Equatorial Currents Research Articles

Global patterns and inferences of tuna movements and trophodynamics from stable isotope analysis

A global dataset of carbon stable isotope (δ13C) values from yellowfin, bigeye, and albacore tuna muscle tissue (n = 4275) was used to develop a novel tool to infer broad-scale movement and residency patterns of these highly mobile marine predators. This tool was coupled with environmental models and lipid content (C:N ratio) of tuna muscle tissues to examine ocean warming impacts on tuna ecology and bioenergetic condition across Longhurst provinces. Over a 16-year study period (2000–2015), latitudinal gradients in tuna δ13C values were consistent, with values decreasing with increasing latitude. Tuna δ13C values, reflecting modelled global phytoplankton δ13C landscapes (“isoscapes”), were largely related to spatial changes in oxygen concentrations at depth and temporal changes in sea surface temperature. Observed tuna isoscapes (δ13CLScorr), corrected for lipid content and the Suess effect (oceanic changes in CO2 over time), were subtracted from model-predicted baseline isoscapes (Δ13Ctuna-phyto) to infer spatial movement and residency patterns of the different tuna species. Stable isotope niche width was calculated for each Longhurst province using Δ13Ctuna-phyto and baseline-corrected nitrogen isotope (δ15Ntuna-phyto) values to further quantify isotopic variability as evidence of movements across isoscapes. A high degree of movement—defined as the deviation from the expected range of Δ13Ctuna-phyto values— was evident in three Longhurst provinces: Guinea current coast, Pacific equatorial divergence, and the North Pacific equatorial counter current. The highest level of population dispersal (variability in Δ13Ctuna-phyto values) was observed in Longhurst provinces within the western and central Pacific Oceans and in the Guinea current coast. While lipid content was low in yellowfin and bigeye, high and variable lipid stores in albacore muscle were consistent with seasonal movements between productive foraging and oligotrophic spawning habitats. Our ability to characterize tuna movement patterns without ambiguity remains challenged by uncertainty in trophic discrimination factors and ecological (e.g. diet variability) processes. However, this study illustrates that model-corrected δ13C values are a valuable, relatively cost-effective tool for identifying potential areas of mixing across management zones, particularly when electronic tagging studies are limited or absent. Stable isotope analyses of tuna tissues can therefore be an additional tool for guiding spatial stock assessments on top predator movement, dispersal patterns, and how they may be altered under a changing climate.

Revisit of seasonal variability of subsurface temperature in the tropical Pacific with Argo data

Abstract Seasonal variability of temperature in the tropical Pacific is studied with Argo data from 2001 to 2017. It is found that the strongest seasonal variation is in the subsurface layer corresponding to the thermocline, which is located in three zonal bands centering at 12°N, 5°N and 5°S across the entire tropical Pacific. Further analysis indicates that seasonal variation of subsurface temperature is caused by the vertical movement of the thermocline, and the signal is generated in the eastern Pacific and propagates westward to the western Pacific. A linear wind-driven first-mode baroclinic Rossby wave model is then employed to investigate the seasonal fluctuation of thermocline. The seasonal shoaling and deepening of thermocline and associated temperature variations are well captured by this Rossby wave model. Basin-scale seasonal anomalies of wind stress curl in the tropical Pacific produce significant thermocline fluctuations, which propagate westward in the form of annual Rossby wave, and cause the seasonal variation of subsurface temperature. Thermocline fluctuations are revealed to be tightly associated with the meridional shift of equatorial currents such as North Equatorial Current, North Equatorial Countercurrent and South Equatorial Current.

Distribution of anguillid leptocephali and possible spawning areas in the South Pacific Ocean

