Meridional Wind Components Research Articles

The Work Done by the Wind on the Oceanic General Circulation

Abstract A new estimate is made using altimeter data of the rate at which the wind works on the oceanic general circulation. The value of about 1 TW is lower than previously estimated and is dominated by the work done by the mean zonal wind in the Southern Ocean. The meridional component of the mean wind contributes primarily in the eastern upwelling regions of the ocean. Fluctuating component contributions are small. A comparison with the results of a numerical model produces both the same total work as well as the same general geographical patterns but with detailed differences. Both observations and model show that the subtropical gyres are regions where the atmosphere is braking the ocean circulation. The input of wind energy is shown to be qualitatively consistent with estimates of the rates of decay of barotropic and baroclinic mesoscale variability. If most of the energy input into the Southern Ocean is dissipated there, this region could be a dominant factor in mixing the global ocean.

Evidence of two regimes of easterly waves over West Africa and the tropical Atlantic

Synoptic‐scale easterly waves at 700 hPa have been studied over West Africa and the tropical Atlantic for the summers of 1979–1995. Spectral analyses (Fast Fourier and Wavelet Transforms) of 700 hPa meridional wind component along 17.5°N enable to point out two band‐periods, between 3 and 5 days and between 6 and 9 days. An example of each wave is shown. Composite analysis confirms the evidence of two such regimes of easterly waves during summer over West Africa and the tropical Atlantic. The 6–9‐day wave regime differ from the 3–5‐day wave regime by larger anticyclonic cells originated from the Libyan and the Azores areas, and by more northern tracks.

Diurnal and Semidiurnal Variations of the Surface Wind Field over the Tropical Pacific Ocean

The climatological large-scale patterns of diurnal and semidiurnal near-surface wind variations over the tropical Pacific Ocean are documented using 3 yr of hourly measurements from the Tropical Atmosphere–Ocean moored buoy array. Semidiurnal variations account for 68% of the mean daily variance of the zonal wind component, while diurnal variations account for 82% of the mean daily variance of the meridional wind component. The spatially uniform amplitude (0.15 m s−1) and phase (0300 LT) of the semidiurnal zonal wind variations are shown to be consistent with atmospheric thermal tidal theory. The diurnal meridional wind variations on either side of the equator are approximately out of phase. This pattern results in a diurnal variation of wind divergence along the equator, with maximum divergence in the early morning (∼0800 LT). The average amplitude of the diurnal cycle in zonal mean divergence is 0.45 × 10−6 s−1, which corresponds to a day–night change of 45% relative to the daily mean. The relative day–night changes in near-surface equatorial wind divergence are larger in the western Pacific (78%) than in the eastern Pacific (31%) due mainly to differences in the daily mean divergence. The diurnal amplitude of equatorial wind divergence changes seasonally and interannually in proportion to the strength of the mean divergence. It is suggested that diurnal heating of the sea surface may contribute to the zonally symmetric diurnal cycle of equatorial wind divergence.

Horizontal meridional thermospheric winds over King George Island, Antarctica, during the June 1991 Geomagnetic storm

Abstract Diurnal variations of the magnetic meridional component of the thermospheric neutral wind have been derived from both a servo model based algorithm with ionosonde data input, and the HWM-90 empirical wind model for the 4–6 and 11–13 June 1991 geomagnetic storm at King George Island (62.2°S, 58.8°W). While the HWM-90 winds are predominantly equatorward, the servo model winds are predominantly poleward during the storm, with especially strong poleward winds in the pre-noon hours. Such strong poleward winds are not expected to occur during a major storm at such a low geomagnetic latitude (51.4°S), which is equatorward of direct ion drag forcing by magnetospheric convection electric fields. We speculate that this atypical thermospheric wind response may be associated with particle heating in the South Atlantic Anomaly.

