Central Indian Ocean Research Articles

DNA barcoding confirms the first record of a Desmodema polystictum (Ogilby, 1898) egg and all-time high adult catches in the Indian Ocean

The eggs of Polka-dot ribbonfish Desmodema polystictum have been recorded for the first time in the Indian Ocean. Although the only previous information on eggs from this species consists of line drawings from 1973 (Pacific Ocean), the identification was possible by combining morphological and molecular analyses. As far as we are aware, only few confirmed records of adult individuals have been previously reported from the Indian Ocean. We found eggs in the proximity of numerous adults (57 and 42 individuals at two stations) indicating that the Central Indian Ocean is potentially an important spawning ground of D. polystictum.

Nassarius foveolatus (Gastropoda, Nassariidae), a new record of an exotic species in Brazil

Exotic species are those that occur in an area beyond their natural limit and they are considered invasive when they cause harm to the economy, environment, or human health. In coastal environments, ballast water and inlays on the hull and other parts of vessels are the main ways of introducing invasive aquatic alien species. Nassarius foveolatus (Dunker, 1847) is native from the Central and East Indian Ocean to the East China Sea. The first specimens (empty shells) of N. foveolatus were collected manually on November 11, 2017 on the Rocio footbridge, located in the Paranaguá Estuarine Complex, on the coast of the State of Paraná, southern Brazil. Posteriorly, live specimens were collected in other localities of this bay. It is already possible to infer that the specimens of N. foveolatus occur together with the native specimens of N. vibex (Say, 1822), having the same niche. As previously only N. vibex existed in that place, at least a displacement of this native species has been occurred. However, certainly future ecological studies may confirm this displacement and additional consequences to the local ecosystem, as nassariids can be predators and scavengers. Control procedures should be also greatly implemented.

On the potential of using sound sources of opportunity recorded on the Comprehensive Nuclear-Test-Ban Treaty hydroacoustic network for historical ocean thermometry

The globally distributed CTBT hydrophones form an existing network to measure temperature structure of the world oceans. Ocean tomography (or thermometry, asserting the primary driver in sound speed variability is temperature) typically operates by deploying active sources in precisely known locations and projecting signals at precisely known times. Ocean temperature is inferred from the travel time to distributed receivers. Here, we investigate the potential for using sound sources of opportunity within the nearly 20 year record of CTBT recordings. While some of these historical events have known locations and time, multiple hydroacoustic stations can localize the origin of intense sounds without this information. This study will focus primarily on sounds generated by seismic events detected on the CTBT station Diego Garcia in the central Indian Ocean. Hydroacoustic-based localization is possible through the consideration of multiple refracted and diffracted arrivals caused by the complicated bathymetry near this archipelago. These multiple arrivals are used to form ‘virtual’ stations to precisely triangulate the sound origin and establish the event time. Comparison of sound source locations to the event epicenters, determined by land-based seismic stations, are discussed in terms of sound generation mechanisms and uncertainties relevant to ocean thermometry.

Subspace detectors for long-term analysis of blue whale vocal activity in the Indian Ocean

The Comprehensive Test Ban Treaty Organization (CTBTO) deep water hydrophones around Diego Garcia island (H08) in the central Indian Ocean observe a number of blue whale calls. The hydrophones operational since 2002 are deployed in two groups of three (triads). The first one H08S is to the south, and the second H08N to north of the island. Not much is known about the distribution of whales in the region. The CTBTO observations are hence an opportunity to address this knowledge gap. This work builds subspace detectors to track the whales. The subspaces use the time-frequency signatures of the respective calls. There were however implementation challenges. The call-frequencies drifted across the 16 year dataset. The observed SNRs across hydrophones varied much due to fading. There were also interference issues due to multiple species vocalizing at the same time. This work extends the subspace approach to deal with these issues. The new approach tracks changing frequencies, fuses calls across each triad, and rejects interfering calls. Test results show that the new detectors are relatively robust to false alarm and improve detection rates. The work then applies the detectors to all the H08S and H08N recordings to track whale seasonality and suggest migration patterns.

