Tropical Thermocline Research Articles

Improving Equatorial Upper Ocean Vertical Mixing in the NOAA/GFDL OM4 Model

AbstractDeficiencies in upper ocean vertical mixing parameterizations contribute to tropical upper ocean biases in global coupled general circulation models, affecting their simulated ocean heat uptake and ENSO variability. To better understand these deficiencies, we develop a suite of ocean model experiments including both idealized single column models and realistic global simulations. The vertical mixing parameterizations are first evaluated using large eddy simulations as a baseline to assess uncertainties and evaluate their implied turbulent mixing. Global models are then developed following NOAA/GFDL's 0.25° nominal horizontal grid spacing OM4 (uncoupled) configuration of the MOM6 ocean model, with various modifications that target biases in the original model. We test several enhancements to the existing mixing schemes and evaluate them against observational constraints from Tropical Atmosphere Ocean moorings and Argo floats. In particular, we find that we can improve the diurnal variability of mixing in OM4 via modifications to its surface boundary layer mixing scheme, and can improve the net mixing in the upper thermocline by reducing the background vertical viscosity, allowing for more realistic, less diffuse currents. The improved OM4 model better represents the mixing, leading to improved diurnal deep‐cycle variability, a more realistic time‐mean tropical thermocline structure, and a better Pacific Equatorial Undercurrent.

A Pacific Tropical Decadal Variability Challenge for Climate Models

AbstractUnderstanding and forecasting Tropical Pacific Decadal‐scale Variability (TPDV) strongly rely on climate model simulations. Using a Linear Inverse Modeling (LIM) diagnostic approach, we reveal Coupled Model Intercomparison Project Phase 6 models have significant challenges in reproducing the spatial structure and dominant mechanisms of TPDV. Specifically, while the models' ensemble mean pattern of TPDV resembles that of observations, the spread across models is very large and most models show significant differences from observations. In observations, removing the coupling between extratropics and tropics reduces TPDV by ∼60%–70%, and removing the tropical thermocline variability makes the central tropical Pacific a key center of action for TPDV and El Niño Southern Oscillation variability. These characteristics are only confirmed in a subset of models. Differences between observations and simulations are outside the range of natural internal TPDV noise and pose important questions regarding our ability to model the impacts of natural internal low‐frequency variability superimposed on long‐term climate change.

The Agulhas Current Transports Signals of Local and Remote Indian Ocean Nitrogen Cycling

AbstractThe greater Agulhas Current region is an important component of the climate system, yet its influence on carbon and nutrient cycling is poorly understood. Here, we use nitrate isotopes (δ15N, δ18O, Δ(15–18) = δ15N–δ18O) to trace regional water mass circulation and investigate nitrogen cycling in the Agulhas Current and adjacent recirculating waters. The deep and intermediate waters record processes occurring remotely, including partial nitrate assimilation in the Southern Ocean and denitrification in the Arabian Sea. In the thermocline and surface, tropically sourced waters are biogeochemically distinct from adjacent subtropically sourced waters, confirming inhibited lateral mixing across the current core. (Sub)tropical thermocline nitrate δ15N is lower (4.9–5.8‰) than the sub‐thermocline source, Subantarctic Mode Water (6.9‰); we attribute this difference to local N2 fixation. Using a one‐box model to simulate the newly fixed nitrate flux, we estimate a local N2 fixation rate of 7–25 Tg N.a−1, with the upper limit likely biased high. In the mixed layer, nitrate δ15N and δ18O rise in unison, indicating that phytoplankton nitrate assimilation dominates in surface waters, with nitrification restricted to deeper waters. Because nitrate assimilation and nitrification are vertically decoupled, the rate of nitrate assimilation plus N2 fixation can be used to approximate carbon export. Thermocline and mixed‐layer nitrate Δ(15–18) is low, due to both N2 fixation and coupled partial nitrate assimilation and nitrification. Similarly low‐Δ(15–18) nitrate in Agulhas rings indicates leakage of low‐δ15N nitrogen into the South Atlantic, which should be recorded in the organic matter sinking to the seafloor, providing a potential tracer of past Agulhas leakage.

