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A review is presented on physical oceanographic features based on expendable CTD data collected in the Indian Ocean sector of the Southern Ocean. The thermohaline structure is dominated by Circumpolar Deep Water. The temperature and salinity are affected by cyclonic circulation in the Weddell Sea and Prydz Bay. High chlorophyll-a blooms (2-4 mg m-3) evolve during austral summer due to stratification which is caused by freshwater generated from the sea ice melt and the glacial outflow which traps phytoplankton in a shallow mixed layer, where they are exposed to higher irradiances of photosynthetically active radiation. Attempts have been made to relate the physical characteristics to biomass inferred from data published from previous Indian Scientific expeditions. More in-situ observations related to biophysical and chemical are recommended in the near future projects. [ABSTRACT FROM AUTHOR]

In the eastern Indian Ocean, the Kimberley region off north-western Australia is characterised by a wide continental shelf, complex coastal topography and very large tides (>10 m). Diversity of fishes is high although little is known about the distribution of their larvae in this dynamic region. This study investigated spatial variation in ichthyoplankton assemblages along four coastal-oceanic transects and related the results to tidal cycles and other environmental variables. Larvae representing 92 neritic and 21 mesopelagic teleost families were collected; this far exceeds the diversity described for other comparable tropical continental shelf and oceanic systems. Distinct ichthyoplankton assemblages characterised inner shelf (Engraulidae and Gobiidae were dominant families), outer shelf / slope (mixed neritic and mesopelagic taxa) and oceanic waters (mesopelagic taxa, particularly Myctophidae, Gonostomatidae and Phosichthyidae), and were correlated with changes in environmental variables (water depth, seawater density, mixed layer depth and zooplankton). The spring-neap tidal cycle and subsequent tides (same stations sampled 12 h apart) had no significant influence on larval fish assemblages. Coastal waters were delineated by a density front and larval fish assemblages were significantly different from offshore assemblages. This study provides new insights into the pelagic ecosystem in the tropical eastern Indian Ocean and contributes to the understanding of the role of environmental variables and physical forcing in the structuring of larval fish assemblages. [ABSTRACT FROM AUTHOR]

Highlights • We use drifting Fish Aggregating Devices (FADs) to derive near-surface ocean currents. • FADs and oceanographic drifters have similar velocities in the Indian Ocean (IO). • FADs have lower velocities than oceanographic drifters in the Atlantic Ocean (AO). • We hypothesize that longer FAD tails in the AO than in the IO slow them down. • Fisher-collected data can become a major contributor of oceanographic information. Abstract Knowledge of ocean surface dynamics is crucial for oceanographic and climate research. The satellite-tracked movements of hundreds of drifters deployed by research and voluntary observing vessels provide high-frequency and high-resolution information on near-surface currents around the globe. Consequently, they constitute a major component of the Global Ocean Observing System (GOOS). However, maintaining this array is costly and in some oceanic regions such as the tropics, spatio-temporal coverage is limited. Here, we demonstrate that the GPS-buoy equipped fish aggregating devices (FADs) used in tropical tuna fisheries to increase fishing success are also capable of providing comparable near-surface current information. We analyzed millions of position data collected between 2008 and 2014 from more than 15,000 FADs and 2,000 drifters, and combined this information with remotely-sensed near-surface current data to demonstrate that the surface velocity components of FADs and drifters are highly correlated in the Atlantic and Indian Oceans. While it was noted that the subsurface structures of FADs did slow them down relative to the drifters, particularly in the Atlantic Ocean, this bias was measurable and could be accounted for in future studies. Our findings show that the physical meteorological and oceanographic data collected by fishermen could provide an invaluable source of information to the GOOS. Furthermore, by forging closer collaborations with the fishing industry and ensuring their contributions to global ocean databases are properly acknowledged, there is significant scope to capture this data more effectively. [ABSTRACT FROM AUTHOR]

Precipitation in southern China during boreal summer (June to August) shows a substantial interdecadal variability on the timescale longer than 8 years. In this study, based on the analysis of singular value decomposition, we diagnose the leading mode of interdecadal covariability between the observational precipitation in southern China and the sea surface temperature (SST) in the Indian Ocean. Results indicate that there exist a remarkable southern China zonal dipole (SCZD) pattern of interdecadal variability of summer precipitation and an interdecadal Indian Ocean basin mode (ID-IOBM) of SST. It is found that the SCZD is evidently covaried with the ID-IOBM, which may induce anomalous inter-hemispheric vertical circulation and atmospheric Kelvin waves. During the warm phase of the ID-IOBM, an enhanced lower-level convergence and upper-level divergence exist over the tropical Indian Ocean, which is a typical Gill-Matsuno-type response to the SST warming. Meanwhile, the accompanied upper-level outflow anomalies further converge over the Indo-China peninsula, resulting in a lower-level anticyclone that contributes to reduction of the eastward moisture transport from the Bay of Bengal to the west part of southern China. In addition, the Kelvin wave-like pattern, as a response of the warm ID-IOBM phase, further induces the lower-level anticyclonic anomaly over the South China Sea–Philippines. Such an anticyclonic circulation is favorable for more water vapor transport from the East China Sea into the east part of southern China. Therefore, the joint effects of the anomalous inter-hemispheric vertical circulation and the Kelvin wave-like pattern associated with the ID-IOBM may eventually play a key role in generating the SCZD pattern. [ABSTRACT FROM AUTHOR]

Using reanalysis data, the role of initial signals in the tropical Pacific Ocean in predictions of negative Indian Ocean Dipole (IOD) events were analyzed. It was found that the summer predictability barrier (SPB) phenomenon exists in predictions, which is closely related to initial sea temperature errors in the tropical Pacific Ocean, with type-1 initial errors presenting a significant west-east dipole pattern in the tropical Pacific Ocean, and type-2 initial errors showing the opposite spatial pattern. In contrast, SPB-related initial sea temperature errors in the tropical Indian Ocean are relatively small. The initial errors in the tropical Pacific Ocean induce anomalous winds in the tropical Indian Ocean by modulating the Walker circulation in the tropical oceans. In the first half of the prediction year, the anomalous winds, combined with the climatological winds in the tropical Indian Ocean, induce a basin-wide mode of sea surface temperature (SST) errors in the tropical Indian Ocean. With the reversal of the climatological wind in the second half of the prediction year, a west-east dipole pattern of SST errors appears in the tropical Indian Ocean, which is further strengthened under the Bjerknes feedback, yielding a significant SPB. Moreover, two types of precursors were also identified: a significant west-east dipole pattern in the tropical Pacific Ocean and relatively small temperature anomalies in the tropical Indian Ocean. Under the combined effects of temperature anomalies in the tropical Indian and Pacific oceans, northwest wind anomalies appear in the tropical Indian Ocean, which induce a significant west-east dipole pattern of SST anomalies, and yield a negative IOD event. [ABSTRACT FROM AUTHOR]

