Search for: All records

In the tropical Pacific, weak ventilation and intense microbial respiration at depth give rise to a low dissolved oxygen (O₂) environment that is thought to be ventilated primarily by the equatorial current system (ECS). The role of mesoscale eddies and vertical mixing as potential pathways of O₂supply in this region, however, remains poorly known due to sparse observations and coarse model resolution. Using an eddy resolving simulation of ocean circulation and biogeochemistry, we assess the contribution of these processes to the O₂budget balance and find that vertical mixing of O₂, which is modulated by the surface wind speed and the vertical shear of the eddying currents, contributes substantially to the replenishment of O₂in the upper equatorial Pacific thermocline, complementing the advective supply of O₂by the ECS and meridional circulation at depth. These transport processes vary seasonally in conjunction with the wind: mixing of O₂into the upper thermocline is strongest during boreal summer and fall when the vertical shear and eddy kinetic energy are intensified. The relationship between eddy activity and the downward mixing of O₂arises from the modulation of equatorial turbulence by Tropical Instability Waves via their impacts on the vertical shear. This interaction of processes across scales sustains a local pathway of O₂delivery into the equatorial Pacific interior and highlights the need for adequate observations and models of turbulent mixing and mesoscale processes for understanding and predicting the fate of the tropical Pacific O₂content in a warmer and more stratified ocean.

 

Estimates suggest that over 4 gigatons per year of carbon dioxide (Gt-CO₂year⁻¹) be removed from the atmosphere by 2050 to meet international climate goals. One strategy for carbon dioxide removal is seaweed farming; however its global potential remains highly uncertain. Here, we apply a dynamic seaweed growth model that includes growth-limiting mechanisms, such as nitrate supply, to estimate the global potential yield of four types of seaweed. We estimate that harvesting 1 Gt year⁻¹of seaweed carbon would require farming over 1 million km²of the most productive exclusive economic zones, located in the equatorial Pacific; the cultivation area would need to be tripled to attain an additional 1 Gt year⁻¹of harvested carbon, indicating dramatic reductions in carbon harvest efficiency beyond the most productive waters. Improving the accuracy of annual harvest yield estimates requires better understanding of biophysical constraints such as seaweed loss rates (e.g., infestation, disease, grazing, wave erosion).

 

Pinatubo erupted during the first decadal survey of ocean biogeochemistry, embedding its climate fingerprint into foundational ocean biogeochemical observations and complicating the interpretation of long‐term biogeochemical change. Here, we quantify the influence of the Pinatubo climate perturbation on externally forced decadal and multi‐decadal changes in key ocean biogeochemical quantities using a large ensemble simulation of the Community Earth System Model designed to isolate the effects of Pinatubo, which cleanly captures the ocean biogeochemical response to the eruption. We find increased uptake of apparent oxygen utilization and preindustrial carbon over 1993–2003. Nearly 100% of the forced response in these quantities are attributable to Pinatubo. The eruption caused enhanced ventilation of the North Atlantic, as evidenced by deep ocean chlorofluorocarbon changes that appear 10–15 years after the eruption. Our results help contextualize observed change and contribute to improved constraints on uncertainty in the global carbon budget and ocean deoxygenation.

 

We examine the effects of the submesoscale in mediating the response to projected warming of phytoplankton new production and export using idealized biogeochemical tracers in a high‐resolution regional model of the Porcupine Abyssal Plain region of the North Atlantic. We quantify submesoscale effects by comparing our control run to an integration in which submesoscale motions have been suppressed using increased viscosity. Annual new production is slightly reduced by submesoscale motions in a climate representative of the early 21st‐century and slightly increased by submesoscale motions in a climate representative of the late 21st‐century. The warmer climate at the end of the 21st century reduces resolved submesoscale activity by a factor of 2–3. Resolving the submesoscale, however, does not strongly impact the projected reduction in annual production under representative warming. Organic carbon export from the surface ocean includes both direct sinking of detritus (the biological gravitational pump) and advective transport mediated pathways; the sinking component is larger than advectively mediated vertical transport by up to an order of magnitude across a wide range of imposed sinking rates. The submesoscales are responsible for most of the advective carbon export, however, which is thus largely reduced in a warmer climate. In summary, our results demonstrate that resolving more of the submesoscale has a modest effect on present‐day new production, a small effect on simulated reductions in new production under global warming, and a large effect on advectively mediated export fluxes.

 

Abstract The California Current System (CCS) sustains economically valuable fisheries and is particularly vulnerable to ocean acidification, due to its natural upwelling of carbon-enriched waters that generate corrosive conditions for local ecosystems. Here we use a novel suite of retrospective, initialized ensemble forecasts with an Earth system model (ESM) to predict the evolution of surface pH anomalies in the CCS. We show that the forecast system skillfully predicts observed surface pH variations a year in advance over a naive forecasting method, with the potential for skillful prediction up to five years in advance. Skillful predictions of surface pH are mainly derived from the initialization of dissolved inorganic carbon anomalies that are subsequently transported into the CCS. Our results demonstrate the potential for ESMs to provide skillful predictions of ocean acidification on large scales in the CCS. Initialized ESMs could also provide boundary conditions to improve high-resolution regional forecasting systems. 

