simulation of the offshore IOD. Distinguishing these two IODs has implications for accurate diagnosis and prediction of the summer climate surrounding the TIO. Significance Statement The Indian Ocean dipole (IOD) significantly influences the monsoon climate. Previous studies use a unified IOD index based on all-season sea surface temperature (SST) data of tropical Indian Ocean (TIO), resulting in a representation biased toward the autumn peak season. However, understanding SST patterns during summer is crucial for Asian summer monsoon precipitation, and what are the characteristic patterns of summer IOD remains unclear. This study fundamentally advances the understanding of the IOD’s seasonal diversity by identifying two distinct dipole patterns in the summer TIO. These patterns differ from the conventional IOD in terms of spatial characteristics, evolutions, ENSO relationships, and precipitation impacts. Distinguishing between these two IODs is anticipated to improve the diagnosis and prediction of summer monsoon rainfall.

Quaternary floodings in the Zanzibar Channel (NW Indian Ocean, Tanzania) – Identifying palaeoceanographic patterns and palaeoenvironment using a multiproxy study

Multiproxy palaeoenvironmental analyses represent a promising tool for complex reconstructions of continent-sea or air-sea interactions in shallow marine environments. This is in particular illustrated by the case of strong climatic variations in the late Pleistocene to mid-Holocene. A unique combination of geochemical proxies (143Nd/144Nd, 87Sr/86Sr, δ18O and δ13C isotope analyses, n-alkane analysis and biomarker study) together with micropalaeontological data (foraminifera, calcareous nannoplankton and diatoms) allowed to precisely characterise two marine floodings (mid-Holocene and late Pleistocene) in the Zanzibar Channel (NW Indian Ocean, Tanzania). The climate of the late Pleistocene interval was arid compared to the Holocene. The fully oligotrophic environment characteristic of the late Pleistocene flooding (∼115–130 ka) was episodically interrupted by intervals of intense rainfall with episodes of increased nutrient input during the mid-Holocene (∼5–10 ka). The dominance of seaweed meadows in the Holocene and late Pleistocene contrasts with the modern environment dominated by seagrass ecosystems. Relatively non-radiogenic εNd signatures in the Zanzibar Channel during the Holocene and late Pleistocene indicate a strong influence of riverine input draining the Precambrian basement on the African mainland. The main inflow of seawater was from the south, consistent with the flow direction of the East African Coastal Current and the directions of the March to October monsoon winds. A promising tool for future applications as indicators of seagrass/seaweed meadow type ecosystems may be the presence of specific diatom taxa.

Contrasting responses of Indian summer monsoon rainfall and Arabian Sea upwelling to orbital forcing

Upwelling-induced productivity in the Arabian Sea infers an out-of-phase relation between the Indian summer monsoon and Northern Hemisphere summer insolation at the precession band but an in-phase relation at the obliquity band. These records contrast speleothem oxygen records showing an in-phase relation between the monsoon and summer insolation in two bands, leaving the driving mechanism for the Indian summer monsoon still unknown. Here we use an isotope-enabled Community Earth System Model, showing that the Indian summer monsoon rainfall and water isotopes respond directly to Northern Hemisphere summer insolation in two bands. This contrasts with the Arabian Sea upwelling which is weakened at precessional minimum but strengthened at obliquity maximum by a stronger northward shift of the monsoon wind due to the stronger summer insolation at precessional minimum compared with obliquity maximum. This paper resolves the longstanding question, supporting the Northern Hemisphere-driving mechanism for the Indian summer monsoon at orbital timescale.

