Does the Solar Cycle Increase or Decrease the Periodof the Quasi-Biennial Oscillation? A Modeling Study

Le Kuai1, Runlie Shia1, Xun Jiang2, Ka-Kit Tung3, Yuk L. Yung1
1 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125
2 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109
3 Department of Applied Mathematics, University of Washington, Seattle, WA 98195

* To whom all correspondence should be addressed. E-mail:

Le Kuai, Division of Geological and Planetary Sciences, California Institute of Technology, MC150-21, 1200 E. California Blvd., Pasadena, CA 91125, USA. ()

Runlie Shia, Division of Geological and Planetary Sciences, California Institute of Technology, MC150-21, 1200 E. California Blvd., Pasadena, CA 91125, USA. ()

Xun Jiang, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA. ()

Ka-Kit Tung, Department of Applied Mathematics, University of Washington, Seattle, WA 98195, USA. ()

Yuk L. Yung, Division of Geological and Planetary Sciences, California Institute of Technology, MC150-21, 1200 E. California Blvd., Pasadena, CA 91125, USA. ()

Abstract

During the period of 1960s, 70s, 80s and early 90s, the westerly phase of the Quasi-Biennial Oscillation has been observed by a number of authors to have extended periods during the solar minimum. This is also the time when the stratosphere is contaminated by the aerosols frommajor volcanic eruptions. For the one solar cycles prior and one after this period, the QBO period appears to increase during solar maximum and decrease during solar minimum. The true dependence of QBO period on the solar cycle in the absence of volcanic aerosol heating cannot be determined statistically due to the short observational data record. We perform a long model simulation of the QBO without volcanoes using the THINAIR model. The analysis of the model simulation indicatesa positive correlation between the length of QBOperiod and the solar cycle flux.

1. Introduction

Quasi-Biennial Oscillation (QBO) is now understood to be an internal oscillation of the equatorial stratosphere involving wave-mean flow interactions. It period averages to about 28 months, but is known to have interannual variations of about the mean. The equatorial QBO is known to affect significantly the polar stratosphere during winter, with the easterly phase creating the condition for a more perturbed and warmer polar vortex. Therefore, the variation of QBO’s period takes on an added importance (especially with respect to the timing of its phase relative to northern winter.)

Recently, Salby and Callaghan [2000], using the radiosonde data from Free University of Berlin (FUB) near the equator at 45 hPa from 1956-1996, and found that the duration of the QBO’s westerly phase tends to be longer during solar minimum. This result is confirmed by Soukharev and Hood [2001] using composite mean analysis, and Pascoe et al [2005] on the mean descent rate of the easterly shear zone prior to 1990s. On the other hand, Hamilton [2002]and Fischer and Tung [2007] studied a longer period of FUB data and found the opposite behavior in the late 1990s and 2000s and also the 1950s. For the longer period of 1953-2005, Fischer and Tung [2007] found that the correlation coefficient between the period of the QBO and a solar cycle index to be zero. It should be mentioned that these later publications did not contradict the findings of the earlier authors. They merely pointed out that the behavior of the 60s, 70s, 80s and early 90s were reversed in other decades not included in the period studied by Salby and Callaghan [2000].

The descent rate of QBO shear zones is affected by a number of external factors, some local and some remote. Local heating due to volcanic aerosols at equator can temporarily induce an upwelling in the lower stratosphere that can affect the descent rate of the easterly shear zone. At equilibrium the increase in temperature may introduce a horizontal temperature gradient anomaly. By the thermal wind relationship, different vertical shears in the zonal wind may be introduced above and below the region of heating, which may affect the apparent descent of the QBO shear zone. There were three major volcanic eruptions whose aerosols reached the equatorial stratosphere, Agung in 1963, El Chichon in 1982 and Pinatubo in 1991. Due to the difference in timing, location and height of the injection of the aerosols, the effect on the QBO period may be different after each eruption. Angel [1986] argued that the increase in westerly period after 1963 was due to the Agung eruption, and pointed out that the temperature increasing due to aerosol heating was seen at 50 hPa dnd 30 hPa at the Balboa station. The possibility exists that the inverse relationship between the westerly QBO period in the lower stratosphere and the solar cycle flux found by Salby and Callaghan [2000] and others may be anomalous; although the behavior was attributed to solar influence, the volcanic aerosols during the period studies may have masked the true solar influence. During the solar minimum of 1997, when the volcanic aerosols from the 1991 eruption of Pinatubo has cleared, the QBO period reached a low of 25 months. Since then, the period increased as the solar cycle advanced from minimum to maximum, with positive correlation [Fischer and Tung, 2007]. A few more decades without major volcanoes are needed to obtain a statistical significant correlation with the solar cycle. We take another approach in this paper in an attempt to resolve the above controversy. We presentastudy onhowthe solar cycle influences QBO using along model simulation without volcanoes.

