Jaime B. Palter

The effect of advection on the nutrient reservoir in the North Atlantic subtropical gyre

Though critically important in sustaining the oceans biological pump, the cycling of nutrients in the subtropical gyres is poorly understood. The supply of nutrients to the sunlit surface layer of the ocean has traditionally been attributed solely to vertical processes. However, horizontal advection may also be important in establishing the availability of nutrients. Here we show that the production and advection of North Atlantic Subtropical Mode Water introduces spatial and temporal variability in the subsurface nutrient reservoir beneath the North Atlantic subtropical gyre. As the mode water is formed, its nutrients are depleted by biological utilization. When the depleted water mass is exported to the gyre, it injects a wedge of low-nutrient water into the upper layers of the ocean. Contrary to intuition, cold winters that promote deep convective mixing and vigorous mode water formation may diminish downstream primary productivity by altering the subsurface delivery of nutrients.

Role of Mesoscale Eddies in Cross-Frontal Transport of Heat and Biogeochemical Tracers in the Southern Ocean

AbstractThis study examines the role of processes transporting tracers across the Polar Front (PF) in the depth interval between the surface and major topographic sills, which this study refers to as the “PF core.” A preindustrial control simulation of an eddying climate model coupled to a biogeochemical model [GFDL Climate Model, version 2.6 (CM2.6)– simplified version of the Biogeochemistry with Light Iron Nutrients and Gas (miniBLING) 0.1° ocean model] is used to investigate the transport of heat, carbon, oxygen, and phosphate across the PF core, with a particular focus on the role of mesoscale eddies. The authors find that the total transport across the PF core results from a ubiquitous Ekman transport that drives the upwelled tracers to the north and a localized opposing eddy transport that induces tracer leakages to the south at major topographic obstacles. In the Ekman layer, the southward eddy transport only partially compensates the northward Ekman transport, while below the Ekman layer, the sout...

On the source of Gulf Stream nutrients

[1] Along density surfaces, nutrient concentrations in the Gulf Stream are elevated relative to concentrations to either side of the current. We assess the source of these elevated nutrient concentrations in the western boundary current using historical hydrographic data. The analysis is extended to the separated Gulf Stream with four hydrographic sections recently occupied as part of the Climate Variability and Predictability Program (CLIVAR) Mode Water Dynamics Experiment. The results of this analysis suggest that imported, extrasubtropical waters are the primary source of the elevated nutrient concentrations. Because the high nutrient signature is likely imported, diapycnal mixing need not be invoked to explain the Gulf Stream’s high nutrient concentrations, as had been proposed in the past. Moreover, nutrients do not increase along the length of the stream, further suggesting that the stream’s high nutrient signature is imported rather than manufactured by processes within the current. The imported nutrients are likely advected into the North Atlantic within the low-salinity water masses that contribute to the shallow limb of the meridional overturning circulation. Thus the availability of nutrients in the North Atlantic may be linked to upstream processes in the tropics and possibly the Southern Hemisphere as well as to variability in the volume of imported water and its distribution in density space.

The supply of excess phosphate across the Gulf Stream and the maintenance of subtropical nitrogen fixation

with estimated rates between 1 and 20 × 10 11 mol nitrogen fixed annually. However, the region’s nutrient reservoir beneath the euphotic zone is so enriched in nitrate relative to phosphate that it is perplexing how fixation might be sustained there. Here, we investigate whether the physical transport of excess phosphate into the subtropical gyre is sufficient to sustain nitrogen fixation in the gyre. Specifically, we assess the Ekman advection and isopycnal mixing of excess phosphate to the subtropical North Atlantic, using detailed hydrographic and nutrient sections occupied across the Gulf Stream combined with satellite wind data. Ekman advection and along‐isopycnal mixing provide a source of approximately 2 × 10 10 mol yr −1 of excess phosphate in the northwestern subtropics, a physical mechanism that has the potential to support more than 3 × 10 11 mol yr −1 of biological nitrogen fixation, after accounting for alternative sinks of excess phosphate. This excess phosphate supply across the gyre’s northern boundary and high nitrogen fixation there offers a mechanism that can explain both the maintenance of subtropical North Atlantic nitrogen fixation in a phosphate‐poor environment and help account for the weak gradients in the proxies of fixation observed along interior circulation pathways of the gyre.

Large-scale ocean circulation-cloud interactions reduce the pace of transient climate change

Changes to the large-scale oceanic circulation are thought to slow the pace of transient climate change due, in part, to their influence on radiative feedbacks. Here we evaluate the interactions between CO2-forced perturbations to the large-scale ocean circulation and the radiative cloud feedback in a climate model. Both the change of the ocean circulation and the radiative cloud feedback strongly influence the magnitude and spatial pattern of surface and ocean warming. Changes in the ocean circulation reduce the amount of transient global warming caused by the radiative cloud feedback by helping to maintain low cloud coverage in the face of global warming. The radiative cloud feedback is key in affecting atmospheric meridional heat transport changes and is the dominant radiative feedback mechanism that responds to ocean circulation change. Uncertainty in the simulated ocean circulation changes due to CO2 forcing may contribute a large share of the spread in the radiative cloud feedback among climate models.

