Indian Ocean Basin Research Articles

Decreasing trend in destructive potential of tropical cyclones in the South Indian Ocean since the mid-1990s

Tropical cyclone activity often leads to many adverse impacts and assessing their destructiveness is a crucial scientific concern. Here we investigated changes in the destructiveness of tropical cyclones worldwide using the power dissipation index and found that there is no clear trend in most basins, but a significant decrease in power dissipation index has been detected in the South Indian Ocean basin since 1994, which is almost entirely due to a decrease in both tropical cyclone frequency and duration in this basin. The decrease in tropical cyclone frequency is influenced by increased atmospheric stability. The decrease in tropical cyclone duration can be attributed to the changes in tropical cyclone locations. In addition, the weakened subtropical high is only observed in the South Indian Ocean (i.e., the Mascarene High), which is also associated with the decrease in tropical cyclone destructiveness. These findings have implications for assessing the destructive potential of future tropical cyclones.

A New Find of Anorthoclase Crystals-lined Vug in a Deep-sea Basalt, Central Indian Ocean Basin

ABSTRACT We detail a new and first find of a vug in a deep-sea basalt that is lined with well-formed anorthoclase crystals. The specimen was dredged from the seafloor (water depth 4,440 m) near a seamount in the Central Indian Ocean Basin. The basalt has a thin veneer of glass covered with ferromanganese (FeMn) oxides while the interior is quite fresh. The geochemical analysis of the basalt depicts different degrees of alteration that vary from the outermost layer with FeMn oxides to the interior of the specimen. In general, the TiO2 (<1.5 wt%) and MgO (~5 wt%) contents are low while CaO, Na2O, K2O and P2O5 contents change within the specimen. The FeMn oxides have Fe2O3 (21 wt%) and MnO (8–27 wt%) and appear as botryoids and occur near and/or over the anorthoclase crystals. The trace and rare earth elements do not show significant variations from the outer to the interior of the specimen. Microscopy (binocular and scanning electron) and energy dispersive spectrometry of the vug revealed a dominance of drusy anorthoclase crystals. The formation of the anorthoclase crystals is ascribed to precipitation from hydrothermal solutions that seeped into the basalt through cracks. This is attested by the co-existing crystals of phillipsite and feldspar and FeMn oxides.

Inter‐Basin Versus Intra‐Basin Sea Surface Temperature Forcing of the Western North Pacific Subtropical High's Westward Extensions

AbstractZonal extensions of the Western Pacific subtropical high (WPSH) strongly modulate extreme rainfall activity and tropical cyclone (TC) landfall over the Western North Pacific (WNP) region. These zonal extensions are primarily forced on seasonal timescales by inter‐basin zonal sea surface temperature (SST) gradients. However, despite the presence of large‐scale zonal SST gradients, the WPSH response to SSTs varies from year to year. In this study, we force the atmosphere‐only NCAR Community Earth System Model version 2 simulations with two real‐world SST patterns, both featuring the large‐scale zonal SST gradient characteristic of decaying El Niño‐developing La Niña summers. For each of these patterns, we performed four experimental sets that tested the relative contributions of the tropical Indian Ocean, Pacific, and Atlantic basin SSTs to simulated westward extensions over the WNP during June–August. Our results indicate that the subtle differences between the two SST anomaly patterns belie two different mechanisms forcing the WPSH's westward extensions. In one SST anomaly pattern, extratropical North Pacific SST forcing suppresses the tropical Pacific zonal SST gradient forcing, resulting in tropical Atlantic and Indian Ocean SSTs being the dominant driver. The second SST anomaly pattern drives a similar westward extension as the first pattern, but the underlying SST gradient driving the WPSH points to intra‐basin forcing mechanisms originating in the Pacific. The results of this study have implications for understanding and predicting the impact of the WPSH's zonal variability on tropical cyclones and extreme rainfall over the WNP.

