Large freshwater anomalies clearly exist in the Arctic Ocean. For example, liquid freshwater has accumulated in the Beaufort Gyre in the decade of the 2000s compared to 1980-2000, with an extra ≈ 5000 km 3-about 25%-being stored. The sources of freshwater to the Arctic from precipitation and runoff have increased between these periods (most of the evidence comes from models).

The Arctic Ocean is a fundamental node in the global hydrological cycle and the ocean's thermohaline circulation. We here assess the system's key functions and processes: (1) the delivery of fresh and low-salinity waters to the Arctic Ocean by river inflow, net precipitation, distillation during the freeze/thaw cycle, and Pacific Ocean inflows; (2) the disposition (e.g., sources, pathways, and storage) of freshwater components within the Arctic Ocean; and (3) the release and export of freshwater components into the bordering convective domains of the North Atlantic. We then examine physical, chemical, or biological processes which are influenced or constrained by the local quantities and geochemical qualities of freshwater; these include stratification and vertical mixing, ocean heat flux, nutrient supply, primary production, ocean acidification, and biogeochemical cycling. Internal to the Arctic the joint effects of sea ice decline and hydrological cycle intensification have strengthened coupling between the ocean and the atmosphere (e.g., wind and ice drift stresses, solar radiation, and heat and moisture exchange), the bordering drainage basins (e.g., river discharge, sediment transport, and erosion), and terrestrial ecosystems (e.g., Arctic greening, dissolved and particulate carbon loading, and altered phenology of biotic components). External to the Arctic freshwater export acts as both a constraint to and a necessary ingredient for deep convection in the bordering subarctic gyres and thus affects the global thermohaline circulation. Geochemical fingerprints attained within the Arctic Ocean are likewise exported into the neighboring subarctic systems and beyond. Finally, we discuss observed and modeled functions and changes in this system on seasonal, annual, and decadal time scales and discuss mechanisms that link the marine system to atmospheric, terrestrial, and cryospheric systems.

A hierarchy of hydrodynamical instabilities controlling the transfer of buoyancy through the oceanic mixed layer is reviewed. If a resting ocean of horizontally uniform stratification is subject to spatially uniform buoyancy loss at the sea surface, then gravitational instability ensues in which buoyancy is drawn from depth by upright convection. But if spatial inhomogeneities in the ambient stratification or the forcing are present (as always exist in nature), then horizontal density gradients will be induced and, within a rotation period, horizontal currents in thermal-wind balance with those gradients will be set up within the mixed layer. There are two important consequences on the convective process: 1) Upright convection will become modified by the presence of the thermal wind shear; fluid parcels are exchanged not along vertical paths but, rather, along slanting paths in symmetric instability. Theoretical considerations suggest that this slantwise convection sets the potential vorticity of the mixed layer fluid to zero but, in general, will leave it stably stratified in the vertical. 2) The convective process ultimately gives way to a baroclinic instability of the horizontal mixed layer density gradients. The resulting baroclinic waves are important agents of buoyancy transport through the mixed layer and can be so efficient that the convective process all but ceases. The authors illustrate and quantify these ideas by numerical experiment with a highly resolved nonhydrostatic Navier-Stokes model. Uniform spatial cooling at the surface of a resting, stratified fluid in a 2½-dimensional model on an f plane, in which zonal strips of fluid conserve their absolute momentum, causes energetic vertical overturning. A well-mixed boundary layer develops over a depth that is accurately predicted by a simple 1D law. In contrast, differential surface cooling induces a mixed layer front. Fluid parcels, made dense at the surface, sink along slanting trajectories in intense nonhydrostatic plumes. After cooling ceases the Ertel potential vorticity within the convective layer is indeed found to be vanishingly small, corresponding to convective neutrality measured in the absolute momentum surfaces that are tilted from the vertical by the horizontal vorticity of the thermal wind. In analogous fully three-dimensional calculations, the absolute momentum constraint is broken, and the convection at first coexists with, but is ultimately dominated by, a baroclinic instability of the mixed layer. For typical mixed layer depths of 500 m stability analysis predicts, and our explicit calculations confirm, that baroclinic waves with length scales O(5 km) develop with timescales of a day or so. By diagnosis of fully developed mixed layer turbulence, the authors assess the importance of the baroclinic eddy field as an agency of lateral and vertical buoyancy flux through the layer. A novel scaling for the lateral buoyancy flux due to the baroclinic eddies is suggested. These ideas are based on analysis of several experiments in which the ...

