Seychelles-Chagos Thermocline Ridge Research Articles

Particulate organic carbon export fluxes across the Seychelles-Chagos thermocline ridge in the western Indian Ocean using 234Th as a tracer

We investigated the export flux of particulate organic carbon (POC) using 234Th as a tracer in the western Indian Ocean along 60°E and 67°E transects in 2017 and 2018. The Seychelles-Chagos Thermocline Ridge (SCTR), where production is relatively high due to nutrient replenishment by upwelling of subsurface water, was observed at 3°S – 12°S in 2017 and 4°S – 13°S both 60°E and 67°E in 2018. POC fluxes in 2017 showed no differences between the SCTR and non-SCTR regions. However, in 2018, the POC fluxes in the SCTR regions (8.52 ± 7.89 mmol C m–2 d–1) were one order of magnitude higher than those observed in the non-SCTR regions (0.63 ± 0.07 mmol C m–2 d–1), which appeared to be related to the strong upwelling of subsurface water. These POC fluxes were comparable to those observed under bloom conditions, and thus, are important for estimating the efficiency of carbon sequestration in the ocean.

Acoustic characterization of fish and macroplankton communities in the Seychelles-Chagos Thermocline Ridge of the southwest Indian ocean

In this study, we describe the dynamics of the sound scattering layers (SSLs), particularly those of fish and macroplankton communities in the epipelagic layer, in the Seychelles-Chagos Thermocline Ridge (SCTR) of the southwest Indian Ocean using hydroacoustic data, net sampling, and oceanographic information. Overall, the acoustic backscattering values of the fish community were considerably higher than those of the macroplankton. Both communities were more densely distributed in the SCTR than in its surrounding region. On the vertical profile, the acoustic peak of the fish community was at 17 m during the day; however, considerably high values of up to 82 m were observed at night. Below 26 m, macroplankton was seldom found, regardless of the time. Hydrographic properties, such as temperature, salinity, dissolved oxygen (DO), and chlorophyll fluorescence, in the SCTR, were similar; however, an area at 5–8°S, 67°E was cooler, saltier, and had slightly lower DO. Weak or moderate positive correlations were observed between acoustic and hydrographic features. Based on all net samples, the most abundant taxon in terms of the total number of samples was found to be krill (Euphausiacea, 81%), followed by lantern fish (Myctophum punctatum, 12%). Understanding the dynamics of SSLs, particularly epipelagic organisms, will help to better clarify the important ecological roles of these organisms and their ability to facilitate vertical incorporation into marine food webs.

Recent changes in the upper oceanic water masses over the Indian Ocean using Argo data

Utilizing Argo data from 2003 to 2019, we examine thermohaline changes in the Indian Ocean within the upper 700 m. Widespread warming is observed except in the Southern Indian Ocean. Increasing salinity is obtained over all regions except the Bay of Bengal and Southern Indian Ocean. Thermohaline trends in regional water masses at various depths are first decomposed into spice and heave components, and then linked to processes like pure heave, pure freshening and pure warming. Three consistent patterns across all seven regions are: (1) Below 300 m spice dominates heave; (2) The freshening process within the spice component is the primary driver below 300 m; (3) Spice primarily influences salinity changes along isobars. The warming of Arabian Sea’s Subsurface Minima and the Indian Equatorial Water are primarily dictated by spice and heave, respectively. Freshening of the Bay of Bengal Water is linked to heave changes under pure freshening and pure heave processes. In the upper 250 m of the western equatorial, southern Indian Ocean, and Seychelles–Chagos Thermocline Ridge, salinity rises due to spice under pure freshening. The southern Indian Ocean’s advected mode water shows freshening and cooling trends due to pure freshening.

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.

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.

