Northeast Winter Monsoon Research Articles

ENSO‐Modulated Variability in Winter Shelf Circulation of the Northern South China Sea

AbstractWe combined long‐term observational data from 1999 to 2021 with numerical simulations to study the seasonal changes in the currents along the continental shelf in the Northern South China Sea (NSCS) during winter and how these changes relate to the El Niño‐Southern Oscillation (ENSO). Our results indicate that during El Niño events, the eastward movement of the warm pool in the tropical Pacific Ocean not only leads to colder sea surface temperatures and a stronger Kuroshio current intrusion but also significantly affects the formation of a high‐pressure system in the subtropics over the NSCS. In El Niño years, an anomalous anticyclonic wind stress curl in the South China Sea weakens the northeast winter monsoon, which in turn weakens the cyclonic shelf circulation. The first principal component from the multivariate empirical orthogonal function decomposition, which accounts for 49.02% of the total variance, shows a significant correlation of 0.64 with the Niño 3.4 index, indicating the circulation's sensitivity to tropical climate changes. Our analysis of the winter shelf circulation, based on the along‐isobath depth‐integrated vorticity equation, reveals that the exchange of water across the isobaths over the shelf is mainly controlled by the nonlinear advection of relative vorticity, with wind stress curl and bottom stress curl playing a less significant role in regulating the structure of these exchanges. The combined effect of baroclinic forces and topography likely governs the dynamics over the slope.

Thermal fronts in coastal waters regulate phytoplankton blooms via acting as barriers: A case study from western Guangdong, China

Oceanic fronts play an important role in regulating the distributions of phytoplankton and are subject to climate change. However, the relationship between climate change, oceanic fronts, and coastal phytoplankton blooms requires clarification. Using long-term satellite data, the seasonal, interannual, and decadal variations of the thermal fronts were examined, along with their effects on the distribution of phytoplankton in western Guangdong. Thermal fronts between the cold western Guangdong coastal current waters and the warm South China Sea (SCS) waters were found along the coast of western Guangdong during winter due to northeast winter monsoon. At decadal timescales, an increase (decrease) in frontal activities occurred during the negative (positive) Pacific Decadal Oscillation (PDO) phase. This pattern suggests that the positive (negative) PDO activity decreases (increases) sea level anomaly (SLA) and sea surface temperature in the SCS, thereby weakens (enhances) frontal activities along the coast. Accordingly, extensions and positive anomalies (> 1.5 mg/m3) in chlorophyll-a (Chl a) were observed during the positive PDO phase, whereas Chl a was concentrated in the nearshore areas during the negative PDO phase. These processes are primarily related to the barrier effects of enhanced thermal fronts and positive SLA on offshore materials transport during the negative PDO period. This study highlights the need to incorporate PDO activity into phytoplankton production in coastal oceans.

Hydro-climatic variability and consequent vegetation response during CE 1219–1942 from the Western Ghats, India

We present a pollen proxy record of hydro-climatic variability in the Somwarpet area of Karnataka State (Western Ghats), India, receiving monsoon rainfall from both the southwest summer monsoon (SWM; June to September: JJAS) and the northeast winter monsoon (NEM; October to December: OND). This record covers the last 723 years (CE 1219–1942) and is based on high-resolution lacustrine sediment pollen proxy data. The pollen evidence indicates that between 731 and 576 cal yr BP (CE [AD] 1219–1374), the landscape was dominated by mixed moist/semi-evergreen–dry tropical deciduous forests under a warm and moist climate, possibly indicating increased monsoon rainfall. This phase is globally correlated with the Medieval Climate Anomaly (MCA; CE 750–1200). Between 576 and 279 cal yr BP (CE 1374–1671), dense mixed moist/semi-evergreen–dry tropical deciduous forests came into existence and transformed the existing forests of the previous phase under a warm and more moist climate with further increase in monsoon rainfall. Finally, between 279 and 8 cal yr BP (CE 1671–1942), mixed moist/semi-evergreen–dry tropical deciduous forests again came into being, replacing the existing dense mixed forests of the previous phase under a warm and comparatively less moist climate with increased monsoon (may be NEM) rainfall, corresponds to the global Little Ice Age (LIA; CE 1550–1850). The hydro-climatic variability during CE 1219–1942 offers valuable insights into the impact of monsoonal variability and human activities on ecosystems in the Western Ghats (India), which are subject to dual monsoon systems (SWM and NEM).

