Western Equatorial Indian Ocean Research Articles

Aridity record of the Arabian Peninsula for the last 200 kyr: Environmental magnetic evidence from the western equatorial Indian ocean

The evolution of aridity and its forcing mechanisms in the Arabian Peninsula are of significance for better understanding the environmental response of the region to global change and the impact of past climate shifts in human evolution. Paleoclimatic reconstructions for the region are predominantly derived from fragmentary terrestrial records. Here, we present a detailed environmental magnetic record for marine sediments from the western equatorial Indian Ocean, which constitute the terminal sink for eolian material originated from the Arabian Peninsula. The dominant magnetic minerals identified in the studied sediments include magnetite, maghemite, and hematite. The hard isothermal remanent magnetization (HIRM), widely used as a proxy for the abundances of the high coercivity mineral hematite, has been used to track variations in the concentration of Arabian dust over the last 200 kyr. Dust concentrations are lowest during interglacial periods of maximum boreal summer isolation, and coincide with intervals during which lake and fluvial systems developed and speleothems formed across most of southern and central Arabia. Our results point to a sharp decrease in the production of dust due to the expansion of vegetation cover up to 30°N during these periods. West African Monsoon appears to be the major source of increased rainfall across Arabia, while the Indian Summer monsoon likely contributed to enhance rainfall in the south and southeastern parts of the peninsula. Variation in the supply of Arabian dust were also driven by changes in high-latitude ice volume and atmospheric CO2, as indicated by highest dust concentration found during glacial periods. We interpret that the expansion of high-latitude ice sheets increased the global meridional temperature gradient, weakened the monsoon system, and resulted in a more southerly, colder and descending subtropical westerly jet, leading to the retreat of vegetation cover and the enhanced production of dust from the Arabian deserts. Our results highlight a link between the production of Arabian dust and orbital-scale climate variability, including both monsoon and northern hemisphere ice sheets dynamics.

Seasonal dynamics of global marine heatwaves over the last four decades

Marine heatwaves (MHWs), prolonged periods of abnormally high sea temperature, have greater devastating impacts on marine ecosystem services and socioeconomic systems than gradual long-term ocean warming. Despite growing evidence of increases in MHW frequency, duration, and intensity, their interseasonal variations remain unclear. Using satellite-derived daily sea surface temperature (SST) data from 1982 to 2022, this work reveals a strong seasonality in MHWs. Typically, the highest cumulative intensity, characterizing total impacts on ecosystems, occurs during the local warm seasons in most oceans, leading to a significant interseasonal difference between warm and cold seasons. The interseasonal difference is predominantly driven by air-sea heat flux, rather than oceanic horizontal advection and vertical process. An increase in these interseasonal differences is observed in mid and high latitudes, with a significant increase in the warm season and a weaker trend in the cold season. In the Equatorial Pacific and Western Equatorial Indian Ocean, intense MHWs are primarily exacerbated by the El Niño-Southern Oscillation (ENSO), which also determines interseasonal variations in MHWs. Understanding the seasonality of MHWs can help better formulate corresponding policies to reduce economic and ecological losses caused by these events and can improve the accuracy of future predictions.

Modelling water isotopologues (1H2H16O, 1H217O) in the coupled numerical climate model iLOVECLIM (version 1.1.5)