Abstract. While there is agreement that global warming over the 21st century is likely to influence the biological pump, Earth system models (ESMs) display significant divergence in their projections of future new production. This paper quantifies and interprets the sensitivity of projected changes in new production in an idealized global ocean biogeochemistry model. The model includes two tracers that explicitly represent nutrient transport, light- and nutrient-limited nutrient uptake by the ecosystem (new production), and export via sinking organic particles. Globally, new production declines with warming due to reduced surface nutrient availability, as expected. However, the magnitude, seasonality, and underlying dynamics of the nutrient uptake are sensitive to the light and nutrient dependencies of uptake, which we summarize in terms of a single biological timescale that is a linear combination of the partial derivatives of production with respect to light and nutrients. Although the relationships are nonlinear, this biological timescale is correlated with several measures of biogeochemical function: shorter timescales are associated with greater global annual new production and higher nutrient utilization. Shorter timescales are also associated with greater declines in global new production in a warmer climate and greater sensitivity to changes in nutrients than light. Future work is needed to characterize more complex ocean biogeochemical models in terms of similar timescale generalities to examine their climate change implications. 

The Southern Ocean plays an important role in determining atmospheric carbon dioxide (CO 2 ), yet estimates of air-sea CO 2 flux for the region diverge widely. In this study, we constrained Southern Ocean air-sea CO 2 exchange by relating fluxes to horizontal and vertical CO 2 gradients in atmospheric transport models and applying atmospheric observations of these gradients to estimate fluxes. Aircraft-based measurements of the vertical atmospheric CO 2 gradient provide robust flux constraints. We found an annual mean flux of –0.53 ± 0.23 petagrams of carbon per year (net uptake) south of 45°S during the period 2009–2018. This is consistent with the mean of atmospheric inversion estimates and surface-ocean partial pressure of CO 2 ( P co 2 )–based products, but our data indicate stronger annual mean uptake than suggested by recent interpretations of profiling float observations. 

Large volcanic eruptions drive significant climate perturbations through major anomalies in radiative fluxes and the resulting widespread cooling of the surface and upper ocean. Recent studies suggest that these eruptions also drive important variability in air‐sea carbon and oxygen fluxes. By simulating the Earth system using two initial‐condition large ensembles, with and without the aerosol forcing associated with the Mt. Pinatubo eruption in June 1991, we isolate the impact of this volcanic event on physical and biogeochemical properties of the ocean. The Mt. Pinatubo eruption forced significant anomalies in surface fluxes and the ocean interior inventories of heat, oxygen, and carbon. Pinatubo‐driven changes persist for multiple years in the upper ocean and permanently modify the ocean's heat, oxygen, and carbon inventories. Positive anomalies in oxygen concentrations emerge immediately post‐eruption and penetrate into the deep ocean. In contrast, carbon anomalies intensify in the upper ocean over several years post‐eruption, and are largely confined to the upper 150 m. In the tropics and northern high latitudes, the change in oxygen is dominated by surface cooling and subsequent ventilation to mid‐depths, while the carbon anomaly is associated with solubility changes and eruption‐generated El Niño—Southern Oscillation variability. We do not find significant impact of Pinatubo on oxygen or carbon fluxes in the Southern Ocean; but this may be due to Southern Hemisphere aerosol forcing being underestimated in Community Earth System Model 1 simulations.

 

Abstract. Interannual variations in air–sea fluxes of carbon dioxide (CO₂) impactthe global carbon cycle and climate system, and previous studies suggest thatthese variations may be predictable in the near term (from a year to a decadein advance). Here, we quantify and understand the sources of near-termpredictability and predictive skill in air–sea CO₂ flux on global andregional scales by analyzing output from a novel set of retrospective decadalforecasts of an Earth system model. These forecasts exhibit the potential topredict year-to-year variations in the globally integrated air–sea CO₂flux several years in advance, as indicated by the high correlation of theforecasts with a model reconstruction of past CO₂ flux evolution. Thispotential predictability exceeds that obtained solely from foreknowledge ofvariations in external forcing or a simple persistence forecast, with thelongest-lasting forecast enhancement in the subantarctic Southern Ocean andthe northern North Atlantic. Potential predictability in CO₂ fluxvariations is largely driven by predictability in the surface ocean partialpressure of CO₂, which itself is a function of predictability in surfaceocean dissolved inorganic carbon and alkalinity. The potentialpredictability, however, is not realized as predictive skill, as indicated bythe moderate to low correlation of the forecasts with anobservationally based CO₂ flux product. Nevertheless, our results suggestthat year-to-year variations in ocean carbon uptake have the potential to bepredicted well in advance and establish a precedent for forecasting air–seaCO₂ flux in the near future.