Persistence of Cold Wedges in the Somali Current System

AbstractThe Somali Current system in the western Arabian Sea reverses seasonally with the South Asian Monsoon and is associated with localized upwelling cells or cold wedges during the summer southwest monsoon. Drifter trajectories in boreal summer and fall 2014 provide rare observational evidence that the northward Somali Current and associated cold wedges can persist into the boreal fall intermonsoon period. The near‐surface circulation and sea surface temperatures further suggest that the wedge‐like surface signatures may intermittently be capped and then reappear at a later time. Our observations show that the northward Somali Current system rapidly decayed within 1 week after the onset of the winter northeast monsoon and the arrival of a cyclonic eddy at the coast in early November 2014. This eddy may not only have affected regional ocean‐atmosphere interactions but also biogeochemical processes and the marine ecosystem through the transport of water properties and locally induced upwelling.

Investigation on the reconstruction of wave bimodal spectrum in the lagoon near islands and reefs

Due to the intricate topography and geomorphology of coral reefs, waves propagating from deep water areas to transitional or shallow water near islands and reefs will exhibit features of wave spectra that are very different from those in the deep sea. Thus, a reconstruction of the wave spectra is necessary. In this study, double Gaussian functions are first proposed to identify the characteristic frequencies of wave bimodal spectra, such as peak frequencies corresponding to the low frequency part and high frequency part, and separation frequencies differentiating between the low frequency part and the high frequency part, based on the observed data of wave spectra in the lagoon near islands and reefs of the South China Sea (SCS). The wave bimodal spectra close to the islands and reefs are then rebuilt using the Ochi-Hubble spectrum type. The high frequency and low frequency portions of the bimodal wave spectra are used to determine the significant wave heights, peak frequency and shape characteristics, respectively. The wave motions exhibit basically linear characteristics when two typical sea conditions, such as summer typhoon waves and winter Northeast monsoon waves, are investigated in detail. The superposition of wind waves and swells on the one hand, and the presence of the bottom on the other, have an impact on the generation of the bi-modal spectrum. The bi-modal spectra that were reconstructed and those that were observed have a correlation coefficient that is greater than 90%. The study on the reconstruction of the bimodal spectrum in the paper will provide a reference for the description of the bimodal spectrum near islands and reefs.

High resolution luminescence chronology of coastal dune deposits near Chumphon, Western Gulf of Thailand

The development of coastal dunes is linked to environmental controls such as sea-level variability, climatic conditions, and coastal morphology. Understanding the spatial and temporal variations of dunes is crucial for predicting how coastal landscapes may react to future climate changes and sea-level rise. However, there are very few detailed studies on the longer time-scale evolution (centennial to millennial) of coastal dunes from subtropical and tropical regions. Here, we combine a high-resolution luminescence chronology with sedimentological analyses to study the depositional history of a transverse coastal dune located within the Bang Berd dune field, Western Gulf of Thailand. While luminescence dating of uniform aeolian deposits is normally straight forward, we observe strong variations in the natural dose rate which are likely explained by the enrichment of accessory minerals in some laminae. Deposition of the dune started at least around 3000 years ago and coincides with a regional sea level drop. Sedimentary structures indicate deposition occurring predominantly in relation to the northeasterly winter monsoon. As the sea-level rise and increased storm intensity in the future may lead to stronger erosion along the coast, this study is highlighting the importance of the Bang Berd dune system as natural protection against coastal inundation.

