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The impact of tropical Atlantic Ocean variability modes in the variability of the upper-ocean circulation has been investigated. For this purpose, we use three oceanic reanalyses, an interannual forced-ocean simulation, and satellite data for the period 1982–2018. We have explored the changes in the main surface and subsurface ocean currents during the emergence of Atlantic meridional mode (AMM), Atlantic zonal mode (AZM), and AMM–AZM connection. The developing phase of the AMM is associated with a boreal spring intensification of North Equatorial Countercurrent (NECC) and a reinforced summer Eastern Equatorial Undercurrent (EEUC) and north South Equatorial Current (nSEC). During the decaying phase, the reduction of the wind forcing and zonal sea surface height gradient produces a weakening of surface circulation. For the connected AMM–AZM, in addition to the intensified NECC, EEUC, and nSEC in spring, an anomalous north-equatorial wind curl excites an oceanic Rossby wave (RW) that is boundary-reflected into an equatorial Kelvin wave (KW). The KW reverses the thermocline slope, weakening the nSEC and EUC in boreal summer and autumn, respectively. During the developing spring phase of the AZM, the nSEC is considerably reduced with no consistent impact at subsurface levels. During the autumn decaying phase, the upwelling RW-reflected mechanism is activated, modifying the zonal pressure gradient that intensifies the nSEC. The NECC is reduced in boreal spring–summer. Our results reveal a robust alteration of the upper-ocean circulation during AMM, AZM, and AMM–AZM, highlighting the decisive role of ocean waves in connecting the tropical and equatorial ocean transport.

This study uses two reanalysis datasets and ocean model experiments to examine the dynamics of the interannual variability of the equatorial currents in the Indian Ocean, and to quantify the effects of the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) climate modes on the currents. Strong interannual variability of the equatorial currents mainly occurs in the upper-central basin (UCB) where the Wyrtki jets are located, and in the subsurface-eastern basin (SEB) where the equatorial undercurrent is located. Equatorial waves directly forced by equatorial winds dominate the interannual current anomalies in both the UCB and the SEB, and the reflected waves have a secondary role. The reflected waves tend to weaken the current anomalies in the UCB but intensify the currents in the SEB. In general, ENSO and the IOD have a comparable effect on the interannual current anomalies in the SEB, but the IOD has a larger role than ENSO in the UCB. In some years, either ENSO or the IOD may play a dominant role. Composite analysis suggests that the interannual current anomalies occur about two months prior to the peaks the climate modes. As ENSO and the IOD have apparent seasonality, the current anomalies mainly occur during August to October for pure IOD years, and during October to December for pure ENSO years. The co-occurrence of ENSO and the IOD enhances their respective impact, resulting in the surface current anomalies lasting from July to December and the subsurface current anomalies from July to March of the following year.

The tropical Southeast Indian Ocean (SEIO) is a key area linking the global freshwater and heat exchanges. The Indian Ocean Dipole (IOD) fundamentally modulates the Indian Ocean circulation and thus regulates the basin‐wide freshwater balance. However, our knowledge of this effect remains limited. Using observational‐based data sets, this study suggests that extreme positive IOD events have notable signatures on the three‐dimensional freshwater content of the SEIO, leading to the vertically opposite salinity anomalies in the surface and subsurface layers. The wind changes led to the northwestward extension of the South Equatorial Current and intensified Sumatra‐Java upwelling. The changing horizontal and vertical currents jointly result in the complicated salinity anomalies. The Equatorial Undercurrent serves as the conduit for water exchange between the equator and the SEIO. This work highlights a strong coupling between the equatorial circulation and the three‐dimensional freshwater inventory of the SEIO within the framework of the IOD. Plain Language Summary The tropical Southeast Indian Ocean (SEIO) connects the three oceans and contributes to the global mass and heat exchanges. The changing freshwater storage and heat content in the SEIO impact regional and global climates. Positive Indian Ocean Dipole (pIOD) is a pattern of internal variability with anomalously low sea surface temperature off Sumatra and high sea surface temperature in the western Indian Ocean, with accompanying wind and precipitation anomalies. The IOD‐related wind anomalies drive the ocean circulation and thus regulate the freshwater content. However, the related process is not clear. This study suggests a framework of IOD regulating the three‐dimensional freshwater content in the SEIO. During the extreme pIOD phase, the abnormal horizontal and vertical currents jointly modulate mass and material distributions in the SEIO, leading to the salting anomalies in the surface layer but the freshening anomalies in the subsurface layer. This study also provides evidence for water exchange between the subsurface layer of the Indian Ocean equator and the surface layer of the SEIO. These findings are a crucial step toward fully understanding the three‐dimensional freshwater content in this critical area. Key Points Extreme positive Indian Ocean Dipole has notable signatures on the three‐dimensional freshwater content in the Southeast Indian Ocean (SEIO) The combined effect of horizontal and vertical currents results in the vertically opposite salinity anomalies The Equatorial Undercurrent serves as an important conduit for water exchange between the equator and the SEIO

