Mean Eddy Research Articles

Effects of a melted greenland ice sheet on climate, vegetation, and the cryosphere

This paper investigates the possible implications for the earth-system of a melting of the Greenland ice-sheet. Such a melting is a possible result of increased high latitude temperatures due to increasing anthropogenic greenhouse gas emissions. Using an atmosphere-ocean general circulation model (AOGCM), we investigate the effects of the removal of the ice sheet on atmospheric temperatures, circulation, and precipitation. We find that locally over Greenland, there is a warming associated directly with the altitude change in winter, and the altitude and albedo change in summer. Outside of Greenland, the largest signal is a cooling over the Barents sea in winter. We attribute this cooling to a decrease in poleward heat transport in the region due to changes to the time mean circulation and eddies, and interaction with sea-ice. The simulated climate is used to force a vegetation model and an ice-sheet model. We find that the Greenland climate in the absence of an ice sheet supports the growth of trees in southern Greenland, and grass in central Greenland. We find that the ice sheet is likely to regrow following a melting of the Greenland ice sheet, the subsequent rebound of its bedrock, and a return to present day atmospheric CO2 concentrations. This regrowth is due to the high altitude bedrock in eastern Greenland which allows the growth of glaciers which develop into an ice sheet.

The oceanic boundary layer driven by wave breaking with stochastic variability. Part 1. Direct numerical simulations

We devise a stochastic model for the effects of breaking waves and fit its distribution functions to laboratory and field data. This is used to represent the space-time structure of momentum and energy forcing of the oceanic boundary layer in turbulence-resolving simulations. The aptness of this breaker model is evaluated in a direct numerical simulation (DNS) of an otherwise quiescent fluid driven by an isolated breaking wave, and the results are in good agreement with laboratory measurements. The breaker model faithfully reproduces the bulk features of a breaking event: the mean kinetic energy decays at a rate approaching t -1 , and a long-lived vortex (eddy) is generated close to the water surface. The long lifetime of this vortex (more than 50 wave periods) makes it effective in energizing the surface region of oceanic boundary layers. Next, a comparison of several different DNS of idealized oceanic boundary layers driven by different surface forcing (i.e. constant current (as in Couette flow), constant stress, or a mixture of constant stress plus stochastic breakers) elucidates the importance of intermittent stress transmission to the underlying currents. A small amount of active breaking, about 1.6% of the total water surface area at any instant in time, significantly alters the instantaneous flow patterns as well as the ensemble statistics. Near the water surface a vigorous downwelling-upwelling pattern develops at the head and tail of each three-dimensional breaker. This enhances the vertical velocity variance and generates both negative- and positive-signed vertical momentum flux. Analysis of the mean velocity and scalar profiles shows that breaking effectively increases the surface roughness z o by more than a factor of 30; for our simulations z o /λ0.04 to 0.06, where λ is the wavelength of the breaking wave. Compared to a flow driven by a constant current, the extra mixing from breakers increases the mean eddy viscosity by more than a factor of 10 near the water surface. Breaking waves alter the usual balance of production and dissipation in the turbulent kinetic energy (TKE) budget; turbulent and pressure transports and breaker work are important sources and sinks in the budget. We also show that turbulent boundary layers driven by constant current and constant stress (i.e. with no breaking) differ in fundamental ways. The additional freedom provided by a constant-stress boundary condition permits finite velocity variances at the water surface, so that flows driven by constant stress mimic flows with weakly and statistically homogeneous breaking waves.

Mean Flow and Variability in the Upper Portion of the East Sea Proper Water in the southwestern East Sea with APEX Floats

Drift data from 17 Argo profiling floats in the East Sea are used to understand the mean flow and its variability in the upper portion of the East Sea Proper Water (UESPW) (around 800 m). The flow penetrates into the Ulleung basin (UB) through two paths: an extension of the southward flowing of the North Korean Cold Water along the east coast of Korea and between Ulleung Island and Dok island. Flows at 800 m are observed in the range of from 0.2 to 4.29 cms-1 and the variability in the north of the UB is larger than that in the south of the UB. In the UB, cyclonic flows from 0.3 to 1.6 cms-1 are observed with the bottom topography. We found that the mean kinetic energy (MKE) and the mean eddy kinetic energy (EKE) are 1.3 and 2.1 cm2s-2 respectively.

