The integrated enstrophy budget of the winter stratosphere diagnosed from LIMS data
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The quasi-geostrophic integrated enstrophy budget for the 1978 to 1979 winter has been analyzed from 10-0.1 mb using LIMS data. During January and late February periods a significant imbalance in the budget appears at 10mb. This imbalance is attributed to Rossby wave breaking. It is produced by the irreversible transfer of enstrophy to smaller scales not resolved by LIMS. The imbalance episodes correspond well to the appearance of Ertel vorticity filaments shown by McIntyre and Palmer (1984). From a seasonal viewpoint, the integrated enstrophy shows an average (although irregular) transfer from a zonal mean reservoir to waves which are then dissipated. On a shorter time scale the integrated enstrophy sloshes back and forth between the waves and mean flow in early winter; then, beginning with the January sudden warming, the total enstrophy is reduced more rapidly. Between 10 mb and 1 mb this reduction is more or less continuous until the end of February. However, in the mesosphere the total enstrophy decrease is very short lived, being quickly restored after the January warming. Even though the zonal mean integrated enstrophy is large, only about 10% can be utilized by the waves. The available integrated potential enstrophy is introduced, which is a better measure of how close the flow is to saturation by Rossby waves. The largest amount of available potential enstrophy in early January is at 1 mb with decreasing amounts above and below. Saturation of the flow by Rossby waves occurs below 1 mb only coincident with sudden warmings; however, at mesospheric heights the flow appears to be nearly saturated throughout the winter.Keywords:
Enstrophy
Energy budget
Mean flow
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Abstract This study shows that gravity wave (GW) forcing (GWF) plays a crucial role in the barotropic/baroclinic instability that is frequently observed in the mesosphere and considered an origin of planetary waves (PWs) such as quasi-2-day and quasi-4-day waves. Simulation data from a GW-resolving general circulation model were analyzed, focusing on the winter Northern Hemisphere where PWs are active. The unstable field is characterized by a significant potential vorticity (PV) maximum with an anomalous latitudinal gradient at higher latitudes that suddenly appears in the midlatitudes of the upper mesosphere. This PV maximum is attributed to an enhanced static stability that develops through the following two processes: 1) strong PWs from the troposphere break in the middle stratosphere, causing a poleward and downward shift of the westerly jet to higher latitudes, and 2) strong GWF located above the jet simultaneously shifts and forms an upwelling in the midlatitudes, causing a significant increase in . An interesting feature is that the PV maximum is not zonally uniform but is observed only at longitudes with strong GWF. This longitudinally dependent GWF can be explained by selective filtering in the stratospheric mean flow modified by strong PWs. In the upper mesosphere, the Eliassen–Palm flux divergence by PWs has a characteristic structure, which is positive poleward and negative equatorward of the enhanced PV maximum, attributable to eastward- and westward-propagating PWs, respectively. This fact suggests that the barotropic/baroclinic instability is eliminated by simultaneous generation of eastward and westward PWs causing PV flux divergence.
Barotropic fluid
Middle latitudes
Atmospheric wave
Jet stream
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Barotropic fluid
Zonal flow (plasma)
Mean flow
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<p>The climatology of residual mean circulation &#8211; a main component of the Brewer&#8211;Dobson circulation &#8211; and the potential contribution of gravity waves (GWs) are examined for the annual mean state and each season in the whole stratosphere based on the transformed-Eulerian mean zonal momentum equation using four modern reanalysis datasets. Resolved and unresolved waves in the datasets are respectively designated as Rossby waves and GWs, although resolved waves may contain some GWs. First, the potential contribution of Rossby waves (RWs) to residual mean circulation is estimated from Eliassen&#8211;Palm flux divergence. The rest of residual mean circulation, from which the potential RW contribution and zonal mean zonal wind tendency are subtracted, is examined as the potential GW contribution, assuming that the assimilation process assures sufficient accuracy of the three components used for this estimation. The GWs contribute to drive not only the summer hemispheric part of the winter deep branch and low-latitude part of shallow branches, as indicated by previous studies, but they also cause a higher-latitude extension of the deep circulation in all seasons except for summer. This GW contribution is essential to determine the location of the turn-around latitude. The autumn circulation is stronger and wider than that of spring in the equinoctial seasons, regardless of almost symmetric RW and GW contributions around the Equator. This asymmetry is attributable to the existence of the spring-to-autumn pole circulation, corresponding to the angular momentum transport associated with seasonal variation due to the radiative process. The potential GW contribution is larger in September to November than in March-to-May in both hemispheres. The upward mass flux is maximized in the boreal winter in the lower stratosphere, while it exhibits semi-annual variation in the upper stratosphere. The boreal winter maximum in the lower stratosphere is attributable to stronger RW activity in both hemispheres than in the austral winter. Plausible deficiencies of current GW parameterizations are discussed by comparing the potential GW contribution and the parameterized GW forcing.</p>
Circulation (fluid dynamics)
Atmospheric Circulation
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The wind fields generated by the ECMWF FGGE III-b analyses at ten mandatory levels were used to examine the tropical enstrophy budget in the spectral domain during the 1979 northern summer (June–August). The seasonal mean analysis shows that the wave enstrophy has its maximum value in the upper troposphere and its major content in the low wavenumber regime. The wave enstrophy is nonlinearly transferred from the low to the large wavenumber regime. The wave enstrophy in the upper troposphere is supplied through the beta effect, while the upward transport of the wave enstrophy is generated by vortex stretching in the lower troposphere. The zonal enstrophy also has its maximum value in the upper troposphere and is supplied by the beta vortex stretching effects in the upper troposphere. Both wave and zonal enstrophies in the tropics exhibited a 40–50 day variation in the 1979 northern summer. The time series of various enstrophy variables suggests that the time variation of the wave enstrophy is maintained by the upward transport of this quantity generated in the lower troposphere. The time variation of the zonal enstrophy is supported by this quantity generated in the upper troposphere.
