Dynamic Seasonal Transition from Winter to Summer in the Northern Hemisphere Stratosphere

ZHANG Yu-Li, LIU Yi, and LIU Chuan-Xi

1University of Chinese Academy of Sciences, Beijing 100049, China

2Key Laboratory of Middle Atmosphere and Global Environmental Observation (LAGEO), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China

Dynamic Seasonal Transition from Winter to Summer in the Northern Hemisphere Stratosphere

ZHANG Yu-Li1,2, LIU Yi2, and LIU Chuan-Xi2

1University of Chinese Academy of Sciences, Beijing 100049, China

2Key Laboratory of Middle Atmosphere and Global Environmental Observation (LAGEO), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China

This study applied the modified spatial similarity coefficient method to define the seasonal transition (ST) from winter to summer in the extratropical stratosphere of the Northern Hemisphere. The features of the ST were examined using European Centre for Medium-Range Weather Forecasts (ECMWF) Interim reanalysis data; and the results showed that the time and duration of the ST, which is affected by the activity of planetary waves (PW) in the stratosphere, largely depended on the geophysical locations. This study also investigated the interannual variability of the ST and its relationship with stratospheric sudden warming (SSW) and the quasi-biennial oscillation (QBO). It was shown that the late-onset SSW events (after 22 January) are close to the start of the ST. An easterly (westerly) QBO hastens (delays) the onset of the ST in high and low latitudes, whereas it delays (hastens) the ST in midlatitudes. The duration of the ST is significantly affected by the QBO. The influence of SSW and the QBO have different significance in different latitudes, so they are both important and irreplaceable factors.

seasonal transition, stratosphere, stratospheric sudden warming, quasi-biennial oscillation

1 Introduction

In the extratropical stratosphere, winter and summer alternate every year with two relatively short transition periods in between. Compared to the summer-to-winter seasonal transition (ST), the winter-to-summer ST is much sharper and quicker (Black and McDaniel, 2007), so we only discuss the winter-to-summer ST in this paper. The winter-to-summer ST witnesses the breakdown of the polar vortex and the buildup of summertime circumpolar easterlies. Because of the dynamic coupling between the stratosphere and the troposphere, the ST is dynamically linked to the mid-range weather prediction in the troposphere (e.g., Baldwin et al., 2007; Wang et al., 2012). Al-though many studies have examined stratospheric variations in winter and summer (e.g., Hess and Holton, 1985; Labitzke and Vanloon, 1988, 1992), fewer have focused on the ST process. An improved understanding of the ST and its controlling factors will definitely help to predict the general circulation adjustments and the variations of chemical species in both the stratosphere and the troposphere.

The basic features of stratospheric dynamics, such as the stratospheric structure and dynamic coupling with the troposphere, have been well-documented using several definitions of the stratospheric ST (e.g., Karpetchko et al., 2005). Most studies (e.g., Sherstyukov et al., 1997; Choi et al., 2008) have followed the traditional method that evenly divides a year into four seasons, as used in the tropospheric studies. Some recent studies (e.g., Waugh and Rong, 2002; Black and McDaniel, 2007) began to realize the limitation of the traditional definition and they defined the winter-to-summer ST as the date when the average wind velocity along the vortex edge falls below a threshold value. This is also known as stratospheric final warming (SFW). However, SFW strongly depends on the definition of the polar vortex edge and the threshold chosen.

Numerous studies have investigated the interannual variability of the stratospheric ST (e.g., Labitzke, 1982; Black and McDaniel, 2007). In early transition, once the vortex has broken down, the remnants of the vortex persist for longer than in late transition when the vortex disappears quickly (Waugh and Rong, 2002). Studies on the long-term trend have shown that between the mid-1980s and the late 1990s, the springtime polar vortex was stronger and colder, and lasted for a longer time (e.g., Waugh et al., 1999; Zhou et al., 2000).

It is widely accepted that the activity of planetary waves (PW) plays a crucial role in modulating the occurrence of the ST (e.g., Newman et al., 2001; Polvani and Waugh, 2004). And PW activity is modulated by factors such as stratospheric sudden warming (SSW) (e.g., Liu et al., 2009) and the quasi-biennial oscillation (QBO) (e.g., Holton and Tan, 1982; Reid, 1994).

