Alison R. Gray

A Global Analysis of Sverdrup Balance Using Absolute Geostrophic Velocities from Argo

Using observations from the Argo array of profiling floats, the large-scale circulation of the upper 2000 decibars (db) of the global ocean is computed for the period from December 2004 to November 2010. The geostrophic velocity relative to a reference level of 900db is estimated from temperature and salinity profiles, and the absolute geostrophic velocity at the reference level is estimated from the trajectory data provided by the floats. Combining the two gives the absolute geostrophic velocity on 29 pressure surfaces spanning the upper 2000db of the global ocean. These velocities, together with satellite observations of wind stress, are thenusedtoevaluateSverdrupbalance,thesimplecanonicaltheoryrelatingmeridionalgeostrophictransport to wind forcing. Observed transports agree well with predictions based on the wind field over large areas, primarily in the tropics and subtropics. Elsewhere, especially at higher latitudes and in boundary regions, Sverdrup balance does not accurately describe meridional geostrophic transports, possibly due to the increased importance of the barotropic flow, nonlinear dynamics, and topographic influence. Thus, while it provides an effective framework for understanding the zero-order wind-driven circulation in much of the global ocean, Sverdrup balance should not be regarded as axiomatic.

Calculating surface ocean pCO2 from biogeochemical Argo floats equipped with pH: An uncertainty analysis

U.S. National Science Foundations Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project under the NSF [PLR-1425989]; NASA [NNX14AP49G]; U.S. Argo through NOAA/JISAO grant [NA17RJ1232]; Ocean Observations and Monitoring Division, Climate Program Office, National Oceanic and Atmospheric Administration, U.S. Department of Commerce; David and Lucile Packard Foundation; NOAA Climate and Global Change postdoctoral fellowship; ARCS Foundation Oregon Chapter

Spiraling pathways of global deep waters to the surface of the Southern Ocean

Upwelling of global deep waters to the sea surface in the Southern Ocean closes the global overturning circulation and is fundamentally important for oceanic uptake of carbon and heat, nutrient resupply for sustaining oceanic biological production, and the melt rate of ice shelves. However, the exact pathways and role of topography in Southern Ocean upwelling remain largely unknown. Here we show detailed upwelling pathways in three dimensions, using hydrographic observations and particle tracking in high-resolution models. The analysis reveals that the northern-sourced deep waters enter the Antarctic Circumpolar Current via southward flow along the boundaries of the three ocean basins, before spiraling southeastward and upward through the Antarctic Circumpolar Current. Upwelling is greatly enhanced at five major topographic features, associated with vigorous mesoscale eddy activity. Deep water reaches the upper ocean predominantly south of the Antarctic Circumpolar Current, with a spatially nonuniform distribution. The timescale for half of the deep water to upwell from 30° S to the mixed layer is ~60–90 years.Deep waters of the Atlantic, Pacific and Indian Oceans upwell in the Southern Oceanbut the exact pathways are not fully characterized. Here the authors present a three dimensional view showing a spiralling southward path, with enhanced upwelling by eddy-transport at topographic hotspots.

A method for multiscale optimal analysis with application to Argo data

This study presents an optimal analysis method for estimating the large- and small-scale components of a field from observations. This technique relies on an iterative generalized least squares procedure to determine the statistics of the small-scale fluctuations directly from the data and is thus especially valuable when such information is not known a priori. The use of spherical radial basis functions in fitting the large-scale signal is suggested, particularly when the domain is sufficiently large. Two test cases illustrate several of the properties of this procedure, demonstrate its utility, and provide practical guidelines for its use. This method is then applied to observations collected by the Argo array of profiling floats to produce global gridded absolute geostrophic velocity estimates.