Orbital-scale thermocline temperature variability in the western equatorial Pacific during the last 370 kyr

Tropical subsurface circulation plays a key role in regulating heat and salt transport between the hemispheres, and in ocean–atmosphere interactions (e.g., El Niño-Southern Oscillation; ENSO). Therefore, understanding the past behavior of tropical thermoclines is important for predicting long-term global climate change. Here, we provide an orbital-scale thermocline temperature (TT) record based on Mg/Ca measurements of planktonic foraminifer Pulleniatina obliquiloculata in sediment cores obtained from the western equatorial Pacific (WEP) for the last 370 kyr. The vertical temperature gradient between sea surface temperature (SST) and TT in WEP showed weak positive correlations with the tropical Pacific zonal SST gradient in long-term trend and obliquity signal. The relationship of the two gradients does not coincide with modern ENSO variability, suggesting that the long-term TT variation depend primarily on another mechanism. Spectral analysis indicated that TT variability has strong signals in both the obliquity and precession bands. Since the obliquity signal is very weak in local insolation, the significant obliquity in TT variability implies an extratropical influence on tropical thermocline circulation. Therefore, our findings suggest that the subsurface circulation connecting the tropics and extratropics plays an essential role in orbital-scale tropical thermocline variability.

Simulation, precursor analysis and targeted observation sensitive area identification for two types of ENSO using ENSO-MC v1.0

Abstract. The global impact of an El Niño–Southern Oscillation (ENSO) event can differ greatly depending on whether it is an eastern Pacific (EP)-type event or a central Pacific (CP)-type event. Reliable predictions of the two types of ENSO are therefore of critical importance. Here we construct a deep neural network with multichannel structure for ENSO (named ENSO-MC) to simulate the spatial evolution of sea surface temperature (SST) anomalies for the two types of events. We select SST, heat content and wind stress (i.e., three key ingredients of Bjerknes feedback) to represent coupled ocean–atmosphere dynamics that underpin ENSO, achieving skilful forecasts for the spatial patterns of SST anomalies out to 1 year ahead. Furthermore, it is of great significance to analyse the precursors of EP-type or CP-type events and identify targeted observation sensitive areas for the understanding and prediction of ENSO. Precursors analysis is to determine what type of initial perturbations will develop into EP-type or CP-type events. Sensitive area identification is to determine the regions where initial states tend to have the greatest impacts on the evolution of ENSO. We use the saliency map method to investigate the subsurface precursors and identify the sensitive areas of ENSO. The results show that there are pronounced signals in the equatorial subsurface before EP events, while the precursory signals of CP events are located in the northern Pacific. It indicates that the subtropical precursors seem to favour the generation of the CP-type El Niño and that the EP-type El Niño is more related to the tropical thermocline dynamics. Furthermore, the saliency maps show that the sensitive areas of the surface and the subsurface are located in the equatorial central Pacific and the equatorial western Pacific respectively. The sensitivity experiments imply that additional observations in the identified sensitive areas can improve forecasting skills. Our results of precursors and sensitive areas are consistent with the previous theories of ENSO, demonstrating the potential usage and advantages of the ENSO-MC model in improving the simulation, understanding and observations of the two ENSO types.