Changes in the particulate organic carbon pump efficiency since the Last Glacial Maximum in the northwestern Philippine Sea

Changes in bottom and pore water oxygenation over glacial – interglacial cycles have influenced the ocean's capacity to store particulate organic carbon regardless of its source, either the marine primary productivity or the continent-to-ocean transfer of terrestrial organic matter. In the Philippine Sea, east off Taiwan, despite being currently oligotrophic, the enhanced East Asian Winter Monsoon during the Last Glacial Maximum and the Heinrich Stadial 1 might have altered the nutrient budget in surface waters by providing nutrients from the Eurasian loess dust and deepening the vertical mixing, bringing nutrients from the nutrient-enriched Kuroshio Current subsurface waters to the surface. During the deglaciation, previous studies also suggest an overall weakening of the marine biological pump during the Heinrich Stadial 1, and the rise in sea level is expected to have led to a global significant decline in the ability of continents to bury their particulate organic carbon in marine sediments. However, changes in the continent-ocean transfer of terrestrial organic matter and on the marine biological pump around Taiwan remain poorly constrained.In the present study, we have thus aimed to reconstruct bottom – pore water oxygenation, past marine primary productivity and continental-ocean transfer of terrestrial particulate organic carbon to the ocean since the end of the Last Glacial Maximum, in order to better constrain the ability of marine sediments to capture atmospheric carbon over the past 20,000 years. To this end, sediment core MD18-3523 has been recovered from a levee of Hoping Canyon, north-east of Taiwan, in the Ryukyu forearc basin. The reconstructions were made possible by the application of multivariate statistics and transfer functions on benthic foraminiferal assemblages, by the measurement of total organic carbon concentration and by the investigation of chemical element ratios obtained from X-ray fluorescence (XRF).We observed a transition across the Bølling–Allerød and the Younger Dryas from suboxic-dysoxic bottom – pore waters during Heinrich Stadial 1 to oxic-suboxic during the Holocene, and revealed an increase in marine primary productivity during Heinrich Stadial 1 in all probability due to intensified East Asian Winter Monsoon winds. We have also identified periods of enhanced terrestrial particulate organic carbon transfer to the ocean driven by short-lived extreme events, most likely typhoons, during the Bølling–Allerød, at the beginning of the Early Holocene and the end of the Late Holocene, when the typhoon dynamics affecting Taiwan were intensified. Overall, these findings suggest an enhanced marine biological pump during the Heinrich Stadial 1 and an efficient carbon turbidity pump during the Bølling–Allerød, the Early and Late Holocene, contrasting with the western coast of Taiwan.

The impact of monsoon on the landfalling tropical cyclone persistent precipitation in South China

Interactions between landfalling tropical cyclones (TCs) and monsoons in South China significantly influence precipitation duration, leading to severe disasters. Previous studies have primarily been individual cases, lacking systematic large-scale statistical analysis of the monsoon and landfalling tropical cyclone persistent precipitation (LTCPP) relationship. This study quantitatively investigated the relationship between monsoonal wind intensity before TCs landfall and post-landfall persistent precipitation induced by TCs in South China, employing the ERA5 reanalysis data and the best track data of 147 TCs from 1979 to 2018. The LTCPP was characterized by the frequency of persistent precipitation events during 0–72 h after TC landfall within a 500 km radius from the TC center. TCs were subdivided into weak and strong LTCPP groups based on the category-specific median of Frequency of 24 h Landfalling Tropical Cyclone Persistent Precipitation (FLTCPP24): 2705 h for TS, 6007 h for STS, and 6419 h for TY. A South China Tropical Cyclone Precipitation Monsoon Index (SCTCPM) was proposed to quantify monsoonal wind intensity derived from zonal winds at 850 hPa over two regions located in the Indian Ocean and Northwestern Pacific Ocean, within 5 d before TC landfall. The results reveal that SCTCPM < 9 m s−1 yields a 72% probability of weak LTCPP occurrence, which increases to 77% when SCTCPM < 6 m s−1. Conversely, SCTCPM > 18 m s−1 corresponds to an 80% probability of strong LTCPP. SCTCPM is an effective indicator for monsoonal wind that impacts LTCPP. Enhanced monsoonal winds, quantified by higher SCTCPM, result in post-landfall changes in horizontal wind speed, moisture transport, convective activity and upward motion, ultimately increasing LTCPP. This study deepens our understanding of the monsoon-TC relationship, emphasizing the crucial role of monsoonal wind in LTCPP in South China and offering valuable insights for disaster preparedness and risk mitigation.

