scholarly journals The Role of the Divergent Circulation for Large-Scale Eddy Momentum Transport in the Tropics. Part I: Observations

2019 ◽  
Vol 76 (4) ◽  
pp. 1125-1144 ◽  
Author(s):  
Pablo Zurita-Gotor
Keyword(s):  
Momentum Flux ◽  
Large Scale ◽  
Zonal Flow ◽  
Stationary Wave ◽  
Hadley Cell ◽  
Momentum Transport ◽  
Mean Flow ◽  
Meridional Transport ◽  
The Tropics

Abstract This work investigates the role played by the divergent circulation for meridional eddy momentum transport in the tropical atmosphere. It is shown that the eddy momentum flux in the deep tropics arises primarily from correlations between the divergent eddy meridional velocity and the rotational eddy zonal velocity. Consistent with previous studies, this transport is dominated by the stationary wave component, associated with correlations between the zonal structure of the Hadley cell (zonal anomalies in the meridional overturning) and the climatological-mean Rossby gyres. This eddy momentum flux decomposition implies a different mechanism of eddy momentum convergence from the extratropics, associated with upper-level mass convergence (divergence) over sectors with anomalous westerlies (easterlies). By itself, this meridional transport would only increase (decrease) isentropic thickness over regions with anomalous westerly (easterly) zonal flow. The actual momentum mixing is due to vertical (cross isentropic) advection, pointing to the key role of diabatic processes for eddy–mean flow interaction in the tropics.

The Role of the Divergent Circulation for Large-Scale Eddy Momentum Transport in the Tropics. Part II: Dynamical Determinants of the Momentum Flux

2019 ◽  
Vol 76 (4) ◽  
pp. 1145-1161 ◽  
Author(s):  
Pablo Zurita-Gotor
Keyword(s):  
Momentum Flux ◽  
Large Scale ◽  
Opposite Sign ◽  
Rotational Momentum ◽  
Divergence Field ◽  
Flux Response ◽  
Open Question ◽  
The Tropics ◽  
Main Factor

Abstract This paper investigates the coupling between the rotational and divergent circulations aiming to explain the observations that show that the tropical eddy momentum flux is due to correlations between divergent eddy meridional velocities and rotational eddy zonal velocities. A simple linear model in which the observed eddy divergence field is used to force the vorticity equation can reproduce quite well the observed tropical eddy momentum flux. The eddy momentum flux in the model shows little sensitivity to the basic-state winds and is mainly determined by the characteristics of the divergent forcing. Vortex stretching and divergent advection of planetary vorticity produce eddy momentum flux contributions with the same sign but the former forcing dominates. It is shown that the main factor affecting the direction of the eddy momentum flux response to both forcings is the meridional tilt of the divergence phase lines, albeit with an opposite sign to the classical relation between rotational momentum flux and streamfunction phase tilt. How this divergent structure is determined remains an open question.

The large-scale tilt of the eddy divergence in the tropics

2020 ◽  
Author(s):  
Pablo Zurita-Gotor
Keyword(s):  
Momentum Flux ◽  
Large Scale ◽  
Hadley Cell ◽  
Coarse Grained ◽  
Wave Source ◽  
Summer Hemisphere ◽  
Divergence Field ◽  
Upper Level ◽  
The Tropics

<p>This work is concerned with the large-scale structure of the upper-level divergence/precipitation field in the deep tropics. Once the fine ITCZ structure is filtered out, the coarse-grained eddy divergence field is found to tilt eastward moving away from its maximum near the equator in the summer hemisphere. This robust tilt (observed for both hemispheres and seasons) is also present in the classical Gill solution.</p><p>In this presentation we show that the sign of the tilt is intimately linked to the direction of the eddy momentum flux. The observed eastward tilt is such that the momentum flux is directed towards the wave source, suggesting that the observed tilt is determined by wave propagation.</p><p>We also discuss the determination of the tilt in the simple Gill model and its sensitivity to the meridional Hadley flow. We show that the increase in the cross-equatorial momentum flux when the Hadley cell strengthens is associated with an increased tilt of the divergence field in the downstream direction of the flow, supporting the conjecture that the tilt is associated with propagation. </p>

Delineating the Eddy–Zonal Flow Interaction in the Atmospheric Circulation Response to Climate Forcing: Uniform SST Warming in an Idealized Aquaplanet Model

2013 ◽  
Vol 70 (7) ◽  
pp. 2214-2233 ◽  
Author(s):  
Gang Chen ◽  
Jian Lu ◽  
Lantao Sun
Keyword(s):  
Wave Propagation ◽  
Momentum Flux ◽  
Surface Friction ◽  
Zonal Flow ◽  
Finite Amplitude ◽  
Wave Activity ◽  
Hadley Cell ◽  
Climate Forcing ◽  
Sst Warming ◽  
Poleward Shift