Pliocene evolution of the tropical Atlantic thermocline depth

Abstract. It has been hypothesized that global temperature trends are tightly linked to tropical thermocline depth, and that thermocline shoaling played a crucial role in the intensification of late Pliocene Northern Hemisphere glaciation. The Pliocene thermocline evolution in the Pacific Ocean is well documented and supports this hypothesis, but thermocline records from the tropical Atlantic Ocean are limited. We present new planktonic foraminiferal Mg/Ca, δ18O, and δ13C records from the late Pliocene interval at Ocean Drilling Program Site 959 in the Eastern Equatorial Atlantic (EEA), which we use to reconstruct ocean temperatures and relative changes in salinity and thermocline depth. Data were generated using surface-dwelling Globigerinoides ruber and subsurface-dwelling Neogloboquadrina dutertrei. Reduced gradients between the surface and subsurface records indicate deepening of the EEA thermocline at the end of the mid-Piacenzian Warm Period (mPWP; ∼ 3.3–3.0 Ma). We connect our late Pliocene records to previously published early Pliocene δ18O data from Site 959 and compare these to the Site 1000 in the Caribbean Sea. Over the course of the Pliocene, thermocline changes in the EEA and Caribbean Sea follow similar patterns, with prominent step-wise thermocline deepening between ∼ 5.5 and 4.0 Ma and gradual shoaling up to the mPWP, followed by minor deepening at the end of the mPWP. The tropical thermocline depth evolution of the tropical Atlantic differs from the Pacific, which is characterized by gradual basin-wide shoaling across the Pliocene. These results potentially challenge the hypothesized link between tropical thermocline depth and global climate. The mechanisms behind the periodically divergent Pacific and Atlantic thermocline movements remain speculative. We suggest that they are related to basin geometry and heterogenous temperature evolutions in regions from where thermocline waters are sourced. A positive feedback loop between source region temperature and tropical cyclone activity may have amplified tropical thermocline adjustments.

Tropical and Subtropical Pacific Sources of the Asymmetric El Niño‐La Niña Decay and Their Future Changes

AbstractEl Niños (EN) are known to decay more rapidly, while La Niñas (LN) tend to decay more slowly. Observational analyses and coupled model experiments are conducted to show that sea surface temperature (SST) anomalies over the subtropical northeastern Pacific (SNEP) and equatorial western Pacific (EWP) are key factors to determine the decay pace of the EN and LN and their asymmetry. In the present climate the LN produces larger cold SST anomalies over the regions than the warm SST anomalies produced by the EN. The magnitude difference over the SNEP and EWP helps to slow down the LN decay via subtropical footprinting and tropical thermocline variation mechanisms, respectively. Coupled Model Intercomparison Project Phase 6 models project the magnitude differences of SNEP SST anomalies between EN and LN to reduce in the future warming world, causing the asymmetric EN‐LN decay to weaken.

Sensitivity of Global Ocean Deoxygenation to Vertical and Isopycnal Mixing in an Ocean Biogeochemistry Model

AbstractLarge‐scale loss of oxygen under global warming is termed “ocean deoxygenation” and is caused by the imbalance between physical supply and biological consumption of oxygen in the ocean interior. Significant progress has been made in the theoretical understanding of ocean deoxygenation; however, many questions remain unresolved. The oxygen change in the tropical thermocline is poorly understood, with diverging projections among different models. Physical oxygen supply is controlled by a suite of processes that transport oxygen‐rich surface waters into the interior ocean, which is expected to weaken due to increasing stratification under global warming. Using a numerical model and a series of sensitivity experiments, the role of ocean mixing is examined in terms of effects on the mean state and the response to a transient warming. Both vertical and horizontal (isopycnal) mixing coefficients are systematically varied over a wide range, and the resulting oxygen distributions in equilibrated and transient simulations are examined. The spatial patterns of oxygen loss are sensitive to both vertical and isopycnal mixing, and the sign of tropical oxygen trend under climate warming can reverse depending on the choice of mixing parameters. An elevated level of isopycnal mixing disrupts the vertical advective‐diffusive balance of the tropical thermocline, increasing the mean state oxygen as well as the magnitude of the transient oxygen decline. These results provide first‐order explanations for the diverging behaviors of simulated tropical oxygen with respect to ocean mixing parameters.

