Northeast Winter Monsoon Research Articles

Physical and geographical origins of the South China Sea Warm Current

We examine the formation mechanism of the South China Sea Warm Current in winter, using a high‐resolution, numerical model. The current, noted for its ability to flow against the prevailing northeast monsoon in winter, has received considerable attentions in recent years. The collective wisdom from previous models points to two likely generation scenarios: occasional wind relaxation or the Kuroshio intrusion. The present model consistently points to the wind relaxation as the dominant mechanism. When comparing differences between previous models and ours, we also conclude that the Kuroshio intrusion helps, but is not chiefly responsible. Tracing the current to the source, we identify the elevated sea level in the Gulf of Tonkin, induced by the northeast monsoon, as the ultimate driving force. The presence of Hainan Island bears little importance in generating the current.

Basin‐scale wind‐forced dynamics of the seasonal southern South China Sea gyre

Using observed wind‐forcing, the seasonal development and disappearance of the near‐surface anticyclonic gyre in the southern South China Sea is modeled, along with associated dynamic features. Forced by monsoon winds, the ocean circulation in the South China Sea alternates between a single cyclonic gyre during the winter northeast monsoon and a double gyre, cyclonic/anticyclonic in the northern/southern South China Sea, during the summer southwest monsoon, reversing the western boundary current along Vietnam with each shift of the monsoon regime. During the summer southwest monsoon, the poleward western boundary current converges with the equatorward western boundary current of the northern gyre, turning offshore to create an eastward jet and flanking dipole recirculation near 13°N. The development and timing of the eastward jet and recirculations are consistent with large‐scale basin dynamics. The subsequent transition to the winter northeast monsoon regime destroys the recirculation cells and eastward jet. Employing high‐resolution 1.5‐layer and 2.0‐layer numerical models, we demonstrate the applicability of large‐scale dynamics to the deeper central basin of the South China Sea, specifically showing that the gyre‐scale circulation governs the development of the eastward jet and dipole recirculations. The destruction of the recirculations by the monsoon winds each autumn is also modeled. This effort highlights the dominance of monsoon wind‐forcing in the South China Sea's basin‐scale dynamics.

Winter Eddy Genesis in the Eastern South China Sea due to Orographic Wind Jets

Abstract Generation of mesoscale eddies in the eastern South China Sea (SCS) in winters during August 1999 to July 2002 is studied with a reduced-gravity model. It is found that the orographic wind jets associated with the northeast winter monsoon and the gaps in the mountainous island chain along the eastern boundary of the SCS can spin up cyclonic and anticyclonic eddies over the SCS. Results suggest that direct wind forcing could be an important generation mechanism for the rich eddy activity in the SCS, and that to simulate this mechanism the resolution of the wind forcing has to be high enough to resolve the local wind jets induced by orographic effects.

A 28‐ka history of sea surface temperature, primary productivity and planktonic community variability in the western Arabian Sea

Uranium series radionuclides and organic biomarkers, which represent major groups of planktonic organisms, were measured in western Arabian Sea sediments that span the past 28 ka. Variability in the past strength of the southwest and northeast monsoons and its influence on primary productivity, sea surface temperature (SST), and planktonic community structure were investigated. The average alkenone‐derived SST for the last glacial period was ∼3°C lower than that measured for the Holocene. Prior to the deglacial, the lowest SSTs coincide with the highest measured fluxes of organic biomarkers, which represent primarily a planktonic suite of diatoms, coccolithophorids, dinoflagellates, and zooplankton. We propose that intensification of winter northeast monsoon winds during the last glacial period resulted in deep convective mixing, cold SSTs and enhanced primary productivity. In contrast, postdeglacial (<17 ka) SSTs are warmer during times in which biomarker fluxes are high. Associated with this transition is a planktonic community structure change, in which the ratio of the average cumulative flux of diatom biomarkers to the cumulative flux of coccolithophorid biomarkers is twice as high during the deglacial and Holocene than the average ratio during the last glacial period. We suggest that this temporal transition represents a shift from a winter northeast monsoon‐dominated (pre‐17 ka) to a summer southwest monsoon‐dominated (post‐17 ka) wind system.

