Tropical Upwelling Region Research Articles

New Estimate of Organic Carbon Export From Optical Measurements Reveals the Role of Particle Size Distribution and Export Horizon

AbstractExport of sinking particles from the surface ocean is critical for carbon sequestration and to provide energy to the deep biosphere. The magnitude and spatial patterns of this export have been estimated in the past by in situ particle flux observations, satellite‐based algorithms, and ocean biogeochemical models; however, these estimates remain uncertain. Here, we use a recent machine learning reconstruction of global ocean particle size distributions (PSDs) from Underwater Vision Profiler 5 measurements to estimate carbon fluxes by sinking particles (35 μm–5 mm equivalent spherical diameter) from the surface ocean. We combine global maps of PSD properties with empirical relationships constrained against in situ flux observations to calculate particulate carbon export from the euphotic zone (5.8 ± 0.1 Pg C y−1) and annual maximum mixed layer depths (6.1 ± 0.1 Pg C y−1). The new flux reconstructions suggest a less variable seasonal cycle in the tropical ocean and a more persistent export in the Southern Ocean than previously recognized. Smaller particles (less than 418 μm) contribute most of the flux globally, while larger particles become more important at high latitudes and in tropical upwelling regions. Export from the annual maximum mixed layer exceeds that from the euphotic zone over most of the low‐latitude ocean, suggesting shallow particle recycling and net heterotrophy in the deep euphotic zone. These estimates open the way to fully three‐dimensional global reconstructions of particle fluxes in the ocean, supported by the growing database of in situ optical observations.

Fates of Paleo‐Antarctic Bottom Water During the Early Eocene: Based on a Lagrangian Analysis of IPSL‐CM5A2 Climate Model Simulations

AbstractBoth deepwater formation and the obduction processes converting dense deep water to lighter surface water are the engine for the global meridional overturning circulation (MOC). Their spatio‐temporal variations effectively modify the ocean circulation and related carbon cycle, which affects climate evolution throughout geological time. Using early‐Eocene bathymetry and enhanced atmospheric CO2 concentration, the IPSL‐CM5A2 climate model has simulated a well‐ventilated Southern Ocean associated with a strong anticlockwise MOC. To trace the fates of these paleo‐Antarctic Bottom Water (paleo‐AABW), we conducted Lagrangian analyses using these IPSL‐CM5A2 model results and tracking virtual particles released at the lower limb of the MOC, defined as an initial section at 60°S below 1,900 m depth. Diagnostic analysis of these particles trajectories reveals that most paleo‐AABW circulates back to the Southern Ocean through either the initial section (43%) or the section above (31%), the remaining (>25%) crossing the base of the mixed layer mostly in tropical regions (up to half). The majority of water parcels ending in the mixed layer experience negative density transformations, intensified in the upper 500 m and mostly occurring in tropical upwelling regions, with a spatial pattern consistent with the wind‐driven Ekman pumping, largely determined by the Eocene wind stress and continental geometry. In the same way as present‐day North Atlantic Deep Water upwells in the Southern Ocean, our results suggest that the strong tropical and equatorial upwelling during the Eocene provides an efficient pathway from the abyss to the surface, but at much higher temperature, with potential implications for the oceanic carbon cycle.

Estimation of Physiological Factors Controlling Carbon Isotope Fractionation in Coccolithophores in Photic Zone and Core‐Top Samples

AbstractEstimates of past pCO2 prior to ice‐core records have often been sought by measuring carbon isotopic fractionation during photosynthesis (Ɛp) in alkenone‐producing algae present in the fossil record. Photic zone and core‐top Ɛp show strong latitudinal gradients, which have been interpreted as an effect of growth rate. The factor b, used in previous studies to account for the changes in growth rate, shows high dependence on [CO2]aq in culture experiments, making difficult the deconvolution of nutrients versus other effects in the field. We combine coccolithophore growth rate output from a global model, limited by sea surface temperature (SST) and [PO43−] with a statistical model that include [CO2]aq, cell size and light to predict Ɛp, and examine the influence of different environmental and physiological variables on the observed expression of Ɛp in field samples. Back calculations of the growth rate necessary to match the observed Ɛp in photic zone and core‐top samples using the statistical model show high correlation with modeled growth rates using [PO43−] and SST in temperate regions. For Southern Ocean and tropical upwelling regions, with colder and warmer SST, respectively, there might be additional factors controlling growth rate not accounted for in the model, such as grazing or nutrient recycling. We evaluate the potential to reconstruct past growth rates using planktonic foraminifera‐ and coccolithophore‐based proxies related to water column structure. We found best application of these proxies as potential indicators of past [PO43−], which can subsequently be used in combination with proxy temperature, to estimate past coccolithophore growth rates.

