scholarly journals The Effect of Lower Stratospheric Shear on Baroclinic Instability

2007 ◽  
Vol 64 (2) ◽  
pp. 479-496 ◽  
Author(s):  
Matthew A. H. Wittman ◽  
Andrew J. Charlton ◽  
Lorenzo M. Polvani
Keyword(s):  
Growth Rate ◽  
Lower Stratosphere ◽  
Baroclinic Instability ◽  
Phase Speed ◽  
Finite Amplitude ◽  
Reanalysis Data ◽  
Life Cycles ◽  
Vertical Shear ◽  
Northern Annular Mode ◽  
Jet Structure

Abstract Using a hierarchy of models, and observations, the effect of vertical shear in the lower stratosphere on baroclinic instability in the tropospheric midlatitude jet is examined. It is found that increasing stratospheric shear increases the phase speed of growing baroclinic waves, increases the growth rate of modes with low synoptic wavenumbers, and decreases the growth rate of modes with higher wavenumbers. The meridional structure of the linear modes, and their acceleration of the zonal mean jet, changes with increasing stratospheric shear, but in a way that apparently contradicts the observed stratosphere–troposphere northern annular mode (NAM) connection. This contradiction is resolved at finite amplitude. In nonlinear life cycle experiments it is found that increasing stratospheric shear, without changing the jet structure in the troposphere, produces a transition from anticyclonic (LC1) to cyclonic (LC2) behavior at wavenumber 7. All life cycles with wavenumbers lower than 7 are LC1, and all with wavenumber greater than 7 are LC2. For the LC1 life cycles, the effect of increasing stratospheric shear is to increase the poleward displacement of the zonal mean jet by the eddies, which is consistent with the observed stratosphere–troposphere NAM connection. Finally, it is found that the connection between high stratospheric shear and high-tropospheric NAM is present by NCEP–NCAR reanalysis data.

scholarly journals Characteristics of Baroclinic Wave Packets during Strong and Weak Stratospheric Polar Vortex Events

2010 ◽  
Vol 67 (10) ◽  
pp. 3190-3207 ◽  
Author(s):  
Ian N. Williams ◽  
Stephen J. Colucci
Keyword(s):  
Zonal Wind ◽  
Lower Stratosphere ◽  
Baroclinic Instability ◽  
Phase Speed ◽  
Vertical Wind Shear ◽  
Polar Vortex ◽  
Vertical Shear ◽  
Stratospheric Polar Vortex ◽  
The Pacific ◽  
Observational Record

Abstract This study tested a numerical and theoretical prediction that the stratosphere and troposphere are coupled through the effect of stratospheric vertical wind shear on baroclinic waves. Wavelengths, phase speeds, and background quasigeostrophic potential vorticity gradients were analyzed over the Pacific and Atlantic during strong and weak stratospheric polar vortex events and interpreted in terms of the counterpropagating Rossby wave perspective on baroclinic instability. Effects of zonal variations in the background flow were included in the analysis of phase speeds. Observed changes in wave packet average wavelength and phase speed support the vertical shear hypothesis for stratosphere–troposphere coupling; however, changes in the intrinsic phase speed contradict the hypothesis. This inconsistency was resolved by considering the change in zonal wind speed in the lower stratosphere, which accounts for most of the change in phase speeds during strong and weak vortex events. Changes in the average wavelengths and meridional wave activity flux are also consistent with this modified hypothesis involving the stratospheric zonal wind. The results demonstrate that a simple mechanism for stratosphere–troposphere coupling can be found in the observational record.

scholarly journals Tropospheric eddy feedback to different stratospheric conditions in idealised baroclinic life cycles

2021 ◽  
Vol 2 (1) ◽  
pp. 111-128
Author(s):  
Philip Rupp ◽  
Thomas Birner
Keyword(s):  
Lower Stratosphere ◽  
Polar Vortex ◽  
Surface Friction ◽  
Life Cycles ◽  
Synoptic Scale ◽  
Final State ◽  
Northern Annular Mode ◽  
Wind Structure ◽  
Basic Set ◽  
Set Up

