Weak Southwest Monsoon Research Articles

Impact of local and remote winds on the surface waves in the coastal waters of eastern Arabian Sea

This research aims to provide analysis of wave propagation and the interaction between local and remote winds on surface waves in the coastal waters of the eastern Arabian Sea. The analysis is based on measurements from a buoy deployed at 23 m water depth along with the data from the ERA5 re-analysis. By employing lead-lag correlation analysis techniques, we identified the origin and propagation routes of waves in this region. The analysis identified the western Indian Ocean as the primary source of the waves. However, the impact of northerly systems such as Shamal and Makran winds cannot be ignored, as they play a prominent role during the pre- and post-monsoon seasons and weak southwest monsoon periods. Moreover, the study focuses on event-based analysis of tropical cyclone Tauktae based on cyclone's track, correlation analysis of its impact, and the spatial lag correlation of the associated wave system. The study builds upon existing knowledge concerning the generation and propagation of swells in the Indian Ocean. The findings of this research will contribute to a better understanding of wave dynamics in the eastern Arabian Sea and have implications for coastal engineering and marine operations in the region.

Abnormal depletion of terrestrial water storage and crustal uplift owing to the 2019 drought in Yunnan, China

SUMMARYDroughts are natural disasters that cause severe economic and social impacts. Analysing changes in terrestrial water storage (TWS) before and after typical drought events is important for understanding the principles of the regional terrestrial water transport law and drought response, and for providing scientific agricultural production guidance. Yunnan is a drought-prone region, and high temperatures and low rainfall in 2019 led to a severe drought. Precipitation data showed that the wet season in Yunnan started 1 month later than usual in 2019. Moreover, 52 per cent (205 000 km2) of the total area of Yunnan experienced a cumulative precipitation anomaly (PA; in percentage) of −30 per cent between January and May. In this study, we used data from the Gravity Recovery and Climate Experiment (GRACE) and Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) missions to identify the spatial and temporal variations in TWS before and after the 2019 drought. Our results show that the drought led to an average reduction of 125 mm (equivalent water height) in the TWS, with the greatest loss in southern Yunnan, with a maximum loss of 272 mm. The reduced TWS caused by the delayed and anomalously weak southwest monsoon in 2019 gradually spread from the southwest to the northeast. By comparing with the drought in 2009–2010, we find that the southern Yunnan with abundant rainfall is more vulnerable to drought and has a relatively larger deficit in TWS. Furthermore, we used GRACE and GRACE-FO data to invert the crustal uplift caused by terrestrial water unloading in the 2019 drought, and found a maximum uplift of 8.4 mm. This showed a consistent spatial distribution with displacement variations recorded by 26 continuous Global Positioning System (GPS) stations in Yunnan during the same period, albeit slightly different in magnitude, indicating the potential of GPS for monitoring regional TWS changes.

Palaeoecological changes from 580 to 1220 CE from the Indo-Burma region: A biotic assessment from the Barak Valley of Assam, northeast India

The pollen and diatom analyses of soil samples from a 80-cm deep sedimentary core from the Barak valley of Assam provides an explicit understanding of vegetation, climatic and ecological change in the Indo-Burma region from 580 CE (1370 cal. years BP) to 1220 CE (730 cal. years BP). The impact of the Medieval Warm Period (MWP) has been well documented from palynological records. Based on changes in vegetation succession, the Mean Annual Temperature (MAT) and Mean Annual Precipitation (MAP) during four pollen phases CP (1–4) were calculated using the coexistence approach. Between 580 and 760 CE (pre-MWP), the occurrence of major riparian tree pollen taxa like Barringtonia, Duabanga and Sapotaceae along with scanty diatom occurrence indicates a warm and relatively less humid climate attributable to a weak Southwest monsoon with low seasonality. This changed to a strikingly enhanced seasonality from 760 to 940 CE (beginning of MWP), indicating the establishment of the dense forest around the lake under increased warm and humid climatic conditions. During the interval from 940 to 1220 CE (MWP peak), the average values of maximum MAT and MAP were the highest recorded (≈31 °C and ≈2250 mm, respectively). From 1220 CE onwards (post MWP), the decline in arboreal pollen coincides with an exponential rise in cereal pollen clumps and shrubby pollen like Melastoma, Clerodendrum and Justicia adhatoda along with the presence of anthropogenic indicator diatom taxa like Ulnaria ulna and Gomphonema indicating increased landscape changes due to human impact under a relatively less warm and humid climate. In many parts of the world, regional data coverage pertinent to the MCA warming has now reached a point which allows compiling palaeoclimate maps for well-defined time intervals. In this direction, future studies need to address the major climatic data gaps from the Indian sub-continent especially during the last millennia.

