A Study On The Trends Of Quasi-Biennial Oscillation And Brewer-Dobson Circulation And Its Relationship

Using the ERA-Interim reanalysis data from 1979-2009, long-term trends of Brewer-Dobson circulation(B-D circulation) and the quasi-biennial oscillation(QBO) as well as their relationship are analyzed. Some results are obtained as follows:1. The changes in QBO over the past 30 years are analyzed using different diagnosis variables including QBO amplitude, period and downward speed associated with the method of harmonic analysis. It’s found that the amplitude of QBO exhibits significant decreasing trend. As for the period of QBO, different definition criteria are used here. One is the period between the occurrences of maximum easterly(westerly) speed and its adjacent maximum westerly(easterly) speed. The other one is defined by the transition time from eastward(westward) wind into westward(eastward) wind. Both of the periods show evidently shortening over these years than before, except the circumstance that we choose time interval between the extreme of adjacent east wind as the period of QBO. In addition, the downward speed of QBO in the west phase also shows a decrease trend for the period 1979-2009.2. The vertical velocity of BD circulation at 5 hPa、10 hPa and 100 hPa exhibit a negative trend(significant test at a 95% confident level) in the tropics, indicating that the upwelling(downwelling) branch in the tropic(middle latitudes) has been weakened over the recent years. As for the meridional velocity of BD circulation, it shows significantly positive trend at 5 hPa and significantly negative trend at 10 hPa and 100 hPa in the northern hemisphere midlatitude and vise verse in the southern hemisphere. It’s apparent from above analysis that poleward motions strengthen in the upper stratosphere and weaken in the lower stratosphere at midlatitude. Further analysis of various branches of BD circulation in the tropical stratosphere show that the deep branch above the altitude of 30 hPa, shallow branch between 30-70 hPa and transition branch between 70-100 hPa are all significantly weakened, implying that the upwelling of BD circulation decreases in lower tropical stratosphere. We further analyzed the planetary wave which drives the BD circulation over the past 30 years and found that there exists a positive trend in the vertical component of EP flux between 45-75 oN. In addition, planetary wave propagating into the upper stratosphere at high latitudes exhibits a significantly enhanced, which may be the main drive for the strengthening of BD circulation.3. BD circulation under different phases of QBO in the tropics presents a significant difference. There exhibit three forced meridional circulations along the equator zonal symmetric distribution in different phase of QBO. During QBO east(west) phase, there are clockwise(counterclockwise) secondary circulation in the upper and lower stratosphere and a reverse secondary circulation in the middle stratosphere. Due to the thermal-wind relation, the anomalous upward(downward) motions caused by the anomalous convergence(divergence) will be compensated in the upper(lower) stratosphere by the anomalous divergence(convergence). Meanwhile, the eastward(westward) wind shear corresponds to negative(positive) temperature anomalies, which caused by the adiabatic cooling(adiabatic heating) associated with anomalous upward(downward) motions. QBO can influence the strength of BD circulation through modulating the secondary circulation, however, this effect is weak. As a result, the BD circulation is totally enhanced(weakened) during QBO east(west) phase. At mid and high latitudes, QBO mainly affect BD circulation by modulating the planetary waves. Compared with QBO west phase, there are anomalous EP flux convergence in high latitude stratosphere during the QBO east phase. In addition, there are positive anomalies of refractive index in the middle latitudes, leading to more planetary waves into the stratosphere and the strengthening of BD circulation.