Tropical Pacific climate variability under solar geoengineering: impacts on ENSO extremes

Many modelling studies suggest that the El Niño-Southern Oscillation (ENSO), in interaction with the tropical Pacific background climate, will change with rising atmospheric greenhouse gas concentrations. Solar geoengineering (reducing the solar flux from outer space) has been proposed as a means to counteract anthropogenic climate change. However, the effectiveness of solar geoengineering concerning a variety of aspects of Earth's climate is uncertain. Robust results are particularly challenging to obtain for ENSO because existing geoengineering simulations are too short (typically ∼ 50 years) to detect statistically significant changes in the highly variable tropical Pacific background climate. We here present results from a 1000-year-long solar-geoengineering simulation, G1, carried out with the coupled atmosphere-ocean general circulation model HadCM3L. In agreement with previous studies, reducing the solar irradiance (4 %) to offset global mean surface warming in the model more than compensates the warming in the tropical Pacific that develops in the 4 × CO2 scenario. We see an overcooling of 0.3 °C and a 0.23 mm d-1 (5 %) reduction in mean rainfall over the tropical Pacific relative to preindustrial conditions in the G1 simulation, owing to the different latitudinal distributions of the shortwave (solar) and longwave (CO2) forcings. The location of the Intertropical Convergence Zone (ITCZ) in the tropical Pacific, which moved 7.5° southwards under 4 × CO2, is restored to its preindustrial position. However, other aspects of the tropical Pacific mean climate are not reset as effectively. Relative to preindustrial conditions, in G1 the time-averaged zonal wind stress, zonal sea surface temperature (SST) gradient, and meridional SST gradient are each statistically significantly reduced by around 10 %, and the Pacific Walker Circulation (PWC) is consistently weakened, resulting in conditions conducive to increased frequency of El Niño events. The overall amplitude of ENSO strengthens by 9 %-10 % in G1, but there is a 65 % reduction in the asymmetry between cold and warm events: cold events intensify more than warm events. Notably, the frequency of extreme El Niño and La Niña events increases by ca. 60 % and 30 %, respectively, while the total number of El Niño events increases by around 10 %. All of these changes are statistically significant at either 95 or 99 % confidence level. Somewhat paradoxically, while the number of total and extreme events increases, the extreme El Niño events become weaker relative to the preindustrial state, while the extreme La Niña events become even stronger. That is, such extreme El Niño events in G1 become less intense than under preindustrial conditions but also more frequent. In contrast, extreme La Niña events become stronger in G1, which is in agreement with the general overcooling of the tropical Pacific in G1 relative to preindustrial conditions

Rare earth elements (REEs) in the tropical South Atlantic and quantitative deconvolution of their non-conservative behavior

This study presents new concentration measurements of dissolved rare earth elements (dREEs) along a full-depth east–west section across the tropical South Atlantic (∼12°S), and uses these data to investigate the oceanic cycling of the REEs. Enrichment of dREEs, associated with the redox cycling of Fe–Mn oxides, is observed in the oxygen minimum zone (OMZ) off the African shelf. For deeper-waters, a multi-parameter mixing model was developed to deconvolve the relative importance of physical transport (i.e., water mass mixing) from biogeochemical controls on the dREE distribution in the deep Atlantic. This approach enables chemical processes involved in REE cycling, not apparent from the measurements alone, to be distinguished and quantified. Results show that the measured dREE concentrations below ∼1000 m are dominantly controlled (>75%) by preformed REE concentrations resulting from water mass mixing. This result indicates that the linear correlation between dREEs and dissolved Si observed in Atlantic deep waters results from the dominantly conservative behavior of these tracers, rather than from similar chemical processes influencing both dREEs and Si. Minor addition of dREEs (∼10% of dNd and ∼5% of dYb) is observed in the deep (>∼4000 m) Brazil Basin, resulting from either remineralization of particles in-situ or along the flow path. Greater addition of dREEs (up to 25% for dNd and 20% for dYb) is found at ∼1500 m and below ∼4000 m in the Angola Basin near the African continental margin. Cerium anomalies suggest that different sources are responsible for these dREE addition plumes. The 1500 m excess is most likely attributed to dREE release from Fe oxides, whereas the 4000 m excess may be due to remineralization of calcite. Higher particulate fluxes and a more sluggish ocean circulation in the Angola Basin may explain why the dREE excesses in this basin are significantly higher than that observed in the Brazil Basin. Hydrothermal venting over the mid-Atlantic ridge acts as a regional net sink for light REEs, but has little influence on the net budget of heavy REEs. The combination of dense REE measurements with water mass deconvolution is shown to provide quantitative assessment of the relative roles of physical and biogeochemical processes in the oceanic cycling of REEs