Seasonal variations in carbon biomass of bacteria, thraustochytrids and microzooplankton in the Northern Arabian Sea

Abstract As a part of the JGOFS-India Programme, abundance of heterotrophic bacteria (Hbac; 0–2000 m), thraustochytrids (0–2000 m), microzooplankton (0–200 m occasionally up to 300 m) as well as bacterial growth rates (0–500 m) were studied in the Northern Arabian Sea (NAS) during Spring (SpIM) and Fall intermonsoon (FIM), Northeast Winter (Win) and Summer (SuM) monsoon seasons. Hbac abundance was mostly in the range of 108–109 cells l−1 in the surface (0–100 m) and intermediate (150–400 m) depths. Thraustochytrids (heterotrophic protists rich in polyunsaturated fatty acids) were roughly in the range of 103 l−1 (nil to ∼2.7×105). Below 200 m, they decreased drastically to values between nil and 10 l−1. Seasonal differences in carbon biomass of Hbac, thraustochytrids and microzooplankton were more pronounced than those seen in the mesozooplankton. The highest Hbac C biomass was observed during FIM. Bacterial carbon demand (BCD, mg C m−3 d−1; at an assumed 33% conversion efficiency) varied from 0.45 to 40.6 in FIM, 23.43 to 100 in SpIM, 2.31 to 17.5 in Win and from 6.6 to 25.4 in SuM in the top 100 m. In general, with increasing depth the BCD decreased quite rapidly ranging from 0.007 to 1.65 mg C m−3 d−1 below 100 m. In the NAS, heterotrophic bacteria appear to play a significant role in sustaining microzooplankton and the so-called ‘Arabian Sea mesozooplankton stable-biomass paradox’ through microbial loop. Sizable quantities of Hbac, thraustochytrids and microzooplankton C might be suggestive that Hbac support these microheterotrophs, in mesopelagic zone in particular. Their role in sustaining a variety of deep-water micro- and mesozooplanktonic communities as well as in some ways, the myctophids ingesting marine snow (enriched with bacteria, thraustocytrids and dead organic matter besides zooplankters) is brought to attention.

PM mass concentrations in comparison with aerosol optical depths over the Arabian Sea and Indian Ocean during winter monsoon

Abstract An analysis of PM2.5 mass concentrations and 0.5 μm aerosol optical depths (AODs) during the Northeast winter monsoon seasons of 1996–2000 is performed and intercompared. AODs are found to show diurnal variations over Coastal India (CI) (west coast) while they are relatively smooth over the Arabian Sea (AS) (5–20°N) and tropical Indian Ocean (TIO) (5°N–20°S). PM2.5, PM10 and total mass concentrations show less variations in a day over these oceanic regions. Columnar AODs are found to increase with an increase in the marine boundary layer aerosol concentrations over CI and AS while an opposite trend is seen over TIO. The yearly-mean AODs and mass concentrations are found to increase over CI and AS, over TIO the mass concentrations increased while the AODs decreased during 1996–2000. It is found from the 7-days air back trajectory analyses that at different altitudes air masses can originate from different source regions leading to changes in chemical, physical and optical characteristics of the aerosol between the surface and column. The differences in the surface and columnar measurements could also occur due to changes in the meteorological conditions, wind patterns, in addition to changes in production and subsequently the transport of aerosols. Least-squares fits to the above intercomparison resulted in intercepts of 0.24 and 0.22 over CI and AS indicating that the background AODs over these oceanic regions are higher. An examination of the daily-mean wind speeds and PM2.5 mass concentrations yielded an index of wind dependence of 0.04 for AS and 0.07 for TIO. The background PM2.5 mass concentrations are also found to be high at 36 and 25 μg m−3 over AS and TIO, respectively, indicating a stronger influence from the continent. Frequency distribution figures show that 28% of the PM2.5 values over CI lie in the 60–80 μg m−3 range. Over AS the dominant mode of distribution is 40–60 μg m−3 with a peak value of 42%. Over TIO PM2.5 values are found to peak in the lower mass bin of 0–20 μg m−3 at 33%. A latitudinal gradient is seen in the peak bin value of PM2.5 as the ship moves away from the coast. About half the days over CI and 20% over AS and TIO, PM2.5 values are found unhealthy indicating the influence of anthropogenic pollution and long-range transport of pollutants from the surrounding continental locations across these oceanic regimes. AODs are found to peak in the 0.2–0.4 bin at 52% over CI and 47% over AS. Over AS 32% of the AODs are found to be α values are found to peak in the 1.5–2 range over CI at 55% while they peak in the lower range of 1–1.5 at 49% and 26% over AS and TIO, respectively. Over the Pacific and Atlantic Oceans in more than 90% of the cases α was ⩽1.5, indicating that the amount of smaller and larger particles are higher over the Indian Ocean when compared to the Pacific and Atlantic. The spread in PM2.5 and AOD indicates that it is a challenging task to obtain a good relation without additional inputs on the vertical distribution of aerosols as varied kinds of aerosols from different source regions contribute at different heights over these oceanic regions.