Strong vertical shears occur in the upper Equatorial Ocean as the trade winds drive the South Equatorial Current westward above the eastward flowing Equatorial Undercurrent. An extremely large “effective viscosity” or vertical momentum transport is required to maintain the speed‐differential between the currents as observed. In the 2012 EquatorMix Experiment data from a 1.8 km optical fiber temperature array and a surface scattering radar were combined with high‐resolution shipboard profiling CTD and Doppler sonar measurements to determine the directionality of energetic ∼600 m wavelength internal waves existing above the Undercurrent. A large vertical momentum flux is found (∼10−4 m2 s−2), with waves excited by nocturnal sea surface convection and maintained by near‐surface critical layer over‐reflection. The net downward‐westward momentum flux is an index of the energy lost during reflection below the Undercurrent. Together with near‐surface‐turbulence, these waves provide the momentum transport needed to balance the large‐scale forcing of the equatorial current system. Plain Language Summary The trade winds push equatorial surface waters westward over the eastward flowing Equatorial Undercurrent ∼100 m below. Given the known basin‐scale forcing, the observed velocity difference between these opposing flows, ∼1.5 m s−1, is understandable provided the upper ocean has an “effective viscosity” roughly equivalent to that of honey. Observed turbulence levels are insufficient to support this level of viscosity at depth. In the 2012 EquatorMix Experiment, sea surface spatial observations from a 1.8 km optical fiber temperature‐sensing array and a Doppler radar were combined with rapidly‐sampled vertical profiles of ocean density and velocity to identify a class of ∼600 m wavelength internal gravity waves that exist above the Undercurrent. These exchange the westward momentum of the sea surface with the Undercurrent's eastward momentum. The waves are triggered by convection resulting from the nocturnal cooling of the sea surface. They propagate downward and westward, reflecting below the Undercurrent Core. The net momentum deposition is associated with the degree of dissipation in the deep reflection process. The upward‐reflected waves arrive at the surface and subsequently reflect back downward, receiving additional energy and momentum from the wind‐driven shear in a process known as critical layer over‐reflection. Key Points Energetic internal waves are found in the highly sheared region above the Equatorial Undercurrent in the Eastern Equatorial Pacific The waves support a large momentum exchange between the westward flowing S. Equatorial Current and the eastward moving Undercurrent below The waves are triggered by nocturnal convection, fueled by wind driven shear, and maintained by over‐reflection at a near‐surface critical layer

Variability of oceanic salinity, an indicator of the global hydrological cycle, plays an important role in the basin-scale ocean circulation. In this study, interannual to decadal variability of salinity in the upper layer of the Indian Ocean is investigated using Argo observations since 2004 and data assimilating model outputs (1992–2015). The southeastern Indian Ocean shows the strongest interannual to decadal variability of upper-ocean salinity in the IndianOcean.Westward propagation of salinity anomalies along isopycnal surfaces is detected in the southern IndianOcean and attributed to zonal salinity advection anomalies associated with the Indonesian Throughflow and the South Equatorial Current. Composite and salinity budget analyses show that horizontal advection is a major contributor to the interannual to decadal salinity variability of the southern Indian Ocean, and the local air–sea freshwater flux plays a secondary role. The Pacific decadal oscillation (PDO) and El Niño–Southern Oscillation (ENSO) modulate the salinity variability in the southeastern Indian Ocean, with low salinity anomalies occurring during the negative phases of the PDO and ENSO and high salinity anomalies during their positive phases. The Indonesian Throughflow plays an essential role in transmitting the PDO- and ENSO-related salinity signals into the Indian Ocean. A statistical model is proposed based on the PDO index, which successfully predicts the southeastern Indian Ocean salinity variability with a lead time of 10 months.

The strong El Niño of 2014–16 was observed west of the Galápagos Islands through sustained deployment of underwater gliders. Three years of observations began in October 2013 and ended in October 2016, with observations at longitudes 93° and 95°W between latitudes 2°N and 2°S. In total, there were over 3000 glider-days of data, covering over 50 000 km with over 12 000 profiles. Coverage was superior closer to the Galápagos on 93°W, where gliders were equipped with sensors to measure velocity as well as temperature, salinity, and pressure. The repeated glider transects are analyzed to produce highly resolved mean sections and maps of observed variables as functions of time, latitude, and depth. The mean sections reveal the structure of the Equatorial Undercurrent (EUC), the South Equatorial Current, and the equatorial front. The mean fields are used to calculate potential vorticity Q and Richardson number Ri. Gradients in the mean are strong enough to make the sign of Q opposite to that of planetary vorticity and to have Ri near unity, suggestive of mixing. Temporal variability is dominated by the 2014–16 El Niño, with the arrival of depressed isopycnals documented in 2014 and 2015. Increases in eastward velocity advect anomalously salty water and are uncorrelated with warm temperatures and deep isopycnals. Thus, vertical advection is important to changes in heat, and horizontal advection is relevant to changes in salt. Implications of this work include possibilities for future research, model assessment and improvement, and sustained observations across the equatorial Pacific.

