Northern Hemisphere Stratospheric Polar Vortex Extremes in February ...

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ATMOSPHERIC AND OCEANIC SCIENCE LETTERS, 2012, VOL. 5, NO. 3, 183−188

Northern Hemisphere Stratospheric Polar Vortex Extremes in February under the Control of Downward Wave Flux in the Lower Stratosphere WEI Ke and CHEN Wen Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100190, China Received 2 December 2011; revised 11 January 2012; accepted 30 January 2012; published 16 May 2012

Abstract Using ECWMF ERA-40 and Interim reanalysis data, the planetary wave fluxes associated with the February extreme stratospheric polar vortex were studied. Using the three-dimensional Eliassen-Palm (EP) flux as a measure of the wave activity propagation, the authors show that the unusual warm years in the Arctic feature an anomalous weak stratosphere-troposphere coupling and weak downward wave flux at the lower stratosphere, especially over the North America and North Atlantic (NANA) region. The extremely cold years are characterized by strong stratosphere-troposphere coupling and strong downward wave flux in this region. The refractive index is used to examine the conception of planetary wave reflection, which shows a large refractive index (low reflection) for the extremely warm years and a small refractive index (high reflection) for the extremely cold years. This study reveals the importance of the downward planetary wave propagation from the stratosphere to the troposphere for explaining the unusual state of the stratospheric polar vortex in February. Keywords: stratospheric sudden warming, atmospheric dynamics, middle atmosphere, planetary wave reflection Citation: Wei, K., and W. Chen, 2012: Northern Hemisphere stratospheric polar vortex extremes in February under the control of downward wave flux in the lower stratosphere, Atmos. Oceanic Sci. Lett., 5, 183–188.

Recently, the “stratospheric bridge” concept was used to indicate the upward planetary propagation from the troposphere to stratosphere above the Eurasian continent and North Pacific (ECNP) and the downward propagation from the stratosphere to the troposphere over the North America and North Atlantic (NANA) in late winter (January-February) (Zyulyaeva and Zhadin, 2009; Jadin et al., 2010). In contrast to numerous studies (e.g., Dunkerton and Baldwin, 1991; Polvani and Waugh, 2004; Labitzke and Kunze, 2009), the three-dimensional (3-D) Eliassen-Palm (EP) fluxes (Plumb, 1985) were used to identify the downward wave signal, the generation of which can be associated with a reflection of planetary waves in the upper stratosphere (Zyulyaeva and Zhadin, 2009). The variation of the stratospheric bridge is coupled with the stratosphere-troposphere interaction processes; the downward wave flux, especially, can be a key indicator of the total wave flux and the strength of the stratospheric polar vortex in late winter. Thus, the stratospheric bridge variability can be a possible mechanism responsible for the strength variations of the stratospheric polar vortex. The aim of this work is to examine the unusual and extreme Februaries in the Arctic stratosphere from the viewpoint of the stratospheric bridge.

2 1

Introduction

Data and method

The variations of the stratospheric polar vortex are influenced by external factors. This influence can be illustrated by the Holton-Tan (HT) relationship (Holton and Tan, 1980), which exhibits an association between the warm (cold) polar vortex and the easterly (westerly) Quasi-Biennial Oscillation (QBO), and the Labitzke and van Loon (LvL) correlation (Labitzke and van Loon, 1988), which shows negative (positive) correlations between the stratospheric polar vortex and the solar activity in the westerly (easterly) QBO years. The variations of the stratospheric polar vortex are also influenced by internal atmospheric dynamics (Charney and Drazin, 1961; Matsuno, 1970; Plumb, 1985; Andrews et al., 1987). Labitzke and Kunze (2009) suggested that the internal atmospheric variability is important and most likely the factor that determines the state of the polar vortex in extreme years; however, the mechanism of internal variability is not yet completely understood.

The atmospheric general circulation data were taken from the European Centre for Medium-Range Weather Forecasts’ (ECMWF) 45-year (1957–2002) ERA-40 reanalysis dataset (Uppala et al., 2005). The data after August 2002 were derived from the ECMWF Interim reanalysis dataset (Simmons et al., 2007). The monthly and daily mean temperature, geopotential height, and horizontal velocity were used. As a diagnostic tool, the EP flux is a measure of the wave activity propagation (Andrews et al., 1987). We used both two-dimensional (2-D) and 3-D EP flux (Plumb, 1985) in this study. Because the vertical component of the monthly mean EP fluxes (EPz) has a well-defined longitudinal structure (Zyulyaeva and Zhadin, 2009), we averaged them in 50−70°N for simplicity. The EPz is defined by the formula ⎧ 2Ω sin 2ϕ ⎡ p 1 ∂ (T 'φ ') ⎤ ⎫ EPz = cos ϕ ⎨ ⎢v ' T '− 2Ω a sin 2ϕ ∂λ ⎥ ⎬ , p0 S ⎣ ⎦⎭ ⎩