Effects of lateral mixing on the mean state and eddy activity of an equatorial ocean

Numerical experiments are performed using an ocean general circulation model (OGCM) to assess the impact of the form of lateral mixing of momentum and tracers on the state of an equatorial ocean. It is found that the large‐scale structure of both the velocity and temperature fields are very sensitive to the imposed parameterization of lateral mixing in the model. With uniform values for the viscosity and diffusion coefficient across the domain, a decrease in these coefficients increases the activity of tropical instability waves (TIW), resulting in an increase in the sea surface temperature of the cold tongue, and thus overcoming the cold bias often found in ocean models of the equatorial Pacific. However, there is an associated increase in the strength of the Equatorial Under‐Current (EUC) to unrealistic levels. It is found that the strength of the EUC can be limited, while little affecting the TIW activity, by applying an enhanced level of mixing in the vicinity of the equator. This enhanced mixing is used to model the effect of the observed interleaving of water masses across the equator. Both the level and distribution of eddy kinetic energy (associated with TIWs) varies with the level and orientation of the lateral mixing. An examination of the main energy conversion terms of mean to eddy energy reveals that both the barotropic and baroclinic energy conversion terms are important for the production of TIWs. The relative role of each of these conversion terms changes as the lateral mixing is decreased. At low levels of lateral mixing, barotropic instability is found to dominate the production of total eddy energy. Including enhanced mixing at the equator decreases this barotropic production, but surprisingly increases the baroclinic production away from the equator.

A Note on the Maintenance of Asian Summer Monsoon Circulation

In this paper, the maintenance of Asian summer monsoon circulation is compared in the National Centres for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) reanalysis and National Centre for Medium Range Weather Forecasting (NCMRWF), India operational analysis. The time mean summer monsoon circulation is bifurcated into stable mean and transient eddy components. The mean component of the summer monsoon circulation is elucidated through the study of large-scale balances of kinetic energy, heat and moisture. Although the balance requirements are fairly satisfied by both NCEP/NCAR and NCMRWF fields, a major difference is noticed. Strong monsoon circulation is evinced by NCEP/NCAR over the Oceanic regions and NCMRWF over the landmass. The various mean budgets under consideration indicated this feature invariably.

Quasi-Decadal Variability of the Tropical Lower Stratosphere: The Role of Extratropical Wave Forcing

Column ozone and satellite-derived temperature records with lengths >20 yr are consistent with the existence of a long-term, quasi-decadal oscillation (QDO) of the tropical lower stratosphere. Using a one-dimensional model for the quasi-biennial oscillation (QBO) of ozone and temperature, it is found that decadal variability of the QBO can account for, at most, only a minor fraction of the tropical lower-stratospheric QDO. One additional source of long-term variability in the Tropics is extratropical wave forcing, which is an important driver of the Brewer–Dobson circulation. To investigate possible long-term variability of extratropical wave forcing, daily and monthly mean meridional eddy heat fluxes are calculated at a series of lower-stratospheric pressure levels over a 23-yr period using National Centers for Environmental Prediction (NCEP) reanalysis data. A decadal variation of the low-pass-filtered extratropical eddy heat flux is present in the Northern Hemisphere with an amplitude that increases with increasing altitude. In the extratropical Southern Hemisphere, a decadal variation is also present but is less regular, possibly owing to reduced radiosonde data coverage. A simplified model of the contribution of extratropical wave forcing to long-term variations in tropical lower-stratospheric ozone and temperature is then formulated based on the ozone chemical continuity and thermodynamic energy equations. Using this model together with empirically derived regression relationships between short-term changes in extratropical eddy heat flux and tendencies in both tropical column ozone and lower-stratospheric temperature, it is found that decadal variations of extratropical wave forcing in both hemispheres may be sufficient to explain much of the amplitude and the phase of the observed QDO of the tropical lower stratosphere.