A Prolonged High-Salinity Event in the Northern Arabian Sea during 2014–17

AbstractA prolonged high-salinity event in the northern Arabian Sea, to the east of the Gulf of Oman, during 2014–17 was identified based on Argo datasets. The prolonged event was manifested as enhanced spreading of the surface Arabian Sea high-salinity water and the intermediate Persian Gulf water. We used satellite altimetric data and geostrophic current data to understand the oceanic processes and the salt budget associated with the high-salinity event. The results indicated that the strengthened high-salinity advection from the Gulf of Oman was one of the main causes of the salinity increase in the northern Arabian Sea. The changes of the seasonally dependent eddies near the mouth of the Gulf of Oman dominated the strengthened high-salinity advection during the event as compared with the previous 4-yr period: the westward shifted cyclonic eddy during early winter stretched to the remote western Gulf of Oman, which carried the higher-salinity water to the northern Arabian Sea along the south coast of the Gulf. An anomalous eddy dipole during early summer intensified the eastward Ras Al Hadd Jet and its high-salinity advection into the northern Arabian Sea. In addition, the weakened low-salinity advection by coastal currents along the Omani coast caused by the weakened southwest monsoon contributed to the maintenance of the high-salinity event. This prolonged high-salinity event reflects the upper-ocean responses to the monsoon change and may affect the regional hydrography and biogeochemistry extensively.

Mesoscale eddy variability and its linkage to deep convection over the Bay of Bengal using satellite altimetric observations

In the Bay of Bengal (BoB), surface circulation is strongly influenced by the Indian Monsoon and notable local eddying that modulates the East India Coastal Current (EICC). In this study, we apply an eddy tracking algorithm to 25 years (1993–2018) of satellite altimetric observations to identify, analyze, and track mesoscale eddies in the BoB from their generation to dissipation. We compare the characteristics and trajectories of these eddies during the southwest (SW) and northeast (NE) monsoon seasons and during the pre- and post-SW monsoon periods to better understand the seasonality of the local eddy field. We find high eddy generation in the eastern BoB associated with instability induced by coastal Kelvin waves and the westward propagating Rossby waves, but we found the most robust eddies in the western BoB around the EICC. Ocean heat content (OHC) in the BoB was compared with warm-core eddies and we find that warm-core eddies drive changes in OHC. We also compared the eddy field during strong and weak SW monsoon forcing and with varying Indian Ocean Dipole (IOD) and El Niño-Southern Oscillation (ENSO) conditions, finding a statistically significant relationship with warm-core eddies and sea surface temperatures (SSTs; R2 = 91%) and outgoing longwave radiation (OLR; R2 = 88%) during periods of strong ENSO and IOD forcing. To understand the impact of these eddies on local atmospheric convection we found the composite surface structure of anticyclonic and cyclonic eddies of warm-core and cold-core SSTs, finding a close relationship between the eddy-composite SSTs and OLR.