Abstract. Stable water isotopes are used to infer changes in the hydrological cycle for different climate periods and various climatic archives. Following previous developments of δ18O in the coupled climate model of intermediate complexity, iLOVECLIM, we present here the implementation of the 1H2H16O and 1H217O water isotopes in the different components of this model and calculate the associated secondary markers deuterium excess (d-excess) and oxygen-17 excess (17O-excess) in the atmosphere and ocean. So far, the latter has only been modelled by the atmospheric model LMDZ4. Results of a 5000-year equilibrium simulation under preindustrial conditions are analysed and compared to observations and several isotope-enabled models for the atmosphere and ocean components. In the atmospheric component, the model correctly reproduces the first-order global distribution of the δ2H and d-excess as observed in the data (R=0.56 for δ2H and 0.36 for d-excess), even if local differences are observed. The model–data correlation is within the range of other water-isotope-enabled general circulation models. The main isotopic effects and the latitudinal gradient are properly modelled, similarly to previous water-isotope-enabled general circulation model simulations, despite a simplified atmospheric component in iLOVECLIM. One exception is observed in Antarctica where the model does not correctly estimate the water isotope composition, a consequence of the non-conservative behaviour of the advection scheme at a very low moisture content. The modelled 17O-excess presents a too-important dispersion of the values in comparison to the observations and is not correctly reproduced in the model, mainly because of the complex processes involved in the 17O-excess isotopic value. For the ocean, the model simulates an adequate isotopic ratio in comparison to the observations, except for local areas such as the surface of the Arabian Sea, a part of the Arctic and the western equatorial Indian Ocean. Data–model evaluation also presents a good match for the δ2H over the entire water column in the Atlantic Ocean, reflecting the influence of the different water masses.

An updated checklist of Myctophids of Western Indian Ocean with 6 new records

The Western Indian Ocean (WIO) encompass distinct ecosystems with differing oceanographic and biotic characteristics. This paper provides a comprehensive documentation on the myctophid diversity of WIO at depths between 80 and 600 m, based on field data from 74 stations. A total of 59 species of myctophids were recorded from WIO during the study which includes 6 new records. The new records include Bolinichthys nikolayi, Diaphus theta, Lampanyctus crocodilus, Lampanyctus macdonaldi and Lampadena urophaos from Western Equatorial Indian Ocean, and Diaphus similis from South Eastern Arabian Sea. Based on sampling data and the information from secondary data, the number of myctophid species in the WIO is revalidated to 159 species. The myctophid species from the Western Indian Ocean north of 15°S clearly indicates presence of distinct myctophid assemblages in the study area, representing the eco-regions North Eastern Arabian Sea (NEAS), South eastern Arabian Sea (SEAS) and Western equatorial Indian Ocean (WEIO). Present study reports 16 species from NEAS, 45 species from SEAS and 55 species from WEIO. WEIO accounts for 70–90% of the observed species diversity of WIO. Average dissimilarity in species assemblages between NEAS and SEAS was 64%, between SEAS and EIO 75%, and between NEAS and WEIO was 84%. This is the first detailed report on the myctophid species diversity along the 23.5°N latitude (lat), 55°E longitude (long) and 23.5°S lat, 74°E long. Updated checklist of myctophids in the Western Indian Ocean is also included in the paper.

Contribution of aged organic carbon to suspended particulate organic carbon in the western equatorial Indian Ocean

We investigated dual carbon isotopes within the vertical water column at sites 67-1 and 67-2 of the western equatorial Indian Ocean to determine the source and age of particulate organic carbon (POC) and thus evaluated the contributions of modern and fossil (aged) POC. The concentration of POC ranged from 7 to 47.3 μgC L−1, δ13CPOC values ranged from –31.8 to –24.4‰, and Δ14CPOC values ranged from –548 to –111‰. Higher values of δ13CPOC and Δ14CPOC near the surface indicated an influence of autochthonous POC, whereas decreasing trends toward the bottom suggested a contribution of aged OC sources to the total POC pool. The contribution of fossil POC was lower near the surface, accounting for only 12% and 6% of the total POC at sites 67-1 and 67-2, respectively; however, in the deeper layers below 1,000 m, the contribution of fossil POC increased to 52% and 44% of the total POC at the two sites. Mechanisms for the increased contributions of fossil OC within deeper POC include the inflow of aged OC from sediments resuspended near slopes, the adsorption of old dissolved organic carbon in deep water masses, and the impact of aged OC that may originate from hydrothermal sources. This study highlights the importance of aged OC in the carbon cycle of the equatorial Indian Ocean.