Distribution and physical–biological controls of dimethylsulfide in the western tropical Indian Ocean during winter monsoon

New field observation on distribution, turnover, and sea–air flux of three dimethylated sulfur compounds (dimethylsulfide (DMS), dimethylsulfoniopropionate, and dimethylsulfoxide) in the western tropical Indian Ocean (WTIO; 4°N–10°S, 61°–65°E) were conducted under the major Global Change and Air–Sea Interaction Program during the 2021/2022 Northeast Monsoon (December 21, 2021 to January 11, 2022). Significantly high surface concentrations of DMS were identified in the region of the Seychelles–Chagos Thermocline Ridge (SCTR; 5°–10°S). This occurred because the shallow thermocline/nitracline and associated upwelling fueled biological production of DMS in the subsurface, which was brought to the surface through vertical mixing. The calculated sea–air DMS flux was also significantly strong in the SCTR region during the Northeast Monsoon owing to combination of high wind speed and high surface concentration of DMS. This finding is similar to results obtained previously during the Southwest Monsoon, suggesting that the SCTR region is an area of active DMS emission during both the Northeast Monsoon and the Southwest Monsoon. Microbial consumption was the dominant pathway of DMS removal, accounting for 74.4% of the total, whereas the processes of photolysis (17.7%) and ventilation (7.9%) were less important. Future work should be undertaken in the WTIO to establish how DMS emission is linked to aerosol properties and climate change.

Interannual variability of sea surface chlorophyll a in the southern tropical Indian Ocean: Local versus remote forcing

Phytoplankton are crucial to marine ecosystems and the global carbon cycle. This study investigates the characteristics and causes of the interannual variation of the surface chlorophyll a (chl a) in the southern tropical Indian Ocean (STIO). We find that in addition to the Seychelles-Chagos thermocline ridge (SCTR) upwelling zone in the southwestern basin, large interannual variability also shows up in the southeastern tropical Indian Ocean (SETIO; 6°–18°S, 70°–92°E). The chl a in the SCTR shows two dominant periods of four years and two years, whereas it shows a dominant period of six years and a weaker period of three years in the SETIO. Surface wind forcing anomalies within the STIO, but not the Indonesian throughflow, cause the interannual chl a anomaly. In the SETIO region, the thermocline dynamics control the chl a variation via a combination of equal contributions from local Ekman pumping and remote Rossby waves generated by the wind stress curl from the east. In the SCTR region, the thermocline dynamics explain only about 67% of the chl a variation, with a larger contribution from the remotely generated Rossby waves than local Ekman pumping. The circulation in the western basin also contributes to the chl a variation by transporting western equatorial chl a to this region during the boreal summer-fall. Both the Indian Ocean Dipole and the El Niño–Southern Oscillation act to modulate the chl a variation in the STIO. They make comparable contributions in the SETIO, while the Indian Ocean Dipole plays a more important role in the SCTR.

Seasonal Structure and Interannual Variation of the South Equatorial Current in the Indian Ocean

AbstractThe South Equatorial Current (SEC) in the south Indian Ocean (SIO) contributes to mass and heat exchanges among the Pacific, Indian, and Atlantic Oceans. By analyzing satellite and in situ observations, this study examines the seasonal structure and the interannual variability of the SEC. The SEC is mainly part of the Seychelles‐Chagos thermocline ridge (SCTR) circulation during December to April, and is composed of the internal SCTR circulation and the intrusive Indonesian Throughflow during May to November. The SEC thus presents straight and meandering routes in these two periods, respectively. Furthermore, observations and model experiments show that the SEC's meandering route has obvious interannual variability, with exceptional northwestward extension in 1994, 1997, and 2019 during the past three decades. These exceptional routes are caused primarily by extreme positive Indian Ocean Dipole (IOD) events, which induce a strengthened westward transport of the fresh water. These exceptional routes thus reflect the abnormal freshening in the SIO. Mooring observations reveal that a significant long‐term freshening event occurred in the SIO from August 2019 to at least July 2020. The persistent low salinity confirms that there is substantial water exchange in the SIO during the extreme positive IOD years.

