Meridional Wind Components Research Articles

Steadiness of the upper winds up to 100 hPa over Turkey between 1980 and 1984

Turkish radiosonde data obtained at the meteorological stations in Istanbul, Ankara and Izmir have further been analysed in order to study the steadiness of the upper winds for the seven pressure levels from 1000 hPa to 100 hPa between 1970 and 1984. The computed standard deviations (σu, σu) of the zonal and the meridional wind components were employed in order to furnish a measure of the total kinetic energy of the wind perturbations. The results revealed a marked seasonal difference. The winds in the troposphere over Turkey are found to be steady, in particular, in summer. This confirms the extent of the subtropical jet stream over Turkey in summer.

An observational study of the 30–50 day atmospheric oscillations part II: Temporal evolution and hemispheric interaction across the equator

In this part, the temporal evolution and interaction across the equator of 30–50 day oscillation in the atmosphere are investigated further. The annual variation of 30–50 day oscillation is quite obvious in the mid-high latitudes. In the tropical atmosphere, the obvious interannual variation is an important property for temporal evolution of 30–50 day oscillation. The low-frequency wavetrain across the equator over the central Pacific and central Atlantic area, the movement of the long-lived low-frequency system across the equator and the meridional wind component across the equator will obviously show the interaction of 30–50 day oscillation in the atmosphere across the equator.

Equatorial thermospheric wind changes during the solar cycle: Measurements at Arequipa, Peru, from 1983 to 1990

Fabry‐Perot interferometer measurements of Doppler shifts in the nightglow 630‐nm emission line have been used to determine near‐equatorial thermospheric wind velocities at Arequipa, Peru, over ∼2/3 of a solar cycle. Monthly‐average nocturnal variations in the meridional and zonal wind components were calculated from the nightly data to remove short term (day‐to‐day) variability, facilitating display of seasonal changes in the wind patterns, as well as any additional changes introduced by the progression of the solar cycle. The measured seasonal variations in the wind patterns are more pronounced than the solar cycle variations and are more readily understandable in terms of the expected, underlying forcing and damping processes. For most of the years, at the winter solstice, there is a weak (≤100 m/s) transequatorial flow from the summer to the winter hemisphere in the early and the late night, with essentially zero velocities in between. At the equinoxes, an early‐night poleward (southward) flow at solar minimum (1986) is replaced by an equatorward (northward) flow at solar maximum (1989–1990). The zonal flows are predominantly eastward throughout the night, except for the solar minimum equinoxes, where brief westward flows appear in the early and the late night. The peak eastward velocities increase toward solar maximum; at the winter solstice, they are ∼100–130 m/s in 1983, 1984 and 1986, reaching ∼200 m/s in 1988, 1989 and 1990. The present equatorial thermospheric wind determinations agree in some respects with the satellite‐data‐based horizontal wind model HWM‐87 and the vector spherical harmonic form of the thermospheric general circulation model. They also are generally consistent with earlier studies at Arecibo, Puerto Rico, and at Kwajalein, Marshall Islands, when compared at similar parts of the solar cycle.

The Southern Oscillation in the Australian Region. Part I: Anomalies at the Extremes of the Oscillation

Abstract The response of the tropospheric circulation over Australia and the southwest Pacific to extremes of the Southern Oscillation is documented using correlation, regression, and compositing techniques applied to seasonal means of rawinsonde station data. A strong and seasonally varying signal is found in the four fields of geopotential height, temperature, zonal, and meridional wind components. The composite analysis shows that while the response to extremes of the oscillation is in general linear, some significant local nonlinear departures are observed.