Dissolved aluminium cycling in the northern, equatorial and subtropical gyre region of the Indian Ocean

Dissolved aluminium (hereafter, dAl) concentrations have been determined on 34 full vertical water column profiles, sampled along the two separate GEOTRACES-India transects (GI-01 and GI-06) to understand the biogeochemical controls on dAl distribution in the northern (the Bay of Bengal, the Andaman Sea and the Arabian Sea), equatorial and subtropical gyre region of the Indian Ocean. Al released due to the partial dissolution of and/or Al desorption from the suspended lithogenic sediments, supplied in a huge amount to the Bay of Bengal (BoB) waters by the Ganga-Brahmaputra (G-B) river system, Indian peninsular rivers and sediment resuspension from the continental shelf and slope, predominantly controls the dAl distribution in the BoB and results in an overall increase in the dAl concentrations throughout the water column towards the northern BoB and the eastern coast of India. Considering the steady-state balance between the dAl input from the lithogenic sediment flux in the upper water column and first-order scavenging removal of dAl, the fractional solubility of Al from the lithogenic sediments in the BoB surface waters is estimated to be in the range of 1.1–4.7%. Advection of Indonesian Sea surface waters through the Strait of Malacca and dAl input from the partial dissolution of resuspended terrigenous sediments, sourced from the insular shelf of the Nicobar Islands, set major controls on the dAl distribution in the surface and upper thermocline waters of the southern Andaman Sea and the southern BoB region near the Nicobar Islands. Rapid renewal of and vertical mixing in the deeper waters (>1000 m) relative to scavenging removal of dAl results in near-uniform dAl concentrations (∼3.7 nM) in the deeper waters at the center of the southern Andaman Sea. In the south-eastern Arabian Sea, the continental outflow of mineral dust and advection of dAl enriched Bay of Bengal surface waters control the dAl distribution in the surface water layer. dAl in the surface waters of the equatorial Indian Ocean, during the late winter sampling period of the GI-06 cruise, is predominantly determined by the mixing between dAl-rich surface waters of the southern BoB and relatively, dAl-depleted surface waters of the southern Arabian Sea under the influence of the Northeast Monsoon Current. The scavenging residence time of dAl in the deep waters of the equatorial Indian Ocean is estimated using a 1-D scavenging-advection–diffusion model and found to be in the range of 92–141 years. Deposition of Australian dust and advection of Indonesian Throughflow Water translates to the dAl enrichment in the upper water column (<500 m) at the northern end of the Indian Subtropical Gyre. Sediment resuspension near the Central Indian Ridge probably enrich the dAl in the ambient deep water depth (2000–3500 m) and this signal dampens as the deep waters progress north-westward in the Central Indian Ocean Basin. The bottom water (>3500 m) advects across the Ninety East Ridge from the Western Australian Basin to bring the dAl-rich waters to the northern end of the Central Indian Ocean Basin.

Indonesian throughflow controlled the westward extent of the Indo-Pacific Warm Pool during glacial-interglacial intervals

