Southwest Monsoon Winds Research Articles

A 70 kyr Record of Monsoon-induced Marine Productivity Changes, Terrigenous Flux and Weathering Variations in the Southern Bay of Bengal

Abstract The southwest (SW) and northeast (NE) monsoon winds significantly affect the oceanographic and climatic changes over the Bay of Bengal (BoB). We present here detailed geochemical and isotopic records from two sediment cores (SK157 14; 5°11′ N: 90°5′ E and SK157-16; 8°46′ N: 90°18′ E), collected from the southern BoB. In this study, we mainly focus on variations in paleoproductivity and terrigenous fluxes introduced into the BoB since last ~70 kyr: We have reconstructed biogenic barium (Babio), calcium carbonate (CaCO3) content, and total organic content (TOC) in both sediment cores to infer paleoproductivity during the period of study. The distributions of productivity proxies in both these cores indicate that the productivity may have been significantly high during MIS 1 and MIS 3 (warmer interglacial periods), when SW monsoon was intensified. The terrigenous proxies (Al%, Ti%, d(0.9)) show that the southern BoB may have received higher terrigenous input during the MIS 2 and MIS 4 (glacial periods) coinciding with the intensification of NE monsoon. The weathering proxy (K/Al), in contrast, shows no significant changes during MIS 2, 3 and 4. However; a significant shift is seen during glacial to Holocene transition.

Understanding the Mechanism of Currents through the Malacca Strait Study Case 2020 – 2022 : Mean state, Seasonal and Monthly Variation

The Malacca Strait is the importance waters for international shipping routes and more 200 cases of ship collisions in the period 1970-2015 in this strait. That is related to some a shallow depth waters with many small islands and strong current which is making this strait have a highest risk in certain seasons. The CMEMS data used to understanding of mechanism of current, illustrate the pattern and tranport volume. The research uses descriptive and the statistical methods to understanding of mechanism and variability of current. The analysis of current patterns and water level shows that during the monsoon winds active, the current always moves to the Andaman Sea, we called the southeast current system. The Northeast (NE) and Southwest (SW) monsoon wind is strongly influence to the mechanism of formation southeast current system, which causes the varition in sea level hight in eastern and western of Malacca Strait. During the NE monsoon season, the water masses of the South China Sea are push away into the Karimata Strait, causing the sea level in the eastern part to be higher than the western part and will increase the current speed. On the other hand, during the SW monsoon season, the water masses from the Karimata Strait is pushed away from the eastern part of the Malacca Strait which causes the sea level decrease and causes the current speed to weaken. Numbers eddies phenomenon were found at the Malacca Strait each season. The anticyclonic eddies occurred almost every season centered at 3° - 5° N, 97° - 100° E. Meanwhile anticyclonic eddies appear around the northern part of Sumatra centered at 4° - 5°N, 99° - 101°E during SW Monsoon season and the eddies formation also existed during the second transition monsoon season.

Trajectory-geospatial approach using HYSPLIT Model in determining air pollution sources at Pasir Gudang industrial areas

Industrial air pollutants emissions can be hazardous to human health and the environment. This study aims to determine the source of air pollution exposure in Malaysia's busiest industrial area, Pasir Gudang by using the trajectory-geospatial approach. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to determine the pathway of air backwards-trajectory. While the geospatial analysis for air pollution sources identification in Pasir Gudang was conducted using Geographic Information System (GIS). Three days of air mass backwards-trajectories were successfully obtained, where the dominant air mass pathways direction came from West Sumatra, Indonesia, influenced by the Southwest monsoon wind. The air mass backwards-trajectories also show there are influenced by nearby factories' emissions. From the geospatial buffer analysis, about 17 to 60 factories were identified to be possible sources of air pollution exposure since the location of factories are within a 1 km buffer zone of six affected schools in Pasir Gudang. The meteorological factors that influence the flow of high emissions of air masses pollutant from nearby factories have worsened the exposure level of air pollution until impacted the health of school’s students. From the findings, a standard of procedure (SOP) for air pollution source identification using a trajectory-geospatial approach has been developed as a guideline for all the stakeholders involved and essential in understanding better strategies in monitoring and mitigating industrial air pollution, particularly in Pasir Gudang as the buffer zones required by the authorities are not actively functioning to mitigate the air pollution exposures. Keywords: Air pollution, buffer zones, HYSPLIT, industrial area, Pasir Gudang, trajectory-geospatial approach