Changes in surface hydrography of the western equatorial Indian Ocean during the Pleistocene: Implications for East African climate variability

Surface and subsurface dynamics of the western equatorial Indian Ocean (WEIO) significantly impacted the East African climate, and played an important role in hominin evolution and dispersal. Planktic foraminiferal abundance and stable isotope data from Site 241, helped to ascertain cool subsurface and thermocline shoaling in the WEIO along with a strong zonal subsurface δ18O gradient in the equatorial Indian Ocean between 2.3 and 0.4 Ma. This east-west contrast in the Indian Ocean triggered an intensification of the equatorial westerlies, which enhanced the subsidence of air convection over the WEIO and hampered moisture transport to East Africa, thus expansion of C4 grassland during the Pleistocene. The intensification of the East African monsoon was governed by positive wind-evaporation feedback during ~1.9–1.7 Ma, 1.2–0.9 Ma, and 0.14–0.09 Ma, where increasing wind intensity of Findlater Jet enhanced the evaporation over WEIO. The wind-evaporation feedback drove short-lived wet-dry phases that influenced hominin speciation, adaptation, and dispersal.

Assessment of thermocline depth bias in the Seychelles-Chagos Thermocline Ridge of the Southwestern Indian Ocean simulated by the CMIP6 models

The Seychelles-Chagos Thermocline Ridge (SCTR, 5°S-10°S, 50°E-80°E) is a unique open-ocean upwelling region in the southwestern Indian Ocean. Due to the negative wind stress curl between the equatorial westerlies and southeasterly trade winds, SCTR is known as a strong upwelling region with high biological productivity, providing a primary fishing zone for the surrounding countries. Given its importance in shaping the variability of the Indian Ocean climate by understanding the sea-air interaction and its dynamics, the simulation of SCTR is evaluated using outputs from the Coupled Model Intercomparison Project Phase Sixth (CMIP6). Compared to observations, 23 out of 27 CMIP6 models tend to simulate considerably deeper SCTR thermocline depth (defined as the 20°C isotherm depth (D20))– a common bias in climate models. The deep bias is related to the easterly wind bias in the equatorial to southern Indian Ocean, which is prominent in boreal summer and fall. This easterly wind bias produces a weak annual mean Ekman pumping, especially in the boreal fall. Throughout the year, the observed Ekman pumping is positive and is driven by two components: the curl term, is associated with the wind stress curl, leads to upwelling during boreal summer to fall; the beta term, is linked to planetary beta and zonal wind stress, contributes to downwelling during boreal spring to fall. However, the easterly wind bias in the CMIP6 increases both the positive curl and negative beta terms. The beta term bias offsets the curl term bias and reduces the upwelling velocity. Furthermore, the easterly wind bias is likely caused by the reduced east-west sea surface temperature (SST) difference associated with a pronounced warm bias in the western equatorial Indian Ocean, accompanied by the east-west mean sea level pressure gradient over the Indian Ocean. Furthermore, this study finds local wind-induced Ekman pumping to be a more dominant factor in thermocline depth bias than Rossby waves, despite CMIP6 models replicating Rossby wave propagation. This study highlights the importance of the beta term in the Ekman pumping simulation. Thus, reducing the boreal summer-to-fall easterly wind bias over the Indian Ocean region may improve the thermocline depth simulation over the SCTR region.

A stacked record of relative palaeointensity for past 500 ka from western equatorial Indian Ocean sediments