Volume Transport Through the Taiwan Strait: A Numerical Study

A fine grid resolution model with realistic bathymetry was constructed to study the spatial and temporal structures of flow through the Taiwan Strait where observations are limited. The model covers an expanded domain that includes the entire East China Sea and South China Sea, as well as the region occupied by the Kuroshio. The fine-resolution model derives its open boundary conditions from a larger scale Western Pacific Ocean model. Two numerical weather products from the European Center for Medium-Range Weather Forecasts and National Centers for Environmental Prediction, and one satellite observation-based wind set (QSCAT) are used to force the ocean model. Model experiments suggested that the best simulation is achieved when the model is driven by the QSCAT wind forcing. Several important features are reproduced in the model simulation. The volume transport is northward and largest in summer while minimal volume transport is southward and occurs in fall and winter. The general trend of volume transport is related to the seasonal reversal of monsoon winds. The present model also suggests that flow in the entire Taiwan Strait is to the southwest during periods of strong northeasterly wind. The annual average transport based on the present model is 1.09 Sv (1 Sv=10^6 m^3S^(-1)), which is smaller than most published values based on shipboard Acoustic Doppler Current Profiler (sb-ADCP) observations. The result suggests that sb-ADCP observations are biased toward estimates in summer and fair weather since bad weather during the winter northeast monsoon often prevents seagoing observations.

The China Coastal Current as a driving force for transporting Calanus sinicus (Copepoda: Calanoida) from its population centers to waters off Taiwan and Hong Kong during the winter northeast monsoon period

Calanus sinicus is a copepod with wide geographical distribution in the continental shelf waters of China. Its occurrence in oceans around Taiwan and Hong Song is limited by temperature to winter and spring. We present evidence to show that C. sinicus is carried into the coastal waters of Taiwan and Hong Kong from population centers in the Yellow Sea and the East China Sea by the China Coastal Current during the northeast monsoon period in winter.

Groundwater δ18O and δD from central Indian Peninsula: influence of the Arabian Sea and the Bay of Bengal branches of the summer monsoon

Results of oxygen and hydrogen isotopic investigations of shallow groundwater and river water samples from the Central Indian Peninsula are reported. The study was undertaken to isotopically characterise the near surface water sources and identify the influence of the two branches of the Indian Summer Monsoon (June–September) in this region. The western part of the study area receives vapour flux largely from the Arabian Sea branch, whereas the eastern part receives significant vapour flux from the Bay of Bengal branch. No significant rainfall occurs in the study area during northeast winter monsoon period of October–December or the remaining part of the year. Therefore, the isotopic character of shallow groundwater from this region could be used to study the relative influence of the vapour sources from the Arabian Sea and the Bay of Bengal. The study shows that: (i) The northeast part, receiving significant rainfall from the Bay of Bengal branch, has a distinctly depleted isotopic composition ( δ 18O≈−5±2‰) compared to areas receiving dominant rain from the Arabian Sea branch ( δ 18O≈−1±1‰). (ii) In the west coastal belt, the average δ 18O in groundwater is close to the average δ 18O of precipitation at Bombay (−1.4±1.1‰). But, groundwater samples in the entire Deccan Plateau region show signatures of evaporative enrichment in the form of (a) low slope (∼6.5) of δ 18O– δD regression line; (b) low d-excess (0±5‰) and (c) slightly enriched δ 18O values (−1±1‰) compared to those expected from the limited precipitation data from stations in Maner basin approximately in the middle of the study area. (iii) Depleted ground waters along the central Indian east coast could arise due to continental rainout of the Arabian Sea vapour source as the moisture-laden monsoon winds crossover to the Bay of Bengal. Alternatively, it is necessary to postulate that the surface waters of the Bay of Bengal contributing to inland vapour flux are significantly more depleted during the monsoon period than the known depletion of ∼1‰. It is also possible that both factors are operative.