Evidence of a dual African and Australian biomass burning influence on the vertical distribution of aerosol and carbon monoxide over the southwest Indian Ocean basin in early 2020

Abstract. During the 2020 austral summer, the pristine atmosphere of the southwest Indian Ocean (SWIO) basin experienced significant perturbations. This study examines the variability of aerosols and carbon monoxide (CO) over this remote oceanic region and investigates the underlying processes in the upper troposphere–lower stratosphere (UT-LS). Aerosol profiles in January and February 2020 revealed a multi-layer structure in the tropical UT-LS. Numerical models – the FLEXible PARTicle dispersion model (FLEXPART) and the Modèle Isentropique de transport Mésoéchelle de l'Ozone Stratosphérique par Advection (MIMOSA) – indicated that the lower-stratospheric aerosol content was influenced by the intense and persistent stratospheric aerosol layer generated during the 2019–2020 extreme Australian bushfire events. A portion of this layer was transported eastward by prevailing easterly winds, leading to increased aerosol extinction profiles over Réunion on 27 and 28 January. Analysis of advected potential vorticity revealed isentropic transport of air masses containing Australian biomass burning aerosols from extratropical latitudes to Réunion at the 400 K isentropic level on 28 January. Interestingly, we found that biomass burning (BB) activity in eastern Africa, though weak during this season, significantly influenced (contributed up to 90 % of) the vertical distribution of CO and aerosols in the upper troposphere over the SWIO basin. Ground-based observations at Réunion confirmed the simultaneous presence of African and Australian aerosol layers. This study provides the first evidence of African BB emissions impacting the CO and aerosol distribution in the upper troposphere over the SWIO basin during the convective season.

Economic Waves of Power: India and China’s Strategic Pursuits in the Indian Ocean Basin

Economic Waves of Power: India and China’s Strategic Pursuits in the Indian Ocean Basin

Comparative analysis of microbiome inhabiting oxygenated and deoxygenated habitats using V3 and V6 metabarcoding of 16S rRNA gene

We examine how oxygen levels and the choice of 16S ribosomal RNA (rRNA) tags impact marine bacterial communities using Next-Generation amplicon sequencing. Analyzing V3 and V6 regions, we assess microbial composition in both Oxygen minimum zones (OMZ) and non-OMZ (NOMZ) areas in the Arabian Sea (AS) and the Central Indian Ocean basin (CIOB) respectively. Operational taxonomic units (OTUs) at 97% similarity showed slightly higher richness and diversity with V6 compared to V3. Vertical diversity patterns were consistent across both regions. NOMZ showed greater richness and diversity than OMZ. AS and CIOB exhibited significant differences in bacterial community, diversity, and relative abundance at the order and family levels. Alteromonadaceae dominated the OMZ, while Pelagibacteraceae dominated the NOMZ. Synechococcaceae were found exclusively at 250 m in OMZ. Bacteria putatively involved in nitrification, denitrification, and sulfurylation were detected at both sites. Dissolved oxygen significantly influenced microbial diversity at both sites, while seasonal environmental parameters affected diversity consistently, with no observed temporal variation.

Partial melt in mesoscale upper mantle upwellings beneath ocean basins

How tectonic plate motions are coupled with mantle flows remains an open question. Quasi-periodic 2000 km wavelength undulations aligned with absolute plate motion in the Pacific and Indian Ocean basins, observed in gravity and seafloor topography and coinciding with seismically imaged low shear velocity fingers in the upper mantle, suggest the presence of meso‑scale convection below the lithosphere. However, the correspondence of sub-lithospheric mantle mass excess, seafloor lows and slow upper mantle seismic velocities cannot be explained by temperature variations alone. Here we introduce a simplified system of bi-dimensional convective cells of width ∼1000 km, extending from the base of the lithosphere through the extended mantle transition zone (down to 1000 km depth), at least partly driven from below. From mass balance considerations in a viscous Earth, we show that the density excess required in the hot upwelling limbs may reflect the formation of stable dense lenses of dehydration-induced partial melt atop the 410 km discontinuity, and upward entrainment of a small fraction of quasi-buoyant partially molten and recrystallized material across the upper mantle. Our model provides an explanation for the thin low shear velocity layer detected intermittently above the 410 km discontinuity in some parts of ocean basins away from subducted slabs, and supports the presence of water in the transition zone.

