Zonal Wind Oscillation Research Articles

The role of ozone‐induced diabatic heating anomalies in the quasi‐biennial oscillation

AbstractThe diabatic effects of the quasi‐biennial oscillation (QBO) in ozone on the oscillation of temperature and zonal mean winds are investigated using a 2‐dimensional model with full interaction between dynamics, radiation and photochemistry. It is found that the temperature anomaly caused by the ozone QBO makes a significant (25%) contribution to the total temperature anomaly of the QBO although the corresponding change in wind is smaller (about 10%). It is shown that the diabatic effects of the ozone QBO should not be neglected in attempts to simulate the QBO in other parameters.

Seasonal and interannual temperature variations in the tropical stratosphere

Temperature variations in the tropical lower and middle stratosphere are influenced by at least five distinct driving forces. These are (1) the mechanism of the regular seasonal cycle, (2) the quasi‐biennial oscillation (QBO) in zonal winds, (3) the semiannual zonal wind oscillation (SAO) at higher levels, (4) El Niño‐Southern Oscillation (ENSO) effects driven by the underlying troposphere, and (5) radiative effects, including volcanic aerosol heating. Radiosonde measurements of temperatures from a number of tropical stations, mostly in the western Pacific region, are used in this paper to examine the characteristic annual and interannual temperature variability in the stratosphere below the 10‐hPa pressure level (∼31 km) over a time period of 17 years, chosen to eliminate or at least minimize the effect of volcanic eruptions. Both annual and interannual variations are found to show a fairly distinct transition between the lower and the middle stratosphere at about the 35‐hPa level (∼23 km). The lower stratosphere, below this transition level, is strongly influenced by the ENSO cycle as well as by the QBO. The overall result of the interaction is to modulate the amplitude of the normal stratospheric seasonal cycle and to impose a biennial component on it, so that alternate seasonal cycles are stronger or weaker than normal. Additional modulation by the ENSO cycle occurs at its quasi‐period of 3–5 years, giving rise to a complex net behavior. In the middle stratosphere above the transition level, there is no discernible ENSO influence, and departures from the regular semiannual seasonal cycle are dominated by the QBO. Recent ideas on the underlying physical mechanisms governing these variations are discussed, as is the relationship of the radiosonde measurements to recent satellite remote‐sensing observations.

Fluctuations in the Earth's rotation since 1830 from high-resolution astronomical data

SUMMARY Fluctuations in the Earth's rotation since 1830, as evidenced by changes in the length of the day, are derived from astronomical data having subannual resolution. Before 1955.5, timings of linar occultations are used; after 1955.5, the data are taken from the time series TAI-UT1. Although the data in the earliest period, 1830–90, display decade fluctuations in the length of the day, they are not accurate enough to reveal interannual variations. In this regard, also, the results from 1890–1925 are somewhat dubious. The quality of the data after 1925, though, is such that the temporal behaviour of the interannual fluctuations in the length of the day can be traced with confidence. We present plots of the interannual fluctuations in the period 1890–1987 and the longer-term decade fluctuations in the period 1830–1983. The interannual fluctuations in the length of the day since 1925 are compared with an index of the El Nino/Southern Oscillation (ENSO) phenomenon in the ocean-atmosphere system and are subjected to spectral analysis. The results support the conclusions reached by other authors that these fluctuations are linked to circulation changes in the atmosphere associated with ENSO, and in part to the quasi-biennial oscillation in the equatorial stratosphere's zonal winds. A spectral analysis of our 62 yr series of length of day values since 1925 reveals two significant peaks in the interannual range 2–4 yr. One is roughly biennial and the other is about twice this period, broadly supporting results obtained previously from shorter records. Our analysis of high-resolution data, therefore, contributes to ongoing efforts to establish a close relationship between the length of the day and aspects of the global climate system in the period before modern data became available in 1955.5.