Abstract. A pronounced signature of stratosphere–troposphere coupling is a robust negative anomaly in the surface northern annular mode (NAM) following sudden stratospheric warming (SSW) events, consistent with an equatorward shift in the tropospheric jet. It has previously been pointed out that tropospheric synoptic-scale eddy feedbacks, mainly induced by anomalies in the lowermost extratropical stratosphere, play an important role in creating this surface NAM signal. Here, we use the basic set-up of idealised baroclinic life cycles to investigate the influence of stratospheric conditions on the behaviour of tropospheric synoptic-scale eddies. Particular attention is given to the enhancement of the tropospheric eddy response by surface friction and the sensitivity to wind anomalies in the lower stratosphere. We find systems that include a tropospheric jet only (modelling post-SSW conditions) to be characterised by an equatorward shift in the tropospheric jet in the final state of the life cycle, relative to systems that include a representation of the polar vortex (mimicking more undisturbed stratospheric wintertime conditions), consistent with the observed NAM response after SSWs. The corresponding relative surface NAM signal is increased if the system includes surface friction, presumably due to a direct coupling of the eddy field at tropopause level to the surface winds. We further show that the jet shift signal observed in our experiments is mainly caused by changes in the zonal wind structure of the lowermost stratosphere, while changes in the wind structure of the middle and upper stratosphere have almost no influence.

scholarly journals Instabilities of continuously stratified zonal equatorial jets in a periodic channel model

2002 ◽  
Vol 20 (5) ◽  
pp. 729-740 ◽  
Author(s):  
S. Masina
Keyword(s):  
Channel Model ◽  
Baroclinic Instability ◽  
Phase Speed ◽  
Maximum Velocity ◽  
Equatorial Region ◽  
Eastern Pacific ◽  
Vertical Shear ◽  
Coriolis Parameter ◽  
Vertical Scale ◽  
Surface Jet

Abstract. Several numerical experiments are performed in a nonlinear, multi-level periodic channel model centered on the equator with different zonally uniform background flows which resemble the South Equatorial Current (SEC). Analysis of the simulations focuses on identifying stability criteria for a continuously stratified fluid near the equator. A 90 m deep frontal layer is required to destabilize a zonally uniform, 10° wide, westward surface jet that is symmetric about the equator and has a maximum velocity of 100 cm/s. In this case, the phase velocity of the excited unstable waves is very similar to the phase speed of the Tropical Instability Waves (TIWs) observed in the eastern Pacific Ocean. The vertical scale of the baroclinic waves corresponds to the frontal layer depth and their phase speed increases as the vertical shear of the jet is doubled. When the westward surface parabolic jet is made asymmetric about the equator, in order to simulate more realistically the structure of the SEC in the eastern Pacific, two kinds of instability are generated. The oscillations that grow north of the equator have a baroclinic nature, while those generated on and very close to the equator have a barotropic nature.  This study shows that the potential for baroclinic instability in the equatorial region can be as large as at mid-latitudes, if the tendency of isotherms to have a smaller slope for a given zonal velocity, when the Coriolis parameter vanishes, is compensated for by the wind effect.Key words. Oceanography: general (equatorial oceanography; numerical modeling) – Oceanography: physics (fronts and jets)

scholarly journals Intraseasonal Variability of Cross-Slope Flow in the Northern South China Sea

2020 ◽  
Vol 50 (7) ◽  
pp. 2071-2084
Author(s):  
Qiang Wang ◽  
Weidong Zhou ◽  
Lili Zeng ◽  
Ju Chen ◽  
Yunkai He ◽  
...  
Keyword(s):  
Growth Rate ◽  
Kinetic Energy ◽  
South China Sea ◽  
Northern South China Sea ◽  
Baroclinic Instability ◽  
Intraseasonal Variability ◽  
Mesoscale Eddy ◽  
Vertical Shear ◽  
Instability Growth ◽  
Slope Flow

AbstractCross-slope flow plays an important role in the exchange of material, heat, and momentum between the continental shelf and the open sea. In the northern South China Sea (SCS), long-period observations confirm that there is significant cross-slope flow. The variability of this flow is dominated by the intraseasonal component (i.e., the 10–90-day period band) that contributes 74.6% of the total standard deviation. The 10–90-day bandpassed cross-slope flow exhibits almost the same direction vertically in the observed layers, and its first empirical orthogonal function, whose direction is also not changed, contributes 86.7% to its total variance. The strong 10–90-day bandpassed cross-slope flow is phase locked to the boreal winter half year. The intraseasonal variability of cross-slope flow is mainly associated with mesoscale eddies west to the Luzon Strait. The contrasting baroclinic instability growth rates, strong in winter and weak in summer, result in a seasonal cycle of mesoscale eddy kinetic energy, that is, vigorous in winter and weak in summer, which explains the winter phase lock. The interannual variability of baroclinic instability growth rate is mainly determined by the vertical shear of velocity. The strongest vertical shear of velocity from 2014 to 2016 occurred in the winter of 2016/17 and induced the most rapid baroclinic instability growth rate and consequently the largest mesoscale eddy kinetic energy, which resulted in the strongest intraseasonal variability of cross-slope flow. The vertical shear of velocity in the northern SCS is mainly determined by the Luzon Strait transport.