SUMMARY Relative palaeointensity (RPI) records can reflect the evolution of the Earth's axial dipole field and provide a suitable template for global stratigraphic correlation. Current RPI records are primarily obtained from the Atlantic and Pacific Oceans, particularly in the mid-latitudes of the Northern Hemisphere. Fewer RPI records originate from the Indian Ocean, particularly in low latitude areas, which limits sediment dating and geomagnetic evolution studies in these regions. In this study, we conduct a palaeomagnetic study on four sediment cores recovered from the western equatorial Indian Ocean (WEIO) to establish a new regional RPI stack from the past 500 ka for the global coverage of palaeointensity data, as well as a regional reference for palaeointensity-assisted stratigraphy. To estimate the RPI, the isothermal remanent magnetization is used as a normalizer. A chronological framework is constructed by correlating RPI records and environmental magnetic parameters (anhysteretic remanent magnetizationsaturation/isothermal remanent magnetization, ARM/SIRM) with PISO-1500 and LR04 δ18O curves, respectively. Our RPI stacked curve (WEIO-500) varies according to global and other regional high-resolution records. The five lows in the obtained RPI curve can be correlated with previously reported excursions. In addition, the mean inclination direction of each core is similar to the expected hypothetical geocentric axial dipole direction, with a negative inclination anomaly (mean ΔI) of −2.18° to −4.86°. The low mean ΔI and its reproducibility reflect the reliability of our chronological framework and stacked curve, thereby providing a new reference for correlating and calibrating RPI records from WEIO as well as other equatorial areas.

Clay mineralogy and geochemistry of surface sediments in the equatorial western Indian Ocean and implications for sediment sources and the Antarctic bottom water inputs

Sediments in the equatorial western Indian Ocean are mixed products from biological, terrigenous, autogenic, volcanic, and hydrothermal materials influenced by various circulation and water masses. The sediments record information of sources and sedimentary environments. To identify the different sources of non-carbonate components in the sediments of the equatorial western Indian Ocean and explore the factors influencing their sedimentary characteristics, surface sediments of the equatorial western Indian Ocean were analysed for clay minerals as well as major and rare earth elements following carbonate removal. Palygorskite, as an indicative mineral of aeolian dust from the North Africa-Arabian Peninsula, is ubiquitous in the sediment of the equatorial western Indian Ocean. These non-carbonate materials are mainly derived from the North Africa-Arabian Peninsula as well as the Indus River and Deccan Traps. The study area can be divided into two sedimentary zones according to the water depth, major element content, and related indexes of rare earth elements (δCe, δEu, LREE/HREE). The sediments of Zone II, being influenced by the Antarctic bottom water, are mainly calcareous clay and characterised by a significant positive Ce anomaly and relative MnO enrichment, with relatively high palygorskite, Al2O3 content, and LREE/HREE ratio (mean values of 24%, 13.95% and 14.00). The sediments of Zone I are mainly clayey calcareous ooze, with a relatively weak positive Ce anomaly and a low palygorskite content (20%). Low Al/(Al+Fe+Mn) values in the sediments of Zone I demonstrate slight signs of hydrothermal input. This study revealed that continental weathering debris is an important source of non-carbonate sediments in the equatorial western Indian Ocean and established that the Antarctic bottom water is an integral factor affecting the sedimentary environment in this region. We present a complete regional sedimentological study of the equatorial western Indian Ocean and provide theoretical support for the reconstruction of the paleo-oceanic environment.

Distribution pattern of planktonic and benthic foraminifera in surface sediments near the equatorial western Indian Ocean and its indications of paleo-environment and productivity

Distribution pattern of planktonic and benthic foraminifera in surface sediments near the equatorial western Indian Ocean and its indications of paleo-environment and productivity

The Atmospheric Input of Dissolvable Pb Based on the Radioactive 210Pb Budget in the Equatorial Western Indian Ocean