The role of wind, mesoscale dynamics, and coastal circulation in the interannual variability of the South Vietnam Upwelling, South China Sea – answers from a high-resolution ocean model

Abstract. The South Vietnam Upwelling (SVU) develops in the South China Sea (SCS) under the influence of southwest monsoon winds. To study the role of small spatiotemporal scales on the SVU functioning and variability, a simulation was performed over 2009–2018 with a high-resolution configuration (1 km at the coast) of the SYMPHONIE model implemented over the western region of the SCS. Its capability to represent ocean dynamics and water masses from daily to interannual scales and from coastal to regional areas is quantitatively demonstrated by comparison with available satellite data and four in situ datasets. The SVU interannual variability is examined for the three development areas already known: the southern (SCU) and northern (NCU) coastal upwelling areas and the offshore upwelling area (OFU). Our high-resolution model, together with in situ observations and high-resolution satellite data, moreover shows for the first time that upwelling develops over the Sunda Shelf off the Mekong Delta (MKU). Our results confirm for the SCU and OFU and show for the MKU the role of the mean summer intensity of wind and cyclonic circulation over the offshore area in driving the interannual variability of the upwelling intensity. They further reveal that other factors contribute to SCU and OFU variability. First, the intraseasonal wind chronology strengthens (in the case of regular wind peaks occurring throughout the summer for SCU or of stronger winds in July–August for OFU) or weakens (in the case of intermittent wind peaks for SCU) the summer average upwelling intensity. Second, the mesoscale circulation influences this intensity (multiple dipole eddies and associated eastward jets developing along the coast enhance the SCU intensity). The NCU interannual variability is less driven by the regional-scale wind (with weaker monsoon favoring stronger NCU) and more by the mesoscale circulation in the NCU area: the NCU is prevented (favored) when alongshore (offshore) currents prevail.

Atmospheric particulate-bound mercury (PBM10) in a Southeast Asia megacity: Sources and health risk assessment

Particulate-bound mercury (PBM) is a global environmental concern owing to its large dry deposition velocities and scavenging coefficients, both of which drive Hg into terrestrial and marine ecosystems. PBM observation studies have been widely conducted over East Asia, but comparable studies in Peninsular Southeast Asia (PSEA) remain scarce. This is the first study reporting PBM concentrations for Ho Chi Minh City (HCMC), the biggest metropolitan area in Vietnam. A total of 222 samples were collected in 2018 and contained an average PBM10 (particulate matter – PM with diameter ≤10 μm) concentration and Hg mass fraction (i.e. PBM/PM) of 67.3 ± 45.9 pg m−3 and 1.18 ± 1.12 μg g−1, respectively. Although PBM concentration was lower than those reported in Chinese megacities, the Hg mass fraction was similar to those in China, suggesting strong enrichment from anthropogenic Hg emissions in HCMC. Traffic-induced particulate emission and deposition processes were major factors governing PBM temporal variation at our site. In addition, the prevailing southwest monsoon winds brought air masses that passed through industrial areas and were associated with a higher Hg mass fraction. Statistically significant positive correlations (R2 = 0.11–0.52, p < 0.01) were observed for PBM with PM and the Hg mass fraction, indicating similar PM and Hg sources or oxidized Hg adsorption onto PM via gas-particle partitioning. Moreover, PCA results revealed a higher contribution of primary sources than secondary sources to PBM concentration variability in HCMC. A health risk assessment indicated that the PBM concentrations at HCMC posed minimal non-carcinogenic risks (HI < 1) for children and adults, but dermal contact may act as an important exposure route since lightweight clothing is common among residents. This PBM dataset over PSEA, a region with high atmospheric Hg emissions, provides a valuable resource for the Hg scientific community to improve our understanding of Hg biogeochemical cycle.

