Westerly Phase Of Quasi-biennial Oscillation Research Articles

Simulations of fall and early winter in the stratosphere

Mechanistic model simulations of fall/early winter in the northern (November and December) and southern (May and June) stratosphere are compared with observational analyses to examine the skill of the model in simulating the state of the stratosphere, including both means and variability in key fields, during six winters. While detailed success varies from year to year, the model produces a realistic climatology of and variability in the evolution of winds, geopotential heights, temperatures and wave propagation in the early-winter stratosphere. The variability and mean fields agree well with those in longer data records. The northern hemisphere (NH) simulations show a small cold bias when averaged over the 6 years, while the southern hemisphere (SH) simulations show a larger warm bias. Greater detailed success in simulations of alternate NH winters suggests the possibility of greater model skill during the westerly quasi-biennial oscillation phase, although the short record and complexity of interactions between tropics and high latitudes preclude definitive identification of such a relationship. A prominent failing of the model when using Rayleigh friction to parametrize gravity-wave drag is an inability to correctly reproduce the latitudinal structure of the stratospheric jet above about 7 hPa; this failing can be alleviated by using a non-orographic gravity-wave drag parametrization, at the expense of frequently degrading agreement of planetary-wave phases and amplitudes. The success of the model in reproducing realistic climatology and variability makes these simulations useful for more detailed studies of transport and vortex evolution in early winter. Interhemispheric comparisons show that the early-winter circulations are qualitatively similar in the NH and SH: they are dominated by a strengthening vortex, with most wave activity being in wave 1; they both have minor warmings in a preferred location and clustered around a preferred time—early December (early June) in the NH (SH); variability in the flow shows a crescent pattern of maximum variations near 60–80° latitude centred at the location of the minor warmings (over the dateline in the NH and in the South Pacific in the SH). Interhemispheric differences are primarily in the magnitude of the variability, with a more symmetric circulation and weaker minor warmings in the SH; this is in contrast to later in the winter, when large qualitative interhemispheric differences have been seen. Copyright © 2002 Royal Meteorological Society

QBO and Pinatubo signals in the mass flux at 100hPa during the period from 1986 to 1995

The residual mean meridional circulation is calculated by the transformed Eulerian-mean (TEM) momentum and continuity equations for 10 years from December 1985 to November 1995. The tropical upward mass flux at 100 hPa is estimated from the extratropical downward mass fluxes in the southern and northern hemispheres. The multiple regression statistical model is used to obtain quantitatively the signal of the quasi-biennial oscillation (QBO) and the trend. The results show that the tropical upward mass flux is enhanced during QBO easterly phase and reduced during QBO westerly phase. The signal of Mount Pinatubo volcanic eruption in June 1991 is detected. An abrupt increase of the mass flux is shown in the latter half of 1991, and the increase ceases at the end of 1993, about 2.5 years after the eruption.

A model study of the influence of the quasi‐biennial oscillation on trace gas distributions in the middle and upper stratosphere

The dominant tracer transport processes in the equatorial and subtropical latitudes of the middle and upper stratosphere are investigated. Distributions of water vapor in Northern Hemisphere winter from the Microwave Limb Sounder onboard the Upper Atmosphere Research Satellite are employed, together with a three‐dimensional Stratosphere Mesosphere Model that incorporates a representation of the quasi‐biennial oscillation (QBO). The model reproduces the observed tracer distributions, in particular a “staircase” feature that is present in northern winter of 1992 (easterly QBO phase) but not in 1993 (westerly QBO phase). This feature is highly asymmetric about the equator. The model circulation is diagnosed to show that while the induced QBO circulation in the lower stratosphere of the model is relatively symmetric about the equator, in the middle and upper stratosphere it is highly asymmetric and in the correct sense to give rise to the staircase feature. Model experiments are compared in which trajectories are advected by (1) the full three‐dimensional circulation and (2) the residual mean circulation only, thereby removing the local effects of isentropic mixing by planetary waves on the trajectory distributions. These confirm the importance of advection by the QBO circulation at equatorial and subtropical latitudes. However, sharpening of the tracer gradients at the subtropical edge of the surf zone by the action of planetary waves is shown to be important in the formation of a subtropical “cliff ” between 10 and 20 mbar at 20°–30°N. The model results also suggest that the prominence of the summer subtropical peak in easterly phase years compared with westerly phase years is not entirely due to increased summer upwelling of the large‐scale global circulation caused by the stronger planetary wave driving. The depression of the winter half of the equatorial peak by the local asymmetric QBO circulation is also shown to be important.