The front on the Northern Flank of Georges Bank in spring: 2. Cross‐frontal fluxes and mixing

Hydrographic observations made with an undulating vehicle carrying a CTD and concurrent shipboard ADCP velocity observations over a 12‐day period are combined to investigate vertical mixing and cross‐frontal fluxes on the Northern Flank of Georges Bank. The CTD density time series is analyzed to detect the presence of vertical overturns, the statistics of which are used to infer vertical mixing parameters. Vertical turbulent buoyancy fluxes are downward and most intense, reaching values of 5 × 10−7 W/kg, near the bottom at the edge of the bank and decrease both on‐ and off‐bank. Horizontal, cross‐bank buoyancy fluxes are partitioned into mean, tidal pumping, and nontidal eddy components and are computed as a function of cross‐isobath/vertical position by averaging in the along‐isobath direction. The tidal pumping component is dominant over most of the cross‐bank section with a peak value of ∼1 × 10−4 W/kg, directed off‐bank near the bank edge. A diagnosed tidal vertical velocity field is used with mean buoyancy gradients to compute the along‐isopycnal skew flux. The horizontal component of this skew flux has similar spatial structure and magnitude to that of the observed tidal pumping flux. The divergent component of the skew flux, at depths above the bottom boundary layer, appears to be convergent north of the bank edge and divergent at the bank edge, suggesting that tidally driven advective processes drive buoyant bank water downward and off‐bank at mid‐depth and force the upwelling of deep, dense water near the bottom at the bank edge.

CFD Simulation of Mixing and Dispersion in Bubble Columns

Liquid phase mixing time was measured in 0.2 and 0.4 m i.d. columns over a wide range of superficial gas velocity (0.07–0.295 ms–1) and height-to-diameter ratio (1–10). A CFD model was developed for the prediction of flow pattern in terms of mean velocity and eddy diffusivity profiles. The predictions agree favourably with all the experimental data published in the literature. Complete energy balance was established in all cases. The validated CFD model was extended for the simulation of the macro-mixing process by incorporating the effects of both the bulk motion and the eddy diffusion. Excellent agreement was observed between the CFD predicted and experimental values of the mixing time over the entire range of D, VG and HD/D covered. The model was further extended for the prediction of residence time distribution and hence the axial dispersion coefficient (DL). The predicted values of DL were found to agree very well with all the experimental data published in the literature.

Cloud‐tracked winds for the first Mars Global Surveyor mapping year

We have measured winds using cloud motion in consecutive Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) wide‐angle global map swaths taken during the first mapping year (Ls 135°–360°–111°). We present a total of ∼11,200 wind vectors collected in the north polar region during Ls 135°–195° (late summer/early fall) and Ls 20°–55° (mid spring) and in the south polar region during Ls 337°–10° (late summer/early fall). For cases with good coverage, we also present the derived mean zonal and meridional winds and the associated eddies. The speed of the zonal winds in 60°N–70°N increases at ∼0.6 m/s/°Ls in late northern summer, and that in 60°S–70°S increases at a rate of ∼0.7 m/s/°Ls in late southern summer. The latitudinal distribution of zonal wind within 50°N–75°N from mid northern summer to early northern fall indicates that winds at higher latitudes are generally weaker than those at lower latitudes, but the rate of increase with time is faster at higher latitudes. There is a cyclonic gyre in the 90°W–0°–30°E sector in the north polar region. There are large‐scale waves in the weekly mean meridional wind and in the biweekly mean eddy momentum flux and eddy kinetic energy fields in the north polar region from mid to late summer. The cloud‐tracked winds in the north are generally consistent with winds calculated by general circulation model at the water condensation level derived from MGS Thermal Emission Spectrometer (TES) observations but appear stronger than the gradient winds derived from TES assuming no flow at the surface.