Air-sea coupling processes over the north Indian Ocean associated with the Indian summer monsoon intraseasonal oscillation (MISO) are investigated. Observations show that MISO convection anomalies affect underlying sea surface temperature (SST) through changes in surface shortwave radiation and surface latent heat flux. In turn, SST anomalies may also affect the MISO precipitation tendency (dP/dt). In particular, warm (cold) SST anomalies can contribute to increasing (decreasing) precipitation rate through enhanced (suppressed) surface convergence associated with boundary layer pressure gradients. These air-sea interaction processes are manifest in a quadrature relation between MISO precipitation and SST anomalies. A local air-sea coupling model (LACM) is formulated based on these observed physical processes. The period of the LACM is proportional to the square root of seasonal mixed layer depth H, assuming other physical parameters remain unchanged. Hence, LACM predicts a relatively short (long) MISO period over the north Indian Ocean during the May-June monsoon developing (July-August monsoon mature) phase when H is shallow (deep). This result is consistent with observed MISO characteristics. A 30-day-period oscillating external forcing is also added to the LACM, representing intraseasonal oscillations propagating from the equatorial Indian Ocean to the north Indian Ocean. It is found that resonance will occur when H is close to 25 m, which significantly enhances the MISO amplitude. This process may contribute to the higher MISO amplitude during the monsoon developing phase compared to the mature phase, which is associated with the seasonal cycle of H. [ABSTRACT FROM AUTHOR]

Ocean heat uptake is the primary heat sink of the globe and modulates its surface warming rate. In situ observations during the past half century documented obvious multidecadal variations in the upper-ocean heat content (0-400 m; OHC400) of the Indian Ocean (IO). The observed OHC400 showed an increase of (5.9 ± 2.5) 3 1021 J decade-1 during 1965-79, followed by a decrease of (-5.2 ± 2.5) 3 1021 J decade-1 during 1980-96, and a rapid increase of (13.6 ± 1.1) 3 1021 J decade21 during 2000-14. These variations are faithfully reproduced by an Indo-Pacific simulation of an ocean general circulation model (OGCM), and insights into the underlying mechanisms are gained through OGCM experiments. The Pacific wind forcing through the Indonesian Throughflow (ITF) was the leading driver of the basin-integrated OHC400 increase during 1965-79 and the decrease during 1980-96, whereas after 2000 local wind and heat flux forcing within the IO made a larger contribution. The ITF heat transport is primarily dictated by Pacific trade winds. It directly affects the south IO, after which the signatures can enter the north IO through the meridional heat transport of the western boundary current. The prevailing warming of the western-tocentral IO for 2000-14 was largely induced by equatorial easterly wind trends, Ekman downwelling off the equator, and northeasterly wind trends over the west Asia-East Africa coastal region. The increasing downward longwave radiation, probably reflecting anthropogenic greenhouse gas forcing, overcame the decreasing surface shortwave radiation and also made a significant contribution to the rapid upper-IO warming after 2000. [ABSTRACT FROM AUTHOR]

Decadal variability of the subsurface ocean heat content (OHC) in the Indian Ocean is investigated using a coupled climate model experiment, in which observed eastern tropical Pacific sea surface temperature (EPSST) anomalies are specified. This study intends to understand the contributions of external forcing relative to those of internal variability associated with EPSST, as well as the mechanisms by which the Pacific impacts Indian Ocean OHC. Internally generated variations associated with EPSST dominate decadal variations in the subsurface Indian Ocean. Consistent with ocean reanalyses, the coupled model reproduces a pronounced east-west dipole structure in the southern tropical Indian Ocean and discontinuities in westwardpropagating signals in the central Indian Ocean around 1008E. This implies distinct mechanisms by which the Pacific impacts the eastern and western Indian Ocean on decadal time scales. Decadal variations of OHC in the eastern Indian Ocean are attributed to 1) western Pacific surface wind anomalies, which trigger oceanic Rossby waves propagating westward through the Indonesian Seas and influence Indonesian Throughflow transport, and 2) zonal wind anomalies over the central tropical Indian Ocean, which trigger eastwardpropagating Kelvin waves. Decadal variations of OHC in the western Indian Ocean are linked to conditions in the Pacific via changes in the atmospheric Walker cell, which trigger anomalous wind stress curl and Ekman pumping in the central tropical Indian Ocean. Westward-propagating oceanic Rossby waves extend the influence of this anomalous Ekman pumping to the western Indian Ocean. [ABSTRACT FROM AUTHOR]

Based on reanalysis data, we find that the Indian Ocean Dipole (IOD) plays an important role in the variability of wave climate in the equatorial Northern Indian Ocean (NIO). Significant wave height (SWH) in the equatorial NIO, especially over the waters southeast to Sri Lanka, exhibits strong interannual variations. SWH anomalies in the waters southeast to Sri Lanka correlate well with dipole mode index (DMI) during both summer and autumn. Negative SWH anomalies occur over the oceanic area southeast to Sri Lanka during positive IOD events and vary with different types of IOD. During positive prolonged (unseasonable) IOD, the SWH anomalies are the strongest in autumn (summer); while during positive normal IOD, the SWH anomalies are weak in both summer and autumn. Strong easterly wind anomalies over the southeast oceanic area of Sri Lanka during positive IOD events weaken the original equatorial westerly wind stress, which leads to the decrease in wind-sea waves. The longer wave period during positive IOD events further confirms less wind-sea waves. The SWH anomaly pattern during negative IOD events is nearly opposite to that during positive IOD events. [ABSTRACT FROM AUTHOR]

This paper investigates the features of the Equatorial Intermediate Current (EIC) in the Indian Ocean and its relationship with basin resonance at the semiannual time scale by using in situ observations, reanalysis output, and a continuously stratified linear ocean model (LOM). The observational results show that the EIC is characterized by prominent semiannual variations with velocity reversals and westward phase propagation and that it is strongly influenced by the pronounced second baroclinic mode structure but with identifiable vertical phase propagation. Similar behavior is found in the reanalysis data and LOM results. The simulation of wind-driven equatorial wave dynamics in the LOM reveals that the observed variability of the EIC can be largely explained by the equatorial basin resonance at the semiannual period, when the second baroclinic Rossby wave reflected from the eastern boundary intensifies the directly forced equatorial Kelvin and Rossby waves in the basin interior. The sum of the first 10 modes can reproduce the main features of the EIC. Among these modes, the resonant second baroclinic mode makes the largest contribution, which dominates the vertical structure, semiannual cycle, and westward phase propagation of the EIC. The other 9 modes, however, are also important, and the superposition of the first 10 modes produces downward energy propagation in the equatorial Indian Ocean. [ABSTRACT FROM AUTHOR]

The processes leading to surficial enrichment of transition elements have been studied in continental slope and deep-sea sediments from the western Bay of Bengal (~500-4500 m water depth). The lower continental slope and rise (1800-3000 m) sediments are characterized by enrichment of Mn/Al ratios by 2-3 orders of magnitude and transition elements (Fe, Ni, Co, Cr and V) by a factor of 2-3 compared to the upper continental slope (500-1100 m) sediments. The occurrence of micronodules rich in Fe and Mn and elements associated with Mn-oxides, confirmed by scanning electron microscope and electron probe micro analysis, are responsible for the accumulation of transition elements in the lower continental slope sediments. The freshwater stratification, low carbonate productivity, fine grain size, low porosity, thin oxidized surficial layer and high sedimentation are conducive for diagenetic enrichment of Fe, Mn and elements associated with Mn-oxides in the lower slope sediments. The middepth oxygen minimum zone, resuspension and absence of oxidized surface layer enhance metal release to water column in the upper slope sediments. The oceanographic settings and high sedimentation rates control metal cycling in the Bay of Bengal sediments. [ABSTRACT FROM AUTHOR]