To estimate the atmospheric deposition flux of 210Pb in the equatorial western Indian Ocean, we determined the dissolved (<0.45 μm) and particulate 210Pb (>0.45 μm) in the water column. In addition, we calculated the atmosphere-derived dissolvable Pb in seawater using the budget of 210Pb. The dissolved 210Pb and total 210Pb were higher in the surface layer and, overall, showed a decreasing distribution with depth. In particular, radioactive 210Pb activities in the surface-to-upper layer (<1000 m depth) were 1.5 to 2 times higher than those reported in the 1970s (in nearby regions), suggesting that there has been additional 210Pb input in recent years. Based on the mass balance of the total 210Pb budget in the water column, we estimated the atmospheric deposition flux of 210Pb and the residence time of Pb for the first time in this region. The atmospheric deposition flux of 210Pb was estimated to be 0.1–0.5 dpm cm−2 yr−1, and these values agreed with the general global estimations for the major oceans (0.1–0.7 dpm cm−2 yr−1). Considering the residence time of 210Pb (29–41 years) in the water column (estimated from the 210Pb inventory and 234Th-based Pb scavenging rate), the atmospheric input of seawater-dissolvable Pb was quantified to be 0.08–0.1 nmol cm−2 yr−1, which is about eight times higher than the estimated input in the early 1990s in the region. Therefore, these results imply that radioactive 210Pb could be a useful tracer for quantifying Pb flux in seawater.

Trend of annual and semiannual harmonics of sea surface temperature in the tropical Indian Ocean, 1979–2018

In the Indian Ocean (IO), the annual and semiannual oscillations of the sea surface temperature (SST) contribute more than 80% of the total variance. It is known that the SST in the tropical IO has warmed at a rate of 0.15 °C/decade since 1979. However, such an estimate of the decadal trend of the annual and semiannual harmonics of SST remains unknown, despite being a strong component of the SST signal. Here we use a widely accepted data product (TropFlux) to quantify the annual and semiannual harmonics in the tropical IO for the first time. The northern IO has a distinctly strong semiannual cycle (1–1.8 °C), which is showing an amplifying trend (∼ 0.04 °C/decade) since 1979. Furthermore, a damping trend in annual amplitude exists over much of the IO, except along the northwestern Bay of Bengal, the Seychelles-Chagos thermocline ridge, the Persian Gulf, and the Indonesian throughflow region. The estimated damping of the annual amplitude is highest over the Arabian Sea. In contrast, the southern tropical IO has a predominant annual cycle, the amplitude of which has weakened during 1979–2018. In the north IO, net air-sea heat flux and vertical processes show a dominant semiannual harmonic oscillation similar to SST. Additionally, the annual amplitude is influential over southern IO. Horizontal advection contributes significantly along the boundary-current regions and the western equatorial IO. Our analysis of combined annual and semiannual cycles shows significant warming in the tropical IO during October–November and cooling during July–August owing to the amplification of semiannual oscillation. These changes in the SST can influence air-sea interaction processes such as cyclogenesis and monsoon.

Hydroclimate Variability in the Equatorial Western Indian Ocean for the Last 250,000 Years

AbstractIndian Ocean sea surface temperatures impact precipitation across the basin through coupled ocean‐atmosphere responses to changes in climate. To understand the hydroclimate response over the western Indian Ocean and equatorial east Africa to different forcing mechanisms, we present four new proxy reconstructions from core VM19‐193 (2.98°N, 51.47°E) that span the last 250 ky. Sub‐surface water temperatures (Sub‐T; TEX86) show strong precessional (23 ky) variability that is primarily influenced by maximum incoming solar radiation (insolation) during the Northern Hemisphere spring season, likely indicating that local insolation dominates the upper water column at this tropical location over time. Leaf waxes, on the other hand, reflect two different precipitation signals: δ13Cwax (in phase with boreal fall insolation) is likely reflecting vegetation changes in response to local rainfall over east Africa, whereas δDprecip (primarily driven by boreal summer insolation) represents changes in regional circulation associated with the summer monsoon. Glacial‐interglacial changes in ocean temperatures support glacial shelf exposure over the Maritime Continent in the eastern Indian Ocean and the subsequent weakening of the Indian Walker Circulation as a mechanism driving 100 ky climate variability across the tropical Indo‐Pacific. Additionally, the 100 ky spectral power in δDprecip supports a basin‐wide weakening of summer monsoon circulation in response to glacial climates. Overall, the proxy records from VM19‐193 indicate that both precession and glacial‐interglacial cycles exert control over hydroclimate at this tropical location.

