Mechanism for Southward Shift of Zonal Wind Anomalies During the Mature Phase of ENSO

2021 ◽  
pp. 1-45
Author(s):  
Yuhan Gong ◽  
Tim Li
Keyword(s):  
Zonal Wind ◽  
General Circulation ◽  
Boreal Summer ◽  
Positive Contribution ◽  
Antisymmetric Mode ◽  
Wind Anomaly ◽  
Zonal Wind Anomaly ◽  
Model Experiments ◽  
Southward Shift ◽  
Background State

AbstractThe cause of southward shift of anomalous zonal wind in the central equatorial Pacific (CEP) during ENSO mature winter was investigated through observational analyses and numerical model experiments. Based on an antisymmetric zonal momentum budget diagnosis using daily ERA-Interim data, a two-step physical mechanism is proposed. The first step involves advection of the zonal wind anomaly by the climatological mean meridional wind. The second step involves the development of an antisymmetric mode in the CEP, which promotes a positive contribution to the observed zonal wind tendency by the pressure gradient and Coriolis forces. Two positive feedbacks are responsible for the growth of the antisymmetric mode. The first involves the moisture–convection–circulation feedback, and the second involves the wind–evaporation–SST feedback. General circulation model experiments further demonstrated that the boreal winter background state is critical in generating the southward shift, and a northward shift of the zonal wind anomaly is found when the same SST anomaly is specified in boreal summer background state.

Solar Influences on Stratospheric Circulation Patterns

2020 ◽  
Author(s):  
Yonatan Givon ◽  
Chaim Garfinkel
Keyword(s):  
Zonal Wind ◽  
Wind Anomaly ◽  
Zonal Wind Anomaly ◽  
Circulation Patterns ◽  
Downward Propagation ◽  
P Flux ◽  
Stratospheric Circulation ◽  
The Tropics ◽  
The Impact ◽  
Transformed Eulerian Mean

<p>The impact of the solar cycle on the NH winter stratospheric circulation is analyzed using<br>simulations of a Model of an idealized Moist Atmosphere (MiMA). By comparing solar minimum<br>periods to solar maximum periods, the solar impact on the stratosphere is evaluated: Solar<br>maximum periods are accompanied by warming of the tropics that extends into the midlatitudes<br>due to an altered Brewer Dobson Circulation. This warming of the subtropics and the altered<br>Brewer Dobson Circulation leads to an increase in zonal wind in midlatitudes, which is then<br>followed by a decrease in E-P flux convergence near the winter pole which extends the enhanced<br>westerlies to subpolar latitudes.<br>We use the transformed Eulerian mean framework to reveal the processes that lead to the<br>formation of this sub-polar zonal wind anomaly and its downward propagation from the top of the<br>stratosphere to the tropopause.</p>

scholarly journals The Development of Upper-Tropospheric Wind over the Western Hemisphere in Association with MJO Convective Initiation

2015 ◽  
Vol 72 (8) ◽  
pp. 3138-3160 ◽  
Author(s):  
Naoko Sakaeda ◽  
Paul E. Roundy
Keyword(s):  
Zonal Wind ◽  
Western Hemisphere ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Kelvin Wave ◽  
Wind Anomaly ◽  
Convective Initiation ◽  
Zonal Wind Anomaly ◽  
Easterly Wind ◽  
Driving Mechanisms