Comparative VGP models to analyse the primary productivity in the north Indian ocean and the linkages with rising sea surface temperature

The Primary Productivity of an ecosystem is an indicator of the health of the marine ecosystem and fishing yield. Primary productivity across the continental sea of the North Indian Ocean (NIO) basin was estimated using two vertically generalised production models. We examine the productivity using two non-parametric tests and present the changes using a gridded basin map. In the Behrenfeld and Falkowski (BF) approach, estimated primary productivity shows a significant decline of over 6550 km2 from 2003 to 2020, recorded spatially 100 km2 adjacent to the coastline. However, an area of 825 km2 across the exclusive economic zone of the Indian sub-continent has seen a significant rise in primary productivity. The Kameda and Ishizaka (KI) model limits productivity reduction to 2500 km2 across the NIO basin. The seasonal trends indicate reduced summer productivity across the basin. Both models point towards a significant reduction of productivity observed across the Bay of Bengal (BoB) and Arabian Sea (ARS) coastal regions owing to the damaging effects of anthropogenic-induced global warming. Secondly, simultaneous regression analysis using climate-based indicators and satellite-based NPP and Chlr-a data shows a significant reduction with high magnitude in the levels of NPP across the BoB and ARS between 1997 and 2020 due to rising sea surface temperature and a reduction of 9% and 6% in chlorophyll-a levels, respectively. These results point towards the inverse linkage between the rising SST on the primary productivity of the NIO basin and its devastating impacts on marine fisheries, especially in the BoB. These findings underscore the need for urgent action to mitigate the impacts of climate change and promote sustainable management of marine resources.

Trend and interannual variability of summer marine heatwaves in the tropical Indian ocean: Patterns, mixed layer heat budget, and seasonal prediction

Marine heatwaves (MHWs) are extreme sea surface temperature (SST) events in all ocean basins, with far-reaching impacts on marine ecosystems and socio-economy. The leading patterns, trend, and interannual variability of summer MHWs in the tropical Indian Ocean (TIO) are investigated in this study. The first empirical orthogonal function (EOF) mode of frequency of MHWs exhibits a monopole pattern over the entire basin. This mode is highly associated with the concurrent Indian Ocean Basin warming, indicating a remarkable trend over the past four decades. The linear trend in MHW properties largely relates to increased summer Indian Ocean mean SST. The second EOF mode exhibits a zonal dipole with the MHW numbers increasing in the west and decreasing in the east. On the interannual timescale, the first two EOF modes are remotely affected by antecedent and concurrent El Niño events, respectively. The ocean mixed layer budget is utilized for examining the formation of different summer MHW patterns. During the preceding spring, the surface heat flux is important for the development of MHWs, while the ocean advections play a secondary role in the South Indian Ocean for the MHW monopole. Once the SST anomaly rises in summer, the ocean advections play a dominant role in maintaining the SST. Last, we assess the prediction skill of summer TIO MHWs by performing a bilinear seasonal statistical prediction model. Our results suggest the frequency of summer MHWs in the TIO could be predicted one season in advance. This study has great implications for understanding and predicting ocean extreme events in the TIO.

Role of the Indian Ocean basin mode in driving the interdecadal variations of summer precipitation over the East Asian monsoon boundary zone

