Stratospheric “wave hole” and interannual variations of the stratospheric circulation in late winter

doi:10.4236/ns.2011.34033

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Vol.3, No.4, 259-267 (2011) Natural Science

http://dx.doi.org/10.4236/ns.2011.34033

Stratospheric “wave hole” and interannual variations of

the stratospheric circulation in late winter

Evgeny A. Jadin

P. P. Shirshov Institute of Oceanology, Moscow, Russia; [email protected]

Received 11 November 2010; revised 20 December 2010; accepted 20 January 2011.

ABSTRACT

Using the monthly mean NCEP dataset, the

analysis of the upward and downward propaga-

tion of planetary waves was conducted by

means of the three-dimensional Eliassen-Palm

(EP) fluxes in the stratosphere. It is shown that

the upward/downward EP fluxes are observed in

different regions of the atmosphere: their well-

known upward propagation takes place over

North Eurasia, while the downward one revealed

over North Atlantic and Canada in a region of

the so-called stratospheric “wave hole”. Gen-

eration of the downward wave signal may be

associated with a reflection of planetary waves

in the upper stratosphere. It is shown that the

downward EP flux responsible for the sink of

eddy energy from the stratosphere to the tro-

posphere is important in late winter (Janu-

ary-February) for an understanding of the stra-

tosphere-troposphere coupling on the interan-

nual and decadal timescales, in particular the

11-year solar cycle influence on the strato-

sphere. Results presented can explain the un-

usual behavior of a few winters in the Arctic

stratosphere, which are outlier from the known

Labitzke, van Loon’s correlations of strato-

spheric parameters with the 11-year solar cycle

under separation in the west/east phases of the

equatorial quasi-biennial oscillation.

Keywords: Stratosphere; Planetary Wave Fluxes

1. INTRODUCTION

During last decade, a big attention is paid to investi-

gations of the stratosphere and its linkage with the tro-

posphere and weather events. It is connected with recent

understanding that stratospheric processes can affect the

climate change not only due to the ozone layer depletion,

but also because of the stratosphere-troposphere dy-

namical interaction. In addition, there is a possibility to

improve the extended-range forecast of extreme weather

events using stratospheric predictors [1]. The lifetime of

dynamical disturbances in the stratosphere is much long-

er (~ one month) than that in the troposphere (~3 - 10

days) and the downward propagation of the zonal wind,

temperature and geopotential height is observed from the

stratosphere to the troposphere [2]. This is a basis to

improve the predictions of extreme weather events in

distinct regions using stratospheric predictors.

In spite of a large progress in research of the strato-

sphere-troposphere coupling, the causes of the down-

ward propagation of the stratospheric signal are not good

understood [3]. Planetary waves responsible for the stra-

tosphere-troposphere coupling can result in the down-

ward signal propagation similar to that during strato-

spheric warmings. However, the downward propagation

of stratospheric parameters is observed not only during

stratospheric warmings, but also for usual conditions in

the stratosphere [4]. The two-dimensional (2D) Elias-

sen-Palm flux diagnostics of planetary wave propagation

and their forcing on the zonal wind can not describe the

downward planetary wave signal from the stratosphere

to the stratosphere because it is responsible only for the

upward wave signal from the troposphere [5]. Zyulyaeva

and Jadin [6], Jadin and Zyulyaeva [7] used the three-

dimensional (3D) Eliassen-Palm (EP) fluxes [8] to sepa-

rate the upward/downward wave signals and obtained

the first direct confirmation of the downward propaga-

tion of planetary waves in the stratosphere. Analysis of

the monthly mean 3D vertical EP fluxes indicated the

existence of the “stratospheric bridge” forming by the

upward planetary wave propagation from the tropo-

sphere over North Eurasia in early winter (December)

and the downward wave signal from the stratosphere to

the troposphere over North Atlantic and Canada in late

winter (January-February). It was also shown that the

interannual variations of planetary wave activity in early

winter influence strongly variations of the stratospheric

circulation in subsequent January.