Suppressed Upwelling Events in the Seychelles–Chagos Thermocline Ridge of the Southwestern Tropical Indian Ocean

A prominent subsurface upwelling over the Seychelles–Chagos Thermocline Ridge (SCTR) in the southwestern tropical Indian Ocean is suppressed when downwelling Rossby waves propagate from the eastern Indian Ocean during the positive phase of the Indian Ocean Dipole (IOD) or El Niño periods. Recent studies have suggested that the upwelling can be further suppressed during the co-occurrence years of positive IOD and El Niño, associated with the strong easterly wind anomalies in the eastern Indian Ocean. This study examines the temporal variations in the SCTR upwelling strength during 1968–2017, identifies strong upwelling suppression events, and analyzes their characteristics by focusing on the role of both remote and local wind forcing, not limited to the remote influence linked to the IOD and the El Niño Southern Oscillation. A total of nine events were identified for the 50-year period, with seven of them occurring 3–6 months after the co-occurring peaks of positive IOD and El Niño in the eastern Indian Ocean. However, only IOD exhibited a positive phase before the 2011–2012 suppression event associated with the downwelling-favorable anticlockwise local wind anomalies in the SCTR. Moreover, the 1978–1979 suppression event was primarily caused by the strong anticlockwise wind anomalies in the SCTR, without any significant influence of the positive IOD or El Niño. The results suggest that the role of local winds, as well as the remote forcing, are significant in the upwelling suppression over the SCTR, which would contribute to understanding the potential predictability of the upwelling variations.

Interannual Variability of Yellowfin Tuna (Thunnus albacares) and Bigeye Tuna (Thunnus obesus) Catches in the Southwestern Tropical Indian Ocean and Its Relationship to Climate Variability

This study investigated the interannual variability of yellowfin tuna (Thunnus albacares) and bigeye tuna (Thunnus obesus) catches in the southwestern tropical Indian Ocean (SWTIO) over 25 years and its relationship to climate variability. The results indicate that the catch amount in the northern SWTIO exhibits a significant relationship with the temperature, salinity, and current variability in the upper ocean (< 400 m), associated with a significant subsurface upwelling variability, which is prominent only in the northern region. An increase of the tuna catches in the northern region is associated with the deepening of the thermocline depth and 20°C isotherm depth of the Seychelles–Chagos Thermocline Ridge, indicating suppression of the subsurface upwelling. Further analysis reveals that the catch amounts in the SWTIO tend to increase during the positive phase of the Indian Ocean Dipole. However, the catch variability in the northern SWTIO is more closely related to the El Niño–Southern Oscillation than the Indian Ocean Dipole. Favorable conditions for catches seem to develop in the northern region during El Niño years and continue throughout the following years. This relationship suggests the potential predictability of catch amounts in the northern SWTIO, an energetic region with strong subsurface upwelling variability.

Mesozooplankton community variability in the Seychelles–Chagos Thermocline Ridge in the western Indian Ocean