Observational evidence on the coastal upwelling along the northwest coast of India during summer monsoon.

The lesser known coastal upwelling in the North Eastern Arabian Sea (NEAS) during summer monsoon, its associated dynamics and forcing mechanisms is elucidated for the first time using basin scale monthly time-series in-situ and satellite data. The presence of cool upwelled waters along northwest coast of India from July to early October with an associated increase in productivity was evident in both data. The low level Findlater jet blows towards west coast of India with high wind magnitude (10-12m/s) during summer monsoon generates strong Ekman transport (1416kg/m/s) at offshore and Ekman pumping velocity (1.349m/s) at coastal region initiates upwelling. It was identified that the currents and remote forcing also regulate upwelling along the region. Although upwelling seems to exist along the northwest coast, it was weaker (25.5°C) compared to the southwest coast where the SST dropped to 24°C. The upwelling was observed in the south during June as a surface process, while it was observed along the northwest coast of India by the end of August. Even though the onset of upwelling in the NEAS and South Eastern Arabian Sea (SEAS) had a lag of two months, the recession of upwelling happened during late and early September respectively. The cause for the lag in the onset and cessation of upwelling between SEAS and NEAS is attributed to the propagation of Kelvin waves and southwest monsoon winds. The study also reveals that temperature and chlorophyll profiles show bi-modal peaks of high and low associated with winter cooling (winter) and upwelling (summer).

A 145 kyr record of upstream changes in Indian monsoon circulation and its link to southern high-latitude climate

The influence of northern high latitude climate variability on the South Asian Summer Monsoon has been extensively studied using both instrumental and proxy based climate data. In comparison, only a few studies have attempted to explore the southern high latitude association of the South Asian Summer Monsoon. Here we present a 145 kyr long new data (oxygen and carbon isotopic abundance of two depth-stratified species of foraminifera viz. Globigerinoides ruber and Globorotalia menardii) from a sediment core on sub-millennial to millennial scale resolution from the southwestern tropical Indian Ocean - a region swept by the southeasterly wind during boreal summer (referred to as ‘summer’ hereafter), which transforms to the southwest monsoon wind after crossing the equator. The available records of past summer monsoon variability are predominantly based on reconstruction of downstream hydrology, which is identifiable with the thermodynamics of the system. The present study is a reconstruction of the dynamics of the summer monsoon circulation from an upstream region which is ideally suited for exploring its link with the southern high-latitude climate. We identified several occurrences of interchanging intense and weak monsoon circulation events during the 145 kyr period. We further find that warm (cold) conditions in Antarctica show a near one-to-one coupling with weak (strong) monsoon phases, suggesting a strong mechanistic link between them during the period.

Contribution of moisture sources to precipitation changes in the Three Gorges Reservoir Region

Abstract. Precipitation changes in the Three Gorges Reservoir Region (TGRR) play a critical role in the operation and regulation of the Three Gorges Dam (TGD) and the protection of residents and properties. The potential impacts of the TGD on local and regional circulation patterns, especially the precipitation patterns, have received considerable attention since its construction. However, how the moisture transport affects precipitation changes in the TGRR spatially and temporally remains obscure. In this study, we investigate the long-term moisture sources of precipitation and their contributions to precipitation changes over the TGRR using an atmospheric moisture tracking model. Results suggest that although there is seasonal variation, the moisture contributing to the TGRR precipitation primarily originates from the areas southwest of the TGRR dominated by the Indian summer monsoon. In particular, the sources with the highest annual moisture contribution are the southwestern part of the Yangtze River basin and the southeastern tip of the Tibetan Plateau (TP). On average, 41 %, 56 %, and 3 % of the TGRR precipitation originates from ocean, land, and local recycling, respectively. In addition, the decreased precipitation over the TGRR during 1979–2015 is mainly attributed to the significantly decreased moisture contribution from the source regions southwest of the TGRR (especially around the southeastern tip of the TP). Compared to dry years, the higher precipitation in the TGRR during wet years is contributed by the extra moisture from the southwestern source regions that is delivered by the intensified southwesterly monsoon winds.