Utility of data from autonomous profiling floats for the validation of satellite ocean colour products from current satellite ocean colour sensors was assessed using radiometric and chlorophyll a fluorescence data from biogeochemical profiling floats (BGC-Argo) deployed in the subtropical gyre of the Indian Ocean. One of the floats was equipped with downward irradiance and upwelling radiance sensors, allowing the remote sensing reflectance, R rs , to be determined. Comparisons between satellite and in situ R rs indicated good agreement for the shorter wavelengths, but weak relationships for both satellites for the 555 nm channel, and showed that radiometers deployed on multipurpose, off-the-shelf BGC-Argo floats can provide validation-quality measurements. About 300 chlorophyll a concentration match-ups were achieved within 18 months, which increased the number of validation data points available for the Indian Ocean as a whole by a factor of ~4 from the previous historical record. Generally, the satellite data agreed with the float-derived chlorophyll concentration within the uncertainty of ±35%, for the band-difference (OCI) and band-ratio (OC3) algorithms, but not for a semianalytical ocean colour model (GSM) that exhibited significantly higher chlorophyll values (>100% mean difference). Our results indicate that autonomous float-based measurements provide substantial potential for improving regional validation of satellite ocean colour products in remote areas. [ABSTRACT FROM AUTHOR]

The Southern Hemisphere subtropical convergence zones are important regions of rainfall in the subtropics. The south Indian Ocean convergence zone (SICZ) has the strongest seasonality and exhibits substantial interannual variability in strength and position during austral summer. On synoptic time scales, the SICZ is a preferred region for the formation of tropical–extratropical (TE) cloud bands with local maxima over the southern African mainland and Madagascar. This study investigates how the seasonality in satellite-observed cloud band frequency emerges from the interplay between the asynchronous seasonal cycles in convective instability and upper-level flow, as represented by reanalysis data. These atmospheric mean states are diagnosed with a gross convective instability metric and a method to distinguish between subtropical and eddy-driven jet axes. Month-by-month analysis of these diagnostics elucidates how mean-state perturbations during ENSO events modify cloud band likelihood. Typically, 150%–200% more cloud bands develop during La Niña seasons supported by 5°–10° latitudinal separation between the local subtropical and eddy-driven jets and higher values of convective instability, especially in semiarid parts of mainland southern Africa. During El Niño events, fewer cloud bands develop over southern Africa in a more convectively stable environment without a distinct subtropical jet. However, east of Madagascar cloud bands are 150% more likely. Plausible teleconnection pathways based on these ENSO-related perturbations are discussed. The paper concludes with a conceptual framing of the seasonal cycle in the mean-state pertinent to TE cloud band likelihood. [ABSTRACT FROM AUTHOR]

Observations show that decadal (10–20 yr) to interdecadal (>20 yr) variability of the tropical Indian Ocean (TIO) sea surface temperature (SST) closely follows that of the Pacific until the 1960s. Since then, the TIO SST exhibits a persistent warming trend, whereas the Pacific SST shows large-amplitude fluctuations associated with the interdecadal Pacific oscillation (IPO), and the decadal variability of the TIO SST is out of phase with that of the Pacific after around 1980. Here causes for the changing behavior of the TIO SST are explored, by analyzing multiple observational datasets and the recently available large-ensemble simulations from two climate models. It is found that on interdecadal time scales, the persistent TIO warming trend is caused by emergence of anthropogenic warming overcoming internal variability, while the time of emergence occurs much later in the Pacific. On decadal time scales, two major tropical volcanic eruptions occurred in the 1980s and 1990s causing decadal SST cooling over the TIO during which the IPO was in warm phase, yielding the out-of-phase relation. The more evident fingerprints of external forcing in the TIO compared to the Pacific result from the much weaker TIO internal decadal–interdecadal variability, making the TIO prone to the external forcing. These results imply that the ongoing warming and natural external forcing may make the Indian Ocean more active, playing an increasingly important role in affecting regional and global climate. [ABSTRACT FROM AUTHOR]

The distribution of ocean salinity controls the density field and thereby plays a major role in influencing the ocean dynamics. It has been a challenging task to understand the variability of salinity structure in the regions of large fresh water discharge and high precipitation such as Bay of Bengal (BoB). Recent advancement in satellite technology has made possible the measurement of sea surface salinity (SSS). Aquarius is the satellite which measured the global SSS for the period 2011 to 2015. In the present study, we assimilated Aquarius SSS in the Global Ocean Data Assimilation System based on 3DVAR technique. The assimilation of Aquarius SSS resulted in reduced biases in salinity not only at the surface, but also in the vertical distribution of salinity and better captured the temporal variations of salinity structure in sensitive regions, such as the Bay of Bengal. In addition, the assimilation of SSS showed marginal improvement in ocean thermal structure over data sparse regions of Indian Ocean. It is also shown that the assimilation of Aquarius SSS has improved the stratification in the upper Ocean which is the key factor in the observed improvement in ocean analysis. [ABSTRACT FROM AUTHOR]

During the breakup of Gondwana the Mozambique Ridge, a Large Igneous Province emplaced between 140 and 125 Ma, was located in the evolving African–Southern Ocean gateway. Therefore, it represents an archive of the evolving exchange of water masses between the South Atlantic and Indian Ocean via the development of surface, intermediate, and bottom circulation. Two Cretaceous seismic units (S1 and S2a) were deposited on top of magmatic basement separated by a hiatus. Unit S1 mostly shows seismic reflections parallel to the top of the basement and no indications of current activity. The occurrence of several sediment drifts within seismic unit S2a indicates the onset of current controlled sedimentation. Based on our observations we propose deposition under partly euxinic conditions in the area of the Mozambique Ridge until ~ 100 Ma. The onset of a strong shallow circulation affecting deposition at the Mozambique Ridge is inferred by the Late Cretaceous ~ 25 Myr hiatus reported by drilling results and documented in the seismic records, whereas black shales deposited in the nearby deep Transkei Basin indicate a restricted deep circulation at least until ~ 85–80 Ma. We propose that the observed hiatus might be a consequence of a late Early Cretaceous uplift of the Mozambique Ridge and the progressive opening of the Agulhas Passage allowing inflow of surface (Upper Pacific Water, Upper North Atlantic Water) and intermediate water (Intermediate Southern Ocean Water) into the study area. The intense circulation that caused the hiatus seems to have weakened in Campanian times, which is documented by the occurrence of sediment drifts in seismic unit S2a. We suggest that the onset of current controlled sedimentation was caused by palaeogeographic modifications in the Atlantic Ocean along with relocation of circulation pathways. Our results illustrate the crucial role of the African–Southern Ocean gateway in the commencing water mass exchange between the Atlantic and Indian Ocean and highlight the complex interactions that eventually lead to the initiation of a proto–Antarctic Circumpolar Current in Turonian times. [ABSTRACT FROM AUTHOR]

The major shipping routes of the South China Sea (SCS) and northern Indian Ocean (NIO) are analyzed via ship observations compiled by the International Comprehensive Ocean-Atmosphere Data Set. In the SCS, one route from the East China Sea moves through the Taiwan Strait toward Hong Kong and Singapore. The other route from the Pacific moves passed the east coast of Taiwan and through the Bashi Channel/Luzon Strait to Singapore. After Singapore, the shipping routes pass through the Strait of M alacca to Sri Lanka. In the Arabian Sea, ships from Sri Lanka go either northwestward toward the Persian Gulf or westward toward the Red Sea. A northeast-southwest route connects the Persian Gulf and the Red Sea. The resultant routes exhibit a triangular pattern in the Arabian Sea. Monsoonal influences on shipping routes are evident in the Arabian Sea, but indiscernible in the SCS. In summer, the southwest monsoon causes strong winds and waves over the western-central Arabian Sea. Ships detour from the southern side of Socotra Island to the northern side. The northern side is on the leeside of the intruding southwesterly flows and has weaker winds and waves than the windward southern side. In the SCS, the winter northeasterly flows intensify winds and waves over two strait regions: the Taiwan Strait and the Luzon Strait. These straits are at the conjunction of the Pacific and the SCS. Routes passing through these straits have the advantage of a short, straight navigation path. Moreover, the winds and waves over these regions are not as strong as those occurring in the Arabian Sea during summer. The maritime conditions in the SCS are not dangerous enough to necessitate a detour. [ABSTRACT FROM AUTHOR]

