Zonal Wind Components Research Articles

Simulated wave number 4 structure in equatorial F‐region vertical plasma drifts

This article investigates the wave number 4 longitudinal structure in equatorial vertical E × B plasma drifts (V⊥), using the Theoretical Ionospheric Dynamo Model, Institute of Geology and Geophysics, Chinese Academy of Sciences, Version II model and the DE3 tide wind from the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED)/TIMED Doppler interferometer (TIDI) observations. We simulate this longitudinal structure and examine its intra‐annual variations and the effect of different DE3 tide components and of different current components on it. We find that this longitudinal structure shows obvious intra‐annual variation, being strongest in northern summer and autumn and weakest in northern winter. This result is consistent with the intra‐annual variation observed by Ren et al. (2009b). Our simulations also suggest that the eastward propagation of the DE3 tide can cause the eastward shift of the wave number 4 structure in V⊥. The wave number 4 structures in V⊥ are mainly driven by a zonal wind component of DE3 tide, which contributes much more than the meridional wind component of DE3 tide to the wave number 4 structure in V⊥. With the influence of the coupling of ionospheric electric fields along the highly conducting magnetic field lines, the symmetric wind component of DE3 tide plays a more important role in the wave number 4 structure in V⊥ than the antisymmetric component does. The intra‐annual variations of the wave number 4 structure in V⊥ are mainly controlled by the symmetric wind component of DE3 tide. Our simulations also suggest that the wave number 4 structures in V⊥ are mainly driven by the Hall current.

Correction to “Three‐dimensional nonlinear evolution of equatorial ionospheric bubbles with gravity wave seeding and tidal wind effects”

[1] In the paper ‘‘Three-dimensional nonlinear evolution of equatorial ionospheric bubbles with gravity wave seeding and tidal wind effects’’ by M. J. Keskinen and Sharon L. Vadas (Geophysical Research Letters, 36, L12102, doi:10.1029/2009GL037892), Keskinen and Vadas [2009, hereafter KV09] presented the three-dimensional nonlinear evolution of equatorial ionospheric bubbles using a realistic lower atmospheric gravity wave (GW) source. There are a number of misrepresentations and errors by KV09 in regards to the presentation and discussion of the GW source function that was used in the plasma model. The figures and discussion of KV09 refer to a different GW specification, despite the fact that the correct GW specification was used in the plasma model, results of which are presented in Figure 4 of KV09. As a result, the model data shown in Figures 1, 2, and 3 of KV09 were not used to seed the 3D plasma model described by KV09. Correct Figures 1–3 are supplied in this erratum. Additionally, part of paragraph 8 and all of paragraphs 9, 10, and 11 of KV09 should be disregarded and replaced with the text below. [2] The dispersion relation discussed in the latter portion of paragraph 8 in KV09 was not the GW dispersion relation, but was rather the full complex dispersion relation derived from the compressible fluid equations by Vadas and Fritts [2005, hereafter VF2005]. Due to formula errors in KV09, readers interested in the compressible dispersion relation should refer directly to VF2005. The last half of paragraph 8 in VF09 should be replaced by the following: For anelastic GWs with horizontal phase speeds much less than the speed of sound, the GW dispersion relation is given by equation (26) from VF2005 or by equation (47) from Vadas and Fritts [2009, hereafter VF2009]. [3] Vadas and Liu [2009, Figure 1] shows a GOES-12 satellite image on 01 October, 2005, at 21:22 UT, in Brazil. For the KV09 study, we simulated the GWs excited by the energetic convective cluster at 59.0 W and 13.5 S. This cluster contained several tightly-clumped convective plumes, each having an approximate full-width horizontal diameter of 20 km and updraft velocities of 40 m/s. We defined a convective cluster to be composed of 3 convective plumes in an equilateral triangle configuration, with a separation of 50 km between the plume centers (VF2009). Figure 1 shows the GW spectrum excited by this cluster. Figure 1 replaces KV09’s Figure 1. This cluster excites GWs with horizontal wavelengths of lH > 100 km and vertical wavelengths of lz > 50 km, which are capable of penetrating well into the thermosphere prior to dissipating [Vadas, 2007; Fritts and Vadas, 2008]. [4] The ray trace model we used was described by VF2009. The hyperbolic tanh zonal wind and temperature models shown in Figure 2 of KV09 were not used in either the ray trace or plasma models. Instead, the wind and temperature models we used were determined mainly from the Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model. Vadas et al. [2009] describe the background wind and temperature models used. Because they include semidiurnal and diurnal tides, both zonal and meridional background wind components were present. [5] The GWs whose spectrum is plotted in Figure 1 were located at 59.0 Wand 13.5 S at an altitude of z = 13.6 km at 21:22 UT. Those GWs were then ray traced into the middle and upper atmosphere through the spatially and temporallyvarying background winds and temperatures. We then reconstructed the GW wind, temperature, and density perturbations as a function of (x, y, z, t) using the method described by VF2009. We extracted the vertical profiles (cuts) of this solution at 10 different locations and times. These profiles were inputted into the 3D plasma model