Intra-seasonal variations of kinetic energy of lower tropospheric zonal waves during northern summer monsoon

Space spectral analysis of zonal (u) and meridional (v) components of wind and time spectral analysis of kinetic energy of zonal waves at 850 hPa during monsoon 1991 (1st June 1991 to 31st August 1991) for the global belt between equator and 40°N are investigated. Space spectral analysis shows that long waves (wavenumbers 1 and 2) dominate the energetics of Region 1 (equator to 20°N) while over Region 2 (20°N to 40°N) the kinetic energy of short waves (wavenumbers 3 to 10) is more than kinetic energy of long waves. It has been found that kinetic energy of long waves is dominated by zonal component while both (zonal and meridional) the components of wind have almost equal contribution in the kinetic energy of short waves. Temporal variations of kinetic energy of wavenumber 2 over Region 1 and Region 2 are almost identical. The correlation matrix of different time series shows that (i) wavenumber 2 over Regions 1 and 2 might have the same energy source and (ii) there is a possibility of an exchange of kinetic energy between wavenumber 1 over Region 1 and short waves over Region 2. Wave to wave interactions indicate that short waves over Region 2 are the common source of kinetic energy to wavenumber 2 over Regions 1 and 2 and wavenumber 1 over Region 1. Time spectral analysis of kinetic energy of zonal waves indicates that wavenumber 1 is dominated by 30-45 day and bi-weekly oscillations while short waves are dominated by weekly and bi-weekly oscillations. The correlation matrix, wave to wave interaction and time spectral analysis together suggest that short period oscillations of kinetic energy of wavenumber 1 might be one of the factors causing dominant weekly (5-9 day) and bi-weekly (10-18 day) oscillations in the kinetic energy of short waves.

Forecasting the onset date of southwest monsoon over Delhi

Using latest 32 years (1964-95) data, upper air temperatures and zonal and meridional components of winds of several selected stations for various standard isobaric levels (850 to 10 hPa) are screened for the pre-monsoon months of April & May in order to study their association with onset date of southwest monsoon at Delhi. Data for temperature and wind components for May for several stations exhibit significant correlations with onset-date. Some well known parameters presently in use in long range forecast models of monsoon seasonal rainfall have also been screened similarly. With a multiple regression technique, equations have been developed using suitable parameters from those which showed significant linear correlations.

Variability and anisotropy of mesospheric wind spectra

The winter mesosphere over Japan was observed with the new MF radars at Wakkanai (45.4°N, 141.7°E) and Yamagawa (31.2°N, 130.6°E). The wind spectra show dominance of different tidal waves at the two sites at the same time. The temporal evolution of the wind spectra is suggestive of resonant nonlinear interactions between planetary waves, tides, and gravity waves. At Wakkanai, the decay of the 24‐h and 12‐h waves was observed in the spectra of the zonal wind component. It is remarkable that the 12‐h wave of the meridional wind component remained strong while the zonal 12‐h wave disappeared. Further anisotropies of the meridional and zonal frequency spectra are described, as well as the slopes of the wind power spectra.

An improved technique to determine neutral winds in the auroral mesosphere using the EISCAT VHF incoherent scatter radar

Abstract The EISCAT VHF radar can be operated to obtain simultaneous measurements of the zonal and meridional components of the mesospheric neutral wind with an altitude resolution of the order of 1 km. We describe the application of this technique and introduce some initial results from an interval near the autumn equinox. These include 21 hours of continuous observations of the zonal wind in the mesosphere, showing predominantly semi-diurnal tidal characteristics in the observed altitude range of 75–95 km. Mean winds, and the amplitude and phase of the 12-h zonal component as a function of altitude, are deduced. Significant differences from published climatological results are found. Evidence of shorter period wave activity is also apparent. The vertical wind component is demonstrated to have a negligible effect on the derived horizontal velocities in this data set, using 5-min averages. Clockwise rotation of the horizontal wind vector with increasing altitude, with a vertical wavelength of 8–12 km, is a fundamental property of the wind field at these heights.

Wavelet transform technique to study the behavior of the 10–20-day and 25–50-day modes during indian summer monsoon onset

Wavelet transform (WT) has been employed to study the behavior of the 10–20-day and 25–50-day modes during Indian monsoon onset. Daily time-series of the zonal and meridional components of surface wind over off the Somali coast and onset dates over south Kerala during 23 years (1954–1976) have been used. In 63% of cases the results of this analysis indicate a link between summer monsoon onset and the 10–20-day and 25–50-day bands expressed by the simultaneously occurrence of fluctuations in these two bands. This suggests the presence of undulatory behavior within the Somali jet whose periods are situated in the 10–20-day and 25–50-day bands during summer monsoon onset.

