Vertical Zonal Wind Shear Research Articles

Possible Linkage between the Monsoon Trough Variability and the Tropical Cyclone Activity over the Western North Pacific

Abstract The present study investigates the influence of the monsoon trough (MT) on the interannual variability of tropical cyclone (TC) activity over the western North Pacific during July–November for the period 1979–2007. It is shown that the TC activity is closely related to the MT location. During the years when the MT extends eastward (retreats westward), more (less) TCs form within the southeastern quadrant of the western North Pacific. Such a relationship can be explained by the changes in large-scale environmental factors associated with the movement of the MT. An eastward extension of the MT coincides with warmed ocean surface, enhanced convection, increased relative humidity in the lower and midtroposphere, reduced vertical shear of zonal wind, intensified upper-level divergence, and low-level anomalous cyclonic vorticity over the southeast quadrant of the western North Pacific. These conditions associated with the eastern extension of the MT are favorable for TC genesis, while those associated with the westward retreat of the MT are not. Diagnosis of the barotropic energy conversion indicates that synoptic-scale disturbances moving westward from tropical eastern Pacific will gain the energy from the mean flow when they meet with the eastward-extending MT. This is an important reason for the linkage between MT variability and TC genesis over the western North Pacific.

Model Evaluation and Projection on the Linkage between Hadley Circulation and Atmospheric Background Related to the Tropical Cyclone Frequency over the Western North Pacific

AbstractThe performance of climate models in simulating the linkage of the spring Hadley circulation (HC) to the vertical zonal wind shear and atmospheric divergence in the lower and upper troposphere, which are related to the tropical cyclone frequency over the western North Pacific (WNPTCF) during June–September (JJAS), is evaluated on the basis of the 20th century climate simulations (20C3M). It is found that four models can simultaneously reproduce the pattern revealed in the observation, with the spring HC in the Northern Hemisphere being positively correlated to the vertical zonal wind shear in the major tropical cyclone (TC) genesis region and negatively (positively) correlated to the atmospheric divergence in the upper (lower) troposphere over the western North Pacific (WNP) in the following JJAS. These four models are further used to project their relationship in the late 21st century under the A1B scenario. The results show that the association of spring HC with the vertical zonal wind shear and...

An observational evidence of climate change during global warming era

The relationships of the Frequency of the Cyclonic Systems (FCS) over the Bay of Bengal in the Summer Monsoon (SM) season (June through September) with the North Atlantic Oscillation Index (NAOI) and the Southern Oscillation Index (SOI) from January through May for the period 1950–2005 are examined. The NAOI (January and February) and the SOI (April) showed significant inverse and positive relationships respectively with the FCS generated in SM season over the Bay of Bengal. These relationships are marked on a decadal scale. In the Global Warming (GW) era, a significant reduction in the FCS is associated with a significant increase of NAOI, relaxation of SOI and a weak vertical zonal wind shear in the domain 5°–22°N and 80°–100°E.

The role of vertical shear of the meridional winds in the northward propagation of ITCZ

[1] A simple linear model was proposed by Jiang et al. (2004) to highlight the mechanism of scale selection during the northward propagation of cloud bands over Bay of Bengal. The easterly shear in zonal winds was shown to be an essential parameter in scale selection. This model was criticized for the use of unrealistic values of the friction and diffusion parameters. The present study extends this model by adding baroclinic vertical shear in the meridional mean winds. The correct rate of propagation is obtained with reasonable values of friction and diffusion parameters. In the present model, the direction of propagation is essentially determined by easterly vertical shear of zonal winds, while vertical shear of meridional wind also contributes to the observed propagation phase speed and instability. The correct phase speed is obtained for southerly mean meridional vertical shear.