Abstract. Based on long-term observational and reanalysis datasets from 1901 through 2014, this study investigates the characteristics and physical causes of the interdecadal variations in the summer precipitation over the East Asian monsoon boundary zone (EAMBZ), which is a peculiar domain defined from the perspective of the interplay between climatic systems (i.e., mid-latitude westerly and East Asian summer monsoon). Observational evidence reveals that, similarly to previous studies, the EAMBZ precipitation featured prominent interdecadal fluctuations, e.g., with dry summers during the periods preceding 1927, 1939–1945, 1968–1982, and 1998–2010 and wet summers during the periods of 1928–1938, 1946–1967, and 2011 onwards. Further analyses identify that, amongst the major interdecadal oceanic forcings (e.g., Atlantic multidecadal oscillation and Pacific decadal oscillation), the Indian Ocean basin mode (IOBM) is a significant oceanic forcing responsible for the interdecadal variations of the EAMBZ precipitation, playing an independent and critical modulation role. When the cold phase of the IOBM occurs, an anomalous cyclonic circulation is excited around the northeast corner of the tropical Indian Ocean, which further induces a north-low–south-high meridional seesaw pattern over the Northeast China–subtropical western Pacific (SWP) sector. Such seesaw pattern is conducive to the enhanced EAMBZ precipitation by linking favorable environments for the transportation of water vapor from the SWP and the convergence over the EAMBZ at interdecadal timescales. For this reason, a physical–empirical model for the EAMBZ precipitation is developed in terms of the IOBM cooling. Despite the fact that the extreme summer EAMBZ precipitation cannot be captured by this model, it can still well capture its interdecadal fluctuations and reflect their steady relationship. The key physical pathway connecting the IOBM cooling with the interdecadal variations of the summer EAMBZ precipitation is supported by the numerical results based on the large ensemble experiment and the Indian Ocean pacemaker experiment. Our findings may provide new insights into the understanding of the causes of the interdecadal variations in the summer EAMBZ precipitation, which may favor the long-term policy decision-making for the local hydrometeorological planning.

Assessment of the Decadal Prediction Skill of Sahel Rainfall in CMIP5 and CMIP6

Abstract Accurately predicting the decadal variations in Sahel rainfall has important implications for the lives and economy in the Sahel. Previous studies found that the decadal variations in sea surface temperature (SST) in the Atlantic, Mediterranean Sea, Indian Ocean, and Pacific contribute to those in Sahel rainfall. This study evaluates the decadal prediction skills of Sahel rainfall from all the available hindcasts contributing to phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6), in comparison with the related uninitialized simulations. A majority of the prediction systems show high skill with regard to Sahel rainfall. The high skill may be partly attributed to external forcings, which are reflected in good performance of the respective uninitialized simulations. The decadal prediction skills of the key SST drivers and their relationships with the Sahel rainfall are also assessed. Both the hindcasts and the uninitialized simulations generally present high skill for the Atlantic multidecadal variability (AMV) and Mediterranean Sea SST indices and low skill for the Indian Ocean basin mode (IOBM) and interdecadal Pacific variability (IPV) indices. The relationship between the Sahel rainfall and the AMV or Mediterranean Sea SST index is reasonably captured by most prediction systems and their uninitialized simulations, while that between the Sahel rainfall and the IOBM or IPV index is captured by only a few systems and their uninitialized simulations. The high skill of the AMV and Mediterranean Sea SST indices as well as the reasonable representations of their relationships with the Sahel rainfall by both the hindcasts and uninitialized simulations probably plays an important role in predicting the Sahel rainfall successfully. Significance Statement Predicting Sahel rainfall on the decadal time scale is of great importance. This study provides a thorough evaluation of the decadal prediction skills of Sahel rainfall in the current decadal prediction systems participating in phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6). A majority of the systems achieve high prediction skill of Sahel rainfall, which probably results from the high prediction skill of some key sea surface temperature (SST) drivers, especially in the Atlantic and Mediterranean Sea SST, and their relationships with Sahel rainfall. This study provides a reference for better understanding the predictability of Sahel rainfall.

