Meridional Eddy Diffusion Research Articles

Photochemistry, mixing and transport in Jupiter’s stratosphere constrained by Cassini

In this work, we aim at constraining the diffusive and advective transport processes in Jupiter’s stratosphere, using Cassini/CIRS observations published by Nixon et al. (2007,2010). The Cassini–Huygens flyby of Jupiter on December 2000 provided the highest spatially resolved IR observations of Jupiter so far, with the CIRS instrument. The IR spectrum contains the fingerprints of several atmospheric constituents and allows probing the tropospheric and stratospheric composition. In particular, the abundances of C2H2 and C2H6, the main compounds produced by methane photochemistry, can be retrieved as a function of latitude in the pressure range at which CIRS is sensitive to. CIRS observations suggest a very different meridional distribution for these two species. This is difficult to reconcile with their photochemical histories, which are thought to be tightly coupled to the methane photolysis. While the overall abundance of C2H2 decreases with latitude, C2H6 becomes more abundant at high latitudes. In this work, a new 2D (latitude-altitude) seasonal photochemical model of Jupiter is developed. The model is used to investigate whether the addition of stratospheric transport processes, such as meridional diffusion and advection, are able to explain the latitudinal behavior of C2H2 and C2H6. We find that the C2H2 observations are fairly well reproduced without meridional diffusion. Adding meridional diffusion to the model provides an improved agreement with the C2H6 observations by flattening its meridional distribution, at the cost of a degradation of the fit to the C2H2 distribution. However, meridional diffusion alone cannot produce the observed increase with latitude of the C2H6 abundance. When adding 2D advective transport between roughly 30 mbar and 0.01 mbar, with upwelling winds at the equator and downwelling winds at high latitudes, we can, for the first time, reproduce the C2H6 abundance increase with latitude. In parallel, the fit to the C2H2 distribution is degraded. The strength of the advective winds needed to reproduce the C2H6 abundances is particularly sensitive to the value of the meridional eddy diffusion coefficient. The coupled fate of these methane photolysis by-products suggests that an additional process is missing in the model. Ion-neutral chemistry was not accounted for in this work and might be a good candidate to solve this issue.

Direct Estimate of Lateral Eddy Diffusivity Upstream of Drake Passage

Abstract The first direct estimate of the rate at which geostrophic turbulence mixes tracers across the Antarctic Circumpolar Current is presented. The estimate is computed from the spreading of a tracer released upstream of Drake Passage as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). The meridional eddy diffusivity, a measure of the rate at which the area of the tracer spreads along an isopycnal across the Antarctic Circumpolar Current, is 710 ± 260 m2 s−1 at 1500-m depth. The estimate is based on an extrapolation of the tracer-based diffusivity using output from numerical tracers released in a one-twentieth of a degree model simulation of the circulation and turbulence in the Drake Passage region. The model is shown to reproduce the observed spreading rate of the DIMES tracer and suggests that the meridional eddy diffusivity is weak in the upper kilometer of the water column with values below 500 m2 s−1 and peaks at the steering level, near 2 km, where the eddy phase speed is equal to the mean flow speed. These vertical variations are not captured by ocean models presently used for climate studies, but they significantly affect the ventilation of different water masses.

Lateral diffusivity from tracer release experiments in the tropical North Atlantic thermocline

Lateral diffusivity is computed from a tracer release experiment in the northeastern tropical Atlantic thermocline. The uncertainties of the estimates are inferred from a synthetic particle release using a high‐resolution ocean circulation model. The main method employed to compute zonal and meridional components of lateral diffusivity is the growth of the second moment of a cloud of tracer. The application of an areal comparison method for estimating tracer‐based diffusivity in the field experiments is also discussed. The best estimate of meridional eddy diffusivity in the Guinea Upwelling region at about 300 m depth is estimated to be m2 s−1. The zonal component of lateral diffusivity is estimated to be m2 s−1, while areal comparison method yields areal equivalent zonal diffusivity component of m2 s−1. In comparison to Ky, Kx is about twice larger, resulting from the tracer patch stretching by zonal jets. Employed conceptual jet model indicates that zonal jet velocities of about m s−1 are required to explain the enhancement of the zonal eddy diffusivity component. Finally, different sampling strategies are tested on synthetic tracer release experiments. They indicate that the best sampling strategy is a sparse regular sampling grid covering most of the tracer patch.