A Shallow Thermocline Bias in the Southern Tropical Pacific in CMIP5/6 Models Linked to Double‐ITCZ Bias

AbstractA basin‐wide strong shallow bias in the thermocline of the southern tropical Pacific is found in an ensemble of models from the coupled model intercomparison project phases 5 and 6 (CMIP5/6). The bias is present in over 90% of models examined and is equally represented in CMIP5 and CMIP6 models. In contrast to observations, where the southern thermocline is far deeper than its northern counterpart, models have a hemispherically symmetric tropical thermocline. The shallow thermocline bias is closely linked to the well known double intertropical convergence zone (ITCZ) bias; more so for the physical thermocline (i.e., depth of maximal vertical thermal gradient) than the commonly used C isotherm thermocline proxy. A shallow thermocline bias is further found to be associated with climatic conditions including wider separation of double ITCZ peaks, a pronounced cold tongue, and a spurious south equatorial counter current.

The Geography of Numerical Mixing in a Suite of Global Ocean Models

AbstractNumerical mixing, defined here as the physically spurious tracer diffusion due to the numerical discretization of advection, is known to contribute to biases in ocean models. However, quantifying numerical mixing is nontrivial, with most studies utilizing targeted experiments in idealized settings. Here, we present a water mass transformation‐based method for quantifying numerical mixing that can be applied to any conserved variable in general circulation models. Furthermore, the method can be applied within individual fluid columns to provide spatial information. We apply the method to a suite of global ocean model simulations with differing grid spacings and subgrid‐scale parameterizations. In all configurations numerical mixing drives diathermal heat transport of comparable magnitude to that associated with explicit parameterizations. Numerical mixing is prominent in the tropical thermocline, where it is sensitive to the vertical diffusivity and resolution. At colder temperatures numerical mixing is sensitive to the presence of explicit neutral diffusion, suggesting that it may act as a proxy for neutral diffusion when it is explicitly absent. Comparison of otherwise equivalent 1/4° and 1/10° configurations with grid‐scale dependent horizontal viscosity shows only a modest enhancement in numerical mixing at 1/4°. However, if the lateral viscosity is maintained while resolution is increased then numerical mixing is reduced by almost 35%. This result suggests that the common practice of reducing viscosity in order to maximize permitted variability must be considered carefully. Our results provide a detailed view of numerical mixing in ocean models and pave the way for improvements in parameter choices and numerical methods.

The Thermocline Biases in the Tropical North Pacific and Their Attributions

AbstractThe tropical thermocline plays an important role in regulating equatorial sea surface temperature (SST); at present, it is still poorly simulated in the state-of-the-art climate models. In this paper, thermocline biases in the tropical North Pacific are investigated using the newly released CMIP6 historical simulations. It is found that CMIP6 models tend to produce an overly shallow thermocline in the northwestern tropics, accompanied by a deep thermocline in the northeastern tropics. A pronounced thermocline strength bias arises in the tropical northeastern Pacific, demonstrating a dipole structure with a sign change at about 8°N. These thermocline biases are accompanied with biases in the simulations of oceanic circulations, including a too weak North Equatorial Countercurrent (NECC), a reduction in water exchanges between the subtropics and the equatorial regions, and an eastward extension of the equatorward interior water transport. The causes of these thermocline biases are further analyzed. The thermocline bias is primarily caused by the model deficiency in simulating the surface wind stress curl, which can be further attributed to the longstanding double-ITCZ bias in the tropical North Pacific. Besides, thermocline strength bias can be partly attributed to the poor prescription of oceanic background diffusivity. By constraining the diffusivity to match observations, the thermocline strength in the tropical northeastern Pacific is greatly increased.

Half-precessional cycle of thermocline temperature in the western equatorial Pacific and its bihemispheric dynamics

The El Niño-Southern Oscillation (ENSO), which is tightly coupled to the equatorial thermocline in the Pacific, is the dominant source of interannual climate variability, but its long-term evolution in response to climate change remains highly uncertain. This study uses Mg/Ca in planktonic foraminiferal shells to reconstruct sea surface and thermocline water temperatures (SST and TWT) for the past 142 ky in a western equatorial Pacific (WEP) core MD01-2386. Unlike the dominant 100-ky glacial-interglacial cycle recorded by SST and δ18O, which echoes the pattern seen in other WEP sites, the upper ocean thermal gradient shows a clear half-precessional (9.4 ky or 12.7 ky) cycle as indicated by the reconstructed and simulated temperature (ΔT) and δ18O differences between the surface and thermocline waters. This phenomenon is attributed to the interplay of subtropical-to-tropical thermocline anomalies forced by the antiphased meridional insolation gradients in the two hemispheres at the precessional band. In particular, the TWT shows greater variability than SST, and dominates the ΔT changes which couple with the west-east SST difference in the equatorial Pacific at the half-precessional band, implying a decisive role of the tropical thermocline in orbital-scale climate change.