Evolution of Antarctic Intermediate Water during the Plio-Pleistocene and implications for global climate: Evidence from the South Atlantic

Formed in the Southern Ocean, Antarctic Intermediate Water (AAIW) plays a fundamental role in the modern climate system as an important sink for atmospheric CO2, and as a mid-depth conduit supplying nutrient-rich waters to subtropical and tropical upwelling regions. During the transition from warm Pliocene ‘greenhouse’ conditions to Pleistocene ‘icehouse’ conditions around 3 Ma, model simulations suggest dramatic circulation changes in the Southern Ocean AAIW formation region due to cooling and expanded sea ice cover around Antarctica. However, the history and properties of AAIW over this time period are still poorly understood. Here, we trace AAIW source water provenance, temperature, and salinity changes over the last 4 Myr using Nd isotopes of Fe-Mn-oxyhydroxide encrusted foraminifera, benthic foraminiferal Mg/Ca, and stable isotopes from southwest Atlantic DSDP Site 516. Our results show that modern AAIW properties emerged gradually over the last 3 Myr, as evidenced by gradual cooling (6 °C) and freshening of intermediate waters. Over this same interval, εNd-values decrease and diverge away from those of Pacific-sourced waters and converge on values of present-day Circumpolar Deep Water (CDW). These observations are in accordance with model simulations indicating increased deep vertical mixing at the Polar Front associated with sea ice extension. These modified AAIW source waters penetrated increasingly northward into the Atlantic at intermediate depths, enhancing nutrient supply and contributing to surface cooling along subtropical coastal upwelling sites. These circulation changes may have facilitated ocean CO2 storage, providing a positive feedback amplifying global cooling and intensification of Northern Hemisphere Glaciation.

ICON-ART 2.1: a flexible tracer framework and its application for composition studies in numerical weather forecasting and climate simulations

Abstract. Atmospheric composition studies on weather and climate timescales require flexible, scalable models. The ICOsahedral Nonhydrostatic model with Aerosols and Reactive Trace gases (ICON-ART) provides such an environment. Here, we introduce the most up-to-date version of the flexible tracer framework for ICON-ART and explain its application in one numerical weather forecast and one climate related case study. We demonstrate the implementation of idealised tracers and chemistry tendencies of different complexity using the ART infrastructure. Using different ICON physics configurations for weather and climate with ART, we perform integrations on different timescales, illustrating the model's performance. First, we present a hindcast experiment for the 2002 ozone hole split with two different ozone chemistry schemes using the numerical weather prediction physics configuration. We compare the hindcast with observations and discuss the confinement of the vortex split using an idealised tracer diagnostic. Secondly, we study AMIP-type integrations using a simplified chemistry scheme in conjunction with the climate physics configuration. We use two different simulations: the interactive simulation, where modelled ozone is coupled back to the radiation scheme, and the non-interactive simulation that uses a default background climatology of ozone. Additionally, we introduce changes of water vapour by methane oxidation for the interactive simulation. We discuss the impact of stratospheric ozone and water vapour variations in the interactive and non-interactive integrations on the water vapour tape recorder, as a measure of tropical upwelling changes. Additionally we explain the seasonal evolution and latitudinal distribution of the age of air. The age of air is a measure of the strength of the meridional overturning circulation with young air in the tropical upwelling region and older air in polar winter downwelling regions. We conclude that our flexible tracer framework allows for tailor-made configurations of ICON-ART in weather and climate applications that are easy to configure and run well.