This article investigates the dynamic stratospheric ST and is organized as follows: Section 2 describes the data and methods. A definition of the ST can be found in section 3. The time and duration of the ST are presented in section 4. The interannual variability of the ST is presented in section 5, and its relationship with the SSW andthe QBO are presented in sections 5.1 and 5.2, respectively. The conclusion is summarized in section 6.

2 Data and method

The daily mean European Centre for Medium-Range Weather Forecasts (ECMWF) ReAnalysis Interim (ERAInterim) dataset (1979-2011) (Dee et al., 2011) was used with a horizontal resolution of 1.5°×1.5°. We identified the SSW (e.g., Andrews et al., 1987) following the World Meteorological Organization, which requires the reversal of the westerlies at 60°N and 10 hPa. As a quasi-periodic oscillation of the equatorial zonal wind between easterlies and westerlies in the tropical stratosphere (e.g., Baldwin et al., 2001), the QBO is defined by the average zonal wind at 30 hPa over the equator.

3 Definition of ST

We modified the method of spatial similarity coefficient (Zeng and Zhang, 1992; Xue et al., 2002) to define the ST. Essentially, this approach measures the similaritybetween a meteorological field at any time and a typical winter field, so we can judge how far this moment is from winter.

The zonal wind velocity,U(θ,λ,p,t), varies with longitude (θ), latitude (λ), pressure (p), and time (t). The stratospheric winter-to-summer ST is actually a change of zonal wind from strong westerly to easterly. Traditionally, the typical winter field,Uw, and summer field,Us, are defined as the averageUin January and July, respectively. But as shown in Fig. 1a, an example of zonal wind evolution at 70 hPa in 1989-1990, January is not always characterized by the strongest westerly wind which actually occurs in different times at different latitudes. So we selected 31 consecutive days in which the averageUwas maximum (period between two blue lines), then we defined the averageUin these 31 days as the winter field at this level of this year. By calculating the average of the winter fields in 33 years, the typical winter field,Uw(θ,λ,p), was eventually found to be a variable of space. Using a similar method, we defined the typical summer field (Us) by looking for the minimum of the zonal wind.

To highlight the anomaly, the average of the typical winter and summer fields,U*= (Uw+Us)/2, was subtracted. This gave

within a given latitudinal areaA((θ)∈A), the similarity betweenU′ andUw′ is

Figure 1 (a) The evolution of zonal-mean zonal wind (contour) at 70 hPa (units: m s?1), 1989-1990. The period between the two blue (red) lines is a month (31 days) with the strongest westerly (easterly) wind. (b) The evolution of similarity between daily zonal wind and winter zonal wind field (Rw) at 70 hPa and 51°N in 1990.

where the inner product and the norm betweenU′ andare defined as follows:

Also, the similarity betweenU′ and′is

It is obvious thatRw=?Rs, and it can be proved that if the two fields,andU′, are exactly the same, thenRw= +1; if the fields are similar in reverse,Rw=?1 andRw= 0 if they are not relevant.

Therefore, we can useRwonly to define the seasons as follows:

In this paper, we calculatedRw(λ,p,t) for each latitude belt (1.5° of latitude). For example, Fig. 1b shows the evolution ofRwat 70 hPa and 51°N in 1990 in whichRwdecreases from about 1.0 in January to about ?1.0 in July. However, the fluctuating variation of the ST shows up in the evolution ofRw, which does not decrease monotonically. Actually,Rwexceeds the transition value (?0.5 or 0.5) multiple times. Here, we defined the last time it was smaller than 0.5 (?0.5) (red dots) as the start (end) of the ST (red lines). So, the time of the ST depends on time, latitude, and pressure.

4 Time and duration of the winter-to-summer ST in the stratosphere

The climatological mean (1979-2011) time and duration of the ST (Fig. 2) change with year, pressure level and latitude. The start of the ST (Fig. 2a) occurs first in February in the region of 50-60°N at about 10 hPa. This is because of the strong PW activity in this region which accelerates the decay of the westerly wind. The farther away from this region, the later the ST occurs. The latest ST occurs below 30 hPa south of 30°N, and a large horizontal gradient appears in the latitudes around 40°N. The end of the ST has a dominant horizontal difference (Fig. 2b) with the largest gradient between 40°N and 50°N. The ST ends earlier at higher latitudes in the upper stratosphere and later at around 30°N in the middle and lower stratosphere. We obtained the duration of the ST (Fig. 2c) by subtracting the start date of the ST from the end date. The ST has a long duration of more than 75 days in the region of 40-50°N between 10 and 30 hPa, but in most areas it is less than 60 days.