scholarly journals Tropospheric eddy feedback to different stratospheric conditions in idealised baroclinic life cycles

2020 ◽  
Author(s):  
Philip Rupp ◽  
Thomas Birner
Keyword(s):  
Lower Stratosphere ◽  
Polar Vortex ◽  
Surface Friction ◽  
Life Cycles ◽  
Synoptic Scale ◽  
Sudden Stratospheric Warming ◽  
Final State ◽  
Northern Annular Mode ◽  
Wind Structure ◽  
Winter Time

Abstract. A pronounced signature of stratosphere-troposphere coupling is a robust negative anomaly in the surface northern annular mode (NAM) following major sudden stratospheric warming (SSW) events, consistent with an equatorward shift of the tropospheric jet. It has previously been pointed out that tropospheric eddy feedbacks, mainly induced by anomalies in the lowermost extratropical stratosphere, play an important role in creating this surface NAM-signal. We use the basic setup of idealised baroclinic life cycles to investigate the influence of stratospheric conditions on the behaviour of tropospheric synoptic-scale eddies. Particular focus is hereby given on the enhancement of the tropospheric eddy response by surface friction, as well as the sensitivity to wind anomalies in the lower stratosphere. We find systems that include a tropospheric jet only (modelling post-SSW conditions) to be characterised by an equatorward shift of the tropospheric jet in the final state of the life cycle, relative to systems that include a representation of the polar vortex (mimicking more undisturbed winter-time conditions), consistent with the observed NAM-response after SSWs. The corresponding surface NAM-signal is increased if the system includes surface friction, presumably associated with a direct coupling of the eddy field at tropopause level to the surface winds. We further show that the jet shift signal observed in our experiments is mainly caused by changes in the zonal wind structure of the lowermost stratosphere, while changes in the wind structure of the middle and upper stratosphere have almost no influence.

Inertia–Gravity Waves Spontaneously Generated by Jets and Fronts. Part I: Different Baroclinic Life Cycles

2007 ◽  
Vol 64 (7) ◽  
pp. 2502-2520 ◽  
Author(s):  
Riwal Plougonven ◽  
Chris Snyder
Keyword(s):  
Life Cycle ◽  
Gravity Waves ◽  
Lower Stratosphere ◽  
Baroclinic Instability ◽  
Life Cycles ◽  
Weather Research And Forecasting ◽  
Spontaneous Generation ◽  
Upper Level ◽  
Upper Level Jet ◽  
Inertia Gravity Waves

Abstract The spontaneous generation of inertia–gravity waves in idealized life cycles of baroclinic instability is investigated using the Weather Research and Forecasting Model. Two substantially different life cycles of baroclinic instability are obtained by varying the initial zonal jet. The wave generation depends strongly on the details of the baroclinic wave’s development. In the life cycle dominated by cyclonic behavior, the most conspicuous gravity waves are excited by the upper-level jet and are broadly consistent with previous simulations of O’Sullivan and Dunkerton. In the life cycle that is dominated by anticyclonic behavior, the most conspicuous gravity waves even in the stratosphere are excited by the surface fronts, although the fronts are no stronger than in the cyclonic life cycle. The anticyclonic life cycle also reveals waves in the lower stratosphere above the upper-level trough of the baroclinic wave; these waves have not been previously identified in idealized simulations. The sensitivities of the different waves to both resolution and dissipation are discussed.

Effects of MJO Vertically Tilted Structure on Its Phase Speed from the Moisture Mode Theory Perspective

2021 ◽  
pp. 1-55
Author(s):  
Feng Hu ◽  
Tim Li
Keyword(s):  
Boundary Layer ◽  
Phase Speed ◽  
Lower Troposphere ◽  
Propagation Speed ◽  
Reanalysis Data ◽  
Life Cycles ◽  
Overturning Circulation ◽  
Pressure Anomaly ◽  
Static Energy ◽  
The One

