West Phase Of Quasi-biennial Oscillation Research Articles

The QBO Modulation on CO Distribution in the UTLS Over the Asian Monsoon Region During Boreal Summer

Using the Microwave Limb Sounding measurements and the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis product, we have investigated the response of carbon monoxide (CO) mixing ratio within the Asian Summer Monsoon Anticyclone (ASMA) region to the Quasi-Biennial Oscillation (QBO) and the potential dynamical mechanisms behind it. The results showed that the mixing ratio of CO within the ASMA is significantly higher at both 100 hPa and 147 hPa in the east phase of QBO (EQBO) years than in the west phase of QBO (WQBO) years, especially in the eastern part of the ASMA. The CO difference between EQBO and WQBO years is consistent with the interannual variation of the location and strength of the ASMA. Specifically, during the EQBO years, the area of the ASMA reduces to the southeast in the upper troposphere, while the westerly jet on the north side of the anticyclone and the southerly winds on the west side of the anticyclone both strengthen. These variations are associated with tropospheric circulation anomalies. The ascending motions over the Tibetan Plateau (TP) are enhanced during the EQBO years, which are correlated with the relatively high CO at 100 hPa. Consistently, an anomalous cyclonic circulation is found at 500 hPa over the TP when the QBO is on its east phase, which promotes the convergence of air mass and carries CO from the surface to the Upper Troposphere and Lower Stratosphere (UTLS).

Impact of the cross-tropopause wind shear on tropical cyclone genesis over the Western North Pacific in May

The role of the 50- to 200-hPa zonal wind shear in modulating tropical cyclone (TC) genesis over the western North Pacific (WNP) in May is investigated in this study. Concurrent with the strong cross-tropopause shear over the key region (0°–5°N, 160°–180°E), suppressed convection was observed over the tropical WNP, especially over the South China Sea and the Philippines. The monsoon trough (MT) was confined westward. However, enhanced convection occurred in the weak shear years and the MT extended eastward. This cross-tropopause wind shear is negatively correlated with TC genesis in May, with a decreased (increased) number of TCs corresponding to strong (weak) cross-tropopause wind shear. This cross-tropopause wind shear can be treated as the combined impacts of the El Nino-Southern Oscillation (ENSO) events and the stratospheric quasi-biennial oscillation (QBO). When decaying El Nino events coupled with the easterly phase of the QBO were noted, the cross-tropopause wind shear was stronger with weakened convection, and an enhanced western Pacific subtropical high was observed. TCs are rarely generated during these years. In contrast, the modulation of the QBO westerly phase on decaying La Nina events is limited. Affected by the QBO westerly phase, TC genesis in the May following La Nina events is only slightly enhanced. The energy analysis indicates that the combined impacts of the decaying El Nino events and the QBO easterly phase might suppress the barotropic eddy kinetic energy conversion in May, whereas the decaying La Nina events and the QBO west phase act in an opposite manner.

The millennium water vapour drop in chemistry–climate model simulations

Abstract. This study investigates the abrupt and severe water vapour decline in the stratosphere beginning in the year 2000 (the "millennium water vapour drop") and other similarly strong stratospheric water vapour reductions by means of various simulations with the state-of-the-art Chemistry-Climate Model (CCM) EMAC (ECHAM/MESSy Atmospheric Chemistry Model). The model simulations differ with respect to the prescribed sea surface temperatures (SSTs) and whether nudging is applied or not. The CCM EMAC is able to most closely reproduce the signature and pattern of the water vapour drop in agreement with those derived from satellite observations if the model is nudged. Model results confirm that this extraordinary water vapour decline is particularly obvious in the tropical lower stratosphere and is related to a large decrease in cold point temperature. The drop signal propagates under dilution to the higher stratosphere and to the poles via the Brewer–Dobson circulation (BDC). We found that the driving forces for this significant decline in water vapour mixing ratios are tropical sea surface temperature (SST) changes due to a coincidence with a preceding strong El Niño–Southern Oscillation event (1997/1998) followed by a strong La Niña event (1999/2000) and supported by the change of the westerly to the easterly phase of the equatorial stratospheric quasi-biennial oscillation (QBO) in 2000. Correct (observed) SSTs are important for triggering the strong decline in water vapour. There are indications that, at least partly, SSTs contribute to the long period of low water vapour values from 2001 to 2006. For this period, the specific dynamical state of the atmosphere (overall atmospheric large-scale wind and temperature distribution) is important as well, as it causes the observed persistent low cold point temperatures. These are induced by a period of increased upwelling, which, however, has no corresponding pronounced signature in SSTs anomalies in the tropics. Our free-running simulations do not capture the drop as observed, because a) the cold point temperature has a low bias and thus the water vapour variability is reduced and b) because they do not simulate the appropriate dynamical state. Large negative water vapour declines are also found in other years and seem to be a feature which can be found after strong combined El Niño/La Niña events if the QBO west phase during La Niña changes to the east phase.