The variability of chlorophyll-a and its relationship with dynamic factors in the basin of the South China Sea

Abstract Satellite observations from 2002 to 2017 were used to investigate the spatial distribution and temporal variability of chlorophyll-a (Chl-a) in the South China Sea (SCS). High levels of Chl-a were mostly found near the coasts of China and Vietnam, with a significantly long-term increasing trend. Large seasonal variability was found in the northern SCS and southeast of Vietnam. The Chl-a levels reached the maximum in winter and minimum in summer for majority of the SCS, while the seasonal variability to the southeast of Vietnam was out of phase. The monsoon winds and sea surface temperatures were the most important determinants impacting the distribution and variability of Chl-a along with other associated and influential environmental drivers, e.g., wind stress curl, frontal activity, and sea level anomalies. High correlation coefficients for the seasonal variability between Chl-a and other factors were found in a majority of the SCS, especially in the northern and central parts. The coefficients to the southeast of Vietnam were not valid at seasonal scales, but Chl-a and other factors were significantly correlated at the monthly anomalous fields. This observation occurred because the nutrient supply was mainly determined by local dynamics, e.g., wind-induced coastal upwelling and offshore transport. The interannual variability indicated low levels of Chl-a southeast of Vietnam during El Nino years because of the weakened southwest monsoon. The study offered the first comprehensive description of Chl-a in the SCS at seasonal, anomalous, and interannual variability scales and an analysis of the potential contributing dynamical processes.

Late Holocene long arid phase in the Indian subcontinent as seen in shallow sediments of the eastern Arabian Sea

Multi-proxy record of benthic and planktic foraminifera, total organic carbon and carbon and oxygen isotope ratios from Core SK291/GC15, off the coast of Goa, eastern Arabian Sea reveals significant paleoceanographic and paleoclimatic turnovers during ~6000 to 1700 calibrated years before the Present (cal yr BP). Benthic foraminiferal census data was analyzed with multivariate techniques including factor and cluster analyses of highest ranked species from the studied core that enabled to identify seven biofacies. The record suggests centennial to millennial-scale changes in the surface conditions driven by monsoon-linked upwelling in the eastern Arabian Sea during the studied interval. Various benthic biofacies combined with isotope and planktic foraminiferal data suggest that depletion of dissolved oxygen and increase in organic productivity in the study area was caused by intense monsoonal upwelling since the middle Holocene. The results suggest that during 5400–4700 and 3000–2500 cal yr BP the southwest (SW) monsoon was strong leading to intense upwelling in the study area as reflected by increased Globigerina bulloides percentages. These were generally warmer intervals in the northern hemisphere. The SW monsoon significantly weakened (abrupt decrease in G. bulloides population) during 4700–3400 cal yr BP roughly coinciding with a long arid phase in the Indian subcontinent and a cold interval in Europe. Our record suggests an abrupt increase in SW monsoon intensity during ~3000 to 2500 cal yr BP, when a pronounced change in benthic biofacies is also noted that suggests a major shift in the sub-surface conditions. Our data document a prolong weak SW monsoon phase beginning at ~4600 cal yr BP, little earlier than 4.2 ka event, in the eastern Arabian Sea.

Study of temporal variability and mass closure of PM2.5 and its chemical constituents during weak south-west monsoon