2. Model description

The THINAIR model is a two-and-a-half-dimensional dynamics model. It has zonally averaged dynamics plus three longest planetary waves, which are prescribed from observations at the tropopause level. It uses isentropic vertical coordinate above 350 K. Below 350 K a hybrid coordinate is used to avoid intersection of the coordinate layers with the ground. The model version used in this study has 29 layers from the ground up to ~100 km for dynamics and 17 layers from ground up to ~60 km for chemistry. The model has 19 horizontal grid points evenly distributed from pole to pole.

The QBO-source term in the momentum equation uses parameterization of wave momentume fluxes from Kevin, Rossby-gravity and gravity waves [Kinnersley, 1996]. UARS/SOLSTICE spectral irradiance observation (Figure 1) has been used as the 11-year solar cycle input. It consists of the solar spectrum in UV 119-400 nm during 1991-2002, with 1-nm resolution. The monthly data has been extended to 1947-2005 using F10.7-cm as a proxy [Jackman et al., 1996]. For the longer runs beyond 2005, the solar cycle is simply repeated using the previous period.

3. Solar cycle influence on the length of QBO period:

The composite analysis of Soukhrarew and Hood [2001, Figure 12]is here reproduced using 82 years of THINAIR model results. Figure 2 shows the mean vertical-time cross section of the westerly QBO of the lowpass filtered equatorial zonal wind for solar maximum conditions (Figure 2 a), solar minimum conditions (Figure 2 b) and the difference of them (Figure 2 c). The duration of the westerly QBO phase is found to be longer in solar maximum conditions than in solar minimum conditions, in contrast to Soukhrarew and Hood’s conclusion. Fischer and Tung [2007] showed that the anti-correlation with the solar cycle mentioned by Salby and Calleghan [2000] breaks down in the solar minimum in 1997 with QBO period being as low as about 25 months when there were no major volcanic perturbations. Our model, exclusive of the volcanic influence, supports Fischer and Tung’s prediction that the in-phase relationship will be found during the solar cycle without the volcanic eruptions.

We have also performed a set of calculations where the solar radiation is held either in perpetual solar minimum or solar maximum conditions. This allows the use of the Fourier analysis to determine the QBO period unambiguously. Figure 3a is the 82 year run under perpetual solar minimum conditions, and Figure 3b shows the corresponding run under perpetual solar maximum conditions. In Figure 3c we show the case with 5 times solar maximum condition. Their Fourier frequency spectra are shown in Figure 3d. It shows unambiguously that as the solar radiation is increased, the period of the QBO lengthens.

In Figure 4, we show the zonal mean streamfunction in isentropic coordinates, which theoretically is closest to the Lagrangian mean circulation. It is seen that during solar maximum conditions, there is a stronger Brewer-Dobson circulation with stronger downwelling in polar stratosphere and upwelling in the equatorial lower stratosphere. The latter slows the descent of both the easterly and westerly shear zones and thus lengthens the QBO period.

4. Conclusion and discussion

Camp and Tung [2007] found in NCEP data that during solar maximum conditions there are more Stratospheric Sudden Warmings in the polar stratosphere during late winter. Consequently the polar stratosphere is warmer and the Brewer-Dobson circulation is more downward. This then could remotely force a stronger upwelling branch of the Brewer-Dobson circulation over the equator, which then slows the descent of the QBO shear zone. Fischer and Tung [2007] found that above 30 hPa it is the easterly phase of the QBO which is lengthened while below 30 hPa it is the westerly phase. Both follow the variation of the whole period of the QBO. Because of the self induced secondary circulation of the QBO itself is upward for the easterly phase, the easterly phase is more vulnerable to the slowing and eventual stalling, which usually occurs near 30 hPa. Below the stalling level, the westerly phase persists without being replaced by the descending easterlies, leading to a longer westerly duration. In this model there is no local heating due to volcanic aerosols, and so the anomalous upwelling over the equator is probably remotely forced by the Sudden Warming over the polar stratosphere. This mechanism then argues for a lengthening of the QBO period during solar maximum in the absence of volcanoes.

Acknowledgements.We would like to thank Alexander Ruzmaikin and John Lawrence for their contribution in the useful discussions about these works. We also thank graduate student King-Fai Li of Caltech for his solar cycle input and helpful suggestions. This work was supported by …..

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