Impact of Monsoon-Transported Anthropogenic Aerosols and Sun-Glint on the Satellite-Derived Spectral Remote Sensing Reflectance in the Indian Ocean

Spectral remote sensing reflectance (Rrs(λ), sr−1) is one of the most important products of ocean color satellite missions, where accuracy is essential for retrieval of in-water, bio-optical, and biogeochemical properties. For the Indian Ocean (IO), where Rrs(λ) accuracy has not been well documented, the quality of Rrs(λ) products from Moderate Resolution Imaging Spectroradiometer onboard both Terra (MODIS-Terra) and Aqua (MODIS-Aqua), and Visible Infrared Imaging Radiometer Suite onboard the Suomi National Polar-Orbiting Partnership spacecraft (VIIRS-NPP), is evaluated and inter-compared based on a quality assurance (QA) system, which can objectively grade each individual Rrs(λ) spectrum, with 1 for a perfect spectrum and 0 for an unusable spectrum. Taking the whole year of 2016 as an example, spatiotemporal pattern of Rrs(λ) quality in the Indian Ocean is characterized for the first time, and the underlying factors are elucidated. Specifically, QA analysis of the monthly Rrs(λ) over the IO indicates good quality with the average scores of 0.93 ± 0.02, 0.92 ± 0.02 and 0.92 ± 0.02 for VIIRS-NPP, MODIS-Aqua, and MODIS-Terra, respectively. Low-quality (~0.7) data are mainly found in the Bengal Bay (BB) from January to March, which can be attributed to the imperfect atmospheric correction due to anthropogenic absorptive aerosols transported by the northeasterly winter monsoon. Moreover, low-quality (~0.74) data are also found in the clear oligotrophic gyre zone (OZ) of the south IO in the second half of the year, possibly due to residual sun-glint contributions. These findings highlight the effects of monsoon-transported anthropogenic aerosols, and imperfect sun-glint removal on the Rrs(λ) quality. Further studies are advocated to improve the sun-glint correction in the oligotrophic gyre zone and aerosol correction in the complex ocean–atmosphere environment.

Extreme Wave Height Analysis in Natuna Sea Using Peak-Over Threshold Method

Extreme wave height analysis has been conducted in Natuna Sea, Indonesia, using 25 years (1991-2015) significant wave height (SWH) data from WAVEWATCH 3 (WW3) with a spatial resolution of 1/8°. The Natuna Sea is geographically connected to South China Sea (SCS) and is often crossed by tropical cyclones. These cyclones may have contributed to the existences of high waves in the SCS, which can propagate as swell waves to the Natuna Sea and can lead to extreme waves in this region. The extreme analysis has been done by classifying extreme events of SWHs using Peak-Over Threshold (POT) method with a fixed threshold level at quantile 0.93 and a minimum time separation of 48 hours between two successive extreme events. Furthermore, Generalized Pareto (GP) distribution has been applied to estimate return values of the extreme SWHs for several return periods. Parameters of the GPD have been estimated by Maximum Likelihood Estimation (MLE) method.Characteristics of extreme SWHs in the Natuna Sea have been explained by maximum and seasonal distribution plots. The maximum value of extreme SWHs in the SCS can reach 13 m and around 3-5 m in the Natuna Sea. The seasonal distributions of extreme waves indicate that the occurrences of extreme waves in the SCS during Northeast Winter Monsoon (NWM) are higher than those during Southwest Summer Monsoon (SSM). Seasonal mean and maxima of extreme SWHs in the Natuna Sea are also high during the NWM and low during the SSM.To examine the effects of swell waves from the SCS and also future extreme waves in the Natuna Sea, we have analyzed the characteristics and calculated return values of extreme waves in front and behind of Bunguran Island, which the former faces directly to the SCS. There were 172 (331) extreme waves from 1991-2015 in the front (behind) of Bunguran Island and mainly from the northeast (southwest). Most of them were around 2 – 4 (0.5 – 2) m with mean periods of 6 – 10 (3 – 6) s. Based on the return values in the front (behind) of the Bunguran Island, there are possibilities of extreme waves with values 4.70, 4.87, and 4.96 (2.08, 2.20, and 2.27) m for return periods of 25, 50, and 75-year, successively.