Reply to comments on “Variation of cosmic ray flux and global cloud coverage — a missing link in solar–climate relationships

The occurrence of coldest region in the lower and middle stratosphere has been investigated using COSMIC/FORMASAT-3 radio occultation measurements. Observations from January 2007 to December 2011, comprising of 2,871,811 numbers of occultations uniformly spread over land and sea, have been used in this study. Using vertical profiles of temperature upto 40 km altitude, zonally averaged at each 5° latitude band between 90°N and 90°S, it is shown that the coldest region in the upper atmosphere occurs during winter in high latitude stratosphere (latitudes >45°) in both the hemispheres with southern hemisphere (temperature less than <−85 °C) cooler than northern hemisphere (temperature ~−75 °C). The spatial extent of the region of low temperature region found between 10 km and 30 km altitude, indicating a 20 km vertical thick layer of cold temperature. In the southern hemisphere, such a region of coldest temperature remains for more than six months (April–October), in the Northern hemispheric polar region (~−75 °C) it is seen mostly during four winter months between October and January. Using NCEP-DOE reanalysis data, we show that cold temperature in the stratospheric region coexists with the jet streams prevalent in those regions. Strong wind jet is surmised to make stratosphere colder. The absence of sunlight in the coldest region is known to cause jet streams. Impact of stratospheric quasi-biennial oscillation (QBO) on the sharpness of tropical tropopause (stability) has also been investigated. Observations suggest that during westerly phase of QBO, the stability of the tropopause increases.

Effects of an Imposed Quasi-Biennial Oscillation in a Comprehensive Troposphere–Stratosphere–Mesosphere General Circulation Model

A 48-yr integration was performed using the Geophysical Fluid Dynamics Laboratory SKYHI troposphere‐ stratosphere‐mesosphere GCM with an imposed zonal momentum forcing designed to produce a quasi-biennial oscillation (QBO) in the tropical stratosphere. In response to this forcing, the model generates a QBO in the tropical circulation that includes some very realistic features, notably the asymmetry between the strength of the descending easterly and westerly shear zones, and the tendency for the initial westerly accelerations to appear quite narrowly confined to the equator. The extratropical circulation in the Northern Hemisphere (NH) winter stratosphere is affected by the tropical QBO in a manner similar to that observed. In particular, the polar vortex is generally weaker in winters in which there are easterlies in the tropical middle stratosphere. Roughly twothirds of the largest midwinter polar warmings occur when the equatorial 30-mb winds are easterly, again in rough agreement with observations. Despite this effect, however, the total interannual variance in the zonalmean extratropical circulation in the model apparently is slightly decreased by the inclusion of the tropical QBO. The observed QBO dependence of the winter-mean stratospheric extratropical stationary wave patterns is also quite well reproduced in the model. The QBO was also found to have a profound influence on stratospheric stationary waves at low latitudes. Near and above 10 mb the NH stationary waves were found to penetrate across the equator during the westerly QBO phase, but to be restricted to latitudes poleward of ;108N during the easterly phase. This means that the equatorial QBO in prevailing wind near and above 10 mb has a significant zonally asymmetric component. If this is also true in the real atmosphere, there are important implications for the adequacy of the current observational rawindsonde network near the equator. Analysis of the zonal-mean zonal momentum budget in the tropical stratosphere reveals that the resolved waves in the model are strong enough to force the realized accelerations through much of the QBO cycle. The exception appears to be the easterly acceleration phase below about 20 mb. The implications of this for the generation of a self-consistent QBO by GCMs will be considered. The effects of the imposed QBO on the troposphere were found to be very modest. There does appear to be a statistically significant weakening (by ; 1ms 21) of the high-latitude winter vortex in the middle and upper troposphere. Given the very high predictability of the stratospheric QBO itself, this effect could possibly be used to enhance the skill of seasonal weather forecasts. No significant QBO influence was found in the model precipitation field.