Marine phytoplankton stoichiometry mediates nonlinear interactions between nutrient supply, temperature, and atmospheric CO<sub>2</sub>

Abstract. Marine phytoplankton stoichiometry links nutrient supply to marine carbon export. Deviations of phytoplankton stoichiometry from Redfield proportions (106C : 1P) could therefore have a significant impact on carbon cycling, and understanding which environmental factors drive these deviations may reveal new mechanisms regulating the carbon cycle. To explore the links between environmental conditions, stoichiometry, and carbon cycling, we compared four different models of phytoplankton C : P: a fixed Redfield model, a model with C : P given as a function of surface phosphorus concentration (P), a model with C P given as a function of temperature, and a new multi-environmental model that predicts C : P as a function of light, temperature, and P. These stoichiometric models were embedded into a five-box ocean circulation model, which resolves the three major ocean biomes (high-latitude, subtropical gyres, and tropical upwelling regions). Contrary to the expectation of a monotonic relationship between surface nutrient drawdown and carbon export, we found that lateral nutrient transport from lower C : P tropical waters to high C : P subtropical waters could cause carbon export to decrease with increased tropical nutrient utilization. It has been hypothesized that a positive feedback between temperature and pCO2, atm will play an important role in anthropogenic climate change, with changes in the biological pump playing at most a secondary role. Here we show that environmentally driven shifts in stoichiometry make the biological pump more influential, and may reverse the expected positive relationship between temperature and pCO2, atm. In the temperature-only model, changes in tropical temperature have more impact on the Δ pCO2, atm (∼ 41 ppm) compared to subtropical temperature changes (∼ 4.5 ppm). Our multi-environmental model predicted a decline in pCO2, atm of ∼ 46 ppm when temperature spanned a change of 10 °C. Thus, we find that variation in marine phytoplankton stoichiometry and its environmental controlling factors can lead to nonlinear controls on pCO2, atm, suggesting the need for further studies of ocean C : P and the impact on ocean carbon cycling.

Kinematic and diabatic vertical velocity climatologies from a chemistry climate model

Abstract. The representation of vertical velocity in chemistry climate models is a key element for the representation of the large-scale Brewer–Dobson circulation in the stratosphere. Here, we diagnose and compare the kinematic and diabatic vertical velocities in the ECHAM/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) model. The calculation of kinematic vertical velocity is based on the continuity equation, whereas diabatic vertical velocity is computed using diabatic heating rates. Annual and monthly zonal mean climatologies of vertical velocity from a 10-year simulation are provided for both kinematic and diabatic vertical velocity representations. In general, both vertical velocity patterns show the main features of the stratospheric circulation, namely, upwelling at low latitudes and downwelling at high latitudes. The main difference in the vertical velocity pattern is a more uniform structure for diabatic and a noisier structure for kinematic vertical velocity. Diabatic vertical velocities show higher absolute values both in the upwelling branch in the inner tropics and in the downwelling regions in the polar vortices. Further, there is a latitudinal shift of the tropical upwelling branch in boreal summer between the two vertical velocity representations with the tropical upwelling region in the diabatic representation shifted southward compared to the kinematic case. Furthermore, we present mean age of air climatologies from two transport schemes in EMAC using these different vertical velocities and analyze the impact of residual circulation and mixing processes on the age of air. The age of air distributions show a hemispheric difference pattern in the stratosphere with younger air in the Southern Hemisphere and older air in the Northern Hemisphere using the transport scheme with diabatic vertical velocities. Further, the age of air climatology from the transport scheme using diabatic vertical velocities shows a younger mean age of air in the inner tropical upwelling branch and an older mean age in the extratropical tropopause region.

The morphology of the Brewer-Dobson circulation and its response to climate change in CMIP5 simulations

This article analyses the annual mean vertical and latitudinal structure of the Brewer–Dobson circulation in the CMIP5 models. The strength of the tropical mass upwelling is found to increase at all altitudes throughout the stratosphere due to climate change. However, the width of the tropical upwelling region narrows below about 20 hPa, and widens above 20 hPa, suggesting different physical mechanisms may play a role in this change above and below 20 hPa. In the lower stratosphere, an equatorward shift in the stationary wave critical line allows waves to propagate further into the Tropics. However, in the upper stratosphere, where the behaviour is dominated by what happens during the winter, an increase in the extratropical zonal mean westerly jet leads to a reduced equatorward refraction of planetary waves. The seasonal cycle of the change in the Brewer–Dobson circulation is also considered, and differences are found in the latitudinal structure of the increased extratropical downwelling between the Northern and Southern Hemispheres in winter.