Abstract This study examines the structure and driving mechanisms of upper-tropospheric intraseasonal zonal wind anomalies over the Western Hemisphere (WH) during the convective initiation of the Madden–Julian oscillation (MJO) over the Indian Ocean using composite and budget analyses. The initiating MJO convection is more often associated with WH upper-tropospheric intraseasonal easterly wind anomalies, and when it is, it tends to develop a stronger and zonally broader envelope of enhanced convection than events associated with westerly wind anomalies. The WH upper-tropospheric zonal wind anomaly associated with the MJO is often described as a dry Kelvin wave radiated from MJO convection, but the results show that both the structure and driving mechanisms are different from the ones of theoretical Kelvin waves. Unlike the theoretical Kelvin wave, the zonal wind anomaly is not driven mainly by the zonal pressure gradient force and it is strongly coupled with rotational wind associated with subtropical and equatorward-propagating midlatitude Rossby waves. The intraseasonal zonal wind anomaly amplifies over the eastern Pacific and Atlantic basins because of advection of the background wind by intraseasonal wind in the presence of background zonal wind convergence, which allows acceleration in the same sign of the present intraseasonal zonal wind anomaly. A part of the WH intraseasonal easterly wind initiates in the lower stratosphere and is advected downward as it merges with eastward-propagating easterly wind in the upper troposphere. The initial sources of the lower-stratospheric intraseasonal easterly wind include equatorward intrusion of midlatitude waves and an equatorial Rossby wave.

scholarly journals Seasonal Variation of the Seychelles Dome

2008 ◽  
Vol 21 (15) ◽  
pp. 3740-3754 ◽  
Author(s):  
Takaaki Yokoi ◽  
Tomoki Tozuka ◽  
Toshio Yamagata
Keyword(s):  
Seasonal Variation ◽  
Indian Ocean ◽  
Wind Stress ◽  
Zonal Wind ◽  
General Circulation ◽  
Indian Monsoon ◽  
Circulation Model ◽  
The Other ◽  
Boreal Summer ◽  
Zonal Wind Stress

Abstract Using an ocean general circulation model (OGCM), seasonal variation of the Seychelles Dome (SD) is investigated for the first time. The SD is an oceanic thermal dome located in the southwestern Indian Ocean, and its influence on sea surface temperature is known to play an important role in the Indian monsoon system. Its seasonal variation is dominated by a remarkable semiannual cycle resulting from local Ekman upwelling. This semiannual nature is explained by different contributions of the following two components of the Ekman pumping: one term that is proportional to the planetary beta and the zonal wind stress and the other term that is proportional to the wind stress curl. The former is determined by the seasonal change in the zonal component of the wind stress vector above the SD; it is associated with the Indian monsoon and causes downwelling (upwelling) during boreal summer (boreal winter). The latter, whose major contribution comes from the meridional gradient of the zonal wind stress, also shows a clear annual cycle with strong upwelling during boreal summer and fall. However, it remains almost constant for 5 months from June to October, even though the zonal wind stress itself varies significantly during this period. The above overall feature is due to the unique location of the SD; it is located between the following two regions: one is dominated by the seasonal variation in wind stress resulting from the Indian monsoon, and the other is dominated by the southeasterly trade winds that prevail throughout a year. The above uniqueness provides a novel mechanism that causes the strong semiannual cycle in the tropical Indian Ocean.

Variability of Intraseasonal Kelvin Waves in the Equatorial Pacific Ocean

2008 ◽  
Vol 38 (5) ◽  
pp. 921-944 ◽  
Author(s):  
Toshiaki Shinoda ◽  
Paul E. Roundy ◽  
George N. Kiladis
Keyword(s):  
Wind Stress ◽  
General Circulation ◽  
Phase Speed ◽  
Circulation Model ◽  
Kelvin Waves ◽  
Upper Ocean ◽  
Equatorial Pacific Ocean ◽  
Model Experiments ◽  
Stress Changes ◽  
Background State