The formation of the ocean’s anthropogenic carbon reservoir

The shallow overturning circulation of the oceans transports heat from the tropics to the mid-latitudes. This overturning also influences the uptake and storage of anthropogenic carbon (Cant). We demonstrate this by quantifying the relative importance of ocean thermodynamics, circulation and biogeochemistry in a global biochemistry and circulation model. Almost 2/3 of the Cant ocean uptake enters via gas exchange in waters that are lighter than the base of the ventilated thermocline. However, almost 2/3 of the excess Cant is stored below the thermocline. Our analysis shows that subtropical waters are a dominant component in the formation of subpolar waters and that these water masses essentially form a common Cant reservoir. This new method developed and presented here is intrinsically Lagrangian, as it by construction only considers the velocity or transport of waters across isopycnals. More generally, our approach provides an integral framework for linking ocean thermodynamics with biogeochemistry

Particle-seawater interaction of neodymium in the North Atlantic

Dissolved neodymium (Nd) isotopes (expressed as εNd) have been widely used as a water mass tracer in paleoceanography. However, one aspect of the modern biogeochemical cycle of Nd that has been sparsely investigated is the interplay between dissolved and particulate phases in seawater. We here present the first regional data set on particulate Nd isotope compositions (εNdp) and concentrations ([Nd]p) from five stations in the western North Atlantic Ocean along the GEOTRACES GA02 transect, in conjunction with previously published dissolved Nd isotope compositions (εNdd) and concentrations ([Nd]d)1. Key observations and interpretations from our new particulate data set include the following: (1) A low fractional contributions of [Nd]p to the total Nd inventory per volume unit of seawater (~5%), with significant increases of up to 45% in benthic boundary layers. (2) Increasing Nd concentrations in suspended particulate matter ([Nd]SPM) and fractions of lithogenic material with water depth, suggesting the removal of Nd poor phases. (3) Different provenances of particulates in the subpolar and subtropical gyres as evidenced by their Nd isotope fingerprints reaching from εNdp ≈ -20 near the Labrador Basin (old continental crust), over εNdp ≈ -4 between Iceland and Greenland (young mafic provenance), to values of εNdp ≈-13 in the subtropics (similar to African dust signal). (4) Vertical heterogeneity of εNdp, as well as large deviations from ambient seawater values in the subpolar gyre, indicate advection of lithogenic particles in this area. (5) Vertically homogenous εNdp values in the subtropical gyre, indistinguishable from εNdd values, are indicative of predominance of vertical particulate supply. The process of reversible scavenging only seems to influence particulate signatures below 3 km. Overall, we do not find evidence on enhanced particle dissolution, often invoked to explain the observed increase in dissolved Nd in the North Atlantic

The large-scale evolution of neodymium isotopic composition in the global modern and Holocene ocean revealed from seawater and archive data

Neodymium isotopic compositions (143Nd/144Nd or εNd) have been used as a tracer of water masses and lithogenic inputs to the ocean. To further evaluate the faithfulness of this tracer, we have updated a global seawater εNd database and combined it with hydrography parameters (temperature, salinity, nutrients and oxygen concentrations), carbon isotopic ratio and radiocarbon of dissolved inorganic carbon. Archive εNd data are also compiled for leachates, foraminiferal tests, deep-sea corals and fish teeth/debris from the Holocene period (< 10,000 years). At water depths ≥ 1500 m, property-property plots show clear correlations between seawater εNd and the other variables, suggesting that large-scale water mass mixing is a primary control of deepwater εNd distribution. At ≥ 200 m, basin-scale seawater T-S-εNd diagrams demonstrate the isotopic evolution of different water masses. Seawater and archive εNd values are compared using property-property plots and T-S-εNd diagrams. Archive values generally agree with corresponding seawater values although they tend to be at the upper limit in the Pacific. Both positive and negative offsets exist in the northern North Atlantic. Applying multiple regression analysis to deep (≥ 1500 m) seawater data, we established empirical equations that predict the main, large-scale, deepwater εNd trends from hydrography parameters. Large offsets from the predicted values are interpreted as a sign of significant local/regional influence. Dominant continental influence on seawater and archive εNd is observed mainly within 1000 km from the continents. Generally, seawater and archive εNd values form gradual latitudinal trend in the Atlantic and Pacific at depths ≥ 600 m, consistent with the idea that Nd isotopes help distinguish between northern/southern sourced water contributions at intermediate and deep water depths