On quasi–biennial oscillation in air–sea system

From the COADS (Comprehensive Ocean–Atmosphere Data Set) I and the COADS II, we got a monthly data set of sea surface temperature (SST), zonal and meridional wind components at sea level (U,V) and sea level pressure (SLP) with 4° × 4° grid system covering the period from Jan. 1950 to Dec. 1987 to study the evolutional features of the quasi-biennial oscillation (QBO) in the air-sea system. The analytic method of complex empirical orthogonal function (CEOF) is used to obtain the composite temporal sequences of amplitude (six phases for half a period) for the first and the second main components of SST, U, V and SLP. It is shown from the results that the main characteristics for different phases of the sea surface temperature anomaly’s (SSTA) QBO are warm water / cold water in the equator of the eastern Pacific (EEP). There are two warm or cold water centers of the SSTA in the EEP, which are located in the equator of the central Pacific (ECP) and the east part of the EEP. The features of the source propagation and the influence of these two centers on atmospheric circulation are discussed and it can be seen that in the formation of these two centers, there are different features in oceanic and atmospheric circulations and air-sea coupled process.

Neutral winds in the thermosphere at mid‐latitudes over a full solar cycle: A tidal decomposition

The magnetic meridional component of the thermospheric neutral wind at the F2 peak at Boulder, Colorado, is derived for days with magnetic index Ap ≤ 20 over a full solar cycle (July 1975–June 1986). The servo model technique and ionosonde measurements of hmF2 are used, and the results are corrected for ionization drifts due to electric fields. A Fourier decomposition of the mean daily variation is carried out for each month. This reveals significant components with wave periods of 24, 12, 8, and 6 hours. The importance of ion drag in causing seasonal and solar cycle variations in the winds is inferred. Specifically, the conclusion of previous work that the diurnal amplitude is larger at solar minimum than at solar maximum is confirmed by the present study. The diurnal amplitude is generally in the range 60–80 m s−1 at solar minimum but only 25–45 m s−1 at solar maximum. The diurnal phase maximum of the northward wind lies in the range 1115–1625 LT with latest values in winter and at solar maximum, when the daytime effects of ion drag are greatest. The amplitude of the semidiurnal component shows little solar cycle variation but tends to be smaller in summer compared to other seasons. It has a mean value of 41 m s−1 over the solar cycle. The semidiurnal phase maximum is remarkably close to 0800 LT for all seasons throughout the solar cycle. The amplitudes of the 8 and 6 hour waves are both in the range 4–25 m s−1. The 8 and 6 hour waves both have later phase maxima in summer than in winter. The present results are generally consistent with current models of atmospheric tides, except for the large solar cycle variation in the diurnal component.

Indian summer monsoon rainfall and 200‐mbar meridional wind index: Application for long‐range prediction

AbstractVerma and Kamte (1980) and Joseph et al. (1981) identified the association between Indian monsoon rainfall and 200‐mbar meridional wind index for the month of May, and indicated its potential for prediction of the seasonal rainfall.A detailed investigation has been made of this relationship between the monsoon rainfall over India and the meridional wind index (arithmetic average of 200‐mbar meridional wind component for May at Bombay, Delhi, Madras, Nagpur, and Srinagar) based on the data for the period 1964–1988.The relationship between all‐India monsoon (June‐September) rainfall (R) and the wind index (Vm) is negative (correlation coefficient = −0.72) and is significant at the 0.1 per cent level and the regression equation based on this data, for the period 1964–1988, is R = 86.98–2.44 Vm. The stability and consistency of the relationship between rainfall and the wind index have been examined over sliding widths of 15 and 21 years, and the relationship is found to be significant at the 1 per cent level for all windows of 21‐year widths. The meridional wind index shows a good potential in the long‐range prediction scheme of the Indian monsoon rainfall, along with other parameters.

Wind characteristics in the lowest 340 m of the atmospheric boundary layer at Pune, India

Wind characteristics in the lowest 340 m (agl) of the atmospheric boundary layer at Pune (18°32′N, 73°51′E, 559 m ASL) have been investigated using the pilot balloon wind observations obtained during the summer monsoon seasons of 1976, 1979 and 1980. Variations in the zonal and meridional components of wind at the surface, 40, 150 and 340 m (agl) have been described. Frequency distribution of the zonal component showed an unimodal character at the surface, which gradually approached a multimodal character at 340 m. The distribution pattern of the wind components was, by and large, normal. Spectral analysis of the wind components showed that the spectral energy was predominantly shared by 5–7 and 2–3 day periods.