Enstrophy
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We provide statistical evidence of the effect of the solar wind dynamic pressure ( P sw ) on the northern winter and spring circulations. We find that the vertical structure of the Northern Annular Mode (NAM), the zonal mean circulation, and Eliassen‐Palm (EP)‐flux anomalies show a dynamically consistent pattern of downward propagation over a period of ~45 days in response to positive P sw anomalies. When the solar irradiance is high, the signature of P sw is marked by a positive NAM anomaly descending from the stratosphere to the surface during winter. When the solar irradiance is low, the P sw signal has the opposite sign, occurs in spring, and is confined to the stratosphere. The negative P sw signal in the NAM under low solar irradiance conditions is primarily governed by enhanced vertical EP‐flux divergence and a warmer polar region. The winter P sw signal under high solar irradiance conditions is associated with positive anomalies of the horizontal EP‐flux divergence at 55°N–75°N and negative anomalies at 25°N–45°N, which corresponds to the positive NAM anomaly. The EP‐flux divergence anomalies occur ~15 days ahead of the mean‐flow changes. A significant equatorward shift of synoptic‐scale Rossby wave breaking (RWB) near the tropopause is detected during January–March, corresponding to increased anticyclonic RWB and a decrease in cyclonic RWB. We suggest that the barotropic instability associated with asymmetric ozone in the upper stratosphere and the baroclinic instability associated with the polar vortex in the middle and lower stratosphere play a critical role for the winter signal and its downward propagation.
Tropopause
Anticyclone
Anomaly (physics)
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The global distribution of the forcing of the time-mean flow due to large-scale, horizontal Reynolds stresses (u′ u′, v′ v′, u′ v′) is determined from upper wind statistics for the period 1968–73. The role of this forcing in the maintenance of the vorticity and enstrophy of the time-mean flow is discussed. The most striking effect of transient eddy stresses is the tendency to shift the subtropical maxima in the time-mean flow and the associated vorticity patterns poleward. However, significant longitudinal Variations in forcing occur, also. Calculations of the dominant terms in vorticity budgets of the North Pacific Low, the North Atlantic Low, and the Siberian High, which may he called the centers of action of winter-time circulation at sea level in the Northern Hemisphere, are presented. In all three cases, transient eddies are found to be important in maintaining the centers against the dissipative action of surface friction. In terms of the enstrophy budget, the hemispheric and global-mean effects of transient eddies on the mean flow are small. both in December–February and June–August. In the Northern Hemisphere, where the results are most reliable, the eddies are weakly dissipative with a time scale on the order of several months. When separating the time-mean flow into the contributions from the axisymmetric component and from the stationary disturbances, it is found that the transient eddy stresses tend to maintain the axisymmetric mean flow, but to weaken the stationary disturbances. There are significant latitudinal variations in the enstrophy forcing of the stationary disturbances. Thus eddy forcing is an important factor in maintaining the enstrophy of stationary disturbances in the extratropies, while it tends to destroy their enstrophy in the tropics.