Seasonal control on coastal dune morphostratigraphy under a monsoon climate, Mui Ne dunefield, SE Vietnam

Having developed in the later Quaternary Period, the Mui Ne dunefield, representing the upper-most portion of a major coastal barrier complex in SE Vietnam, formed by wind regimes associated with the Asian monsoon climate. The barrier complex is 100 km long and higher than 150 m in elevation but despite its large dimensions, research on its geomorphological and geological characterization is limited. Several active dune fields extend 2–10 km landwards from the coast while the rest of the barrier surface is stabilized. We applied ground-penetrating radar (GPR) and quartz optically-stimulated luminescence (OSL) dating to the transverse dunes of the Mui Ne dunefield to characterize the morphostratigraphy and recent aeolian sedimentation history. Currently the dunes have a relief of typically 5–10 m and a wavelength of 50–100 m. They are driven by the northeasterly winter monsoon winds and reversed with minor relief in response to the southwesterly summer monsoon. GPR profiles and OSL ages define three morphostratigraphic units: the modern, Holocene, and Pleistocene dune units. The GPR data of the modern dune unit is dominated by a series of steep reflections that dip exclusively towards the west reflecting the winter monsoon and is punctuated by reactivation surfaces. Evidence for the wind reversal is not preserved in the sediment successions but is represented by reactivation surfaces. Thus a subunit bounded by the reactivation surfaces defines annual deposition. The preservation potential of a subunit varies with the migration rate of the transverse dune, and in the fastest migrating dunes, approximately one subunit was preserved every year (1.1 ± 0.6 subunit/yr). The modern dunes are generally younger than 100 years and have migrated WSW at net rates of 0.6–2.5 m/yr. The Holocene dune unit is as thick as the modern unit although it spans a period, ten times longer, reflecting the lower preservation potential of the transverse dunes as sedimentary records, due to erosion associated with their migration. The rate of dune migration suggests that an aeolian contribution of coastal sands has occurred following the sea level highstand of the Middle Holocene, accounting for the extent of the active dunefields. This mode of the barrier accumulation is considered as an analogue for earlier highstand events. However, the dunefields only cover a fraction of the entire barrier. It is thus inferred that barrier formation occurred over a long period, probably encompassing several glacial-interglacial cycles. Sand accumulation during the lowstand, either by transport of sand from the subaerially exposed shelf and/or reworking of sand emplaced during the prior highstand, may contribute significantly to the barrier development.

Distribution and variability of sea surface temperature fronts in the south China sea

High-resolution satellite observations of sea surface temperature (SST) were used to derive the daily SST fronts and monthly frontal probability (FP) in the South China Sea (SCS). A gradient-based frontal detection method was applied to increase the accuracy for frontal identification. The current study offered a comprehensive description for the variability of SST front and the associated dynamics in the SCS. Large variability was found along the coast of China and Vietnam, northwest of Luzon Island, and the corresponding seasonal cycle captured highest (lowest) values in winter (fall). The wind pattern was found highly influential for frontogenesis. The northeast monsoon in winter drove onshore Ekman transport near the coast of China, which induced convergence between coastal cold water and onshore warm water. The southwest monsoon during summer induced offshore transport, which brought upwelling near the coast of southeast Vietnam and east of Hainan Island. There were fronts generated between cold coastal water and offshore warm water during both monsoon patterns for impacted regions. The integrated effect from intrusion of the Kuroshio Current, wind stress curl and eddy activity were found favorable for generating fronts near Luzon Island. The dynamical relationship was more prominent at anomalous field that higher FP was associated with larger wind stress. And the interannual variability of fronts in the basin was clearly impacted by El Niño related large-scale atmospheric signals.