Simulation of equatorial wind field patterns with TAPM during the 1997 haze episode in Peninsular Malaysia

The large‐scale forest fires that occurred during the major El Niño event in 1997 can be counted among the worst environmental disasters in Southeast Asia. This study investigates atmospheric mesoscale features over Sumatra and the Klang Valley in Peninsular Malaysia during the resultant haze episode of September 1997 utilizing a limited area mesoscale three‐dimensional meteorological and dispersion model, The Air Pollution Model (TAPM).Mesoscale features that would not be highlighted by global numerical prediction models, such as the daily land and sea breeze conditions at the selected air pollution stations located near the west coast of Peninsular Malaysia, were predicted with an Index of Agreement of 0.3, which implied a moderate conformity between the predicted and observed values. Tracer analysis of air particles at a selected location in the Straits of Malacca verified the existence of the landward and shoreward movement of the air during the simulation of the low‐level wind field. Air pollutants from the burning areas of neighbouring Sumatra just across the straits were transported towards the Klang valley during the daytime and seawards during the nighttime, highlighting the recirculation features of aged and newer air particles over the seven days (13–18 September) of the model simulation. The presence of the central Main Range east of the Klang valley to a certain extent limited further eastward movement of the air particles. Near calm conditions at low levels were simulated from midnight to midday on 14 September, where the movement of the polluted air particles from the uncontrolled burning in Sumatra was confined within the Straits of Malacca. Turbulence within the planetary boundary layer in terms of the total kinetic energy was found to be weak from 14 to 15 September, congruent with the weak strength of low level winds that reduced the ability of the air to transport the pollutants during the period of severe smoke haze.Statistical evaluation showed that parameters such as the systematic RMSE (root mean square error) and unsystematic RMSE for the zonal wind component were slightly higher than for the meridional one, indicating higher errors between the observed and simulated zonal values. Otherwise, the equatorial meteorological parameters such as wind speed and temperature were successfully simulated by the model with comparatively high correlation coefficients, lower RMSEs and moderately high indices of agreement with observed values.

Testing the influence of western Pacific subtropical high...

In this paper, two seasonal scale simulations were conducted for the abnormal climate event in China in the summer of 1998 using a regional climate model (RegCM3). One is the control run, the other is nudging run, which was performed for zonal and meridional wind components, temperature, and humidity data for the region east of 120° E in the model domain to ensure that the simulated activity of western Pacific subtropical high (WPSH) in summer followed those of reanalysis data, while the interaction between the WPSH and the surrounding circulation systems was still maintained partially. Comparisons between the simulated regional circulation systems and the extension/withdrawal of the rain belt over eastern China as well as the activity of the WPSH were carried out. The results show that the relationship between the precipitation over eastern China and WPSH can be reproduced well in the nudging run. However, though the extension/withdrawal of the rain belt over eastern China is mainly dominated by that of WPSH, as pointed out by so many research works, the detailed precipitation scenario is not solely determined by the intensity and position of WPSH, and the precipitation discrepancy between simulation and observation is significant to some extent, which suggests that it is important to improve the precipitation physical process of the model in simulating the detailed precipitation scenario over eastern China.