Acoustic detections and sightings of blue whales Balaenoptera musculus in the Seychelles, western tropical Indian Ocean (2020-2022)

Historically, the Seychelles archipelago was an opportunistic whaling ground for fleets en route to and from the Antarctic. Soviet whalers illegally killed 500 blue whales near the Seychelles in the 1960s. Since then, no dedicated research has occurred to understand the ecological importance of this region for blue whales. Based on opportunistic sightings, we undertook 2 expeditions to assess the occurrence of blue whales. The overall goals were to determine blue whale distribution, obtain photo-identification data and collect the first acoustic data on this species in this region using a hydrophone deployed for a year. The expeditions consisted of vessel-based visual surveys that focused on the slope habitat (500-2000 m) off the northern portion of the Mahé Plateau. Over the 2 expeditions, a total of 5 sightings of up to 10 animals were seen. The results of our acoustic monitoring off Seychelles demonstrate that blue whales occur there regularly, primarily from December to April, and that the acoustic population identity matches that from near Sri Lanka. Published records and the results of our work suggest that blue whales from the northwestern Indian Ocean are seasonally present in the western equatorial Indian Ocean.

A new species of Bathysphyraenops from the Indian Ocean with comments on the genus (Teleostei: Acropomatiformes: Howellidae).

A new species, Bathysphyraenops radhae, is described from the equatorial western Indian Ocean. It resembles Bathysphyraenops declivifrons in the shape of snout and number of pseudobranchial filaments in adults but can be readily distinguished from all known members of Howellidae by its characteristically armed preopercle and in strong reduction or absence of spinules on scales around the pectoral fin. The genus Bathysphyraenops is rediagnosed, and the characteristics useful in diagnosing the howellid genera are briefly discussed. The nominal genus Schistoperca is synonymized with Bathysphyraenops. A key to the species of Bathysphyraenops is provided.

Hominin evolution and dispersal: Insight from surface and subsurface dynamics of the western equatorial Indian Ocean

Hominin evolution and dispersal: Insight from surface and subsurface dynamics of the western equatorial Indian Ocean

Characteristics and Drivers of Marine Heatwaves in the Western Equatorial Indian Ocean

AbstractThe spatio‐temporal characteristics of the interannual variability and long‐term trend of the marine heatwaves (MHWs) and related dynamic mechanisms in the western equatorial Indian Ocean (WEIO) are investigated using satellite observations. A prominent MHW hot spot is found in a region of the WEIO (48°E−54°E, 2°S–2°N), with a mean MHWs' intensity, duration, and frequency of 1.54°C, 13.33 days, and 1.97 times, respectively. MHWs in the hot spot region have significant interannual variability after removing the long‐term trend, associated with Indo‐Pacific major climate modes. In 1982/1983, 1983/1984, 1987/1988, 1997/1998, 2006/2007, 2009/2010, 2011/2012, 2012/2013, 2014/2015, 2015/2016, and 2019/2020, the MHWs occurred with longer duration, higher frequency, and more total days. These years correspond to a positive Indian Ocean Dipole, or an El Niño event, or both. The occurrence of MHWs accompanied by anomalous positive sea surface height suggests that oceanic planetary wave processes modulate MHWs in the WEIO. Westward‐propagating downwelling equatorial Rossby waves triggered by anomalous equatorial easterly winds drive the convergence of warm upper‐ocean water and weaken the upwelling of cool subsurface water, which favor anomalously warm sea surface temperature (SST) and the occurrence of MHWs. In addition, the westward‐propagating off‐equatorial downwelling Rossby waves in the southern tropical Indian Ocean also affect MHWs in the WEIO through the propagation and reflection of waves. The annual MHW frequency, duration, and total days in the hot spot region increase up to 1.56 times, 4.95 days, and 31.72 days per decade, respectively, related to the significant increase in mean SST under global warming.