Kinematic properties of the mean circulation over southern Africa

Harmonic analysis is used to study the temporal and spatial distributions of monthly-mean zonal and meridional wind components, and their standard deviations, above southern African radiosonde stations. Thermal and mechanical influences of the underlying subcontinent have substantial impacts on the form of the flow. Below the plateau inversion mechanical effects are the predominant influence but above inversion levels winds are primarily modified by spatially-varying pressure gradients related to differential surface-atmosphere heat fluxes. The flow below is decoupled from that above the coastal inversion, as is the flow across the plateau inversion. Two independent high pressure systems influence the circulation of either side of the coastal inversion. Two standing waves may be detected above the inversion. The first, modulated on an annual cycle, is marked by a local Hadley Cell over the west coast and a local Ferrel Cell over the east coast. The second, modulated on a semi-annual cycle, is marked by meridional component reversals over the interior of the subcontinent. With these reversals there is a shift in the latitude of the subtropical jet.

Characteristics of North African 6–9 Day Waves during Summer 1981

Abstract This study applies compositing methods to ECMWF model analyses during July and August 1981 to determine the structure of synoptic scale waves with periods of 6–9 days. Over northern Africa, these ware intermittent in time, are most regular in period and amplitude west of the Greenwich meridian in a narrow longitude band, though there is a suggestion of their existence between 15°E and 0°. The average wavelength is 5000 km and the westward velocity is 6°–7° longitude day−1. The horizontal wind pattern in midtroposphere (700 and 500 hPa) consists of two vortices of opposite signs on either side of 12.5°N, with maximum perturbation of the zonal wind component at this latitude, and maximum perturbation of the meridional wind component at 5°–7.5°N and at 17.5°–20°N. Temperature is modulated by the passage of the waves throughout the troposphere. This structure is very different from that of the 3‐4 day African wave.

Global Forecast Error Correlation. Part 1: Isobaric Wind and Geopotential

Abstract Results of a thorough study of the correlation structure of observation-minus-forecast increments for mandatory pressure level radiosonde observations of zonal and meridional wind components and geopotential, differenced with NMC's 6-hour global forecasts, are reported. Our work focused on the selection of a representation for spatial lag-correlations to be used in updating the multivariate statistical objective analysis algorithm of the global data assimilation system, with attention given to regional and seasonal dependence of the correlation structure, and on the degree to which the increments are in the same geostrophic balance as the signal and forecast fields individually. We compare the performance of several candidates for representing autocorrelations of geopotential increments, on the one hand, and the auto- and cross-correlations of the wind component increments, on the other, for five mandatory pressure levels, for four regions of the Northern Hemisphere and for the Southern Hemispher...

Correlative radar and airborne sodium lidar observations of the vertical and horizontal structure of gravity waves near the mesopause

In November 1986, joint radar and Na lidar observations were conducted in Colorado to study mesopause dynamics. The radar observations were conducted at Platteville, Colorado with a Stratosphere Troposphere (ST) radar in a meteor echo detection mode. The Na lidar observations included ground‐based observations at Broomfield and Denver, Colorado, and airbome observations during three flights over the Great Plains, Rocky Mountains, and Pacific Coast. Both radar and lidar identified a 6‐hour wave that was remarkably coherent during the November campaign. The vertical wavelength of the 6‐hour wave estimated from the radar data was 8 to 9 km, the amplitude of the zonal wind component was 10 m S−1, and the amplitude of the meridional wind component was 4 m s−1. The vertical wavelength estimated from the ground‐based and airborne lidar data was 7.3 km, and the amplitude of the scalar horizontal wind ranged from 13 to 16 m s−1. The zonal wavelength of this wave was approximately 772 km, and the intrinsic zonal phase velocity was 35 m s−1 westward. The 6‐hour wave was dominant in the height range of 80–90 km, which corresponds to the bottomside of the Na layer. An additional 2‐hour wave was observed with the Na lidar during both the ground‐based and airborne observations. The vertical wavelength of this wave was about 12 km, and the zonal wavelength was 260 km. The intrinsic zonal phase velocity was 43 m s−1 westward. This 2‐hour wave was also present only on the bottomside of the Na layer.