Corresponding author: WEI Ke, [email protected]

where a is the radius of the Earth, Ω is the Earth’s rotation rate, (ϕ, λ) are latitude and longitude, p is pressure,

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p0=1000 hPa, v is meridional wind, T is temperature, primes denote zonal deviation and overbars denote zonal average. The term S is the static stability ∂Tˆ κ Tˆ S= + , ∂z H where the caret denotes an areal average (the region poleward of 20°N), and κ is the ratio of the gas constant R to the specific heat at constant pressure Cp, H is a constant-sale height (7 km). The extreme warm (weak) polar vortex years are 1958, 1987, 2004, 2006, 2009, and 2010, and the extreme cold (strong) polar vortex years are 1964, 1967, 1974, 1976, 1997, and 2000. The Februaries of these years correspond to the classification of Labitzke and Kunze (2009) and Gray et al. (2010), where 1987, 2004, and 2006 are the eQBO years and 1964, 1974, and 1976 are the wQBO

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years under low solar (LS) activity. These correlations are in good accordance with those of the LvL. However, February 2009 is a wQBO/LS year that does not correspond to either the HT relationship or the LvL correlations. The same is true for February 1997 (eQBO/LS year), which had a very cold polar vortex. The years 1958 and 2000 are wQBO years under high (HS) and mid-solar activity.

3

Results and discussion

To understand the cause of the unusual polar vortex from the viewpoint of the stratospheric bridge, we plotted the EPz values averaged between 50°N and 70°N in January, February, and March for the extremely warm and cold polar vortex years (Fig. 1). For the chosen extremely

Figure 1 EPz averaged between 50−70°N at 50 hPa for extremely warm polar vortex years (left column) and extremely cold polar vortex years (right column). The upper panels are for the preceding January, the middle panels are for February, and the lower panels are for the following March. The extremely warm years (left column) are 1987 (blue), 2004 (green), 2009 (red), 2010 (yellow), 2006 (purple), and 1958 (orange). The extremely cold years (right column) are 1967 (yellow), 1974 (green), 1976 (red), 1997 (blue), 1964 (purple), and 2000 (orange). The shading in each panel denotes the range of the EPz climatology value with a ±1 standard deviation. The two gray rectangles outline the longitudes for the Eurasian continent and North Pacific (ECNP) and the longitudes for North America and the North Atlantic (NANA), respectively.

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warm February years, the stratospheric sudden warming occurred entirely in January. Therefore, January is featured with a stronger upward wave flux over the upward EPz flux region (the Eurasian continent and the North Pacific (ECNP)). The downward EPz flux over the North America and North Atlantic (NANA) region is smaller than the climatological average or reversal on the upward wave flux, which is most prominent in January 2009. This result means that there is a very small transfer of the wave energy from the stratosphere to the troposphere together with the wave energy’s large upward transport from the troposphere to the stratosphere over north Eurasia in January for these warm years. In the selected warm Februaries, the coupling between the stratosphere and the troposphere are very weak, especially in the climatological downward flux region. The EPz fluxes over the NANA region are very small and, in most years, are positive, opposite the climatology value. The upward fluxes over the ECNP region are also weak, indicating a decoupling between the stratosphere and troposphere. The opposite upward flux over the NANA region forms a “blocking regime” of the stratospheric bridge (Jadin et al., 2010), which helps to conserve the eddy energy in the stratosphere and favors the maintenance of the warming state of the stratospheric polar vortex. This decoupling lasts from February to March for these extremely warm February years, with a very weak wave flux between the stratosphere and the troposphere. A downward wave flux is hardly observed over the NANA region, indicating the continuity of the blocking regime of the stratospheric bridge and the warm state of the polar vortex. The unusual February 2009 features a strong upward EP flux over the NANA region, which is actually the strongest among these extremely warm polar vortex years, making February 2009 the warmest month in recorded history (Labitzke and Kunze, 2009; Manney et al., 2009). In March 2009, the strong upward EP flux over the North Atlantic was maintained, favoring the unusual prolongation of the warm polar vortex. For the extremely cold years, the stratosphere and the troposphere are strongly coupled in February, with a very strong downward EPz flux over the NANA region. The EPz flux over the other regions has large variations among the extremely cold years. A cold February is led by a very strong downward EP flux over the North Atlantic in January. January features a weaker upward EP flux over the ECNP region in most extremely cold years. Therefore, a cold and strong stratospheric polar vortex is preconditioned with less wave energy into the stratosphere from the troposphere and with more wave energy output from the stratosphere to the troposphere. The main difference between the warm and cold years is that the much stronger transfer of the wave energy from the stratosphere to the troposphere for the cold years in the downward EPz flux region indicates a “ventilation regime” of the stratosphere for these years (Jadin et al., 2010). The net EPz flux is the total wave flux effect in the middle stratosphere, represented by the difference be-