[1] Measurements of chemical tracers whose spatial gradients are primarily due to the time dependence of sources and/or sinks are often used to define ''tracer ages'' in an effort to diagnose transport. However, a major problem with interpreting these tracer ages is that different tracers can yield different ages, and at present, it is not clear what aspects of the transport are measured by the different tracers. We use the concept of a distribution of transit times to compare the timescales derived from different tracers, including CFCs, tritium-helium, and radioactive tracers. By performing a systematic study over a range of transit time distributions we examine under what conditions two tracers yield similar or different ages. It is shown that there can be significant differences in tracer ages and that in general, tracer ages are not fundamental timescales of the flow. Furthermore, even if ages from two tracers are similar these ages can be very different from the mean (ideal) age or the age of a third tracer. It is also shown that significant temporal variations in tracer ages can occur for steady transport and that these changes are of similar magnitude to the changes in CFC and tritium-helium ages observed in the North Atlantic and North Pacific over the 1980s and 1990s. Accounting for the changes in tracer ages caused by steady transport is necessary before attributing changes in tracer ages to changes in transport. The possibility of using the differences in ages from different tracers to infer information about the transit time distribution is also examined. It is shown that two tracer ages can constrain the first two moments (mean age and width) of the distribution, but how tightly these are constrained depends on the tracers used, the certainty of the age calculations, and the flow characteristics.

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[1] We present a technique to infer concentrations of anthropogenic carbon in the ocean from observable tracers and illustrate the technique using synthetic data from a simple model. In contrast to several recent studies, the technique makes no assumptions about transport being dominated by bulk advection and does not require separation of the small anthropogenic signal from the large and variable natural carbon cycle. Mixing is included naturally and implicitly by using observable tracers in combination to estimate the distributions of transit times from the surface to interior points. The time-varying signal of anthropogenic carbon in surface waters is propagated directly into the interior by the transit time distributions (TTDs) without having to consider background natural carbon. The TTD technique provides estimates of anthropogenic carbon, as simulated directly in the model, that are more accurate than techniques relying on single tracer ''ages'' (e.g., CFC age) to represent transport. In general, the TTD technique works best when at least two tracers are used in combination, and the tracers have significantly different timescales in either their surface temporal variation or radioactive decay. Possibilities are a CFC or CCl 4 in combination with natural D 14 C or 39 Ar. However, even for a CFC alone the TTD technique results in less bias for anthropogenic carbon estimates than use of a CFC age.

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This paper addresses some fundamental methodological issues concerning the sensitivity analysis of chaotic geophysical systems. We show, using the Lorenz system as an example, that a naïve approach to variational (“adjoint”) sensitivity analysis is of limited utility. Applied to trajectories which are long relative to the predictability time scales of the system, cumulative error growth means that adjoint results diverge exponentially from the “macroscopic climate sensitivity”(that is, the sensitivity of time‐averaged properties of the system to finite‐amplitude perturbations). This problem occurs even for time‐averaged quantities and given infinite computing resources. Alternatively, applied to very short trajectories, the adjoint provides an incorrect estimate of the sensitivity, even if averaged over large numbers of initial conditions, because a finite time scale is required for the model climate to respond fully to certain perturbations. In the Lorenz (1963) system, an intermediate time scale is found on which an ensemble of adjoint gradients can give a reasonably accurate (O(10%)) estimate of the macroscopic climate sensitivity. While this ensemble‐adjoint approach is unlikely to be reliable for more complex systems, it may provide useful guidance in identifying important parameter‐combinations to be explored further through direct finite‐amplitude perturbations.