Monsoon Forcing Research Articles

Monsoon Forcing

Monsoon Forcing

Seasonal and Mesoscale Variability of the Kuroshio Near Its Origin

The Kuroshio is the most important current in the North Pacific. Here, we present historical data and recent observations of the Kuroshio off the coasts of Taiwan and the Philippine Archipelago, with a focus on its origins. Seasonal climatologies from shipboard hydrographic and velocity measurements, and from surface drifters, demonstrate changes in the Kuroshio caused by the monsoon. In particular, seasonal monsoon forcing affects the degree of penetration of the Kuroshio through Luzon Strait. Data from surface drifters and underwater gliders describe its mesoscale variability. Velocities derived from drifters make clear the mesoscale variability associated with the Subtropical Countercurrent east of the Kuroshio. Underwater gliders document mesoscale structure prominent in salinity extrema associated with water masses. The evolution of these water masses as they progress northward near the Kuroshio indicates strong mixing in the region.

An overview of physical and biogeochemical processes and ecosystem dynamics in the Taiwan Strait

The Taiwan Strait is an important channel in the west Pacific Ocean transporting water and chemical constituents between the East China Sea and the South China Sea. Due to its complex bottom topography, alternating monsoon forcing and conjunction of several current systems [such as the Zhejiang–Fujian (Zhe–Min) Coastal Current, the Kuroshio intrusion and the extension of the South China Sea Warm Current], the physical and biogeochemical processes and ecosystem dynamics in the Taiwan Strait vary significantly both in space and in time. Our recent interdisciplinary studies, combining in situ and remote sensing observations with numerical modeling, allow us to address several important issues concerning the Taiwan Strait. The temporal and spatial variation of circulation in the Taiwan Strait is modulated by strong monsoon forcing, complex topography and circulation in the northern South China Sea as well as coastal water input and the Kuroshio intrusion. The biogeochemical processes of carbon and nutrients in the Taiwan Strait depend largely on the physical forcing (external input) and the community structure (internal cycling). The primary producers in the Taiwan Strait are dominated by nano- and pico-phytoplankton, and the contribution of the microbial food web to the traditional food web is estimated to be about 30%, implying the fundamental significance of the microbial food web in this subtropical region. Upwelling is a predominant feature in the Taiwan Strait and shows dynamic short-term, seasonal and interannual variations. Combined hydrographic and satellite-derived information provides evidence on the teleconnection between the Taiwan Strait upwelling variation and the El Nino-Southern Oscillation (ENSO) climate variability. Upwelling has a tremendous impact on biogeochemical processes, biological productivity and ecosystem structure. Not only the biological productivity, but also dramatic changes in the phytoplankton community structure reveal the dynamic ecosystem responses to the variations in upwelling, which should have significant impact on the fishery resources. In this overview, we summarize the hydrographical features with an emphasis on upwelling, which is the key driver of biogeochemical processes and ecosystem dynamics in the Taiwan Strait.

An open‐ocean forcing in the East China and Yellow seas

Recent studies have demonstrated that the annual mean barotropic currents over the East China and Yellow seas (ECYS) are forced primarily by the oceanic circulation in the open‐ocean basin through the Kuroshio Current (KC), the western boundary current of the subtropical gyre in the North Pacific Ocean. The local wind stress forcing plays an important but secondary role. Those previous results were mainly qualitative and from a simple barotropic model forced by a steady wind stress field. They remain to be tested in a more complete 3‐D model with both wind stress and buoyancy fluxes. In addition, the seasonal variability of major ECYS currents may involve different forcing mechanisms than their annually averaged fields do, and this can only be addressed when a seasonally varying forcing is used in the model. In this paper, we will address these issues by using a 3‐D baroclinic model. Our results confirm the finding from the previous studies that the KC is the primary forcing mechanism for major annually mean currents in the ECYS, which include the Taiwan Strait Current, the Tsushima Warm Current, and the Yellow Sea Warm Current (YSWC), etc. However, the local monsoonal forcing plays a prominent role in modulating the seasonal variability of all major currents in the region. A deep northwestward intrusion of the YSWC in winter, for instance, is mainly due to a robustly developed China Coastal Current and Korea Coastal Current, which draw water along the Yellow Sea Trough to feed the southward flows along the west and east coasts of the Yellow Sea.