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tween EPz at 50 hPa and 10 hPa (EPz at 50 hPa minus EPz at 10 hPa), which is an indication of the convergence of vertical wave energy in this region. At each level, the EPz flux comprises upward (+) and downward (−) components. The net EPz flux in the middle stratosphere is plotted for the extremely warm (Fig. 2a) and cold (Fig. 2b) polar vortex years from the preceding October to the following April. The net EPz flux is usually positive in the extended winter months (ONDJFM), with a maximum value in January, which is balanced by an EP flux in the meridional direction and the radiative cooling effect. The net EPz flux has the largest variation in January, corresponding to the mid-winter wave anomalies, which are usually associated with mid-winter stratospheric sudden warming. For years with an extremely warm polar vortex in February, the net EPz flux is normal in the preceding October and November, indicating that the late winter months are not related to the early winter months. However, the net EPz flux has negative anomalies in the preceding December. The total net EPz flux is less than the climatology values in these extremely warm polar vortex years, which favors the acceleration of the polar vortex, indicating that the polar vortex warming in January is usually preconditioned by a stronger cold polar region. In January and February, the net EPz flux has a greater convergence than the climatology values, which leads to the stratospheric warming in January and the prolongation of the warm polar vortex in February. The upward EPz flux at 50 hPa (Fig. 2c) is larger than the climatology in January, indicating the importance of the upward EPz at 50 hPa for the forcing of the stratospheric polar warming in January, while most of the years have smaller values than the climatology in February. The downward EPz flux at 50 hPa (Fig. 2e) features a weaker downward EP flux in January and February; therefore, the stratosphere-troposphere dynamical coupling in January is mainly in the upward EPz region. The stratosphere is decoupled with the troposphere and features both weaker upward and downward EPz fluxes in February. The total EPz wave flux (Fig. 2g) at 50 hPa dominates the variation of the net EPz flux in the extremely warm years, while the EPz flux at the upper 10 hPa helps to shape the exact feature in some years. For example, the total wave flux at 10hPa (Fig. 2m) is anomalously large in December 2004, resulting in the weak net flux in Fig. 2a. The upward flux is weakest in February and March 2009 at 10 hPa (Fig. 2i), together with the weakest downward EPz flux at 50 hPa (Fig. 2e), which helps to confine the energy in the stratosphere, leading to the warmest polar vortex. There are no systematic features for the net flux in the cold polar vortex years; therefore, the cold polar vortex is not determined by the vertical component of the EP flux, suggesting the possible importance of the horizontal component of the EP flux. It is worth noting that the downward EPz is anomalously weak for all the years in February and March at 50 hPa (Fig. 2f), corresponding to a stronger coupling between the stratosphere and troposphere in the downward EPz region, i.e., the NANA region.

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Figure 2 (From top to bottom) (a and b): the net EPz flux (50 hPa minus 10 hPa) averaged between 50−70°N; (c and d) the upward EPz flux at 50 hPa; (e and f) the downward EPz flux at 50 hPa; (g and h) the total EPz flux at 50 hPa; (i and j) the upward EPz flux at 10 hPa; (k and l) the downward EPz flux at 10 hPa; (m and n) the total EPz flux at 10 hPa. The left column shows the extremely warm polar vortex years: 1987 (blue), 2004 (green), 2009 (red), 2010 (yellow), 2006 (purple), and 1958 (orange). The right column shows the extremely cold polar vortex years: 1967 (yellow), 1974 (green), 1976 (red), 1997 (blue), 1964 (purple), and 2000 (orange). The lines with a plus sign denote the climatology with a range of ±1 standard deviation (shading).