QBO and Pinatubo signals in the mass flux at 100 hPa and stratospheric circulation

The residual mean meridional circulation is calculated by the transformed Eulerian‐mean (TEM) momentum and continuity equations for the period from December 1988 to November 1995. Our method does not rely on radiative calculations. The tropical upward mass flux at 100 hPa is estimated from the extratropical downward mass fluxes in the southern and northern hemispheres. The multiple regression statistical model is used to obtain quantitatively the signal of the quasi‐biennial oscillation (QBO) and the trend. The results show that the tropical upward mass flux is enhanced during QBO easterly phase and reduced during QBO westerly phase. As a residual, the signal of Mount Pinatubo volcanic eruption in June 1991 is detected. This reflects the aerosol effects to the residual circulation in the lower stratosphere after the eruption. An abrupt increase of the mass flux is shown in the latter half of 1991, and the increase ceases at the end of 1993, about 2.5 years after the eruption.

Seasonal, semiannual, and interannual variability seen in measurements of methane made by the UARS Halogen Occultation Experiment

Variability in atmospheric methane on semiannual, seasonal, and interannual timescales is investigated using measurements of CH4 made by the Upper Atmosphere Research Satellite Halogen Occultation Experiment (HALOE). It is shown that there is a strong correlation between the appearance of the stratopause double peak in methane mixing ratio (which is thought to be associated with the semiannual oscillation (SAO)) and the phase of the quasi‐biennial oscillation (QBO) in the equatorial lower stratosphere. In a descending westerly QBO phase the double peak is seen clearly in the HALOE methane data, but during the onset of the easterly phase the double peak is not observed. This is the opposite of that which might be expected if the cause was a straightforward modulation of the strength of the SAO due to wave filtering by the QBO. The intra‐annual variability in HALOE methane is shown to be broadly consistent with previous observations. One interesting feature is the apparent occurrence of descent in the summer high latitudes near the stratopause. It is shown that this feature is, in fact, due to enhanced photochemical destruction in this region in the presence of weak transport.

Modeling quasi‐biennial variability in the semiannual double peak

The interannual variability of the stratopause semiannual double peak as seen in methane measurements made by the Halogen Occultation Experiment (HALOE) is investigated using a two‐dimensional isentropic model. It is found that the observed variability is reproduced when the quasi‐biennial oscillation (QBO) of the tropical lower stratosphere is introduced into the model and that there are two distinct means by which the QBO affects stratopause tracer values. First, the QBO modulates the westerly phase of the stratopause semiannual oscillation (SAO) by the selective damping of the parameterized waves responsible for producing this phase of the SAO in the model. This results in a modulation of the strength and duration of the SAO circulation, with it being stronger when the QBO is easterly than when it is westerly. However, when the QBO is easterly, the effect of stronger downwelling in the tropics is negated by subsequent upwelling; the overall effect is to reduce or eliminate the trough of the double peak. For a westerly QBO phase the downwelling, although weaker, is also more persistent, giving a deeper trough. Second, the tropical QBO influences the propagation of planetary waves in the winter hemisphere, thereby producing an extratropical QBO in the winter stratosphere and mesosphere. This alters the circulation in this region, giving more upwelling in the winter extratropics above 0.5 hPa when the QBO (defined at 15 hPa) is westerly. This causes the winter lobe of the double peak to form only when the QBO is westerly. The combined result of the two processes is to produce a double peak when the QBO winds are westerly but a single peak in the summer hemisphere when the QBO winds are easterly. This modeled result agrees well with the interannual variability displayed by the HALOE methane observations.