Indian Ocean near-equatorial symmetric stability from satellite observations: an elusive connection to atmospheric convection

The north–south symmetry of the low level near-equatorial atmospheric flow is examined in relation to the position of the Intertropical Convergence Zone (ITCZ) over the Indian Ocean with the aid of satellite and reanalysis data. Absolute vorticity (AV) and convergence fields on temporal scales of 10 days are derived from 7 years of Quick Scatterometer (QuikSCAT) observations and related statistically to organized convection deduced from outgoing longwave radiation (OLR) measured by the National Oceanic and Atmospheric Administration (NOAA) polar-orbiting satellites. The AV observations reveal a belt of symmetric instability on the summer side of the equator. The evolution of this symmetric instability with respect to the cross-equatorial flow (CEF), the location of the ITCZ, and monsoonal forcing is investigated. We identify three distinct dominant convective regimes. We show that during the active Indian monsoon season, symmetric instability and the displacement of the zero-AV line off the equator are directly linked to organized convection and the off-equatorial location of the ITCZ. The advective (inertial) regime interacts with and is controlled by monsoon and CEF. In periods of transition between summer and winter monsoon, the lower atmosphere is symmetrically stable and two other convective regimes can be identified: a double ITCZ, and localized convection near the equator.

Late Glacial–Holocene climate variability at the south-eastern margin of the Aegean Sea

New micropaleontological, palynological, and geochemical results from a relatively shallow (∼ 500 m) sediment core (NS-14) in the south-eastern Aegean Sea provide a detailed picture of the regional expression of sapropel S1 formation in this sub-basin of the eastern Mediterranean Sea. Specifically, freshwater input during ∼ 10.6–10.0 ka BP has preceded the deposition of S1. Further decrease in surface water salinity is evidenced between 10.0 and 8.5 ka BP at the lower part of S1a, which in respect to S1b, is featured by warmer (∼ 19.5 °C) and more productive surface waters associated with dysoxic bottom conditions. A series of coolings detected within the S1 depositional interval, may be linked to outbursts of cold northerly air masses and relevant pulses in the deep-intermediate water ventilation that caused the S1 interruption between 7.9 and 7.3 ka BP and culminated during the deposition of S1b, with the decline of deep chlorophyll maximum (DCM) at ∼ 6.5 ka BP. The climate instability and the relevant absence of anoxia weakened the organic matter preservation in the shallow south-eastern Aegean margin during the S1 times. NS-14 record provides evidence for a distinct mid Holocene warm (up to ∼ 25 °C) and wet phase associated with the deposition of the sapropel-like layer SMH (Sapropel Mid Holocene), between 5.4 and 4.3 ka BP. The SMH layer could represent evidence of on-going, albeit weak, African monsoon forcing, only expressed at the south-eastern edge of the Aegean Sea. Its end is associated with the 4.2 ka BP Northern Hemisphere mega-drought event and the termination of the African Humid Period at 3.8 ka BP.

Direct estimates of the Indonesian Throughflow entering the Indian Ocean: 2004–2006