The above wave flux analysis illustrates the importance of the atmospheric internal dynamics in the state of the late winter polar vortex. Previous studies (Harnik and Lindzen, 2001; Perlwitz and Harnik, 2003) showed that

the wave propagation can be influenced by the state of the zonal flow, as represented by the refractive index: U (2 hPa) − U (10 hPa) , where U is the zonal-

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mean wind averaged over 58−74°N and over time, where a negative index corresponding to a reflective state and a polar night jet peaking in the mid-stratosphere and a positive index corresponds to a non-reflective state with the zonal wind at mid to high latitudes increasing with increasing height. The climatology reflective index (Table 1) has its maximum value in October (14.7 m s−1) and decreases from early winter to late winter with its minimum value occurring in March (1.2 m s−1). The reflective index has large interannual variation. The standard deviations in January (7.5 m s−1), February (9.9 m s−1), and March (8.8 m s−1) are much larger than those in the other months. Therefore, the upward planetary waves are less reflected in early winter and the polar vortex is less disturbed, while the upward planetary waves are more reflected and the polar vortex is more disturbed in mid to late winter. The reflective state is very different in the extremely warm and cold polar vortex years. In the extremely warm years (Fig. 3a), the reflective indexes have positive anomalies in February and March, indicating a nonreflective state with less wave energy reflected downward to the troposphere. For the extremely warm polar vortex in February and March 2009, the reflective index is 17 m s−1 and 27 m s−1 in February and March, much larger than the climatology values of 3.8 m s−1 and 1.2 m s−1. Therefore, the wave propagates upward without downward reflection and downward energy flux, which favors wave energy convergence in the stratosphere and warming. For the extremely cold years (Fig. 3a), the reflective indexes are biased to negative values in February and March, indicating the favored condition for the reflective state, which leads to downward wave flux from the stratosphere to the troposphere, wave energy divergence and atmospheric cooling. Table 1 The average (Ave) and standard (Std) deviations of the refractive index (Ri) in the extended winter months. Ri

Oct

Nov

Dec

Jan

Feb

Mar

Apr

Ave

14.7

10.8

8.2

4.4

3.8

1.2

3.9

Std

3.4

3.5

5.7

7.5

9.9

8.8

6.3

4

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Concluding remarks

This study investigates the internal dynamics of the extremely warm and cold polar vortexes. Wave flux analysis shows that the stratosphere-troposphere wave coupling is weak in the extremely warm polar vortex years, especially the weak downward flux over the NANA region, which favors wave energy convergence in the stratosphere and helps maintain the warming state. Meanwhile, the stratosphere-troposphere wave coupling is strong in the extremely cold polar vortex years, with stronger downward EP flux over the NANA region, favoring wave energy divergence and polar vortex cooling. The wave flux budget analysis shows that a warm February is usually preceded by a strong stratospheric wave flux convergence (causing stratospheric warming) in January and is accompanied by a wave flux convergence in February. A consistently weak downward (near zero) wave energy flux occurs for all the extremely warm years at the lower stratosphere. The cold February, however, is characterized by a stronger downward wave flux at the lower stratosphere. For the strongest and most prolonged warming that occurred in February 2009, there are unusual upward wave fluxes over the NANA region in January and February 2009 (Figs. 1a and 1c), where downward EPz fluxes are usually observed. These conditions resulted in the absence of the transfer of wave energy from the stratosphere to troposphere in January and February, which helps to accumulate the wave energy in the stratosphere. This unusual absence of the downward wave energy could be the reason why February 2009 is an outlier of both the HT relationship and the LvL correlations (Labitzke and Kunze, 2009). Although external factors such as the QBO, solar cycles, and El Niño can influence the state of the polar vortex, the internal dynamical processes act to modulate the final state of the polar vortex, especially the internal dynamics over the region where the downward wave fluxes dominate. This downward wave flux may have an effect on the stratosphere-troposphere coupling process associated with the North Atlantic Oscillation; however, further analysis is needed in a future study.

Figure 3 The reflective index for (a) the extremely warm polar vortex years: 1987 (blue), 2004 (green), 2009 (red), 2010 (yellow), 2006 (purple), and 1958 (orange); and (b) the extremely cold polar vortex years: 1967 (yellow), 1974 (green), 1976 (red), 1997 (blue), 1964 (purple), and 2000 (orange). The black line with plus sign represents the climatology value and the shadings indicating a range of ±1 standard deviation.

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Acknowledgements. We would like to thank two anonymous reviewers for their helpful comments and suggestions. This study is supported by the National Basic Research Program of China (973 Program) (Grant No. 2010CB428603), the National Natural Science Foundation of China (Grant Nos. 40805017 and 41175041).

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