Impact of climate change on sea surface wind in Southeast Asia, from climatological average to extreme events: results from a dynamical downscaling

Southeast Asia (SEA) climate shows a large range of variability scales, from extreme events to interannual variability. Understanding its answer to climate change is of primary scientific and socio-economic importance. IPCC 5th assessment report however pointed the lack of knowledge in regional climate change and its impact in SEA. In particular little is known about the answer of wind to climate change. The impact of climate change on sea surface wind speed is investigated here, examining changes in daily and extreme event scales, interannual variability and climatological average between the twentieth and twenty-first centuries. For that the RegCM4 regional model was used to perform a dynamical downscaling of CMIP5 simulations done with CNRM-CM5 global climate model under RCP4.5 and RCP8.5 hypothesis. Comparisons with QuikSCAT satellite data show that the downscaled simulations perform overall better than the global simulations. Both models produce regionally and seasonally contrasted results in terms of daily wind speed answer to climate change. Global simulations produce mostly weak and non-significant changes, only suggesting an intensification of northeast winter monsoon in the northern SEA. Conversely regional downscaled simulations suggest from March to November in the northern South China Sea and Pacific regions a significant weakening of wind speed from climatological to daily scales (summer monsoon to extreme values), for regions and periods of initially strong values, associated with a 40–50% decrease of tropical cyclones frequency. These changes result from the increase of mean meridional south to north sea level pressure gradient and decrease its daily variability.

Evaluation of satellite precipitation products over Central Vietnam

A comprehensive validation of three satellite precipitation datasets (SPDs), including (1) the Climate Prediction Center Morphing algorithm (CMORPH), (2) Global Satellite Mapping of Precipitation (GSMaP) Reanalysis, and (3) Tropical Rainfall Measuring Mission multi-satellite precipitation analysis (TRMM) 3B42, was conducted using the rain gauge-based Vietnam Gridded Precipitation dataset (VnGP) and rain gauge station data for Central Vietnam. The three SPDs were compared and evaluated for two contrasting topographic regions, i.e., the Vietnam Central Highlands (VCH) and the Vietnam Central Coast (VCC), during the rainy seasons from 2001 to 2010 at different spatial (grid and regional) and temporal (daily and monthly) scales. Widespread heavy rainfall (WHR) days caused by the Northeast Winter Monsoon (NM), the Inter-tropical Convergence Zone (ITCZ), and tropical cyclones (TCs) were also identified, and the accuracies of the SPDs in detecting heavy rainfall during the WHR days were evaluated. TRMM was shown to exhibit advantages over the other SPDs, regardless of the spatial and temporal scales. Although the CMORPH and GSMaP datasets appeared to correlate moderately well with the VnGP dataset and proved able to capture the spatial distribution of rainfall characteristics in the VCC, they significantly underestimated rainfall in the VCH. Regarding the capability to reproduce WHR events, the three SPDs exhibited better performance for TCs and the ITCZ than for the NM. TRMM exhibited the best performance among the three datasets, especially for rainfall thresholds ranging from 25 to 80 mm day−1. The GSMaP and CMORPH biases showed a clear dependence on elevation and zonal wind speed, indicating the need to improve correction methods.

Erosion of the coastal Mekong delta: Assessing natural against man induced processes

The Lower Mekong Delta Coastal Zone (LMDCZ) is emblematic of the coastal erosion problem facing many tropical deltas. Over the last 3500 years, large river sediment fluxes expanded the delta seaward, and waves and currents formed the Ca Mau Peninsula to the southwest. Since the middle of the 20th century, the LMDCZ is affected by various human activities that include reduction of river fluxes due to damming and sand mining, land subsidence due to groundwater extraction, and reduction of protective coastal mangroves in favor of agriculture and aquaculture. Coastal erosion is observed along many sections of the delta, with a rate of up to 50 m per year in some areas. However, the role of human activities remains difficult to assess because of its complexity. The present modeling study is designed to sort out the contribution of natural hydrodynamic redistribution of sediments from man-induced erosion. The modeling system used is based on CROCO, forced by global reanalyses at the boundaries and at the surface, including wave statistics (required by the sediment transport model). Tides and realistic river forcing are also included. Calibration and validation relies on a combination of in situ and remotely-sensed observations, and laboratory experiments. Once validated, coastal dynamics are investigated by performing sensitivity experiments for both the hydro- and sediment dynamics. The results suggest that while wind is the main factor driving the coastal currents, the sediment dynamics is essentially the result of re-suspension due to wave-induced bed shear stress. The suspended sediments are then redistributed by coastal currents that are not limited to the nearshore zone. Strong seasonality of the process is observed with the northeast winter monsoon being the season of strongest re-suspension and sediment redistribution. The annual sediment budget is characterized by important local disequilibrium, with alongshore patterns that are in agreement with the observed shoreline evolution. The effect of a decrease in river sediment supply is difficult to evaluate because the estuarine zone is still in accretion, apart from the particular case of Go Cong shores. Far from the estuarine zone, subsidence is an additional strong candidate to explain erosion in areas that should naturally be accreting. Synthesizing these results, the study proposes a first attempt at a “taxonomy and geography” of processes along the coastal Mekong delta that can explain the recent observations of shoreline changes and help design protection measures.