Developing climate services for vulnerable islands in the Southwest Indian Ocean: A combined statistical and dynamical CMIP6 downscaling approach for climate change assessment

Climate change is a global challenge necessitating adaptation at the local level. Small island developing states (SIDS) in the southwest Indian Ocean (SWIO) basin are particularly vulnerable and already facing significant challenges due to climate variability and extreme weather events like tropical cyclones (TCs). Tailored climate services, catering to their specific needs and contexts, are crucial for formulating appropriate adaptation strategies. This study aims to fill the gap of localized and reliable information for climate services in the SWIO region. A 12-km resolution regional climate model is dynamically downscaled from a CMIP6 model to capture the climatology of tropical cyclones. Outputs are bias-corrected at kilometer-scale resolution over multiple islands. A subset of CMIP6 simulations is also statistically downscaled over La Réunion to quantify model uncertainties at the local scale and compare statistical and dynamical downscaling methods.CMIP6 models project an average increase in daily mean temperatures over small islands ranging from 1.2 °C (SSP1-2.6) to 3.7 °C (SSP5-8.5) by the end of the century compared to the reference period of 1981–2010, and up to 4.4 °C on Madagascar (SSP5-8.5). The dry season in the SWIO region is anticipated to become significantly drier, with precipitation deficits ranging from 10 % to 40 %, primarily due to a delayed onset of the rainy season. The cyclonic risk is expected to increase due to stronger cyclone intensities, a higher proportion of strong TCs, and the poleward migration of very intense TCs by one to two degrees latitude, further amplifying the risk faced by the Mascarene islands.

An insight into the prokaryotic diversity from a polymetallic nodule-rich region in the Central Indian Ocean Basin using next generation sequencing approach.

Deep sea is a vast, dark, and difficult-to-access terrain and is now looked upon as a unique niche harboring diverse microorganism. We used a metataxonomic approach to decipher the microbial diversity present in the water column (surface to near bottom), water overlaying the sediments, and the deep-sea sediments (up to 35 cm) from the Indian Contract Region (ICR) in the Central Indian Ocean Basin (CIOB). Samples were collected from #IRZ (Impact Reference Zone), #PRZ (Potential Reference Zone), and #BC20 (Control site, outside potential mining area) with an average water depth of 5,200 m. 16S rRNA (V3-V4 region) amplicon sequencing on the MiSeq platform resulted in 942,851 ASVs across 65 water and sediment samples. Higher prokaryotic diversity was observed below 200 m in the water column to the seafloor. Proteobacteria was the most dominant bacterial phylum among all the water samples while Firmicutes, Actinobacteria and, Bacteroidota dominated the sediments. Sediment (below 10 cm) was co-dominated by Firmicutes. Thermoplasmata was the dominant archaeal group in the water column while Crenarchaeota was in the sediments. BC20 was less diverse than IRZ and PRZ. Deep Sea microorganisms could play a vital role in the mineralization processes, nutrient cycling, and also different biogeochemical cycles.

Community-Based Strategies for Disaster Preparedness in Mauritius

Mauritius is Small Island Developing State (SIDS) of volcanic origin which lies in the South-West Indian Ocean Basin and is vulnerable to the consequences of global environmental challenges. The fourth priority action of the Sendai Framework for Disaster Risk Reduction 2015-2030 is to consolidate disaster preparedness for an effective response and to have a Build Back Better agenda that will improve recovery, rehabilitation, and reconstruction. This research aimed at examining the disaster management system in Mauritius through community-based approaches. The National Disaster Risk Reduction and Management Centre’s (NDRRMC) involvement with the community living in high-risk zones was investigated. This study used quantitative data gathered from secondary sources and included the number of field training simulation exercises, training of volunteers under the Community Disaster Response Programme (CDRP), awareness campaigns and formulation of contingency plans based on documentation across Mauritius. The NDRRMC identified 109 high-risk zones across Mauritius out of which 13.7 per cent benefitted from all four forms of mitigation activities. Results indicated that contingency plans in Mauritius were evidence-based and had been developed after identification of disaster risk in selected regions, and therefore, they were limited to some types of disasters only. Findings also revealed that flooding was the most common type of simulated disaster exercise undertaken. The greatest number of field simulation exercises was carried out in Poste de Flacq. Results also demonstrated that only 38.5% of the community were exposed to disaster-related awareness campaigns, making it the least common form of disaster risk reduction (DRR) activity in Mauritius.