Variations of mean winds and tides in the upper middle atmosphere over a solar cycle, Saskatoon, Canada, 52°N, 107°W

Saskatoon (52 N, 107 W) medium frequency (MF) radar data from 1979 to 1990 have been analyzed to investigate the solar activity effects on upper middle atmospheric winds and tidal amplitudes. The period of study covers two solar maxima and a solar minimum; the continuous data allow a systematic analysis of solar cycle dependence on mean winds and tides. The height region of 79–97 km sampled in the study shows an apparent but very weak dependence of mean winds and tidal amplitudes on solar activity variation. The observed features are fairly consistent with the early results reported by Sprenger and Schmindkr [(1969) J. atmos. terr. Phy. 31, 217). The mean zonal wind and the semidiurnal tidal amplitudes appear to exhibit positive and negative correlations with the solar activity, respectively; the statistical significances of these correlations are generally low. There is a biennial periodicity evident in the zonal wind oscillations but this docs not have a consistent phase relationship with the equatorial stratospheric wind oscillations (QBO). The meridional winds and the tidal amplitudes are characterized with different and quite irregular periods of oscillations (2–5 yr). The diurnal tidal variations over the solar cycle are small and irregular, although amplitudes are slightly larger during the solar minimum years.

Long-period motions in the equatorial mesosphere

The seasonal variations in winds measured in the equatorial mesosphere and lower thermosphere are discussed, and oscillations in zonal winds in the 3–10 day period range are examined. The observations were made between January 1990 and June 1991 with a spaced-antenna MF radar located on Christmas Island (2°N, 157°W). The seasonal variations are analyzed in terms of the mean, annual, and semiannual (SAO) harmonic components. The SAO is the dominant component in the zonal winds, with the amplitude and phase characteristics being in good agreement with earlier rocketsonde measurements at Kwajalien (9°N) and Ascension Island (8°S). The annual and semiannual oscillations combine to produce a stronger change in zonal wind strength in the first half-year (January–June) than in the second half-year (July–December). An annual cycle dominates the meridional winds with maximum velocities (5–10m s −1) attained at about 90km. The meridional circulation at the solstices is consistent with a flow from the summer to the winter pole. Power spectral analyses indicate that motions in the 3–10 day period range occur mainly in the zonal winds, behavior which is interpreted as being due to eastward propagating Kelvin waves. Despite the intermittent nature there is an overall semiannual variation in Kelvin-wave activity. Maximum amplitudes are achieved at the mesopause in January/February and August/September which are times when the zonal winds are westward.

Zonal-Wind Oscillations over the Western Hemisphere during Winter: Further Evidence of a Zonal-Eddy Relationship

Abstract The European Centre for Medium-Range Weather Forecasts (ECMWF) 500-mb height analyses and 7-day forecasts are examined during ten winters (1981–90) for oscillations of geostrophic zonal wind over the western hemisphere. A well-defined zonal-wind oscillation is isolated in the first two eigenvectors. This zonal-wind oscillation accounts for about 55% of the total zonal-wind variance over the western hemisphere during this ten-winter period. The oscillation is characterized by zonal-wind anomalies that are in phase between 30° and 70°N and out of phase with zonal-wind anomalies along 50°N. The oscillation clearly displays southward propagation from 85° through 30°N, with standing components along 30°, 50°, and 70°N. The dominant temporal period associated with the oscillation is found to be in the range of 15–35 days with large interannual variability. Composites of 500-mb heights through 25 cycles of zonal-wind oscillations over ten winters were performed for unfiltered and 15–39-day filtered data...

Predicting Atlantic Seasonal Hurricane Activity 6–11 Months in Advance

Abstract A surprisingly strong long-range predictive signal exists for Atlantic-basin seasonal tropical cyclone activity. This predictive skill is related to two measures of West African rainfall in the prior year and to the phase of the stratospheric quasi-biennial oscillation of zonal winds at 30 mb and 50 mb, extrapolated ten months into the future. These predictors, both of which are available by 1 December, can be utilized to make skillful forecasts of Atlantic tropical cyclone activity in the following June-November season. Using jackknife methods to provide independent testing of datasets, it is found that these parameters can be used to forecast nearly half of the season-to-season variability for seven indices of Atlantic seasonal tropical cyclone activity as early as late November of the previous year.