A Homogeneous Stochastic Model of the Madden–Julian Oscillation

Abstract This paper presents a new empirical model to simulate the Madden–Julian oscillation (MJO), which is the most prominent mode of tropical intraseasonal variability. Zonal wind components at 850 and 200 hPa from reanalysis (1948–2007) and outgoing longwave radiation from satellites (1979–2007) are used to identify MJOs and characterize their statistical properties. The temporal variability of the MJO is represented with a nine-state first-order Markov chain in which state 0 represents quiescent days and states 1–8 are phases of the MJO when it is active. Transition probabilities are estimated based on the historical record of MJO events, and sensitivity tests were performed to evaluate the best estimates for a homogeneous model. Once the model simulates time series of phase transitions, composites of convective and circulation anomalies determine the spatial structure of the events. The amplitudes of the MJOs are stochastically generated with an amplitude factor that has a Gaussian frequency distribution. MJO events generated by the homogeneous stochastic model occur irregularly in time and can appear as single isolated events or sequences of successive MJOs. The MJO in the model can have different eastward propagations and the zonal scale is consistent with the observations. The simulated MJOs have different durations (30–90 days), and each event can be stronger or weaker than the mean composite according to a normal distribution. The results show that the homogeneous stochastic model simulates the irregularity of the MJO and model biases are small. Possible applications and future extensions of the homogeneous stochastic model are discussed.

The spatial pattern of summertime subtropical anticyclones over Asia and Africa: A climatological review

AbstractThe National Centers for Environmental Prediction‐National Center for Atmospheric Research (NCEP‐NCAR) monthly mean reanalysis dataset has been used to analyze spatial variations of summertime subtropical anticyclones over the Asia–Africa region. The geopotential height and zonal wind components of 1000, 500, 200, and 100 hPa in a 30‐year period (1971–2000) have been used to determine the spatial and temporal variations of the anticyclone centres, their monthly frequency and latitudinal axis variations during April–October.The results revealed that there is a clear difference in the location of the summer anticyclone centres in lower, middle and upper levels of the troposphere. In the lower levels, the Azores subtropical anticyclone is located at the east of North Atlantic. In the middle levels, the frequencies of anticyclone centre are concentrated over the northwest of Africa, Arabian Peninsula and Iranian Plateau. In the upper troposphere, the geographical location of the anticyclone centres and their frequencies in the summer season exhibit a scattered pattern from south of China up to western Iran at 200 hPa, and a bimodal pattern over the Tibetan and the Iranian Plateaus at 100 hPa. In fact, in the entire study domain, the Iranian Plateau is a preferable location of the middle and upper troposphere anticyclones.The highest observed latitude of the subtropical anticyclone at 100, 200 and 500 hPa levels have been seen over north of Tibetan plateau, a large area from east to west of Asia and Iran during August, July–August and July, respectively. The maximum monthly variation in the latitude of the ridgeline is seen at 500, 200, and 100 hPa from June to July which goes even up to 10 degrees at some longitudes. Copyright © 2009 Royal Meteorological Society

First mesospheric turbulence study using coordinated rocket and MST radar measurements over Indian low latitude region

Abstract. A campaign to study turbulence in the mesosphere, over low latitudes in India, using rocket-borne measurements and Indian MST radar, was conducted during July 2004. A rocket-borne Langmuir probe detected a spectrum of electron density irregularities, with scale sizes in the range of about 1 m to 1 km, in 67.5–78.0 km and 84–89 km altitude regions over a low latitude station Sriharikota (13.6° N, 80.2° E). A rocket-borne chaff experiment measured zonal and meridional winds about 30 min after the Langmuir probe flight. The MST radar located at Gadanki (13.5° N, 79.2° E), which is about 100 km west of Sriharikota, also detected the presence of a strong scattering layer in 73.5–77.5 km region from which radar echoes corresponding to 3 m irregularities were received. Based on the region of occurrence of irregularities, which was highly collisional, presence of significant shears in zonal and meridional components of wind measured by the chaff experiment, 10 min periodicity in zonal and meridional winds obtained by the MST radar and the nature of wave number spectra of the irregularities, it is suggested that the observed irregularities were produced through the neutral turbulence mechanism. The percentage amplitude of fluctuations across the entire scale size range showed that the strength of turbulence was stronger in the lower altitude regions and decreased with increasing altitude. It was also found that the amplitude of fluctuations was large in regions of steeper electron density gradients. MST radar observations showed that at smaller scales of turbulence such as 3 m, (a) the thickness of the turbulent layer was between 2 and 3 km and (b) and fine structures, with layer thicknesses of about a km or less were also embedded in these layers. Rocket also detected 3-m fluctuations, which were very strong (a few percent) in lower altitudes (67.5 to 71.0 km) and small but clearly well above the noise floor at higher altitudes. Rocket and radar results also point to the possibility of existence of thin layers of turbulence (<450 m). The turbulence parameters estimated from rocket-borne measurements of electron density fluctuations are consistent with those determined from MST radar observed Doppler spectra and the earlier works.