Abstract The tropical atmospheric circulation is strongly influenced by the sea surface temperature (SST) of the warmest region of the world ocean, the Indo-Pacific Warm Pool (IPWP). The effect of recent anomalous Indian Ocean warming on the extent of the IPWP remains uncertain. Past SST records from the tropical Indian Ocean, associated with different temporal boundary conditions, can help shed light on how climate change impacts the IPWP and associated monsoonal processes. Here, we reconstruct the seawater temperature and precipitation changes (from Mg/Ca and δ18O of surface-dwelling planktic foraminifera Globigerinoides ruber) of the last ~184 kyr BP from the westernmost margin of the IPWP. The data is then used to reconstruct the spatial extent and intensity of the IPWP structure as well as its relationship with other regional phenomena. A consistently warmer (>28 °C) central equatorial Pacific Ocean and significantly cooler equatorial western Pacific and the Indian Ocean suggests that the warm pool extent reduced considerably during MIS6 as well as the last glacial interval, leading to a strong La-Nina like condition. The most enriched seawater δ18O (δ18Osw) during MIS6, was in the westernmost IPWP, suggesting weakened precipitation, despite La-Lina like conditions. The westernmost part of the IPWP became warmer than the threshold 28 °C during MIS5e and the early Holocene, suggesting the westward extension of the IPWP. The eastern equatorial Pacific Ocean also warmed considerably during MIS5e, suggesting a weaker Walker Circulation and a stronger El-Nino. The difference in δ18Osw decreased considerably during MIS5e and is attributed to increased precipitation/freshwater runoff in the central equatorial Indian Ocean. The increase in δ18Osw (~1.5‰) between ~55 kyr until the last glacial maximum, in a majority of the Indian Ocean records, suggests progressively weakened precipitation. A majority of the records have a uniform SST anomaly throughout the Holocene. A warmer equatorial Indian Ocean during interglacials suggests a strong modulation of the IPWP extent by the transport of warm water through the Indonesian throughflow region. The large drop in δ18Osw despite the comparable change in SST during the last glacial interval suggests that monsoon forcing has been much stronger in altering the surface hydrography in the Indian Ocean than the central Pacific Ocean.

Re-description of type material of Xenia Lamarck, 1816 (Octocorallia: Xeniidae).

In this study we examined the original types of 21 of the 56 morphospecies of Xenia currently listed in WoRMS. Unfortunately, our attempts to locate additional type material failed, and we conclude that they have been lost. The type localities of the material examined include the Red Sea, West and Central Indian Ocean, the South Pacific, Indonesia, The Philippines, and the Bismarck Sea, reflecting the extensive biogeographical distribution of this genus throughout the Indo-Pacific Ocean. A neotype was designated from the northern Red Sea for X. umbellata, and sequencing established its position in the phylogenetic tree in relation to other genera of the family Xeniidae. In addition, X. flexibilis n. sp. was described from The Philippines. The findings from the current study led to the synonymizing of X. actuosa with X. sansibariana and of X. plicata with X. blumi. Overall, the study has re-described the types of 17 species considered to be valid based on morphological characters. Validation of the morphological features of original type material along with species-diagnostic genetic characters are required in order to facilitate our ability to correctly employ Latin binomials, both in phylogenetic studies and in any other biological surveys.

Holocene sea levels in Southeast Asia, Maldives, India and Sri Lanka: The SEAMIS database

We assembled a database of Holocene relative sea-level index points (n = 213) and marine (n = 211) and terrestrial (n = 122) limiting points for the broader South and Southeast Asian region including the Maldives, India and Sri Lanka. The standardized review of published age-elevation information from corals, deltaic, estuarine and mangrove deposits, beachrocks and tidal notches, yielded a new suite of relative sea-level index and limiting points produced according to a standardized protocol. Expected spatial variability in Holocene relative sea-level change due to glacial isostatic adjustment was accounted for, by first subdividing the study area into ten geographic sub-regions from the Central Indian Ocean to the Western Tropical Pacific, and second by comparing sub-regional relative sea-level data to model predictions of glacial isostatic adjustment. Results show that some of the regionally constrained relative sea-level data are characterized by significant inconsistencies that cannot be explained by glacial isostatic adjustment. Such inconsistencies of standardized relative sea-level data become particularly obvious in areas around the Red River Delta in Vietnam, the Gulf of Thailand, the northwest coast of Malaysia and the Spermonde Archipelago in Indonesia. Based on a critical evaluation of the reviewed relative sea-level indicators, we discuss possible sources of local divergence and identify regions where data are currently insufficient to constrain glacial isostatic adjustment predictions. The remaining quality-controlled and consistent relative sea-level data show that glacial isostatic adjustment and syn-/post-formational influences such as tectonic uplift, subsidence and compaction were the dominant local drivers of Holocene relative sea-level change. Collectively, the results of this review suggest that Holocene sea levels in South and Southeast Asia and surrounding regions have been controlled by a variety of global and local drivers and imply that additional index points from the Java Sea in Indonesia would be valuable to better assess the spatial variability, and to calibrate geophysical models of glacial isostatic adjustment.