The Indian Ocean Deep Meridional Overturning Circulation in Three Ocean Reanalysis Products

AbstractThe time mean Indian Ocean (IO) deep meridional overturning circulation (MOC) is compared across three ocean reanalysis products (ORAS4, GECCO2, and GFDL). The MOC stream functions obtained by vertically integrating the mass flux across a latitude‐depth section in three products are found to be significantly different from each other. Detailed analysis suggests that ORAS4 delivers the best depiction of IO MOC. The inferred IO deep MOC consists of two deep and strong counterclockwise cells located south of 30°S and around 10°S, respectively. The geostrophic component along with the barotropic or external mode dominates the former, and a combination of Ekman and geostrophic components dominates the latter. GECCO2 depicts a steady decline in the northward meridional transport in the bottom layer and a consequent reduction in the MOC strength. The tropical thermocline in GECCO2 responds to this MOC variability leading to rapid and monotonic warming of the tropical IO.

Marine N2O emissions during a Younger Dryas-like event: the role of meridional overturning, tropical thermocline ventilation, and biological productivity

Past variations in atmospheric nitrous oxide (N2O) allow important insight into abrupt climate events. Here, we investigate marine N2O emissions by forcing the Bern3D Earth System Model of Intermediate Complexity with freshwater into the North Atlantic. The model simulates a decrease in marine N2O emissions of about 0.8 TgN yr−1 followed by a recovery, in reasonable agreement regarding timing and magnitude with isotope-based reconstructions of marine emissions for the Younger Dryas Northern Hemisphere cold event. In the model the freshwater forcing causes a transient near-collapse of the Atlantic Meridional Overturning Circulation (AMOC) leading to a fast adjustment in thermocline ventilation and an increase in O2 in tropical eastern boundary systems and in the tropical Indian Ocean. In turn, net production by nitrification and denitrification and N2O emissions decrease in these regions. The decrease in organic matter export, mainly in the North Atlantic where ventilation and nutrient supply is suppressed, explains the remaining emission reduction. Modeled global marine N2O production and emission changes are delayed, initially by up to 300 years, relative to the AMOC decrease, but by less than 50 years at peak decline. The N2O perturbation is recovering only slowly and the lag between the recovery in AMOC and the recovery in N2O emissions and atmospheric concentrations exceeds 400 years. Thus, our results suggest a century-scale lag between ocean circulation and marine N2O emissions, and a tight coupling between changes in AMOC and tropical thermocline ventilation.

Assessment of time of emergence of anthropogenic deoxygenation and warming: insights from a CESM simulation from 850 to 2100 CE