Energia estática sobre o norte e nordeste do Brasil relacionada com a temperatura da superfície do mar

A partir de fontes aparentes de calor e do sumidouro aparente de umidade, foram investigadas as características da circulação atmosférica de grande escala, nas regiões Norte e Nordeste do Brasil (NEB), relacionadas com a variabilidade da temperatura da superfície do mar (TSM) nos oceanos Atlântico e Pacífico Tropicais. Foram utilizados dados mensais das variáveis temperatura, umidade específica, componente zonal e meridional do vento e altura geopotencial (gpm) gerados pelo National Centers for Environment Prediction/National Center for Atmospheric Research (NCEP/NCAR). Os dados utilizados encontram-se espaçados numa grade de 2,5°x 2,5° latitude e longitude, para os níveis padrão de 925 a 100 hPa. Além disso, também foram utilizados dados de TSM, numa grade de 2°x2° de latitude e longitude. Foram escolhidos quatro episódios de El Niño-Oscilação Sul (ENOS) e quatro de padrão de dipolo. O estudo refere-se ao trimestre de fevereiro-abril do período chuvoso. Durante os episódios quentes (frios), El Niño e dipolo positivo (La Niña e dipolo negativo), a atmosfera das áreas 1 e 2 (área 1 - centro Norte e área 2 - centro do Nordeste do Brasil) apresentaram-se menos (mais) úmidas. As variações observadas acima (abaixo) da média na energia estática seca, foram mais fortes durante os anos de El Niño (La Niña) do que durante os anos de dipolo positivo (dipolos negativos). Foram encontradas, menores (maiores) áreas de convergência de energia estática em baixos níveis e divergências em altos níveis durante os anos de El Niño (La Niña). Durante a fase positiva (negativa) dos dipolos, a faixa de convergência de energia estática sobre o Oceano Atlântico, foi observada mais ao norte (ao sul). A região Amazônica, apesar da atuação dos episódios de El Niños, La Niña, dipolos positivos e negativos, comportou-se como um sumidouro de vapor d'água. Já, região do NEB, como uma fonte aparente de vapor d'água durante os anos de El Niño e dipolo positivo e sumidouro para os anos de La Niña e Fase negativa do dipolo.

Venus upper atmospheric CO, temperature, and winds across the afternoon/evening terminator from June 2007 JCMT sub-millimeter line observations