Oceanographic fronts are strong discontinuities in water mass properties and can result in differences in biota over short horizontal distances. The vertical structuring of epipelagic euphausiids was investigated across an ephemeral thermohaline front between Leeuwin Current Water (LCW) and Sub Tropical Surface Water (STSW) in the south-east Indian Ocean. Vertical ontogenetic layering was evident and euphausiid larvae were most abundant in the 0–50 m stratum in both water masses. Differences in assemblages between water masses were mostly driven by the numerical abundance of shared species; most species were in greater abundance in LCW. LCW was characterized by a uniform water column and no significant structuring of assemblages with depth. Structuring was, however, evident in STSW and analyses indicated this was a result of stratification of temperature, salinity and dissolved oxygen. Higher chlorophyll a concentrations and zooplankton settled volumes were evident in LCW, as well as the upper 100 m of STSW. Decreases in temperature and salinity were correlated with the variation in assemblages at >100 m depth in STSW. This is the first study to examine vertical structuring and environmental correlations of euphausiid life stages and assemblages across a thermohaline front in the south-east Indian Ocean. [ABSTRACT FROM AUTHOR]

The article reports that the oceanic studies requires new critical methods and new geographic and spatial scales. New paradigms such as the Atlantic world and the Indian Ocean world have proved effective when it comes to dislodging national boundaries in literary scholarship and they have also had the unintended effect of shoring up hemispheric divisions.

The seamounts of the southern Indian Ocean remain some of the most poorly studied globally and yet have been subject to deep-sea fishing for decades and may face new exploitation through mining of seabed massive sulphides in the future. As an attempt to redress the knowledge deficit on deep-sea benthic and pelagic communities associated mainly with the seamounts of the South West Indian Ridge two cruises were undertaken to explore the pelagic and benthic ecology in 2009 and 2011 respectively. In this volume are presented studies on pelagic ecosystems around six seamounts, five on the South West Indian Ridge, including Atlantis Bank, Sapmer Seamount, Middle of What Seamount, Melville Bank and Coral Seamount and one un-named seamount on the Madagascar Ridge. In this paper, existing knowledge on the seamounts of the southwestern Indian Ocean is presented to provide context for the studies presented in this volume. An account of the overall aims, approaches and methods used primarily on the 2009 cruise are presented including metadata associated with sampling and some of the limitations of the study. Sampling during this cruise included physical oceanographic measurements, multibeam bathymetry, biological acoustics, and net sampling of phytoplankton, macrozooplankton and micronekton/nekton. The studies that follow reveal new data on the physical oceanography of this dynamic region of the oceans, and the important influence of water masses on the pelagic ecology associated with the seamounts of the South West Indian Ridge. New information on the pelagic fauna of the region fills an important biogeographic gap for the mid- to high-latitudes of the oceans of the southern hemisphere. [ABSTRACT FROM AUTHOR]

Exploratory surveys of six seamounts in the Southwest Indian Ocean provide a description of physical processes induced by seamounts along the Southwest Indian Ridge. Mean currents (15–25 cm s −1 ) in the vicinity of each seamount were dominated by mesoscale eddies. The dominant seamount-driven process was the generation of internal tides by the barotropic tide interacting with the seamount crests. This led to enhanced shear in the vicinity of the crests resulting in mixing where stratification was weak, for example in the core of an anticyclonic mesoscale eddy or where there had been a winter mixed layer. Tidally driven up- and downwelling was observed at the seabed with associated variability in bottom temperature of up to 3 °C over a tidal cycle. Vertical displacement of isopycnals by internal tidal waves reached 200 m peak to trough. Fluorescence in the surface (eutrophic) layer could thus extend down to the seamount crest on each tidal cycle. Apparently spatial variations in short conductivity/temperature/depth sections across each seamount were probably aliased temporal variations from the strong tidal signal. Evidence for Taylor caps or other potential trapped circulations at the seamount crest was weak, most likely because currents associated with mesoscale eddies were too strong to allow their formation. [ABSTRACT FROM AUTHOR]

The aesthetic experience is often characterized as ‘working’ when the self loses its bearings in feelings of immersion. The text presented here experiments with immersion as engagement and ontological entanglement. This fragmentary piece has seven numbered sections spelling out the recipe for a genre—fictocriticism—that narrates without reproducing the familiar patterns of literary fiction. Each has actors representing different ontological modes: history (2), literary aesthetics (3) (in which a Jonathan Franzen character is given new life), myth (4), factual writing (5), performance (6) and economics (7). The writing makes explicit the pathways of research that could play a part in an impossible project: writing the Indian Ocean. All these actors are linked into networks that precede the writing, yet continue to traverse it as well. This heterogeneous text experiments with immersion in these different ontological modes, and no doubt fails, unless the reader responds better to more complex relations than to simpler subject–object ones. [ABSTRACT FROM PUBLISHER]

Micronekton constitutes the largest unexploited marine biomass worldwide. It is one of the most conspicuous and ecologically important components of the still poorly known mesopelagic ecosystem. Acoustic data were collected from both fishing and research vessels along 18 transects for a total of 47 682 linear kilometers to investigate large-scale distribution of micronekton over a long latitudinal gradient (20–50°S) and two contrasted seasons (summer and winter) in the South-Western Indian Ocean. Acoustic backscatter at 38 kHz was used as a proxy of mid-water organisms' abundance (0–800 m depth). Two consistent features were diel vertical migration of backscatters and vertical distribution of micronekton in three distinct layers, namely the surface (SL), intermediate (IL) and deep (DL) layers. Satellite remote sensing data was used to position oceanic fronts, and hence define water masses, from the tropical to low Antarctic zones. A key finding of this study was the significant correlation observed between abundance and distribution of acoustic backscatter and position relative to these front and water masses. Total backscatter peaked in the subtropical zone, with low abundances in the colder Polar Frontal Zone. The high overall abundances in subtropical waters resulted mainly from high backscatters in the IL and DL that contrasted with low SL values, especially during the day (2–11%). The warmer the waters, the higher SL backscatter was, with the highest absolute and relative (38–51% of the total abundance) values observed at night in the Tropical Zone and the lowest abundance in the Antarctic Zone. No significant seasonal pattern was found, but SL backscatters were very low in winter compared to summer in the Polar Frontal Zone. Moreover, the Northern winter shift of the fronts induced a Northern latitudinal shift of the peak in abundance from summer to winter. The present study highlights the value of building large acoustic databases collected from both research and fishing vessels. The method provides unique opportunities to gather basic information on micronekton and is an essential step to describe oceanic zones of relevant biological interest in terms of trophic ecology. [ABSTRACT FROM AUTHOR]