Ambient PM2.5 (n = 42) samples have been collected during a weak South-west (SW) monsoon (July‒September, 2015) from central part of the Indo-Gangetic Plain (IGP; at Kanpur). Organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), water-soluble inorganic species (WSIS) and metals have been measured in this study. Air-mass back trajectory (AMBT) analysis reveals atmospheric transport from three major pathways indicated as Type-1 (n = 24; traversing through western region), Type-2 (n = 8; traversing eastern region) and Type-3 (n = 10; terrestrial air-masses). Total carbonaceous aerosols (TCA), mineral dust (MD) and WSIS in Type-1 air-mass contributed 18.7 ± 8%, 40.3 ± 14.5% and 35.2 ± 13.4%, respectively to the PM2.5 mass. TCA, MD and WSIS mass fractions constituted 27.1 ± 13.2%, 29.1 ± 7.9% and 34.9 ± 10.7% in Type-2 air-mass and 25.2 ± 14.1%, 34.6 ± 14% and 33.3 ± 9.8% in Type-3 air-mass, respectively. Mineral dust percentage fraction and composition (based on Fe/Al ratio) suggests significant differences between Type-1 and Type-2 air-masses. Based on statistical two-tailed t-test, the mineral dust composition of Type-1 versus Type-3 and Type-2 versus Type-3 air-masses look near similar. However, average mass ratios of OC/EC (6.2–6.4), WSOC/OC (≈0.50; exception being difference in Type-2 versus Type-3) and K+BB/OC (0.05–0.07; K+BB: biomass burning derived potassium) look near similar in PM2.5 associated with all the three air-masses; indicating carbonaceous aerosols from well-mixed sources with dominant contribution from biomass burning emission. A significant abundance of organic aerosols and mineral dust in presence of high moisture content and their mixing over IGP has implications to perturbation in regional scale radiative forcing.

Synergistic effect in absorption properties of brown carbon and elemental carbon over IGP during weak south-west monsoon

Recent ground-based measurements reveal that model-based observations are underestimating absorption properties and direct radiative forcing (DRF) due to carbonaceous species by a factor of 2–3 over South and East Asia. Thus, to better constraint these parameters associated with carbonaceous species, seasonal variability records through ground-based measurements are very essential. In this context, we report herein the absorption properties of water-extractable brown carbon (BrC), elemental carbon (EC), and BrC + EC and DRF of BrC and BrC + EC (relative to EC) over the Indo-Gangetic Plain (IGP) during a weak monsoon season [July–September 2015; influenced by El Nino and PDO (Pacific Decadal Oscillation)]. PM2.5 (particulate matter with aerodynamic diameter ≤ 2.5 μm) samples (n = 31) have been assessed from central IGP location at Kanpur. Absorption coefficient at 365 nm (b abs-365) of BrC, EC, and BrC + EC centers at 3.6, 8.1, and 11.4 Mm−1, respectively. Strong linear regression correlation (R = 0.8) of b abs-365 of BrC with sea-salt and mineral dust corrected potassium ( $${\text{K}}^{ + }_{\text{BB}}$$ ) indicates biomass burning as the predominant source of BrC over the region in this study. Synergistic effect in b abs of BrC + EC (relative to that of EC) increases conspicuously (enhanced by a factor varying from 1.05 to 1.21) with an increase in wavelength from UV (365 nm) to visible region (660 nm). DRF (relative to EC) of BrC during monsoon season ranges from 3.9 to 23.8 (13.0 ± 5.0) %, whereas total DRF (BrC + EC) ranges from 111.0 to 148.2 (126.6 ± 10.0) %. Individual contribution of BrC and EC to total DRF has been estimated as 10 and 79.5%, respectively. The remaining contribution (10.5%) to total DRF has been attributed to synergism in absorption properties (i.e., b abs) of BrC + EC.

A modeling study of the characteristics and mechanism of the westward coastal current during summer in the northwestern South China Sea

The characteristics and dynamical mechanism of summer-time coastal current over the North South China Sea shelf have been investigated based on a high resolution unstructuredgrid finite volume community ocean model (FVCOM). Modeldata comparison demonstrates that the model describes and explains well the coastal dynamics over the North South China Sea shelf. The coastal current on the North South China Sea shelf is greatly influenced by monsoon and the freshwater discharge of the Pearl River. Strong southwesterly wind drives the coastal current northeastward. However, under weak southwest monsoon, the coastal current west of the Pearl River estuary (PRE) advects toward the southwest, and splits into two parts when reaching east of the Qiongzhou Strait, with one branch entering the Gulf of Tonkin through the Qiongzhou Strait, transporting low salinity water into the Gulf of Tonkin, and the other part flows cyclonic and interacts with the northeastward current around southeast of Hainan Island, forming a cyclonic eddy east of the Qiongzhou Strait. A variety of model experiments focused on freshwater discharge, wind forcing, tidal rectification, and stratification are performed to study the physical mechanism of the southwestward coastal current which is usually against the summer wind. Process-oriented experiment results indicate that the southwest monsoon and freshwater discharge are important factors influencing the formation of southwestward coastal current during summer.