Abstract Previous observational work has demonstrated that the phase speed of oceanic equatorial Kelvin waves forced by the Madden–Julian oscillation (MJO) appears to vary substantially. Processes that are responsible for systematic changes in the phase speed of these waves are examined using an ocean general circulation model. The model was integrated for 26 yr with daily wind stress derived from the NCEP–NCAR reanalysis. The model is able to reproduce observed systematic changes of Kelvin wave phase speed reasonably well, providing a tool for the analysis of their dynamics. The relative importance of the upper ocean background state and atmospheric forcing for phase speed changes is determined based on a series of model experiments with various surface forcings. Systematic changes in phase speed are evident in all model experiments that have different slowly varying basic states, showing that variations of the upper ocean background state are not the primary cause of the changes. The model experiments that include and exclude intraseasonal components of wind stress in the eastern Pacific demonstrate that wind stress changes to the east of the date line can significantly alter the speed of Kelvin waves initially generated over the western Pacific, which often results in a phase propagation faster than the free wave speed. These faster waves contribute to the systematic changes of phase speed evident in observations. Similar results are also obtained using a linear stratified model, eliminating nonlinearity as a possible cause of the phase speed changes.

Local time variation of equatorial temperature and zonal wind anomaly (ETWA)

1998 ◽  
Vol 60 (6) ◽  
pp. 631-642 ◽  
Author(s):  
R. Raghavarao ◽  
R. Suhasini ◽  
W.R. Hoegy ◽  
H.G. Mayr ◽  
L. Wharton
Keyword(s):  
Local Time ◽  
Zonal Wind ◽  
Time Variation ◽  
Wind Anomaly ◽  
Zonal Wind Anomaly

Zonal displacement of western Pacific warm pool and zonal wind anomaly over the Pacific Ocean

2007 ◽  
Vol 25 (3) ◽  
pp. 277-285 ◽  
Author(s):  
Qilong Zhang ◽  
Qinghua Zhang ◽  
Yijun Hou ◽  
Jianping Xu ◽  
Xuechuan Weng ◽  
...  
Keyword(s):  
Pacific Ocean ◽  
Zonal Wind ◽  
Warm Pool ◽  
Western Pacific ◽  
Western Pacific Warm Pool ◽  
Wind Anomaly ◽  
Zonal Wind Anomaly ◽  
The Pacific ◽  
Pacific Warm Pool ◽  
The Pacific Ocean

scholarly journals Transient Extratropical Response to Solar Ultraviolet Radiation in the Northern Hemisphere Winter

2021 ◽  
pp. 1-51
Author(s):  
Yonatan Givon ◽  
Chaim I. Garfinkel ◽  
Ian White
Keyword(s):  
Uv Radiation ◽  
Zonal Wind ◽  
General Circulation ◽  
Solar Rotation ◽  
Rotation Period ◽  
Circulation Model ◽  
Solar Variability ◽  
Solar Ultraviolet Radiation ◽  
Hemisphere Winter ◽  
Solar Ultraviolet

AbstractAn intermediate complexity General Circulation Model is used to investigate the transient response of the NH winter stratosphere to modulated ultraviolet (UV) radiation by imposing a step-wise, deliberately exaggerated UV perturbation and analyzing the lagged response. Enhanced UV radiation is accompanied by an immediate warming of the tropical upper stratosphere. The warming then spreads into the winter subtropics due to an accelerated Brewer Dobson Circulation in the tropical upper stratosphere. The poleward meridional velocity in the subtropics leads to an increase in zonal wind in midlatitudes between 20N and 50N due to Coriolis torque. The increase in mid-latitude zonal wind is accompanied by a dipole in Eliassen-Palm flux convergence, with decreased convergence near the winter pole and increased convergence in mid-latitudes (where winds are strengthening due to the Coriolis torque); this dipole subsequently extends the anomalous westerlies to subpolar latitudes within the first ten days. The initial radiatively-driven acceleration of the Brewer-Dobson circulation due to enhanced shortwave absorption is replaced in the subpolar winter stratosphere by a wave-driven deceleration of the Brewer-Dobson circulation, and after a month the wave-driven deceleration of the Brewer-Dobson circulation encompasses most of the winter stratosphere. Approximately a month after UV is first modified, a significant poleward jet shift is evident in the troposphere. The results of this study may have implications for the observed stratospheric and tropospheric responses to solar variability associated with the 27-day solar rotation period, and also to solar variability on longer timescales.

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