Diurnal migrating tide as seen by the high‐resolution Doppler imager/UARS: 1. Monthly mean global meridional winds

The high‐resolution Doppler imager (HRDI) instrument on board the upper atmosphere research satellite measures global winds in the mesosphere and lower thermosphere on a day‐to‐day basis. The horizontal coverage of the HRDI data is excellent and provides a unique opportunity to study global‐scale dynamic phenomena; however, the local time resolution and coverage are limited because of the nature of the satellite sampling. The lack of local time coverage makes conventional methods of data analysis (e.g., Fourier analysis) both difficult and erroneous. An original method of analysis, based on a numerical model of atmospheric thermal tides, is proposed and applied to the HRDI data. The tidal model is solved for the tidal oscillations in the meridional wind component. The simulated diurnal meridional tidal winds are used as a first guess in the analysis. The results of the model are adjusted to give meridional migrating tidal winds that have maximum consistency with HRDI measurements. This technique is used to derive monthly mean tidal oscillations of the meridional velocity. The derived tidal amplitudes show profound seasonal changes that seem to be consistent with gravity wave breaking theory. The results are compared with MF radar data. It is found that in the upper mesosphere and lower thermosphere, the tidal amplitudes obtained by HRDI can be bigger than those from MF radars by a factor of 2.

Climate variability modes due to ocean—atmosphere interaction in the central Atlantic

Sea surface temperature (SST) and surface wind patterns from the central Atlantic are studied using empirical orthogonal function and canonical correlation analysis. The data set, which consists of monthly means of SST, zonal and meridional wind components, spans the interval 1965—1987. North Atlantic Oscillation (NAO) effects are revealed by the analysis of the tropical Atlantic variability. The persistence of a NAO phase for several years seems to be related to the persistence of SST and wind anomalies through a feedback mechanism which takes place in the northern tropical Atlantic.

Potential of continental-scale circulation for the determination of local daily surface variables

This study deals with one aspect of statistical downscaling, viz. links between the continental-scale upper-air circulation and surface weather variables on a daily scale. The circulation-to-weather links are expressed in terms of multiple regression between either grid point values or intensities of circulation variability modes and weather elements, including temperature variables, relative humidity, cloudiness, sunshine duration, zonal and meridional wind components, precipitation, and atmospheric pressure. The upper-air circulation influences the surface pressure, temperature and zonal wind most strongly. The relative humidity, cloudiness and sunshine duration appear not to be connected with circulation in winter at all. The low-frequency part of circulation (i.e. processes with periods longer than 10 days) is most efficient in specifying surface weather variables. The circulation-to-weather links manifest considerable intra- and interdecadal variations, posing doubts on the applicability of the ‘downscaling-from-circulation’ method of constructing climate change scenarios.

Tropical atmospheric stability and wind structures as inferred from the Indian MST radar observations

The MST (Mesosphere-Stratosphere-Tropospher) Radar Facility at Gadanki (13.47° N, 79.18° E), near Tirupati, Andhra Pradesh, India has been operated over seven diurnal cycles—three in November 1994, one in September 1995 and three in January–February 1996 with an objective to study the wind and stability characteristics in the troposphere and lower-stratosphere. The radar-measured height profiles of both zonal (EW) and meridional (NS) wind components and near-simultaneous radiosonde measurements from Madras (13.04° N, 80.7° E) and Bangalore (12.85° N, 77.58° E), the two stations close to either side of the radar site, have been compared and they are found to be in gross agreement within the limitations of the sensing techniques. The results of the study also indicated multiple stable and turbulent structures/stratification throughout the height region from about 4 to 30 km. It is noticed that the stable layers are well marked around the altitudes 4 km, 12 km and the tropopause while the turbulent layers exist a few kilometers below the tropopause. These stable and turbulent layer structures showed good correspondence with the radar-measured wind gradients and also with the radiosonde-derived temperature and wind distributions over Madras. The maximum positive gradient in the signal-tonoise ratio (SNR) which corresponds to ‘radar tropopause’ is found to coincide with the greater potential temperature gradient and smaller wind gradient. The time evolution of atmospheric stability structure, derived from the SNR, spectral width and vertical wind revealed a ‘diffused tropopause’ or ‘tropopause weakening’ which is found to be associated with broader spectral width and larger gradients of winds. This feature is considered to be due either to the instability associated with large vertical gradients in horizontal winds (dynamical instability) or to the instability generated by the convection (convective instability).