Response of the Ocean Natural Carbon Storage to Projected Twenty-First-Century Climate Change

The separate impacts of wind stress, buoyancy fluxes, and CO2 solubility on the oceanic storage of natural carbon are assessed in an ensemble of twentieth- to twenty-first-century simulations, using a coupled atmosphere‐ocean‐carbon cycle model. Time-varying perturbations for surface wind stress, temperature, and salinity are calculated from the difference between climate change and preindustrial control simulations, and are imposed on the ocean in separate simulations. The response of the natural carbon storage to each perturbation is assessed with novel prognostic biogeochemical tracers, which can explicitly decompose dissolved inorganic carbon into biological, preformed, equilibrium, and disequilibrium components. Strong responses of these components to changes in buoyancy and winds are seen at high latitudes, reflecting the critical role of intermediate and deep waters. Overall, circulation-driven changes in carbon storage are mainly due to changes in buoyancy fluxes, with wind-driven changes playing an opposite but smaller role. Results suggest that climate-driven perturbations to the ocean natural carbon cycle will contribute 20PgC to the reduction of the ocean accumulated total carbon uptake over the period 1860‐2100. This reflects a strong compensation between a buildup of remineralized organic matter associated with reduced deep-water formation (196PgC) and a decrease of preformed carbon (2116PgC). The latter is due to a warming-induced decrease in CO2 solubility (252PgC) and a circulation-induced decrease in disequilibrium carbon storage (264PgC). Climate change gives rise to a large spatial redistribution of ocean carbon, with increasing concentrations at high latitudes and stronger vertical gradients at low latitudes.

The Deep Ocean Buoyancy Budget and Its Temporal Variability

AbstractDespite slow rates of ocean mixing, observational and modeling studies suggest that buoyancy is redistributed to all depths of the ocean on surprisingly short interannual to decadal time scales. The mechanisms responsible for this redistribution remain poorly understood. This work uses an Earth system model to evaluate the global steady-state ocean buoyancy (and related steric sea level) budget, its interannual variability, and its transient response to a doubling of CO2 over 70 years, with a focus on the deep ocean. At steady state, the simple view of vertical advective–diffusive balance for the deep ocean holds at low to midlatitudes. At higher latitudes, the balance depends on a myriad of additional terms, namely mesoscale and submesoscale advection, convection and overflows from marginal seas, and terms related to the nonlinear equation of state. These high-latitude processes rapidly communicate anomalies in surface buoyancy forcing to the deep ocean locally; the deep, high-latitude changes th...

How Does Labrador Sea Water Enter the Deep Western Boundary Current

Abstract Labrador Sea Water (LSW), a dense water mass formed by convection in the subpolar North Atlantic, is an important constituent of the meridional overturning circulation. Understanding how the water mass enters the deep western boundary current (DWBC), one of the primary pathways by which it exits the subpolar gyre, can shed light on the continuity between climate conditions in the formation region and their downstream signal. Using the trajectories of (profiling) autonomous Lagrangian circulation explorer [(P)ALACE] floats, operating between 1996 and 2002, three processes are evaluated for their role in the entry of Labrador Sea Water in the DWBC: 1) LSW is formed directly in the DWBC, 2) eddies flux LSW laterally from the interior Labrador Sea to the DWBC, and 3) a horizontally divergent mean flow advects LSW from the interior to the DWBC. A comparison of the heat flux associated with each of these three mechanisms suggests that all three contribute to the transformation of the boundary current a...

Impact of Weddell Sea deep convection on natural and anthropogenic carbon in a climate model

A climate model is used to investigate the influence of Weddell Sea open ocean deep convection on anthropogenic and natural carbon uptake for the period 1860–2100. In a three-member ensemble climate change simulation, convection ceases on average by year 1981, weakening the net oceanic cumulative uptake of atmospheric CO2 by year 2100 (−4.3 Pg C) relative to an ocean that has continued convection. This net weakening results from a decrease in anthropogenic carbon uptake (−10.1 Pg C), partly offset by an increase in natural carbon storage (+5.8 Pg C). Despite representing only 4% of its area, the Weddell Sea is responsible for 22% of the Southern Ocean decrease in total climate-driven carbon uptake and 52% of the decrease in the anthropogenic component of oceanic uptake. Although this is a model-specific result, it illustrates the potential of deep convection to produce an intermodel spread in future projections of ocean carbon uptake.

The Role of the Gulf Stream in European Climate

The Gulf Stream carries the warm, poleward return flow of the wind-driven North Atlantic subtropical gyre and the Atlantic Meridional Overturning Circulation. This northward flow drives a significant meridional heat transport. Various lines of evidence suggest that Gulf Stream heat transport profoundly influences the climate of the entire Northern Hemisphere and, thus, Europes climate on timescales of decades and longer. The Gulf Streams influence is mediated through feedback processes between the ocean, atmosphere, and cryosphere. This review synthesizes paleoclimate archives, model simulations, and the instrumental record, which collectively suggest that decadal and longer-scale variability of the Gulf Streams heat transport manifests in changes in European temperature, precipitation, and storminess. Given that anthropogenic climate change is projected to weaken the Atlantic Meridional Overturning Circulation, associated changes in European climate are expected. However, large uncertainty in the magnitude of the anticipated weakening undermines the predictability of the future climate in Europe.