Interannual variations of tropical cyclone activity over the north Indian Ocean

AbstractAn examination of the interannual variations of tropical cyclone (TC) activity over the North Indian Ocean during 1983–2008 has been carried out. The results suggest that instead of local sea surface temperatures, such variations, at least over the Bay of Bengal (BB) during October‐November‐December (OND), can be attributed to similar variations in the atmospheric flow patterns and moist static energy that are apparently forced largely by the El Niño/Southern Oscillation (ENSO). In an El Niño year, conditions for TC genesis and development, including 850‐hPa relative vorticity, 200–850‐hPa vertical shear of zonal wind, moist static energy, 500‐hPa zonal wind, 500‐hPa and 850‐hPa geopotential height and 200‐hPa divergence, are generally less favourable in BB and fewer intense cyclones are observed during OND. The reverse occurs during a La Niña event. However, causes of the variability of TC activity over BB during April‐May‐June and that over Arabian Sea have yet to be found, which may be due to the small sample size. Copyright © 2011 Royal Meteorological Society

Neutral wind-driven gradient drift instability in the low-latitude daytime E region

[1] We present strong evidence for the wind-driving, daytime E region field-aligned irregularities (FAIs) on the basis of Sanya VHF radar observations on 29 June 2009. The observed sporadic E (Es) and echoing layers show a clear diurnal tidal descending trend and a gravity-wave-scale oscillation with a period of 40–50 min, which are consistent with the diurnal tidal height evolution seen from the meteor wind observations. The most important finding is that the daytime continuous FAIs are closely related to the negative vertical zonal wind shear, which also forms the dense ion layer. Associated observations suggest that the FAIs are generated on both sides of the Es layer. The echoes form two layers with 10 km vertical distance, about half the vertical wavelength of the diurnal tide. Our results, for the first time, demonstrate that the daytime E region continuous FAIs can be formed by the wind-driven gradient drift instabilities. We suggest that the latitudinal variation of the daytime E region FAI morphologies is caused by the dip angle dependence of the instability growth rate. Continuous echoes observed elsewhere at low latitudes and midlatitudes may also be attributed to the wind effects.

Changes in Jupiter’s zonal velocity between 1979 and 2008

We show that the peak velocity of Jupiter’s visible-cloud-level zonal winds near 24°N (planetographic) increased from 2000 to 2008. This increase was the only change in the zonal velocity from 2000 to 2008 for latitudes between ±70° that was statistically significant and not obviously associated with visible weather. We present the first automated retrieval of fast (∼130 m s −1) zonal velocities at 8°N planetographic latitude, and show that some previous retrievals incorrectly found slower zonal winds because the eastward drift of the dark projections (associated with 5-μm hot spots) “fooled” the retrieval algorithms. We determined the zonal velocity in 2000 from Cassini images from NASA’s Planetary Data System using a global method similar to previous longitude-shifting correlation methods used by others, and a new local method based on the longitudinal average of the two-dimensional velocity field. We obtained global velocities from images acquired in May 2008 with the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). Longer-term variability of the zonal winds is based on comparisons with published velocities based on 1979 Voyager 2 and 1995–1998 HST images. Fluctuations in the zonal wind speeds on the order of 10 m s −1 on timescales ranging from weeks to months were found in the 1979 Voyager 2 and the 1995–1998 HST velocities. In data separated by 10 h, we find that the east–west velocity uncertainty due to longitudinal fluctuations are nearly 10 m s −1, so velocity fluctuations of 10 m s −1 may occur on timescales that are even smaller than 10 h. Fluctuations across such a wide range of timescales limit the accuracy of zonal wind measurements. The concept of an average zonal velocity may be ill-posed, and defining a “temporal mean” zonal velocity as the average of several zonal velocity fields spanning months or years may not be physically meaningful. At 8°N, we use our global method to find peak zonal velocities of ∼110 m s −1 in 2000 and ∼130 m s −1 in 2008. Zonal velocities from 2000 Cassini data produced by our local and global methods agree everywhere, except in the vicinity of 8°N. There, the local algorithm shows that the east–west velocity has large variations in longitude; vast regions exceed ∼140 m s −1. Our global algorithm, and all of the velocity-extraction algorithms used in previously-published studies, found the east–west drift velocities of the visible dark projections, rather than the true zonal velocity at the visible-cloud level. Therefore, the apparent increase in zonal winds between 2000 and 2008 at 8°N is not a true change in zonal velocity. At 7.3°N, the Galileo probe found zonal velocities of 170 m s −1 at the 3-bar level. If the true zonal velocity at the visible-cloud level at this latitude is ∼140 m s −1 rather than ∼105 m s −1, then the vertical zonal wind shear is much less than the currently accepted value.