Observations of the diurnal tide in the mesosphere and lower thermosphere over Christmas Island

Atmospheric tidal variations in the mesosphere wind field have been studied extensively using radars. Most of the climatological studies of the tide have been done at middle and high latitudes. Previous tropical observations of the diurnal oscillation derived from campaign studies have shown the dominance of the migrating diurnal tide in the mesosphere and lower thermosphere. By using wind measurements obtained with a meteor radar system located at Christmas Island, the diurnal tidal behavior in the upper atmosphere of the equatorial Pacific has been characterized over several years. The data collected from August 1988 to December 1993 are presented in this paper and compared to various model results. Our observations show the meridional wind component of the diurnal tide maximizes in April near 94 km. The observed meridional vertical wavelengths are large and indicate upward propagating energy. Even though the vertical wavelengths for the zonal diurnal component are also large, the phase progression is upward corresponding to downward propagating energy. There is considerable interannual and intraseasonal variation in the diurnal amplitudes throughout the year.

Comparison of wind measurements between Yamagawa MF Radar and the MU Radar

A new MF radar was installed at Yamagawa (31.20°N, 130.62°E), Japan in August, 1994 by the Communications Research Laboratory, and the first comparison of wind measurements taken with the Yamagawa MF radar and meteor wind measurements with the MU radar at Shigaraki (34.85°N, 136.10°E) are carried out. In spite of their spatial separation of approximately 650 km, reasonable agreement between wind velocities by the two radars at 80–92 km in altitude is confirmed, except for small scale perturbations that are probably due to gravity waves enhanced for the period from 17 h LT on September 13 to 7 h LT on September 14, 1994. It is found that the winds of the MF radar above 92 km tend to be smaller than the winds obtained with the MU radar. Receiver saturation and interference are possible causes of the reduced wind velocities observed with the MF radar. The height structures of semidiurnal tides are also compared. The amplitude and phase of zonal components agree at most heights between the two radars. For the meridional component the amplitudes fluctuate above 88 km with a significant difference and the phase from the MU radar leads that of the MF radar by less than several hours at 86 km to 92 km. An enhanced gravity wave seems to pass over the MU radar almost from north to south, resulting in a contamination of the meridional wind component of the semidiurnal variation.

Observed Interannual Variability in the Hadley Circulation and Its Connection to ENSO

Abstract Based on a 26-yr set of daily global upper-air wind data for the period January 1964–December 1989, the interannual variability in the strength of the tropical Hadley cells is investigated. Although several measures of the intensity of the zonal-mean cells are discussed, the main focus is on the maximum in the streamfunction in the northern and southern Tropics. The streamfunction was computed from observed monthly mean latitude versus pressure cross sections of the zonal-mean meridional wind component. Significant seasonal variations are found in the strength, latitude, and height of the maximum streamfunction for both Hadley cells. Significant correlations are also observed between the Hadley cells and the El Nino-Southern Oscillation phenomenon. During the extreme seasons, only one “winter” Hadley cell dominates the Tropics, with the rising branch in the summer hemisphere and the sinking branch in the winter hemisphere. Superimposed on this “normal” one-cell winter Hadley circulation in the Tr...