The Seychelles–Chagos Thermocline Ridge (SCTR), is formed by strong air-ocean interactions that cause open-ocean upwelling in the western Indian Ocean (WIO). The SCTR enhances upper-ocean nutrient levels, which promotes phytoplankton growth, leading to high zooplankton abundance and a strong nursery habitat for fish. To evaluate the evidence of significant upwelling in the SCTR, upwelling indicators associated with variability in the mesozooplankton community and other environmental factors were compared between SCTR and non-SCTR regions. We investigated mesozooplankton and physicochemical factors using multiple opening/closing net and environmental sensing system at three epipelagic (0–200 m) depth strata along 67°E (16°S–5°N) in the central and WIO. Survey stations were clustered into six environmental regions, including SCTR regions (I and II) and non-SCTR regions of the Southwest Monsoon Currents (SMC), Cold-Core Eddy (CCE), and the South Equatorial Current regions North(N) and South(S). Higher abundances of mesozooplankton were observed in the surface mixed layer (SML) (mean: 1275 inds. m−3). In the SML, Noctiluca scintillans were distributed ubiquitously, regions in the CCE were significantly different (p < 0.05). The middle layer (ML) of SCTR-II, there were significant peaks (p < 0.05) in abundance (962 inds. m−3) with significantly lower temperatures (mean: 17.7°C), and N. scintillans and Oncaea copepodites were numerically dominant. At the non-SCTR sites in the ML, there were significantly lower abundances, with a dominance of copepodites of Scolecithricella and Scolecithrix. Mesozooplankton abundance (mean: 57 inds. m−3) was significantly variable in the bottom layer (BL). The BL was dominated by either ostracods or Scolecithricella copepodites. Opportunistic copepods such as Oncaea spp. peaked in upwelled waters due to enhanced nutrients and associated high chlorophyll-a concentrations. A principal component analysis of the environmental factors and numerically dominant mesozooplankton groups showed that the dominant groups were affected by significantly elevated temperatures (25.3–29.8°C, p < 0.05) in the SML and by high salinity (34.8–35.7) in the ML. In the BL, there was no significant association between the mesozooplankton groups with any environmental factors. These results suggest the middle layer of the SCTR acts as a hotspot for opportunistic Oncaea spp., and is associated with open-ocean upwelling in the western Indian Ocean.

Sound Scattering Layers Within and Beyond the Seychelles-Chagos Thermocline Ridge in the Southwest Indian Ocean

In global oceans, ubiquitous and persistent sound scattering layers (SL) are frequently detected with echosounders. The southwest Indian Ocean has a unique feature, a region of significant upwelling known as the Seychelles-Chagos Thermocline Ridge (SCTR), which affects sea surface temperature and marine ecosystems. Despite their importance, sound SL within and beyond the SCTR are poorly understood. This study aimed to compare the characteristics of the sound SL within and beyond the SCTR in connection with environmental properties, and dominant zooplankton. To this end, the region north of the 12°S latitude in the survey area was defined as SCTR, and the region south of 12°S was defined as non-SCTR. The results indicated contrasting oceanographic properties based on the depth layers between SCTR and non-SCTR regions. Distribution dynamics of the sound SL differed between the two regions. In particular, the diel vertical migration pattern, acoustic scattering values, metrics, and positional properties of acoustic scatterers showed two distinct features. In addition, the density of zooplankton sampled was higher in SCTR than in the non-SCTR region. This is the first study to present bioacoustic and hydrographic water properties within and beyond the SCTR in the southwest Indian Ocean.

Diverse impacts of Indian Ocean Dipole on El Niño-Southern Oscillation

AbstractUnderstanding the impact of the Indian Ocean Dipole (IOD) on El Niño-Southern Oscillation (ENSO) is important for climate prediction. By analyzing observational data and performing Indian and Pacific Ocean pacemaker experiments using a state-of-the-art climate model, we find that a positive IOD (pIOD) can favor both cold and warm sea surface temperature anomalies (SSTA) in the tropical Pacific, in contrast to the previously identified pIOD-El Niño connection. The diverse impacts of the pIOD on ENSO are related to SSTA in the Seychelles-Chagos thermocline ridge (SCTR; 60°E-85°E and 7°S-15°S) as part of the warm pole of the pIOD. Specifically, a pIOD with SCTR warming can cause warm SSTA in the southeast Indian Ocean, which induces La Niña-like conditions in the tropical Pacific through interbasin interaction processes associated with a recently identified climate phenomenon dubbed the “Warm Pool Dipole”. This study identifies a new pIOD-ENSO relationship and examines the associated mechanisms.