Here, we present planktonic foraminiferal, benthic foraminiferal and other proxy profiles from well-dated sediment cores in the Andaman Sea showing changes in climate and oceanography at Dansgaard-Oeschger (D/O) and Heinrich scales. The large temporal variations in the abundances of total benthic foraminifera, Globigerina rubescens and Neogloboquadrina dutertrei suggest substantial changes in the surface to bottom hydrography of the Andaman Sea. G. rubescens abundance minima during the last glacial cycle correspond to warm interstadials (D/O events 1 to 14) while maxima correspond to Heinrich events (H1 to H4), the last glacial maximum (LGM) and the Younger Dryas. D/O events are marked by very low G. rubescens and high N. dutertrei abundances which indicate freshened surface water related to increased direct precipitation (over evaporation) and strengthened Irrawaddy outflow. Lower abundance of N. dutertrei (and higher abundance of G. rubescens ) during North Atlantic Heinrich events, the deglacial, the YD and the mid- to late-Holocene reflect reduced influx of fresh water as a result of weakened summer monsoon freshwater input. The timing of these Andaman Sea monsoonal changes indicate a strong teleconnection to North Atlantic climate change. [ABSTRACT FROM AUTHOR]

The oblique-banded grouper, Epinephelus radiatus (day, 1968), occurred within the whole indo-West Pacific where it is harvested by small-scale coastal fisheries. The aim of this study was to estimate the growth of this species at Reunion island (south-West indian ocean) since no study investigated this topic so far. Fifty-seven individuals were sampled in the landings of the French local artisanal fisheries from March 2014 to March 2015. The relationships between two types of body length (total and standard lengths, cm) and total weight (g) were significant (p < 0.05). Total length-weight relationship was described by the following parameters: a = 0.01 and b = 3.098. For each individual, morphometry of the whole otolith was described with several parameters (OL: otolith length; OWI: otolith width; OA: otolith area; Ow: otolith weight). Age was determined on a transversal section of otolith. Among the morphometric parameters of otoliths, only the weight (p = 0.002) was significantly correlated with the age of E. radiatus, with no difference between left and right otolith (p = 0.67). Von bertalanffy growth equations were TLt = 65.99(1 - e-0.20(t)) and Wt = 4526.44(1 - e-0.20(t))³. [ABSTRACT FROM AUTHOR]

A decade of newly available Argo float data for the period 2004-13 are used to investigate the three-dimensional structures of upper-ocean seasonality with emphasis on the vertical aspects of annual and semiannual cycles, yielding three main findings with oceanographic implications. First, the vertical evolution of the horizontal pattern of annual and semiannual amplitudes appears to be highly 'nonlinear,' suggesting that the thermodynamic causes are depth dependent. The global ocean seasonality exhibits a vertically varying pattern in space, including midlatitude maxima in the near-surface layer due to solar forcing, zonal 'strips' in the subsurface layer due to the equatorial current system, and systematic westward phase propagation in the intermediate layer due to annual Rossby waves. Second, a zone of 500 ± 300-m depths along with a 6-month periodicity are chosen as appropriate space-time 'windows' for detecting eddy signatures via Argo-derived temperature amplitude and phase, respectively. It is revealed that the eddy-induced 'blobby' pattern observed previously by satellite altimeter appears in the Agro result as 'woodsy' bulks, which can be well illustrated in the semiannual amplitude and phase maps at window depths. Meanwhile, six eddy deserts paired in each ocean basin have also been identified. Third, the existence of a dozen vertical quasi-annual amphidromes is first reported, with cophase lines that may radiate toward the ~2000-m lower limit of Argo measurement. The well-known global meridional overturning circulation and the pseudozonal overturning currents in the equatorial Pacific, Atlantic, and Indian Oceans may possibly contribute to the observed vertical amphidromes. [ABSTRACT FROM AUTHOR]

The different impacts of El Niño during peak phases with and without a positive Indian Ocean Dipole (P-IOD) on the Northwest Pacific circulation were studied. The authors focused on the Northwest Pacific circulation features in the mature phase of El Niño from September to February of the next year. Composite maps and simulations demonstrate that the atmospheric circulation under the impact of El Niño with and without P-IOD exhibits large differences in temporal evolution and intensity. In single El Niño (SE) years without a P-IOD, an anomalous low-level anticyclonic circulation around the Philippines (PSAC) is instigated by the single El Niño-induced Indonesian subsidence. However, during the years when El Niño and a P-IOD matured simultaneously, a much greater anomalous subsidence over the western Pacific and the Maritime Continent occurred. The PSAC tends to occur earlier, is much stronger and has a longer lifetime than that during SE. More importantly, the PSAC shows a characteristic of an eastward movement from the southern South China Sea (SCS) to the Philippine Sea. This characteristic does not appear during SE. These patterns imply that a positive IOD event tends to exert a prominent influence on the PSAC during El Niño events and there is a combined impact of El Niño and P-IOD on the development of the PSAC. [ABSTRACT FROM AUTHOR]

The Global Ocean Data Assimilation System configured at Indian National Centre for Ocean Information Services (INCOIS-GODAS) has been forced with satellite-based QuikSCAT gridded winds (QSCAT) to obtain accurate operational ocean analysis, particularly ocean currents, as compared to the default National Centers for Environmental Prediction-Reanalysis 2 (NCEP-R2) wind forcing in the tropical Indian Ocean (TIO). However, after termination of QuikSCAT mission in November 2009, an alternate wind forcing was required for providing operational ocean analysis. The present study examines the suitability of an Advanced Scatterometer (ASCAT)-based daily gridded wind product (DASCAT) for the INCOIS-GODAS. Experiments were performed by forcing INCOIS-GODAS with three different momentum fluxes derived from QSCAT, DASCAT, and NCEP-R2 wind products. Simulated ocean currents from these experiments are validated with respect to in situ current measurements from Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) buoys. Results suggested that the quality of simulated ocean currents from the daily DASCAT forcing is on par with the QSCAT forcing in the TIO, except for the equatorial Indian Ocean (EIO). Although QSCAT-forced current simulations are slightly better than DASCAT-forced simulations, both QSCAT and DASCAT provide a much better result than NCEP-R2. Our analysis shows that the better simulations of currents over the EIO, with the QSCAT forcing compared to DASCAT forcing, can be attributed to the smoothening of the wind field in the DASCAT compared to QSCAT. The impact of the error in the DASCAT on ocean current analysis is, however, limited to local scales and upper 100 m of water column only. Thus, our study demonstrated that, in the absence of QSCAT, DASCAT is a better alternative for INCOIS-GODAS ocean analysis than the NCEP-R2. [ABSTRACT FROM AUTHOR]

The reef fauna connectivity of the West Indian Ocean ( WIO) is one of the least studied globally. Here we use genetic analyses of the grouper Epinephelus merra ( Bloch 1793) to determine patterns of connectivity and to identify barriers to dispersal in this WIO marine area. Phylogeographic and population-level analyses were conducted on cytochrome b sequences and microsatellites (13 loci) from 557 individuals sampled in 15 localities distributed across the West Indian Ocean. Additional samples from the Pacific Ocean were used to benchmark the WIO population structure. The high level of divergence revealed between Indian and Pacific localities (of about 4.5% in sequences) might be the signature of the major tectonic and climatic changes operating at the Plio- Pleistocene transition, congruently with numerous examples of Indo-Pacific speciation. In comparison, the E. merra sequences from the Indian Ocean constitute a monophyletic clade with a low average genetic distance (d < 0.5%). However both genetic markers indicated some structure within this ocean. The main structure revealed was the isolation of the Maldives from the WIO localities (a different group signature identified by clustering analysis, great values of differentiation). Both marker types reveal further significant structure within the WIO, mainly the isolation of the Mascarene Islands (significant AMOVA and isolation-by-distance patterns) and some patchy structure between the northernmost localities and within the Mozambique Channel. The WIO genetic structure of E. merra appeared congruent with main biogeographic boundaries and oceanographic currents. [ABSTRACT FROM AUTHOR]