Spectral aerosol optical characteristics during the northeast monsoon over the Arabian Sea and the tropical Indian Ocean: 2. Ångström parameters and anthropogenic influence

Ångström parameters are estimated from the spectral aerosol optical depths (AODs) measured over the Arabian Sea (AS) and tropical Indian Ocean (TIO) during the northeast winter monsoon seasons of 1996–2000, and their yearly and monthly mean variabilities are investigated. The 5‐year mean Ångström wavelength exponent α is found to be 1.5 over coastal India (CI), AS, and TIO. The 5‐year mean β, turbidity parameter, is found to be about 0.14 over CI, 0.10 over AS, and about 0.05 over TIO indicating the less polluted nature of TIO. The α and β values display year‐to‐year and monthly mean variations. The 5‐year mean α and β values over two land locations (Trivandrum (TVM), Vishakapatnam (VSK)) in peninsular India and marine locations (Minicoy (MCY), Kaashidhoo (KCO)) are found to be around 1 and 0.2. An examination of the daily mean wind speeds and the 0.5 μm AODs obtained during 1996–2000 yielded an index of wind dependence 0.05 for AS and 0.11 over TIO. The scaling distance, the distance up to which the effects of continental aerosols are noticeable over the oceanic regions, is estimated to be 1500 km over the Indian Ocean region. The 5‐year mean anthropogenic contribution to the 0.5 μm AODs over CI, AS, and TIO estimated by comparing with the wind speed dependent sea salt aerosol is found to be above 90% over CI and AS, while it is 74% over TIO. The 5‐year mean anthropogenic contribution over TVM and VSK is in excess of 80%. Over MCY/KCO the anthropogenic contribution is lower at 70%.

Spectral aerosol optical characteristics during the northeast monsoon over the Arabian Sea and the tropical Indian Ocean: 1. Aerosol optical depths and their variabilities

From the cruise expeditions conducted during 1996–2000 in the northeast winter monsoon seasons over the Arabian Sea (AS), tropical Indian Ocean (TIO) aerosol optical depths (AODs) in the 0.4–0.85 μm wavelength region are measured, and their yearly, monthly mean variabilities and trends are analyzed. The AODs are found to show a steep latitudinal gradient as the ship moves away from the coast towards the pristine oceanic region. AODs are found to exhibit an increasing trend over coastal India (CI) and AS, while the AODs are found to decrease over TIO during the 5‐year period. The monthly mean AODs from December to April are found to increase over CI and AS, while they decrease over TIO. AODs obtained over two land locations (Trivandrum (TVM), Vishakapatnam (VSK)) in peninsular India, and two marine locations (Minicoy (MCY), Kaashidhoo (KCO)), situated in the downwind direction of the Indian subcontinent for the 1996–2000 period are analyzed and compared. The 5‐year mean AOD spectra over TVM, VSK, and MCY/KCO are found to exhibit a less steeper gradient and the AODs over these locations are comparable. The AODs are found to show an increasing trend in the 4‐year period over TVM. Over VSK no definite positive or negative trends are visible in the AODs. The yearly mean AODs over MCY/KCO are found to exhibit a decreasing trend quite consistent with the trends seen over the Indian Ocean. The monthly mean AODs are found to increase over the land and marine locations during the December–April period.

General circulation model estimates of aerosol transport and radiative forcing during the Indian Ocean Experiment

Aerosol sources, transport, and sinks are simulated, and aerosol direct radiative effects are assessed over the Indian Ocean for the Indian Ocean Experiment (INDOEX) Intensive Field Phase during January to March 1999 using the Laboratoire de Météorologie Dynamique (LMDZT) general circulation model. The model reproduces the latitudinal gradient in aerosol mass concentration and optical depth (AOD). The model‐predicted aerosol concentrations and AODs agree reasonably well with measurements but are systematically underestimated during high‐pollution episodes, especially in the month of March. The largest aerosol loads are found over southwestern China, the Bay of Bengal, and the Indian subcontinent. Aerosol emissions from the Indian subcontinent are transported into the Indian Ocean through either the west coast or the east coast of India. Over the INDOEX region, carbonaceous aerosols are the largest contributor to the estimated AOD, followed by sulfate, dust, sea salt, and fly ash. During the northeast winter monsoon, natural and anthropogenic aerosols reduce the solar flux reaching the surface by 25 W m−2, leading to 10–15% less insolation at the surface. A doubling of black carbon (BC) emissions from Asia results in an aerosol single‐scattering albedo that is much smaller than in situ measurements, reflecting the fact that BC emissions are not underestimated in proportion to other (mostly scattering) aerosol types. South Asia is the dominant contributor to sulfate aerosols over the INDOEX region and accounts for 60–70% of the AOD by sulfate. It is also an important but not the dominant contributor to carbonaceous aerosols over the INDOEX region with a contribution of less than 40% to the AOD by this aerosol species. The presence of elevated plumes brings significant quantities of aerosols to the Indian Ocean that are generated over Africa and Southeast and east Asia.