Stratification governs the plankton community structure and trophic interaction in the Southwestern tropical Indian Ocean during boreal summer

Seychelles–Chagosthermocline ridge is one of the major upwelling area in the tropical Indian Ocean, where planktons are known to have large impacts on ocean ecosystem and biogeochemical cycles. Time series assessments of surface zooplankton abundance, biovolume, and community composition were carried out in the Indian Ocean during June 2014. In this paper, we explore the impact of phytoplankton size class biomass on trophic interaction in the thermocline ridge of the southwestern tropical Indian Ocean. Relatively, nitrate was a dominant contributor than the nitrite in the dissolved inorganic nitrogen pool. Size fractionated chlorophyll-a concentration clearly showed that the study area was a typical oligotrophic open ocean, in which picophytoplankton biomass was dominated, accounted for approximately 72% of total Chl a. We assumed that picophytoplankton biomass was most likely influenced by dissolved inorganic nutrients (chiefly nitrite), that showed a strong linear relationship with picophytoplankton biomass (r2=0.41). These results support the key role of nitrite is not only supplying and promoting the growth of smaller phytoplankton biomass, but also controls the structure of zooplankton communities. As a result surface waters were dominated by picophytoplankton biomass, in this condition result in zooplankton being dominated by Poecilostomatoida, constituting approximately 68% of the total zooplankton count. These organisms were widely distributed and occurred in large numbers and displayed a strong linear relationship with picophytoplankton biomass (r2=0.87). These empirical results suggest that abundance of poecilostomatoids presumably supports tertiary trophic levels, suggesting they might play a key role as carbon drivers in the vicinity of the thermocline ridge of the Indian Ocean.

Enhanced subsurface mixing due to near-inertial waves: observation from Seychelles-Chagos Thermocline Ridge

The Seychelles-Chagos Thermocline Ridge (SCTR) is a key region which plays a major role in various regional climatic processes at all time scales. The vertical mixing at the base of mixed layer may be climatically important at this region, where a shallow thermocline and highly variable wind activity exists. In this study, we provide evidence of strong vertical mixing at the base of mixed layer and thermocline associated with near-inertial waves (NIWs) using the microstructure profiler time series data during boreal summer 2014. During the study, it was observed that the surface turbulent layer was mostly confined in the mixed layer with turbulent kinetic energy dissipation rate of the order of 10−6 Wkg−1. Elevated dissipation rate in the order of 10−7–10−6 Wkg−1 was also observed at the base of the mixed layer and thermocline (60–100 m) during the entire period of observation. The estimated vertical diffusive heat flux across the mixed layer base (100 Wm−2) was strong and a moderate downward flux (~ 30 Wm−2) was noticed in the thermocline region. The analysis showed that the strong NIW triggered by a strong wind event were responsible for the high dissipation rate at the base of mixed layer and at deeper depths. The higher-mode NIW create elevated vertical shear at the base the mixed layer, which leads to the enhanced mixing. The study also showed the possible role of internal tides on the subsurface mixing in the region. The NIW-induced mixing may have important role on the mixed layer heat balance in a climatically active SCTR.

Dynamic partitioning of tropical Indian Ocean surface waters using ocean colour data — management and modelling applications

Over the past few decades, partitioning of the surface ocean into ecologically-meaningful spatial domains has been approached using a range of data types, with the aim of improving our understanding of open ocean processes, supporting marine management decisions and constraining coupled ocean-biogeochemical models. The simplest partitioning method, which could provide low-latency information for managers at low cost, remains a purely optical classification based on ocean colour remote sensing. The question is whether such a simple approach has value. Here, the efficacy of optical classifications in constraining physical variables that modulate the epipelagic environment is tested for the tropical Indian Ocean, with a focus on the Chagos marine protected area (MPA). Using remote sensing data, it was found that optical classes corresponded to distinctive ranges of wind speed, wind stress curl, sea surface temperature, sea surface slope, sea surface height anomaly and geostrophic currents (Kruskal–Wallis and post-hoc Tukey honestly significantly different tests, α = 0.01). Between-class differences were significant for a set of sub-domains that resolved zonal and meridional gradients across the MPA and Seychelles–Chagos Thermocline Ridge, whereas between-domain differences were only significant for the north–south gradient (PERMANOVA, α = 0.01). A preliminary test of between-class differences in surface CO2 concentrations from the Orbiting Carbon Observatory-2 demonstrated a small decrease in mean pCO2 with increasing chlorophyll (chl), from 418 to 398 ppm. Simple optical class maps therefore provide an overview of growth conditions, the spatial distribution of resources — from which habitat fragmentation metrics can be calculated, and carbon sequestration potential. Within the 17 year study period, biotic variables were found to have decreased at up to 0.025%a−1 for all optical classes, which is slower than reported elsewhere (Mann-Kendall-Sen regression, α = 0.01). Within the MPA, positive Indian Ocean Dipole conditions and negative Southern Oscillation Indices were weakly associated with decreasing chl, fluorescence line height (FLH), eddy kinetic energy, easterly wind stress and wind stress curl, and with increasing FLH/chl, sea surface temperature, SSH gradients and northerly wind stress, consistent with reduced surface mixing and increased stratification. The optical partitioning scheme described here can be applied in Google Earth Engine to support management decisions at daily or monthly scales, and potential applications are discussed.