Nannofossil data from ODP Site 711 (equatorial Indian Ocean) yield a set of consistent, reliable biohorizons that form the basis of a revised calcareous nannofossil biostratigraphy for the low-latitude Eocene-Oligocene. We discuss 31 biohorizons occurring over an 11 myr time interval which we correlate to previous magnetostratigraphic data. Calcareous nannofossils from the middle Eocene through the lower Oligocene of the studied section are characterized by moderately well preserved assemblages consisting largely of low latitude and cosmopolitan species. A significant nannofossil dissolution interval is evidenced at the middle Eocene Climatic Optimum (MECO). We document a significant increase in late Eocene nannoplankton exhibiting a eutrophic preference. Analysis of the assemblage suggests important changes in the equatorial oceanic regime just before the onset of the Eocene-Oligocene transition (EOT), that foreshadow the more dramatic climatic shift of the early Oligocene. [ABSTRACT FROM AUTHOR]

Processes governing productivity at the base of the pelagic food web of the southern Indian Ocean are influenced primarily by physical--chemical conditions with implications for the structure and function of the entire pelagic food web. Here, we report observations along a great circle transect from Cape Town, South Africa, to Broome in north western Australia. Primary production was tightly linked to water column stability and nutrient availability, with high productivity (1144 mg C m-2 day-1) in the sub-tropical convergence zone, and falling off by an order of magnitude in the sub-tropical gyre and tropical waters off northwest Australia. Primary production was largely confined to the GF/F fraction (GF/F > 75% of total production and usually much higher) and the photosynthetic parameters showed adaption to the prevailing light levels. Bacterial production ranged from 19 to 155 mg C m-2 day-1. No relationships were found between bacterial production and primary production or phytoplankton biomass and bacterial production seem more related to the state of the phytoplankton community as high bacterial production was observed in a post-bloom situation. The average BP:PP ratio was 31% (range 3.5-71%). [ABSTRACT FROM AUTHOR]

To reconstruct the Cenozoic paleoceanographic evolution, in particular, temporal changes in nutrient levels, we studied calcareous nannofossil assemblages from Ocean Drilling Program holes 1210A, located in the northwestern Pacific Ocean, and 762B, situated in the eastern Indian Ocean. At each site we focused on the relationships among the nannofossil accumulation rate (NAR), relative abundance of Discoaster , and coccolith size of the genus Reticulofenestra . The co-occurrence of a low NAR, high Discoaster abundance and presence of large Reticulofenestra suggest a deep thermocline and nutricline, typical of oligotrophic conditions. Conversely, the co-occurrence of a high NAR, low Discoaster abundance and presence of small Reticulofenestra indicate a shallow thermocline and nutricline, typical of eutrophic conditions. The combination of these three parameters suggests that the gradual eutrophication and collapse of sea surface stratification occurred in the northwestern Pacific Ocean at 7.9, 6.4, and 5.0 Ma. In contrast, abrupt eutrophication is indicated at 8.8 Ma in the eastern Indian Ocean. Although the timing of eutrophication differs between the Pacific and Indian Ocean sites, it roughly coincides with tectonic and/or climatic events around the two oceans. This finding suggests that tectonic and climatic events are capable of causing an increase in the terrestrial input and/or coastal upwelling of nutrients that subsequently leads to the eutrophication of sea surface waters. [ABSTRACT FROM AUTHOR]

Sea surface salinity (SSS) is one of the key components of the Earth's global water cycle. Reliable information on SSS is very important for ocean modelling, data assimilation, and ocean and climate research applications. In this study, SSS variability in the tropical Indian Ocean (TIO) was analysed using the Aquarius instrument on board the SAC-D satellite and in situ observations from the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) buoys and Array for Real-Time Geostrophic Oceanography (ARGO) data sets for the period 2012-2013. Comparison of two recent versions (V2 and V3) of Aquarius-based SSS estimates to nine RAMA buoys on a daily timescale showed excellent mutual agreement. The systematic underestimation of SSS by satellite-based V2 products over the TIO shows a clear advantage for the new version product (V3). A larger root-meansquare error of the order of 0.50 psu in the satellite-based SSS was observed over the highly variable (larger standard deviation) Bay of Bengal region as compared with ARGO data sets. In the eastern equatorial Indian Ocean region, satellite-based SSS overestimated SSS below 34 psu and underestimated SSS of 34-35 psu as compared with ARGO data. However, the V3 SSS from Aquarius showed marginal improvement over V2 SSS. Monthly variation and fast Fourier analysis of the satellite-based SSS estimates are in reasonably good agreement with in situ observations which suggest that Aquarius is able to capture SSS variability in the TIO. The Aquarius-based V3 SSS showed a temporal autocorrelation of 0.6 over most parts of the TIO up to day 10, and decreased gradually with time. Overall analysis suggests that Aquarius-derived V3 SSS can detect variability in SSS satisfactorily in the TIO and is in reasonably good agreement with in situ observations. [ABSTRACT FROM AUTHOR]

The claim for a zonal-dipole structure in interannual variations of the tropical Indian Ocean (IO) SSTs-the Indian Ocean dipole (IOD)-is reexamined after accounting for El Niño-Southern Oscillation's (ENSO) influence. The authors seek an a priori accounting of ENSO's seasonally stratified influence on IO SSTs and evaluate the basis of the related dipole mode index, instead of seeking a posteriori adjustments to this index, as common. Scant observational evidence is found for zonal-dipole SST variations after removal of ENSO's influence from IO SSTs: The IOD poles are essentially uncorrelated in the ENSO-filtered SSTs in both recent (1958-98) and century-long (1900-2007) periods, leading to the breakdown of zonal-dipole structure in surface temperature variability; this finding does not depend on the subtleties in estimation of ENSO's influence. Deconstruction of the fall 1994 and 1997 SST anomalies led to their reclassification, with a weak IOD in 1994 and none in 1997. Regressions of the eastern IOD pole on upper-ocean heat content, however, do exhibit a zonal-dipole structure but with the western pole in the central-equatorial IO, suggesting that internally generated basin variability can have zonal-dipole structure at the subsurface. The IO SST variability was analyzed using the extended-EOF technique, after removing the influence of Pacific SSTs; the technique targets spatial and temporal recurrence and extracts modes (rather than patterns) of variability. This spatiotemporal analysis also does not support the existence of zonal-dipole variability at the surface. However, the analysis did yield a dipole-like structure in the meridional direction in boreal fall/winter, when it resembles the subtropical IOD pattern (but not the evolution time scale). [ABSTRACT FROM AUTHOR]

A recent finding is the significant impact of the sea surface temperature anomaly (SSTA) over the east Indian Ocean (EIO) on the genesis frequency of tropical cyclones (TCs) over the western North Pacific (WNP). In this study it is shown that such an impact is significant only after the late 1970s. The results based on both data analysis and numerical model experiments demonstrate that prior to the late 1970s the EIO SSTA is positively correlated with the equatorial central Pacific SSTA and the latter produces an opposite atmospheric circulation response over the WNP to the former. As a result, the impact of the EIO SSTA on the TC genesis over the WNP is largely suppressed by the latter. After the late 1970s, the area coverage of the EIO SSTA is expanding. This considerably enhances the large-scale circulation response over the WNP to the EIO SSTA and significantly intensifies the impact of the EIO SSTA on TC genesis frequency over the WNP. The results from this study have great implications for seasonal prediction of TC activity over the WNP. [ABSTRACT FROM AUTHOR]

