The nighttime poleward wind responses to SAPS simulated by TIEGCM: a universal time effect

2020 ◽  
Author(s):  
Kedeng Zhang ◽  
Hui Wang ◽  
Wenbin Wang ◽  
Jing Liu ◽  
Shunrong Zhang ◽  
...  
Keyword(s):  
Coriolis Force ◽  
General Circulation ◽  
Opposite Effect ◽  
Circulation Model ◽  
Meridional Wind ◽  
Universal Time ◽  
Coordinate Systems ◽  
Large Component ◽  
Meridional Winds ◽  
Channel Orientation

<p>The present work investigates the nighttime meridional wind (30º-50º magnetic latitude and 19-22 magnetic local time) in response to subauroral polarization streams (SAPS) that commence at different universal time (UT) by using Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) under geomagnetically disturbed conditions that are closely related to the southward interplanetary magnetic field (IMF) carried by the solar wind. The SAPS effects on the meridional winds show a remarkable UT variation, with larger magnitudes at 00 and 12 UT than at 06 and 18 UT. The strongest poleward wind emerges when SAPS commence at 06 UT, and the weakest poleward wind develops when SAPS occur at 00 UT. A diagnostic analysis of model results shows that the pressure gradient is more prominent for the developing of the poleward wind at 00 and 12 UT. Meanwhile, the effect of the ion drag is important in the modulation of the poleward wind velocity at 06 and 18 UT. This is caused by the misalignment of the geomagnetic and geographic coordinate systems, resulting to a large component of ion drag in geographically northward (southward) direction due to the SAPS channel orientation at 06 and 18 UT (00 and 12 UT). The Coriolis force effect induced by westward winds maximizes (minimizes) when SAPS commence at 12 UT (00 UT). The centrifugal force due to the accelerated westward winds shows similar UT variations as the Coriolis force, but with an opposite effect.</p>

“Modes of Variability” and Climate Change

2009 ◽  
Vol 22 (10) ◽  
pp. 2639-2658 ◽  
Author(s):  
Grant Branstator ◽  
Frank Selten
Keyword(s):  
Climate Change ◽  
Stochastic Model ◽  
General Circulation ◽  
Climate Changes ◽  
Circulation Model ◽  
Meridional Wind ◽  
Model Atmosphere ◽  
Linear Pattern ◽  
Modes Of Variability ◽  
Mean State

Abstract A 62-member ensemble of coupled general circulation model (GCM) simulations of the years 1940–2080, including the effects of projected greenhouse gas increases, is examined. The focus is on the interplay between the trend in the Northern Hemisphere December–February (DJF) mean state and the intrinsic modes of variability of the model atmosphere as given by the upper-tropospheric meridional wind. The structure of the leading modes and the trend are similar. Two commonly proposed explanations for this similarity are considered. Several results suggest that this similarity in most respects is consistent with an explanation involving patterns that result from the model dynamics being well approximated by a linear system. Specifically, the leading intrinsic modes are similar to the leading modes of a stochastic model linearized about the mean state of the GCM atmosphere, trends in GCM tropical precipitation appear to excite the leading linear pattern, and the probability density functions (PDFs) of prominent circulation patterns are quasi-Gaussian. There are, on the other hand, some subtle indications that an explanation for the similarity involving preferred states (which necessarily result from nonlinear influences) has some relevance. For example, though unimodal, PDFs of prominent patterns have departures from Gaussianity that are suggestive of a mixture of two Gaussian components. And there is some evidence of a shift in probability between the two components as the climate changes. Interestingly, contrary to the most prominent theory of the influence of nonlinearly produced preferred states on climate change, the centroids of the components also change as the climate changes. This modification of the system’s preferred states corresponds to a change in the structure of its dominant patterns. The change in pattern structure is reproduced by the linear stochastic model when its basic state is modified to correspond to the trend in the general circulation model’s mean atmospheric state. Thus, there is a two-way interaction between the trend and the modes of variability.

scholarly journals Fundamental Causes of Propagating and Nonpropagating MJOs in MJOTF/GASS Models

2017 ◽  
Vol 30 (10) ◽  
pp. 3743-3769 ◽  
Author(s):  
Lu Wang ◽  
Tim Li ◽  
Eric Maloney ◽  
Bin Wang
Keyword(s):  
Vertical Velocity ◽  
General Circulation ◽  
Circulation Model ◽  
Vertical Gradient ◽  
Meridional Wind ◽  
Kelvin Wave ◽  
Zonal Distribution ◽  
The Poor ◽  
Fundamental Causes ◽  
Zonal Asymmetry