Enstrophy
Eddy
Mean flow
Forcing (mathematics)
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A method based on contour advection is introduced that aims to quantify the formation of filaments both equatorward and poleward of the subtropical barrier. It is applied to diagnosed potential vorticity fields for every day of January and February in 1997 and 1998. An isosurface of modified potential vorticity is found to represent the region of largest isentropic gradients of potential vorticity and is hence assumed as the tropopause. Tropopause‐penetrating structures (“filaments”) forming in contour‐advected potential vorticity fields are identified, and statistics of the abundance of such structures are derived. Filamentation as measured by this method exhibits a large temporal variability on the scale of days to weeks. For example, tropospheric cutoff systems developing in the wake of Rossby wave breaking events cause strong filamentation to occur in the lowermost stratosphere. The timescales governing filamentation are a function of altitude, reflecting the differing types and amplitudes of waves inducing filamentation at different isentropic levels. Zonal asymmetries arise as Rossby waves favorably break near the end of the North Atlantic storm track. A difference in the intensities of filamentation in January and February of 1997 and 1998 suggests that the relatively low values of ozone in the lowermost stratosphere observed in 1997 are related to increased filamentation‐induced stratosphere‐troposphere exchange, compared to 1998. For example, around 30% more filaments are found in the hemispheric and bimonthly mean at the 330 K isentropic surface in January and February of 1997 than during the same months of 1998. Single events can represent most of the filamentation occurring in a month, and hence interannual variability in the frequency of filamentation can be considerable.
Filamentation
Tropopause
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In the winter stratosphere of the Northern Hemisphere, the disruption of the westerly vortex and associated warming of polar latitudes is a well known phenomenon. It has become apparent that some important dynamical processes in the stratosphere are highly nonlinear and are best thought of locally rather than in terms of the interaction between the zonal-mean flow and harmonic waves around latitude circles. The importance of nonlinear processes was suggested by McIntyre and Palmer (1983, 1984) who used isentropic maps of Ertel's Potential Vorticity to show that during disturbed episodes material lines may become strongly and irreversibly deformed in certain places. They adopted the term planetary wave breaking to describe this process. Isentropic maps of Q are used to follow the evolution of a Canadian warming in November - December 1981 and a particularly strong warming in January 1982. The advection of Q over large distances on isentropic surfaces was a striking feature of the flow during each event. This could be identified because of our ability to follow the movement of material lines due to the approximate conservation of Q over several days. The advection of Q was a nonlinear process because its changing distribution affected the advecting wind field. The Canadian warming did not lead to a permanent change in the structure of the westerly vortex, as defined by the coarse-grain field of Q, whereas the January event was accompanied by a substantial loss of resolved Q which was never fully recovered.
Sudden stratospheric warming
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The correlation lengths of vorticity anomalies from temporal averages are examined in the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis dataset. It is shown that, in the annual mean, eddies in the Southern Hemisphere are significantly larger than those in the Northern Hemisphere. The eddy vorticity lengths exhibit a strong seasonal cycle, with the largest scales occurring in the winter season. The maximum zonal eddy lengths closely follow the contours of the strong upper-level winds, while the maximum meridional lengths are found in jet exit regions and in the stratosphere.
Eddy
Atmospheric Circulation
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Abstract. The westerly phase of the stratospheric Quasi-Biennial Oscillation (QBO) was reversed during Northern Hemisphere winter 2015/2016 for the first time since records began in 1953. Recent studies proposed that Rossby waves propagating from the extratropics played an important role during the reversal event in 2015/2016. Building upon these studies, we separated the extratropical Rossby waves into different wavenumbers and time-scales by analyzing the combined ERA-40 and ERA-Interim reanalysis zonal wind, meridional wind, vertical velocity and potential vorticity daily mean data from 1958 to 2017. We find that both synoptic and quasi-stationary Rossby waves are dominant contributors to the reversal event in 2015/2016 in the tropical lower stratosphere. By comparing the results for 2015/2016 with two additional events (1959/1960 and 2010/2011), we find that the largest differences in Rossby wave momentum fluxes are related to synoptic-scale Rossby waves of periods from 5–20 days. We demonstrate for the first time, that these enhanced synoptic Rossby waves at 40 hPa in the tropics in February 2016 originate from the extratropics as well as from local wave generation. The strong Rossby wave activity in 2016 in the tropics happened at a time with weak westerly zonal winds. This coincidence of anomalous factors did not happen in any of the previous events. In addition to the anomalous behavior in the tropical lower stratosphere in 2015/16, we explored the forcing of the unusually long-lasting westerly zonal wind phase in the upper stratosphere (at 20 hPa). Our results reveal that mainly enhanced Kelvin wave activity contributed to this feature. This was in close relation with the strong El Niño event in 2015/2016, which forced more Kelvin waves in the equatorial troposphere. The easterly or very weak westerly zonal winds present around 30–70 hPa allowed these Kelvin waves to propagate vertically and deposit their momentum around 20 hPa, maintaining the westerlies there.
Extratropical cyclone
Forcing (mathematics)
Quasi-biennial oscillation
Westerlies
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