The mean and variable transport of the Indonesian Throughflow (ITF) are determined from full‐depth velocity measurements in the three major exit passages of Lombok Strait, Ombai Strait, and Timor Passage from January 2003 through December 2006. Collectively, these passages convey the full‐depth transport and stratification profile of the ITF from the Pacific Ocean to the Indian Ocean. To first order, the seasonal cycle of transport in the thermocline (∼100–150 m) in all three exit straits is dominated by regional monsoon forcing, with maximum ITF during the southeast monsoon. During the northwest monsoon, the surface transport relaxes in Timor and weakly reverses in Ombai and Lombok, so the main core of the ITF is subsurface. Below the thermocline, semiannual reversals occur in all three straits during the monsoon transitions in response to the passage of Indian Ocean wind‐forced Kelvin waves. However, the reversals occur over different depth levels in each passage reflecting the influence of different sill depths along the coastal waveguide. The seasonal cycle of depth‐integrated transports in Lombok and Ombai are strongly out of phase with Timor Passage, suggesting that the subthermocline flow is largely gated by these Kelvin waves. Despite the different seasonal transport phases, interannual anomalies in all three passages are remarkably similar, particularly during the strong positive Indian Ocean Dipole event in 2006 when transport in the surface layer is toward the Indian Ocean and reversed below. The deep reversals are likely in response to a series of Kelvin waves driven by anomalous zonal winds in the equatorial Indian Ocean. Total mean transport over the 3‐year period is −2.6 Sv in Lombok Strait (i.e., toward the Indian Ocean), −4.9 Sv in Ombai Strait, and −7.5 Sv in Timor Passage. The transport in Timor Passage is nearly twice as large as historical estimates and represents half of the −15 Sv full‐depth ITF transport that enters the Indian Ocean.

The Study of the Yellow Sea Warm Current and Its Seasonal Variability

The Yellow Sea Warm Current (YSWC) penetrates northward along the Yellow Sea Trough, and brings warm and saline water towards the Bohai Sea. The YSWC becomes much less intrusive in summer and is limited mostly in the southern trough, contrasting with a deep winter penetration well into the trough. The seasonal variability of the YSWC has prompted a debate regarding which controls the YSWC and its seasonal variability. In this article, the annual mean and seasonal variability of the YSWC was examined by using a 3-D ocean model together with several experiments. The results show that in the annual mean the YSWC is a compensating current firstly for the southward Korea Coastal Current (KCC), which is mainly caused by the Kuroshio Current (KC). The local wind-stress forcing plays an important but secondary role. However, the local monsoonal forcing plays a prominent role in modulating the seasonal variability. A deep northwestward intrusion of the YSWC in winter, for instance, is mainly due to a robustly developed China Coastal Current (CCC) which draws water along the Yellow Sea trough to feed a southward flow all the way from the Bohai Sea to the Taiwan Strait.

Holocene glacier fluctuations and migration of Neolithic yak pastoralists into the high valleys of northwest Bhutan

Here we present geomorphologic, palaeoenvironmental and archaeo-botanical data which elucidate the Late Pleistocene and Holocene glacial history of the high, mountain-locked Himalayan valleys in northwest Bhutan and provide one of the earliest proofs of human activity yet known for the High Himalaya range. In this area, difficult to access, close linkage between climatic change, glacier fluctuations and human migration patterns has been discovered. Glacier systems in the studied area are characterized by avalanching and debris mantled glacier snouts, with the significant local influence of the Indian summer monsoon causing decoupling of glacier responses from temperature changes but supporting the idea of monsoonal forcing. Geomorphologic mapping, together with Optically Stimulated Luminescence (OSL) and radiocarbon dating of ice-proximal sediments, has been used to construct a local glacial chronology. Local ice-stream networks developed during the Early Holocene (ca 10,000–9000 a ago) and during the early part of the Mid Holocene (6710 ± 90–4680 ± 155 cal a BP) at which times there were ice advances of about 5 km from the modern glacier termini. At such times, the intensity of pro- and periglacial processes would have intensified and ice-dammed lakes were probably common as well, rendering human colonization of the high valleys in northwest Bhutan impossible. An abrupt shift to dry climatic conditions on the Tibetan Plateau between 5000 and 4500 a BP coincided with glacial decay and the onset of morphodynamically stable conditions on the broad valley floors of the high valleys in this part of the Himalaya. Palynological data suggest that the sudden disappearance of juniper and rhododendron pollen, the immediate onset of pollen input from cereals (confirmed by detailed SEM analysis) and a clear pattern of over-grazing, trampling and peat deterioration can be linked to human arrival in the valleys at ca 4280 ± 130 cal a BP. Extensive charcoal horizons dating to 4745 ± 250 and 4680 ± 155 cal a BP are interpreted as evidence for human use of fire and forest clearances and agree spatially and temporally with the pollen-based picture. Charcoal occurrences as old as 6710 ± 90 cal a BP might be linked to yet earlier exploration of these Himalayan valleys during phases of low glacial activity. We provide an account of the colonization of these high valleys in response to glacial and monsoonal change and argue that the most likely founder societies come from the Tibetan Plateau, where yak and barley based pastoralism and Neolithic settlements are known to have existed since the Mid Holocene.