Modification of Saharan air layer and environmental shear over the eastern Atlantic Ocean by dust‐radiation effects

This study investigates the influence of dust‐radiation effects on the modification of the Saharan air layer (SAL) and environmental shear. A tracer model based on the Weather Research and Forecast model was developed to examine the influence using a dust outbreak event. Two numerical experiments were conducted with (ON) and without (OFF) the dust‐radiation effects. Both simulations reasonably reproduced SAL's features. However, the 700 hPa maximum temperature within SAL was slightly underestimated and shifted northwestward from OFF. These were improved from ON, but the maximum temperature became slightly overestimated, which might be due to inaccurate optical properties. The dust‐radiation interactions mainly warmed the dusty air between 750 and 550 hPa because dust shortwave absorption dominated dust longwave cooling. Another major warming area was found near the surface over the ocean due to longwave radiative heating by dust aloft. The modification of temperature resulted in an adjustment of the vertical wind shear. To the south of SAL, where easterly wave disturbances and tropical storms usually occur, the vertical zonal wind shear increased by about 1∼2.5 m s−1 km−1 from 750 to 550 hPa, resulting in a maximum wind change of 3∼5 m s−1, a 30∼40% increase, around the top of this layer. The enhancement of the vertical shear in this layer could potentially have an impact on TC genesis and development. The dust‐radiation effects also modified the moisture and dust distribution, which can have a feedback (i.e., a secondary effect) on the heating profile and the vertical shear.

Sea surface temperature east of Australia: A predictor of tropical cyclone frequency over the western North Pacific?

The relationship between sea surface temperature (SST) east of Australia and tropical cyclone frequency over the western North Pacific (WNPTCF) is analyzed by use of observation data. The WNPTCF from June to October is correlated negatively to spring SST east of Australia. When the spring SST is in the positive phase, a cyclonic circulation anomaly in the upper troposphere and an anticyclonic circulation anomaly in the lower troposphere prevail over the western North Pacific from June to October, concurrent with an anomalous atmospheric subsidence and an enlarged vertical zonal wind shear. These conditions are unfavorable for tropical cyclone genesis, and thus WNPTCF decreases. The negative phase of the spring SST east of Australia leads to more tropical cyclones over the western North Pacific. The spring SST east of Australia may give rise to simultaneous change in tropical atmospheric circulation via the teleconnection wave train, and then subsequently affect atmospheric circulation variation over the western North Pacific.

The Madden–Julian Oscillation Simulated in the NCEP Climate Forecast System Model: The Importance of Stratiform Heating

Abstract This study investigates the capability for simulating the Madden–Julian oscillation (MJO) in a series of atmosphere–ocean coupled and uncoupled simulations using NCEP operational general circulation models. The effect of air–sea coupling on the MJO is examined by comparing long-term simulations from the coupled Climate Forecast System (CFS T62) and the atmospheric Global Forecast System (GFS T62) models. Another coupled simulation with a higher horizontal resolution model (CFS T126) is performed to investigate the impact of model horizontal resolution. Furthermore, to examine the impact on a deep convection scheme, an additional coupled T126 run (CFS T126RAS) is conducted with the relaxed Arakawa–Schubert (RAS) scheme. The most important factors for the proper simulation of the MJO are investigated from these runs. The empirical orthogonal function, lagged regression, and spectral analyses indicated that the interactive air–sea coupling greatly improved the coherence between convection, circulation, and other surface fields on the intraseasonal time scale. A higher horizontal resolution run (CFS T126) did not show significant improvements in the intensity and structure. However, GFS T62, CFS T62, and CFS T126 all yielded the 30–60-day variances that were not statistically distinguishable from the background red noise spectrum. Their eastward propagation was stalled over the Maritime Continent and far western Pacific. In contrast to the model simulations using the simplified Arakawa–Schubert (SAS) cumulus scheme, CFS T126RAS produced statistically significant spectral peaks in the MJO frequency band, and greatly improved the strength of the MJO convection and circulation. Most importantly, the ability of MJO convection signal to penetrate into the Maritime Continent and western Pacific was demonstrated. In this simulation, an early-stage shallow heating and moistening preconditioned the atmosphere for subsequent intense MJO convection and a top-heavy vertical heating profile was formed by stratiform heating in the upper and middle troposphere, working to increase temperature anomalies and hence eddy available potential energy that sustains the MJO. The stratiform heating arose from convective detrainment of moisture to the environment and stratiform anvil clouds. Therefore, the following factors were analyzed to be most important for the proper simulation of the MJO rather than the correct simulations of basic-state precipitation, sea surface temperature, intertropical convergence zone, vertical zonal wind shear, and lower-level zonal winds: 1) an elevated vertical heating structure (by stratiform heating), 2) a moisture–stratiform instability process (a positive feedback process between moisture and convective–stratiform clouds), and 3) the low-level moisture convergence to the east of MJO convection (through the appropriate moisture and convective–stratiform cloud processes–circulation interactions). The improved MJO simulation did improve the global circulation response to the tropical heating and may extend the predictability of weather and climate over Asia and North America.