Sedimentary phosphorus record from the Oman margin: New evidence of high productivity during glacial periods

The northern region of the Arabian Sea is one of the biologically most fertile regions of the world oceans, with present productivity rates varying between 150 and 2500 mgC/m2 × day [Madhupratap et al., 1996]. This is related to the influence of the southwesterly summer monsoon which causes vigorous upwelling along the Oman margin. Upwelling ceases during northeasterly winter monsoon activity; productivity rates, however, remain relatively high (about 800 mgC/m2 × day), related to deep water mixing [Madhupratap et al., 1996]. The goal of this study is to verify if during the last glacial period, a period in which winter monsoon conditions prevailed, productivity rates were similarly high. With an analysis of phosphorus phases, stable nitrogen isotopes, organic matter content, and bulk mineralogy of the upper 10 m of the cores of ODP Hole 724C (corresponding to the last 140,000 years, sample resolution is ∼5 kyr), we provide new evidence of high productivity during this last glacial period (marine isotopic stages 2, 3, and 4). This was probably related to the combined effect of (1) increased eolian input of iron‐containing dust due to dryness on the adjacent continent and stronger winter monsoon, and (2) regeneration and diffusion of dissolved phosphorus from the sediments to the water column due to variations in the position and intensity of the Oxygen Minimum Zone. These findings suggest that there is no one‐to‐one relationship between summer monsoon activity and productivity, which emerges to be a quasi‐persistent phenomenon across glacial and interglacial stages.

Seasonal variation of volume transport in the major inflow region of the Taiwan Strait: the Penghu Channel

Abstract Eight cruises of current measurements along a zonal transect (∼31.84 km) across the major inflow region of the Taiwan Strait, the Penghu Channel, were carried out using the shipboard Acoustic Doppler Current Profiler during 1999–2001. On each cruise, the measurement was repeated twice along the transect with a time lag of 6 h and 12 min, and the repeated data were averaged to eliminate the dominant semidiurnal tidal currents. Velocities, after removing semidiurnal tides, suggest a strong northward flow in the channel, with a speed of about 100 cm/s in the upper 50 m in summer. The northward flow becomes much weaker in winter. The calculated throughflow transports vary seasonally and are correlated with the change of the East Asia monsoon. The estimated transport is around 0 during the peak northeast monsoon in winter, increases from 0.5 to 1 Sv (1 Sv=106 m3/s) as the northeast monsoon weakens in spring, peaks to 1.5 Sv at the end of southwest monsoon in summer, and decreases rapidly from 1.5 to 0 Sv when the northeast monsoon intensifies in fall. The error, mostly induced by unfiltered diurnal tidal currents, is estimated to be±0.20 Sv in the transport calculation.

Seasonality and composition of downward particulate fluxes at the continental shelf and Okinawa Trough in the East China Sea

Abstract As part of the Marginal Sea Flux Experiment in the West Pacific (MASFLEX) program, conventional sediment traps were deployed at three mooring sites on the East China Sea shelf for 1–2 days during 4 seasonal cruises. Time-series sediment traps also were deployed in the East China Sea, one at the northern part of the Okinawa Trough (Stn. SST 1) and the other to the north-west of the Okinawa Island (SST 2), for 1 year from March 1993 to February 1994. The settling particles were analyzed for total mass flux and the abundance of major biogenic constituents (organic carbon, nitrogen, inorganic carbon and opal). On the inner shelf, particulate fluxes were large and rapidly increased near the bottom throughout the year, probably due to resuspended bottom sediment and suspended particles from the Changjiang, forming a bottom turbid layer (BTL). On the central shelf, particulate fluxes in the BTL were extremely high in summer when Changjiang discharge peaks, possibly indicating a seasonal supply of the Changjiang particles to the central shelf. At the shelf edge, the fluxes in the BTL were significantly higher in autumn and winter than in spring and summer. At the Okinawa Trough stations (ca. 1100 m ), particulate fluxes at 600 m depth were extremely low but increased noticeably with depth (800 and 50 m above the bottom). The flux generally peaked from late autumn to early spring and was lowest during summer, with an unusually large peak in September at SST1. Considering that there is no strong seasonality of the primary productivity and phytoplankton biomass in the surface layers at the Okinawa Trough, we hypothesize that the seasonality of particle fluxes at the deep waters of the Trough is linked to seasonal events in shelf waters. Downwelling and seaward bottom flow induced by the northeast monsoon in winter favor the offshore transport of particles in the BTL, while in summer a transverse circulation pattern may inhibit it (Hu, 1994; Yanagi et al., 1996). Near-bottom transport may be a key process for shelf-to-deep sea export of biogenic/lithogenic particles. As a result, the continental slope and the Okinawa Trough may play an important role as a carbon sink for the East China Sea.