The Venus mesosphere constitutes a highly variable transition region between the zonal rotation of the lower atmosphere and the diurnal circulation of the upper atmosphere. It further serves as the primary photochemical region of the Venus atmosphere. We obtained James Clerk Maxwell Telescope (JCMT, Mauna Kea Hawaii) sub-millimeter line observations of mesospheric 12CO and 13CO during coordinated space (MESSENGER and Venus Express) and ground-based observations of Venus in June of 2007. Such CO spectra line measurements support temperature, CO mixing ratio, and wind retrievals over the 80–110 km altitude range, encompassing the upper mesosphere and lower thermosphere of Venus. Five-point beam integrations were obtained across the observed Venus disk, allowing distinction of afternoon (noon to 6 p.m.) versus evening (6 p.m. to midnight) local times and northern (0–60N) versus southern (0–60S) latitudes. Distinctive diurnal variations (noon to midnight) are retrieved for both temperatures above 95 km and CO mixing ratios above 85 km altitudes. Separate CO line maps obtained on (UT) June 2, 3, 6, and 11 indicate moderate daily variability in afternoon and evening CO mixing ratios (20–50%) and temperatures (5–10 K). Average Venus mesospheric temperatures over this period were 10 K warmer than returned from 1978 to 1979 Pioneer Venus or 2000–01 sub-millimeter measurements, without evidence for the very large temperature inversions indicated by Venus Express SPICAV measurements at 90–100 km altitudes (Bertaux, J.L., Vandaele, A.-C., Korablev, O., Villard, E., Fedorova, A., Fussen, D., Quémerais, E., Belyaev, D., Mahieux, A., Montmessin, F., Muller, C., Neefs, E., Nevejans, D., Wilquet, V., Dubois, J.P. Hauchecorne, A., Stepanov, A., Vinogradov, I., Rodin, A., Bertaux, J.-L., Nevejans, D., Korablev, O., Montmessin, F., Vandaele, A.-C., Fedorova, A., Cabane, M., Chassefière, E., Chaufray, J.Y., Dimarellis, E., Dubois, J.P., Hauchecorne, A., Leblanc, F., Lefèvre, F., Rannou, P., Quémerais, E., Villard, E., Fussen, D., Muller, C., Neefs, E., Van Ransbeeck, E., Wilquet, V., Rodin, A., Stepanov, A., Vinogradov, I., Zasova, L., Forget, F., Lebonnois, S., Titov, D., Rafkin, S., Durry, G., Gérard, J.C., Sandel, B., 2007. A warm layer in Venus’ cryosphere and high-altitude measurements of HF, HCl, H 2O and HDO. Nature 450, 646–649). Measured Doppler shifts associated with June 2 and 11 12CO line center absorptions indicate nearly supersonic (200 m/s, Mach 1) afternoon-to-evening (retrograde) circulation; composed of additive subsolar-to-antisolar (SSAS) and zonal retrograde wind components, which are not separable due to the particular observational geometry.

Observations of the diurnal tide during seven intensive radiosonde campaigns in Australia and Indonesia

Seven intensive radiosonde campaigns of between five days and one month duration were conducted in northern Australia and the Indonesian region between 2001 and 2006. These data were used to study the diurnal tide below 30 km altitude. Inter‐campaign comparisons were made at Darwin (12°S, 131°E) and Koto Tabang (0°S, 100°E). The amplitudes of the diurnal tide did not depend upon the strength of tropospheric convection at Koto Tabang or whether there were monsoon conditions at Darwin. However, the amplitude of the zonal wind component at Koto Tabang during the different campaigns was influenced by the background QBO zonal wind structure. Stratospheric amplitudes exceeded 2.0 m s−1 at 16–17 km and 24–26 km, where the zonal wind shear was greater than 15 m s−1 km−1 during November–December 2005, but were 0.3 m s−1 when the wind shear was close to zero above 25 km during April–May 2004. The local time phase structures at Darwin and Koto Tabang were similar between campaigns and also similar to previous observations in the region, suggesting a uniform regional scale heating source. Vertical wavelengths of the stratospheric diurnal tide were generally 5–20 km. This, together with the tidal interaction with the background wind shear, suggests the presence of non‐migrating components.

A Climatology of Diurnal and Semidiurnal Surface Wind Variations over the Tropical Pacific Ocean Based on the Tropical Atmosphere Ocean Moored Buoy Array

Abstract Hourly measurements from 51 moored buoys in the Tropical Atmosphere Ocean array (9°N–8°S, 165°E–95°W) during 1993–2004 are used to document the climatological seasonal and annual mean patterns of diurnal and semidiurnal near-surface wind variability over the tropical Pacific Ocean. In all seasons, the amplitude of the semidiurnal harmonic is approximately twice as large as the diurnal harmonic for the zonal wind component, while the diurnal harmonic is at least 3 times as large as the semidiurnal harmonic for the meridional wind component, both averaged across the buoy array. Except for the eastern equatorial Pacific, the semidiurnal zonal wind harmonic exhibits uniform amplitude (∼0.14 m s−1) and phase [maximum westerly wind anomalies ∼0325/1525 local time (LT)] across the basin in all seasons. This pattern is well explained by atmospheric thermal tidal theory. The semidiurnal zonal wind signal is diminished over the cold surface waters of the eastern equatorial Pacific where it is associated with enhanced boundary layer stability. Diurnal meridional wind variations tend to be out of phase north and south of the equator (maximum southerly wind anomalies ∼0700 LT at 5°N and ∼1900 LT at 5°S), while a noon southerly wind anomaly maximum is observed on the equator in the eastern Pacific particularly during the cold season (June–November). The diurnal meridional wind variations result in enhanced divergence along the equator and convergence along the southern border of the intertropical convergence zone ∼0700 LT (opposite conditions ∼1900 LT); the amplitude of the divergence diurnal cycle is ∼5 × 10−7 s−1. The diurnal meridional wind variations are largely consistent with the diurnal pressure gradient force.