Abstract The mechanisms of the atmospheric response to climate forcing are analyzed using an example of uniform SST warming in an idealized aquaplanet model. A 200-member ensemble of experiments is conducted with an instantaneous uniform SST warming. The zonal mean circulation changes display a rapid poleward shift in the midlatitude eddy-driven westerlies and the edge of the Hadley cell circulation and a slow equatorward contraction of the circulation in the deep tropics. The shift of the poleward edge of the Hadley cell is predominantly controlled by the eddy momentum flux. It also shifts the eddy-driven westerlies against the surface friction, at a rate much faster than the expectation from the natural variability of the eddy-driven jet (i.e., the e-folding time scale of the annular mode), with much less feedback between the eddies and zonal flow. The transient eddy–zonal flow interactions are delineated using a newly developed finite-amplitude wave activity diagnostic of Nakamura. Applying it to the transient ensemble response to uniform SST warming reveals that the eddy-driven westerlies are shifted poleward by permitting more upward wave propagation in the middle and upper troposphere rather than reducing the lower-tropospheric baroclinicity. The increased upward wave propagation is attributed to a reduction in eddy dissipation of wave activity as a result of a weaker meridional potential vorticity (PV) gradient. The reduction allows more waves to propagate away from the latitudes of baroclinic generation, which, in turn, leads to more poleward momentum flux and a poleward shift of eddy-driven winds and Hadley cell edge.

Equatorial deep jets and their influence on the equatorial mean circulation in an idealised model forced by intraseasonal momentum flux convergence

2020 ◽  
Author(s):  
Swantje Bastin ◽  
Martin Claus ◽  
Peter Brandt ◽  
Richard J. Greatbatch
Keyword(s):  
Momentum Flux ◽  
Intraseasonal Variability ◽  
Zonal Flow ◽  
Wind Forcing ◽  
Vertical Resolution ◽  
Mean Flow ◽  
Geophysical Research ◽  
Zonal Jets ◽  
Total Magnitude ◽  
Idealised Model

<p>Equatorial deep jets (EDJ) are vertically stacked, downward propagating zonal jets that alternate in direction with depth. In the tropical Atlantic, they have been shown to influence both surface conditions and tracer variability. Despite their importance, the EDJ are absent in most ocean models, most likely due to a lack of vertical resolution. However, when the vertical resolution is sufficiently high, EDJ oscillating at a period of about 4.5 years are present in idealised model configurations driven by steady wind forcing. We have used such a model for a basin-wide reconstruction of the intraseasonal eddy momentum flux convergence, which has recently been shown to be a large contributor to the EDJ maintenance (Greatbatch et al., 2018). When we apply the diagnosed momentum flux convergence that is oscillating at the EDJ period as forcing in a model without wind forcing, EDJ develop, allowing us to verify the mechanism proposed in Greatbatch et al. (2018). Additionally, we can isolate the nonlinear effect that the EDJ have on the time mean zonal flow at intermediate depths. We can show that the EDJ drive time mean zonal flow that is similar in structure to the mean flow measured by Argo floats at 1000 m depth, contributing (in our idealised setup) about one fifth of the total magnitude of the observed flow, the rest likely resulting from direct forcing by downward propagating intraseasonal waves as shown before in other studies.</p><p> </p><p>References:</p><p>Greatbatch, R. J., M. Claus, P. Brandt, J.-D. Matthießen, F. P. Tuchen, F. Ascani, M. Dengler, J. Toole, C. Roth, J. T. Farrar, 2018: Evidence for the Maintenance of Slowly Varying Equatorial Currents by Intraseasonal Variability. <em>Geophysical Research Letters</em>, <strong>45</strong>, 1923-1929.</p>

Investigating the Dynamics of Hothouse Earth Climates with a Simplified GCM

2020 ◽  
Author(s):  
Matthew McKinney ◽  
Jonathan Mitchell
Keyword(s):  
Large Scale ◽  
Hadley Cell ◽  
Three Dimensions ◽  
Status Report ◽  
Atmospheric Moisture ◽  
Surface Liquid ◽  
The Tropics ◽  
Large Scale Atmospheric Circulation ◽  
Atmospheric Moisture Content ◽  
Saturation Vapor