Winter to summer monsoon variation of aerosol optical depth over the tropical Indian Ocean

The northern Indian Ocean undergoes a major transition from anthropogenic aerosols during the northeasterly winter monsoon season to mineral dust and sea salt during the southwest summer monsoon. The former is dominated by low‐level transport from south and southeast Asia, while the latter results from low to mid tropospheric transport from the African continent and the Arabian Peninsula. During the winter monsoon, low‐level transport from the Indian subcontinent and neighboring nations is particularly important. This paper uses 5 years of satellite‐derived aerosol optical depths (AODs) to document the large seasonal variations in AODs modulated by the monsoons. In situ data are used to develop and validate the procedure we employ to derive visible AODs from the radiances observed by the advanced very high resolution radiometer (AVHRR) onboard polar‐orbiting satellites. Monthly mean AOD over the Arabian Sea displays a clear annual cycle with a maximum (0.60 ± 0.10) in July and a minimum (0.20 ± 0.05) in January. The summer monsoon maximum is due both to the southwest winds bringing dust from the Horn of Africa over the Arabian Sea and mid tropospheric transport of dust from the Arabian Peninsula. Such long‐range transports also lead to aerosol loading south of the equator. We also show that the Indonesia forest fires during the 1997 El Niño led to a large increase in AOD over most of the equatorial Indian Ocean.

Processes controlling interannual variations in wintertime (Northeast Monsoon) primary productivity in the central Arabian Sea

Three years of ocean color observations obtained by SeaWiFS reveal significant interannual variation in surface chlorophyll a (Chl a) concentrations in the central Arabian Sea during the Northeast (winter) Monsoon (NEM). Consistent with previous findings in the literature, no obvious relation to sea-surface temperature is apparent. A strong relationship with interannual variability in thermocline depth has been established using an interannually forced ocean general circulation model (OGCM). This relationship consists of reduced Chl a concentration associated with a deeper thermocline. Deeper winter convection is generally associated with higher nutrient concentrations and therefore higher phytoplankton biomass. Both in situ observations from the US JGOFS Arabian Sea Expedition and net wintertime nutrient entrainment estimated with the OGCM simulation indicate that mixed-layer concentrations are always sufficiently high to be non-limiting for phytoplankton growth. This raises two questions. What process(es) check phytoplankton growth? What leads to the observed relationship between deeper thermocline and reduced chl a concentration? A prominent feature of the NEM is a large-amplitude diurnal cycle of the mixed layer that is evident in moored temperature time-series. We hypothesize that the night-time penetration of this diurnal mixing, which is defined by the interannually varying thermocline depth, determines the magnitude of phytoplankton biomass that will be retained in the euphotic zone for the following photoperiod. This daily dilution acts to check the accumulation of phytoplankton biomass and prevents a full phytoplankton bloom. A simple 1-D model has been developed to quantify this process. An excellent correspondence exists between model-predicted mixed-layer Chl a concentration as it varies with thermocline depth and the similarly represented SeaWiFS observations.