Relação entre o Multivariate Enso Index (MEI) e a TSM das regiões dos Niños com a precipitação em regiões homogêneas do Estado do Rio Grande do Sul

On Rio Grande do Sul the seasons of the year are well defined being felt, in its peculiar characteristics, in the winter, in the spring, in the summer and in the autumn. The pluviometric regime is quite regular and the precipitations are well distributed during all the year on the State. The Multivariate Enso Index (MEI) lacks of a study about its relations with the precipitation. It is a numeric index that integrates the action of different factors that characterize the phenomenon and that oscilate between positive values for the warm phase, the El Niño, and negative values for the cold phase, the La Niña. It considers, in its composition, the following variables: sea level pressure, zonal and meridional wind components at the surface, the Sea Surface Temperature (SST), the air temperature at the surface and a cloudiness indicator. This work had the objective to study the relations between the MEI and the SST of the Niño regions with the precipitations on Rio Grande do Sul State. For this, it were utilized total monthly data of precipitation from 40 meteorological stations of Rio Grande do Sul, bimonthly data of MEI and SST of the Niño regions for the period 1950 to 2002. The correlation coefficients between the precipitation of the Rio Grande Do Sul with the MEI and the regions of the Niños showed low values due to the fact of if using only the months of the beginning and end of the event. The MEI, although to be a more complex index of the methodologic point of view, it does not improve the coefficients of correlation with the precipitation of the State of the Rio Grande do Sul, and it always presents lesser or equal values to obtained when using the TSM of the regions of the Niños in the out/nov and nov/dez coupled of months. The MEI and the Niños regions 3 and 3.4 present the highest correlation coefficient with the Rio Grande do Sul State precipitation for the bimonths oct/nov and nov/dec.

Zonal Wind Calculations from Mars Global Surveyor Accelerometer and Rate Data

TheMars Global Surveyor spacecraft was initially placed into a high-eccentricity, nearly polar orbit about Mars with a 45-h period. To accomplish the science objectives of the mission, a 2-h circular orbit was required. Using a method known as aerobraking, numerous passes through the upper atmosphere slowed the spacecraft, thereby reducing the orbital period and eccentricity. To successfully perform aerobraking, the spacecraft was designed to be longitudinally, aerodynamically stable in pitch and yaw. Because the orbit was nearly polar, the yaw orientation of the spacecraft was sensitive to disturbances caused by the zonal components of wind (east to west or west to east) acting on the spacecraft at aerobraking altitudes. Zonal wind velocities were computed by equating the aerodynamic and inertia-related torques acting on the spacecraft. Comparisons of calculated zonal winds with those computed from the Mars thermospheric general-circulation model are discussed.

Observational evidence for the generation of a 4-day oscillation in the low-latitude middle atmosphere associated with an anomalous stratospheric cooling

Abstract. Altitude profiles of temperature in the stratosphere and lower mesosphere over Gadanki (13.5° N, 79.2° E), based on lidar observations during the Equatorial Wave Study campaign (29 February–2 April 2000), showed an anomalous cooling of the stratosphere, lasting for about 4–5 days. Immediately following this stratospheric cooling, a prominent ~4-day period oscillation is observed in the zonal, as well as meridional wind in the 30–50 km altitude region, obtained from simultaneous rocketsonde and balloon wind measurements. The amplitude of this wave is in the range of 5–15 ms−1 in this altitude region. The altitude profiles of phase of this 4-day oscillation show a constant phase with height indicating that the wave is not propagating vertically. The causative mechanism for the sudden cooling is investigated by examining the day-to-day variation in the total atmospheric column ozone. Zonal and meridional wind components of this oscillation are approximately in quadrature phase, with zonal wind leading the meridional wind. Strikingly, while the 4-day wave is clearly observable in the zonal and meridional winds, it is almost absent in the temperature.