Seven South Pacific anguillid eel species live from New Guinea to French Polynesia, but their spawning areas and life histories are mostly unknown despite previous sampling surveys. A July–October 2016 research cruise was conducted to study the spawning areas and times, and larval distributions of South Pacific anguillid eels, which included a short 155°E station-line northeast of New Guinea and five long transects (5–25°S, 160°E–140°W) crossing the South Equatorial (SEC) and other currents. This survey collected nearly 4000 anguilliform leptocephali at 179 stations using an Isaacs-Kidd Midwater Trawl accompanied by 104 CTD casts. Based on morphometric observations and DNA sequencing, 74 anguillid leptocephali were collected, which in the southern areas included 29 larvae of six species: Anguilla bicolor pacifica, A. marmorata, A. australis, A. reinhardtii, A. megastoma,and A. obscura (all anguillid species of the region were caught except A. dieffenbachii). Small A. australis (9.0–16.8 mm) and A. reinhardtii (12.4, 12.5 mm) leptocephali were collected south of the Solomon Islands, other A. australis (10.8–12.0 mm) larvae were caught northwest of Fiji along with an A. obscura (20.0 mm) larva, and an A. marmorata (7.8 mm) larva was collected near Samoa. Considering collection sites, larval ages from otolith analysis, and westward SEC drift, multiple spawning locations occurred from south of the Solomon Islands and the Fiji area (16–20 days old larvae) to near Samoa (19 days old larva) during June and July in areas where high-salinity Subtropical Underwater (STUW, ~150 m depth) and the warm, low-salinity surface Fresh Pool were present. Five long hydrographic sections showed the strong Fresh Pool in the west and the STUW formation area in the east.

Dissolved inorganic Radiocarbon content of the Western Coral sea: Implications for Intermediate and Deep Water Circulation

ABSTRACTThe South Pacific Ocean contributes to the global carbon cycle by exchanging CO2 between the atmosphere and intermediate to deep water masses. The path of the Antarctic Intermediate Water (AAIW) in the South Pacific gyre has been inferred from salinity, oxygen, and nutrient measurements, but radiocarbon (14C) measurements—a direct tracer of the carbon cycle—remain sparse. Here, we present the first radiocarbon profiles in the western Coral Sea and compare our measurements with South Pacific stations from GLODAPv2, a database of ocean hydrochemistry. Surface and subsurface waters in the Coral Sea cannot be attributed to a single source based on their Δ14C signatures, and we observe a penetration of bomb-produced 14C. AAIW in the western Coral Sea shows Δ14C values comparable to those in the South Pacific gyre, consistent with circulation of AAIW in the lower part of the southern equatorial current. The deep waters of the western Coral Sea have significantly higher 14C than the South Pacific at the same isopycnal, consistent with a northward intrusion of Circumpolar Deep Water from the Tasman Sea, along with a westward influx of deep waters from the Central Pacific. In accordance with silicate concentrations published previously, this shows the dual origin of deep waters in the Coral Sea.

The Structure of Planetary Period Oscillations in Saturn's Equatorial Magnetosphere: Results From the Cassini Mission

AbstractSaturn's magnetospheric magnetic field, planetary radio emissions, plasma populations, and magnetospheric structure are all known to be modulated at periods close to the assumed rotation period of the planetary interior. These oscillations are readily apparent despite the high degree of axisymmetry in the internally produced magnetic field of the planet and have different rotation periods in the northern and southern hemispheres. In this paper we study the spatial structure of (near‐)planetary period magnetic field oscillations in Saturn's equatorial magnetosphere. Extending previous analyses of these phenomena, we include all suitable data from the entire Cassini mission during its orbital tour of the planet so as to be able to quantify both the amplitude and phase of these field oscillations throughout Saturn's equatorial plane, to distances of 30 planetary radii. We study the structure of these field oscillations in view of both independently rotating northern and southern systems, finding spatial variations in both magnetic fields and inferred currents flowing north‐south that are common to both systems. With the greatly expanded coverage of the equatorial plane achieved during the latter years of the mission, we are able to present a complete survey of dawn‐dusk and day‐night asymmetries in the structure of the oscillating fields and currents. We show that the general structure of the rotating currents is simpler than previously reported and that the relatively enhanced nightside equatorial fields and currents are due in part to related periodic vertical motion of Saturn's magnetotail current sheet.

A Self‐Regulating Equilibrium Magnetopause Model With Applications to Saturn

AbstractWe describe a new method of constructing an equilibrium magnetopause surface based on the pressure balance between external and internal pressure sources. Details are given for every step of the procedure, including the initial pressure balance condition (which determines magnetopause morphology), the related underlying assumptions, and the final optimized magnetopause surface. Our method produces a final equilibrium magnetopause surface that satisfies the local balance between the solar wind dynamic pressure and planetary magnetic pressure with an error no greater than 1%. We also discuss the contribution of hot plasma pressure and equatorial ring currents and their effects on magnetopause morphology. For each optimized magnetopause boundary, we give coefficients for fitted polynomial approximations to allow simple and accurate reproductions for practical applications. We specifically discuss the potential application to the modeling of Saturn's magnetopause and describe how the equilibrium magnetopause model can be used to estimate the compressibility of the boundary.