Annual, quasi‐biennial, and El Niño‐Southern Oscillation (ENSO)time‐scale variations in equatorial total ozone

Equatorial total ozone variations with time scales of annual, quasi‐biennial, and about 4‐year periodicities are described by paying attention to their longitudinal structure. Analyses are made for 11 years from 1979 to 1989, using the global total ozone data derived from the total ozone mapping spectrometer on board the Nimbus 7 satellite. Over the equator an annual cycle in total ozone is conspicuous. Zonal mean values are maximum around September and minimum around January. The longitudinal structure shows a zonal wavenumber 1 pattern with minimum values around 140°E to the date line all year‐round, indicating a close relationship to a region where the convective cloud activity is vigorous. By removing the climatological annual cycle from the original data, there appears the quasi‐biennial oscillation in total ozone. This variation is characterized by zonally uniform phase changes and is strongly coupled with the quasi‐biennial oscillation of the equatorial zonal wind in the lower stratosphere. Moreover, subtracting zonal mean values from the anomaly data mentioned above, we see an east‐west seesaw variation with a nodal longitude around the date line. This east‐west variation, having a characteristic time scale of about 4 years, is clearly related to the El Niño and the Southern Oscillation cycle. During El Niño events the longitudinal anomaly field in total ozone is positive in the western Pacific and negative in the eastern Pacific; the anomaly pattern is reversed during anti‐El Niño events. Because the active region of convective clouds is located relatively in the eastern Pacific sector during El Niño events, it is suggested that the stronger upwelling and the higher tropopause associated with the convective cloud activity bring about less total ozone.

A study of quasi-biennial oscillation in the tropical stratosphere

The characteristics of the quasi-biennial oscillation in zonal wind and temperature at Trivandrum (8.5°N, 77°E) have been studied using data covering 16 years. Similar study has been carried out for zonal wind at Balasore (21.5°N, 87°E) using data covering 9 years. The cycle to cycle variation of amplitudes, their altitude variation, periods and descent rates of the westerly and easterly regimes have been studied.

Kelvin Wave Variability in the Upper Stratosphere Observed in SBUV Ozone Data

The Solar Backscatter UV ozone data collected for the eight years between 1979 and 1986 are used to analyze the signatures of equatorially trapped Kelvin waves in the upper stratosphere. The data reveal a strong semiannual modulation of Kelvin wave activity, confirming the results of previous rocketsonde observations. A comparison of the eight-year-average ensemble spectra to the semiannual oscillation in the stratospheric zonal winds revealed a seasonal asymmetry in the strength of Kelvin waves, which mimics the asymmetry observed in the zonal winds. No consistent relationship was observed with the quasi-biennial oscillation in the lower stratosphere, while correlations with the upper stratospheric winds are weak or nonexistent.

On the wave forcing of the semi-annual zonal wind oscillation

The mean zonal wind in the tropical atmosphere is characterised by the presence of two dominant long-period oscillations namely the quasi-biennial oscillation (QBO) between 18–35 km altitude and the semi-annual oscillation (SAO) at altitudes between 35 and 60 km. These oscillations cannot be accounted for on the basis of thermal forcing, as is the case for the annual oscillation (AO) in extratropical regions. It is now established that wave forcing in the troposphere is responsible for the generation of both the QBO and SAO through wave-mean flow interactions. Though the nature of the wave modes required to explain the QBO are known with some certainty, the same cannot be said for the wave modes in the upper stratosphere which are responsible for producing the SAO. This paper gives observational evidence for the presence of rather long period waves (22–60 days) at upper stratospheric heights (30–60 km), particularly during the winter months over Indian tropical stations. The implications of these wave modes in producing the easterly phase of the SAO are discussed. A 2-D dynamic wind model has been used to simulate both the westerly and easterly phases of the SAO.