The Interdecadal Weakening of the Relationship Between Indian Ocean Sea Surface Temperature and Summer Precipitation in Central Asia

AbstractThe close relationship between the Indian Ocean Basin mode (IOBM) and summer precipitation in Central Asia (CA) has been documented in several studies. Nonetheless, this relationship has weakened since the 1990s and varies with the Atlantic Multi‐decadal Oscillation (AMO) phase transition. During the cold phase of the AMO (1970–1998), precipitation in CA was significantly positively correlated with the IOBM. Conversely, during the warm phase of the AMO (1999–2019), this correlation became insignificant. The decrease in the interannual variation in the IOBM resulted in the weakening of the atmospheric heat source variation over the North Indian continent and the south‐north movement of the subtropical westerly jet (SWJ). Along with the southerly SWJ, the IOBM exhibited only a weak positive correlation with precipitation in southern CA after the 1990s. This remarkable contrast in the impact of IOBM during different phases of the AMO offers intriguing possibilities for improving climate prediction in CA.

Locating the Littoral: Michael N. Pearson in the Study of the Northern Bay of Bengal

Abstract This article takes up Michael Pearson’s concept of littoral societies and discusses how it provides us with a lens to study a specific estuarine place, a mangrove forest called Sundarbans that is found along the coastlines of India and Bangladesh and opens onto the Bay of Bengal. Inhabitants cope with constant risks and hazards, maintaining distinctive folk traditions practiced by both the region’s Hindus and Muslims. This article discusses the outsider and insider perceptions of this place and its distinctive material culture, which may provide a window for studying other littoral cultures across the Indian Ocean basin.

Constraints from GPS measurements on plate coupling within the Makran subduction zone and tsunami scenarios in the western Indian Ocean

SUMMARY Plate-coupling estimates and previous seismicity indicate that portions of the Makran megathrust of southern Pakistan and Iran are partially coupled and have the potential to produce future magnitude 7+ earthquakes. However, the GPS observations needed to constrain coupling models are sparse and lead to an incomplete understanding of regional earthquake and tsunami hazard. In this study, we assess GPS velocities for plate coupling of the Makran subduction zone with specific attention to model resolution and the accretionary prism rheology. We use finite element model-derived Green's functions to invert for the interseismic slip deficit under both elastic and viscoelastic Earth assumptions. We use the model resolution matrix to characterize plate-coupling scenarios that are consistent with the limited spatial resolution afforded by GPS observations. We then forward model the corresponding tsunami responses at major coastal cities within the western Indian Ocean basin. Our plate-coupling results show potential segmentation of the megathrust with varying coupling from west to east, but do not rule out a scenario where the entire length of the megathrust could rupture in a single earthquake. The full subduction zone rupture scenarios suggest that the Makran may be able to produce earthquakes up to Mw 9.2. The corresponding tsunami model from the largest earthquake event (Mw 9.2) estimates maximum wave heights reaching 2–5 m at major port cities in the northern Arabian Sea region. Cities on the west coast of India are less affected (1–2 m). Coastlines bounding eastern Africa, and the Strait of Hormuz, are the least affected (<1 m).

A review of the Indian Ocean carbon dynamics, acidity, and productivity in a changing environment