Coral bleaching impacts from back-to-back 2015\u20132016 thermal anomalies in the remote central Indian Ocean

Studying scleractinian coral bleaching and recovery dynamics in remote, isolated reef systems offers an opportunity to examine impacts of global reef stressors in the absence of local human threats. Reefs in the Chagos Archipelago, central Indian Ocean, suffered severe bleaching and mortality in 2015 following a 7.5 maximum degree heating weeks (DHWs) thermal anomaly, causing a 60% coral cover decrease from 30% cover in 2012 to 12% in April 2016. Mortality was taxon specific, with Porites becoming the dominant coral genus post-bleaching because of an 86% decline in Acropora from 14 to 2% cover. Spatial heterogeneity in Acropora mortality across the Archipelago was significantly negatively correlated with variation in DHWs and with chlorophyll-a concentrations. In 2016, a 17.6 maximum DHWs thermal anomaly caused further damage, with 68% of remaining corals bleaching in May 2016, and coral cover further declining by 29% at Peros Banhos Atoll (northern Chagos Archipelago) from 14% in March 2016 to 10% in April 2017. We therefore document back-to-back coral bleaching and mortality events for two successive years in the remote central Indian Ocean. Our results indicate lower coral mortality in 2016 than 2015 despite a more severe thermal anomaly event in 2016. This could be caused by increased thermal resistance and resilience within corals surviving the 2015 thermal anomaly; however, high bleaching prevalence in 2016 suggests there remained a high sensitivity to bleaching. Similar coral mortality and community change were seen in the Chagos Archipelago following the 1998 global bleaching event, from which recovery took 10 yr. This relatively rapid recovery suggests high reef resiliency and indicates that the Archipelago’s lack of local disturbances will increase the probability that the reefs will again recover over time. However, as the return time between thermal anomaly events becomes shorter, this ability to recover will become increasingly compromised.

Diversity and extracellular enzyme activities of heterotrophic bacteria from sediments of the Central Indian Ocean Basin

Sedimentary bacteria play a role in polymetallic nodule formation and growth. There are, however, limited reports on bacterial diversity in nodule-rich areas of the Central Indian Ocean Basin (CIOB). In this study, bacterial abundance in thirteen sediment cores collected from the CIOB was enumerated, followed by phylogenetic characterisation and, screening of select heterotrophic bacteria for extracellular enzyme activities. Total bacterial counts (TBC) were in the order of 107 cells g−1; there was a significant difference (p > 0.05) among the cores but not within the sub-sections of the cores. The retrievable heterotrophic counts ranged from non-detectable to 5.33 × 105 g−1; the heterotrophic bacteria clustered within the phyla Firmicutes, Proteobacteria and Actinobacteria. Bacillus was the most abundant genus. The extracellular enzyme activities were in the order: amylase > lipase > protease > phosphatase > Dnase > urease. Major findings are compared with previous studies from the CIOB and other areas.