Human health risk of mercury in street dust: A case study of children in the vicinity of compact fluorescence lamp factory, Hanoi, Vietnam.

Urban dust has been contaminated with toxic heavy metals from various diffused sources. On August 28, 2019, the compact fluorescence lamp (CFL) factory that generates light for Rang Dong Light Source and Vacuum Flask in Hanoi City in Vietnam was accidentally fired. The assessment for Hg contamination and its human health impact 2 months after the incident was conducted in autumn 2019. The heavy Hg pollution is still found near CFL. Additionally, localized Hg pollution by air and wind direction was identified at sites in the northeast direction from CFL. Namely, in the area located downstream of the southwest monsoon wind, Hg had a trend higher than other directions. The hazard quotient (HQ) and the hazard index (HI) were evaluated by four routes of Hg exposure pathways. The HI values for children and adults in the most polluted site (Site 15) were 0.968 and 0.468, respectively. This meant that children who lived around this site and commuted to schools were the most at risk regarding the potential adverse health effects of Hg in street dust around the polluted areas. This study also showed that Hg levels were affected by wind direction and that the higher Hg levels in the most polluted sites resulted from the wind flow to the sites. This study revealed that street dust may be a useful tool for the assessment of human and environmental health.

Particulate organic matter dynamics and its isotopic signatures (δ13CPOC and δ15NPN) in relation to physical forcing in the central Arabian Sea during SW monsoon (2017–2018)

Monsoonal changes in particulate organic matter (POM) and its isotopic signatures (δ13CPOC and δ15NPN) were studied (Aug 2017 and 2018) in the central Arabian Sea (21–11°N 64°E) . A low-level atmospheric jet (Findlater Jet) forms during the south-west (SW) monsoon and results in “open ocean upwelling” to the north of the jet's axis. Higher wind speed and jet-induced wind stress curl coupled with low sea surface temperature (SST) and higher nutrients were noticed in 2018 than in 2017. The region to the north of the jet was characterized by substantially shallower Mixed Layers Depths (MLDs) and higher POM contents relative to the jet's axis and southern stations. The molar ratio between particulate organic carbon (POC) and nitrogen (PN) (6.2 ± 1.9in 2017; 6.4 ± 0.9 in 2018) was very close to the canonical Redfield ratio (6.63). The δ13CPOC values (−26.3 ± 1.4‰ in 2017; 25.5 ± 1.4‰ in 2018) exhibited typical marine signature reported from this area and revealed a noticeable inter-annual difference. Relatively higher δ15NPN values noticed in the north (7.68 ± 2.6‰ in 2017; 9.24 ± 3‰ in 2018) indicated regenerated dissolved inorganic nitrogen coming from the oxygen minimum zone (OMZ) into the well-lit zone. The lower δ15NPN values along the jet axis and south of it were attributed to lateral advection of nitrogen from the Somali upwelling. Despite the higher nutrient availability, POC contents in 2018 did not exceed the values in 2017. Considering the total consumption of nitrogen (according to C:N: P = 106:16:1), the potential POC development in 2018 could be double the value in 2017. The interannual differences in SW monsoon onset and wind speed seem to directly control the nutrient supply, affecting plankton community structure and POM variability. Thus, any future change in the physical forcing may directly influence the POC pool and consequent export flux to the mesopelagic.

Sources of atmospheric lead (Pb) after quarter century of phasing out of leaded gasoline in Bangkok, Thailand