Suppression of Eddy Diffusivity across Jets in the Southern Ocean

AbstractGeostrophic eddies control the meridional mixing of heat, carbon, and other climatically important tracers in the Southern Ocean. The rate of eddy mixing is typically quantified through an eddy diffusivity. There is an ongoing debate as to whether eddy mixing in enhanced in the core of the Antarctic Circumpolar Current or on its flanks. A simple expression is derived that predicts the rate of eddy mixing, that is, the eddy diffusivity, as a function of eddy and mean current statistics. This novel expression predicts suppression of the cross-jet eddy diffusivity in the core of the Antarctic Circumpolar Current, despite enhanced values of eddy kinetic energy. The expression is qualitatively and quantitatively validated by independent estimates of eddy mixing from altimetry observations. This work suggests that the meridional eddy diffusivity across the Antarctic Circumpolar Current is weaker than presently assumed because of the suppression of eddy mixing by the strong zonal current.

Lagrangian and Tracer Evolution in the Vicinity of an Unstable Jet

The dynamics of Lagrangian particles and tracers in the vicinity of a baroclinically unstable zonal jet are investigated in a simple two-layer model with an initially quiescent lower layer. The presence of a growing wave induces a particle drift dominated by Stokes drift rather then the contribution of the wave to the mean Eulerian velocity. Stable and unstable waves have zonal Stokes drift with similar meridional structure while only unstable waves possess meridional drift, which is in the direction of increasing meridional wave displacement. Particle dispersion in the upper layer is maximum at critical lines, where the jet and phase speeds are equal. In the lower layer, dispersion is maximum where the wave amplitude is maximum. Zonal mean tracer evolution is formulated as an advection‐diffusion equation with an order Rossby number advection and an orderone eddy diffusion. The latter is proportional to two-particle dispersion. Finite amplitude simulations of the flow reveal that small amplitude theory has predictive value beyond the range for which it is strictly valid. Mixing (as opposed to stirring) is maximum near cat’s-eye-like recirculation regions at the critical lines. In the lower layer the pattern of convergence and divergence of the flow locally increases tracer gradients, resulting in stirring yet with a much slower mixing rate than in the upper layer. Meridional eddy diffusion (or particle dispersion) alone is not sufficient for prediction of mixing intensity. Rotation, which is quantified by the cross-correlation of meridional and zonal displacements, must also be present for mixing. These results are consistent with observations of tracer and floats in the vicinity of the Gulf Stream.

Analysis of deep‐drogued satellite‐tracked drifter measurements in the northeast pacific

Abstract We analyse the trajectories of 24 deep‐drogued, satellite‐tracked drifters launched between 50 and 52°N in the northeast Pacific during June and October 1987. Three aspects of the observed motions at the drogue depths of 100 to 120 m are studied: (i) the spatial structure of the mean and variance velocity fields; (ii) the dispersion and eddy diffusion characteristics of the fluctuating motions; and (iii) the properties of selected mesoscale eddies. The mean Lagrangian velocity field is consistent with the mean flow pattern derived from the historical dynamic height topography. Fluctuating motions within the region are dominated by mesoscale eddies and meanders. Several instances of persistent O(100 days) westward flowing countercurrents were also observed. Based on the Lagrangian integral time‐ and length scales, drifter motions become decorrelated within a period of 10 days and a separation of 100 km. The mean zonal and meridional integral time‐scales of 4.5 and 3.6 days, respectively, are nearly identical with those obtained by Krauss and Böning (1987) from deep‐drogued drifter tracks in the North Atlantic. Because of the relatively small (<100 cm2 s−2) kinetic energy values in the northeast Pacific, the corresponding mean Lagrangian length scales of 29.4 and 29.9 km are roughly half those for the Atlantic. The observed drifter dispersion is generally consistent with Taylor's (1921) theory for single‐particle dispersion in homogeneous isotropic turbulence. Estimates obtained using 476 pseudo‐drifter tracks generated from the original records indicate that the dispersion increases linearly with time, t, within the first 3 to 5 days of launch and subsequently increases as t1/2 (the random‐walk regime) within 10 days of launch. The respective peak zonal and meridional eddy diffusion coefficients of 4.1 × and 3.8 × 107 cm2 s−1 are reached within 30 days of deployment. Similar estimates for the peak eddy diffusivities are obtained using dispersion curves for sets of 4 drifters launched at the same location during the June and October deployments. The dispersion of these clusters followed an exponential rather than a t1/2 dependence over the first 70 days after release. Eddies are predominantly clockwise rotary and are characterized by radii of 26 ± 16 km, periods of rotation of 16.0 ± 5.2 days, and azimuthal current speeds of 12.7 ± 8.6 cm s−1. One eddy was tracked for over 10 months. Oceanographic data collected during the October deployment period showed the eddies have vertical extents of 500 to 700 m and are linked to isotherm depressions of over 100 m in the main pycnocline. All eddies in the bifurcation zone propagate to the west at roughly 1.5 ± 0.4 cm s−1 counter to the prevailing mean flow and winds. These speeds are consistent with the westward phase speeds of first mode baroclinic planetary (Rossby) waves.