<p>There are records of past Earth climates that were ice-free all the way to the poles (Barron 1983), which can be described as “hothouse” climates. These hothouse climates can be contrasted with an “all-tropics” planet, where the tropics are defined by the atmospheric dynamics, i.e. the Hadley Cell extent (Faulk et al. 2017). This classification is thus primarily dependent on a planet’s rotation, rather than its ice-free extent or surface temperatures. We investigate the parameter space between Earth and an all-tropics world using the open-source GCM Isca, developed by Vallis et al (2018). We take an Earth analog and perform a parameter sweep in three dimensions: global reservoir depth (1000m, 100m, 10m, 1m, 1cm); global saturation vapor pressure (1.5x current, 1.4x, 1.3x, 1.2x, 1.1x, 1x); and rotation rate (16 days, 8 days, 1 day). The sweep will allow us to explore the effects of surface liquid coverage, atmospheric moisture content, and large-scale atmospheric circulation on an Earth-like climate. In this presentation we provide a status report and analysis of initial findings.</p>

scholarly journals Rossby waves and two-dimensional turbulence in a large-scale zonal jet

1987 ◽  
Vol 183 ◽  
pp. 467-509 ◽  
Author(s):  
Theodore G. Shepherd
Keyword(s):  
Large Scale ◽  
Zonal Flow ◽  
Order Effect ◽  
Spatial Inhomogeneity ◽  
Mean Flow ◽  
Scale Separation ◽  
Zonal Flows ◽  
Beta Plane ◽  
Zonal Wavenumber ◽  
Scale Shear

The theory of homogeneous barotropic beta-plane turbulence is here extended to include effects arising from spatial inhomogeneity in the form of a zonal shear flow. Attention is restricted to the geophysically important case of zonal flows that are barotropically stable and are of larger scale than the resulting transient eddy field.Because of the presumed scale separation, the disturbance enstrophy is approximately conserved in a fully nonlinear sense, and the (nonlinear) wave-mean-flow interaction may be characterized as a shear-induced spectral transfer of disturbance enstrophy along lines of constant zonal wavenumber k. In this transfer the disturbance energy is generally not conserved. The nonlinear interactions between different disturbance components are turbulent for scales smaller than the inverse of Rhines's cascade-arrest scale κβ≡ (β0/2urms)½ and in this regime their leading-order effect may be characterized as a tendency to spread the enstrophy (and energy) along contours of constant total wavenumber κ ≡ (k2 + l2)½. Insofar as this process of turbulent isotropization involves spectral transfer of disturbance enstrophy across lines of constant zonal wavenumber k, it can be readily distinguished from the shear-induced transfer which proceeds along them. However, an analysis in terms of total wavenumber K alone, which would be justified if the flow were homogeneous, would tend to mask the differences.The foregoing theoretical ideas are tested by performing direct numerical simulation experiments. It is found that the picture of classical beta-plane turbulence is altered, through the effect of the large-scale zonal flow, in the following ways: (i) while the turbulence is still confined to KKβ, the disturbance field penetrates to the largest scales of motion; (ii) the larger disturbance scales K < Kβ exhibit a tendency to meridional rather than zonal anisotropy, namely towards v2 > u2 rather than vice versa; (iii) the initial spectral transfer rate away from an isotropic intermediate-scale source is significantly enhanced by the shear-induced transfer associated with straining by the zonal flow. This last effect occurs even when the large-scale shear appears weak to the energy-containing eddies, in the sense that dU/dy [Lt ] κ for typical eddy length and velocity scales.

scholarly journals Mesospheric gravity wave activity estimated via airglow imagery, multistatic meteor radar, and SABER data taken during the SIMONe–2018 campaign

2020 ◽  
Author(s):  
Fabio Vargas ◽  
Jorge L. Chau ◽  
Harikrishnan Charuvil Asokan ◽  
Michael Gerding
Keyword(s):  
Gravity Waves ◽  
Momentum Flux ◽  
Large Scale ◽  
Lower Thermosphere ◽  
Small Scale ◽  
Meteor Radar ◽  
Mean Flow ◽  
Horizontal Scale ◽  
Momentum Fluxes ◽  
Mesosphere And Lower Thermosphere

Abstract. We describe in this study the analysis of small and large horizontal scale gravity waves from datasets composed of images from multiple mesospheric nightglow emissions as well as multistatic specular meteor radar (MSMR) winds collected in early November 2018, during the SIMONe–2018 campaign. These ground-based measurements are supported by temperature and neutral density profiles from TIMED/SABER satellite in orbits near Kühlungsborn, northern Germany (54.1° N, 11.8° E). The scientific goals here include the characterization of gravity waves and their interaction with the mean flow in the mesosphere and lower thermosphere and their relationship to dynamical conditions in the lower and upper atmosphere. We obtain intrinsic parameters of small and large horizontal scale gravity waves and characterize their impact in the mesosphere region via momentum flux and flux divergence estimations. We have verified that a small percent of the detected wave events are responsible for most of the momentum flux measured during the campaign from oscillations seen in the airglow brightness and MSMR winds. From the analysis of small-scale gravity waves in airglow images, we have found wave momentum fluxes ranging from 0.38 to 24.74 m2/s2 (0.88 ± 0.73 m2/s2 on average), with a total of 586.96 m2/s2 (sum over all 362 detected waves). However, small horizontal scale waves with flux > 3 m2/s2 (11 % of the events) transport 50 % of the total measured flux. Likewise, wave events having flux > 10 m2/s2 (2 % of the events) transport 20 % of the total flux. The examination of two large-scale waves seen simultaneously in airglow keograms and MSMR winds revealed relative amplitudes > 35 %, which translates into momentum fluxes of 21.2–29.6 m/s. In terms of gravity wave–mean flow interactions, these high momentum flux waves could cause decelerations of 22–41 m/s/day (small-scale waves) and 38–43 m/s/day (large-scale waves) if breaking or dissipating within short distances in the mesosphere and lower thermosphere region. The dominant large-scale waves might be the result of secondary gravity excited from imbalanced flow in the stratosphere caused by primary wave breaking.

