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Based on a physics-oriented modeling study, we investigate the underlying forcing processes of the North Equatorial Undercurrent (NEUC). Made up of large-scale (~90%) and mesoscale (~10%) components, the NEUC weakens eastward with a longitude-independent seasonality. The large-scale component reflects the effect of the meridional baroclinic pressure gradient force (PGF_BC). The vertical velocity shear forms the eastward NEUC, when the PGF_BC exceeds the meridional barotropic pressure gradient force (PGF_BT). The mesoscale variability with alternating jets is linked to the wind stress curl in different regions of the tropical North Pacific. Spatially, the NEUC has a northern (NEUC_N) and a southern branch (NEUC_S), which are mainly attributed to the transports from Luzon Undercurrent (LUC) and Mindanao Undercurrent (MUC), respectively. The LUC of ~3 Sv (1 Sv ≡ 10 6 m 3 s −1 ) feeds the NEUC_N in summer, while the MUC of ~4 Sv fuels the NEUC_S in autumn and the two branches do not coexist. The total NEUC transport peaks in August/September, and there exist three distinct periods in a 1-yr cycle: the non-NEUC period in winter, the LUC-driven period in summer, and the MUC-driven period in autumn. Based on the layer-integrated vorticity equation, we diagnose quantitatively that the variation of the NEUC is dominated by the lateral planetary vorticity influx from the LUC and the MUC. These external influxes interact with the internal dynamics of pressure torques and stress curls in the NEUC layer, to jointly govern the NEUC and its variability. Meanwhile, the nonlinearity due to relative vorticity advection near the coast modulates the strength of the NEUC.

The general problem of an ocean on a rotating sphere is considered. The governing equations for an inviscid, incompressible fluid, written in spherical coordinates that are fixed at a point on the rotating Earth, together with the free surface and rigid bottom boundary conditions, are introduced. An exact solution of this system is presented; this describes a steady flow that is moving only in the azimuthal direction, with no variation in this direction. However, this azimuthal velocity component has an arbitrary variation with depth (i.e., radius), and so, for example, an Equatorial Undercurrent (EUC) can be accommodated. The pressure boundary condition at the free surface relates this pressure to the shape of the surface via a Bernoulli relation; this provides the constraint on the existence of a solution, although the restrictions are somewhat involved in spherical coordinates. To examine this constraint in more detail, the corresponding problems in model cylindrical coordinates (with the equator “straightened” to become a generator of the cylinder), and then in the tangent-plane version (with the β -plane approximation incorporated), are also written down. Both these possess similar exact solutions, with a Bernoulli condition that is more readily interpreted in terms of the choices available. Some simple examples of the surface pressure, and associated surface distortion, are presented. The relevance of these exact solutions to more complicated, and physically realistic, flow structures is briefly mentioned.

Stable isotopes of oxygen (δ18O) in seawater reflect the combined influences of ocean circulation and atmospheric moisture balance. However, it is difficult to disentangle disparate ocean and atmosphere influences on modern seawater δ18O values, partly because continuous time series of seawater δ18O are rare. Here we present a nearly nine‐year, continuous record of seawater δ18O values from the Galápagos. Seawater δ18O values faithfully track sea surface salinity and salinity along the equator at 50 m depth. Zonal current velocity within the Equatorial Undercurrent (EUC), directly west of the Galápagos, is strongly correlated with Galápagos surface seawater δ18O values with a 1‐month lag. Reconstructions of Galápagos seawater δ18O values could thus provide a window into past variations in the strength of the EUC, an important influence on large‐scale tropical Pacific climate. Plain Language Summary The Equatorial Undercurrent (EUC) flows beneath the surface of the equatorial Pacific Ocean from west to east, transporting cold, salty, nutrient rich waters. When this current hits the Galápagos, it rises to the surface. Its high nutrient levels serve as the foundation for the diverse Galápagos ecosystem and its colder temperature helps set up a strong sea surface temperature gradient that is the foundation of the tropical Pacific climate system. Despite its importance, little is known about how this current has varied prior to the short period of instrumental observations, and it remains challenging to reproduce in climate models. Here we show how Galápagos seawater stable isotope values track the strength of the EUC. Our findings open up possibilities to extend the record of the EUC back in time with isotope‐based paleoclimate proxies from the Galápagos region. Key Points Galápagos seawater δ18O values strongly covary with equatorial cold tongue salinity values Seawater δ18O values are higher with a stronger Pacific Equatorial Undercurrent west of Galápagos