Changing monsoon and midlatitude circulation interactions over the Western Himalayas and possible links to occurrences of extreme precipitation

Historical rainfall records reveal that the frequency and intensity of extreme precipitation events, during the summer monsoon (June–September) season, have significantly risen over the Western Himalayas (WH) and adjoining upper Indus basin since 1950s. Using multiple datasets, the present study investigates the possible coincidences between an increasing trend of precipitation extremes over WH and changes in background flow climatology. The present findings suggest that the combined effects of a weakened southwest monsoon circulation, increased activity of transient upper-air westerly troughs over the WH region, enhanced moisture supply by southerly winds from the Arabian Sea into the Indus basin have likely provided favorable conditions for an increased frequency of certain types of extreme precipitation events over the WH region in recent decades.

Monsoon-influenced variations in productivity and lithogenic flux along offshore Saurashtra, NE Arabian Sea during the Holocene and Younger Dryas: A multi-proxy approach

A sediment core from NE Arabian Sea was studied to reveal the variations in shelf environment of western continental margin of India. A multi-proxy analysis based on planktonic foraminifera abundance (PFA), benthic foraminifera abundance (BFA), planktonic-benthic foraminiferal ratio (P/B), CaCO3 percentage (wt%), Oxygen (δ18OG. ruber) and Carbon (δ18OG. ruber) isotopes of foraminifera as well as sediment X-ray studies were carried out on 3.4m long core SK-240/485 recovered from offshore Saurashtra, NE Arabian Sea. The study aims to understand the monsoon variability in the NE Arabian Sea offshore Saurashtra since the late Pleistocene-Holocene transition and how monsoon climate coupled with sea-level change influenced paleo-productivity pattern. Six AMS radiocarbon dates place the core in the time interval 12–1Ka. Our study reveals that there have been three periods of major changes in the monsoonal climate, productivity and sea level; viz. 12–8Ka (Younger Dryas and early Holocene); 8–4Ka (middle Holocene) and 4–1Ka (late Holocene). The studied time period was also marked by weakened south west monsoon (SWM) during the cold events such as Younger Dryas (YD), Abrupt Event (AE) centring around 9.8Ka as well as 8.2 and 4Ka cold events. The Younger Dryas and early Holocene shows the low sea level stand and less Indus river discharge. Thereafter during middle and late Holocene the monsoon strengthened which resulted in raised sea level and increased Indus river discharge. The proxy records broadly suggest relatively low foraminiferal productivity at the core location during the Younger Dryas and early Holocene. We record a moderate foraminiferal productivity during the middle Holocene between 8 and 4Ka, which suggest the mesotrophic condition. The foraminiferal productivity however was relatively high since 4Ka suggesting the eutrophic conditions at the core site.

Human impact on low-land Vellayani Lake, south India: A record since 3000 yrs BP

Abstract Humans have continuously changed the landscape and vegetation cover of the earth, including deserts and lake margins. Vellayani Lake, Kerala, is being severely affected by human activity in the catchment basin. Population increase, leading to increased demand for agricultural land and water, is driving a positive feedback loop resulting in the accelerating siltation of the lake and lowering of water levels. A 145 cm sediment core represents deposition since 3000 yrs BP, but at least half of the sediment has been deposited since 1650 AD, possibly since 1950 AD. The sediment stratigraphy is disturbed, most likely by slumping since 1970, of an unstable sediment accumulation near the lake margin. The disturbance precludes detailed interpretation of paleoenvironmental proxies, textural variation, chemical weathering index, pollen and phytolith assemblages. The proxies provide evidence of drier climate up to 3000 yrs BP, and wetter conditions at about 1210 yrs BP. Low frequency of phytoliths morph types indicates reduction in vegetation cover and a significant increase in grassland since 2300 yrs BP due to climate warming, weakened southwest monsoon, deforestation reclamation of the lake margins for intensive agricultural practices. Occurrence of diatoms and sponge spicules indicate shallowing of the lake from 2300 yrs BP onwards. Occurrence of charcoal pieces post 3000 yrs BP represents human impact. Integration of all the results show that the Vellayani Lake has contracted because of a weakened Indian southwest monsoon since 3000 yrs BP, intense human impact in the form of deforestation, irrigation and agricultural practices.