Modelling the circulation on the continental shelf of the province Khanh Hoa in Vietnam

A model simulation of the circulation on the continental shelf of the Khanh Hoa province in central Vietnam during the year 2004 is presented. The model, a three-dimensional baroclinic z-coordinate model (the Hamburg Shelf Ocean Model), is implemented with a horizontal resolution of about 1 km. It is initialised with temperature and salinity fields taken from the Levitus data, and by the two main tidal constituents. The model is forced by daily fields of wind stress, air temperature, wind speed, and cloudiness taken from NCEP, and by monthly mean river runoff values. At the open boundary sea surface displacements are prescribed by the tidal variation and by the steric height determined from the density anomalies determined from climatological values of temperature and salinity. The modelled circulation reflects the monsoonal forcing fields, and reveals downwelling during winter and upwelling during summer. The modelled hydrography is compared with measured profiles, and some biases are found. The flushing times of three bays along the Khanh Hoa coast are calculated. The relative influence of river runoff, tides, and weather on the flushing times is discussed.

Highly improved predictive skill in the forecasting of the East Asian summer monsoon

The East Asian summer monsoon greatly influences the lives and property of about a quarter of the people in the world. However, the predictive skill of the monsoon is very low in comparison with that of the Indian summer monsoon because of the complexity of the system which involves both tropical and subtropical climates. Previous monsoon prediction models emphasized ocean factors as the primary monsoon forcing. Here we show that preseason land surface cover is at least as important as ocean indices. New statistical forecast models of the East Asian summer monsoon using land cover conditions in addition to ocean heat sources double and triple, respectively, the predictive skill of the northern and southern East Asian summer monsoon forecasting models relative to models using ocean factors alone. This work highlights the, as yet, undocumented importance of seasonal land cover in monsoon prediction and the role of the biosphere in the climate system as a whole. We also detail the physical mechanisms involved in these land surface forcings.

Monsoon-ocean coupled modes in the South China Sea and their linkage with the eastern Indian Ocean-western Pacific warm pool

Monsoon-ocean coupled modes in the South China Sea (SCS) were investigated by a combined singular value decomposition (CSVD) analysis based on sea surface temperature (SST) and sea surface wind stress (SWS) fields from SODA (Simple Ocean Data Assimilation) data spanning the period of 1950–1999. The coupled fields achieved the maximum correlation when the SST lagged SWS by one month, indicating that the SCS coupled system mainly reflected the response of the SST to monsoon forcing. Three significant coupled modes were found in the SCS, accounting for more than 80% of the cumulative squared covariance fraction. The first three SST spatial patterns from CSVD were: (I) the monopole pattern along the isobaths in the SCS central basin; (II) the north-south dipole pattern; and (III) the west-east seesaw pattern. The expansion coefficient of the SST leading mode showed interdecadal and interannual variability and correlation with the Indo-Pacific warm pool (IPWP), suggesting that the SCS belongs to part of the IPWP at interannual and interdecadal time scales. The second mode had a lower correlation coefficient with the warm pool index because its main period was at intra-annual time scales instead of the interannual and interdecadal scales with the warm pools. The third mode had similar periods to those of the leading mode, but lagged the eastern Indian Ocean warm pool (EIWP) and western Pacific warm pool (WPWP) by five months and one year respectively, implying that the SCS response to the warm pool variation occurred from the western Pacific to the eastern Indian Ocean, which might have been related to the variation of Indonesian throughflow. All three modes in the SCS had more significant correlations with the EIWP, which means the SCS SST varied much more coherently with the EIWP than the WPWP, suggesting that the SCS belongs mostly to part of the EIWP. The expansion coefficients of the SCS SST modes all had negative correlations with the Nino3 index, which they lag by several months, indicating a remote response of SCS SST variability to the El Nino events.