Oxygen in the Southern Ocean From Argo Floats: Determination of Processes Driving Air‐Sea Fluxes

The Southern Ocean is of outsized significance to the global oxygen and carbon cycles with relatively poor measurement coverage due to harsh winters and seasonal ice cover. In this study, we use recent advances in the parameterization of air-sea oxygen fluxes to analyze nine years of oxygen data from a recalibrated Argo oxygen dataset and from air-calibrated oxygen floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project. From this combined dataset of 150 floats, we find a total Southern Ocean oxygen sink of -183 ± 80 Tmol yr−1 (positive to the atmosphere), greater than prior estimates. The uptake occurs primarily in the Polar-Frontal Antarctic Zone (PAZ, -94 ± 30 Tmol O2 yr−1) and Seasonal Ice Zone (SIZ, -111 ± 9.3 Tmol O2 yr−1). This flux is driven by wintertime ventilation, with a large portion of the flux in the SIZ passing through regions with fractional sea ice. The Subtropical Zone (STZ) is seasonally driven by thermal fluxes and exhibits a net outgassing of 47 ± 29 Tmol O2 yr−1 that is likely driven by biological production. The Subantarctic Zone (SAZ) uptake is -25 ± 12 Tmol O2 yr−1. Total oxygen fluxes were separated into a thermal and non-thermal component. The non-thermal flux is correlated with net primary production and mixed layer depth in the STZ, SAZ, and PAZ, but not in the SIZ where seasonal sea ice slows the air-sea gas flux response to the entrainment of deep, low oxygen waters.

Reply to “Comments on ‘A Global Analysis of Sverdrup Balance Using Absolute Geostrophic Velocities from Argo’”

AbstractThis response addresses the three comments by A. Polonsky on “A Global Analysis of Sverdrup Balance Using Absolute Geostrophic Velocities from Argo.”

Lagrangian Timescales of Southern Ocean Upwelling in a Hierarchy of Model Resolutions

In this paper we study upwelling pathways and timescales of Circumpolar Deep Water (CDW) in a hierarchy of models using a Lagrangian particle tracking method. Lagrangian timescales of CDW upwelling decrease from 87 years to 31 years to 17 years as the ocean resolution is refined from 1∘ to 0.25∘ to 0.1∘. We attribute some of the differences in timescale to the strength of the eddy fields, as demonstrated by temporally degrading high-resolution model velocity fields. Consistent with the timescale dependence, we find that an average Lagrangian particle completes 3.2 circumpolar loops in the 1∘ model in comparison to 0.9 loops in the 0.1∘ model. These differences suggest that advective timescales and thus interbasin merging of upwelling CDW may be overestimated by coarse-resolution models, potentially affecting the skill of centennial scale climate change projections. Plain Language Summary In this paper we use a variety of ocean models to investigate how long it takes for deep ocean waters to upwell to the surface of the Southern Ocean around Antarctica. We track virtual particles in our simulated currents and show how they spiral southward and upward toward the surface. We find that this journey takes 87 years in a standard coarse-resolution climate model but only 17 years in a state of the art high-resolution climate model. We argue that the difference between the models is due to vortices which vigorously upwell the virtual particles but are too small to be represented in standard climate models. Particles also only loop around Antarctica 0.9 times in the high-resolution model compared to 3.2 times in the coarse-resolution model, suggesting that different kinds of upwelling waters have less time to mix with each other in coarse-resolution models. These differences in timescale and the number of loops suggest that there exist biases in long-term climate change projections using standard coarse-resolution climate models.

Observing System Simulation Experiments for an array of autonomous biogeochemical profiling floats in the Southern Ocean

This study uses Observing System Simulation Experiments (OSSEs) to examine the reconstruction of biogeochemical variables in the Southern Ocean from an array of autonomous profiling floats. In these OSSEs, designed to be relevant to the Southern Ocean Carbon and Climate Observation and Modeling (SOCCOM) project, the simulated floats move with oceanic currents and sample dissolved oxygen and inorganic carbon. The annual mean and seasonal cycle of these fields are then reconstructed and compared to the original model fields. The reconstruction skill is quantified with the reconstruction error (RErr), defined as the difference between the reconstructed and actual model fields, weighted by a local measure of the spatio-temporal variability. The square of the RErr is small (< 0.5) for 150 floats in most of the domain, which is interpreted to mean that the reconstruction skill is high. An idealized analytical study demonstrates that the RErr depends on the magnitude of the seasonal cycle, spatial gradients, speed of float movement, amplitude of mesoscale variability and number of floats. These factors explain a large part of the spatial variability in the RErr and can be used to predict the reconstruction skill of the SOCCOM array. Furthermore, our results demonstrate that an array size of 150 floats is a reasonable choice for reconstruction of surface properties and annual-mean 2000 m inventories, with the exception of the seasonal cycle in parts of the Indo-Atlantic, and that doubling this number to 300 results in a very modest increase in the reconstruction skill for dissolved oxygen.