Monsoon-influenced variation in productivity and lithogenic sediment flux since 110 ka in the offshore Mahanadi Basin, northern Bay of Bengal

The Indian monsoon drives seasonal changes in precipitation and weathering across India as well as circulation and productivity in the northern Indian Ocean. Variation in paleo-monsoon intensity and its effect on productivity and lithogenic fluxes is poorly constrained in the Bay of Bengal. In this paper, we present analysis of a sediment record from the offshore Mahanadi Basin recovered during the Indian National Gas Hydrate Program Expedition 01 (Site NGHP-01-19B). We reconstruct variation in biogenic and lithogenic components during the last 110 kyr using measurements of total organic carbon (TOC), total nitrogen (TN), TOC/TN, CaCO3, biogenic silica (BSi), δ13TOC, δ15TN, bulk mineralogy from X-ray diffraction, bulk and lithogenic grain size distribution, magnetic susceptibility, bulk density, and Ca, Br, and Zr/Rb from x-ray fluorescence (XRF). The mass-accumulation rate (MAR) of CaCO3, a function of marine productivity, drastically increased between 70 and 10 ka and is correlated to previously-documented elevated Asian dust fluxes and increased Bay of Bengal salinity during a weakened southwest monsoon. Decreased freshwater input over this period likely diminished stratification, allowing for increased mixing and nutrient availability, thus enhancing productivity despite weaker southwest monsoon winds. The MAR of lithogenic material is highest during the Holocene suggesting that sediment supply driven by monsoon intensity is a stronger control on margin sedimentation than sea level at the Mahanadi Basin. Over the entire record, magnetic susceptibility and XRF Zr/Rb are strongly correlated with CaCO3, suggesting higher primary mineral input under a weakened southwest monsoon. TOC/TN and δ13TOC also increase under glacial conditions, suggesting higher relative input of terrestrial C4 organic matter. These results highlight the Mahanadi Basin as a supply-dominated margin where terrigenous sedimentation is strongly influenced by monsoon intensity, and that productivity is limited by variation in monsoon-driven stratification on glacial-interglacial timescales rather than a direct response to monsoon winds.

Latest Pleistocene and Holocene vegetation and climate history inferred from an alpine lacustrine record, northwestern Yunnan Province, southwestern China

Reconstructing past changes of the summer monsoon in southwestern China is key to understanding the paleoclimatic dynamics of the Indian summer monsoon (southwest monsoon), a major tropical system with impacts beyond the region. High-resolution pollen and conifer stoma data are presented from a sediment core (Tiancai Lake), located just below treeline in the Hengduan Mountains, northwestern Yunnan Province of China. These data record changes in vegetation and climate for the latest Pleistocene and Holocene (c. the last 12,230 cal. yr BP), and by comparing these results with the pollen records from the nearby Erhai Lake and Lugu Lake, a regional climate history in Yunnan Province can be inferred. During the period of 12,230–11,510 cal. yr BP, open alpine meadow around Tiancai Lake indicates a relatively cold and dry climate, corresponding to the Younger Dryas cold event (YD). Between 11,510 and ∼10,000 cal. yr BP, the vegetation types around Tiancai Lake changed into Picea and Abies forest and alpine Rhododendron shrubland, reflecting increases in the temperature and humidity. From ∼10,000 to 6100 cal. yr BP, Tsuga forest expanded persistently toward Tiancai Lake, indicating further climate changed towards warmer and wetter conditions. Between 6100 and 3410 cal. yr BP, Tsuga forest was the dominant vegetation type nearest to Tiancai Lake, denoting warm and humid climatic conditions, corresponding to the Holocene climatic optimum in the northwestern Yunnan Province. Tsuga forest in mountains around Lugu Lake shrank quickly at 3410 cal. yr BP, reflecting significant decline of humidity. Since 2930 cal. yr BP, Tsuga forest around Tiancai Lake shrank gradually, indicating a fluctuating decrease of temperature. These climatic changes reflect changes of the southwest monsoon, namely, warm and humid climate indicating strong southwest monsoon, cold and dry climate denoting weak southwest monsoon.