The Influence of Large-Scale Eddy Flux Variability on the Zonal Mean Ozone Distribution

The influence of the variability of large-scale eddy heat and momentum fluxes on interannual and decadal changes in the zonal mean ozone distribution is investigated based on a two-dimensional circulation model with interactively coupled transport and chemistry. The large-scale eddy fluxes are prescribed according to observations, that is, in terms of monthly mean eddy mixing coefficients that are derived from ECMWF reanalysis 1979–93 (ERA-15). The results show that the eddy-forced changes in the lower-stratospheric residual circulation and eddy mixing processes induce coherent changes in temperature, ozone, and other chemical tracers. Significant ozone declines are found mainly in the lower stratosphere during winter, with maximum values of about −1.2 Dobson units (DU) per kilometer per decade at midlatitudes and more than −2 DU km−1 decade−1 at polar latitudes. The seasonal and vertical structure of the observed Northern Hemispheric ozone trend is captured in a remarkable way. It is also shown that, at midlatitudes, maximum declines in the zonal mean lower-stratospheric ozone are mainly related to the eddy mixing processes, and, at polar latitudes, to the induced changes in the lower-stratospheric temperatures.

Float Observations of the Southern Ocean. Part I: Estimating Mean Fields, Bottom Velocities, and Topographic Steering

Autonomous Lagrangian Circulation Explorer (ALACE) floats are used to examine mean flow and eddy fluxes at 900-m depth in the Southern Ocean. Mean temperature and dynamic topography from float data are consistent with earlier estimates from hydrographic surveys, although floats imply warmer temperatures and narrower frontal structures than do atlas data. Differences between hydrographic and ALACE dynamic topography suggest the presence of eastward bottom velocities of about 2 cm s−1 below the eastward-flowing jets of the Antarctic Circumpolar Current. Flow is steered by bathymetry and can be represented as an equivalent barotropic system with an e-folding depth of about 700 m.

Spreading of Labrador sea water: an advective-diffusive study based on Lagrangian data

The pathways and timescales for the spreading of Labrador Sea Water (LSW) in the subpolar North Atlantic are investigated with an advective–diffusive model. The model's mean flow and eddy diffusivity are derived from float measurements, while the region of LSW formation is obtained from hydrographic data. Two main export pathways for LSW are reproduced by the model: eastward into the Irminger Sea and southward via the Deep Western Boundary Current (DWBC). The mean interior flow field in the Labrador Sea is found to play an important role in feeding both pathways. In particular, flow into the Irminger basin is due to a cyclonic recirculation located southwest of Greenland while the export via the DWBC is partially maintained through an internal pathway, transporting LSW across the basin to the west Greenland coast. A region of high eddy kinetic energy west of Greenland tends to increase the flushing rate of LSW, but its impact is found to be limited. The residence time for LSW in the Labrador basin is estimated to be approximately 4–5 years, with 80% leaving via the DWBC and 20% via the Irminger pathway.

Circulation characteristics over the Himalayas during winter season

¶During winter season, atmospheric systems, which traverse from west to east interact with the Himalayan massif and produce widespread rainfall over North India. In this study, we made an endeavor to examine the mean circulation features and large-scale budgets of kinetic energy, heat and moisture over Himalayas and the adjoining domain for winter season. The time-mean circulation is bifurcated into stable mean and transient eddy parts and examined the mean component of the circulation. The uninitialized daily analyses of European Centre for Medium range Weather Forecasts (ECMWF) for five winter seasons (1986–90) comprising December, January and February (DJF) have been used for the purpose.

Tidal Currents in the Tsushima Straits Estimated from ADCP Data by Ferryboat

Tidal currents in the Tsushima Straits have been analyzed using measurements obtained since February 1997 by an acoustic Doppler current profiler (ADCP) mounted on the ferryboat Camellia. Tidal current constituents (M 2, S 2, K 1, O 1) are dominant among the ten tidal current constituents (Q 1, O 1, P 1, K 1, N 2, M 2, S 2, K 2, MSf, Mf), and generally 1.4–2.1 times stronger at the western channel of the straits than those at the eastern channel. The ratio between amplitude of M 2, S 2, K 1 and O 1 averaged along the ferryboat track is 1:0.45:0.59:0.51. The major axis directions of tidal current ellipses are generally SW to NE, exceptionally in the vicinity of the Tsushima Islands. Approaching the Tsushima Islands from the Korean Peninsula side, the major axis gradually rotates clockwise. At the western channel, the M 2 and K 1 constituents change the rotation direction of current vectors from clockwise to counterclockwise at about 90–130 m depth. The contributions of the tidal currents to the mean kinetic energy and the mean eddy kinetic energy along the ferryboat track are, on average, 0.56 and 0.71, respectively. This suggests that tidal current activities are generally more dominant than the mean current activities and much more dominant than eddy activities. The only region where the eddy activities are comparable to the tidal current activities is located on the east side of the Tsushima Islands.