Detection of an Annual Westward Propagating Signal in the Meridional Wind Component along 8°N in the Pacific

A westward propagating signal with the annual period is detected in anomalies of the zonally averaged meridional wind component along 8°N across the Pacific Ocean. The propagating signal in the “eddy” (defined as the departure from the zonally averaged) meridional wind has approximately the same propagation speed as the well-known propagating signal in the zonal wind component along the equator, and the former has larger amplitude than the latter. The eddy SST gradient between the equator and 10°N exhibits a similar westward propagation and is in phase with the eddy meridional wind; the northward eddy wind is accompanied by the warmer eddy SST to the north. The propagating features in both the eddy meridional wind and the SST gradient are quite regular from year to year, except for El Niño years. In the El Niño periods, the regular features are disturbed in the western Pacific, but the in-phase relationship between these two parameters still holds. These relationships indicate that the boundary-layer mechanism is most likely to be of primary importance in the response of the eddy meridional wind to the SST variations.

Importance of meridional winds in. forecasting sub-regional summer monsoon rainfall

ABSTRACT. Based on 35 years' (1959-1993) data, the zonal and meridional wind components of selected Indian RS/RW statiomupto 100 hPa level were analysed for the pre-monsoon months of April and May in order to associate them with sub-regional monsoon rainfall of northeast India. Composite values of monsoon rainfall and meridional components for May for excess and deficient yean have revealed that anomaly of meridional components for the middle and upper troposphere is northerly/southerly preceding excees/deficient monsoon year. The meridional winds at most of the levels of Delhi and some of the levels of Jodhpur Nagpur, Bombay and Madras for the month of May showed significant correlations (significant at 0.1% to 5% level of significance) with sub-divisional monsoon rainfall in northeast India. The temporal behaviour of correlation coefficients for Punjab and Haryana for 16 and 20-year sliding windows has exhibited rasonable temporal stability except for first few years. Multiple rearession equations for 30 and 35 year period for Haryana, Punjab and contiguous northweat India were also developed. The regression model for Punjab sub-division has shown quite good results for the independent period.

Observed and Simulated Northern Hemisphere Intraseasonal Circulation Anomalies and the Effluence of Model Bias

Abstract The ability of an atmospheric general circulation model to simulate the observed primary modes of intraseasonal variability in the Northern Hemisphere upper-tropospheric winds during boreal winter is examined. The model used is the Navy Operational Global Atmospheric Prediction System. The authors examine differences between the observed and modeled modes of variability in the context of various model deficiencies, where the observed modes are derived from the European Centre for Medium-Range Weather Forecasts analyses. Rotated empirical orthogonal function analysis is used to determine the primary modes of variability in the Pacific and Atlantic regions. EOFs are computed for both the zonal and meridional wind components. Time-lagged composite analysis is used to examine the temporal evolution of these modes, as well as their relationship to tropical convection. Wave activity flux vectors are used to examine further the characteristics of these intraseasonal modes and their relationship to tropi...

Structure and properties of summer monsoon stationary B,"Operational use of Ozone over .southern Asia: an observational study

A study of ten-year (1976-1985) mean July climatology of southern Asia and adjoining ocean areas confirms the presence of a well-defined stationary wave, believed to be due mainly to land-sea thermal contrast over the region, in the fields of several meteorological variables. The wave extends laterally over about 10 degrees of latitude with maximum intensity along about 20° N and vertically from surface to about 300 hPa. Its zonal wavelength is about 2000-2500 km and its amplitude in the field of zonal anomaly of temperature and meridional component of wind is 1 oC and 4ms-l respectively. The trough-ridge system of the wave appears to tilt eastward with height from surface to about 700 hPa and westward aloft up to about 300 hPa, while the warmest-coldest anomaly system appears to tilt eastward all the way from surface to about 300 hPa. A phase difference appears to exist between the geopotential and the temperature fields in both the lower and the upper tropospheres. The aforesaid zonal-vertical tilt of the monsoon trough and phase difference between the geopotential and the temperature fields appears to be compatible, through thermal advection, With a direct conversion of eddy available potential energy into eddy kinetic energy via a west-east (clockwise) overturning with warm air rising in the west and cold air sinking in the east in the case of the eastward-tilting lower-tropospheric trough and an east-west (anti-clockwise) overturning with warm air rising in the east and cold air sinking in the west in the case of the westward-tilting middle and upper-tropospheric trough, An enhancement of the thermal advection and hence the vertical circulation may occasionally lead to development of the trough into a I(JW or depression. However, the question of development of the trough and physical factors, which may contribute to such development, needs to be examined by further study.