Large‐Scale Fresh and Salt Water Exchanges in the Indian Ocean

AbstractUpper‐ocean dynamics in the Northern Indian Ocean (NIO) depend on changes in the magnitude and location of the high salinity waters of the Arabian Sea and low salinity waters of the Bay of Bengal. The large sea surface salinity (SSS) differences between these two basins are related to the surface freshwater flux (evaporation minus precipitation), which is positive (negative) in the Arabian Sea (Bay of Bengal). To quantify large‐scale salinity changes on decadal time scale over the whole water column and to study trends in salinity and volume transport, we have analyzed Simple Ocean Data Assimilation (SODA) reanalysis product, HYbrid Coordinate Ocean Model (HYCOM) simulations, European Centre for Medium‐Range Weather Forecasting's ERA‐Interim reanalysis product, and riverine streamflow data from the National Centers for Atmospheric Research's Global River Flow and Continental Discharge Dataset for the NIO. We find increased freshening conditions in the Bay of Bengal and salinification conditions in the Arabian Sea that would support a stronger zonal SSS difference in the NIO but that it is partially compensated by positive (negative) salt transports into the Bay of Bengal (BoB) (Arabian Sea). Empirical orthogonal function analysis of SODA SSS indicates that the main factors of SSS variability are Indian Ocean Dipole and El Niño‐Southern Oscillation and seasonal currents. The trends in the volume transport reveal decadal changes in zonal equatorial currents in HYCOM and Somali Current in SODA.

A Method of Estimating Equatorial Plasma Vertical Drift Velocity and Its Evaluation Using C/NOFS Observations

AbstractTo understand the dynamics of the equatorial ionosphere and mitigate its effect on radio wave propagation, vertical ion drift velocity empirical models have been developed using limited ground‐based and/or space‐based observations. These models, however, have not yet been validated in detail using recent observations for sufficiently different longitudinal sectors. In this paper we have evaluated the performance of two empirical models and also propose a simplified vertical drift velocity model based on basic physics laws (i.e., Ampere's and Ohm's laws) that we call it parameterized drift velocity (PDV) model. These models have been applied to estimate the E region electric field and the associated F region E × B drift velocity using observed horizontal magnetic fields, due to equatorial electrojet current, as model driver input. Drift velocities obtained from these models are compared with the Communication/Navigation Outage Forecasting System (C/NOFS) satellite in situ vertical drift observations for different longitudinal sectors. It is found, for all longitudinal sectors considered in this study, that the vertical drift velocity obtained from a model based on physics laws has shown better agreement with C/NOFS observations as compared to the outputs of other empirical models. Moreover, it is shown that the Anderson empirical model performs better than the International Reference Ionosphere model.

Poleward Shift of the Pacific North Equatorial Current Bifurcation

AbstractThe dynamics of the poleward shift of the Pacific North Equatorial Current bifurcation latitude (NBL) is studied using a 5.5‐layer reduced gravity model. It is found that the poleward shift of the NBL is associated with the asymmetric intensity of the wind stress curl input to the Pacific tropical and subtropical gyres. Stronger wind stress curl in the subtropical gyre leads to equatorward transport in the interior upper ocean across the boundary between the two gyres, causing a poleward transport compensation at the western boundary. In the lower layer ocean, in turn, there is poleward (equatorward) transport at the interior (western boundary) due to Sverdrup balance which requires zero transport at the gyre boundary where zonally integrated wind stress curl is zero. Therefore, the NBL exhibits a titling feature, with its position being more equatorward in the upper layer and more poleward in the lower layer. The equatorial currents bifurcations in other basins are also characterized by the poleward titling vertical structure. The wind stress curl over the subtropical gyre is generally stronger than that over the tropical gyre, resulting in the bifurcations shifting poleward with increasing depth.

СТРУКТУРА ПОЛЯ СКОРОСТИ ЭКВАТОРИАЛЬНОГО ТЕЧЕНИЯ ЛОМОНОСОВА ПО ДАННЫМ ИЗМЕРЕНИЙ ВЕСНОЙ 2014 И ОСЕНЬЮ 2015 гг.