Long-term variations of the mesospheric wind field at mid-latitudes

Abstract. Continuous MF radar observations at the station Juliusruh (54.6° N; 13.4° E) have been analysed for the time interval between 1990 and 2005, to obtain information about solar activity-induced variations, as well as long-term trends in the mesospheric wind field. Using monthly median values of the zonal and the meridional prevailing wind components, as well as of the amplitude of the semidiurnal tide, regression analyses have been carried out with a dependence on solar activity and time. The solar activity causes a significant amplification of the zonal winds during summer (increasing easterly winds) and winter (increasing westerly winds). The meridional wind component is positively correlated with the solar activity during summer but during winter the correlation is very small and non significant. Also, the solar influence upon the amplitude of the semidiurnal tidal component is relatively small (in dependence on height partly positive and partly negative) and mostly non-significant. The derived trends in the zonal wind component during summer are below an altitude of about 83 km negative and above this height positive. During the winter months the trends are nearly opposite compared with the trends in summer (transition height near 86 km). The trends in the meridional wind components are below about 85 km positive in summer (significant) and near zero (nonsignificant) in winter; above this height during both seasons negative trends have been detected. The trends in the semidiurnal tidal amplitude are at all heights positive, but only partly significant. The detected trends and solar cycle dependencies are compared with other experimental results and model calculations. There is no full agreement between the different results, probably caused by different measuring techniques and evaluation methods used. Also, different heights and observation periods investigated may contribute to the detected differences.

Local Hadley circulation over the Asian monsoon region associated with the Tropospheric Biennial Oscillation

The Tropospheric Biennial Oscillation (TBO), a major interannual variation phenomenon in the Indo-Pacific region, is the result of strong ocean-atmosphere coupling over the Asian-Australian monsoon area. Along with other meteorological and oceanographic parameters, the tropical circulation also exhibits interannual oscillations. Even though the TBO is the result of strong air–sea interaction, the circulation cells during TBO years are, as yet, not well understood. In the present study, an attempt has been made to understand the interannual variability of the mean meridional circulation and local monsoon circulation over south Asia in connection with the TBO. The stream function computed from the zonal mean meridional wind component of NCEP/NCAR reanalysis data for the years 1950–2003 is used to represent the mean meridional circulation. Mean meridional mass transport in the topics reverses from a weak monsoon to a strong monsoon in the presence of ENSO, but in normal TBO years mean transport remains weak across the Northern Hemisphere. The meridional temperature gradient, which drives the mean meridional circulation, also shows no reversal during the normal TBO cycle. The local Hadley circulation over the monsoon area follows the TBO cycle with anomalous ascent (descent) in strong (weak) monsoon years. During normal TBO years, the Equatorial region and Indian monsoon areas exhibit opposite local Hadley circulation anomalies.

Observed Changes in the Lifetime and Amplitude of the Madden–Julian Oscillation Associated with Interannual ENSO Sea Surface Temperature Anomalies

Abstract The Madden–Julian oscillation (MJO) is analyzed using the reanalysis zonal wind– and satellite outgoing longwave radiation–based indices of Wheeler and Hendon for the 1974–2005 period. The average lifetime of the MJO events varies with season (36 days for events whose central date occurs in December, and 48 days for events in September). The lifetime of the MJO in the equinoctial seasons (March–May and October–December) is also dependent on the state of El Niño–Southern Oscillation (ENSO). During October–December it is only 32 days under El Niño conditions, increasing to 48 days under La Niña conditions, with similar values in northern spring. This difference is due to faster eastward propagation of the MJO convective anomalies through the Maritime Continent and western Pacific during El Niño, consistent with theoretical arguments concerning equatorial wave speeds. The analysis is extended back to 1950 by using an alternative definition of the MJO based on just the zonal wind component of the Wheeler and Hendon indices. A rupture in the amplitude of the MJO is found in 1975, which is at the same time as the well-known rupture in the ENSO time series that has been associated with the Pacific decadal oscillation. The mean amplitude of the MJO is 16% larger in the postrupture (1976–2005) compared to the prerupture (1950–75) period. Before the 1975 rupture, the amplitude of the MJO is maximum (minimum) under El Niño (La Niña) conditions during northern winter, and minimum (maximum) under El Niño (La Niña) conditions during northern summer. After the rupture, this relationship disappears. When the MJO–ENSO relationship is analyzed using all-year-round data, or a shorter dataset (as in some previous studies), no relationship is found.