In the eastern tropical Indian Ocean, intraseasonal variability (ISV) affects the regional oceanography and marine ecosystems. Mooring and satellite observations documented two periods of unusually weak ISV during the past two decades, associated with suppressed baroclinic instability of the South Equatorial Current. Regression analysis and model simulations suggest that the exceptionally weak ISVs were caused primarily by the extreme El Niño events and modulated to a lesser extent by the Indian Ocean dipole. Additional observations confirm that the circulation balance in the Indo-Pacific Ocean was disrupted during the extreme El Niño events, impacting the Indonesian Throughflow Indian Ocean dynamics. This research provides substantial evidence for large-scale modes modulating ISV and the abnormal Indo-Pacific dynamical connection during extreme climate modes.

This study assesses the biogeographic classification of the Western Indian Ocean (WIO) on the basis of the species diversity and distribution of reef-building corals. Twenty one locations were sampled between 2002 and 2011. Presence/absence of scleractinian corals was noted on SCUBA, with the aid of underwater digital photographs and reference publications for species identification. Sampling effort varied from 7 to 37 samples per location, with 15 to 45 minutes per dive allocated to species observations, depending on the logistics on each trip. Species presence/absence was analyzed using the Bray-Curtis similarity coefficient, followed by cluster analysis and multi-dimensional scaling. Total (asymptotic) species number per location was estimated using the Michaelis-Menten equation. Three hundred and sixty nine coral species were named with stable identifications and used for analysis. At the location level, estimated maximum species richness ranged from 297 (Nacala, Mozambique) to 174 (Farquhar, Seychelles). Locations in the northern Mozambique Channel had the highest diversity and similarity, forming a core region defined by its unique oceanography of variable meso-scale eddies that confer high connectivity within this region. A distinction between mainland and island fauna was not found; instead, diversity decreased radially from the northern Mozambique Channel. The Chagos archipelago was closely related to the northern Mozambique Channel region, and analysis of hard coral data in the IUCN Red List found Chagos to be more closely related to the WIO than to the Maldives, India and Sri Lanka. Diversity patterns were consistent with primary oceanographic drivers in the WIO, reflecting inflow of the South Equatorial Current, maintenance of high diversity in the northern Mozambique Channel, and export from this central region to the north and south, and to the Seychelles and Mascarene islands.

Interannual variabilities of sea level and upper-ocean gyre circulation of the western tropical Pacific Ocean (WTPO) have been predominantly attributed to El Niño–Southern Oscillation (ENSO). The results of the present study put forward important modulation effects by the Indian Ocean dipole (IOD) mode. The observed sea level in the WTPO shows significant instantaneous and lagged correlations (around −0.60 and 0.40, respectively) with the IOD mode index (DMI). A composite of 14 “independent” IOD events for 1958–2017 shows negative sea level anomalies (SLAs) of 4–7 cm in the WTPO during positive IOD events and positive SLAs of 6–8 cm in the following year that are opposite in sign to the El Niño effect. The IOD impacts are reproduced by large-ensemble simulations of a climate model that generate respectively 430 and 519 positive and negative independent IOD events. A positive IOD induces westerly winds over the western and central tropical Pacific and causes negative SLAs through Ekman upwelling, and it facilitates the establishment of a La Niña condition in the following year that involves enhanced Pacific trade winds and causes positive SLAs in the WTPO. Ocean model experiments confirm that the IOD affects the WTPO sea level mainly through modulating the tropical Pacific winds. Variability of the Indonesian Throughflow (ITF) induced by IOD winds has a relatively weak effect on the WTPO. The IOD’s impacts on the major upper-ocean currents are also considerable, causing anomalies of 1–4 Sv (1 Sv ≡ 10⁶ m³ s−1) in the South Equatorial Current (SEC) and North Equatorial Countercurrent (NECC) volume transports.

Indonesian Throughflow (ITF) waters move along multiple pathways within the Indian Ocean. The western route is within the thermocline of the South Equatorial Current (SEC), and the southern route is via injection into the Leeuwin Current (LC) along western Australia. We use gridded Argo data to examine heat content anomaly (HCa) within three boxes in the eastern Indian Ocean, one adjacent to the ITF outflow from the Indonesian Seas (ITF box), the second in the eastern portion of the SEC (SEC box), and the third in the LC (LC box). Although interannual HCa variability in the SEC and ITF boxes is well correlated, a large increase in HCa within the ITF box does not appear in the SEC box in 2011 but is evident in the LC box. The 2011 change in the SEC–LC partitioning is investigated using GODAS reanalysis by examining the strength of the SEC and LC during a 2009 HCa increase within the ITF box and the subsequent increase in 2011. During 2009, a strong SEC and weakened LC spread the increased ITF HCa into the central Indian Ocean, whereas a weak SEC and strengthened LC during 2011 transmit the HCa signal to the south. Near-surface winds and mean sea level pressure from NCEP–NCAR reanalysis reveal that Ningaloo Niño events led to shifts in ocean circulation during 2000 and 2011. LC and SEC exports show a high negative correlation at interannual time scales, indicating that a reduction of outflow from one pathway is partially compensated by an increase from the other.