Role of Ekman transport versus Ekman pumping in driving summer upwelling in the South China Sea

Relative roles of Ekman transport and Ekman pumping in driving summer upwelling in the South China Sea (SCS) are examined using QuikSCAT scatterometer wind data. The major upwelling regions in the SCS are the coastal regions east and southeast of Vietnam (UESEV), east and southeast of Hainan Island (UESEH), and southeast of Guangdong province (USEG). It is shown that the Ekman transport due to alongshore winds and Ekman pumping due to offshore wind stress curl play different roles in the three upwelling systems. In UESEV, Ekman pumping and Ekman transport are equally important in generating upwelling. The Ekman transport increases linearly from 0.49 Sv in May to 1.23 Sv in August, while the Ekman pumping increases from 0.36 to 1.22 Sv during the same period. In UESEH, the mean estimates of Ekman transport and Ekman pumping are 0.14 and 0.07 Sv, respectively, indicating that 33% of the total wind-driven upwelling is due to Ekman pumping. In USEG, the mean Ekman transport is 0.041 Sv with the peak occurring in July, while Ekman pumping is much smaller (0.003 on average), indicating that the upwelling in this area is primarily driven by Ekman transport. In the summers of 2003 and 2007 following El Nino-Southern Oscillation (ENSO) events, both Ekman transport and Ekman pumping decrease in UESEV due to the abnormally weak southwest monsoon. During the same events, however, Ekman transport is slightly enhanced and Ekman pumping is weakened in UESEH and USEG.

Monsoonal changes inferred from the Middle to Late Holocene sediments off Landfall Island, North Andaman

In this paper, sediment texture, clay mineral composition, and δ18O data on Globigerinoides ruber of a sediment core collected from a water depth of 250 m near Landfall Island, Bay of Bengal, is presented to understand paleoenvironmental shifts during the Mid–Late Holocene period. From the sediment core, five organic carbon-rich samples were radiocarbon dated and the reservoir-corrected ages range from 6,078 to 1,658 years BP. The marine sediment core is represented dominantly by clayey silt with incursions of coarser components that occur around 6,000, 5,400, and 3,400 years BP. The sedimentation of the coarser detritus is due to strengthened southwest monsoon (SWM) since 6,500 to 5,400 years BP. Clay minerals are represented by smectite, illite, kaolinite, and chlorite in varying amounts. High kaolinite content and K/C ratio indicate intense SWM and strong bedrock weathering from the hinterland (~6,500–5,400 years BP). Incidence of smectite (48.82 to 25.09 %) and chlorite/illite (C/I) ratio (0.56 to 0.28) indicate an overall weakened southwest monsoon since 6,000 to 2,000 years BP with a brief incursion of extremely reduced SWM around 4,400 to 4,200 years BP. This is corroborated with the oxygen isotope on G. ruber that reveals a significant shift in the isotopic values ~4,300 years BP (−3.39 ‰), indicating weakening in SWM. Subsequently, fluctuations in the intensity of SWM are observed since 2,000 years to present.