Effects of stratospheric variability on El Niño teleconnections

The effects of the tropical Pacific El Niño Southern Oscillation (ENSO) phenomenon are communicated to the rest of the globe via atmospheric teleconnections. Traditionally, ENSO teleconnections have been viewed as tropospheric phenomena, propagating to higher latitudes as Rossby waves. Recent studies, however, suggest an influence of the stratosphere on extra-tropical ENSO teleconnections. The stratosphere is highly variable: in the tropics, the primary mode of variability is the quasi-biennial oscillation (QBO), and in the extra-tropics sudden stratospheric warmings (SSWs) regularly perturb the mean state. Here, we conduct a 10-member ensemble of simulations with a stratosphere-resolving atmospheric general circulation model forced with the observed evolution of sea surface temperatures during 1952–2001 to examine the effects of the QBO and SSWs on the zonal-mean circulation and temperature response to El Niño, with a focus on the northern extra-tropics during winter. We find that SSWs have a larger impact than the QBO on the composite El Niño responses. During El Niño winters with SSWs, the polar stratosphere shows positive temperature anomalies that propagate downward to the surface where they are associated with increased sea-level pressure over the Arctic. During El Niño winters without SSWs, the stratosphere and upper troposphere show negative temperature anomalies but these do not reach the surface. The QBO modulates the El Niño teleconnection primarily in winters without SSWs: the negative temperature anomalies in the polar stratosphere and upper troposphere are twice as large during QBO West compared to QBO East years. In addition, El Niño winters that coincide with the QBO West phase show stronger positive sea-level pressure anomalies over the eastern Atlantic and Northern Europe than those in the QBO East phase. The results imply that the stratosphere imparts considerable variability to ENSO teleconnections.

Northern Hemisphere stratospheric winds in higher midlatitudes: longitudinal distribution and long-term trends

Abstract. The Brewer–Dobson circulation (mainly meridional circulation) is very important for stratospheric ozone dynamics and thus for the overall state of the stratosphere. There are some indications that the meridional circulation in the stratosphere could be longitudinally dependent, which would have an impact on the ozone distribution. Therefore, we analyse here the meridional component of the stratospheric wind at northern middle latitudes to study its longitudinal dependence. The analysis is based on the NCEP/NCAR-1 (National Centers for Environmental Prediction and the National Center for Atmospheric Research), MERRA (Modern Era-Retrospective Analysis) and ERA-Interim (European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis Interim) reanalysis data. The well-developed two-core structure of strong but opposite meridional winds, one in each hemisphere at 10 hPa at higher northern middle latitudes, and a less pronounced five-core structure at 100 hPa are identified. In the central areas of the two-core structure the meridional and zonal wind magnitudes are comparable. The two-core structure at 10 hPa is almost identical for all three different reanalysis data sets in spite of the different time periods covered. The two-core structure is not associated with tides. However, the two-core structure at the 10 hPa level is related to the Aleutian pressure high at 10 hPa. Zonal wind, temperature and the ozone mixing ratio at 10 hPa also exhibit the effect of the Aleutian high, which thus affects all parameters of the Northern Hemisphere middle stratosphere. Long-term trends in the meridional wind in the "core" areas are significant at the 99% level. Trends of meridional winds are negative during the period of ozone depletion development (1970–1995), while they are positive after the ozone trend turnaround (1996–2012). Meridional wind trends are independent of the sudden stratospheric warming (SSW) occurrence and the quasi-biennial oscillation (QBO) phase. The influence of the 11-year solar cycle on stratospheric winds has been identified only during the west phase of QBO. The well-developed two-core structure in the meridional wind illustrates the limitations of application of the zonal mean concept in studying stratospheric circulation.

The quasi‐biennial oscillation: Analysis using ERA‐40 data

The ERA‐40 data set is used to examine the equatorial quasi‐biennial oscillation (QBO). The data set extends from the ground to 0.1 hPa (∼65 km) and covers a 44‐year period (January 1958 to December 2001), including 18.5 QBO cycles. Analysis of this data set of unprecedented spatial and temporal coverage has revealed a threefold structure in height in the QBO zonal wind anomalies at the equator. In addition to the well‐known twofold structure in the lower and middle stratosphere, that is, easterlies overlying westerlies or vice versa, there is a third anomaly in the upper stratosphere. The sign of this upper stratospheric anomaly is the same as the lower stratospheric anomaly, thus forming anomalies of alternating sign throughout the depth of the equatorial stratosphere. The amplitude of this upper stratospheric anomaly is ∼10 m s−1, approximately one third of the amplitude of the lower stratospheric signal. The frequency and descent rates of the east and west QBO phases are analyzed in detail, with particular attention to any 11‐year solar cycle influence. In addition to the observed solar modulation of the duration of the QBO west phase the analysis shows a solar modulation of the mean descent rate of the easterly shear zone. The mean time required for the easterly shear zone to descend from 20 to 44 hPa is 2 months less under solar maximum conditions than under solar minimum conditions (7.4 months versus 9.7 months). This rapid descent of the easterly shear zone cuts short the west phase of the QBO in the lower stratosphere during solar maximum periods.

QBO and solar activity effects on temperatures in the mesopause region

This statistical correlation study is based on two upper mesospheric temperature data sets for the region around 90 km, on the phase of the quasi-biennial oscillation (QBO) and on the solar 10.7-cm radiation. The temperatures are measurements from passive OH∗-spectrometers and lidar sounding experiments carried out between 1980 and 1988 at geographic latitudes between 50°N and 70° N. An anticorrelation exists betwen the 10.7-cm flux and the temperatures. Two different types of significance tests were used to check the confidence level of the correlation. The anti-correlation is highly significant (confidence ⪢ 99.99%) for latitudes around 50°N if the QBO is in its east phase and for the spring/summer temperatures. The anti-correlation is not significant for the QBO west phase and for fall/winter temperatures (confidence < 95%). For higher latitudes, around 79°N, no significant correlation was found.