Rossby wave activity in a two‐dimensional model: Closure for wave driving and meridional eddy diffusivity

A parameterization of the effects of Rossby waves in the middle atmosphere is proposed for use in two‐dimensional models. By adding an equation for conservation of Rossby wave activity, closure is obtained for the meridional eddy fluxes and body force due to Rossby waves. Rossby wave activity is produced in a climatological fashion at the tropopause, is advected by a group velocity which is determined solely by model zonal winds, and is absorbed where it converges. Absorption of Rossby wave activity causes both an easterly torque and an irreversible mixing of potential vorticity, represented by the meridional eddy diffusivity, Kyy. The distribution of Rossby wave driving determines the distribution of Kyy, which is applied to all of the chemical constituents. This provides a self‐consistent coupling of the wave activity with the winds, tracer distributions, and the radiative field. Typical winter stratospheric values for Kyy of 2×106 m2 S−1 are obtained. Poleward tracer advection is enhanced and meridional tracer gradients are reduced where Rossby wave activity is absorbed in the model.

Meridional eddy diffusion model of the transport of atmospheric carbon dioxide: 1. Seasonal carbon cycle over the tropical Pacific Ocean

An extensive set of atmospheric CO2 observations were obtained during the First Global Geophysical Experiment (FGGE) Hawaii to Tahiti Shuttle expedition of 1979–1980. These data have been examined using a one‐dimensional meridional diffusive transport model of the atmospheric circulation. The observed CO2 concentration field was first decomposed into three parts: a seasonal component consisting of two harmonic functions of time, with periods of 1 year and 6 months; a mean annual north‐south concentration profile; and a linear trend with time, independent of latitude. In the first of two papers interpreting these data, we consider which features of the north‐south eddy transport are revealed by the seasonal component of the CO2 concentration field. We assume that the FGGE data represent a zonally and vertically uniform CO2 field, and we restrict our analysis to the latitude belt between 14.5°N and 14.5°S, where seasonal sources and sinks of CO2 have minimal influence on the atmospheric CO2 concentration. The strongest feature of the CO2 data for calibrating the model is the steadily diminishing amplitude of the annual harmonic from north to south. We show that in the absence of sources and sinks the magnitude of the eddy diffusion coefficient as a function of latitude can be prescribed from this amplitude, provided that two additional quantities are specified at a single reference location. These additional quantities are the amplitude of the first harmonic of the seasonal flux and the phase difference between that harmonic and the first harmonic of the local concentration signal. Selecting 14.5°S for this reference location, we find through comparisons of the model prediction with the FGGE CO2 data that the diffusion coefficient depends primarily on the selected amplitude of the seasonal flux and is practically independent of the phase difference. On the basis of the goodness of fit of the model predictions to the CO2 data, we set a lower limit on this flux amplitude, corresponding to an average eddy diffusion coefficient of 4.6 × 109 cm2 s−1. This is equivalent to an upper limit of 1.4 years on the interhemispheric exchange time for CO2. We are unable to set an upper limit on the diffusion coefficient because the phasing of the first harmonic in the FGGE data shifts so slightly with latitude that even very large diffusion coefficients yield satisfactory model predictions of the seasonal CO2 concentration field. The relative variation in the diffusion coefficient with latitude, which is derived solely from the amplitude of the first harmonic of the CO2 data, reveals two zones of resistance to interhemispheric CO2 exchange. One, near 8°N, corresponds to the well‐known intertropical convergence zone of the wind field which exists around the entire earth. The second, near 8°S, correlates with a weaker wind convergence zone in the central Pacific. This regional feature evidently affects the CO2 concentration field in the vicinity of the FGGE data set, but it may not represent a global feature.