The effect of doubled CO2 and model basic state biases on the monsoon‐ENSO system. II: Changing ENSO regimes

AbstractIntegrations of a fully‐coupled climate model with and without flux adjustments in the equatorial oceans are performed under 2 × CO2 conditions to explore in more detail the impact of increased greenhouse gas forcing on the monsoon‐ENSO system. When flux adjustments are used to correct some systematic model biases, ENSO behaviour in the modelled future climate features distinct irregular and periodic (biennial) regimes. Comparison with the observed record yields some consistency with ENSO modes primarily based on air‐sea interaction and those dependent on basinwide ocean wave dynamics. Simple theory is also used to draw analogies between the regimes and irregular (stochastically forced) and self‐excited oscillations respectively. Periodic behaviour is also found in the Asian‐Australian monsoon system, part of an overall biennial tendency of the model under these conditions related to strong monsoon forcing and increased coupling between the Indian and Pacific Oceans. The tropospheric biennial oscillation (TBO) thus serves as a useful descriptor for the coupled monsoon‐ENSO system in this case. The presence of obvious regime changes in the monsoon‐ENSO system on interdecadal timescales, when using flux adjustments, suggests there may be greater uncertainty in projections of future climate, although further modelling studies are required to confirm the realism and cause of such changes. Copyright © 2007 Royal Meteorological Society

Biogeochemical and ecological research in the Indian Ocean

The Indian Ocean has many unique characteristics that give rise to a variety of physical, biogeochemical, and ecological responses that are not observed in other ocean basins. For instance, atmospheric circulation over the ocean is seasonally influenced by the presence of the Asian landmass. The monsoonal forcing results in seasonally reversing surface oceanic circulations throughout the northern Indian Ocean, which are particularly energetic in the Arabian Sea. Unlike the Pacific and Atlantic oceans, there is no strong equatorial upwelling or biological response in the east. Furthermore, the Indian Ocean contains one of the most intense oxygen minimum zones (OMZs) in the world oceans, but unlike the Atlantic and Pacific, its OMZ is located in the north rather than along the eastern boundary.

Observations and proxies of the surface layer throughflow in Lombok Strait

Seasonal to interannual variability of the Lombok Strait surface layer transport is investigated. The geostrophic transport within the surface layer is estimated from the cross‐channel pressure gradient measured by a pair of shallow pressure gauges positioned on opposing sides of Lombok Strait during 1996‐1999. The Ekman transport through Lombok Strait, derived from scatterometer winds, is less than 10% or ∼0.15 Sv of the estimated surface layer geostrophic transport. Monsoonal forcing is clearly evident in the regional sea surface height anomalies (SSHA) as derived from the satellite altimeter measurements. During the southeast monsoon, relatively low sea level is observed to the south of Lombok Strait, with relatively high sea level to the north; conditions reverse during the northwest monsoon. Estimated transports from the cross‐channel pressure gradient, winds, SSHA and thermocline depth anomalies all reveal interannual variability associated with ENSO. Both the thermocline depth anomaly and the SSHA to the south of the East Java coast correlate significantly (r = 0.7) with the Lombok Strait total surface layer throughflow. The difference of SSHA from the south of the East Java coast minus the SSHA north of Lombok shows a higher correlation (r = 0.84). These high correlation values suggest that SSHA and thermocline depth anomalies can be used as proxies for the Lombok Strait surface layer throughflow. Qualitatively, such proxy transports agree with the surface transport inferred from the pressure gauges and Ekman transport in Lombok Strait from 1996 to 1999, and also with direct velocity measurements from current meter data obtained in 1985 and 2004‐2005.