Holton and Tan [9] have shown that the warm (cold)

E. A. Jadin / Natural Science 3 (2011) 259-267

260

vortex in the Arctic stratosphere is usually observed in

winter for years with the east (west) phase of the equato-

rial quasi-biennial oscillation (QBO) (Holton – Tan rela-

tionship). There are large difficulties in the understand-

ing of the Holton – Tan relationship and its decadal vio-

lation [10]. Another unresolved question is the explana-

tion of an influence of the 11-year solar cycle on the

stratosphere under the separation of years with the

west/east phases of the QBO [11]. Labitzke and van

Loon [11] indicated significant correlations (hereafter

the LvL correlations) of the lower stratospheric tem-

perature at North Pole with the 11-year solar cycle (SC)

under the grouping of the years into the west QBO

(wQBO years) and east QBO (eQBO years) categories.

Relations of stratospheric and tropospheric parameters

with the SC are most prominent in January-February, not

in early winter and for the whole years.

The small (~ 1%) solar irradiance changes from the

maximum to the minimum of the 11-year SC in the

spectrum absorbing by ozone cannot lead directly to

such large variations of the temperature at North Pole (~

20˚C) in the lower stratosphere [11]. These changes are

larger in the solar spectrum shorter than 200 nm (~ 7%)

[12], therefore, the SC radiation effects on the upper

stratospheric circulation can be reasonable. Kodera et al.,

[13] suggested that the 11-year solar signal propagates

downward into the lower stratosphere during the winter-

time due to the planetary wave-zonal flow interaction

resulting in a possible SC influence on the stratospheric

dynamics [14,15]. However, an absence of the clear

physical mechanism of the SC influence on the atmos-

phere could not explain why the LvL correlations are

observed mainly in late winter and why we observe

more stratospheric warmings in high solar (HS) activity

for the wQBO years and in low solar (LS) activity for

eQBO years [16]. A possible mechanism explaining

these questions was recently proposed by Jadin et al. [17]

who showed that the 11-year solar cycle could modulate

the downward wave signal, generating by a reflection of

planetary waves in the upper stratosphere [18].

The LvL correlations are stable during last five dec-

ades from late 1950’s onwards [19], however, there are a

few extreme years which outlier from these correlations.

The warm (cold) polar vortex in the Arctic stratosphere

is usually observed in the LS (HS)/eQBO years, but very

cold vortex took place in February 1997 in contrast with

unusual warm vortex in February 1987 (LS/eQBO years).

Most prominent violation from the LvL correlations oc-

curred in February 2009 (LS/wQBO year) with the ex-

treme warm vortex in the Arctic, though cold vortex was

expected [20].

The aim of this study is to specify the simple physical

mechanism of the stratospheric bridge [6,7,17] and ex-

plain the outliers from the LvL correlations with the

point of view of this mechanism.

2. DATA AND METHOD OF THE

ANALYSIS

The monthly mean data from NCEP/NCAR reanalysis

[21] dataset were used in this study covering the years

from 1958 to 2007. This atmospheric dataset has a 2.5 ×

2.5 horizontal resolutions and extends from 1000 to 10

hPa with 17 vertical pressure levels. The three dimen-

sional vertical Eliassen-Palm fluxes were calculated as a

diagnostic tool to measure the wave activity propagation

[8]. After the separation of the longitudinal disturbances,

the flux deviations from the averages (anomalies) were

calculated for each level and month in 1958-2007, i.e.,

seasonal cycle was removed. The empirical orthogonal

functions (EOF) of the vertical wave flux anomalies at

each standard level from 100 to 10 hPa for December,

January and February were used for the analysis.