Lagoon shrimp goby, Cryptocentrus cyanotaenia (Bleeker, 1853) (Teleostei: Gobiidae), an additional fish element for the Iranian waters

Shrimp-associated gobies are burrowing fish of small to medium size that are common inhabitants of sand and mud substrates throughout the tropical Indo-Pacific region. Due to specific habitat preference, cryptic behavior, the small size and sampling difficulties, many gobies were previously overlooked and thus the knowledge about their distribution is rather scarce. This study presents lagoon shrimp goby, Cryptocentrus cyanotaenia , as additional fish element for the Iranian waters in the coast of Hormuz Island (Strait of Hormuz). The distribution range of lagoon shrimp goby was in the Western Central Pacific and eastern Indian Ocean. This species is distinguished by the several traits such as body elongate and compressed, snout truncate, body brownish grey color with 11 vertical narrow whitish blue lines on the sides, largely greenish yellow on head and mandible, head and base of pectoral fin with numerous short blue oblique broken lines and spots with markings on the head and snout. Providing data on C. cyanotaenia which is now widely distributed in the Indo-Pacific, would offer particularly valuable information on the biogeography of species.

An integrated geophysical study east of the southern Chagos Laccadive Ridge Complex, Central Indian Ocean Basin: Implications for the Rodriguez Triple Junction dynamics in the Late Cretaceous

The Central Indian Ocean Basin, east of the southern Chagos Laccadive Ridge complex has evolved due to seafloor spreading in the N-S direction since the Late Cretaceous. The trace of the Rodriguez Triple Junction (RTJ) has been inferred in this basin, but its exact location and dynamics prior to C29o are not known. About 20,000 km of total magnetic intensity data along with satellite gravity mosaic were used to understand the dynamics of the RTJ in the Late Cretaceous.Synthetic seafloor spreading models endorse the presence of Late Cretaceous to early Palaeogene magnetic anomalies C34y to C24o in the study area. Half-spreading rates are initially low (32 mm/yr) between C34y to C33y, and increase up to 78 mm/yr by C28y. Thereafter a decreasing trend is seen (57 mm/yr). Towards the west in the study area, higher spreading rates (120 mm/yr) between C33y and C32ay, and distorted anomalies between chrons C32ay and C30y are observed. Detailed geophysical analysis suggests that the higher spreading rates are due to the presence of transferred crust, and the distorted anomalies are caused by the presence of fragmented tectonic blocks depicting microplate formation and rotation.Plate reconstruction models suggest that the RTJ at C34y time lay southwest of Comorin Ridge which coincided with the northern edge of the Conrad Rise. Until C33y, the RTJ left a linear trace parallel to the fracture zone trend on both the Indian and Antarctic plates, while it gradually moved eastwards on the African plate. The RTJ migrated westwards until C31y causing the transfer of a considerable amount of crust from the African plate to both the Indian and Antarctic plates. Thereafter, the RTJ rapidly reversed its migration direction towards the east until C29o causing the formation and rotation of a microplate on the Indian plate and lengthening of the Southwest Indian Ridge.The present study suggests that this episodic migration of the RTJ may be due to ridge propagation under the influence of the Marion hotspot which was in the vicinity during that period. This RTJ migration resulted in microplate formation, lithospheric transfer and thickened lithosphere similar to the inferences made at many triple junctions in the Pacific Ocean. Thus the origin of anomalous structures such as the Crozet Bank may be related to triple junction dynamics under the influence of Marion hotspot. Further, the counter clockwise rotation of the Southwest Indian Ridge between C33y and C29y may have a causal link with the formation and rotation of the microplate, which occurred during the same period. The present study thus facilitated the inference of the dynamics of the RTJ trace between C34y and C29o on the three plates.

Recent changes in the prominent modes of Indian Ocean dipole in response to the tropical Pacific Ocean SST patterns