Low-Frequency Surface Currents and Generation of an Island Lee Eddy in Panay Island, Philippines

AbstractHigh-frequency Doppler radar (HFDR) and acoustic Doppler current profiler (ADCP) time-series observations during the Philippine Straits Dynamics Experiment (PhilEx) were analyzed to describe the mesoscale currents in Panay Strait, Philippines. Low-frequency surface currents inferred from three HFDR (July 2008–July 2009), reveal a clear seasonal signal concurrent with the reversal of the Asian monsoon. A mesoscale cyclonic eddy west of Panay Island is generated during the winter northeast (NE) monsoon. This causes changes in the strength, depth, and width of the intraseasonal Panay coastal (PC) jet as its eastern limb. Winds from QuikSCAT and from a nearby airport indicate that these flow structures correlate with the strength and direction of the prevailing local wind. An intensive survey in 8–9 February 2009 using 24 h of successive cross-shore conductivity–temperature–depth (CTD) sections, which in conjunction with shipboard ADCP measurements, show a well-developed cyclonic eddy characterized by near-surface velocities of 50 cm s−1. This eddy coincides with the intensification of the wind in between Mindoro and Panay Islands, generating a positive wind stress curl in the lee of Panay, which in turn induces divergent surface currents. Water column response from the mean transects show a pronounced signal of upwelling, indicated by the doming of isotherms and isopycnals. A pressure gradient then is set up, resulting in the spin up of a cyclonic eddy in geostrophic balance. Evolution of the vorticity within the vortex core confirms wind stress curl as the dominant forcing.

The Northeast Winter Monsoon over the Indian Subcontinent and Southeast Asia: Evolution, Interannual Variability, and Model Simulations

AbstractThe northeast monsoon (NEM) brings the bulk of annual rainfall to southeastern peninsular India, Sri Lanka, and the neighboring Southeast Asian countries. This October–December monsoon is referred to as the winter monsoon in this region. In contrast, the southwest summer monsoon brings bountiful rainfall to the Indo-Gangetic Plain. The winter monsoon region is objectively demarcated from analysis of the timing of peak monthly rainfall. Because of the region’s complex terrain, in situ precipitation datasets are assessed using high-spatiotemporal-resolution Tropical Rainfall Measuring Mission (TRMM) rainfall estimates, prior to their use in monsoon evolution, variability, and trend analyses. The Global Precipitation Climatology Center’s in situ analysis showed the least bias from TRMM.El Niño–Southern Oscillation’s (ENSO) impact on NEM rainfall is shown to be significant, leading to stronger NEM rainfall over southeastern peninsular India and Sri Lanka but diminished rainfall over Thailand, Vietnam, and the Philippines. The impact varies subseasonally, being weak in October and strong in November. The positive anomalies over peninsular India are generated by anomalous anticyclonic flow centered over the Bay of Bengal, which is forced by an El Niño–related reduction in deep convection over the Maritime Continent.The historical twentieth-century climate simulations informing the Intergovernmental Panel on Climate Change’s Fifth Assessment (IPCC-AR5) show varied deficiencies in the NEM rainfall distribution and a markedly weaker (and often unrealistic) ENSO–NEM rainfall relationship.