South China Sea

The South China Sea is poorly understood in terms of its marine biota, ecology and the human impacts upon it. What is known is most often contained in reports and workshop and conference documents that are not available to the wider scientific community. The South China Sea has an area of some 3.3 million km 2 and depths range from the shallowest coastal fringe to 5377 m in the Manila Trench. It is also studded with numerous islets, atolls and reefs many of which are just awash at low tide. It is largely confined within the Tropic of Cancer and, therefore, experiences a monsoonal climate being influenced by the Southwest Monsoon in summer and the Northeast Monsoon in winter. The South China Sea is a marginal sea and, therefore, largely surrounded by land. Countries that have a major influence on and claims to the sea include China, Malaysia, the Philippines and Vietnam, although Thailand, Indonesia and Taiwan have some too. The coastal fringes of the South China Sea are home to about 270 million people that have had some of the fastest developing and most vibrant economies on the globe. Consequently, anthropogenic impacts, such as over-exploitation of resources and pollution, are anticipated to be huge although, in reality, relatively little is known about them. The Indo-West Pacific biogeographic province, at the centre of which the South China Sea lies, is probably the world's most diverse shallow-water marine area. Of three major nearshore habitat types, i.e., coral reefs, mangroves and seagrasses, 45 mangrove species out of a global total of 51, most of the currently recognised 70 coral genera and 20 of 50 known seagrass species have been recorded from the South China Sea. The island groups of the South China Sea are all disputed and sovereignty is claimed over them by a number of countries. Conflicts have in recent decades arisen over them because of perceived national rights. It is perhaps because of this that so little research has been undertaken on the South China Sea. What data are available, however, and if Hong Kong is used, as it is herein, as an indicator of what the perturbations of other regional cities upon the South China Sea are like, then it is impacted grossly and an ecological disaster has probably already, but unknowingly, happened.

Atmospheric particle size and composition measurements to support light extinction calculations over the Indian Ocean

The size distribution and chemical composition of the atmospheric aerosol at the Kaashidhoo Climate Observatory (KCO) in the Republic of Maldives was determined during the winter northeast monsoon season to aid in determining the light scattering and light absorption properties of the aerosol particles in that region. These experiments were conducted over 8 two‐day periods during February 11–26, 1999, using filter‐based samplers and cascade impactors operated at ambient relative humidity which was in the range of 80–89% relative humidity over 83% of the period sampled. Fine particle concentrations (Da<1.8 μm) averaged 17.7(±0.22) μg m−3 and varied between 8.4(±0.33)–24.7(±0.21) μg m−3 over the period studied. Sulfate ion and carbonaceous aerosols are the largest contributors to the fine particle mass concentration, accounting for 33–37% and 26–27% of the fine mass, respectively. Calcium carbonate contributes 3% of the mass measured on the impactor stages. Ammonium, nitrate, and chloride ion account for 7–9, 1, and 0–1% of the fine particle mass, respectively. The residual mass of as yet undetermined fine particle material stands at 28–30%. Black elemental carbon particles contribute 6–11% of the fine particle mass concentration and dominate light absorption in the atmosphere at KCO (A. Eldering et al., unpublished manuscript, 2001). These fine particle concentrations are comparable to those found in major cities in the United States and are surprisingly high for a remote location such as the Maldive Islands, which is located downwind of the Indian subcontinent.

Effects of atmosphere-ocean interaction on the interannual variability of winter temperature in Taiwan and East Asia

This study investigated the ocean-atmosphere interaction effect on the winter surface air temperature in Taiwan. Temperature fluctuations in Taiwan and marine East Asia correlated better with a SST dipole in the western North Pacific than the SST in the central/eastern equatorial Pacific. During the warm (cold) winters, a positive (negative) SST anomaly appears in marine East Asia and a negative (positive) SST anomaly appears in the Philippine Sea. The corresponding low-level atmospheric circulation is a cyclonic (anticyclonic) anomaly over the East Asian continent and an anticyclonic (cyclonic) circulation in the Philippine Sea during the warm (cold) winters. Based on the results of both numerical and empirical studies, it is proposed that a vigorous ocean-atmosphere interaction occurring in the western North Pacific modulates the strength of the East Asian winter monsoon and the winter temperature in marine East Asia. The mechanism is described as follows. The near-surface circulation anomalies, which are forced by the local SST anomaly, strengthen (weaken) the northeasterly trade winds in the Philippine Sea and weaken (strengthen) the northeasterly winter monsoon in East Asia during warm (cold) winters. The anomalous circulation causes the SST to fluctuate by modulating the heat flux at the ocean surface. The SST anomaly in turn enhances the anomalous circulation. Such an ocean-atmosphere interaction results in the rapid development of the anomalous circulation in the western North Pacific and the anomalous winter temperature in marine East Asia. This interaction is phase-locked with the seasonal cycle and occurs most efficiently in the boreal winters.