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Chris Wilson and Richard G. Williams

Abstract

The mechanisms controlling the direction of eddy tracer fluxes are examined using eddy-resolving isopycnic experiments for a cyclic zonal channel. Eddy fluxes are directed downgradient on average when either (i) there is a Lagrangian increase in tracer variance or (ii) there is strong dissipation of tracer variance. The effect of the eddies on the mean tracer evolution can be described through an ensemble of eddies that each have a particular life cycle. Local examination of the eddy behavior, such as fluxes, eddy kinetic energy, and tracer variance appears complex, although the cumulative time-mean picture has coherence: eddies are preferentially formed in localized regions with downstream growth and increase in tracer variance concomitant with downgradient eddy tracer fluxes, while eventually the eddies decay with a decrease in tracer variance and upgradient eddy tracer fluxes. During spinup, tracer deformation through flow instability leads to an area-average increase in tracer variance (although locally it is increasing and decreasing with the individual eddy life cycles) and therefore an implied area-average, downgradient tracer flux. At a steady state, part of the pattern in eddy fluxes simply reflects advection of background tracer variance by the time-mean and eddy flows. The eddy flux becomes biased to being directed downgradient if there is a strong sink in the tracer, which is likely to be the case for eddy heat fluxes along isopycnals outcropping in the mixed layer or for eddy nitrate fluxes along isopycnals intersecting the euphotic zone.

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Chris Wilson and Richard G. Williams

Abstract

Eddy fluxes systematically affect the larger-scale, time-mean state, but their local behavior is difficult to parameterize. To understand how eddy fluxes of potential vorticity (PV) are controlled, the enstrophy budget is diagnosed for a five-layer, 1/16°, eddy-resolving, isopycnic model of a wind-driven, flat-bottom basin. The direction of the eddy flux across the mean PV contours is controlled by the Lagrangian evolution of enstrophy, including contributions from the temporal change and mean and eddy advection, as well as dissipation of enstrophy. During the spinup, an overall increase in enstrophy is consistent with eddy fluxes being directed downgradient on average and homogenization of PV within intermediate layers. Enstrophy becomes largest along the flanks of the gyre, where PV gradients are large, and becomes smallest in the interior. At a statistically steady state, there is a reversing pattern of up- and downgradient eddy PV fluxes, which are locally controlled by the advection of enstrophy. A downgradient eddy PV flux occurs only on the larger scale over the gyre flanks and part of the western boundary. These larger-scale patterns are controlled by the eddy advection of enstrophy, which becomes significant in regions of high eddy enstrophy. As a consequence, at a statistically steady state, the eddy PV fluxes are not simply related to the mean fields, and their local, finescale pattern is difficult to parameterize.

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Richard G. Wilson and Wayne R. Rouse

Abstract

Energy balance measurements of evapotranspiration from a developing corn crop are compared with daily equilibrium evapotranspiration estimates to examine the accuracy of the model and the environmental conditions under which it can be applied. Equilibrium estimates compared closely (a standard error of 6%) with the measured values when the surface was moderately dry, a condition which applied to 14 of the 24 days of the experiment. The ratio of actual evapotranspiration to available energy and the Bowen ratio are used to establish moisture and temperature limits for the model. The success of the model was related to a typical diurnal pattern of the difference between actual and equilibrium evapotranspiration which reflects expected variations of moisture stress during daytime hours. The performance of the model was nearly independent of the physical condition of the surface and the height of the required air temperature measurement. An equation is presented which permits easy calculation of equilibrium evapotranspiration from air temperature, net radiation, and soil heat flux data.

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Chris Wilson, Bablu Sinha, and Richard G. Williams

Abstract

The control of atmospheric storm tracks by ocean dynamics, orography, and their interaction is investigated using idealized experiments with a simplified coupled atmosphere–ocean climate model. The study focuses on the quasi–steady state for the storm tracks in the Northern Hemisphere winter mean. The experiments start with a background state without mountains and ocean dynamics, and in separate stages include orography and a dynamic ocean to obtain a more realistic control simulation. The separate effects of ocean dynamics, orography, and their nonlinear interaction are identified for the storm tracks and the surface thermodynamic forcing over the ocean.

The model study suggests that atmospheric storm tracks are a robust feature of the climate system, occurring at midlatitudes even if there is no orographic forcing or ocean heat transport. Ocean dynamics generally lead to a poleward shift in both the storm track and the maximum in atmospheric northward heat transport and induce a northeastward tilt over the Atlantic. This poleward shift is linked to the extra heat transport by the ocean and the tightening of sea surface temperature gradients on the western side of ocean basins. Orographic forcing causes along-track variability with a weakening of the storm track over the continents and induces a northeastward tilt over the western Pacific, which is associated with a stationary planetary wave train generated by the Tibetan Plateau. The interaction between ocean dynamics and orographic forcing plays a localized role, enhancing the contrast between the Atlantic and Pacific. Much of the response to the forcing is eddy mediated and transient eddies help to spread the influence of orographic and ocean forcing.