After global phasing out of leaded gasoline, anthropogenic sources of atmospheric lead (Pb) are dominated by coal combustion emissions, high-temperature metallurgical processes, and vehicle exhausts. Thailand was one of the first countries in Southeast Asia to completely phase out leaded gasoline by the year 1994. This study investigates the sources of atmospheric Pb in Thai aerosols, a quarter-century after phasing out of leaded gasoline using a multiproxy approach, Pb isotopes in conjunction with trace metal composition of PM2.5. Aerosol samples were collected for 1 year from January 2018 to April 2019 from Bangkok and Chonburi to understand the influence of seasonal variation on aerosol chemistry. Bangkok is the only megacity in Thailand and notorious for traffic congestion and Chonburi is a coastal town, 80 km southeast of Bangkok where the inter monsoon winds make landfall. Aerosol Pb concentrations significantly decreased in 25 years from 74 ± 5 ng/m3 in 1994–1995 to 14 ± 13 ng/m3 measured in Bangkok during Northeast (NE) monsoon winds. Concentrations of all the metals are higher in Chonburi compared to Bangkok in the NE monsoon wind, owing to the addition of local emissions from the megacity over the long-range transported metals before the winds reach the coastal town. The Pb isotope ratios have a wide range (206Pb/207Pb range 1.1343–1.1685 and 208Pb/207Pb range 2.4138–2.4450) but the average ratios are not significantly different in Bangkok and Chonburi for all three seasons. However, present day 206Pb/207Pb ratios are more radiogenic than those measured 25 years ago in Bangkok. The Pb isotopes from both locations in all seasons have considerable overlap with unleaded gasoline and diesel used in Southeast Asia indicating automobile exhaust as an important source. Additionally, the NE monsoon winds are influenced by mixing of crustal dust (Al/Pb ratios correlates well with 206Pb/207Pb, r2 = 0.63) with coal combustion emission (strong inter-correlation of Pb, Zn, As and Cd, r2 from 0.72 to 0.94) from China and Vietnam. Pb isotopes of the Southwest (SW) monsoon winds are also influenced by crustal dust and falls on the mixing line between coal combustion and ore processing emission from India in a Pb triple isotope plot. Chemical composition of aerosols in the inter-monsoon winds are similar to SW monsoon winds. After a quarter-century of leaded gasoline banning in Thailand, the atmospheric Pb sources have changed from less radiogenic leaded gasoline exhaust and Chinese ore processing to more radiogenic unleaded gasoline and diesel exhaust and coal combustion.

Seasonal variability of tropospheric CO2 over India based on model simulation, satellite retrieval and in-situ observation

In this study, investigation of the seasonal cycle of the tropospheric CO2 concentration over India was carried out using the GEOS-Chem atmospheric transport model, Greenhouse gas Observation SATellite (GOSAT) retrievals, and in-situ measurements. The model simulation is highly coherent with the satellite and in-situ datasets, and it shows a distinct seasonal cycle of the tropospheric CO2 tendency over India with a negative phase (decreasing concentration) during April–August and a positive phase (increasing concentration) during September–March. The model diagnostics were analyzed to estimate budgets of the surface layer CO2, up to 650 hPa pressure level, for the two-phases of the seasonal cycle. A mean tendency, equivalent to −0.70 ppmv month−1, observed during April–August, which results from the loss of CO2 content in the surface layer through horizontal advection (−2.25 ppmv month−1) and vertical diffusion (−0.20 ppmv month−1), that dominates the gain from vertical advection (1.53 ppmv month−1). The negative contribution of horizontal advection in this period comes from the transport of CO2 depleted air-parcels over the oceanic region to India by the southwest monsoon winds and the positive contributions of vertical advection comes from upwelling of CO2 enriched air-parcels. The mean tendency, equivalent to 1.01 ppmv month−1, during September–March results from the gain through vertical advection (0.78 ppmv month−1) and horizontal advection (0.37 ppmv month−1) and a small contribution of vertical diffusion (−0.15 ppmv month−1). In this period, positive contribution of horizontal advection is due to the transport of CO2 enriched air-parcels from the southeast Asian region to India by north-east monsoon winds. At the annual scale, CO2 content of the surface layer over India has a net gain of 0.75 GtC that comes from 14.31 GtC through vertical advection that exceeds the loss due to horizontal advection (−11.10 GtC) and vertical diffusion processes (−2.46 GtC). This net gain is almost 85% higher than the input of 0.4 GtC through surface fluxes, which composed of 0.61 GtC anthropogenic emission and −0.21 GtC net terrestrial ecosystem exchanges. Additional sensitivity experiment was carried out to elucidate the semi-annual features of the seasonal cycle of CO2 for north India, in contrast to the annual characteristics of the seasonal cycle for south India in relation to the GOSAT observation.