Abstract This study investigates the fundamental causes of differences in the Madden–Julian oscillation (MJO) eastward propagation among models that participated in a recent model intercomparison project. These models are categorized into good and poor groups characterized by prominent eastward propagation and nonpropagation, respectively. Column-integrated moist static energy (MSE) budgets are diagnosed for the good and the poor models. It is found that a zonal asymmetry in the MSE tendency, characteristic of eastward MJO propagation, occurs in the good group, whereas such an asymmetry does not exist in the poor group. The difference arises mainly from anomalous vertical and horizontal MSE advection. The former is attributed to the zonal asymmetry of upper-midtropospheric vertical velocity anomalies acting on background MSE vertical gradient; the latter is mainly attributed to the asymmetric zonal distribution of low-tropospheric meridional wind anomalies advecting background MSE and moisture fields. Based on the diagnosis above, a new mechanism for MJO eastward propagation that emphasizes the second-baroclinic-mode vertical velocity is proposed. A set of atmospheric general circulation model experiments with prescribed diabatic heating profiles was conducted to investigate the causes of different anomalous circulations between the good and the poor models. The numerical experiments reveal that the presence of a stratiform heating at the rear of MJO convection is responsible for the zonal asymmetry of vertical velocity anomaly and is important to strengthening lower-tropospheric poleward flows to the east of MJO convection. Thus, a key to improving the poor models is to correctly reproduce the stratiform heating. The roles of Rossby and Kelvin wave components in MJO propagation are particularly discussed.

scholarly journals New results on equatorial thermospheric winds and the midnight temperature maximum

2008 ◽  
Vol 26 (3) ◽  
pp. 447-466 ◽  
Author(s):  
J. Meriwether ◽  
M. Faivre ◽  
C. Fesen ◽  
P. Sherwood ◽  
O. Veliz
Keyword(s):  
Local Time ◽  
Wind Field ◽  
General Circulation ◽  
Circulation Model ◽  
Meridional Wind ◽  
Optical Observations ◽  
Horizontal Wind ◽  
Temporal Sampling ◽  
The Mean ◽  
Thermospheric Winds

Abstract. Optical observations of thermospheric winds and temperatures determined with high resolution measurements of Doppler shifts and Doppler widths of the OI 630-nm equatorial nightglow emission have been made with improved accuracy at Arequipa, Peru (16.4° S, 71.4° W) with an imaging Fabry-Perot interferometer. An observing procedure previously used at Arecibo Observatory was applied to achieve increased spatial and temporal sampling of the thermospheric wind and temperature with the selection of eight azimuthal directions, equally spaced from 0 to 360°, at a zenith angle of 60°. By assuming the equivalence of longitude and local time, the data obtained using this technique is analyzed to determine the mean neutral wind speeds and mean horizontal gradients of the wind field in the zonal and meridional directions. The new temperature measurements obtained with the improved instrumental accuracy clearly show the midnight temperature maximum (MTM) peak with amplitudes of 25 to 200 K in all directions observed for most nights. The horizontal wind field maps calculated from the mean winds and gradients show the MTM peak is always preceded by an equatorward wind surge lasting 1–2 h. The results also show for winter events a meridional wind abatement seen after the MTM peak. On one occasion, near the September equinox, a reversal was observed during the poleward transit of the MTM over Arequipa. Analysis inferring vertical winds from the observed convergence yielded inconsistent results, calling into question the validity of this calculation for the MTM structure at equatorial latitudes during solar minimum. Comparison of the observations with the predictions of the NCAR general circulation model indicates that the model fails to reproduce the observed amplitude by a factor of 5 or more. This is attributed in part to the lack of adequate spatial resolution in the model as the MTM phenomenon takes place within a scale of 300–500 km and ~45 min in local time. The model shortcoming is also attributed in part to the need for the model to include a hydrodynamical mechanism to describe the merging of the zonal wind with the meridional tidal winds that converge onto the geographical equator. Finally, a conclusion of this work is that the MTM compressional heating takes place along the perimeter of the pressure bulge rather than within the bulge, an issue previously not appreciated.

scholarly journals Influence of the sudden stratospheric warming on quasi-2-day waves