Two examples of measurements of oceanic equatorial currents in the Atlantic using an shipborne acoustic Doppler current profiler SADCP on meridional sections across the equator in spring 2014 at 23°W and autumn 2015 at 34°W are considered. The relevant data of some previous observations in this region are also used for comparison. The methods for studying the Lomonosov Current using ship profiling and moorings are discussed. According to the results of profiling with a two-minute averaging of the source data of the section of September 27, 2015, a structure of the two-dimensional section of the high-resolution velocity field is presented, which is characterized by small-scale fragmentary stratification, which was previously detected only from one-dimensional data of vertical soundings of velocity and other parameters of the ocean environment. Using the example of comparing this detailed section with the versions for other time averages (15 and 60 min) differences in the estimates of parameters of the velocity field due to the averaging are specified. Using the example of vertical profiles through the core of the Lomonosov Undercurrent (EUC) in the springs of 2002 and 2014 and winters of 2002 and 1971 at the locations close to 23°W, the stability of the annual cycle of fluctuations of the intensity of currents in the core and the position of the core in a multiyear aspect is illustrated. The agreement of our results with other previously published data is shown. The main parameters of the velocity field over the equatorial section on April 17, 2014 indicate a high probability of the outcropping of the easterly flow on the surface. Similar episodes of outcropping in 2002 within the PIRATA program together with corresponding intervals of changing the direction of the zonal component of wind speed derived from the Era Interim reanalysis from westward to eastward are demonstrated. A similar correlation with the wind is assumed to occur on April 17, 2014.

Variability of the African equatorial ionization anomaly (EIA) crests during the year 2013

This paper discusses the variability of the position and magnitude of the crests of African Equatorial Ionization Anomaly during noon and post sunset periods. Total electron content data covered the year 2013, and were obtained from a chain of global positioning system receivers in both hemispheres around 37°E longitude. Local magnetometer data were used to infer the direction and magnitude of the E × B drift, while the solar extreme ultraviolet proxy index was used as a measure of solar activity. It was found that the time of formation of both crests varied from 1400 to 1700 local time. Additionally, the position of the crests was found to be asymmetric with respect to the magnetic equator. During the noon period, the position of the northern and southern crests varied from 4.91° to 7.36° and −9.17° to −12.62°, respectively. During the post-sunset period, it varied from 8° to 11.7° and −9° to −16°, respectively. Seasonally, with reference to the magnetic equator, both crests moved poleward during equinoxes and collapsed towards the equator during winter and summer. Equinoxes recorded the greatest crest magnitude followed by winter then summer over both hemispheres during the noon period. However, this trend persisted over the northern crest only during the post-sunset period. Overall, during the noon period, we recorded correlation coefficients of 0.67 and 0.68 between crest magnitudes and ΔH, a proxy for equatorial electrojet current, and 0.88 and 0.81 between crest positions and ΔH, for the northern and southern crests, respectively. During the Halloween day storm of 30 October 2013, a westward electric field inhibited the development of the post-sunset crests.

The tropical-subtropical coupling in the Southeast Atlantic from the perspective of the northern Benguela upwelling system.

In the Benguela upwelling system, the environmental conditions are determined to a large extent by central water masses advected from remote areas onto the shelf. The origin, spreading pathways and fate of those water masses are investigated with a regional ocean model that is analysed using Eulerian passive tracers and on the basis of Lagrangian trajectories. Two major water masses influencing the Benguela upwelling system are identified: tropical South Atlantic Central Water (SACW) and subtropical Eastern South Atlantic Central Water (ESACW). The spreading of tropical waters into the subtropical Benguela upwelling system is mediated by equatorial currents and their continuation in the Southeast Atlantic. This tropical-subtropical connection has been attributed to signal propagation in the equatorial and coastal waveguides. However, there exists an additional spreading path for tropical central water in the open ocean. This mass transport fluctuates on a seasonal scale around an averaged meridional transport in Sverdrup balance. The inter-annual variability of the advection of tropical waters is related to Benguela Niños, as evidenced by the 2010/2011 event. The northern Benguela upwelling system is a transition zone between SACW and ESACW since they encounter each other at about 20°S. Both water masses have seasonal variable shares in the upwelled water there. To summarise the main pathways of central water mass transport, an enhanced scheme for the subsurface circulation in the Southeast Atlantic is presented.