A study of the ocean's water masses using data and models

Water masses are collections of water parcels that share similar histories and fate. The validity and value of the concept of a water mass depends on scale, spatial and temporal, as well as purpose. In spite of pragmatic and conceptual limitations, this dissertation shows that the treatment of the ocean as composed of a finite set of water masses can provide a practical analytical framework in which to describe aspects of the ocean's physical and biogeochemical structure. In particular, the case is made that, given the current biases of ocean models, model diagnostics developed in a water mass reference frame are useful tools to identify, understand and attribute causes for the specific biases. Water masses also provide an intuitive basis for analyzing the response of ocean models to many types of perturbations, be they numerical experiments or analogues of real climatic phenomena. This dissertation is divided in three parts. The first part provides a detailed account of the ocean's water mass system. The second part utilizes the information gathered in part I to develop an analytical framework that is used to evaluate global coupled model simulations. Finally, part III investigates a methodology designed to account for the confounding role of natural variability for the purpose of measuring the change in the oceanic anthropogenic carbon distribution from repeat hydrography sampling programs. Part I A synthesis of the ocean's major water masses is presented that compares and contrasts the water masses of the Northern and Southern polar, subpolar and subtropical regions. The world's most prominent bottom waters, deep waters, intermediate waters and central waters are reviewed sequentially, including a discussion of formation mechanisms, thermohaline and biogeochemical properties, variability, formation rates and their role in ocean circulation or climate. Water masses originating at high polar latitudes are discussed first, with a comparative description of the freshwater and sea ice conditions in the Arctic and Antarctic regions. Low salinity intermediate waters from subpolar regions are described next, followed by high salinity intermediate waters that originate from evaporation basins. The central waters that ventilate the thermocline are discussed last, including a presentation of the five types of mode waters. One recurrent theme of the water mass synthesis presented here is that overflows, entrainment at the overflows, water mass formation and export is often impacted critically by small bathymetric features, such as the Maud Rise in the Weddell Sea, the Orphan Knoll in the Labrador Sea, the depth and width of continental shelves or the presence of canyons cutting through the shelf break. Mode and intermediate water formation are critically dependent on the winds and on the cycling of the mixed layer. Some water masses, such as Eastern Subtropical Mode Waters in the North Pacific, can also depend critically on peculiarities of the seasonal cycle of the buoyancy flux, making these waters an interesting potential early warning diagnostic for climate change. As water mass formation often requires cold winter air bursts, small zonal or meridional shifts in the winds can have a dramatic impact on the formation characteristics of water masses. Part II While global coupled climate models do, for the most part, resolve the main structure of the ocean, that is, models possess bottom waters, deep waters, intermediate waters and central waters, global climate models vary greatly in their ability to resolve the water masses regionally. The temperature and salinity distribution simulated by models also show significant biases. Water masses, although difficult to define quantitatively, can provide a useful framework to evaluate and discuss model simulations and biases. This portion of the dissertation is composed of two chapters. Chapter 3 describes the structure of the overturning streamfunction in a set of global coupled climate models (Climate Model Inter-comparison Project 3, CMIP3). The main result of this chapter indicates that the different strengths of the meridional overturning circulation of the North Atlantic cell in the CMIP3 model set are mostly due to model parameterization issues associated with effective diapycnal mixing or viscosity. In the Southern Ocean, winds take on additional importance. A tight relationship is in fact found between the strength of the abyssal cell and wind intensity in the Southern Ocean across the CMIP3 models. Chapter 4 addresses the effects of stratification. This chapter looks for relationships between specific hydrographic measures of stratification and aspects of the circulation. This study relies on the information presented in part I to analyze the water mass system simulated by global coupled circulation models. Using classical (volumetric T/S analysis, core layers) water mass analysis techniques, the model experiments are compared to the data and to one-another, revealing systematic relationships between model biases found among the CMIP3 models and these models' circulations. It is found that temperature and salinity biases result from differences in the model's circulations more than the differences in the models' circulation depend on the biases of the thermohaline properties. Part III The third section of the dissertation utilizes the concept of water mass more indirectly: as a means to evaluate and select linear regression models. This chapter focuses on the use of statistical regression methods to isolate the anthropogenic carbon decadal change signal from hydrographic data. A synthetic data set derived from ocean biogeochemistry and circulation model simulations, in which the true signals are known, is used as a simulator to optimize algorithms and evaluate the accuracy of the particular methods in their ability to recover the true estimate of the change in the interior anthropogenic carbon concentration. It is shown that regression analysis on repeat hydrographic data sets such as WOCE and CLIVAR can account for water mass variance and short-term variability, and estimate the true 10-year change in anthropogenic carbon inventory with a better than 10% accuracy. More importantly, this chapter outlines a strategy that can be used to select appropriate regression models in the context of non-ideal repeat hydrographic samples