Surface circulation in the eastern North Atlantic, from drifters and altimetry

A description of the near‐surface circulation and its properties is the result of the analysis of a drifting buoy data set in the eastern North Atlantic between the Iberian Peninsula, the Azores, and the Canary Islands. World Ocean Circulation Experiment–Tropical Ocean–Global Atmosphere experiment drifters equipped with holey sock drogues centered at 15 m depth collected a total of 14.4 years of data. The drifters sampled a rather inhomogeneous velocity field with a weak mean flow regime and eddies of different scales. They meandered southward everywhere in the study region, except in the Iberian coastal transition zone north of 41°N where they headed northward. The near‐surface mean velocity field obtained from the drifter data set shows all important mean currents, including the poleward Portugal Coastal Countercurrent during the fall, winter, and early spring off western and northern Iberia, the southward Portugal Coastal Current, the slow offshore southward flow of the Portugal Current during the whole year, the southwestward Canary Current, and the eastward Azores Current, which extends to the vicinity of the African coast near the Gulf of Cadiz. Maps of the eddy kinetic energy field were obtained from the drifters and from satellite altimetry. It provides the largest part of the total kinetic energy. The rate of dispersion is estimated from the Lagrangian statistics of the drifting buoys. The dispersion of the drifters in the study region is well modeled by a simple description of eddy diffusion assuming homogeneous turbulence. Ensemble mean diffusivities K and the Langrangian integral length scales and timescales (L and T) were obtained for the zonal and meridional directions. The sea surface temperature measured along the drifter trajectories is used to produce estimates of the eddy diffusivity, which is compared with the diffusivity estimates obtained from the theory of Taylor. The eddy diffusivity is found to be approximately proportional to the eddy kinetic energy. Discrete eddies and meanders were observed using drifters and altimetry in order to map and describe their geographical distribution and characteristics in the eastern North Atlantic.

A regional tidal/subtidal circulation model of the southeastern Bering Sea: development, sensitivity analyses and hindcasting

Abstract A regional eddy-resolving primitive equation circulation model was used to simulate circulation on the southeastern Bering Sea (SEBS) shelf and basin. This model resolves the dominant observed mean currents, eddies and meanders in the region, and simultaneously includes both tidal and subtidal dynamics. Circulation, temperature, and salinity fields for years 1995 and 1997 were hindcast, using daily wind and buoyancy flux estimates, and tidal forcing derived from a global model. This paper describes the development of the regional model, a comparison of model results with available Eulerian and Lagrangian data, a comparison of results between the two hindcast years, and a sensitivity analysis. Based on these hindcasts and sensitivity analyses, we suggest the following: (1) The Bering Slope Current is a primary source of large ( ∼100 km diameter) eddies in the SEBS basin. Smaller meanders are also formed along the 100 m isobath on the southeastern shelf, and along the 200-m isobath near the shelf break. (2) There is substantial interannual variability in the statistics of eddies within the basin, driven by variability in the strength of the ANSC. (3) The mean flow on the shelf is not strongly sensitive to changes in the imposed strength of the ANSC; rather, it is strongly sensitive to the local wind forcing. (4) Vertical mixing in the SEBS is strongly affected by both tidal and subtidal dynamics. Strongest mixing in the SEBS may in fact occur between the 100- and 400-m isobaths, near the Pribilof Islands, and in Unimak Pass.