Abstract. Marine deoxygenation and anthropogenic ocean warming are observed and projected to intensify in the future. These changes potentially impact the functions and services of marine ecosystems. A key question is whether marine ecosystems are already or will soon be exposed to environmental conditions not experienced during the last millennium. Using a forced simulation with the Community Earth System Model (CESM) over the period 850 to 2100, we find that anthropogenic deoxygenation and warming in the thermocline exceeded natural variability in, respectively, 60 % and 90 % of total ocean area. Control simulations are typically used to estimate the pre-industrial variability level. However, the natural variability of oxygen (O2) and temperature (T) inferred from the last millennium period is systematically larger than the internal variability simulated in the corresponding control simulation. This renders natural variability from control simulations to be biased towards low estimates. Here, natural variability is assessed from the last millennium period (850–1800 CE), thus considering the response to forcing from explosive volcanic eruptions, solar irradiance and greenhouse gases in addition to internal, chaotic variability. Results suggest that in the tropical thermocline, where biological and solubility-driven O2 changes counteract each other, anthropogenic changes in apparent oxygen utilisation (AOU) and in O2 solubility (O2,sol) are detectable earlier than O2 changes. Both natural variability and change in AOU are predominantly driven by variations in circulation with a smaller role for productivity. By the end of the 21st century, ventilation becomes more vigorous in the tropical thermocline, whereas ideal age in deep waters increases by more than 200 years relative to the pre-industrial period. Different methodological choices are compared and the time for an anthropogenic signal to emerge (ToE) is earlier in many thermocline regions when using variability from a shorter period, from the control simulation or estimates from the industrial period instead of the variability from the last millennium. Our results highlight that published methods may lead to deviations in ToE estimates, calling for a careful quantification of variability. They also highlight that realised anthropogenic change exceeds natural variations in many regions.

Upwelling in the Ocean Basins North of the ACC: 2. How Cool Subantarctic Water Reaches the Surface in the Tropics

AbstractLarge volumes of cool water are drawn up to the surface in the tropical oceans. A companion paper shows that the cool water reaches the surface in or near the upwelling zones off northern and southern Africa and Peru. The cool water has a subantarctic origin and spreads extensively across the Atlantic and Pacific basins after it reaches the surface. Here, we look at the spreading in two low‐resolution ocean general circulation models and find that the spreading in the models is much less extensive than observed. The problem seems to be the way the upwelling and the spreading are connected (or not connected) to the ocean's large‐scale overturning. As proposed here, the cool upwelling develops when warm buoyant water in the western tropics is drawn away to become deep water in the North Atlantic. The “drawing away” shoals the tropical thermocline in a way that allows cool subantarctic water to be drawn up to the surface along the eastern margins. The amounts of upwelling produced this way exceed the amounts generated by the winds in the upwelling zones by as much as 4 times. Flow restrictions make it difficult for the warm buoyant water in our models to be drawn away.

Projected Centennial Oxygen Trends and Their Attribution to Distinct Ocean Climate Forcings

AbstractWe explore centennial changes in tropical Pacific oxygen (O2) using numerical models to illustrate the dominant patterns and mechanisms under centennial climate change. Future projections from state‐of‐the‐art Earth System Models exhibit significant model to model differences, but decreased solubility and weakened ventilation together deplete thermocline O2 in middle to high latitudes. In contrast, the tropical thermocline O2 undergoes much smaller changes or even a slight increase. A suite of sensitivity experiments using a coarse resolution ocean circulation and biogeochemistry model show that ocean warming is the leading cause of global deoxygenation in the thermocline across all latitudes with secondary contributions from changes in hydrological cycles and wind stress modulating regional changes in O2. The small O2 changes in the tropical Pacific thermocline reflect near‐complete compensation between the solubility decrease due to warming and reduction in apparent oxygen utilization (AOU). We further quantified the changes in AOU due to contributions from changes in water mass age and biological remineralization from the sensitivity experiments. The two effects almost equally contribute to the reduction of AOU in the tropical Pacific thermocline (43% for physical circulations and 57% for biology). Our results suggest that better understanding of water mass changes in the tropical oceans is key to improving projections and reducing the uncertainties of future O2 changes.

Thermocline state change in the eastern equatorial Pacific during the late Pliocene/early Pleistocene intensification of Northern Hemisphere glaciation