2016 ◽  
Vol 16 (8) ◽  
pp. 4885-4896 ◽  
Author(s):  
Sheng-Yang Gu ◽  
Han-Li Liu ◽  
Xiankang Dou ◽  
Tao Li
Keyword(s):  
General Circulation ◽  
Phase Speed ◽  
Circulation Model ◽  
Meridional Wind ◽  
Equatorial Region ◽  
Wave Number ◽  
Stratospheric Warming ◽  
Sudden Stratospheric Warming ◽  
Middle Latitudes ◽  
Zonal Wave Number

Abstract. The influence of the sudden stratospheric warming (SSW) on a quasi-2-day wave (QTDW) with westward zonal wave number 3 (W3) is investigated using the Thermosphere–Ionosphere–Mesosphere Electrodynamics General Circulation Model (TIME-GCM). The summer easterly jet below 90 km is strengthened during an SSW, which results in a larger refractive index and thus more favorable conditions for the propagation of W3. In the winter hemisphere, the Eliassen–Palm (EP) flux diagnostics indicate that the strong instabilities at middle and high latitudes in the mesopause region are important for the amplification of W3, which is weakened during SSW periods due to the deceleration or even reversal of the winter westerly winds. Nonlinear interactions between the W3 and the wave number 1 stationary planetary wave produce QTDW with westward zonal wave number 2 (W2). The meridional wind perturbations of the W2 peak in the equatorial region, while the zonal wind and temperature components maximize at middle latitudes. The EP flux diagnostics indicate that the W2 is capable of propagating upward in both winter and summer hemispheres, whereas the propagation of W3 is mostly confined to the summer hemisphere. This characteristic is likely due to the fact that the phase speed of W2 is larger, and therefore its waveguide has a broader latitudinal extension. The larger phase speed also makes W2 less vulnerable to dissipation and critical layer filtering by the background wind when propagating upward.

The Multidecadal Variability of the Asymmetric Mode of the Boreal Autumn Hadley Circulation and Its Link to the Atlantic Multidecadal Oscillation

2016 ◽  
Vol 29 (15) ◽  
pp. 5625-5641 ◽  
Author(s):  
Yipeng Guo ◽  
Jianping Li ◽  
Juan Feng ◽  
Fei Xie ◽  
Cheng Sun ◽  
...  
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Hadley Circulation ◽  
Atlantic Multidecadal Oscillation ◽  
Meridional Wind ◽  
Vertical Shear ◽  
Asymmetric Mode ◽  
Multidecadal Variability ◽  
Four Seasons ◽  
Precipitation Difference

Abstract Previous studies show that the first principal mode of the variability of the seasonal mean Hadley circulation (HC) is an equatorial asymmetric mode (AM) with long-term trend. This study demonstrates that the variability of the boreal autumn [September–November (SON)] HC is also dominated by an AM, but with multidecadal variability. The SON AM has ascending and descending branches located at approximately 20°N and 20°S, respectively, and explains about 40% of the total variance. Further analysis reveals that the AM is closely linked to the Atlantic multidecadal oscillation (AMO), which is associated with a large cross-equatorial sea surface temperature (SST) gradient and sea level pressure (SLP) gradient. The cross-equatorial thermal contrast further induces an equatorial asymmetric HC anomaly. Numerical simulations conducted on an atmospheric general circulation model also suggest that AMO-associated SST anomalies can also induce a cross-equatorial SLP gradient and anomalous vertical shear of the meridional wind at the equator, both of which indicate asymmetric HC anomaly. Therefore, the AM of the variability of the boreal autumn HC has close links to the AMO. Further analysis demonstrates that the AMO in SON has a closer relationship with AM than those in the other seasons. A possible reason is that the AMO-associated zonal mean SST anomaly in the tropics has differences among the four seasons, which leads to different atmospheric circulation responses. The AM in SON has inversed impacts on the tropical precipitation, suggesting that the precipitation difference between the northern and southern tropics has multidecadal variability.

scholarly journals Influence of the sudden stratosphere warming on quasi-2 day waves

2016 ◽  
Author(s):  
Sheng-Yang Gu ◽  
Han-Li Liu ◽  
Xiankang Dou ◽  
Tao Li
Keyword(s):  
General Circulation ◽  
Phase Speed ◽  
Circulation Model ◽  
Meridional Wind ◽  
Equatorial Region ◽  
Middle Latitudes ◽  
Zonal Wavenumber ◽  
Westerly Winds ◽  
Summer Hemisphere ◽  
Stationary Planetary Wave