Brill, D.; Pint, A.; Jankaew, K.; Frenzel, P.; Schwarzer, K.; Vött, A., and Brückner, H., 2014. Sediment transport and hydrodynamic parameters of tsunami waves recorded in onshore geoarchives. In regions with a short historical tsunami record, the assessment of long-term tsunami risk strongly depends on geological evidence of prehistoric events. Whereas dating tsunami deposits is already well established, magnitude assessment based on remaining sedimentary structures is still a major challenge. In this study, two approaches were applied to deduce transport processes and hydrodynamic parameters of tsunami events from onshore deposits found in the coastal plain of Ban Bang Sak, SW Thailand: (1) The maximum offshore sediment source was determined using granulometry, geochemistry, mineralogy and foraminifera of the tsunamites, and reference samples from various marine and terrestrial environments, and (2) the onshore flow velocities and flow depths of associated tsunami waves were estimated by means of sedimentation modelling. In the case of the Indian Ocean tsunami (IOT) of 2004, modelled flow velocities of 3.7 to 4.9 m/s, modelled onshore flow depths of up to 5.5 m, and a sediment source from offshore areas shallower than a 45-m water depth-including littoral sediments transported as bedload and suspended load from the shallow subtidal zone-are in agreement with quotations based on survivor videos and posttsunami surveys. For a 500- to 700-year-old predecessor, comparable flow velocities and flow depths of 4.1 to 5.9 m/s and 4.0 to 7.5 m, respectively, were modelled, indicating a similar magnitude as the IOT 2004. Comparable values of maximum transport distance and depth of wave erosion were also found. In the case of three older tsunami candidates, dated to 1180 to 2000 cal BP, the deposits indicate partly similar source areas with water depths of less than 45 m and partly shallower source areas restricted solely to the beach. Whereas the former tsunamis are interpreted as events similar to 2004, the latter are more likely storms or tsunamis of a lower magnitude. [ABSTRACT FROM AUTHOR]

Because conventional observations over the oceans are not available, especially during tropical cyclones, multi-spectral atmospheric motion vectors (AMVs) estimated from geostationary satellites are routinely assimilated in the numerical weather prediction models at different operational centres across the globe. The derived AMVs are generally validated with radiosonde observations available over land at synoptic hours; however, over the ocean there is a limited scope to assess the quality of AMVs. Over ocean, AMVs can be validated with radiosonde data available from opportunistic ships or using dropsonde data available from aircrafts. In this study, the accuracy of the AMVs derived from the geostationary satellites Kalpana-1 and Meteosat-7 is evaluated over the oceanic region. Radiosonde data available from a ship cruise held in the Bay of Bengal during the period 09 July-08 August 2012 and from the Cal/Val site situated at Kavaratti Island (72.62°E, 10.57°N) in the southern Indian Ocean are used to assess the AMV accuracy. In this study, 83 radiosonde profiles are used to validate the Kalpana-1 AMVs, to allow a better understanding of AMV errors over the Indian Ocean. The RMSVD of Kalpana-1 AMVs for the high-, mid- and low-levels are found to be 7.9, 9.4 and 5.3 m s, respectively, while the corresponding RMSVD for Meteosat-7 AMVs are 9.1, 5.5 and 3.7 m s. A similar accuracy is observed when the AMVs are validated against the NCEP analyses collocated with the nearest radiosonde locations. The high RMSVD and bias for Kalpana-1 AMVs at the mid-level and Meteosat-7 AMVs at the high-level are associated with the limitation of satellite winds to resolve the upper-level easterly jet in conjunction with errors in the height assignment. This study could help the numerical modellers to assign appropriate observation error over this region during the assimilation of AMVs into the NWP models. [ABSTRACT FROM AUTHOR]

The ecology of the chaetognath community and its relation to varying environmental factors were studied in the eastern half of the northern Indian Ocean. Analysis of data from two major oceanographic programmes performed over four decades apart suggests a pronounced shift in the community composition and a poleward migration of chaetognaths. This migration has led to an aggregation in the north, as the Eurasian land mass prevents further movement beyond 30°N. [ABSTRACT FROM PUBLISHER]

The western rock lobster, Panulirus cygnus, provides Australia's most valuable wild caught fishery but, in recent years, there has been a dramatic decline in settlement of the post-larval phase into their natal coastal habitat. One hypothesis for this decline was that the oceanographic conditions no longer favour the survival, feeding and growth of the larval (phyllosoma) phase. To explore this, the oceanography and corresponding zooplankton prey field along five latitudinal transects in the south-eastern Indian Ocean were quantified during July 2010. Leeuwin Current Water (LCW) and Sub-Tropical Surface Water (STSW) were distinguished and a prominent front at ∼30°S characterized by strong eastward flow separated them. Although zooplankton abundance increased towards the north, the prey field was unevenly distributed with patches of higher prey concentration associated mainly with LCW. Chaetognaths were the most abundant prey item (means: 17.2 and 4.1 m−3 in LCW and STSW, respectively) and were positively correlated with chlorophyll a in both water masses. Panulirus cygnus phyllosoma had a highly patchy distribution but, despite lower prey concentrations, were more abundant in STSW than LCW, particularly south of the front. Our results suggest that LC meso-scale features with strong fronts may be implicated in phyllosoma aggregations and shoreward transport of late-stage larvae and that this warrants further investigation. [ABSTRACT FROM PUBLISHER]

Abstract: High-resolution records of beryllium (10Be) production and relative paleointensity have been obtained across the Matuyama–Brunhes (M–B) reversal from the equatorial Indian Ocean (Maldives area). Both magnetic and geochemical analyses were performed from the same discrete samples to avoid any artificial depth offset. The authigenic 10Be concentrations were normalized with respect to 9Be in order to correct for potential environmental effects, while the relative paleointensity was derived from the remanent magnetization intensity after accounting for changes in magnetic concentration within the sediment. The relative paleointensity and the 10Be/9Be records are both characterized by large deviations, which culminate in the middle of the reversal. In contrast to most previous studies, and despite relative high deposition rate (4.7 cm/ka), we observed a perfect synchronism between the 10Be/9Be peak, the lowest value of relative paleointensity and the switch in direction, which indicates that bioturbation and post-depositional processes did not affect the magnetic record. This leaves no ambiguity for the stratigraphic position of the reversal located within Marine Isotopic Stage 19 as revealed by the planktonic δ 18O record from the same core. The magnetic data depict a two-phase process with a precursory event preceding the rapid polarity switch, while only the second phase is present in the 10Be record, similarly to other low latitude records from the Indonesian area. Using an orbitally-tuned age model, we obtain an age of for the middle of the transition, while the precursory event occurred almost 20 ka before. We believe that the bimodal distribution emerging from the compilations of the ages of the M-B reversal results from the succession of these two events. Microtektites from the Australasian impact were found at 0.6 m below the transition ( B.P.) and confirm that this large event occurred 12 ka prior to the polarity transition. The distribution of tektite abundance was used to deconvolve the 10Be/9Be signal. The results confirm that the beryllium changes are concentrated during the transitional period, thus likely in presence of a multipolar geomagnetic field (or in the vicinity of a geomagnetic pole) that favored the penetration of cosmic rays and consequently increased the 10Be production. The absence of 10Be during the precursor indicates that the present site and the Indonesian ones were far away from a geomagnetic pole and that interlatitudinal atmospheric mixing was limited. The geomagnetic pole positions above the Indonesian sites during the precursor would thus be incompatible with the corresponding inclined dipolar field during this period, and suggest the dominance of low-degree harmonics. [Copyright &y& Elsevier]