Java–Sumatra Niño/Niña and Its Impact on Regional Rainfall Variability

Abstract A phenomenon referred to here as Java–Sumatra Niño/Niña (JSN or JS Niño/Niña) is characterized by the appearance of warm/cold sea surface temperature anomalies (SSTAs) in the coastal upwelling region off Java–Sumatra in the southeastern equatorial Indian Ocean. JSN develops in July–September and sometimes as a precursor to the Indian Ocean dipole, but often without corresponding SSTAs in the western equatorial Indian Ocean. Although its spatiotemporal evolution varies considerably between individual events, JSN is essentially an intrinsic mode of variability driven by local atmosphere–ocean positive feedback, and thus does not rely on remote forcing from the Pacific for its emergence. JSN is an important driver of climate variability over the tropical Indian Ocean and the surrounding continents. Notably, JS Niña events developing in July–September project onto the South and Southeast Asian summer monsoons, increasing the probability of heavy rainfall and flooding across the most heavily populated regions of the world.

The Characteristics and Variability of Intraseasonal Coastal Kelvin Waves in the Bay of Bengal under Hindcast Conditions and the RCP8.5 Scenario

Abstract The characteristics and variability of intraseasonal internal coastal Kelvin waves (CKWs) along the Bay of Bengal (BoB) waveguide are investigated in the context of global warming by employing a regional ocean model. The analyzed period covers 120 years from 1980 to 2099, which includes the historical scenario and the RCP8.5 scenario. CKW information is successfully extracted from the temperature anomalies along the pycnocline by applying a newly developed methodology. The analysis reveals that intraseasonal CKWs in the BoB are highly in accordance with the intraseasonal zonal wind stress in the western equatorial Indian Ocean; the downwelling CKW lags the equatorial intraseasonal westerly winds, and the upwelling CKW lags the equatorial intraseasonal easterly winds. The CKWs significantly affect subsurface characteristics at the eastern BoB boundary, and the weakening of CKWs near the Irrawaddy Delta tip is a general feature occurring in the subsurface. With respect to the long-term scale, the occurrence of significant CKWs is predicted to be more frequent in the future under the high emissions pathway. Remarkably, the monthly climatology of CKWs varies over time; unlike the first two 30-yr analyzed periods, significant CKWs are predicted to mainly occur around August during the last two 30-yr periods due to the corresponding variabilities in the equatorial wind field, suggesting that the BoB characteristics may greatly deviate from the current climatological state.

Maintenance of the Basin-Dependent Quasi-Biweekly Mode in the Indian Ocean during Summer

AbstractThe atmospheric 10–20-day quasi-biweekly mode (QBWM) significantly modulates the active–break spells of the South Asian monsoon. Current knowledge, however, is limited concerning the diversity of the QBWM in the Indian Ocean (IO). Based on extended empirical orthogonal function analysis, two dominant summer modes are constructed in the IO. The first mode (QBWM1) generally depicts IO basin-dependent variability, while the second mode (QBWM2) exhibits a close relationship with the northwestern Pacific. QBWM1 initiates in the equatorial western IO and propagates toward the eastern IO along the equator. Two Rossby wave cells evolve in the off-equatorial eastern IO when convection encounters the Maritime Continent, and subsequently the northern cell develops and moves westward in the South Asian monsoon region. In contrast, QBWM2 originates in the northwestern Pacific and passes westward across the South Asian monsoon region in the form of convectively coupled Rossby waves. The maintenance mechanism of the peculiar IO basin-dependent QBWM1 is understood in terms of moisture dynamics. Significant moisture anomalies are found to precondition convection initiation in the western IO and subsequent eastward movement along the equator. Afterward, two off-equatorial moisture centers are generated in the double Rossby wave cells along with convection dissipation in the eastern IO, and the moisture anomalies are delivered from the southern cell toward the convection initiation area in the western IO via a moisture conveyor belt without coupling with convection. Moisture budget analysis indicates that the horizontal moisture advection associated with QBWM1 is regulated by the mean clockwise circulation in the tropical IO.