Relative contributions of heat flux and wind stress on the spatiotemporal upper-ocean variability in the tropical Indian Ocean

High-resolution ocean general circulation model (OGCM) simulations are employed to investigate interannual variability of the upper-ocean temperature in the tropical Indian Ocean (20°S–20°N). The seasonal cycle and interannual variability in the upper-ocean temperature in the tropical Indian Ocean in the forced ocean simulation are in good agreement with available observation and reanalysis products. Two further sensitivity OGCM simulations are used to separate the relative contributions of heat flux and wind stress. The comparison of the model simulations reveals the depth-dependent influences of heat flux and wind stress on the ocean temperature variability in the tropical Indian Ocean. Generally, heat flux dominates the temperature variability in the top 30 m, while wind stress contributes most strongly to the subsurface temperature variability below 30 m. This implies that a transition depth should exist at each location, where the dominant control of the ocean temperature variability switched from heat flux to wind stress. We define the depth of this transition point as the ‘crossing depth’ and make use of this concept to better understand the depth-dependent impacts of the heat flux and wind stress on the upper-ocean temperature variability in the tropical Indian Ocean. The crossing depth tends to be shallower in the southern tropical Indian Ocean (20°S-EQ), including the Seychelles-Chagos Thermocline Ridge (SCTR) and the eastern part of the Indian Ocean Dipole (IOD), suggesting the dominance of forcing due to wind stress and the resulting ocean dynamical processes in the temperature variability in those regions. The crossing depth also shows prominent seasonal variability in the southern tropical Indian Ocean. In the SCTR, the variability of the subsurface temperature forced by the wind stress dominates largely in boreal winter and spring, resulting in the shallow crossing depth in these seasons. In contrast, the intensified subsurface temperature variability with shallow crossing depth in the eastern part of the IOD is seen during boreal autumn. Overall, our results suggest that the two regions within the tropical Indian Ocean, the SCTR and the eastern part of the IOD, are the primary locations where the ocean dynamics due to wind-stress forcing control the upper-ocean temperature variability.

The Role of Oceanic Processes in the Initiation of Boreal Winter Intraseasonal Oscillations Over the Indian Ocean

AbstractObservational analyses and ocean general circulation model experiments were performed to understand the influence of oceanic processes on warm sea surface temperature anomalies (SSTAs) prior to convection initiation of boreal winter intraseasonal oscillations (ISOs), including the Madden‐Julian Oscillation (MJO), in the tropical Indian Ocean. We found 39 strong ISOs that passed over the Indian Ocean Warm Pool (WP) region during the November–April season of the 2001–2012 period. Seventeen of them (44%) initiated over the Seychelles‐Chagos Thermocline Ridge (SCTR) before propagating eastward to the WP and Maritime Continent. Including only global‐scale MJOs, 71% (24%) of the remaining MJO events initiated from the SCTR (WP). Four (seven) SCTR (WP) ISO events were preceded by SSTAs that were strongly influenced by wind stress‐driven oceanic processes; all four (2/7) SCTR (WP) events were MJOs. Composite analyses of the four oceanic process influenced SCTR MJO events showed that wind stress‐driven oceanic processes contributed about 30% (between 0.1 and 0.2 °C) of the preconvection warming in the SCTR, which coincides with the Intertropical Convergence Zone with mean SST &gt; 28 °C. Reduced upwelling and entrainment played a more important role than horizontal advection in elevating the +SSTAs. Case studies revealed that the two ocean process‐influenced primary MJO SCTR events were also associated with oceanic equatorial Rossby waves. Furthermore, results of a linear atmospheric mixed layer model indicated that boundary layer processes related to SSTAs contributed approximately half of the total convergence during MJO initiation, relative to the effect of the free troposphere, for the Rossby wave‐influenced events.