Bloom of the diatom, Biddulphia sp. and ecology of Pulicat lagoon, Southeast India in the aftermath of the 2015 north east monsoonal rainfall.

Pulicat lagoon in the south-east coast of India is recharged with the highest fresh water influx annually during north-east winter monsoon. An abrupt heavy rainfall in November-December 2015 was a flood calamity in the region that inundated the lagoon. We investigated the physico-chemical characteristics and palynological profile of the surface sediments from the lagoon at water depth between 0.5 and 2m to understand the impact of the 2015 event. On the basis of a 'marine index', three zones were demarcated, the 'LC zone' (Araniar river-lagoon confluence, south), the 'LM zone' and the 'LL zone' (Kalangi river confluence, north) showing values of 4.55, 1.25 and 0.25, respectively, indicating the extent of tidal influence. The dissolved oxygen, pH and salinity ranged between 6.4 and 9.6mg/L, 7.96-9.23 and average ~ 12.3ppt at different sites, respectively. The highest salinity (31 and 31.8ppt) was in the LC zone along with the highest dissolved inorganic nitrogen (DIN) concentrations (275-288mmol/m3) too. The dissolved inorganic phosphates (DIP) ranged between 8.5 and 29.5mmol/m3, which was relatively high. Sandy sediment, high sodium (Na+) and chloride (Cl-) ions too indicate the landward extension of seawater influx. Forsterite (Mg2SiO4) and beta-uranophane (Ca[(UO2)SiO3(OH)]2.(H2O) minerals show higher values with the dominance of olivine (MgFe)2SiO4) and quartz (SiO2) in this zone indicating magnitude of fresh water influx too through Araniar river. The highest calcium, magnesium and potassium ions were also observed in LC zone. Fe-rich goethite, calcite and aragonite were recorded at all sites but with dominance of illite in LL zone. Halite, an evaporite recorded from all sites shows inundation of the entire lagoon during flooding event. Results show a bloom of Biddulphia pulchella, B. biddulphiana and B. laevis in association with Cladophora in LC zone which serves as potential indicators of physico-chemical characteristics of the lagoon showing intense response to catastrophic events of floods due to above normal monsoon variability.

Effects of monsoon activity on monthly phytoplankton blooms in the gulf of Thailand in El Nino year 2002

ABSTRACT: The Gulf of Thailand is a semi-closed Gulf on the west and southwest side of the Indochina Penisula and experienced reversal monsoon. The object of the present study is to investigate monthly and spatial distributions of the phytoplankton in the Gulf of Thailand during whole El Nino year 2002 by using remote-sensing measurements of chlorophyll-a (Chl-a) and surface wind vectors. Results show that monthly and spatial variations of the phytoplankton blooms are primarily associated with the monsoonal winds. In general, the average monthly Chl-a concentrations were quite low (<0.5 mg m-3) most area of the Gulf, with a belt of higher Chl-a concentrations along the coast during throughout year. Phytoplankton blooms extensively offshore in the near-coastal area of the Gulf in January and February, which is consistent with the winter northeast monsoon. In particular, one peak of Chl-a concentrations was observed in December. Areas with higher Chl-a concentrations along the coast were observed in both winter and summer monsoon months.
 KEY WORDS: Phytoplankton blooms, Monsoon, Gulf of Thailand, El Nino

Multiscale processes in the genesis of a near‐equatorial tropical cyclone during the Dynamics of the MJO Experiment: Results from partial lateral forcing experiments