The present study examines the prominent patterns of Indian Ocean dipole (IOD) associated with the recent changes in the Pacific Ocean variability centered around 1998. EOF analysis of boreal fall (September through November (SON)) Indian Ocean sea surface temperature anomaly (SSTA) reveals that EOF1 (32% variability) is dominated by East Pacific El Nino-IOD co-occurring pattern during 1980–1998 (period 1). During period 2 (1999–2016), central Pacific El Nino also influence EOF1 (26% variability) and the IOD co-occurrence is reduced. EOF2 with 22% variability has central Indian Ocean dipole pattern co-occurred with El Nino Modoki during period 1, and it became basin-wide south-north dipole pattern in period 2 with reduced (20%) variability. At the same time, local Indian Ocean variability induced by SST anomalies centered on western side of Australia (Ningaloo Nino/Nina) is found to be related to EOF2 in both the periods and its influence is stronger after 1998, when its association with El Nino weakens. Third mode is ENSO independent IOD pattern in both the epochs and has more variability in period 2. Thus, during period 1, both East Pacific and central Pacific El Nino events are well separated as major two modes of tropical Pacific Ocean. These two flavors are associated with two different IOD patterns. But in the second period, first mode of Pacific Ocean is strongly related to both the El Nino flavors and reduced the co-occurrence of IOD with the El Nino modes and the effect of local Indian Ocean processes in the IO variability is intensified.

Detailed Structure and Plate Reconstructions of the Central Indian Ocean Between 83.0 and 42.5 Ma (Chrons 34 and 20)

AbstractThe Central Indian Ocean, namely the Central Indian, Crozet, and Madagascar basins, formed by rifting and subsequent drifting of India (now Capricorn), Antarctica, and Africa (now Somalia). We gathered a comprehensive set of sea surface magnetic anomaly profiles over these basins and revisited location and identification of magnetic isochrons between C34ny (83.0 Ma) and C20ny (42.536 Ma) using the objective analytic signal technique. We present high‐resolution magnetic isochrons for 29 periods based on ~1,400 magnetic anomaly picks. From the conjugate sets of picks, we derive two‐plate finite rotation parameters for both the Capricorn‐Antarctica and Capricorn‐Somalia motions. These finite rotations are compared to three‐plate reconstructions of the plate boundaries between the Capricorn, Antarctica, and Somalia plates, constrained by the closure of the Indian Ocean Triple junction. In general, the three‐plate reconstructions slightly overrotate the reconstructed isochrons with respect to the Capricorn‐Antarctica and Capricorn‐Somalia two‐plate reconstructions. Conversely, the two‐plate reconstructions for Somalia‐Antarctica slightly underrotate the isochrons compared to the Capricorn‐Antarctica‐Somalia three‐plate reconstructions. We suspect that the discrepancies between the two‐plate and three‐plate methods result from the recent seafloor deformation in the Capricorn‐India diffuse plate boundary and/or from the contrasted nature and geometry of magnetic isochrons at different spreading rates (i.e., magnetic structure of the three spreading centers). Three‐plate reconstructions better constrain the closure of the triple junction but spread any misfit among all three plate boundaries. When enough quality data are available, two‐plate reconstructions may lead to more realistic plate motion estimates from which additional geological problems can be identified and solved.

Massive corals maintain a positive carbonate budget of a Maldivian upper reef platform despite major bleaching event

Coral reefs experienced the third global bleaching event in 2015–2016 due to high sea-surface temperature (SST) anomalies. Declines in net carbonate production associated with coral bleaching are implicated in reef structural collapse and cascading impacts for adjacent coral reef islands. We present the first carbonate budget study of a reef platform surface (reef crest and reef flat) in the southern Maldives and the first record of upper reef flat condition in the central Indian Ocean post the 2015–2016 coral bleaching event. Scleractinian corals were the primary carbonate producers, with live coral cover averaging between 11.1 ± 6.5 and 31.2 ± 21.8% and dominated by massive corals. Gross carbonate production rates averaged 5.9 ± 2.5 G (kg CaCO3 m2 yr−1). Bioerosion was estimated at 3.4 ± 0.4 G, resulting in an average net carbonate production rate of 2.5 ± 2.4 G. Comparison of results with a study of the fore-reef slope highlights major differences in post-bleaching carbonate budget state between the fore-reef slope and the reef platform surface. The positive reef flat carbonate budget is attributed to the persistence of massive corals (Porites spp. and Heliopora spp.) through the bleaching event.