Effects of Climate Modes on Interannual Variability of Upwelling in the Tropical Indian Ocean

AbstractThis paper investigates interannual variability of the tropical Indian Ocean (IO) upwelling through analyzing satellite and in situ observations from 1993 to 2016 using the conventional Static Linear Regression Model (SLM) and Bayesian Dynamical Linear Model (DLM), and performing experiments using a linear ocean model. The analysis also extends back to 1979, using ocean–atmosphere reanalysis datasets. Strong interannual variability is observed over the mean upwelling zone of the Seychelles–Chagos thermocline ridge (SCTR) and in the seasonal upwelling area of the eastern tropical IO (EIO), with enhanced EIO upwelling accompanying weakened SCTR upwelling. Surface winds associated with El Niño–Southern Oscillation (ENSO) and the IO dipole (IOD) are the major drivers of upwelling variability. ENSO is more important than the IOD over the SCTR region, but they play comparable roles in the EIO. Upwelling anomalies generally intensify when positive IODs co-occur with El Niño events. For the 1979–2016 period, eastern Pacific (EP) El Niños overall have stronger impacts than central Pacific (CP) and the 2015/16 hybrid El Niño events, because EP El Niños are associated with stronger convection and surface wind anomalies over the IO; however, this relationship might change for a different interdecadal period. Rossby wave propagation has a strong impact on upwelling in the western basin, which causes errors in the SLM and DLM because neither can properly capture wave propagation. Remote forcing by equatorial winds is crucial for the EIO upwelling. While the first two baroclinic modes capture over 80%–90% of the upwelling variability, intermediate modes (3–8) are needed to fully represent IO upwelling.

Variability of Sea Level and Upper-Ocean Heat Content in the Indian Ocean: Effects of Subtropical Indian Ocean Dipole and ENSO

Abstract In this study, the Indian Ocean upper-ocean variability associated with the subtropical Indian Ocean dipole (SIOD) is investigated. We find that the SIOD is associated with a prominent southwest–northeast sea level anomaly (SLA) dipole over the western-central south Indian Ocean, with the north pole located in the Seychelles–Chagos thermocline ridge (SCTR) and the south pole at southeast of Madagascar, which is different from the distribution of the sea surface temperature anomaly (SSTA). While the thermocline depth and upper-ocean heat content anomalies mirror SLAs, the air–sea CO2 flux anomalies associated with SIOD are controlled by SSTA. In the SCTR region, the westward propagation of oceanic Rossby waves generated by anomalous winds over the eastern tropical Indian Ocean is the major cause for the SLAs, with cyclonic wind causing negative SLAs during positive SIOD (pSIOD). Local wind forcing is the primary driver for the SLAs southeast of Madagascar, with anticyclonic winds causing positive SLAs. Since the SIOD is correlated with ENSO, the relative roles of the SIOD and ENSO are examined. We find that while ENSO can induce significant SLAs in the SCTR region through an atmospheric bridge, it has negligible impact on the SLA to the southeast of Madagascar. By contrast, the SIOD with ENSO influence removed is associated with an opposite SLA in the SCTR and southeast of Madagascar, corresponding to the SLA dipole identified above. A new subtropical dipole mode index (SDMI) is proposed, which is uncorrelated with ENSO and thus better represents the pure SIOD effect.