Glacial marine nutrient and carbon redistribution: Evidence from the tropical ocean

The nature and timing of marine nutrient and carbon redistribution through a glacial cycle remains unclear. Understanding transfers to and from the surface and deep ocean reservoirs is important to explaining Pleistocene variation in atmospheric CO2 content. Observations in the modern ocean show that the nutrient supply to the tropical upwelling regions depends on content of deep reservoirs and vertical mixing in the Southern and subantarctic oceans. Previous work in the Pacific demonstrated that nutrient supply to the tropics was reduced during the Glacial, consonant with reduced vertical mixing in the Southern Ocean. We examine the glacial record of the tropical Atlantic with the same methods used in the Pacific (N. dutertrei carbon isotope data combined with export production estimates to evaluate changes in thermocline nutrient content). In contrast to the Pacific, we find evidence for an increase in tropical Atlantic nutrient supply under glacial conditions. The source of nutrients can be traced to subantarctic surface waters and ultimately to an enriched abyssal reservoir. Bathymetrically forced vertical mixing could account for the transfer of nutrients from this reservoir in the S. Atlantic. The enriched reservoir developed in early MIS 4 (75 ka) and persisted until about 14.5 ka (Bolling/Allerod). This timing corresponds to shifts in atmospheric CO2 content from intermediate to minimum (full glacial) levels at 75 ka and from intermediate to Holocene concentrations near 14.5 ka.

Microbial Diversity of a Brazilian Coastal Region Influenced by an Upwelling System and Anthropogenic Activity

BackgroundUpwelling systems are characterised by an intense primary biomass production in the surface (warmest) water after the outcrop of the bottom (coldest) water, which is rich in nutrients. Although it is known that the microbial assemblage plays an important role in the food chain of marine systems and that the upwelling systems that occur in southwest Brazil drive the complex dynamics of the food chain, little is known about the microbial composition present in this region.Methodology/Principal FindingsWe carried out a molecular survey based on SSU rRNA gene from the three domains of the phylogenetic tree of life present in a tropical upwelling region (Arraial do Cabo, Rio de Janeiro, Brazil). The aim was to analyse the horizontal and vertical variations of the microbial composition in two geographically close areas influenced by anthropogenic activity (sewage disposal/port activity) and upwelling phenomena, respectively. A lower estimated diversity of microorganisms of the three domains of the phylogenetic tree of life was found in the water of the area influenced by anthropogenic activity compared to the area influenced by upwelling phenomena. We observed a heterogenic distribution of the relative abundance of taxonomic groups, especially in the Archaea and Eukarya domains. The bacterial community was dominated by Proteobacteria, Cyanobacteria and Bacteroidetes phyla, whereas the microeukaryotic community was dominated by Metazoa, Fungi, Alveolata and Stramenopile. The estimated archaeal diversity was the lowest of the three domains and was dominated by uncharacterised marine Crenarchaeota that were most closely related to Marine Group I.Conclusions/SignificanceThe variety of conditions and the presence of different microbial assemblages indicated that the area of Arraial do Cabo can be used as a model for detailed studies that contemplate the correlation between pollution-indicating parameters and the depletion of microbial diversity in areas close to anthropogenic activity; functional roles and geochemical processes; phylogeny of the uncharacterised diversity; and seasonal variations of the microbial assemblages.

Narrowing of the upwelling branch of the Brewer‐Dobson circulation and Hadley cell in chemistry‐climate model simulations of the 21st century

Changes in the width of the upwelling branch of the Brewer‐Dobson circulation and Hadley cell in the 21st Century are investigated using simulations from a coupled chemistry‐climate model. In these model simulations the tropical upwelling region narrows in the troposphere and lower stratosphere. The narrowing of the Brewer‐Dobson circulation is caused by an equatorward shift of Rossby wave critical latitudes and Eliassen‐Palm flux convergence in the subtropical lower stratosphere. In the troposphere, the model projects an expansion of the Hadley cell's poleward boundary, but a narrowing of the Hadley cell's rising branch. Model results suggest that eddy forcing may also play a part in the narrowing of the rising branch of the Hadley cell.

