QUASI-BIENNIAL OSCILLATION

QUASI-BIENNIAL OSCILLATION Quasi-periodic oscillation of equatorial zonal wind between e’lies and w’lis in the stratosphere. 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 e’lies more irregular. Amplitude of easterly phase is twice as strong as that of westery phase.

QBO Mean zonal winds in the stratosphere near 30 hpa 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 qbo or 26 month oscillation is generally accepted.

QBO Similar oscillations have been noticed in the other parameters also, for example, temperature field, ozone concentration and 500 hpa 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.

QBO At equatorial latitudes (7° lat on either side of equator), qbo in zonal wind is the dominant dynamical feature of the lower stratosphere. Qbo is characterised 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 qbo has a significant influence on middle atmospheric dynamics on a global scale.

QBO For example, recent studies show that no major mid-winter warmings in the high latitude stratosphere occurred when the sunspot number was low and qbo was in westerly phase whereas, major mid-winter warmings occur in the easterly phase of the qbo irrespective of solar activity. Further the presence of strong easterly or westerly winds associated with the qbo modulate the upward propagation of gravity waves in the stratosphere and the wave flux reaching the mesospheric heights.

MEAN MONTHLY ZONAL WIND COMPONENT AT 50 hPa

GENERAL CHARACTERSTICS OF QBO The periodicity of the oscillations is 26 months (varies between 20 & 32 months). E’lies during the odd years and are stronger. W’lies during even years and are weaker. Its max amplitude is at 30 hpa and over the equator. It decreases downward and poleward. It is hardly noticeable below 100 hpa.

GENERAL CHARACTERSTICS OF QBO Average amplitude of wind ocillation near the equator is 15 m per sec at 60 hpa and 25 m per sec at 30 hpa. Its amplitude decreases rapidly with latitude at the rate of 1 m per sec for every 2° latitude at 60 hpa. 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 QBO 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 hpa to 7 hpa level. Vertical progression of qbo at higher latitudes is greater than that over tropics ( 1 km per month). Simultaneous occurrence of maxima or minima are found at neighbouring levels.

GLOBAL ASPECTS OF QBO The magnitude of qbo 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 midlatitudes a general eastward progression is noticed.

EFFECTS OF QBO ON TROPOSPHERIC CIRCULATIONS Ebden (1975) examined the mean tropospheric circulation patterns of jan, apr, jul & oct together with qbo at 30 hpa over canton and gan 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.

EFFECTS OF QBO ON TROPOSPHERIC CIRCULATIONS When qbo is in w’ly phase, tropospheric w’lies are stronger with pronounced w’ly maxima over 45°-55°n between 200 and 300 hpa. At 500 hpa, when qbo is of w’ly phase, the average lat of the strongest flow is 35°n in jan. Individual maxima lies between 35°n & 45°n. Whereas, when qbo is in e’ly 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 qbo is in e’ly phase and reverse is the case when it is in w’ly phase.

THEORY OF QBO Qbo is an internal oscillation of the mean flow resulting from wave mean flow interaction with the vertically propagating kelvin & rossby-gravity (rg) waves. There is continuous field of gravitational wave energy from the troposphere into the stratosphere in the near equatorial region. Feed by kelvin waves and mixed rg waves. These two classes of waves carry momentum upwards. Kelvin waves carry westerly momentum and mixed rg waves carry easterly momentum.

THEORY OF QBO 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 rg 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.

Dynamical overview of the QBO during northern winter Dynamical overview of the QBO during northern winter. The propagation of various tropical waves is depicted by orange arrows, with the QBO driven by upward propagating gravity, inertia-gravity, Kelvin, and Rossby-gravity waves. The propagation of planetary-scale waves (purple arrows) is shown at middle to high latitudes. Black contours indicate the difference in zonal-mean zonal winds between easterly and westerly phases of the QBO, where the QBO phase is defined by the 40-hPa equatorial wind. Easterly anomalies are light blue, and westerly anomalies are pink. In the tropics the contours are similar to the observed wind values when the QBO is easterly. The mesospheric QBO (MQBO) is shown above ;80 km, while wind contours between ;50 and 80 km are dashed due to observational uncertainty..

Harmonic analysis of 30-hPa zonal wind, showing the amplitude of the annual cycle (squares), semiannual cycle (triangles), and residual deseasoned component (circles). Symbols show individual rawinsonde station amplitudes. Solid lines are based on binned data.

LIMITATIONS OF THE THEORY THE MOMENTUM TRANSFERRED BY THE WAVES IS NOT CONFIRMED BY OBSERVATIONS. NOT ENOUGH OBSERVATIONS AVAILABLE. 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 RG WAVES TRANSFER MOMENTUM TO THE LOWER TROPOSPHERE.

LIMITATIONS OF THE THEORY THE THEORY DO NOT EXPLAIN QBO 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.

QBO AND SW MONSOON RANJIT SINGH (1986) Berson westerlies (bw) along with the easterlies execute qbo 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 qbo over gan island (00°41’s, 73°09’e) and trivandrum together, the withdrawing phase of bw from trivandrum and gan island may be followed by draught or below normal rainfall over india.

QBO AND SW MONSOON RANJIT SINGH (1986) The setting phase of bw over gan island and/or their strengthening with time mostly coincides with a good monsoon. Taking trivandrum alone, the presence of bw 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 have occurred during easterly phase also.

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