3. UPWARD AND DOWNWARD

PROPAGATION OF PLANETARY

WAVES

The stratosphere undergoes itself large internal varia-

tions on the interannual and shorter timescales, which

are connected with the wave forcing on zonal flow and

the accommodation to the radiation equilibrium (vacilla-

tion cycle) [22]. Planetary wave forcing is usually de-

scribed by means of the 2D EP flux diagnostics showing

the upward wave propagation from the troposphere and

its influence on the stratospheric circulation. In contrast

with the 2D Eliassen-Palm fluxes, the 3D Plumb wave

fluxes allow to indicate the downward propagation of

planetary waves in the stratosphere even on the monthly

mean timescales. Moreover, there are significant differ-

ences between wave forcing in early and late winter,

which are associated with the downward wave signal

from the stratosphere to the troposphere [6,7].

Figure 1 shows the monthly mean vertical wave

fluxes (EPz) at 30 hPa, 100 hPa and 500 hPa for Febru-

ary averaged in 1959-2007. In the lower stratosphere,

there are the upward wave propagation from the tropo-

sphere over North Eurasia and the downward EPz flux

over North Atlantic and Canada with a smaller intensity.

The downward wave flux has fewer magnitudes in early

winter (November-December) than that in late winter

(January-February) and it is disappeared in March [6].

Upward and downward propagations of planetary waves

in the stratosphere have almost barotropic structures

over North Eurasia and North Atlantic and Canada (re-

gion of the stratospheric “wave hole”), respectively.

Such seesaw feature is pertaining to the EPz fluxes in the

E. A. Jadin / Natural Science 3 (2011) 259-267

261

Figure 1. Climatology of vertical Plumb wave flux (EPz) (m2/s2 × 103) at 30 hPa (a), 100 hPa (b) and 500 hPa (c) in February

1959-2007.

lower and middle stratosphere up to 5 hPa level, above

which the downward wave flux appears to be not ob-

served on the average [17].

Instead the stratospheric “wave hole”, the upward

wave flux takes place in the middle troposphere (Figure

1(c)), that is consistent with the results of Plumb [8] who

revealed the second source of planetary wave excitation

in the North Atlantic together with the first one con-

nected with the Tibet topography excitation. The spatial

structure of the interannaul variations of the wave fluxes

in the lower and middle stratosphere are similar to their

averages shown in Figure 1 and represent the seesaw

between the North Eurasia and the North Atlantic in late

winter. Because the upward/downward propagation of

planetary waves are mainly in the 40˚N - 90˚N latitu-

dinal belt without significant latitudinal shifts in the

lower stratosphere (Figure 1), we can average the EPz

flux in this belt in order to study the longitudinal EPz

seesaw and its influence on the stratospheric circulation.

Figure 2 shows the first EOF spatial patterns in de-

pendence on the longitude and their principal compo-

nents (PCs) of the EPz anomalies for each month in

1958/59-2003/04. Calculations of the EPz wave flux for

January 2005 indicated an unrealistic feature in the

NCEP data; therefore, the analysis was restricted up to

2004. For convenience, December years denote as the

next January years, i.e. December 1959 means Decem-

ber 1958, for example. These leading modes give the

largest contributions to total variance 53%, 43% and

41% for December, January and February, respectively.

Similar modes are peculiar to 100 hPa height, but with

smaller contributions, therefore, the 30 hPa level is chosen

E. A. Jadin / Natural Science 3 (2011) 259-267

262

Figure 2. First EOF of the EPz anomalies averaged in the 40˚N - 90˚N belt at 30 hPa for December (black-bk), January (red-rd)

and February (blue-bl) in 1958/59-2003/04 (upper panel). Their PCs are shown below (a,b,c). Red (blue) stars show Januaries

(Februaries) with the major stratospheric warmings, respectively.

for an identification of the upward/downward propaga-

tion of planetary waves.

Spatial patterns of the EPz anomalies are almost iden-

tical in January and February, but with different time

series. They describe the interannual seesaw anomalies

between the upward planetary wave propagation over

North Eurasia and the downward ones over North Atlan-

tic and Canada (Figure 1). However, there is a large

difference of the spatial pattern in December with those

in late winter in a region of the stratospheric “wave

hole”. The seesaw is almost absent in December and

interannual wave flux variations behave itself over North

Atlantic in phase with those over North Eurasia, in gen-

erally. This does not mean that the downward wave sig-

nal is not observed in December; it can occur in distinct

years with the negative PC in December.