Low frequency oscillations in wind and circulation fields over India during northern summer monsoon

The study presents result of low frequency oscillations in zonal and meridional wind and divergence and vorticity fields during summer monsoon over India. Harmonic analysis has been adopted for this purpose to detect prominent periodicities; Strength of Hadley circulation has been related to the monsoon rainfall. Inter-annual fluctuations in the oscillations have been examined with the help of Homelier diagrams. The contribution of Kelvin and Rossby waves in the excitation of the oscillations has been discussed. Role of easterly and westerly anomalies in the establishment of the monsoon and the active-break cycles has been projected. The vertical structure of the cycles has been investigated and plausible explanation offered, particularly on its tilt and its relation to monsoon activity. Possible mechanism on generation. Propagation, Intensification and weakening of the oscillations over Indian latitudes has been proposed.

Middle atmosphere dynamics and monsoon variability

Some dynamical parameters of the middle atmosphere such as quasi-biennial oscillation in the low-latitude stratospheric zonal winds, interannual variability of the high-latitude middle atmosphere and vertical motions in the tropical middle atmosphere are studied to examine their roles in determining the variability of rainfall over India during the summer monsoon season. It is believed that the amount of sulphurous gases released in the stratosphere from the great volcanic eruptions modifies in the climate (rainfall). An attempt has also been made in this paper to present the work done so far in this direction.

Intraseasonal Periodicities in Indian Rainfall

Abstract Spectral analysis of a 70-year (1901–70) record of daily precipitation from 3700 stations in the country of India is carried out to search for periodicities on subseasonal time scales during the summer monsoon. Two statistically significant spectral peaks are found. A 40–50 day spectral peak corresponding to the Madden-Julian Oscillation is found over most of the portion of India south of 23°N. The phase of the oscillation is such that the precipitation maximum appears first over the relatively dry southeastern portion of the peninsula. Ten to 12 days later the precipitation peaks simultaneously all along the coast west of the Western Ghats and along a line running across India between 20° and 25°N. The precipitation maximum then spreads slowly northward and loses significance. Cross-spectral analysis shows strong coherence between the precipitation patterns and wind oscillations. The zonal wind oscillations at 850 and 200 mb am about 180 degrees out-of-phase equatorward of about 20°N, but in-pha...

Influence of the Annual Cycle in Meridional Transport on the Quasi-biennial Oscillation in Total Ozone

The equatorial stratospheric quasi-biennial oscillation (QBO) in zonal wind and temperature is observed to be symmetric about the equator. The QBO in column ozone, although it is believed to be caused primarily by vertical displacements due to the meridional circulation associated with the equatorial temperature QBO, is asymmetric with respect to the equator, and is strongly linked to the phase of the annual cycle. In this note a simple one-layer model is used to demonstrate that the gross features of the observed QBO in total ozone can be attributed to meridional advection of the ozone perturbation by the annually reversing mean meridional Hadley circulation. This advection causes a displacement of the equatorial ozone anomaly towards the winter hemisphere, and thus products an asymmetry with respect to the equator. It also modulates the amplitude of the ozone QBO, since the phase of the equatorial wind QBO with respect to the annual cycle may produce either constructive or destructive interference between the effects of the annually reversing meridional transport and the vertical advection by the equatorial wind QBO.

Additional Evidence of an Upper Tropospheric Zonal/Eddy Relationship during Winter

Abstract A zonal wind oscillation with a period of about 15–35 days is isolated over the middle latitudes of the Northern Hemisphere during winter. A systematic relationship between the zonal winds and wind departures from zonal symmetry (eddies) is found for nine prominent zonal wind oscillation cycles taken from six 120-day winters. This zonal/eddy relationship consists of an oscillation between anomalous zonal flow and anomalous meridional flow, which is strongest across North America from 150°W to 0°.