Properties, sensitivity, and stability of the Southern Hemisphere salinity minimum layer in the UKESM1 model

Antarctic Intermediate Water (AAIW) is a water mass originating in the Southern Ocean characterised by its low salinity. The properties of the salinity minimum layer that characterise AAIW in the CMIP6 UKESM1 model and its response to different climate change scenarios are investigated. In UKESM1, the depth of the salinity minimum shoals by 116 m in the SSP5-8.5 run compared to the control run by 2080–2100. The salinity minimum also gets warmer (+ 1.9 °C) and lighter (− 0.4 kg/m3) and surface properties where the salinity minimum outcrops warm, freshen and lighten in all scenarios. In spite of these expected changes in properties, the location where the salinity minimum outcrops does not change in any of the future scenarios. The stability of the outcrop location of the salinity minimum is linked to the relative stability of the position of the Antarctic Circumpolar Current (ACC) in UKESM1. The position of the ACC does not follow the maximum wind stress trend, which intensifies and shifts poleward under radiative forcing. Changes in surface buoyancy fluxes in the region are consistent with the changes in hydrographic properties observed at depth on the salinity minimum mentioned above. However, transformation rates at the density corresponding to the salinity minimum outcrop remain constant in all scenarios. Stability in transformation rates at that density is due to the haline and thermal contributions counteracting one another. This analysis identifies two features (outcrop location, transformation rate) associated with the salinity minimum defining AAIW that show remarkable stability in an otherwise changing world. The effect of model resolution and other parameterisations on these findings have yet to be evaluated

Emergence of deep convection in the Arctic Ocean under a warming climate

International audienceThe appearance of winter deep mixed layers in the Arctic Ocean under a warming climate is investigated with the HiGEM coupled global climate model. In response to a four times increase of atmospheric CO_2 levels with respect to present day conditions, the Arctic Basin becomes seasonally ice-free. Its surface becomes consequently warmer and, on average, slightly fresher. Locally, changes in surface salinity can be far larger (up to 4 psu) than the basin-scale average, and of a different sign. The Canadian Basin undergoes a strong freshening, while the Eurasian Basin undergoes strong salinification. These changes are driven by the spin up of the surface circulation, likely resulting from the increased transfer of momentum to the ocean as sea ice cover is reduced. Changes in the surface salinity field also result in a change in stratification, which is strongly enhanced in the Canadian Basin and reduced in the Eurasian Basin. Reduction, or even suppression, of the stratification in the Eurasian Basin produces an environment that is favourable for, and promotes the appearance of, deep convection near the sea ice edge, leading to a significant deepening of winter mixed layers in this region (down to 1000 m). As the Arctic Ocean is transitioning toward a summer ice-free regime, new dynamical ocean processes will appear in the region, with potentially important consequences for the Arctic Ocean itself and for climate, both locally and on larger scales