Extreme positive PC in December 1959, 1969, 1976,

1984, 1993, 1997 and 2002 correspond very well to the

appearance of major stratospheric warmings and the

warm vortex in the Arctic in next January (Figure 2(a)).

This finding is consistent with results of [6,7] who have

shown that the increase (decrease) of the upward pene-

tration of planetary waves from the troposphere in De-

cember leads to the warm (cold) stratospheric vortex in

the Arctic in next January.

This is similar to the well-known “preconditions” of

the stratospheric warming appearances [5,23,24], but on

the monthly timescales. Such monthly “preconditions”

E. A. Jadin / Natural Science 3 (2011) 259-267

263

are much less remarkable in a linkage of the wave flux

variability in January with the major warming occur-

rence in February (Figure 2(b)) because of the appear-

ance of the stratospheric “wave hole” in January-February.

In spite of its smaller magnitude in comparison with

upward wave fluxes (Figure 1), the downward propaga-

tion of planetary waves over North Atlantic and Canada

plays an important role in the interannual variations of

the stratospheric circulation especially in late winter.

Influence of the interannual variations of stratospheric

“wave hole” on the circulation in late winter is most

prominent for unusual years which are not correspond-

ing to the LvL correlations. There exist the extreme

warm (cold) Februaries in the stratosphere Arctic in

1987 (1997), which belong to the east QBO years and

the low solar activity [20]. Figure 3 shows the zonal

wind anomalies in February 1987, 1997 and the EPz

values at 30 hPa averaged north of 40˚N for January and

February these years. Strong easterly (westerly) anoma-

lies of the polar jet are observed in February 1987 (1997)

that results in anomalous warm (cold) vortex in the Arc-

tic stratosphere. Notice, that there are no extreme peaks

in the PCs of the EPz anomalies in January-February

1987 and 1997 (Figure 2(b), (c)). Taking into account

the long lifetime of the wave-zonal mean flow interac-

tion, this can imply that the polar jet in February de-

pends on the wave activity in previous January. In addi-

tion, the first EOF of the EPz anomalies cannot describe

(a) (b)

Figure 3. Zonal wind anomalies (m/s) at 30 hPa in February 1987 (a) and 1997 (b) and the EPz values (m2/s2 × 103) averaged in the

40˚N - 90˚N belt (c, d), respectively.

E. A. Jadin / Natural Science 3 (2011) 259-267

264

anomalous behaviors of the stratospheric “wave hole”

for extreme years in late winter, because it is responsible

for the seesaw of the upward/downward wave signal.

Indeed, Figure 3 shows the anomalous large upward

wave flux in January together with an absence of the

stratospheric “wave hole” both in January and February

1987. Analysis of the interannaul variations of the EPz

values showed that such situation is very rare in

1959-2007. In contrast with 1987, the strong downward

wave signal takes place over North Atlantic in January -

February 1997 that leads to the anomalous cold vortex in

February 1997.

As mentioned above, the extreme warm vortex in the

Arctic stratosphere was observed in February 2009 [20].

This year belongs to the west QBO and low solar cate-

gory and, therefore, the cold vortex is expected. Thus,

the unusual behavior of the polar vortex in February

2009 contradicts to the Holton – Tan relationship and the

LvL correlations. Can this abnormal event be explained

with the point of view of the stratospheric bridge con-

cept? Figure 4 shows the EPz values at 50 hPa in Feb-

ruary 1976 and 2009 in comparison with 1987 and 1997

(Figure 3(c), (d)). The polar vortex in February 1976

was very cold that is consistent with the LvL correlations

[20]. The general feature of the warmest polar vortices in

February 1987 (LS/eQBO year) and 2009 (LS/wQBO

year) is an absence of the downward wave fluxes in pre-

vious January as well as in February. In contrast, the

coldest vortices in February 1976 (LS/wQBO year) and

1997 (LS/eQBO year) had the large downward fluxes

from the stratosphere to the troposphere in January and

February. Influence of the large downward flux in De-

cember 2009 on the polar vortex in February 2009 is

probably small because of the restricted lifetime (~ one

month) in the planetary wave and zonal flow interaction

[2].