Abstract. The late Pliocene/early Pleistocene intensification of Northern Hemisphere glaciation (iNHG) ∼2.5 million years ago (marine isotope stages, MIS, 100–96) stands out as an important tipping point in Earth's climate history, which strongly influenced oceanographic and climatic patterns including trade wind and upwelling strength in the eastern equatorial Pacific (EEP). The thermocline depth in the EEP, in turn, plays a pivotal role in the Earth's climate system: small changes in its depth associated with short-term climate phenomena such as the El Niño–Southern Oscillation can affect surface-water properties and the ocean–atmosphere exchange. However, thermocline dynamics in the EEP during the iNHG still remain unclear. While numerous studies have suggested a link between a thermocline shoaling in the EEP and Northern Hemisphere ice growth, other studies have indicated a stable thermocline depth during the iNHG; consequently, a causal relationship between thermocline dynamics and ice-sheet growth has been excluded. In light of these contradictory views, we have generated geochemical (planktic foraminiferal δ18O, δ13C and Mg ∕ Ca), sedimentological (sand accumulation rates) and faunal (abundance data of thermocline-dwelling foraminifera) records for Ocean Drilling Program Site 849 located in the central region of the EEP. Our records span the interval from ∼2.75 to 2.4 Ma (MIS G7–95), which is critical for understanding thermocline dynamics during the final phase of the iNHG. Our new records document a thermocline shoaling from ∼2.64 to 2.55 Ma (MIS G2–101) and a relatively shallow thermocline from ∼2.55 Ma onwards (MIS 101–95). This indicates a state change in thermocline depth at Site 849 shortly before the final phase of the iNHG. Ultimately, our data support the hypothesis that (sub-)tropical thermocline shoaling may have contributed to the development of large Northern Hemisphere ice sheets.

Patterns of deoxygenation: sensitivity to natural and anthropogenic drivers.

Observational estimates and numerical models both indicate a significant overall decline in marine oxygen levels over the past few decades. Spatial patterns of oxygen change, however, differ considerably between observed and modelled estimates. Particularly in the tropical thermocline that hosts open-ocean oxygen minimum zones, observations indicate a general oxygen decline, whereas most of the state-of-the-art models simulate increasing oxygen levels. Possible reasons for the apparent model-data discrepancies are examined. In order to attribute observed historical variations in oxygen levels, we here study mechanisms of changes in oxygen supply and consumption with sensitivity model simulations. Specifically, the role of equatorial jets, of lateral and diapycnal mixing processes, of changes in the wind-driven circulation and atmospheric nutrient supply, and of some poorly constrained biogeochemical processes are investigated. Predominantly wind-driven changes in the low-latitude oceanic ventilation are identified as a possible factor contributing to observed oxygen changes in the low-latitude thermocline during the past decades, while the potential role of biogeochemical processes remains difficult to constrain. We discuss implications for the attribution of observed oxygen changes to anthropogenic impacts and research priorities that may help to improve our mechanistic understanding of oxygen changes and the quality of projections into a changing future.This article is part of the themed issue 'Ocean ventilation and deoxygenation in a warming world'.

A long history of equatorial deep-water upwelling in the Pacific Ocean

Cold, nutrient- and CO2-rich waters upwelling in the eastern equatorial Pacific (EEP) give rise to the Pacific cold tongue. Quasi-periodic subsidence of the thermocline and attenuation in wind strength expressed by El Niño conditions decrease upwelling rates, increase surface-water temperatures in the EEP, and lead to changes in regional climates both near and far from the equatorial Pacific. EEP surface waters have elevated CO2 concentrations during neutral (upwelling) or La Niña (strong upwelling) conditions. In contrast, approximate air–sea CO2 equilibrium characterizes El Niño events. One hypothesis proposes that changes in physical oceanography led to the establishment of a deep tropical thermocline and expanded mixed-layer prior to 3 million years ago. These effects are argued to have substantially reduced deep-water upwelling rates in the EEP and promoted a “permanent El Niño-like” climate state. For this study, we test this supposition by reconstructing EEP “excess CO2” and upwelling history for the past 6.5 million years using the alkenone-pCO2 methodology. Contrary to previous assertions, our results indicate that average temporal conditions in the EEP over the past ∼6.5 million years were characterized by substantial CO2 disequilibrium and high nutrient delivery to surface waters — characteristics that imply strong upwelling of deep waters. Upwelling appears most vigorous between ∼6.5 to 4.5 million years ago coinciding with high accumulation rates of biogenic material during the late Miocene – early Pliocene “biogenic bloom”.