The eastern equatorial Atlantic hosts a productive marine ecosystem that depends on upward supply of nitrate, the primary limiting nutrient in this region. The annual productivity peak, indicated by elevated surface chlorophyll levels, occurs in the Northern Hemisphere summer, roughly coinciding with strengthened easterly winds. For enhanced productivity in the equatorial Atlantic, nitrate-rich water must rise into the turbulent layer above the Equatorial Undercurrent. Using data from two trans-Atlantic equatorial surveys, along with extended time series from equatorial moorings, we demonstrate how three independent wind-driven processes shape the seasonality of equatorial Atlantic productivity: (1) the nitracline shoals in response to intensifying easterly winds; (2) the depth of the Equatorial Undercurrent core, defined by maximum eastward velocity, is controlled by an annual oscillation of basin-scale standing equatorial waves; and (3) mixing intensity in the shear zone above the Equatorial Undercurrent core is governed by local and instantaneous winds. The interplay of these three mechanisms shapes a unique seasonal cycle of nutrient supply and productivity in the equatorial Atlantic, with a productivity minimum in April due to a shallow Equatorial Undercurrent and a productivity maximum in July resulting from a shallow nitracline coupled with enhanced mixing. The seasonal timing of peak primary productivity in the eastern equatorial Atlantic is a result of wind-driven processes coinciding with increased surface nitrate supply, according to transect and mooring observations.

The eastern equatorial Pacific (EEP) is a source of atmospheric CO2 during the last deglaciation, but the associated oceanic dynamics in the broader low‐latitude Pacific is not fully understood. Here, we report 30,000‐year‐long surface and subsurface pCO2 records for the western equatorial Pacific (WEP), based on boron isotopes in two planktonic foraminiferal species from core MD10‐3340. Our results show that the WEP surface became a significant atmospheric CO2 sink despite that its subsurface waters were enriched by CO2 during the last deglaciation to early Holocene. Combined with EEP proxy data and model results, we suggest that a deglacial‐early Holocene zonal seesaw of sea‐air CO2 exchange across the equatorial Pacific led to a net CO2 outgassing much greater than the modern situation. This can be ascribed to strengthened Subtropical‐Tropical Circulation, resulting in stronger upper ocean stratification in the WEP concurrent with enhanced upwelling of CO2‐rich subsurface waters in the EEP. Plain Language Summary The modern equatorial Pacific is one of the main sources of CO2 released from the ocean into the atmosphere, with the strongest outgassing in the central‐eastern part and nearly neutral state in the western part. However, according to our boron isotope data of planktonic foraminifera, the western equatorial Pacific was turned into a significant atmospheric CO2 sink rather than a source between 16,000 and 7,000 years ago. At the same time, the eastern equatorial Pacific released more CO2, resulting in a zonal seesaw of sea‐air CO2 exchange across the equatorial Pacific that generally accelerated the CO2 outgassing and favored global warming. We further argue that these deglacial‐early Holocene CO2 changes were ascribed to the strengthened Subtropical‐Tropical Circulation, in which the accelerated eastward Equatorial Undercurrent leads to stronger upper ocean stratification in the WEP, and also converges seawater CO2 into the upper thermocline and finally releases at the surface EEP. This finding helps us understand and quantitatively assess future global carbon budget, particularly how the upper ocean circulation modulates the sea‐air CO2 flux in a manner similar to the past. Key Points Strengthened upper ocean stratification led to significant atmospheric CO2 sink in the western equatorial Pacific during ∼16–7 ka The zonal seesaw of sea‐air CO2 exchange in the equatorial Pacific resulted in a net CO2 outgassing much greater than the modern situation The sea‐air CO2 exchange across the equatorial Pacific was modulated by the Subtropical‐Tropical Circulation on orbital timescales

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

This study presents aerosol iron isotopic compositions (δ56Fe) in the western and central equatorial and tropical Pacific Ocean. Aerosols supply iron (Fe), a critical element for marine primary production, to the open ocean. Particulate aerosols, > 1 µm, were sampled during the EUCFe (Equatorial Undercurrent Fe) cruise (RV Kilo Moana, PI: James W. Murray, 2006). One aerosol sample was isotopically lighter than the crust (δ56Fe = −0.16 ± 0.07 ‰, 95 % confidence interval), possibly originating from combustion processes. The nine other aerosol samples were isotopically heavier than the crust, with a rather homogeneous signature of +0.31 ± 0.21 ‰ (2 SD, n= 9). Given (i) this homogeneity compared to the diversity of their modeled geographic origin and (ii) the values of the Fe/Ti ratios used as a lithogenic tracer, we suggest that these heavy δ56Fe signatures reflect isotopic fractionation of crustal aerosols caused by atmospheric processes. Using a fractionation factor of Δsolution-particle= −1.8 ‰, a partial dissolution of ≈ 13 % of the initial aerosol iron content, followed by the removal of this dissolved fraction, would explain the observed slightly heavy Fe isotope signatures. Such fractionation has been observed previously in laboratory experiments but never before in a natural environment. The removal of the dissolved fraction of the aerosols has not been previously documented either. This work illustrates the strong constraints provided by the use of iron isotopes for atmospheric process studies.