Modeling the Influence of Spring Hadley Circulation on the Summer Tropical Cyclone Frequency in the Western North Pacific

AbstractThe spring Hadley circulation is observed to be negatively correlated to the summer tropical cyclone frequency in the western North Pacific. This relationship is linked by the summer atmospheric circulation anomalies resulting from the anomalous spring Hadley circulation. To examine the authenticity of such a relationship indicated by the observation, a simulation on the strong spring Hadley circulation scenario by IAP9L‐AGCM is performed, and the response of the environmental conditions affecting the tropical cyclone genesis is analyzed in the present study. Results show that in relation to the strong spring Hadley circulation, larger magnitude of the vertical zonal wind shear and the anomalous low‐level divergence and upper‐level convergence are predominated in the western North Pacific in summer. The East Asian summer monsoon is also weakened. Such an atmospheric circulation background is unfavorable for the tropical cyclone genesis, and thus the tropical cyclone frequency in the western North Pacific is decreased. The numerical simulation supports the observation, demonstrating the existence of the out‐of‐phase relationship between the spring Hadley circulation and the summer tropical cyclone frequency in the western North Pacific. So, the variation of the spring Hadley circulation can be a potential indicator in predicting summer tropical cyclone activities in the western North Pacific.

Characteristics of the Tropical Easterly Jet: Long‐term trends and their features during active and break monsoon phases

Using high–vertical resolution Indian mesosphere‐stratosphere‐troposphere (MST) radar and GPS sonde observations from Gadanki (13.5°N, 79.2°E), an attempt has been made for the first time to delineate the characteristics of Indian summer monsoon winds with special emphasis on the Tropical Easterly Jet stream (TEJ), its day‐to‐day, monthly, and interannual variability using observations available for more than a decade (1996–2008). For comparison, data available during 1996–2008 from routine radiosonde observations from nearby India Meteorological Department (IMD) observatory located at Chennai (13.0°N, 80.1°E) and National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP‐NCAR) reanalysis data sets have been considered. Relatively poor vertical resolution data from IMD lead to underestimation of the jet characteristics like jet width, vertical shear above and below the jet, and the wind reversal height. Although reanalysis data sets represented well the spatial variability, they underestimate the peak zonal winds by about 5 m/s when compared to Indian MST radar and GPS sonde observations. Large day‐to‐day variability of TEJ characteristics (peak jet speed, jet height, jet width, zonal wind reversal height, and vertical shear of zonal wind) over Gadanki is noticed. No significant trend is observed in peak jet speed using reanalysis data sets either over Gadanki grid or over jet core region in Indian summer monsoon months. However, MST radar and GPS sonde observations show increasing trend (∼1 m/s/yr) in peak jet speed particularly from the year 2000. Characteristics of the TEJ during active and break phases showed quite different features. Thus, statistics generated using the long‐term database from Indian MST radar and GPS sonde provide valuable information for studying the TEJ characteristics and will be useful to incorporate in the future reanalysis data sets.