Microzooplankton grazing of phytoplankton in a tropical upwelling region

We measured grazing by herbivorous zooplankton ( 80% of phytoplankton production. A comparison of incubations with and without added inorganic nutrients showed no consistent stimulation of phytoplankton growth (slope of enriched versus unenriched treatments not significantly different from 1). Estimates from microscopic counts of heterotrophic organisms >10 μm indicated that copepod nauplii comprised the largest share of the microzooplankton biomass (mean 62.4 ± 5.8% SE). Grazing estimates were not correlated with microzooplankton biomass, whether or not nauplii were included, suggesting that most of the grazing was done by nano-sized zooplankton.

Modeling Ulva spp. dynamics in a tropical upwelling region

The development and application of a model to represent the seasonal dynamics of Ulva spp. biomass at the Cabo Frio tropical upwelling region is here described. In addition to biomass, the model simulates nitrogen and phosphorus internal concentrations in the algae. The factors considered to affect algal growth at Cabo Frio are sunlight, water temperature, nitrate, ammonium and phosphate, and the alga carrying capacity. Peaks in algal abundance (up to 1000 g d.wt./m 2) were observed to occur and were simulated by the model during the upwelling season. The inclusion of a variable optimum temperature was found to be necessary to avoid a phase lag between model and observations. Calibration of some of the model parameters with a simplex method has further improved the model results. The simulations shown here suggest that the initial growth of Ulva, after a downwelling period, is stimulated by the increase in nutrient concentrations in the water column during upwelling.

Airborne observations of DMSO, DMS, and OH at marine tropical latitudes

This paper reports the first simultaneous fast‐time resolution measurements of dimethylsulfide (DMS), dimethylsulfoxide (DMSO), and the hydroxyl radical (OH) at marine tropical latitudes. These observations were recorded during the PEM‐Tropics B field program on NASA's P‐3B aircraft. The observations of DMSO, using a Selected Ion Chemical Ionization Mass Spectrometry (SICIMS) technique, are of particular significance. They have revealed two unique findings: 1) average midday‐tropical levels of DMSO are significantly higher than those predicted from current models when constrained by observed DMS and OH levels (e.g., 10 pptv versus 1 to 3 pptv); and 2) DMSO concentration profiles are significantly out‐of‐phase with model predictions, maximum values being seen under near dark conditions and minimum values being observed at midday. Although no simple explanation has yet been found for these unusual results, the fact that no evidence points to a problem in the measurements suggests that others may exist. Clearly, if the observations are correct, they indicate that at least for tropical upwelling regions the atmospheric sulfur budget may need to be adjusted to accommodate additional sources of DMSO.

Bulk properties of isentropic mixing into the tropics in the lower stratosphere

Timescales for mixing of midlatitude air into the tropical lower stratosphere are deduced from observations of long‐lived tracers N2O and CCl3F. Bulk mixing between tropical and midlatitude regions is assumed to be isentropic and relatively slow compared with local mixing within each region. The mean value of the mixing timescale ranges from 12 to 18 months near 20 km. There is a tendency for shorter mixing times at higher and lower altitudes, although vertical profiles of mixing cannot be definitively established by the data. A more robust quantity is given by the fraction of midlatitude air entrained into the tropical upwelling region. Implied mixing fractions exceed 50% above 22 km.

Sea Surface Temperatures Computed by a Simple Ocean Mixed Layer Coupled to an Atmospheric GCM

Abstract An ocean mixed layer with fixed depth (50 m) is coupled to a global atmospheric general circulation model (GCM). The mixed layer is a simple slab that allows for seasonal heat storage but has no dynamics (advection or diffusion). The an surface temperatures generated by the model are compared to observations for January and July. The overall simulation of ocean temperature is reproduced even with this simple ocean model. However, there are important Discrepancies. Because of the lack of dynamics, the tropical upwelling regions are too warm in the model. Most Northern Hemisphere midlatitude ocean areas are too cool during summer due to the absence of shallowing of the mixed layer at that time of year. In Southern Hemisphere midlatitudes where deep mixed layers persist year round, the model is warmer than observed for most of the year. Standard deviations of daily SSTs from the computed mixed layer are less than observed in most open ocean areas and oven smaller near western boundary current region...