Prediction of Northeast Brazil Rainfall Anomalies

Abstract Expanding on earlier research, empirically based methods are developed for predicting March-September rainfall in northern Northeast Brazil. From a network of 27 raingage stations in Brazil's Nordeste, new rainfall index series are constructed for March-September (MS) and October-January (OJ). Data input to stepwise multiple regression models further include January values of the Tahiti minus Darwin pressure index, an index of sea surface temperature (SST) in the equatorial Pacific (PWT), and indices of the fields of meridional (v) and zonal (u) wind components and of SST in the tropical Atlantic between 30°N and 30°S (AFV, ARJ, and AFT, respectively). Empirical orthogonal function (EOF) analysis of the v, u, and SST fields was used in constructing the latter three indices. Throughout the study, a sharp distinction is kept between a “dependent” dataset (1921–42 and 1948–57) used as a training period and an “independent” portion of the record (1958–87) reserved for prediction experiments. Preseaso...

Structure of the marine boundary layer over north western Indian Ocean during 1983 summer monsoon

The spatial variability of the structure of the lower troposphere over the northwestern Indian Ocean for the period 12th July to 2nd September, 1983 has been studied using upper air data collected during the first scientific cruise of ORV ‘Sagar Kanya’. An analysis of thermodynamic structure and kinematics of the marine boundary layer for different zonal and meridional sections revealed the following features: (a) Temperature and humidity inversions were generally absent over the study area except over a few locations in the western region; (b) Large-scale subsidence was found over the central equatorial Indian Ocean; (c) The convective activity over the western Indian Ocean was found to be moderately suppressed as compared to the eastern region; (d) The zonal and meridional components of winds along the equator and 10° N zonal section exhibited a mirror-image-like distribution.

Low‐latitude thermospheric neutral winds determined from AE‐E measurements of the 6300‐Å nightglow at solar maximum

Atmosphere Explorer E (AE‐E) measurements of the O(¹D) 6300‐Å emission in the nighttime equatorial thermosphere are used to infer the height of the F2 layer peak as a function of latitude and local time. The investigation is conducted both for northern hemisphere winter solstice and for spring equinox, under solar maximum conditions. The layer heights are used to derive magnetic meridional components of the transequatorial neutral wind, in conjunction with the MSIS‐86 model and previous Jicamarca incoherent scatter measurements of the zonal electric field. The AE‐E wind estimates indicate a predominant summer to winter flow for the winter solstice case. Comparisons are made with the empirical horizontal wind model HWM87 and with winds generated by the thermospheric general circulation model. The model predictions and experimental results are generally in good agreement, confirming the applicability of visible airglow data to studies of the global neutral wind pattern. Perhaps the most significant discrepancy occurs near midnight, where some of the data indicate a sharp decrease in the magnitude of the neutral wind. This feature, which has been observed in previous experimental studies, is not reproduced by either of the two models.

Thermosphere‐ionosphere coupling: An experiment in interactive modeling

The National Center for Atmospheric Research thermosphere general circulation model (TGCM) is utilized to perform a series of controlled experiments aimed at better understanding the interactive coupling between ionospheric plasma densities and thermospheric neutral winds. The experiments are simple and controlled so as to facilitate identification of governing mechanisms. The interaction is accomplished by parameterizing the F layer peak height (hmF2) in an empirical ionospheric model in terms of the meridional wind (υsouth) and forcing hmF2 and υsouth to remain mutually coupled in a dynamical calculation. Interactive computations are performed where the TGCM is driven by 30‐kV and 90‐kV cross polar cap potentials and are compared with corresponding reference simulations where hmF2 is constrained to a balance height near 280 km with a small (∼20 km) diurnal variation. Mutual coupling between hmF2 and υsouth is found to be weak during the daytime when the F layer exhibits a broad vertical structure. At night, when the F2 layer is more localized, the neutral dynamical structure is dependent on whether hmF2 is significantly above or below the altitude (approximately 275–300 km) at which ion drag effectively competes with viscosity in the neutral momentum balance. At Arecibo during both levels of high‐latitude forcing, the premidnight elevation (+75 km) of hmF2 relative to a nighttime balance height of approximately 290 km is accompanied by an increase (∼50 m s−1) in the zonal wind above 250 km relative to the noninteractive simulation. A much smaller effect, characterized by an 8‐hour periodicity spanning 24 hours, is seen in the meridional wind component. At Millstone Hill under the 90‐kV forcing, a 40‐km ascent of hmF2 to 330 km around midnight due to southward winds is accompanied by a 40–60 m s−1 increase in the westward wind above 200 km. Interestingly, the southward wind is diminished in magnitude by about 35 m s−1 compared to the noninteractive simulation, opposite to what one would expect if the ion drag presented to the meridional flow were reduced. We interpret this as a Coriolis effect induced by the increased westward wind.