Results shown above have the simple physical inter-

pretation. Because the EPz flux describes the eddy en-

ergy transport [8], the upward wave flux over North

Eurasia is responsible for the source of eddy energy

from the troposphere into the stratosphere, while the

downward one over North Atlantic and Canada implies

Figure 4. The vertical wave flux at 50 hPa in dependence on the longitude in December (blue line), January (green line) and Febru-

ary (red line) for 1976 (a), 1997 (b), 1987 (c) and 2009 (d). Units are arbitrary.

E. A. Jadin / Natural Science 3 (2011) 259-267

265

the eddy energy sink from the stratosphere to the tropo-

sphere. Interannual variations of the stratospheric circu-

lation and polar vortex in the Arctic depend strongly on a

variability of the stratospheric bridge [6] combining the

upward and downward wave fluxes which are response-

ble for the eddy energy exchange between the tropo-

sphere and stratosphere. Formation of the stratospheric

bridge is beginning in early winter (November - Decem-

ber) by the upward wave fluxes over North Eurasia with

small or negligible downward fluxes over North Atlantic

and Canada in the region of the stratospheric “wave

hole” [17]. In late winter (January - February), the

stratospheric circulation is controlled not only by the

eddy energy accumulation in previous month (source of

eddy energy from the troposphere), but also by the eddy

energy sink from the stratosphere to the troposphere

through the stratospheric “wave hole”. The lag between

the upward wave flux forcing and stratospheric circula-

tion response is explained by the long lifetime (~ one

month) of the wave-mean flow interaction and the vacil-

lation cycle. Figure 5 illustrates the scheme of the

stratospheric bridge and stratospheric “wave hole” for

extreme years.

The strong (weak) “pumping” of the eddy energy from

the troposphere in December results in the warm (cold)

vortex in the Arctic in January. Further development of

the stratospheric circulation depends strongly on the

eddy energy sink from the stratosphere to the tropo-

sphere through the stratospheric “wave hole” over North

Atlantic and Canada in January and February. If even the

upward wave flux in January is weak, but the strato-

spheric wave hole is “closed”, then the eddy energy ac-

cumulated in previous December is blocked from a sink

to the troposphere and the warm vortex is observed. This

is “blocking regime” of the stratosphere-troposphere

coupling [17]. In the opposite case, when the strato-

spheric hole is “opened”, the strong sink of eddy energy

from the stratosphere leads to the cold vortex in the Arc-

tic in spite of a strong upward wave flux in January

(“ventilation regime”). This simple mechanism reminds

the pumping of the air-balloon with a hole. Of course,

this is only two extreme regimes; the stratosphere-tro-

posphere interaction is more complex especially in late

winter.

According to this mechanism, very warm vortex in

February 1987 and 2009 is caused by strong penetration

Figure 5. Scheme of the influence of the upward wave flux (red arrows) and downward wave flux (blue arrows) in the lower strato-

sphere on the warm/cold vortex in the Arctic. See text for explanation.