Modeling of tidal circulation and sediment transport near tropical estuary, east coast of India

Estuarine hydrodynamics is a key factor for altering the inlet and its adjacent morphodynamics. Sediment transport in estuaries is controlled by the hydrodynamic action of wind, wave, tide, currents and river runoff. Sediment transport rate near a tropical estuary i.e., Rushikulya, east coast of India was assessed based on field surveys carried out during 2008 and 2009, covering both southwest and northeast monsoon wind field. A depth-averaged two-dimensional model was used to simulate and estimate sediment transport through a validated hydrodynamic and wave model output. The total sediment load, transport direction and bed level changes were estimated using Mike 21 Sediment Transport (ST) model. During the northeast monsoon, the sediment transport on the south varied from 2 × 10−6 to 6 × 10−6m3/s/m while at north of the inlet, it was constant with a value of 2 × 10−6m3/s/m and transport direction ranges from 330 to 350° at south and 280 to 320° at north. During southwest monsoon, the sediment load ranged from 2 x 10−6 to 2 x 10−5 m3/s/m and transport directions are mostly confined to 100°. Beach profiles and shoreline position confirm the onshore–offshore​ sediment transport. The work provides baseline information, can be used for management to protect the world-famous Olive Ridley nesting beaches, adjacent to the Rushikulya estuary, east coast of India.

Southward shift of precipitation extremes over south Asia: Evidences from CORDEX data

Analysis of observed Indian Summer Monsoon precipitation reveals more increase in extreme precipitation (in terms of its magnitude) over south India compared to north and central India during 1971–2017 (base period: 1930–1970). In the future, analysis of precipitation from the Coordinated Regional Downscaling Experiment indicates a southward shift of precipitation extremes over South Asia. For instance, the Arabian Sea, south India, Myanmar, Thailand, and Malaysia are expected to have the maximum increase (~18.5 mm/day for RCP8.5 scenario) in mean extreme precipitation (average precipitation for the days with more than 99th percentile of daily precipitation). However, north and central India and Tibetan Plateau show relatively less increase (~2.7 mm/day for RCP8.5 scenario). Analysis of air temperature at 850 mb and precipitable water (RCP4.5 and RCP8.5) indicates an intensification of Indian Ocean Dipole in future, which will enhance the monsoon throughout India. Moisture flux and convergence analysis (at 850 mb) show a future change of the direction of south-west monsoon winds towards the east over the Indian Ocean. These changes will intensify the observed contrast in extreme precipitation between south and north India, and cause more extreme precipitation events in the countries like Myanmar, Thailand, Malaysia, etc.

Impact of the Boreal Summer Intraseasonal Oscillation on the Diurnal Cycle of Precipitation near and over the Island of Luzon

AbstractDuring the boreal summer, satellite-based precipitation estimates indicate a distinct maximum in rainfall off the west coast of the island of Luzon in the Philippines. Also occurring during the summer months is the boreal summer intraseasonal oscillation (BSISO), a main driver of intraseasonal variability in the region. This study investigates the diurnal variability of convective intensity, morphology, and precipitation coverage offshore and over the island of Luzon. The results are then composited by BSISO activity. Results of this study indicate that offshore precipitation is markedly increased during active BSISO phases, when strong low-level southwesterly monsoon winds bring increased moisture and enhanced convergence upwind of the island’s high terrain. A key finding of this work is the existence of an afternoon maximum in convection over Luzon even during active BSISO phases, when solar heating and instability are apparently reduced due to enhanced cloud cover. This result is important, as previous studies have shown in other areas of the tropics afternoon convection over landmasses is a key component to offshore precipitation. Although offshore precipitation is maximized in the evening hours during active phases, results indicate that precipitation frequently occurs over the ocean around the clock (both as organized systems and isolated, shallow showers), possibly owing to an increase in sensible and latent heat fluxes, vertical wind shear, and convergence of the monsoon flow with land features.