Habitat associations of hawkfish: depth, distribution, and density at two sites in Kenya

Hawkfish (Actinopterygii: Perciformes: Cirrhitidae) are a family of carnivorous, coral-dwelling fishes. Some of these have been identified as obligate coral-dwelling, facultative coral-dwelling, substratum-dwelling, or water column-dwelling, depending on the species and habitat. Research on hawkfish density and habitat associations has been conducted in the Atlantic, Pacific, and the eastern and central Indian oceans. However, little is known about the density or habitat associations on the coast of East Africa. This study investigates distribution and density patterns of hawkfish species off Kenya, as well as relationships with available coral habitats, at both shallow (<5 m) and deeper (5–15 m) coral reef sites. In addition, habitat associations of the two most abundant hawkfish species of the same genus [Paracirrhites arcatus (Cuvier, 1829) and Paracirrhites forsteri (Schneider, 1801)] are identified. Findings showed species-specific macrohabitat associations, whereby higher densities of P. arcatus were found on deep reefs, and higher densities of P. forsteri were found on shallow reefs. In addition, P. arcatus showed a positive association with the hard-branching coral Pocillopora. However, as there was less Pocillopora on the deep reefs, P. arcatus does not prefer the deep reefs simply because there is more Pocillopora present, lending support that these species are facultative coral dwellers. The increased density of hawkfish on deeper reefs could be an indication of mesopredator release, as the study's deep reef locations were within national reserves, which are open to some types of artisanal fishing, reducing the number of apex predators.

Reef‐Scale‐Dependent Response of Massive Porites Corals From the Central Indian Ocean to Prolonged Thermal Stress: Evidence From Coral Sr/Ca Measurements

AbstractHere we present coral Sr/Ca data of biweekly resolution from three modern coral cores drilled from living Porites corals from two different reef settings at Chagos (tropical Indian Ocean). Chagos lies at the eastern margin of the Seychelles‐Chagos thermocline ridge and features open ocean upwelling. In situ temperatures have been recorded by temperature loggers since 2006. High‐resolution satellite temperatures closely track the logger data. Two cores were collected from a patch reef in the lagoon of Peros Banhos, a site characterized by high mean temperatures and low‐temperature variability. An open ocean core was collected at the outer reef slope of Diego Garcia, which experiences larger temperature fluctuations related to open ocean upwelling. The open ocean core shows clear seasonal cycles in Sr/Ca that closely track the satellite temperatures. The Sr/Ca records from the two lagoon corals show good reproducibility. Between 2007 and 2010, Sr/Ca tracks satellite temperatures. However, between 2003 and 2006 the Sr/Ca curves are almost flat and annual coral growth rates are low. During these years, warm sea surface temperatures and coral bleaching have been observed at Chagos. It is likely that the observed reduction in coral growth rates is a response to prolonged warming events, adding to the growing evidence that seemingly healthy and surviving corals are affected by thermal stress. Further, the high correlation between the Sr/Ca records of the two lagoon cores suggests that this effect is not limited to single coral colonies but may affect the entire reef setting.

Interannual Variability of Summer Surface Air Temperature over Central India: Implications for Monsoon Onset

AbstractYear-to-year variability of surface air temperature (SAT) over central India is most pronounced in June. Climatologically over central India, SAT peaks in May, and the transition from the hot premonsoon to the cooler monsoon period takes place around 9 June, associated with the northeastward propagation of intraseasonal convective anomalies from the western equatorial Indian Ocean. Positive (negative) SAT anomalies during June correspond to a delayed (early) Indian summer monsoon onset and tend to occur during post–El Niño summers. On the interannual time scale, positive SAT anomalies of June over central India are associated with positive SST anomalies over both the equatorial eastern–central Pacific and Indian Oceans, representing El Niño effects in developing and decay years, respectively. Although El Niño peaks in winter, the correlations between winter El Niño and Indian SAT peak in the subsequent June, representing a post–El Niño summer capacitor effect associated with positive SST anomalies over the north Indian Ocean. These results have important implications for the prediction of Indian summer climate including both SAT and summer monsoon onset over central India.