AbstractThe effect of vertically tilted structure (VTS) of MJO on its phase propagation speed was investigated through the diagnosis of ERA-I reanalysis data during 1979-2012. A total of 84 eastward propagating MJO events were selected. It was found that all MJO events averaged throughout their life cycles exhibited a clear VTS, and the tilting strength was significantly positively correlated to the phase speed.The physical mechanism through which the VTS influenced the phase speed was investigated. On the one hand, a stronger VTS lead to a stronger vertical overturning circulation and a stronger descent in the front, which caused a greater positive moist static energy (MSE) tendency in situ through enhanced vertical MSE advection. The stronger MSE tendency gradient lead to a faster eastward phase speed. On the other hand, the enhanced overturning circulation in front of MJO convection lead to a stronger easterly/low pressure anomaly at the top of the boundary layer, which induced a stronger boundary layer convergence and stronger ascent in the lower troposphere. This strengthened the boundary layer moisture asymmetry and favored a faster eastward propagation speed.

scholarly journals Baroclinic instability of a symmetric, rotating, stratified flow: a study of the nonlinear stabilisation mechanisms in the presence of viscosity

2002 ◽  
Vol 9 (5/6) ◽  
pp. 487-496 ◽  
Author(s):  
R. Mantovani ◽  
A. Speranza
Keyword(s):  
Parameter Space ◽  
Baroclinic Instability ◽  
Vertical Wall ◽  
Time Integration ◽  
Stable State ◽  
Physical Nature ◽  
Finite Amplitude ◽  
Linear Instability ◽  
Vertical Shear ◽  
Direct Role

Abstract. This paper presents the analysis of symmetric circulations of a rotating baroclinic flow, forced by a steady thermal wind and dissipated by Laplacian friction. The analysis is performed with numerical time-integration. Symmetric flows, vertically bound by horizontal walls and subject to either periodic or vertical wall lateral boundary conditions, are investigated in the region of parameter-space where unstable small amplitude modes evolve into stable stationary nonlinear solutions. The distribution of solutions in parameter-space is analysed up to the threshold of chaotic behaviour and the physical nature of the nonlinear interaction operating on the finite amplitude unstable modes is investigated. In particular, analysis of time-dependent energy-conversions allows understanding of the physical mechanisms operating from the initial phase of linear instability to the finite amplitude stable state. Vertical shear of the basic flow is shown to play a direct role in injecting energy into symmetric flow since the stage of linear growth. Dissipation proves essential not only in limiting the energy of linearly unstable modes, but also in selecting their dominant space-scales in the finite amplitude stage.

scholarly journals Interannual Variability and Trends in Sea Surface Temperature, Lower and Middle Atmosphere Temperature at Different Latitudes for 1980–2019

Atmosphere ◽  
2021 ◽  
Vol 12 (4) ◽  
pp. 454
Author(s):  
Andrew R. Jakovlev ◽  
Sergei P. Smyshlyaev ◽  
Vener Y. Galin
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Lower Stratosphere ◽  
Middle Atmosphere ◽  
Southern Oscillation ◽  
Lower Troposphere ◽  
Reanalysis Data ◽  
Sea Surface ◽  
The Arctic ◽  
The Impact

The influence of sea-surface temperature (SST) on the lower troposphere and lower stratosphere temperature in the tropical, middle, and polar latitudes is studied for 1980–2019 based on the MERRA2, ERA5, and Met Office reanalysis data, and numerical modeling with a chemistry-climate model (CCM) of the lower and middle atmosphere. The variability of SST is analyzed according to Met Office and ERA5 data, while the variability of atmospheric temperature is investigated according to MERRA2 and ERA5 data. Analysis of sea surface temperature trends based on reanalysis data revealed that a significant positive SST trend of about 0.1 degrees per decade is observed over the globe. In the middle latitudes of the Northern Hemisphere, the trend (about 0.2 degrees per decade) is 2 times higher than the global average, and 5 times higher than in the Southern Hemisphere (about 0.04 degrees per decade). At polar latitudes, opposite SST trends are observed in the Arctic (positive) and Antarctic (negative). The impact of the El Niño Southern Oscillation phenomenon on the temperature of the lower and middle atmosphere in the middle and polar latitudes of the Northern and Southern Hemispheres is discussed. To assess the relative influence of SST, CO2, and other greenhouse gases’ variability on the temperature of the lower troposphere and lower stratosphere, numerical calculations with a CCM were performed for several scenarios of accounting for the SST and carbon dioxide variability. The results of numerical experiments with a CCM demonstrated that the influence of SST prevails in the troposphere, while for the stratosphere, an increase in the CO2 content plays the most important role.

Sign in / Sign up

Export Citation Format

Share Document