Turbulence characteristics of wind over a hill with a rough surface

The turbulence characteristics of a boundary layer flow over a two-dimensional hill with a rough surface are investigated through experiments conducted in a wind tunnel under neutral conditions by using a hot wire constant temperature anemometer system with a split-fiber probe. Two types of windbreak fence are used to set on the hill surface. Its porosities φ are 0% and 50%. Measurements analysis includes mean velocity, turbulent velocity, Reynolds stress, turbulent energy and eddy viscosity profiles over the hill surface and in the wake region. The results obtained indicate the following: (a) the mean velocity profiles for the types of windbreak fence slightly change with the different fence arrangements, (b) the turbulent velocity shows an almost uniform distribution across the windward ascent slope, (c) the turbulence velocity becomes higher at the upper wake region behind the hill, (d) the porous fences work as strong windbreaks and weaken the wind velocity near the hill surface.

Extracting Multiyear Surface Currents from Sequential Thermal Imagery Using the Maximum Cross-Correlation Technique

Ocean surface circulation can be estimated by automated tracking of thermal infrared features in pairs of sequential satellite imagery. A 7-yr time series of velocity, extracted from thermal imagery of the East Australian Current using the maximum cross-correlation (MCC) technique, provides enough measurements for a more statistical evaluation of the method than has previously been possible. Excluding 1 yr with extensive cloud cover, the method produces about 8000 velocity estimates per month with some seasonal variation. Method precision is estimated to be between 0.08 and 0.2 m s21 rms, the lower value with more restrictive compositing. Mean flow, time-dependent flow, and eddy kinetic energy from the time series are compared with values derived from a dynamic height climatology, altimeter analyses, and drifter datasets in the region. The observations reproduce similar features in the flow. The differences between the observations are discussed in relation to noise in the methods and differences in the types of velocities they measure.

Eddy mean flow decomposition and eddy diffusivity estimates in the tropical Pacific Ocean: 2. Results

Eddy diffusivity of the surface velocity field in the tropical Pacific Ocean was estimated using satellite‐tracked drifting buoys (1979 through mid‐1996). The tropical Pacific surface current system is characterized by nonstationarity, strong meridional shear, and an energetic mesoscale velocity field. Eddy diffusivity may be defined as the integral of the autocovariance of Lagrangian eddy velocities, requiring both stationary and homogeneous statistics of the eddy field. Eddy velocities were obtained by removing a splined mean field to eliminate mean shear from observations binned (1) spatially to group data that have similar dispersion characteristics and (2) temporally to create stationary eddy statistics. Zonal diffusivity estimates are up to ≈7 times larger than meridional diffusivity estimates in the high eddy energy regions. This anisotropy is associated with the meridional mesoscale wave motion (i.e., by equatorial and tropical instability waves) that increases eddy variance but does not lead to a proportional increase in water parcel diffusion because of the coherent character of the trajectory motion, at least for initial time lags. Simple autoregressive models of first and second order are used to describe and classify the resulting eddy statistics. An independent confirmation of the diffusivity estimate in the central/eastern Pacific was obtained by comparing tracer flux divergence computed from a parameterization using diffusivity estimates of our analysis with that from direct eddy Reynolds stress flux divergence. Our results show that diffusivity can be estimated for regions not considered previously either because of sparse data or the complexities of the velocity field.

Justifying the WKB approximation in pure katabatic flows

Pure katabatic flow is studied with a Prandtl-type model allowing eddy diffusivity/conductivity to varywith height. Recently we obtained an asymptotic solution to the katabatic flow assuming the validity ofthe WKB method, which solves the fourth-order governing equation coupling the momentum and heat transfer. The WKB approximation requires that eddy diffusivity may vary only gradually compared to the calculated quantities, i.e., potential temperature and wind speed. This means that the scale heightfor eddy diffusivity must be higher than that for the calculated potential temperature and wind speed. The ratio between the maximum versus the mean eddy diffusivity should be less than that for the scale heights of the diffusivity versus the calculated quantities (temperature and wind). Here we justify(a posteriori) the WKB method independently based on two arguments: (i) a scaling argument and (ii )a philosophy behind a higher-order closure turbulence modeling. Both the eddy diffusivity maximum and the level of the relevant maximum turbulent kinetic energy are above the strongest part of the near surface inversion and the low-level jet which is required for the WKB validity. Thus, the numerical modeling perspective lends credibility to the simple WKB modeling. This justification is important before other data sets are analyzed and a parameterization scheme written.