Foreign influences on tropospheric ozone over East Asia through global atmospheric transport

Abstract. Tropospheric ozone in East Asia is influenced by the transport of ozone from foreign regions around the world. However, the magnitudes and variations in such influences remain unclear. This study was performed to investigate the influences using a global chemical transport model, GEOS-Chem, through the tagged ozone and emission perturbation simulations. The results show that foreign ozone is transported to East Asia (20–60∘ N, 95–150∘ E) mainly through the middle and upper troposphere. In East Asia, the influence of foreign ozone increases rapidly with altitude. In the middle and upper troposphere, the regional mean concentrations of foreign ozone range from 32 to 65 ppbv, being 0.8–4.8 times higher than its native counterpart (11–18 ppbv). Annually, ∼60 % of foreign ozone in the East Asian middle and upper troposphere comes from North America (5–13 ppbv) and Europe (5–7 ppbv), as well as from foreign oceanic regions (9–21 ppbv). Over the East Asian tropospheric columns, foreign ozone appears most in spring when ozone concentrations in the foreign regions are high and the westerlies are strong and least in summer when the South Asian High blocks eastward foreign ozone from reaching East Asia south of 35∘ N. At the East Asian surface, the annual mean of foreign ozone concentrations is ∼22.2 ppbv, which is comparable to its native counterpart of ∼20.4 ppbv. In the meantime, the annual mean of anthropogenic ozone concentrations from foreign regions is ∼4.7 ppbv, half of which comes from North America (1.3 ppbv) and Europe (1.0 ppbv). Seasonally, foreign ozone concentrations at the East Asian surface are highest in winter (27.1 ppbv) and lowest in summer (16.5 ppbv). This strong seasonality is largely modulated by the East Asian monsoon (EAM) via its influence on vertical motion. The large-scale subsidence prevailing during the East Asian winter monsoon (EAWM) favours the downdraft of foreign ozone to the surface, while widespread convection in the East Asian summer monsoon (EASM) blocks such transport. Interannually, the variation in foreign ozone at the East Asian surface is found to be closely related to the intensity of the EAM. Specifically, the stronger the EAWM is in a winter, the more ozone from North America and Europe reaches the East Asian surface because of the stronger subsidence behind the East Asian trough. In summer, ozone from South and South-east Asia is reduced in strong EASM years due to weakened south-westerly monsoon winds. This study suggests substantial foreign influences on ozone at the East Asian surface and in its tropospheric columns. It also underscores the importance of the EAM in the seasonal and interannual variations in foreign influences on surface ozone in East Asia.

Distinctive characteristics of upwelling along the Peninsular Malaysia's east coast during 2009/10 and 2015/16 El Niños

We investigate the southwest monsoon average structure and interannual variability of upwelling along the Peninsular Malaysia's east coast (PMEC) based on satellite-derived sea surface temperature (SST), reanalysis wind data and cruise observations. The upwelling area prompted by southwest monsoon winds is represented by a patch of cooler water along the coast. This cooler water is also an outcome of the cooler water flooded from the Karimata Strait, nonetheless, localised upwelling conditions demonstrating their substantial impact to further cool down this already cooled water. Results from empirical orthogonal function (EOF) analysis revealed that variation in SST was dominated by the first EOF mode, which accountable of 80.99% of total variance and its loading map demonstrated a persistence of SST front, represented the signature of upwelling. The time series of the first EOF mode was highly correlated with the alongshore wind stress anomalies, indicating the Ekman transport driven nature of upwelling interannual variability. We demonstrate that a delayed El Niño Southern Oscillation (ENSO) effect as an important factor for the interannual variations of upwelling along the PMEC. Our analysis focusing on the two recent strongest El Niños, 2009/10 and 2015/16 revealed that the intense northwest Pacific anticyclone anomaly (NWPAA) and the occurrence of the northerly wind anomalies from the subtropical western Pacific (SWP) in 2010 weakened the upwelling along the PMEC. While in 2016, the occurrence of the ‘C-shape’ wind anomalies generated by the warming of the southeast tropical Indian Ocean (SETIO) weakened the upwelling along the PMEC.