Figure 2 The climatological mean (1979-2011) (a) start dates, (b) end dates, and (c) duration of the stratospheric winter-to-summer seasonal transition (ST) period in the Northern Hemisphere defined by ERA-Interim zonal wind velocity (U) (units: (a, b) day of year; (c) day).

Compared to the traditional method that uses a fixed time (March-April-May) and duration (three months) for the ST, our method shows the spatial differences. These differences reveal the non-uniform evolutions of zonal wind in different areas of the stratosphere during the ST period.

5 Interannual variability of the winter-tosummer ST

Figure 3 The evolution of polar (70.5-90°N) zonal-mean zonal wind (contours) at 10 hPa (units: m s?1) in 32 years: the start (black solid line) and end (black dash line) dates of the ST at 70.5°N, 10 hPa; and the stratospheric final warming (SFW) date (red solid line) and the date of vortex displacement stratospheric sudden warming (SSW) events (black circles) and vortex split SSW events (black dots). All the variables correspond to the left coordinate. The duration of the ST (white dashed line) corresponds to the right coordinate (units: day).

An example (see Fig. 3) of the 10-hPa time and duration of the ST at 70.5°N (the core latitude of the stratospheric polar vortex) shows a pronounced interannual variability which is similar to the trend of the SFW. The SFW event is taken as a sign that the polar vortex has finally broken down and the zonal wind transition has changed from westerlies to easterlies. These are identified as the final time that the 10-hPa zonal-mean zonal wind at 70.5°N drops below zero without returning to the threshold value (10 m s?1). As one of the definitions of the ST, the SFW successfully marks the time that the vortex breaks down and the wind reverses; however, it ignores the duration of the ST. Take 2009 for example: the 10-hPa zonal wind in the polar region (70.5-90°N) began to reverse with the onset of the SSW event in late January. From then, the zonal wind remained either easterly or as a weak westerly, which is exactly a characteristic of the ST, but the SFW event of this year happened in early May (close to the end of the ST). So, the long duration of this ST (nearly 100 days) had not been found by the SFW, but it had been seen clearly between the start and end of the ST.

5.1 Impact of SSW on the winter-to-summer ST

Factors such as SSW and the QBO could have complex influences on the ST. The occurrence of a late SSW (i.e., after 22 January) was found to have a good correlation with the start of the ST. As SSW is characterized by the weakening of the westerly wind, the later the SSW happens, the less likely it can recover. Instead, it keeps on weakening and is finally replaced by the summer easterly wind. In contrast, the wind returns to westerly after early SSW events (before 22 January), such as in 1982 and 1988. The three longest durations of ST happened in 1987, 2001, and 2009 because of the relatively early start of the ST with the onset of SSW events.

In addition, we divided SSW into two categories: vortex-displacement SSW and vortex-split SSW (e.g., Cohen and Jones, 2012). It is likely that the vortex-split SSW events affect the start of ST more frequently than vortex-displacement ones.

5.2 The impact of the QBO on the winter-to-summer ST

Considering the relationship between the ST and the QBO, we performed a composite analysis using the five strongest westerly and easterly QBO winters based on the definition in section 2. We compared the differences of the start/end date anomalies and the zonal-mean zonal wind between these two types. The start date anomaly of the ST displays a sandwich-like distribution (Fig. 4a). More specifically, for a westerly QBO, the ST occurs earlier than the climatological date in midlatitudes and later in high and low latitudes. The distribution for an easterly QBO (Fig. 4b) is a mirror image of the westerly. The influence of the QBO at the end date of the ST (Figs. 4c and 4d) is similar to the influence on the start date, but smaller. Furthermore, Figs. 4a and 4b indicate a similar distribution for the duration of the ST (Fig. 2c) suggesting that the duration of the ST is also dominated by the dynamics related to the QBO.