Atmospheric tides below 80 km

Measurements and theory of diurnal and semidiurnal tidal oscillations between about 25 and 80 km are reviewed. At latitudes greater than about 30°, meridional (N-S) wind components are consistently in quadrature with and similar to the zonal (E-W) components. The tidal structures are interpreted as a superposition of quasi-steady higher-order modes excited in the troposphere by sources of limited extent (1,000–10,000 km). At latitudes less than about 30°, steady or quasi-steady diurnal and semidiurnal components are not necessarily the dominant components of the daily variation. At high latitudes diurnal phases generally show little change with height in comparison with observations at lower latitudes, in accord with the latitudinal properties of diurnal modes with positive and negative equivalent depths.

Harmonic analysis of summer mean wind at 200 mbar level during contrasting monsoon years over India

Summer (June–August) mean zonal and meridional wind components at 200 mbar level are subjected to harmonic analysis for the years 1970, 1971, 1972 and 1979. It is found that the small scale disturbances are intense during normal monsoon years. The westerlies in the belt 10°S to 30°S are stronger during drought years. During normal monsoon years (1970, 1971) the northward transport of westerly momentum by wave number 1 at 19.6°N is large as compared to drought years (1972, 1979). The transport of westerly momentum by standing eddies is northward for all the years between 5°S and 28.7°N but large during the normal monsoon years.

The Influence of Sea-Surface Temperature on Surface Wind in the Eastern Equatorial Pacific: Weekly to Monthly Variability

Abstract Temporal correlations between near-equatorial surface wind and sea-surface temperatures (SST) at 11°W in the eastern Pacific Ocean are investigated using data from an array of moored sensors between 5°N and 5°S. The signature of tropical instability waves with periods of 20–30 days is apparent in time series of SST and both the meridional and zonal wind components. Results indicate the existence of a band of pronounced horizontal divergence in the surface wind field associated with the large meridional SST gradient (equatorial front) normally located just north of the equator. Perturbations of the equatorial front by the instability waves induce fluctuations in the overlying winds. Evidence of the air-sea coupling is stronger in time series of the meridional gradients of wind and SST than between time series of the variables themselves. The meridional differencing serves as a high-pass filter in the space domain, which removes planetary-scale wind fluctuations that are unrelated to the local SST ...

Observations of neutral circulation at mid‐latitudes during the Equinox Transition Study

Measurements of ion drift velocity made by the Millstone Hill incoherent scatter radar have been used to calculate the meridional neutral wind velocity during the September 17–24, 1984, period. Strong daytime southward neutral surges were observed during the magnetically disturbed days of September 19 and 23, in contrast to the small daytime winds observed as expected during the magnetically quiet days. The surge on September 19 was also seen at Arecibo. In addition, two approaches have been used to calculate the meridional wind component from the radar‐derived height of the F layer electron density peak (hmF2). Results confirm the wind surges, particularly when the strong electric fields measured during the disturbed days are included in the calculations. The magnitudes of the surges are uncertain, however, due to the poorly known O+, O collision cross section and neutral composition. The two approaches for the F layer peak wind calculations are applied to the radar‐derived hmF2 as a function of latitude to reveal the large north‐south extent as well as the temporal variation of the southward daytime surges.