Maritime Activities of the Demak Sultanate: Shipping and Trade Route in the Nusantara Network (1478-1546)

This study endeavours to explain the geostrategic conditions of the north coast of Java, as well as the activities and maritime policies of the Demak Sultanate. It adopts a historiographic approach comprising heuristics based on the book Suma Oriental as a primary source and other relevant secondary sources; in-depth verification and interpretation of data from various sources; and methodical historical writing. The results illustrate (1) the geostrategic conditions of the north coast of Java, which is in the middle of the Nusantara spice route, helped make Demak and other port cities an important emporium. The region is traversed by monsoon winds that support shipping activities and has abundant teak wood resources for the shipbuilding industry. (2) Demak’s maritime activities included military expeditions to conquer the Portuguese in Malacca and several important emporiums on the north coast of Java; trade expeditions to export-import various commodities; and establishing trade partnerships with other regions, especially Malacca and Maluku. (3) Raden Patah’s maritime policy included the deployment of military expeditions to conquer the Portuguese in Malacca in 1512, establishing trade partnerships with Malacca, producing Jung ships, and building port cities on the north coast of Java as important emporiums; Pati Unus mobilized a military expedition to conquer the Portuguese in Malacca in 1521 and formed the Demak-Cirebon-Palembang military alliance; and Trenggana conquered Lampung and Banten to control the pepper trade until the conquest of the Sunda Kelapa emporium under the Pajajaran kingdom.

Impact of Asian aerosols on the summer monsoon strongly modulated by regional precipitation biases

Abstract. Reliable attribution of Asian summer monsoon variations to aerosol forcing is critical to reducing uncertainties in future projections of regional water availability, which is of utmost importance for risk management and adaptation planning in this densely populated region. Yet, simulating the monsoon remains a challenge for climate models that suffer from long-standing biases, undermining their reliability in attributing anthropogenically forced changes. We analyze a suite of climate model experiments to identify a link between model biases and monsoon responses to Asian aerosols and associated physical mechanisms, including the role of large-scale circulation changes. The aerosol impact on monsoon precipitation and circulation is strongly influenced by a model's ability to simulate the spatio-temporal variability in the climatological monsoon winds, clouds, and precipitation across Asia, which modulates the magnitude and efficacy of aerosol–cloud–precipitation interactions, an important component of the total aerosol response. There is a strong interplay between South Asia and East Asia monsoon precipitation biases and their relative predominance in driving the overall monsoon response. We found a striking contrast between the early- and late-summer aerosol-driven changes ascribable to opposite signs and seasonal evolution of the biases in the two regions. A realistic simulation of the evolution of the large-scale atmospheric circulation is crucial to realize the full extent of the aerosol impact over Asia. These findings provide important implications for better understanding and constraining the diversity and inconsistencies of model responses to aerosol changes over Asia in historical simulations and future projections.

Incidence of ghost nets in the Tioman Island Marine Park of Malaysia

Ghost nets are major contributor to ocean pollution with extensive social, economic and environmental impacts. Ghost nets trap marine life, build up sediment, and smother and damage sensitive bottom habitats such as coral reefs and seagrass beds. Data on ghost nets are widely available for many coastal locations but there has been very limited information from Malaysia specifically Tioman Island. In 2015, Reef Check Malaysia started training local islanders to locate and remove ghost nets from reefs and beaches around Tioman Island. A reporting hotline was set up to gather information on ghost nets. Once vital information was gathered, the local islanders retrieved them and recorded the ghost nets and sent them for proper disposal. A total of 145 ghost nets weighing over 21 t were retrieved from Tioman Island from 2016 to 2022. The volume of ghost nets retrieved showed an increasing trend and were often found in March, April and September. The ghost nets originated from fishermen operating illegally inside the marine protected area (MPA) and legal fishermen operating outside the MPA. Ghost nets find their way to Tioman Island with strong currents and monsoonal winds. Most of the ghost nets retrieved were inside the MPA and on the western side of Tioman Island. Higher human population, shallower waters, shoreline morphology, dominant coral growth design and reef rugosity on the western side of Tioman Island are reasons for higher ghost nets sightings, thus retrieval and management efforts should focus on this side.