This regional influence can be interpreted using the Holton-Tan oscillation (Holton and Tan, 1980). It suggests that the correlation between the QBO and the extratropical zonal wind in the stratosphere is negative at midlatitudes and positive at both low and high latitudes. So, the easterly QBO strengthens the wind (positive anomaly) in midlatitudes and weakens the zonal wind (negative anomaly) in high latitudes (Fig. 4f). Therefore, the easterly QBO delays the ST in midlatitudes and hastens it in the high and low latitudes. Conversely, a westerly QBO produced exactly the opposite influence (Fig. 4e). Since the Holton-Tan oscillation is most significant in winter, it is no longer evident as we move into summer. As a result, the influence of the QBO on the end of the ST is weaker.

6 Conclusions

On the basis of the modified spatial similarity coefficient, the stratospheric winter-to-summer ST in the Northern Hemisphere was analyzed using ERA-Interim reanalysis data. Affected by PW activity, the ST occurs as early as February in the 50-60°N with the longest duration in the midlatitudes. Compared to the traditional definition, our definition focuses more precisely on the spatial difference of the stratospheric dynamic evolution. Based on this definition, the dynamic and chemical processes that occur in different locations during the ST period will be studied in the future.

Figure 4 The composite zonal-wind-based ST start date anomaly in the Northern Hemisphere in (a) westerly QBO winters and (b) easterly QBO winters, and end date anomalies in the Northern Hemisphere in (c) westerly QBO winters and (d) easterly QBO winters (units: day). The zonal-mean zonal wind anomalies at 30 hPa in the Northern Hemisphere in (e) westerly QBO winters and (f) easterly QBO winters (units: m s?1); the white lines are the start and end date of the ST. The anomaly contours statistically significant at the 0.05 level (based on the Student’st-test) are shown as shadow regions.

The time and duration of the ST display pronounced interannual variability. Compared to SFW, our ST can show the duration of the transition period, and we find it is strongly affected by SSW and the QBO. The late SSWevents (occurring after 22 January) are strongly correlated to the start of the ST. The QBO plays a regionally different role in the ST, which can be explained by the Holton-Tan oscillation. The easterly winter QBO strengthens the zonal wind in midlatitudes and weakens it in the high latitudes. Therefore, an easterly QBO delays the transition in midlatitudes and hastens the onset of the ST in high and low latitudes, while the westerly QBO produces exactly the opposite effect.

As a dramatic event in the middle and high latitudes of stratospheric winter, the onset of SSW significantly determines the start of the ST in high and middle latitudes. And as a tropical phenomenon, the influence of the QBO on the ST in midlatitudes is stronger than in high latitudes. These influencers adjust the stratospheric dynamic through PW activity, and these adjustments will further influence the troposphere and the chemical field which will be studied in the future.

Acknowledgements. This work was funded by the National Basic Research Program of China (Grant No. 2010CB428604), the National Natural Science Foundation of China (Grant No. 41105025), and the Dragon 3 Programme (Grant No. 10577). The ERA-Interim reanalysis was kindly provided by ECMWF.

Andrews, D. G., J. R. Holton, and C. B. Leovy, 1987:Middle Atmosphere Dynamics, Academic Press, London, 489pp.

Baldwin, M. P., M. Dameris, and T. G. Shepherd, 2007: How will the stratosphere affect climate change?Science, 316, 1576-1577.

Baldwin, M. P., L. J Gray, T. J. Dunkerton, et al., 2001: The quasi-biennial oscillation,Rev. Geophys., 39, 179-229.

Black, R. X., and B. A. McDaniel, 2007: Interannual variability in the Southern Hemisphere circulation organized by stratospheric final warming events,J. Atmos. Sci., 64, 2968-2974.

Choi, Y., Y. Wang, T. Zeng, et al., 2008: Springtime transitions of NO2, CO, and O3over North America: Model evaluation and analysis,J. Geophys. Res., 113, D20311, doi:10.1029/2007 JD009632.

Cohen, J., and J. Jones, 2012: Tropospheric precursors and stratospheric warmings,J. Climate, 25, 1780-1790.

Dee, D. P., S. M. Uppala, A. J. Simmons, et al., 2011: The ERAInterim reanalysis: Configuration and performance of the data assimilation system,Quart. J. Roy. Meteor. Soc., 137, 553-597.