Abstract. The influence of the sudden stratosphere warming (SSW) on quasi-2 day wave (QTDW) with westward zonal wavenumber 3 (W3) is investigated using the Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM). The summer easterly jet below 90 km is strengthened during an SSW, which results in a larger refractive index and thus more favorable condition for the propagation of W3. In the winter hemisphere, the Eliassen Palm (EP) flux diagnostics indicate that the strong instabilities at middle and high latitudes in the mesopause region are important for the amplification of W3, which are weakened during SSW periods due to the deceleration or even reversal of the winter westerly winds. Nonlinear interactions between the W3 and the wavenumber 1 stationary planetary wave produce QTDW with westward zonal wavenumber 2 (W2). The meridional wind perturbations of the W2 peak in the equatorial region, while the zonal wind and temperature components maximize at middle latitudes. The EP flux diagnostics indicate that the W2 is capable of propagating upward in both winter and summer hemispheres, whereas the propagation of W3 is mostly confined to the summer hemisphere. This characteristic is likely due to the fact that the phase speed of W2 is larger, and therefore its waveguide has a broader latitudinal extension. The larger phase speed also makes W2 less vulnerable to dissipation and critical layer filtering by the background wind when propagating upward.

scholarly journals Influence of Eddy Momentum Fluxes on the Mean Flow of the Kuroshio Extension in a 1/10° Ocean General Circulation Model

2016 ◽  
Vol 46 (9) ◽  
pp. 2769-2784 ◽  
Author(s):  
Kunihiro Aoki ◽  
Atsushi Kubokawa ◽  
Ryo Furue ◽  
Hideharu Sasaki
Keyword(s):  
Reynolds Stress ◽  
Coriolis Force ◽  
General Circulation ◽  
Kuroshio Extension ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Upstream Region ◽  
Downstream Region ◽  
Momentum Budget ◽  
The Kuroshio

AbstractThis study explores the role of the momentum flux divergence due to mesoscale eddies for the maintenance of the Kuroshio Extension (KE) jet. For that purpose, the zonal momentum budget in a high-resolution ocean general circulation model is examined on the basis of the temporal residual mean framework. The momentum budget analysis is performed for two control volumes: the upstream region of the KE jet flanked by the robust recirculations (33°–38°N and 142.2°–149.4°E) and the downstream region to the east (33°–38°N and 149.4°–160.0°E), both fully covering the meridional width of the KE jet. In both regions the KE jet decelerates to the east, which can be well accounted for by sum of zonal Reynolds stress and Coriolis force on mean ageostrophic flow; the former tends to decelerate the KE jet and the latter to accelerate it in the upstream region, respectively, but these effects are switched in the downstream region. The mean ageostrophic Coriolis force is partially balanced by the horizontal gradient of the eddy kinetic energy, which is the isotropic component of the Reynolds stress. The difference between these terms, that is, net ageostrophic Coriolis force, leads to the final deceleration of the KE jet in the downstream region, overwhelming the acceleration tendency of the anisotropic Reynolds stress. The authors also reinterpret the downstream decay process of an eastward jet in a previous quasigeostrophic experiment in terms of momentum and show that the same features as described above are also likely to be included in that experiment.

scholarly journals Morphological features and variations of temperature in the upper thermosphere simulated by a whole atmosphere GCM

2010 ◽  
Vol 28 (2) ◽  
pp. 427-437 ◽  
Author(s):  
H. Fujiwara ◽  
Y. Miyoshi
Keyword(s):  
High Latitude ◽  
General Circulation ◽  
Circulation Model ◽  
Meridional Wind ◽  
Morphological Features ◽  
Atmospheric Tides ◽  
Low Latitude ◽  
Atmosphere General Circulation Model ◽  
Latitude Region ◽  
Diurnal Temperature Variation