Latitude-dependent finescale turbulent shear generations in the Pacific tropical-extratropical upper ocean

Turbulent mixing, which is critically important for the equilibrium of ocean circulation, is controlled by finescale turbulent shear (S2) of oceanic flows through shear instability. Although the relationship between S2 and mixing is well understood, the latitude-dependent generation processes of S2 remain poorly known due to the lack of geographically extensive, long-term finescale velocity measurements. Here, using one-year ADCP data from 17 moorings along 143°E, we first show that the upper-ocean S2 and its resultant mixing rate have a W-shaped latitudinal distribution in the tropical-extratropical northwest Pacific with peaks at 0–2°N, 12–14°N, and 20–22°N, respectively. Further analyses reveal that these S2 peaks are caused by vertically-sheared equatorial currents, parametric subharmonic instability of diurnal tide, and anticyclonic eddy’s inertial chimney effect, respectively. As climate model simulations are sensitive to the mixing parameterizations, our findings highlight the need to incorporate the latitude-dependent generation mechanisms of S2 to improve climate models’ prediction capabilities.

Relationship between a function of the northward pressure gradient and the Pacific Equatorial Undercurrent

The Equatorial Undercurrent (EUC) plays an important role in ocean circulation and global climate change. Near the equator, as the Coriolis parameter goes to 0, equatorial currents cannot be described by geostrophy in which the pressure gradient force term is balanced by the Coriolis force term. Many previous studies focus on the relationships between the EUC and El Nino-Southern Oscillation (ENSO), the thermocline, sea surface topography, the distribution of equatorial wind stress and other atmosphere-ocean factors. However, little attention has been paid to the northward pressure gradient (NGT), which may also be important to the EUC. The pressure can be regarded as a complex nonlinear function of terms including temperature, salinity and density. This study attempts to reveal the connection between a function of the northward pressure gradient (FNP) and the EUC. The connection is derived from primitive equations, by simplifying the equations with using scaling analysis, and shows that the beta effect may be the main reason why the FNP is important to the EUC. The vertical structure of the EUC can be partially described by the FNP. The NGT has an obvious influence on the EUC while the eastward pressure gradient has a relatively smaller effect.

Evaluation of Interannual Simulations and Indian Ocean Dipole Events During 2000–2014 from a Basin Scale General Circulation Model

Interannual simulations performed using an eddy-permitting ocean general circulation model (OGCM) for years 2000–2014 were validated against buoy measurements over the Indian Ocean (IO). Model-simulated fields were evaluated extensively using multiple statistical metrics to quantify the quality of model in reproducing variability in oceanic surface and subsurface features. The model-simulated sea surface temperature (SST) at different moored buoy locations exhibits high (close to + 0.9) correlation coefficient (R) with the ranges of standard deviation (SD) of simulated SST consistent with the corresponding buoy observations. The root mean square difference (RMSD) estimated between the buoy and simulated SST was found to be less than 0.6 °C at most of the buoy locations. The model-simulated subsurface temperature profile, including the thermocline, resembled good agreement with the buoy profiles. Intraseasonal and interannual variability of 20 °C isotherm (D20) was simulated reasonably well as observed at the respective buoy locations. Mean error in surface currents was low; however, the meridional component from model simulations showed a better agreement (RMSD < 0.25 m/s) with the observations as compared to zonal components (< 0.4 m/s) for the periods of buoy data availability. The Dipole Mode Index (DMI) derived from simulated SST reproduces the positive/negative Indian Ocean Dipole (IOD) events that occurred during the simulation period. Interannual variability in temperature, currents and oceanic mixed layer depth was analyzed in response to IOD events. The anomalies in equatorial currents were found to affect the strength of coastal currents along the Indian coastlines. Model simulations showed the enhanced (suppressed) coastal upwelling process along the Sumatra coast that leads to anomalous cooling (warming) off the Sumatra coast during the positive (negative) IOD events.

Geomagnetically Induced Currents Caused by Interplanetary Shocks With Different Impact Angles and Speeds