Top predators need to develop optimal strategies of resources and habitats utilization in order to optimize their foraging success. At the individual scale, a predator has to maximize his intake of food while minimizing his cost of foraging to optimize his energetic gain. At the ecosystem scale, we hypothesized that foraging strategies of predators also respond to their general energetic constraints. Predators with energetically costly lifestyles may be constrained to select high quality habitats whereas more phlegmatic predators may occupy both low and high quality habitats. The objectives of this study were 1) to investigate predator responses to heterogeneity in habitat quality with reference to their energetic strategies and 2) to evaluate their responses to contemporaneous versus averaged habitat quality. We collected cetacean and seabird data from an aerial survey in the Southwest Indian Ocean, a region characterized by heterogeneous oceanographic conditions. We classified cetaceans and seabirds into energetic guilds and described their habitats using remotely sensed covariates at contemporaneous and time-averaged resolutions and static covariates. We used generalized additive models to predict their habitats at the regional scale. Strategies of habitat utilization appeared in accordance with predators energetic constraints. Cetaceans responded to the heterogeneity in habitat quality, with higher densities predicted in more productive areas. However, the costly Delphininae appeared to be more dependent on habitat quality (showing a 1-to-13 ratio between the lowest and highest density sectors) than the more phlegmatic sperm and beaked whales (showing only a 1-to-3 ratio). For seabirds, predictions primarily reflected colony locations, although the colony effect was stronger for costly seabirds. Moreover, our results suggest that predators may respond better to persistent oceanographic features. To provide a third dimension to habitat quality, cetacean strategies of utilization of the vertical habitat could be related to the distribution of micronekton in the water column. [ABSTRACT FROM AUTHOR]

The Indian Ocean Dipole (IOD) can affect the El Niño-Southern Oscillation (ENSO) state of the following year, in addition to the well-known preconditioning by equatorial Pacific Warm Water Volume (WWV), as suggested by a study based on observations over the recent satellite era (1981-2009). The present paper explores the interdecadal robustness of this result over the 1872-2008 period. To this end, we develop a robust IOD index, which well exploits sparse historical observations in the tropical Indian Ocean, and an efficient proxy of WWV interannual variations based on the temporal integral of Pacific zonal wind stress (of a historical atmospheric reanalysis). A linear regression hindcast model based on these two indices in boreal fall explains 50 % of ENSO peak variance 14 months later, with significant contributions from both the IOD and WWV over most of the historical period and a similar skill for El Niño and La Niña events. Our results further reveal that, when combined with WWV, the IOD index provides a larger ENSO hindcast skill improvement than the Indian Ocean basin-wide mode, the Indian Monsoon or ENSO itself. Based on these results, we propose a revised scheme of Indo-Pacific interactions. In this scheme, the IOD-ENSO interactions favour a biennial timescale and interact with the slower recharge-discharge cycle intrinsic to the Pacific Ocean. [ABSTRACT FROM AUTHOR]

The impact of marine ecosystem on the tropical climate variability in the Indian Ocean is investigated by performing coupled ocean/biogeochemical model experiments, which are forced by realistic surface winds from 1951 to 2010. Results from a suite of chlorophyll perturbation experiments reveal that the presence of chlorophyll can have significant effects on the characteristics of the Indian Ocean Dipole (IOD), including its amplitude and skewness, as well as on the mean state. Specifically, chlorophyll increases mean sea surface temperature due to direct biological heating in regions where the mean mixed layer depth is generally shallow. It is also found that the presence of chlorophyll affects the IOD magnitude by two different processes: One is the amplifying effect by the mean chlorophyll, which leads to shoaling of mean thermocline depth, and the other is the damping effect by the interactively varying chlorophyll coupled with the physical model. There is also a biological impact on the skewness of the IOD, resulting in enhanced positive skewness. This skewness change is primarily caused by the phase dependency of the above two contradicting effects involving the asymmetric thermocline feedback and the nonlinear mixed layer heating. [ABSTRACT FROM AUTHOR]

The mixing characteristics of intermediate water at the Central Indian Ocean Basin (CIOB) and the Arabian Sea using dissolved oxygen (DO) as a tracer were studied. Only a few datasets are available in the literature for north-south mixing in this region. Our studies using DO sensor mounted on a remotely operated vehicle in comparison with array for real-time geostrophic oceanography (ARGO) reveal the influence of sub-Antarctic mode water (SAMW) in the study region. Reduction in second oxygen maximum concentration anomaly from south to north may be due to very low concentrated oxygen minimum zone in the Arabian Sea and influence of Red Sea water and the Persian Gulf water. We observed second oxygen maximum concentration at a depth of 300-700 m at CIOB from 150 to 220 µM, compared to the Arabian Sea profiles in the 25-40 µM range with peak value at a depth at 450 m. The present study highlights the usage of high-resolution DO data as tracer for intermediate water circulation in the Indian Ocean and also shows the influence of SAMW up to 8°N along 75°E. [ABSTRACT FROM AUTHOR]

Abstract: The Arafura and Timor Seas (ATS) are a crucial link between the Indian and Pacific Oceans and play a vital role in global ocean circulation and climate. Some high-standing islands in the ATS are globally significant sources of sediment. Here we derive a synthesis of river catchments and their role as sources of water, sediment, carbon, and nutrients to estimate the pathways, magnitude, and fate of exported riverine materials. The edge of the Australian continental shelf and slope receives riverine sediment from Asia as isotopes suggest a mixing of sediment sources, with some overlap between the Australian and non-Australian material, including evidence of significant input of volcanic material from Indonesia. The catchments bordering both the Arafura and Timor Seas account for ≈12% of sediment and ≈35% of water discharged from tropical Asia. Northern Australia discharges a volume of freshwater comparable to southwest New Guinea, but 50–65% of total sediment (754Mt), DIC (61.1Mt), POC (7.9Mt), DOC (3.5Mt), TN (2.88Mt), and TP (254,264t) delivered annually to the ATS come from New Guinea. The island of Timor discharges much smaller amounts of water (170km3 yr−1) and sediment (133Mtyr−1), but area-specific rates of DIC (1150tkm−2 yr−1), POC (238tkm−2 yr−1), DOC (94tkm−2 yr−1), TN (61tkm−2 yr−1) and TP (4.4tkm−2 yr−1) are higher, reflecting very high rates of deforestation and land degradation. Compared to other tropical rivers, carbon export into the ATS is dominated by DIC. The ATS catchments are being increasingly affected by human activities, and material discharge to the continental margins will likely increase, impacting shelf communities and highly diverse reef, mangrove, and seagrass habitats. [Copyright &y& Elsevier]

The quality of daily gridded Advanced Scatterometer (ASCAT; DASCAT) blended winds is examined in the tropical Indian Ocean using 3-day running mean gridded Quick Scatterometer (QuikSCAT; QSCAT) winds and in situ daily winds from the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA). The primary objective of this study is to examine whether DASCAT is a reliable replacement for the widely used QSCAT wind products. Spatial distributions of DASCAT and QSCAT winds show good agreement in speed and direction, except over a few localized regions. The study finds a significant spatial coherence between rainfall and the regions of discrepancy between DASCAT and QSCAT. Comparison of DASCAT and QSCAT wind products with RAMA moorings indicates that DASCAT better captures the overall wind variability compared to QSCAT, especially during rainy and low wind (<5 m s−1) conditions. The root-mean-square of the RAMA-DASCAT (RAMA-QSCAT) difference during rainfall in the zonal and meridional winds is 1.4 and 1.6 m s−1 (2.7 and 2.0 m s−1), respectively. The present study indicates that the DASCAT blended wind product is a reliable alternative to QSCAT in the tropical Indian Ocean. [ABSTRACT FROM AUTHOR]