scholarly journals Large-scale transport into the Arctic: the roles of the midlatitude jet and the Hadley Cell

2019 ◽  
Vol 19 (8) ◽  
pp. 5511-5528 ◽  
Author(s):  
Huang Yang ◽  
Darryn W. Waugh ◽  
Clara Orbe ◽  
Guang Zeng ◽  
Olaf Morgenstern ◽  
...  
Keyword(s):  
Large Scale ◽  
Climate Model ◽  
Source Region ◽  
Meridional Flow ◽  
Hadley Cell ◽  
The Arctic ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Meridional Transport ◽  
Fixed Lifetime

Abstract. Transport from the Northern Hemisphere (NH) midlatitudes to the Arctic plays a crucial role in determining the abundance of trace gases and aerosols that are important to Arctic climate via impacts on radiation and chemistry. Here we examine this transport using an idealized tracer with a fixed lifetime and predominantly midlatitude land-based sources in models participating in the Chemistry Climate Model Initiative (CCMI). We show that there is a 25 %–45 % difference in the Arctic concentrations of this tracer among the models. This spread is correlated with the spread in the location of the Pacific jet, as well as the spread in the location of the Hadley Cell (HC) edge, which varies consistently with jet latitude. Our results suggest that it is likely that the HC-related zonal-mean meridional transport rather than the jet-related eddy mixing is the major contributor to the inter-model spread in the transport of land-based tracers into the Arctic. Specifically, in models with a more northern jet, the HC generally extends further north and the tracer source region is mostly covered by surface southward flow associated with the lower branch of the HC, resulting in less efficient transport poleward to the Arctic. During boreal summer, there are poleward biases in jet location in free-running models, and these models likely underestimate the rate of transport into the Arctic. Models using specified dynamics do not have biases in the jet location, but do have biases in the surface meridional flow, which may result in differences in transport into the Arctic. In addition to the land-based tracer, the midlatitude-to-Arctic transport is further examined by another idealized tracer with zonally uniform sources. With equal sources from both land and ocean, the inter-model spread of this zonally uniform tracer is more related to variations in parameterized convection over oceans rather than variations in HC extent, particularly during boreal winter. This suggests that transport of land-based and oceanic tracers or aerosols towards the Arctic differs in pathways and therefore their corresponding inter-model variabilities result from different physical processes.

Inertia–gravity waves in a liquid-filled, differentially heated, rotating annulus

2015 ◽  
Vol 782 ◽  
pp. 144-177 ◽  
Author(s):  
Anthony Randriamampianina ◽  
Emilia Crespo del Arco
Keyword(s):  
Prandtl Number ◽  
Gravity Waves ◽  
Large Scale ◽  
Baroclinic Instability ◽  
Zonal Flow ◽  
Small Scale ◽  
Mean Flow ◽  
Baroclinic Waves ◽  
Fluid Properties ◽  
Spatio Temporal

Direct numerical simulations based on high-resolution pseudospectral methods are carried out for detailed investigation into the instabilities arising in a differentially heated, rotating annulus, the baroclinic cavity. Following previous works using air (Randriamampianina et al., J. Fluid Mech., vol. 561, 2006, pp. 359–389), a liquid defined by Prandtl number $Pr=16$ is considered in order to better understand, via the Prandtl number, the effects of fluid properties on the onset of gravity waves. The computations are particularly aimed at identifying and characterizing the spontaneously emitted small-scale fluctuations occurring simultaneously with the baroclinic waves. These features have been observed as soon as the baroclinic instability sets in. A three-term decomposition is introduced to isolate the fluctuation field from the large-scale baroclinic waves and the time-averaged mean flow. Even though these fluctuations are found to propagate as packets, they remain attached to the background baroclinic waves, locally triggering spatio-temporal chaos, a behaviour not observed with the air-filled cavity. The properties of these features are analysed and discussed in the context of linear theory. Based on the Richardson number criterion, the characteristics of the generation mechanism are consistent with a localized instability of the shear zonal flow, invoking resonant over-reflection.

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