AbstractThe occurrence of geomagnetically induced currents (GICs) poses serious threats to modern technological infrastructure. Large GICs result from sharp variations of the geomagnetic field (dB/dt) caused by changes of large‐scale magnetospheric and ionospheric currents. Intense dB/dt perturbations are known to occur often in high‐latitude regions as a result of storm time substorms. Magnetospheric compressions usually caused by interplanetary shocks increase the magnetopause current leading to dB/dt perturbations more evident in midlatitude to low‐latitude regions, while they increase the equatorial electrojet current leading to dB/dt perturbations in dayside equatorial regions. We investigate the effects of shock impact angles and speeds on the subsequent dB/dt perturbations with a database of 547 shocks observed at the L1 point. By adopting the threshold of dB/dt = 100 nT/min, identified as a risk factor to power systems, we find that dB/dt generally surpasses this threshold when following impacts of high‐speed and nearly frontal shocks in dayside high‐latitude locations. The same trend occurs at lower latitudes and for all nightside events but with fewer high‐risk events. Particularly, we found nine events in equatorial locations with dB/dt &gt; 100 nT/min. All events were caused by high‐speed and nearly frontal shock impacts and were observed by stations located around noon local time. These high‐risk perturbations were caused by sudden strong and symmetric magnetospheric compressions, more effectively intensifying the equatorial electrojet current, leading to sharp dB/dt perturbations. We suggest that these results may provide insights for GIC forecasting aiming at preventing degradation of power systems due to GICs.

THE COMPUTATION OF OCEANIC AND METEOROLOGICAL FIELDS OF MOTION WITH FRICTION PROPORTIONAL TO THE VELOCITY

The friction in the hydrodynamical differential equations is taken proportional to the velocity. With it simple relations result, permitting the calculation of the mass transport, without the need to solve differential equations, if the wind and pressure distributions are known. The solution can cross the equator freely. Simultaneously the equations give the vertical components of motion at the lower boundary of the layer under consideration. Numerical values of the friction coefficient are given, and oceanographical and meteorological models are discussed, as for instance the monsoon winds over Southeast Asia, the monsoon currents in the Java and China Seas and the equatorial currents of the Pacific Ocean.

Equatorial Currents in the Indian Ocean Based on Measurements in February 2017

We analyze the results of measurements of the Tareev equatorial undercurrent in the Indian Ocean in February 2017. Sections from 3° S to 3°45′ N along 68° and 65° E crossed the current with measurements of the temperature, salinity, and current velocity at oceanographic stations. The maximum velocity of this eastward flow was recorded precisely at the equator. The velocity at a depth of 50 m was approximately 60 cm/s. The transport of the Tareev Current was estimated at 9.8 Sv (1 Sv = 106 m3/s).

Evidence for the Maintenance of Slowly Varying Equatorial Currents by Intraseasonal Variability

AbstractRecent evidence from mooring data in the equatorial Atlantic reveals that semiannual and longer time scale ocean current variability is close to being resonant with equatorial basin modes. Here we show that intraseasonal variability, with time scales of tens of days, provides the energy to maintain these resonant basin modes against dissipation. The mechanism is analogous to that by which storm systems in the atmosphere act to maintain the atmospheric jet stream. We demonstrate the mechanism using an idealized model setup that exhibits equatorial deep jets. The results are supported by direct analysis of available mooring data from the equatorial Atlantic Ocean covering a depth range of several thousand meters. The analysis of the mooring data suggests that the same mechanism also helps maintain the seasonal variability.

Application of solar quiet (Sq) current in determining mantle conductivity-depth structure in Malaysia

The mantle electrical conductivity-depth structure of Malaysia was determined for the first time using solar quiet day ionospheric current (Sq) variations. Spherical harmonic analysis (SHA) was employed to separate the external and internal field contribution to the Sq variations. A transfer function was applied in estimating the conductivity-depth profile for the paired of external and internal coefficients of the SHA. We observed a downward increase in electrical conductivity with initial magnitude of 0.0065 S/m at a depth of ∼ 56 km which gradually rose to 0.0106 and 0.0140 S/m at 118 and 180 km. Subsequently, the conductivity profile rose to about 0.0228 S/m at 380 km (near the base of the upper mantle) and reached 0.0260 S/m at 435 km, after which a sharp steep increase was observed at 450 km with conductivity profile of 0.0278 S/m. Consequently, the conductivity profile increases significantly to about 0.1367 S/m at a depth of 973 km and reached its peak value 0.1975 S/m at the depth of 1097 km in the lower mantle with no indication of leveling off. An evidence of discontinuity was observed near 390–460 km and 675–746 km. A slight increase in conductivity values at depth between 150 and 300 km corresponds to the region of unusual global low velocity zone with high electrical conductivity. The conductivity profile showed a less steep increase above 450 km below which a steep increase was observed. The present profile showed the deepest penetration depth which may be attributed to the influence of equatorial electrojet current (EEJ) that is actively supported by the conductive properties of the Earth's interior within the study region.