Observational relationship of climatologic beta drift with large‐scale environmental flows

While many numerical studies suggested that large‐scale environmental flows significantly affect beta drift, this influence was not verified observationally and included in assessing changes of tropical cyclone (TC) tracks under the background of global warming. This study investigates the influence of large‐scale environmental flow on climatologic beta drift in the northern West Pacific using the best track dataset from Joint Typhoon Warming Center (JTWC) and the NCEP monthly reanalysis over the period from 1965 to 2007. Consistent with most previous numerical studies, the meridional shear of zonal wind, vertical shear of zonal wind and meridional gradient of relative vorticity, which are mainly associated with the zonal environmental wind, play an important role in affecting beta drift. For the meridional shear of zonal flows, the northward beta drift in an anticyclonic shear is faster than that in a cyclonic shear. A poleward (equatorward) gradient of relative vorticity leads to faster (slower) westward beta drift than in a resting environment. TCs tend to move to the left of the vector of vertical wind shear.

2007년 북서태평양에서의 열대저기압 발생 특징

This study found that tropical cyclones (TCs) formed for fall in 2007 over the western North Pacific were distributed in high-latitudes comparing to 56-year (1951-2006) climatological mean. The frequency and latitude of TC genesis became higher than 56-year climatological mean from September onward in 2007 and all the TCs that formed to the north of 20<TEX>$^{\circ}$</TEX>N was also distributed after September in 2007. These characteristics of TC genesis for fall in 2007 could be confirmed through analyzing various variables, such as a large-scale atmospheric circulation, outgoing longwave radiation (OLR), vertical zonal wind shear, and sea surface temperature (SST). On the other hand, a frequency of the TC that occurred to the north of 200N showed a clear interdecadal variation and its decreasing trend was distinctive in recent years. Its intensity was also weaker that TCs that did to the south of 20<TEX>$^{\circ}$</TEX>N. However, a latitude of TC genesis showed an increasing trend until recent years, whose variation was consistent with trend that through a SST analysis, warm SST went north in recent years.

The Dynamics of the ENSO–Atlantic Hurricane Teleconnection: ENSO-Related Changes to the North African–Asian Jet Affect Atlantic Basin Tropical Cyclogenesis

Abstract The nature of the teleconnection linking ENSO variability with Atlantic basin tropical storm formation is investigated. Solutions of the linearized barotropic vorticity equation forced with August–October El Niño event divergence produce upper-tropospheric vorticity anomalies over the Sahel and at the mouth of the North African–Asian (NAA) jet over the tropical Atlantic. These responses are similar in magnitude and orientation to observed ENSO vorticity variability for this region. Further investigation reveals that the vorticity anomalies over the subtropical Atlantic develop primarily in response to very low wavenumber, westward-propagating stationary Rossby waves excited by El Niño–related convective activity over the equatorial Pacific Ocean. However, the dynamics of this teleconnection change as the Atlantic basin hurricane season progresses. In August and September the response is dominated by the westward-propagating stationary Rossby waves that alter vorticity within the NAA jet and to its south. The upper-tropospheric nondivergent zonal wind anomalies produced by these vorticity anomalies are similar in pattern to observed zonal wind and vertical zonal wind shear anomalies, which suppress Atlantic basin tropical cyclogenesis. By October, eastward-propagating signals also develop over the tropical Atlantic Ocean in response to El Niño conditions. Over the main development region of Atlantic basin tropical cyclogenesis, these eastward-propagating Rossby waves appear to destructively interfere with the vorticity changes produced by the westward-propagating Rossby waves within the NAA jet. In addition, the NAA jet has shifted south by October. Consequently, the resultant upper-tropospheric nondivergent zonal wind perturbations for October are weak and suggest that ENSO should have little effect on rates of Atlantic basin tropical cyclogenesis during October. Statistical analyses of monthly ENSO-related changes in Atlantic basin tropical storm formation support this hypothesis.

Hadley circulation signal in the tropical cyclone frequency over the western North Pacific

The relationship between the boreal spring (March–May) Hadley circulation (HC) and the following summer (June–September) tropical cyclone (TC) frequency expressed as the total number of TCs in the western North Pacific is investigated through using the observed data. Results show that the spring HC is negatively correlated to the summer TC frequency. Such a relationship can be explained by the changes of the atmospheric circulations related to anomalous spring HC. A strong spring HC is followed by the weaker East Asian monsoon, stronger vertical zonal wind shear, and reduced convection over the western North Pacific in summer, which are unfavorable for TC genesis, and vice versa. The potential mechanism of how the spring HC affects the summer atmospheric circulations is also preliminarily identified. It is found that sea surface temperature (SST) anomalies in the Indian Ocean and the South China Sea may play important roles, since an anomalous spring HC can excite simultaneous SST anomalies, which can persist to the following summer and in turn give rise to the atmospheric anomalies associated with TC activities. Thus the variation of the spring HC can be a potential indicator in predicting summer TC activities over the western North Pacific.