The ice shelves in the Amundsen Sea are being melted rapidly by warm Circumpolar Deep Water (CDW), causing sea‐level rise. Ice‐shelf melt variability is controlled by the speed of a shelf‐break undercurrent which transports CDW onto the continental shelf. We study decadal variability of the undercurrent and ice‐shelf melting using new regional ice‐ocean model perturbation experiments. The perturbation experiments suggest that the undercurrent decadal variability is controlled by variable coastal sea‐ice freshwater fluxes, these driven by winds mechanically opening and closing coastal polynyas. With the perturbation experiments we also quantify a positive feedback mechanism between the undercurrent and ice‐shelf melting which is responsible for 25% $25\\%$ of their decadal variability.

The Makassar Strait throughflow (MST) is the major component of the Indonesian Throughflow (ITF), transferring Pacific water into the Indian Ocean. In our previous study, we identified a new zonal pathway, a. k.a. the North Equatorial Subsurface Current (NESC), which carried equatorial water into the MST sub‐thermocline (>300 m) in the summer 2016 following the 2015/16 El Niño. We now show continued strong southward MST in the sub‐thermocline during the winter of 2016–2017, with salinity higher than that in the summer 2016, due to direct South Pacific water intrusion into the Sulawesi Sea. The origin of the intrusion is identified from the New Guinea Coastal Undercurrent (NGCUC) and from an anomalous westward flow along 3°N in the western equatorial Pacific. The identified interannual variability of the western Pacific Ocean circulation is particularly strong in the winter following super El Niño events. Plain Language Summary The Indonesian Throughflow (ITF) transfers Pacific waters into the eastern Indian Ocean through the complex passages of the Maritime Continent, affecting the water properties and heat content in both oceans. The vertical structure of the ITF plays an important role in modulating the Indo‐Pacific Ocean heat content and climate. Understanding the Pacific water mass sources of the ITF and their variations is essential to understanding interocean heat and salt transports. The sub‐thermocline (>300 m) throughflow within the Indonesian Seas, has waters drawn from the relatively salty South Pacific thermocline. To date, the pathway of the South Pacific water into the ITF is not understood well. Here we present evidence showing a new pathway for high salinity South Pacific water flowing into the sub‐thermocline Makassar Strait directly after strong El Nino events, which may become more common in the future. This study helps to understand the importance of the South Pacific water in the variations of the Great Ocean Conveyer Belt and in biogeochemical processes with ecological impacts downstream of the ITF. Key Points Strong anomalous southward flow with higher salinity in the winter of 2016–2017 was observed in the sub‐thermocline Makassar Strait The high salinity is due to direct intrusion of South Pacific water from the western boundary current and an anomalous flow along 3 °N The identified direct intrusion of South Pacific water into the Makassar Strait appears strong in the winter following a super El Niño

Several years of moored turbulence measurements from χ pods at three sites in the equatorial cold tongues of Atlantic and Pacific Oceans yield new insights into proxy estimates of turbulence that specifically target the cold tongues. They also reveal previously unknown wind dependencies of diurnally varying turbulence in the near-critical stratified shear layers beneath the mixed layer and above the core of the Equatorial Undercurrent that we have come to understand as deep cycle (DC) turbulence. Isolated by the mixed layer above, the DC layer is only indirectly linked to surface forcing. Yet, it varies diurnally in concert with daily changes in heating/cooling. Diurnal composites computed from 10-min averaged data at fixed χ pod depths show that transitions from daytime to nighttime mixing regimes are increasingly delayed with weakening wind stress τ . These transitions are also delayed with respect to depth such that they follow a descent rate of roughly 6 m h −1 , independent of τ . We hypothesize that this wind-dependent delay is a direct result of wind-dependent diurnal warm layer deepening, which acts as the trigger to DC layer instability by bringing shear from the surface downward but at rates much slower than 6 m h −1 . This delay in initiation of DC layer instability contributes to a reduction in daily averaged values of turbulence dissipation. Both the absence of descending turbulence in the sheared DC layer prior to arrival of the diurnal warm layer shear and the magnitude of the subsequent descent rate after arrival are roughly predicted by laboratory experiments on entrainment in stratified shear flows.