The Quasi-Biennial Oscillation

The quasi-biennial oscillation (QUASI BIENNIAL OSCILLAION), is a quasiperiodic oscillation of the equatorial zonal wind between easterlies and westerlies in the tropical stratosphere with a mean period of 28 to 29 months. The alternating wind regimes develop at the top of the lower stratosphere and propagate downwards at about 1 km (0.6 mi) per month until they are dissipated at the tropical tropopause.QUASI-BIENNIAL OSCILLATION

The quasi-biennial oscillation (QUASI BIENNIAL OSCILLAION), is a quasiperiodic oscillation of the equatorial zonal wind between easterlies and westerlies in the tropical stratosphere with a mean period of 28 to 29 months. The alternating wind regimes develop at the top of the lower stratosphere and propagate downwards at about 1 km (0.6 mi) per month until they are dissipated at the tropical tropopause.
Downward motion of the easterlies is usually more irregular than that of the westerlies. The amplitude of the easterly phase is about twice as strong as that of the westerly phase. At the top of the vertical QUASI BIENNIAL OSCILLAION domain, easterlies dominate, while at the bottom, westerlies are more likely to be found. At the 30mb level, with regards to monthly mean zonal winds, the strongest recorded easterly was 29.55 m/s in November 2005, while the strongest recorded westerly was only 15.62 m/s in June 1995.
Quasi-periodic oscillation of equatorial zonal wind between easterlies and  westerlies in the stratosphere.

QUASI-BIENNIAL OSCILLATION

Mean period 28-29 months.
Develop at top of lower stratosphere and propagate downwards at about 1km per month.
Dissipated at tropical tropopause.
Downward motion of easterlies more irregular.
Amplitude of easterly phase is twice as strong as that of westerly phase.
Mean zonal winds in the stratosphere near 30 hectopascal(hectopascal) level between 30°n and 30°s show alternating easterly and westerly regimes.
Easterly phase is prevalent during odd years and westerly during even years.
Reed (1964) studied the winds and temperatures over tropics and separated the annual and 26 months component oscillations.
Probert & jones (1964) found the periods varying from 20 to 29 months. It is generally agreed that the oscillation is not truly biennial and the term Quasi Biennial Oscillation or 26 month oscillation is generally accepted.
Similar oscillations have been noticed in the other parameters also, for example, temperature field, ozone concentration and 500 hectopascal contour anomalies etc.
In southern hemisphere (Australia), kovin (1975) found that the sea level pressure anomalies reversed their signs in succeeding years.
The attention in this essay is mainly on the stratospheric circulations.
At equatorial latitudes (7° lat on either side of equator), Quasi Biennial Oscillation in zonal wind is the dominant dynamical feature of the lower stratosphere.
Quasi Biennial Oscillation is characterized by alternating easterly and westerly wind regimes occurring in the lower stratosphere with an average period of 28 months and average maximum amplitude of 20 m per sec at about 20 km. It occurs in the total column of ozone also near the equator.
Many recent studies indicated that the Quasi Biennial Oscillation has a significant influence on middle atmospheric dynamics on a global scale
For example, recent studies show that no major mid-winter warming’s in the high latitude stratosphere occurred when the sunspot number was low and Quasi Biennial Oscillation was in westerly phase whereas, major mid-winter warming’s occur in the easterly phase of the Quasi Biennial Oscillation irrespective of solar activity.
Further the presence of strong easterly or westerly winds associated with the Quasi Biennial Oscillation modulate the upward propagation of gravity waves in the stratosphere and the wave flux reaching the mesospheric heights

GENERAL CHARACTERSTICS OF QUASI BIENNIAL OSCILLAION

The periodicity of the oscillations is 26 months (varies between 20 & 32 months).
Easterlies during the odd years and are stronger.
Westerlies during even years and are weaker.
Its max amplitude is at  30 hectopascal and over the equator.
It decreases downward and pole ward.
It is hardly noticeable below 100 hectopascal.
Average amplitude of wind ocillation near the equator is 15 m per sec at 60 hectopascal and 25 m per sec at 30 hectopascal.
Its amplitude decreases rapidly with latitude at the rate of 1 m per sec for every 2° latitude at 60 hectopascal.
Max amplitude is 20 m per sec around 24 km. Decreases rapidly bet 22km and 17 km.
Its variation about the equator are symmetrical.So it is a phenomenon of either of the hemisphere.