A little spin in the Indian Ocean plate circuit

Divergence of the Australian and East Antarctic plates is well understood from the late Jurassic onset of half graben development on the Australian continental shelf, and from post mid-Eocene (chron 20; 45 Ma) seafloor spreading isochrons further offshore. Relative plate motion between these times is less confidently interpretable from magnetic reversal anomalies landwards of isochron 20 and localised evidence for mid-to-late Cretaceous subsidence and growth strata from the continental shelf and rise south of Australia. A new test of this history examines it within the post-34y (84 Ma) Indian Ocean plate circuit, built using seafloor spreading data from the Wharton and central Indian Ocean basins. The Australian-Antarctic Jurassic-onset rift system is interpreted to have been abandoned before 34y, because motion in the circuit is inconsistent with an active plate boundary during 34y–26y (84–58 Ma). Starting 26y, the model depicts plate divergence distances and azimuths that, after 25y (57 Ma), can be independently confirmed by reassessment of the pre-chron 20 magnetic reversal anomaly pattern. Previous studies have identified evidence for mantle exhumation and focused magmatism in basement marginwards of these anomalies. These processes are not directly or confidently dated, but mantle exhumation is inconsistent with the circuit model's fast plate divergence at 26y–25y. Hence, plate motion during the immediate build-up to post-57 Ma seafloor spreading may have been accommodated by focused magmatism, whilst mantle exhumation may mark the conclusion of the Jurassic-onset rift phase during a slower pre-84 Ma period of plate divergence. Using the new model to make tectonic reconstructions results in a large overlap between Tasmania and Victoria Land that can be explained with reference to Eocene strike-slip faulting and transtension in recently-discovered subglacial basins of Wilkes and George V lands and Terre Adélie.

Wind Limits on Rain Layers and Diurnal Warm Layers

AbstractStratification of the upper few meters of the ocean limits the penetration depth of wind mixing and the vertical distribution of atmospheric fluxes. Significant density stratification at depths ≤ 5 m was observed in 38% of a 2‐month data set from the central Indian Ocean collected during the DYNAMO experiment (Dynamics of the MJO, Madden‐Julian Oscillation). Diurnal warm layers (DWLs) formed by solar heating populated 30% of the data set and rain layers (RLs) populated 16%. Combined contributions from rain and insolation formed RL‐DWLs in 9% of the data set. RLs were detected at values of U10 up to 9.8 m s−1, while DWLs were only detected at U10 &lt; 7.6 m s−1 (99th percentile values), symptomatic of the greater buoyancy flux provided by moderate to high rain rate compared to insolation. From the ocean friction velocity, u*w, and surface buoyancy flux, B, we derived estimates of , stable layer depth, and , the maximum U10 for which stratification should persist at for fixed B. These estimates predicted (1) 36 out of 44 observed stratification events (88% success rate) and (2) the wind limits of these events, which are considered to be the 99th percentile values of U10). This suggests a means to determine the presence of ocean stable layers at depths ≤ 5 m from U10 and B. Near‐surface stratification varied throughout two Madden‐Julian Oscillation (MJO) cycles. In suppressed MJO periods, (U10 ≤ 8 m s−1 with strong insolation), RLs and RL‐DWLs were rare while DWLs occurred daily. During disturbed and active MJO periods, (U10 ≤ 8 m s−1 with increased rain and cloudiness), multiple RLs and RL‐DWLs formed per day and DWLs became less common. When westerly wind bursts occurred, (U10 = 7–17 m s−1 with steady rain), RLs formed infrequently and DWLs were not detected.