Insights into the seasonal changes in the taxonomic and functional diversity of bacteria in the eastern Arabian Sea: Shotgun metagenomics approach

The eastern Arabian Sea (EAS) is known for its unique oceanographic features such as the seasonal monsoonal winds, upwelling of nutrient-rich waters and a significant increase in primary productivity during the monsoon season. In this study, we utilised the shotgun metagenomics approach to determine the seasonal variations in bacterial taxonomic and functional profiles during the non-monsoon and monsoon seasons in the EAS. Significant seasonal variations in the bacterial community structure were observed at the phylum and genera levels. These findings also correspond with seasonal shifts in the functional profiles of the bacterial communities based on the variations of genes encoding enzymes associated with different metabolic pathways. Pronounced seasonal variation of bacterial taxa was evident with an increased abundance of Idiomarina, Marinobacter, Psychrobacter and Alteromonas of Proteobacteria, Bacillus and Staphylococcus of Firmicutes during the non-monsoon season. These taxa were linked to elevated nucleotide and amino acid biosynthesis, amino acid and lipid degradation. Conversely, during the monsoon, the taxa composition changed with Alteromonas, Candidatus Pelagibacter of Proteobacteria and Cyanobacteria Synechococcus; contributing largely to the amino acid and lipid biosynthesis, fermentation and inorganic nutrient metabolism which was evident from functional analysis. Regression analysis confirmed that increased seasonal primary productivity significantly influenced the abundance of genes associated with carbohydrate, protein and lipid metabolism. These highlight the pivotal role of seasonal changes in primary productivity in shaping the bacterial communities, their functional profiles and driving the biogeochemical cycling in the EAS.

Spring circulation characteristics and formation mechanism in the Beibu Gulf

Spring circulation in the Beibu Gulf remains an underinvestigated research topic. Model results validated by observations reveal that the northern and southern parts of the Beibu Gulf are controlled by an enclosed cyclonic gyre and a cyclonic gyre, respectively, while in the vertical, the circulation at the surface differs from those in the middle and deep layers. Contrary to that of the classical view, the modeled Qiongzhou Strait (QS) current flows eastward during some days in spring. This eastward current has an occurrence proportion of 29.3–55.4%, and its longest duration period is 7~18 d. The average current in the QS in spring is approximately westward except in some typical years. In spring, the northern Beibu Gulf is affected by the cold water mass (CWM), which is depicted by the 12°C and 10°C isolines in strong and weak years, respectively. Sensitivity experiments show that the heat flux is dominant in the northern Beibu Gulf via the effect of the CWM, and the monsoon wind favors the formation of the cyclonic gyre. Conversely, the cyclonic gyre in the southern Beibu Gulf is jointly driven by the monsoon wind and heat flux, with the effect of the monsoon wind being more significant. The current off the northern coast of Vietnam (VNC) can be either northeastward or southwestward in spring in different years. The VNC is mainly controlled by the elevation gradient along the northern coast of Vietnam, and the Coriolis force, horizontal advection, and vertical diffusion also play significant roles. Additionally, the CWM is mainly controlled by the heat flux whereas the monsoon wind plays a secondary role. And the VNC, current in the QS, CWM, and cyclonic gyre jointly affect nutrient distributions in the northern Beibu Gulf.