A Model Study of Zonal Forcing in the Equatorial Stratosphere by Convectively Induced Gravity Waves

A two-dimensional cloud-resolving model is used to examine the possible role of gravity waves generated by a simulated tropical squall line in forcing the quasi-biennial oscillation (QBO) of the zonal winds in the equatorial stratosphere. A simulation with constant background stratospheric winds is compared to simulations with background winds characteristic of the westerly and easterly QBO phases, respectively. In all three cases a broad spectrum of both eastward and westward propagating gravity waves is excited. In the constant background wind case the vertical momentum flux is nearly constant with height in the stratosphere, after correction for waves leaving the model domain. In the easterly and westerly shear cases, however, westward and eastward propagating waves, respectively, are strongly damped as they approach their critical levels, owing to the strongly scale-dependent vertical diffusion in the model. The profiles of zonal forcing induced by this wave damping are similar to profiles given by critical level absorption, but displaced slightly downward. The magnitude of the zonal forcing is of order 5 m s21 day21. It is estimated that if 2% of the area of the Tropics were occupied by storms of similar magnitude, mesoscale gravity waves could provide nearly 1/4 of the zonal forcing required for the QBO.

Effects of Solar Activity and Stratospheric QBO on Tropical Monsoon Rainfall.

A study on the influence of solar activity and phases of equatorial quasi-biennial oscillation (QBO) on tropical rainfall over India has been made. The results of the total rainfall over the country indicate that during the period of low solar activity, excess/normal rainfall (frequent floods) are associated with the westerly phase of QBO and deficient/normal rainfall (frequent droughts) corresponded to the easterly phase. During the period ofhigh solar activity only normal rain is observed when QBO is in its westerly phase. Multiple regression analysis has employed to estimate the summer monsoon rainfall using sunspot number and equatorial zonal wind at 15-20 hPa during January February as independent parameters. A study on the spatial variability of monsoon rainfall in response to the solar activity and phases of QBO has also been attempted.

Seasonal dependence of stratospheric temperature variations

Abstract. Stratospheric temperatures show distinct trends, not necessarily monotonically upward or downward. At the North Pole, trends were large only during winter and spring and were different for different months; downward for November, December, mixed for January and upward for February, March and April. For the 10°-90°N belt, the trends were variable, viz., downward during 1971-1975, upward during 1975-1978 and downward again from 1978 onwards up to date, opposite to the upward trend of ground temperature in the Northern hemisphere in recent years. Twelve-monthly running averages revealed strong QBO (quasi-biennial oscillation). For the North Pole, the QBO showed colder (lower) temperatures during 50-mb wind QBO westerly phase maxima. For the 10°-90°N belt, the QBO was similar for 30 mb and 50 mb but the QBO phases did not match well with 50-mb wind QBO phases.

Modeling the Quasi-biennial Oscillation's Effect on the Winter Stratospheric Circulation

The influence of the equatorial quasi-biennial oscillation (QBO) on the winter middle atmosphere is modeled with a mechanistic global primitive equation model. The model's polar vortex evolution is sensitive to the lower stratosphere's tropical winds, with the polar vortex becoming more (less) disturbed as the lower stratospheric winds are more easterly (westerly). This agrees with the observed relationship between wintertime polar circulation strength and the phase of the QBO in the lower stratosphere. In these experiments it is the extratropical planetary Rossby waves that provide the tropical-extratropical coupling mechanism. More easterly tropical winds in the lower stratosphere act to confine the extratropical Rossby waves farther north and closer to the vortex at the QBO altitudes, weakening the vortex relative to the case of westerly QBO phase. While the QBO winds occur in the lower stratosphere, the anomaly in the polar vortex strength is strongest at higher levels.

The interannual variability of polar stratospheric clouds and related parameters in Antarctica during September and October

Antarctic polar stratospheric cloud (PSC) sightings by the orbiting SAM II sensor during September and October show a pronounced Quasi‐Biennial Oscillation (QBO) signal, and October sightings have increased markedly over the past 10 years in years of westerly QBO phase. The QBO in PSC frequency is likely to affect the rate of Antarctic heterogeneous chemical processes and, hence, ozone depletion. Studies of the observed long‐term temperature trend suggest that the decadal PSC trend probably results from the ozone decline through its effect on stratospheric heating rates. A more detailed analysis of data from 1986 and 1987 shows that there were more PSCs in 1987 and that they persisted much later into the spring season as compared to 1986. Qualitatively similar behavior was found for the OClO column abundances and 18‐km ozone depletion observed at McMurdo Station during these 2 years. These observations suggest that both the intensity and duration of heterogeneous chemical processes are likely greater during colder, QBO‐westerly phase years.