A numerical study on amplitude characteristics of the terdiurnal tide excited by nonlinear interaction between the diurnal and semidiurnal tides

Abstract A fully nonlinear numerical tidal model has been developed with the aim of presenting amplitude features of the terdiurnal tide excited by nonlinear interaction between the diurnal and semidiurnal tides. Since the superposition of the migrating diurnal and semidiurnal tidal solutions from the GSWM-00 (Global Scale Wave Model 2000) is taken as the initial disturbance for this nonlinear model, the diurnal and semidiurnal tides are allowed to nonlinearly interact with each other. The analyses on the simulations show that the migrating terdiurnal tide can be significantly excited by the nonlinear interaction between the diurnal and semidiurnal tides in the mesosphere and lower thermosphere (MLT) region, especially above 90 km, and have pronounced amplitudes (wind speeds over 15 m s−1 and temperature over 10 K) in the lower thermosphere (90–110 km). In addition, its amplitudes vary strongly with season and maximize during equinoctial months at low and middle latitudes, and its zonal wind component is larger than the meridional wind component. Simultaneously, the diurnal and semidiurnal tides exhibit evident variations, indicating that the wave-wave and wave-mean flow interactions are the important cause of the tidal variability. Our calculations also illustrate the remarkable alteration of the background fields induced by the mean flow/tidal interaction, implying this interaction should be comprehensively considered in describing global atmospheric dynamic and thermal structures in the MLT region.

Scale-Selective Bias Correction in a Downscaling of Global Analysis Using a Regional Model

Abstract Systematic large-scale errors are often found within the regional domain in the regional dynamical downscaling procedure. This paper proposes a method to suppress such errors using a combination of spectral tendency damping and area average correction of temperature, humidity, and surface pressure in the Regional Spectral Model. The proposed scale-selective bias-correction method reduces the time tendency of the zonal and meridional wind components for the physical scale greater than a predetermined length. In addition, the area mean perturbations of temperature and humidity are forced to zero. The surface pressure difference between the model field and the global field is adjusted from the hydrostatic equation using the mean elevation difference between the two fields and the area mean temperature. Each of these three components of the technique is necessary for the model to effectively reduce large-scale errors in the regional domain. With this method, the downscaled field becomes less dependent on the domain size. Furthermore, the downscaled precipitation compares better with observations, as do the near-surface temperature and wind fields. The scheme allows much weaker lateral boundary relaxation, although it is still an essential part of the regional model. The use of a similar scheme is recommended for any regional model in the application of dynamical downscaling of analysis for climate studies.

Environmental variability in the North Atlantic and Iberian waters and its influence on horse mackerel (Trachurus trachurus) and albacore (Thunnus alalunga) dynamics

Abstract Lavín, A., Moreno-Ventas, X., Ortiz de Zárate, V., Abaunza, P., and Cabanas, J. M. 2007. Environmental variability in the North Atlantic and Iberian waters and its influence on horse mackerel (Trachurus trachurus) and albacore (Thunnus alalunga) dynamics. – ICES Journal of Marine Science, 64: 425–438. We explore the potential impact of climatic and oceanic variables on the dynamics of horse mackerel Trachurus trachurus (coastal distribution) and albacore Thunnus alalunga (oceanic distribution). Principal components analysis of a set of environmental parameters for the years 1966–2000 allowed us to characterize the system by three components. The first consisted mainly of sea surface temperature (SST; 18.5% of variability), the second was determined by the oceanic transport indices, potential energy anomaly (PEA), and the Gulf Stream Index (15.6%), and the third by the meridional wind component and Ekman transport (11.5%). Horse mackerel recruitment was negatively correlated mainly with the first thermal component, whereas albacore age 3 catches were negatively correlated with the second oceanic component and positively with the third wind component. Multiple linear regression confirmed that environmental conditions [SST, PEA, and the zonal (east–west) wind component] explained the availability of age 3 albacore to the surface fisheries for the period 1975–1999. In contrast, cross-validation analysis showed that environmental conditions did not consistently explain horse mackerel recruitment, probably because of the short time-series available (15 y).