A Two-Dimensional Model of the Quasi-biennial Oscillation of Ozone

Abstract The stratospheric quasi-biennial oscillation (QBO) in zonal wind, temperature and column ozone has been successfully modeled in a two-dimensional dynamical/chemical model by the introduction of a parameterization scheme to model the transfer of momentum to the zonal flow associated with the damping of vertically propagating Kelvin and Rossby-gravity waves. The largest amplitudes of the observed QBO in column ozone are found in high latitudes and this must be taken into account in any explanation of the increased depletion of ozone in the southern polar spring during the 1980s. A strong QBO signal in column ozone is evident in the model at all latitudes. The largest anomalies of approximately 20 DU are present at high latitudes. The equatorial ozone QBO is out of phase with the mid- and high-latitude ozone QBO. A positive (negative) ozone anomaly at the equator coincides with the presence of equatorial westerlies (easterlies) at 50 mb, in good agreement with observations. The modeled zonal wind at...

Seasonal Variability of the 40–50 Day Oscillation in Wind and Rainfall in the Tropics

Abstract Time spectral analysis is performed on long records of wind and precipitation from stations in the tropical Indian Ocean-Pacific Ocean are. The spectra are done separately for winter and summer half-years. Statistically significant spectral peaks in the 40–50 day period range show strong seasonal variability. The 40–50 day peaks in the 200 mb zonal wind spectra are stronger and more prevalent during the Northern Hemisphere winter half-year. Spectral peaks in the 850 mb wind show a preference for summer in the Northern Hemisphere. Precipitation spectra show significant 40–50 day peaks at selected locations in the Indonesian region and along the South Pacific convergence zone in the central Pacific during Southern Hemisphere summer. These oscillations in precipitation are coherent with nearby zonal wind oscillations. No significant oscillations in precipitation were found for stations significantly north of the equator during either half-year. In particular, no significant peaks in precipitation sp...

A 40–50 day oscillation in sea-surface temperature along the central California coast

Previous studies of the tropical troposphere indicate the presence of a 40–50 day quasi-periodic oscillation in zonal winds. Spectral analysis of extensive daily sea-surface temperatures (SSTs) reveals a significant increase in variance (i.e. a quasi-periodic oscillation) between 40 and 50 days along the California coast as far south as Santa Barbara (34·3°N) and at least as far north as Port Arena (39°N). Increases in the amplitude of this oscillation at Granite Canyon (36°N) in some cases coincide with local El Nino warming episodes. The local surface winds, which also contain a quasi-periodic oscillation between 40 and 50 days, are coherent with, and lead, sea-surface temperature by approximately 10 days, consistent with local wind forcing. It is suggested that the 40–50 day oscillation in sea-surface temperature off central California may be related to the 40–50 day oscillation in the tropical troposphere through atmospheric teleconnections between the tropics and mid-latitudes.

熱帯季節内振動の三次元線形レスポンスモデル

The propagation and structure of the tropical intraseasonal oscillation are studied by the use of a three-dimensional linear response model and compared with those obtained from the FGGE data.It is assumed that the imposed thermal forcing oscillates with a 40 day period and propagates eastward. Although the amplitude of the forcing is assumed to be large over a certain longitude band and small elsewhere, the responding zonal wind oscillation has significant components that propagate eastward around the earth as observed. This oscillation is also associated with an observed longitudinal node in the region of the maximum forcing. When the imposed forcing is strictly confined over some longitudes, the zonal wind oscillation propagates eastward and westward away from the forced region as in the case of a two-dimensional model. The eastward moving wavenumber 1 component is associated with the observed wave pattern of the combined Kelvin-Rossby mode in the upper troposphere, while it is dominated by that of the Rossby mode in the lower troposphere. This component also takes the observed structure of the Walker cell modified by a frictional meridional convergence in the boundary layer. The dominance of the Rossby mode in the lower troposphere is due to the effect of surface friction.