Modulation of Monsoon Circulations by Cross-Equatorial Ocean Heat Transport

Abstract Motivated by observations of southward ocean heat transport (OHT) in the northern Indian Ocean during summer, the role of the ocean in modulating monsoon circulations is explored by coupling an atmospheric model to a slab ocean with an interactive representation of OHT and an idealized subtropical continent. Southward OHT by the cross-equatorial cells is caused by Ekman flow driven by southwesterly monsoon winds in the summer months, cooling sea surface temperatures (SSTs) south of the continent. This increases the reversed meridional surface gradient of moist static energy, shifting the precipitation maximum over the land and strengthening the monsoonal circulation, in the sense of enhancing the vertical wind shear. However, the atmosphere’s cross-equatorial meridional overturning circulation is also weakened by the presence of southward OHT, as the atmosphere is required to transport less energy across the equator. The sensitivity of these effects to varying the strength of the OHT, fixing the OHT at its annual-mean value, and to removing the land is explored. Comparisons with more realistic models suggest that the idealized model used in this study produces a reasonable representation of the effect of OHT on SSTs equatorward of subtropical continents, and hence can be used to study the role of OHT in shaping monsoon circulations on Earth.

Characteristics of land and sea breezes along the Guinea Coast of West Africa

This study utilized 34-years hourly data of wind speed and direction, precipitation, and air temperature over land to investigate the climatology of land and sea breezes (LSB) at seven coastline stations along the Guinean Coast of West Africa. Daily sea surface temperature from the adjacent Atlantic Ocean and Tropical Rainfall Measurement Mission daily precipitation data was used. Ten years reanalysis data of ERA-Interim, Climate Forecast System Reanalysis version 2 (CFSRv2), and National Centers for Environmental Prediction (NCEP2) were also used for comparison with the observations. Adopted criteria were based on both surface and upper air information to identify LSB days. The results show that LSB is a seasonal phenomenon but most frequent in winter. All reanalyses underestimated the monthly occurrence and seasonal cycle of LSB, although CFSRv2 is better in reproducing the observations than ERA-Interim and NCEP2. The wind roses revealed the existence of night/early morning offshore winds. Except in summer when the southwesterly monsoon winds prevail all day, onshore winds are observed from about 1100 local solar time (LST), with a mean cessation time at 0100 LST, giving a mean duration of about 14 h. However, in Abidjan, they occurred at all hours. The vertical plots confirmed the wind direction reversals for LSB days and give an indication of its depth and speed in different months. It has been found that LSB height is influenced by the West African monsoon as intertropical discontinuity advances northwards from January. This was also seen in LSB increased speed from December to August.

Revolatilisation of soil-accumulated pollutants triggered by the summer monsoon in India

Abstract. Persistent organic pollutants that have accumulated in soils can be remobilised by volatilisation in response to chemical equilibrium with the atmosphere. Clean air masses from the Indian Ocean, advected with the onset of the summer monsoon, are found to reduce concentrations of hexachlorocyclohexane (HCH), dichlorodiphenyltrichloroethane (DDT) and its derivatives, endosulfan and polychlorinated biphenyls (PCBs) in air at a mountain site (all in the range 5–20 pg m−3) by 77 %, 70 %, 82 % and 45 %, respectively. The analysis of fugacities in soil and air suggest that the arrival of summer monsoon triggers net volatilisation or enhances ongoing revolatilisation of the now-banned chemicals HCH and PCBs from background soils in southern India. The response of the air–soil exchange was modelled using a regional air pollution model, WRF-Chem PAH/POP. The results suggest that the air is increasingly polluted during transport by the south-westerly monsoon winds across the subcontinent. Using a multidecadal multimedia mass balance model, it is found that air–surface exchange of HCH and DDT have declined since the ban of these substances from agriculture, but remobilisation of higher chlorinated PCBs may have reached a historical high, 40 years after peak emission.