Spatial and temporal distribution of Trichodesmium blooms in the world's oceans

Nitrogen fixing organisms, such as the cyanobacterium Trichodesmium, directly affect the oceanic nutrient inventory through the addition of new nitrogen to the ocean ecosystem and therefore have an important role in the strength and functioning of the biological carbon pump. Nonetheless, little is known about the distribution of Trichodesmium beyond limited shipboard observations. Even less is known about the occurrence and characteristics of very intense, transient blooms of this organism that have been observed historically throughout the world oceans. A new method for discriminating the occurrence of blooms from satellite ocean color data is used here to make the first global maps of Trichodesmium bloom occurrence and to examine their spatial and temporal distribution. As expected, Trichodesmium blooms are rare, occurring <5–10% of the time over most of the tropical and subtropical oceans for the time period examined (1998–2003). Areas of greatest persistence are found in the eastern tropical Pacific and the Arabian Sea, and reach recurrence levels of >30%. Many of the retrieved patterns are consistent with previously reported blooms, though differences exist. A strong seasonal cycle is observed in the Indian Ocean, probably related to monsoonal forcing, with weaker seasonal changes elsewhere. Estimated global nitrogen fixation rates by Trichodesmium blooms is ∼42 Tg N yr−1 which is biogeochemically significant on regional and global scales. Further, an estimate of the rate of Trichodesmium nitrogen fixation under nonbloom conditions is an additional ∼20 Tg N yr−1 suggesting that Trichodesmium is likely the dominant organism in the global ocean new nitrogen budget.

Monsoon-induced upwelling off the Vietnamese coast

During the southwest monsoon from July 8 to 28, 2003, an interdisciplinary cruise took place in the central area of Vietnamese upwelling with “MV Nghien Cuu Bien” in the South China Sea. Physical observations in the upwelling area are analyzed with respect to local/regional wind forcing and far field forcing. Nutrients and phytoplankton measurements are discussed with respect to exchange processes between different water masses. The wind-induced coastal upwelling by local wind forcing is much weaker than in the previous years due to weaker-than-normal winds. This can be attributed to the far field forcing of the 2002/2003 El Nino event which modulates the upwelling intensity. The atmospheric conditions reflect the typical situation after an El Nino event which weakens the wind-induced coastal upwelling, reduces the latent heat flux, and results in higher-than-normal sea-surface temperatures. The general circulation pattern during SW monsoon is driven by the spatial asymmetry in the monsoon forcing. The flow pattern is characterized by an upwelling-induced northward undercurrent and a recently detected southward countercurrent. The resulting stretching deformation of this flow pattern forms an offshore jet between ∼12°N and 12.5°N and causes a local enhancement of the upwelling intensity. The upwelling due to stretching deformation is a peculiarity, which makes the Vietnamese upwelling area different to other upwelling areas. A budget of the upwelling components is presented: the strongest contribution in 2003 to the Vietnamese upwelling is the dynamical upwelling due to the clockwise rotation of the northward undercurrent. The internal radius of deformation separates the upwelling area from the offshore area as well as different water masses. Mekong River and the Gulf of Thailand waters which are offshore show nutrient depletion. Therefore, high chlorophyll maxima cannot be explained by nutrient supply from river runoff. The dynamical upwelling brings in nutrient-rich Maximum Salinity Water into the euphotic zone. This causes a subsurface chlorophyll maximum between 20 and 40 m water depth along the northward undercurrent. Deflection from the Redfield ratio in the C:N ratio and negative excess nitrogen identifies the region as nitrogen-limited which may favor cyanobacteria blooms. The consequence is a unique feature in new production: in the upwelling area, new production is based on upwelled nitrate, whereas offshore in the nutrient-depleted Mekong and Gulf of Thailand water, new production is based in addition on nitrogen fixation.