Hess, P. G., and J. R. Holton, 1985: The origin of temperal variance in long-lived trace constituents in the summer stratosphere,J. Atmos. Sci., 42, 1455-1463.

Holton, J. R., and H. C. Tan, 1980: The influence of the equatorial quasi-biennial oscillation on the global circulation at 50 Mb,J. Atmos. Sci., 37, 2200-2208.

Holton, J. R., and H. C. Tan, 1982: The quasi-biennial oscillation in the Northern Hemisphere lower stratosphere,J. Meteor. Soc. Japan, 60, 140-148.

Karpetchko, A., E. Kyro, and B. M. Knudsen, 2005: Arctic and Antarctic polar vortices 1957-2002 as seen from the ERA-40 reanalyses,J. Geophys. Res., 110, D21109, doi:10.1029/2005 JD006113.

Labitzke, K., 1982: On the interannual variability of the middle stratosphere during the northern winters,J. Meteor. Soc. Japan, 60, 124-139.

Labitzke, K., and H. Vanloon, 1988: Associations between the 11-year solar-cycle, the QBO and the atomosphere. Part 1. The troposphere and stratosphere in the Northern Hemisphere in winter,J. Atmos. Terr. Phys., 50, 197-206.

Labitzke, K., and H. Vanloon, 1992: Association between the 11-Year solar cycle and the atmosphere. Part 5. Summer,J. Climate, 5, 240-259.

Liu, Y., C. X. Liu, H. P. Wang, et al., 2009: Atmospheric tracers during the 2003-2004 stratospheric warming event and impact of ozone intrusions in the troposphere,Atmos. Chem. Phys., 9, 2157-2170.

Newman, P. A., E. R. Nash, and J. E. Rosenfield, 2001: What controls the temperature of the Arctic stratosphere during the spring?J. Geophys. Res., 106, 19999-20010.

Polvani, L. M., and D. W. Waugh, 2004: Upward wave activity flux as a precursor to extreme stratospheric events and subsequent anomalous surface weather regimes,J. Climate, 17, 3548-3554.

Reid, G. C., 1994: Seasonal and interannual temperaturevariations in the tropical stratosphere,J. Geophys. Res., 99, 18923-18932.

Sherstyukov, O. N., A. D. Akchurin, and E. Yu Zykov, 1997: Spring stratospheric circulation transition and mid-latitude sporadic E-layer,Adv. Space Res., 20, 1313-1316.

Wang, Y., P. Konopka, Y. Liu, et al., 2012: Tropospheric ozone trend over Beijing from 2002-2010: Ozonesonde measurements and modeling analysis,Atmos. Chem. Phys., 12, 8389-8399.

Waugh, D. W., and P. P. Rong, 2002: Interannual variability in the decay of lower stratospheric Arctic vortices,J. Meteor. Soc. Japan, 80, 997-1012.

Waugh, D. W., W. J. Randel, S. P. Paul, et al., 1999: Persistence of the lower stratospheric polar vortices,J. Geophys. Res., 104, 27191-27201.

Xue, F., Y. Lin, and Q. Zeng, 2002: On the seasonal division of atmospheric general circulation and its abrupt change. Part III: Climatology,Sci. Atmos. Sinica, 26, 307-314.

Zeng, Q., and B. Zhang, 1992: On the seasons of general atmospheric circulation and their abrupt changes. Part I: General concept and method,Chinese J. Atmos. Sci.(in Chinese), 16, 641-647.

Zhou, S. T., M. E. Gelman, A. J. Miller, et al., 2000: An inter-hemisphere comparison of the persistent stratospheric polar vortex,Geophys. Res. Lett., 27, 1123-1126.

:Zhang, Y.-L., Y. Liu, and C.-X. Liu, 2014: Dynamic seasonal transition from winter to summer in the Northern Hemisphere stratosphere,Atmos. Oceanic Sci. Lett., 7, 180-185,

10.3878/j.issn.1674-2834.13.0082.

Received 16 October 2013; revised 15 November 2013; accepted 3 December 2013; published 16 May 2014

LIU Yi, liuyi@mail.iap.ac.cn