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Richard G. Williams, Chris Wilson, and Chris W. Hughes

Abstract

Signatures of eddy variability and vorticity forcing are diagnosed in the atmosphere and ocean from weather center reanalysis and altimetric data broadly covering the same period, 1992–2002. In the atmosphere, there are localized regions of eddy variability referred to as storm tracks. At the entrance of the storm track the eddies grow, providing a downgradient heat flux and accelerating the mean flow eastward. At the exit and downstream of the storm track, the eddies decay and instead provide a westward acceleration. In the ocean, there are similar regions of enhanced eddy variability along the extension of midlatitude boundary currents and the Antarctic Circumpolar Current. Within these regions of high eddy kinetic energy, there are more localized signals of high Eady growth rate and downgradient eddy heat fluxes. As in the atmosphere, there are localized regions in the Southern Ocean where ocean eddies provide statistically significant vorticity forcing, which acts to accelerate the mean flow eastward, provide torques to shift the jet, or decelerate the mean flow. These regions of significant eddy vorticity forcing are often associated with gaps in the topography, suggesting that the ocean jets are being locally steered by topography. The eddy forcing may also act to assist in the separation of boundary currents, although the diagnostics of this study suggest that this contribution is relatively small when compared with the advection of planetary vorticity by the time-mean flow.

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M. Susan Lozier, Sheldon Bacon, Amy S. Bower, Stuart A. Cunningham, M. Femke de Jong, Laura de Steur, Brad deYoung, Jürgen Fischer, Stefan F. Gary, Blair J. W. Greenan, Patrick Heimbach, Naomi P. Holliday, Loïc Houpert, Mark E. Inall, William E. Johns, Helen L. Johnson, Johannes Karstensen, Feili Li, Xiaopei Lin, Neill Mackay, David P. Marshall, Herlé Mercier, Paul G. Myers, Robert S. Pickart, Helen R. Pillar, Fiammetta Straneo, Virginie Thierry, Robert A. Weller, Richard G. Williams, Chris Wilson, Jiayan Yang, Jian Zhao, and Jan D. Zika

Abstract

For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017.

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Dennis Baldocchi, Eva Falge, Lianhong Gu, Richard Olson, David Hollinger, Steve Running, Peter Anthoni, Ch. Bernhofer, Kenneth Davis, Robert Evans, Jose Fuentes, Allen Goldstein, Gabriel Katul, Beverly Law, Xuhui Lee, Yadvinder Malhi, Tilden Meyers, William Munger, Walt Oechel, K. T. Paw U, Kim Pilegaard, H. P. Schmid, Riccardo Valentini, Shashi Verma, Timo Vesala, Kell Wilson, and Steve Wofsy

FLUXNET is a global network of micrometeorological flux measurement sites that measure the exchanges of carbon dioxide, water vapor, and energy between the biosphere and atmosphere. At present over 140 sites are operating on a long-term and continuous basis. Vegetation under study includes temperate conifer and broadleaved (deciduous and evergreen) forests, tropical and boreal forests, crops, grasslands, chaparral, wetlands, and tundra. Sites exist on five continents and their latitudinal distribution ranges from 70°N to 30°S.

FLUXNET has several primary functions. First, it provides infrastructure for compiling, archiving, and distributing carbon, water, and energy flux measurement, and meteorological, plant, and soil data to the science community. (Data and site information are available online at the FLUXNET Web site, http://www-eosdis.ornl.gov/FLUXNET/.) Second, the project supports calibration and flux intercomparison activities. This activity ensures that data from the regional networks are intercomparable. And third, FLUXNET supports the synthesis, discussion, and communication of ideas and data by supporting project scientists, workshops, and visiting scientists. The overarching goal is to provide information for validating computations of net primary productivity, evaporation, and energy absorption that are being generated by sensors mounted on the NASA Terra satellite.

Data being compiled by FLUXNET are being used to quantify and compare magnitudes and dynamics of annual ecosystem carbon and water balances, to quantify the response of stand-scale carbon dioxide and water vapor flux densities to controlling biotic and abiotic factors, and to validate a hierarchy of soil–plant–atmosphere trace gas exchange models. Findings so far include 1) net CO2 exchange of temperate broadleaved forests increases by about 5.7 g C m−2 day−1 for each additional day that the growing season is extended; 2) the sensitivity of net ecosystem CO2 exchange to sunlight doubles if the sky is cloudy rather than clear; 3) the spectrum of CO2 flux density exhibits peaks at timescales of days, weeks, and years, and a spectral gap exists at the month timescale; 4) the optimal temperature of net CO2 exchange varies with mean summer temperature; and 5) stand age affects carbon dioxide and water vapor flux densities.

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