Can the climate background of western North Pacific typhoon activity be predicted by climate model?

Based on the observation and reanalysis data through 1948–2004, the vertical shear of zonal wind, outgoing longwave radiation, and divergence fields in the lower and upper troposphere during summer are revealed to correlate significantly with the concurrent western North Pacific (WNP) typhoon frequency, and they therefore can be regarded as predictors for the WNP typhoon activity anomaly. After that, the 34-year (1970–2003) ensemble hindcast experiments are performed by the nine-level atmospheric general circulation model developed at the Institute of Atmospheric Physics Under the Chinese Academy of Sciences (IAP9L-AGCM), aiming to investigate the numerical predictability of the summer vertical shear of zonal wind and divergence field in the lower troposphere. It is found that the temporal correlation coefficients between the hindcast and observation are 0.70 and 0.62 for the vertical shear of zonal wind and divergence field, respectively. This suggests that the model possesses a large potential skill for predicting the large-scale climate background closely related to the WNP typhoon activity, and the model is therefore capable of performing the real-time numerical prediction of the WNP typhoon activity anomaly to some extent.

Relationship between the Asian-Pacific oscillation and the tropical cyclone frequency in the western North Pacific

The relationship between the Asian-Pacific oscillation (APO) and the tropical cyclone frequency over the western North Pacific (WNP) in summer is preliminarily investigated through an analysis of observed data. The result has shown clearly that APO is significantly and positively correlated to the tropical cyclone frequency in the WNP. If APO is above (below) the normal in summer, more (less) tropical cyclones will tend to appear in the WNP. The present study also addresses the large-scale atmospheric general circulation changes underlying the linkage between APO and the WNP tropical cyclone frequency. It follows that a positive phase of summer APO is concurrent with weakened as well as northward and eastward located western Pacific subtropical high (WPSH), low-level convergence and high-level divergence, and reduced vertical zonal wind shear in the WNP, providing favorable environment for the tropical cyclone genesis, and thus more tropical cyclones will come into being, and vice versa.

The Research on Abnormal Features and Formation Causes of Tropical Cyclones Landing Southeastern Coastal Zone in 2005

AbstractUsing 1949~2005 data from the Tropical Cyclone (Typhoon) Year‐Book of CMA, this paper investigates the features of the TC (tropical cyclone) activities in the western Pacific as well as TCs landing on the coastal areas of southeastern China, indicating that the numbers have dropped slightly, while the intensity of the landing TCs increased significantly. In 2005, the number of TCs of the western Pacific origin was fewer than normal but the landing typhoons were greatly stronger than the mean. Analysis of the abnormal phenomena resulting from the large‐scale circulation background shows that prior to the generation, the south‐Asian high (SAH) and the western Pacific subtropical high (sub‐high) are stronger than the mean, together with the weaker sub‐high and East Asian summer monsoon, vertical shear of zonal wind etc. in the TC season all working together prevent TCs from formation. On the other hand, the onland TCs are stronger than normal, which may be the result of abnormal vapor transport in the west part of the western Pacific, weak vertical wind shear, SSTA and the interaction between weather systems at mid‐ and low‐ latitudes.

Relationships between the North Pacific Oscillation and the typhoon/hurricane frequencies

Relationships between the North Pacific Oscillation (NPO) and the typhoon as well as hurricane frequencies are documented. The correlation between NPO index in June–July–August–September and the annual typhoon number in the western North Pacific is 0.37 for the period of 1949–1998. The NPO is correlated with the annual hurricane number in the tropical Atlantic at −0.28 for the same period. The variability of NPO is found to be concurrent with the changes of the magnitude of vertical zonal wind shear, sea-level pressure patterns, as well as the sea surface temperature, which are physically associated with the typhoons and hurricanes genesis. The NPO associated atmospheric circulation variability is analyzed to explain how NPO is linked with variability of the tropical atmospheric circulation in the western Pacific and the tropical Atlantic, via the atmospheric teleconnection.