The Indian Ocean dynamics is governed by the seasonal reversal of monsoon winds and the associated ocean currents. The relatively deep thermocline along the equator due to a lack of steady easterlies, low-latitude connection to the neighbouring Pacific, and a lack of northward heat export due to the position of the Asian continent are important factors in regulating the ocean state. These features make it a unique ecosystem among the world's tropical oceans and determine key features of potential air-sea interaction at different time scales. The pCO2 shows a large seasonal variation linked with monsoon circulation. The Indian Ocean’s northwestern part acts as an atmospheric CO2 source, whereas the northeastern part acts as a net atmospheric CO2 sink. The region between the latitudes of 15°S-50°S in the Indian Ocean is a major subduction zone because of positive wind stress curl. The subducted water masses are transported to the northern Indian Ocean by the cross-equatorial cell (a shallow meridional overturning circulation). Based on the regional studies carried out on the carbonate system in the Indian Ocean, the area north of 15°S is a source of atmospheric CO2, while the area between 15°S and 50°S is a sink. A recent synthesis of models (observational climatology) over different spatial scales provides an estimate of the mean value of CO2 in the north of 37.5°S of the Indian Ocean as − 0.19 ± 0.1 PgC/yr (−0.07 ± 0.14 PgC/yr) during 1985–2018. The estimated decrease in pH (acidification) using model outputs in the Indian Ocean basin is 0.0675 units during 1961–2010, in which the contribution of dissolved inorganic carbon and surface temperature is 69.3 % and 13.8 %, respectively. The range of the Indian Ocean’s annual primary production based on satellite estimates over the last two decades (1998–2018) is 7.72–8.70 Gt C/yr, whereas the climatological mean is 8.24 ± 0.30 Gt C/yr. This paper consolidates the current state of understanding of the Indian Ocean carbon fluxes, acidification, and productivity using available field and satellite-based measurements, model simulations, and re-constructed datasets. It provides an overview of the functioning of the Indian Ocean’s air-to-sea CO2 exchange and highlights its influence on global climate. Finally, it aims to highlight the grey areas of the Indian Ocean carbon cycle that need to be understood.

The role of marine fish-produced carbonates in the oceanic carbon cycle is determined by size, specific gravity, and dissolution rate

Rising CO2 emissions have heightened the necessity for increased understanding of Earth's carbon cycle to predict future climates. The involvement of marine planktonic species in the global carbon cycle has been extensively studied, but contributions by marine fish remain poorly characterized. Marine teleost fishes produce carbonate minerals (‘ichthyocarbonates’) within the lumen of their intestines which are excreted at significant rates on a global scale. However, we have limited understanding of the fate of excreted ichthyocarbonate. We analyzed ichthyocarbonate produced by three different marine teleosts for mol%MgCO3 content, size, specific gravity, and dissolution rate to gain a better understanding of ichthyocarbonate fate. Based on the species examined here, we report that 75 % of ichthyocarbonates are ≤0.91 mm in diameter. Analyses indicate high Mg2+ content across species (22.3 to 32.3 % mol%MgCO3), consistent with previous findings. Furthermore, ichthyocarbonate specific gravity ranged from 1.23 to 1.33 g/cm3, and ichthyocarbonate dissolution rates varied among species as a function of aragonite saturation state. Ichthyocarbonate sinking rates and dissolution depth were estimated for the Atlantic, Pacific, and Indian ocean basins for the three species examined. In the North Atlantic, for example, ~33 % of examined ichthyocarbonates are expected to reach depths exceeding 200 m prior to complete dissolution. The remaining ~66 % of ichthyocarbonate is estimated to dissolve and contribute to shallow water alkalinity budgets. Considering fish biomass and ichthyocarbonate production rates, our results support that marine fishes are critical to the global carbon cycle, contributing to oceanic alkalinity budgets and thereby influencing the ability of the oceans to neutralize atmospheric CO2.

Boosting effect of strong western pole of the Indian Ocean Dipole on the decay of El Niño events

The Indian Ocean Basin (IOB) mode is believed to favor the decay of El Niño via modulating the zonal wind anomalies in the western equatorial Pacific, while the contribution of the Indian Ocean Dipole (IOD) mode to the following year’s El Niño remains highly controversial. In this study, we use the evolution of fast and slow decaying El Niño events during 1950–2020 to demonstrate that the positive IOD with a strong western pole prompts the termination of El Niño, whereas a weak western pole has no significant effect. The strong western pole of a positive IOD leads to a strong IOB pattern peaking in the late winter (earlier than normal), enhancing local convection and causing anomalous rising motions over the tropical Indian Ocean and sinking motions over the western tropical Pacific. The surface equatorial easterly wind anomalies on the western flank of the sinking motions stimulate oceanic equatorial upwelling Kelvin waves, which shoal the thermocline in the eastern equatorial Pacific and rapidly terminate the equatorial warming during El Niño. However, a weak western pole of the IOD induces a weak IOB mode that peaks in the late spring, and the above-mentioned cross-basin physical processes do not occur.