Abstract. In order to illustrate morphological features and variations of temperature in the upper thermosphere, we performed numerical simulations with a whole atmosphere general circulation model (GCM) for the solar minimum and geomagnetically quiet conditions in March, June, September, and December. In previous GCMs, tidal effects were imposed at the lower boundaries assuming dominant diurnal and semi-diurnal tidal modes. Since the GCM used in the present study covers all the atmospheric regions, the atmospheric tides with various modes are generated within the GCM. The global temperature distributions obtained from the GCM are in agreement with ones obtained from NRLMSISE-00. In addition, the GCM also represents localised temperature structures which are superimposed on the global day-night distributions. These localised structures, which vary from hour to hour, would be observed as variations with periods of about 2–3 h at a single site. The amplitudes of the 2–3 h variations are significant at high-latitude, while the amplitudes are small at low-latitude. The diurnal temperature variation is more clearly identified at low-latitude than at high-latitude. When we assume the same high-latitude convection electric field in each month, the temperature calculated in the polar cap region shows diurnal variation more clearly in winter than in summer. The midnight temperature maximum (MTM), which is one of the typical low-latitude temperature structures, is also seen in the GCM results. The MTMs in the GCM results show significant day-to-day variation with amplitudes of several 10s to about 150 K. The wind convergence and stream of warm air are found around the MTM. The GCM also represent the meridional wind reversals and/or abatements which are caused due to local time variations of airflow pattern in the low-latitude region.

scholarly journals Tropospheric Waveguide Teleconnections and Their Seasonality

2017 ◽  
Vol 74 (5) ◽  
pp. 1513-1532 ◽  
Author(s):  
Grant Branstator ◽  
Haiyan Teng
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Wave Model ◽  
Meridional Wind ◽  
Teleconnection Patterns ◽  
Subseasonal Variability ◽  
Point Correlation ◽  
Geographical Position ◽  
The Impact ◽  
Mean State

Abstract One-point correlation maps of the subseasonal variability of 200-hPa meridional wind in nature and an atmospheric general circulation model are systematically analyzed to quantify the impact of the climatological-mean jets on tropospheric covariability as a result of the jets acting as waveguides for the propagation of Rossby waves. As anticipated by linear theory, signatures of jet influence are detected in terms of (i) the geographical position of the strongest teleconnections, (ii) the zonal orientation and extent of prominent patterns of variability, and (iii) the scale of the features that make up those patterns. Further evidence of jet waveguide influence comes from examining the seasonality of these teleconnection attributes. During winter, covariability can be essentially circumglobal, while during summer it tends to be confined within two separate sectors of the globe where the jets are especially strong. Experiments with a multilevel linear planetary wave model confirm that the analyzed characteristics of teleconnections in the waveguides can be attributed to the action of the mean state; no organization to the anomalous forcing of the atmosphere is required to produce these properties. Some attributes, however, depend on the presence of zonal variations in the climatological-mean state that are of similar scale to the teleconnection patterns themselves.

scholarly journals Empirical Localization of Observations for Serial Ensemble Kalman Filter Data Assimilation in an Atmospheric General Circulation Model

2014 ◽  
Vol 142 (5) ◽  
pp. 1835-1851 ◽  
Author(s):  
Lili Lei ◽  
Jeffrey L. Anderson
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
State Variables ◽  
Localization Algorithm ◽  
Localization Function ◽  
Independent Verification ◽  
Community Atmosphere Model ◽  
Verification Period ◽  
Meridional Winds ◽  
Rms Error

Abstract The empirical localization algorithm described here uses the output from an observing system simulation experiment (OSSE) and constructs localization functions that minimize the root-mean-square (RMS) difference between the truth and the posterior ensemble mean for state variables. This algorithm can automatically provide an estimate of the localization function and does not require empirical tuning of the localization scale. It can compute an appropriate localization function for any potential observation type and kind of state variable. The empirical localization algorithm is investigated in the Community Atmosphere Model, version 5 (CAM5). The empirical localization function (ELF) is computed for the horizontal and vertical separately so that the vertical localization is explored explicitly. The horizontal and vertical ELFs are also computed for different geographic regions. The ELFs varying with region have advantages over the single global ELF in the horizontal and vertical, because different localization functions are more effective in different regions. The ELFs computed from an OSSE can be used as the localization in a subsequent OSSE. After three iterations, the ELFs appear to have converged. When used as localization in an OSSE, the converged ELFs produce a significantly smaller RMS error of temperature and zonal and meridional winds than the best Gaspari–Cohn (GC) localization for a dependent verification period using the observations from the original OSSE, and a similar RMS error to the best GC for an independent verification period. The converged ELFs have a significantly smaller RMS error of surface pressure than the best GC for both dependent and independent verification periods.

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