Meridional eddy diffusion model of the transport of atmospheric carbon dioxide: 2. Mean annual carbon cycle

In the second of two papers interpreting atmospheric CO2 observations obtained during the First Global Geophysical Experiment (FGGE) Hawaii to Tahiti Shuttle expedition of 1979–1980, we consider features of the atmospheric CO2 cycle revealed by the mean annual component of the CO2 concentration field. For this purpose the FGGE data, after decomposition into seasonal, secular, and north‐south varying components, were extended to 71°N and to the south pole by included smoothed mean annual data based on CO2 observations at seven land stations. The resulting mean annual north‐south profile was referred to a datum of January 1, 1980. An additional profile for January 1, 1962 was derived from observations at five land stations, at an Arctic ice floe station, and from ships during the period 1960–1963. Both profiles have been examined using a one‐dimensional meridional diffusive transport model of the atmospheric circulation in which the latitudinal dependence of the eddy diffusion coefficient between 14.5°N and 14.5°S has been determined from seasonal variations in atmospheric CO2, and its mean value estimated from halocarbon and 85Kr data. The difference between the two CO2 concentration profiles is explained as being due almost entirely to the combustion of fossil fuels, which caused 2.7 × 1015 g more carbon to be injected into the air in 1980 than in 1962, predominantly north of 14.5°N. A residual profile was obtained by subtracting the predicted effect of the injection of fossil fuel CO2 from the 1980 profile. This residual profile has a peak concentration near the equator which, according to the model, is a result of the release of 5.0 × 1015 g yr−1 of carbon to the atmosphere between 14.5°N and 14.5°S, balanced by an equal removal from the atmosphere poleward of these latitudes. The source‐sink couple inferred to produce this CO2 exchange is consistent with the distribution of CO2 partial pressure in the equatorial ocean surface water, as observed on the FGGE Shuttle Expedition, provided that the air‐sea exchange rate of CO2 there is 30 mol m−2 yr−1. This exaggerated rate probably reflects the lack of vertical resolution in the model, such that the CO2 concentration near the sea surface is assumed to apply to the entire air column. The residual profile also shows a higher average concentration in the southern hemisphere than in the northern. The cause of this difference, of the order of 1 ppm, could not be resolved by the model owing to lack of information on the character of sources and sinks and transport behavior poleward of 14.5°N and 14.5°S; it may be owing to either oceanic or land biospheric CO2 exchange with the air or even to time dependent atmospheric transport.

Radiocarbon from nuclear weapons tests

The distribution in time and space of C14 from the 1961–1962 nuclear weapons tests of the U.S. and the USSR is used as a tracer for atmospheric mixing phenomena and exchange of carbon dioxide between the atmosphere and the sea. The mean residence time of C14 injected into the stratosphere by the tests or produced by cosmic rays is about 1.5 years before entering the troposphere. The C14 enters the troposphere predominantly at midlatitudes in the late spring. Dispersion of the C14 injected into the troposphere is well accounted for by eddy diffusion, with the meridional eddy diffusion coefficients being proportional to the variance of the meridional component of the wind speed. Exchange of carbon dioxide between the atmosphere and the sea (and therefore uptake of excess C14 by the sea) is taken to be proportional to the square of the average wind speed over the sea surface. This leads to the conclusion that the bulk of the C14 will eventually be taken up by the oceans of the southern hemisphere. A mean residence time of 2.5 years for a carbon dioxide molecule in the troposphere before uptake by the sea is deduced from the model, giving a mean residence time of 4 years for a C14 atom in the atmosphere before uptake by the sea.