AbstractTropical cyclone (TC) 05A (TC05A, November 2011) over the Indian Ocean originated near the equator and brought huge damages to Sri Lanka and India. The genesis and propagation of TC05A involves interactions with the Madden–Julian Oscillation (MJO), interannual (IA) variations, and other synoptic (SY) systems. To investigate the impacts of the SY, MJO, and IA forcings across the lateral boundaries on TC05A, regional simulation with the Weather Research and Forecast model through the partial lateral forcing approach is conducted. The control experiment reproduces realistic genesis and movement of TC05A and associated heavy rainfall, as well as the mean states and MJO. Both the SY and IA forcings make a moderate contribution to the initiation of cyclonic vorticity and rainfall enhancement in the central Indian Ocean and moderate modification of the TC05A track. However, it is the MJO forcing that fundamentally reproduces a coherent moist, low‐level convergence, and a cyclonic environment that is favorable for the development of TC05A. The greatest impact of northern boundary intraseasonal (IS) forcing on the genesis location and path of TC05A is further identified, suggesting the importance of the interaction between the MJO and northeast winter monsoon. In contrast, the southern boundary IS forcing is unimportant and the western and eastern boundary IS forcings play a moderate role in slowing down TC05A and modifying its track, intensity, and rainfall. This study demonstrates that the partial lateral forcing method is an effective tool to identify the key factors controlling the extreme events through regional climate modeling.

General Sea State and Drag Coefficient Observed near Shore in Taiwan Strait

The general characteristics of wind waves and drag coefficient are studied using the data from the buoy observation near shore in Taiwan Strait. An algorithm of the bulk aerodynamic method using 10-minute mean wind speed and the temperature difference between the air and sea surface was developed to calculate the equivalent wind speed at 10 m in height and the surface friction velocity. The observation shows that the sea states contains a wide range of wave ages driven by the synoptic wind systems, i.e. the strong northeast monsoon in winter and southwest monsoon in summer, mixed with the thermally diurnal variation across the sea-land boundaries. The large-scale winds generate a number of swells with long fetch and the mesoscale circulation perpendicular to the main streams causing wind and wave misalignments. The drag coefficients display considerable scattering around the linear growth formulation along with the increase of neutral wind speed. This is attributed to the dependence of surface roughness on wave ages. The present observation confirms that the drag coefficients are sensitive to the sea state described using the Charnock constant and hence the wave ages.

Trend analysis of evapotranspiration over India: Observed from long-term satellite measurements

Owing to the lack of consistent spatial time series data on actual evapotranspiration (ET), very few studies have been conducted on the long-term trend and variability in ET at a national scale over the Indian subcontinent. The present study uses biome specific ET data derived from NOAA satellite’s advanced very high resolution radiometer to investigate the trends and variability in ET over India from 1983 to 2006. Trend analysis using the non-parametric Mann–Kendall test showed that the domain average ET decreased during the period at a rate of $$0.22\,\hbox {mm year}^{-1}$$ . A strong decreasing trend ( $$m = -1.75\, \hbox {mm year}^{-1}$$ , $$F = 17.41$$ , $$P$$ 0.01) was observed in forest regions. Seasonal analyses indicated a decreasing trend during southwest summer monsoon ( $$m= -0.320\, \hbox {mm season}^{-1}\,\hbox {year}^{-1})$$ and post-monsoon period ( $$m= -0.188\, \hbox {mm season}^{-1 }\,\hbox {year}^{-1})$$ . In contrast, an increasing trend was observed during northeast winter monsoon ( $$m = 0.156 \,\hbox {mm season}^{-1 }\,\hbox {year}^{-1})$$ and pre-monsoon ( $$m = 0.068\, \hbox {mm season}^{-1 }\,\hbox {year}^{-1})$$ periods. Despite an overall net decline in the country, a considerable increase ( $$4 \,\hbox {mm year}^{-1}$$ ) was observed over arid and semi-arid regions. Grid level correlation with various climatic parameters exhibited a strong positive correlation ( $$r \!>\!0.5$$ ) of ET with soil moisture and precipitation over semi-arid and arid regions, whereas a negative correlation ( $$r$$ $$-0.5$$ ) occurred with temperature and insolation in dry regions of western India. The results of this analysis are useful for understanding regional ET dynamics and its relationship with various climatic parameters over India. Future studies on the effects of ET changes on the hydrological cycle, carbon cycle, and energy partitioning are needed to account for the feedbacks to the climate.