GLOBAL ASPECTS OF QUASI BIENNIAL OSCILLAION

The variability is confined to periods from one year to four years.
Most of them over arctic and mid latitudes of southern hemisphere can be traced from 700 hectopascal to 7 hectopascal level.
Vertical progression of Quasi Biennial Oscillation at higher latitudes is greater than that over tropics ( 1 km per month).
Simultaneous occurrence of maxima or minima is found at neighbouring levels.
The magnitude of Quasi Biennial Oscillation varies irregularly from one cycle to the other outside tropics.
In arctic stratosphere it may range from 1 m per sec to 8 m per sec.
In arctic region, there are hardly any longitudinal variations, whereas, in mid latitudes a general eastward progression is noticed.

EFFECTS OF QUASI BIENNIAL OSCILLAION ON TROPOSPHERIC CIRCULATIONS

Ebden (1975) examined the mean tropospheric circulation patterns of January, April, July & October together with Quasi Biennial Oscillaion at 30 hectopascal over Kanton(located at Pacific ocean) and Gan(the southernmost island of the Maldives) islands. He concluded:-
During easterly phase, the positive pressure anomalies are noticed in high latitudes.
The islandic low is displaced towards south by 5° to 10° with below normal pressure gradients across north Atlantic and NW Europe.
When Quasi Biennial Oscillation is in westerly phase, tropospheric westerlies are stronger with pronounced westerly maxima over 45°-55°n between 200 and 300 hectopascal.
At 500 hectopascal, when Quasi Biennial Oscillation is of westerly phase, the average lat of the strongest flow is 35°n in January. Individual maxima lie between 35°n & 45°n.
Whereas, when Quasi Biennial Oscillation is in easterly phase, the maximum lies at 30°N (5 years mean) and individual maxima between 30°N & 40°N.
In July, the mean surface pressures in high latitudes are above normal when Quasi Biennial Oscillation is in easterly phase and reverse is the case when it is in westerly phase.

THEORY OF QUASI BIENNIAL OSCILLAION

Quasi Biennial Oscillaion is an internal oscillation of the mean flow resulting from wave means flow interaction with the vertically propagating kelvin & Rossby-Gravity (Rossby-Gravity) waves.
There is continuous field of gravitational wave eneRossby-Gravity y from the troposphere into the stratosphere in the near equatorial region.
Feed by Kelvin waves and mixed Rossby-Gravity  waves.
These two classes of waves carry momentum upwards. Kelvin waves carry westerly momentum and mixed Rossby-Gravity waves carry easterly momentum.
During their upward journey, the Kelvin waves can penetrate easterly flow, but get absorbed by westerlies near the transition between lower easterlies and upper strong westerlies. The reverse is true for mixed Rossby-Gravity waves.As a result, the absorption of kelvin waves leads to a build-up of strong westerlies and their gradual descent towards the tropopause and vice-versa

LIMITATIONS OF THE THEORY THE MOMENTUM TRANSFERRED BY THE WAVES IS NOT CONFIRMED BY OBSERVATIONS.

   The above theory has considered the stratosphere at rest and hence continuous injunction of wave energy through this. But stratosphere has its own wave motion.
In the southwest monsoon easterlies dominates over westerlies and hence it is not understood how rossby-gravity waves transfer momentum to the lower troposphere
    The theory do not explain quasi biennial oscillation in other meteorological elements occur over practically the whole globe (for example, temperature, ozone, tropopause height, spring stratospheric warming, surface pressures, Indian monsoon, depth of equatorial winds, index of surface, weather of England and EL NINO ETC.



QUASI BIENNIAL OSCILLAION AND SW MONSOON
RANJIT SINGH (1986)

    Berson westerlies (bw) along with the easterlies execute Quasi Biennial Oscillation in the lower equatorial duration of westerlies decreasing northward.
    This biennial characteristic forms a significant predictive parameter for the performance of SW monsoon over India in terms of rainfall.
    Taking Quasi Biennial Oscillation over Gan island (00°41’s, 73°09’e) and Trivandrum together, the withdrawing phase of Berson westerlies from Trivandrum and Gan island may be followed by draught or below normal rainfall over India.
The setting phase of  over Gan island and/or their strengthening with time mostly coincides with a good monsoon.
    Taking Trivandrum alone, the presence of Berson westerlies over Trivandrum between September and May of the next year and their withdrawal with the onset of monsoon may coincide with below normal rainfall over India (success is just about 60%).
    Easterly phase during this period over Trivandrum has normally coincided with a good rainfall over India. But drought has occurred during easterly phase also.
The quasi-biennial oscillation (QBO) was observed during the 1883 eruption of Krakatoa, when the ash from the volcano was transported around the globe from east to west by stratospheric winds in about two weeks. These winds became known as the "Krakatoa easterlies". It was observed again in 1908, by the German meteorologist Arthur Berson, who saw that winds blow from the west at 15 km (9.32 mi) altitude in tropical Africa from his balloon experiments. These became known as the "Berson westerlies"