E. A. Jadin / Natural Science 3 (2011) 259-267

266

of eddy energy from the troposphere without any sink

from the stratosphere in previous January and the ab-

sence of the stratospheric “wave hole” in February

(Figure 4(c), (d)). In contrast, the large sink of eddy

energy from the stratosphere to the troposphere through

the “wave hole” over North Atlantic in January and

February resulted in very cold vortex in the Arctic in

February 1976 and 1997. The large eddy energy accu-

mulated in January 1987 and 2009 was blocked in the

stratosphere (extreme “blocking regimes” in January and

February) (Figure 5). Because the 11-year solar cycle

cannot modulate the upward wave signal [17], this win-

ter outliers from the LvL correlations. Controversially,

the extreme “ventilation regimes” take place in January

and February 1976 and 1997. A downward displacement

of a reflective surface from the upper stratosphere may

be a cause of the violation of the LvL correlations in this

extreme year. Thus, unusual winters in the Arctic strato-

sphere can be explained by extreme regimes in the be-

havior of the stratospheric bridge in late winter.

4. CONCLUDING REMARKS

Many external and internal factors affect the Earth’s

atmosphere from the bottom and the top. One can take to

the external factors the anthropogenic impacts, oceanic

forcing and the 11-year solar cycle influence. Internal

factors are associated with a large atmospheric variabi-

lity due to the non-linear interaction of zonal mean and

wave processes in the atmosphere. There are numerous

feedbacks between the external and internal factors that

result in large difficulties in the understanding of a rela-

tive role of anthropogenic and natural factors in the in-

terannual and decadal variations of the ozone layer and

climate changes [25,26]. Long-term (“bottom-up”) sig-

nal of an oceanic forcing on the atmosphere can be more

remarkable in the stratosphere than in the troposphere,

because this signal in the troposphere is strongly masked

by the high-frequency weather variability. The “up-down”

influence of the 11-year solar cycle on the stratosphere

also is difficult to be identifying because of its small

irradiance change and a large internal variability of the

stratosphere-troposphere coupling.

Results presented here confirm the concept of the

stratospheric bridge and stratospheric “wave hole” over

North Atlantic and Canada in regions of the downward

planetary wave propagation from the stratosphere to the

upper troposphere [6,7]. This mechanism can be useful

for the better understanding of both the “bottom-up” and

“up-down” influences on the stratosphere-troposphere

coupling not only on the interannual timescales, but also

on decadal ones. The violation of the Holton-Tan rela-

tionship is occurring during a decadal period from the

mid-1970’s to mid-1990’s [10]. Jadin et al. [27] first

indicated that the interannual and decadal variations of

the upward wave flux in December are strongly associ-

ated with the sea surface temperature (SST) anomalies in

the North Pacific (Pacific Decadal Oscillation - PDO

[28]) in early winter during 1958 - 1979 and 1992 - 2007.

It is intriguing that a decadal period from the mid -1970s

to mid-1990s of the Holton-Tan relationship violation

corresponds well to that of the positive PDO phase (an-

omalously cold SSTs in the central North Pacific). No

similar relations between the SST anomalies in the North

Pacific and North Atlantic were found in late winter.

This can mean that two-way ocean-atmosphere interac-

tion is different in early (November - December) and late

(January - March) winter and it is associated with the

development of the stratospheric bridge.

An absence of the modulation of the stratospheric

circulation by the solar cycle in early winter can be also

connected with this oceanic forcing on the thermal exci-

tation of the stationary planetary waves on a longer than

the 11-year decadal timescales. In late winter (January -

February), the 11-year solar cycle can modulate a re-

flecting surface on which the downward wave signal is

generated due to changes of the dynamics in the upper

stratosphere caused by the different ultraviolet absorbing

of the ozone. Jadin et al. [17] indicated significant cor-

relations of the PC1 of the EPz anomalies with the

11-year solar cycle index (F10.7 cm irradiance) corre-

sponding well to the Labitzke and van Loon correlations.

This can mean that the 11-year solar cycle signal can

indeed influence the lower stratosphere, and possibly the

troposphere because of an amplification of this signal in

the stratosphere-troposphere coupling.

Further observational studies and model simulations

are needed for a better understanding of influences of the

ocean and the Sun on the stratosphere and possible rela-

tions between the extreme stratospheric vortices and

extreme weather events.

5. ACKNOWLEDGEMENTS

The author thanks the anonymous referee for useful remarks.

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