Upwelling in the continental shelf of northern South China Sea associated with 1997–1998 El Niño

[1] A strong upwelling off the continental shelf of the northern South China Sea (SCS) in 1998 summer is reinvestigated using a suite of new satellite measurements and numerical modeling. Previous studies indicate that the upwelling in the western SCS, especially off the Vietnam coast, almost disappears during 1998 summer following the El Nino because of the weakened southwest monsoon. This study identifies that the coastal upwelling in the adjacent northern SCS (NSCS) is significantly strengthened during 1998 summer, and the alongshore wind stress is dramatically enhanced over the region. As a result, the offshore Ekman transport in 1998 summer is the strongest, almost twice the average of the other 17 years during 1997–2007. The Chl a concentrations in the representative upwelling regions are much higher than any in other years. Further analysis suggests that two adjacent basin-scale upwellings in the SCS have different responses and maintaining mechanisms because of the anticyclonic atmospheric circulation anomaly over the SCS and northwest Pacific. The northern flank of the atmospheric circulation anomaly intensifies the monsoonal winds off the NSCS coast, while the southern flank suppresses the southwesterly winds along the Vietnam coast.

Development of Phaeocystis globosa blooms in the upwelling waters of the South Central coast of Viet Nam

Blooms of haptophyte algae in the south central coastal waters of Viet Nam often occur in association with upwelling phenomenon during the southwest (SW) monsoon. Depending on the magnitude of the blooms, damage to aquaculture farms may occur. Based on two years of data on biology, oceanography, and marine chemistry, the present study suggests a conceptual model of the growth of the haptophyte Phaeocystis globosa. At the beginning of the bloom, low temperature and abundant nutrient supply, especially nitrate from rain and upwelling, favour bloom development. Diatoms utilize available nitrate and phosphate; subsequently, higher ammonium concentration allows P. globosa to grow faster than the diatoms. At the end of the Phaeocystis bloom, free cells may become available as food for a heterotrophic dinoflagellate species, Noctiluca scintillans. During and after the phytoplankton bloom, remineralization by bacteria reduces dissolved oxygen to a very low concentration at depth, and favors growth of nitrate-reducing bacteria.A Lagrangian Harmful Algal Bloom (HAB) model, driven by a circulation model of the area, realistically simulates the transport of microalgae in surface waters during strong and weak SW monsoon periods, suggesting that it may be a good tool for early warning of HABs in Vietnamese coastal waters.

The variation of the southwest monsoon from the high resolution pollen record in Heqing Basin, Yunnan Province, China for the last 2.78 Ma

A high-resolution pollen record has been obtained from the Heqing Basin of Yunnan Province, China. The 737.72-m-long continuous lacustrine sequence covers the last 2.78 Ma according to the results of AMS 14C and magnetostratigraphy dating. Seven pollen ecological groups, several high percentage pollen taxa and two palynological diversity indices (D-total and H′-total) are combined to reconstruct synthetically the palaeoclimatic evolution in the Heqing region of Yunnan Province. According to the palaeoclimatic changes, eight major variations of the southwest monsoon occurred. During the periods 2.780–2.729 Ma, 2.608–1.447 Ma and 0.342–0.118 Ma, the climate was predominantly relative warm and dry, indicating a rather weak southwest monsoon. Between 2.729 and 2.608 Ma, the climate was mostly humid with increased rainfall seasonality, mirroring a relative strong southwest monsoon. From 1.447 to 0.342 Ma, the southwest monsoon gradually strengthened, and its intensity achieved its maximum strength within wet phases between 0.991 and 0.342 Ma. The southwest monsoon during the period 0.118 Ma–11.83 ka BP fluctuated greatly with a pattern of weak–strong–weak. The spectral analysis of mesic conifer pollen percentages revealed that the climate and the southwest monsoon in the Heqing region have been dominated by the 100-ka eccentricity period, and also influenced by the other major orbital and sub-orbital frequencies including 400-ka eccentricity, obliquity (41- and 29-ka periods), 23-ka precession, and ∼ 10-ka.