Monsoonal forcing of Holocene glacier fluctuations in Ganesh Himal (Central Nepal) constrained by cosmogenic 3He exposure ages of garnets

In the Himalayas, the late Pleistocene glacier oscillations have produced spectacular glacial landforms. Detailed reconstructions of the chronology and extent of these oscillations are essential to document the sensitivity of the Himalayan glaciers to past and future climatic changes. In this paper, we present a new cosmogenic helium 3 ( 3He c) dating on garnets, that were sampled on moraine blocks and ice-scoured surfaces in a small glaciated valley of the Central Nepal (the Mailun valley), and that provided a detailed chronology of Himalayan glacier fluctuations during the Holocene. Soon after the Younger Dryas, the glacier of the Mailun valley underwent a significant retreat around 10 ka. This retreat was followed by relative stability of the extent of the glacier between ∼ 8.5 and ∼ 7.5 ka. A second phase of rapid retreat occurred at ∼ 7 ka, but rapidly slowed down at ∼ 5–6 ka. Finally, a last phase of re-advance occurred between 0 and 1 ka. The interpretation of the Equilibrium Line Altitude (ELA) variation, deduced from this chronology (for the Holocene period) and carbon 14 ( 14C) dating (for the Pleistocene period), shows that the early history of the Mailun valley deglaciation (late Pleistocene) is in good agreement with the global paleoclimatic records. The main extent of the glacier and the major ice volume drop are in phase with the global Last Glacial Maximum (25–17 ka) and with the major worldwide temperature increase following the Younger Dryas, respectively, indicating that the Mailun glacier was primarily driven by temperature oscillations during the late Pleistocene. In contrast, the glacier chronology during the Holocene suggests that the Mailun glacier was modulated by the variation in annual precipitation, and is asynchronous relative to most glaciers of the Northern Hemisphere. The significant sensitivity of the Himalayan glaciers to precipitation might explain the striking lack of synchronism of the Himalayan glaciers both along and across the Himalayan arc.

Monsoon forcing, hydrodynamics of the Kuroshio Current, and tectonic effects on sedimentary carbon and sulfur cycling in the Okinawa Trough since 90 ka

Monsoon forcing and hydrodynamic effects controlled carbon and sulfur biogeochemistry over the last 90 ka in a 43‐m core from the Okinawa Trough (OT). Total organic carbon (TOC) contents co‐vary with summer insolation at 30°N suggesting monsoon forcing of primary productivity and organic carbon burial. Before the last glacial maximum (LGM), total sulfur (TS) contents varied concomitantly with TOC. However, sea level rise and intensified Kuroshio inflow during the Holocene enhanced deepwater ventilation, which resulted in TS‐depleted sediments with low degree of pyritization (DOP). By contrast, DOP values were high during the previous highstand (at ∼80 ka BP), when sea level was similar to that in the Holocene, as well as during the LGM when deepwater circulation was relatively weak. A topographic barrier is proposed to have sufficiently blocked the Kuroshio Current out of the OT during the previous sea level highstand, which weakened deepwater ventilation and led to reducing diagenetic conditions. Tectonic rifting in the southern OT at 60‐30 ka BP enabled the major change in hydrodynamics and sediment biogeochemistry.

Environmental change and the arrival of people in the Australian region

Continuous pollen and charcoal records, predominantly from well dated marine sediments, provide a good spatial coverage of patterns of change in vegetation, climate and biomass burning within the northern Australian - southern Indonesian region through the late Quaternary period. Northern hemisphere ice volume and monsoon forcing is conspicuous on orbital timescales with general expansion of wetter communities under higher precipitation during interglacials and their contraction under drier glacials. However, this picture is regionally complicated by both southern hemisphere monsoon and long term El Nino - Southern Oscillation influences. The charcoal records suggest a complex pattern of burning with generally more frequent or intense fires during climate transitions and also during drier periods except when and where fuel availability became a limitation. Superimposed on this largely cyclical pattern is a trend, within the last 300,000 to 200,000 years, towards more open canopied vegetation and increased...