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Response of the extratropical middle atmosphere to the September 2002 major stratospheric sudden warming
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ScienceDirect Advances in Space Research 53 (2014) 257–265 www.elsevier.com/locate/asr
Response of the extratropical middle atmosphere to the September 2002 major stratospheric sudden warming A. Guharay a,⇑, P.P. Batista a, B.R. Clemesha a, S. Sarkhel b b
a National Institute for Space Research, INPE, Sa˜o Jose´ dos Campos, Sa˜o Paulo, Brazil Radar Space Sciences Laboratory, The Pennsylvania State University, 323 Electrical Engineering East, PA, USA
Received 14 June 2013; received in revised form 18 September 2013; accepted 1 November 2013 Available online 10 November 2013
Abstract The effects of a major stratospheric sudden warming (SSW) at extratropical latitudes have been investigated with wind and temperature observations over a Brazilian station, Cachoeira Paulista (22.7°S, 45°W) during September–October 2002. In response to the warming at polar latitudes a corresponding cooling at tropical and extratropical latitudes is prominent in the stratosphere. A conspicuous signature of latitudinal propagation of a planetary wave of zonal wavenumbers 1 and 2 from polar to low latitude has been observed during the warming period. The polar vortex which split into two parts of different size is found to travel considerably low latitude. Significant air mass mixing between low and high latitudes is caused by planetary wave breaking. The meridional wind exhibits oscillations of period 2–4 days during the warming period in the stratosphere. No wave feature is evident in the mesosphere during the warming period, although a 12–14 day periodicity is observed after 2 weeks of the warming event, indicating close resemblance to the results of other simultaneous investigations carried out from high latitude Antarctic stations. Convective activity over the present extratropical station diminishes remarkably during the warming period. This behavior is possibly due to destabilization and shift of equatorial convective active regions towards the opposite hemisphere in response to changes in the mean meridional circulation in concert with the SSW. Ó 2013 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Stratospheric sudden warming; Planetary waves; Convective activity
1. Introduction Major stratospheric sudden warming (SSW) is a dramatic feature of the winter stratosphere due to a split of the polar vortex leading to an increase of the temperatures by more than 15 K and reversal of the zonal mean zonal wind (Dowdy et al., 2007; Guharay and Sekar, 2012). Following the onset of the SSW event it takes about 4–6 weeks to restore the pre-warming zonal circulation state (Schoeberl, 1978). In general, major warming takes place once every 2 years in the northern hemisphere, but such an occurrence is not observed in the southern
⇑ Corresponding author. Tel.: +55 12302 87182; fax: +55 12302 86990.
E-mail address: [email protected] (A. Guharay).
hemisphere except for the warming event which occurred in September 2002. Such hemispheric disparity of the SSW can be attributed to the strength of the planetary wave activity. Strong forcing due to orography is supposed to cause greater planetary wave activity in the northern hemisphere (Andrews et al., 1987). It is theorized that planetary waves of zonal wavenumbers 1 and 2 from the lower atmosphere are mainly responsible by providing the necessary energy through wave mean flow interaction for sustaining such warming in the stratosphere (Matsuno, 1971). Matsuno (1971) explained that such interaction between planetary waves and mean flow decelerates the eastward winter stratospheric jet and forces downward flow in the stratosphere, thus creating adiabatic heating, and in the mesosphere it causes upward circulation leading to adiabatic cooling.
0273-1177/$36.00 Ó 2013 COSPAR. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.asr.2013.11.002
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The large amplitude Rossby waves (westward planetary waves) propagating through the eastward wind regime can break and cause intrusion of stratospheric wind of high potential vorticity (PV) into the tropical upper troposphere region of low PV (Waugh and Polvani, 2000). Waugh and Polvani (2000) also found a close relationship between PV and deep convection, with PV intrusion preceding the convection most of the time over the tropical eastern Pacific. For the last couple of decades a handful of investigations of the SSW event have been carried out using lidar (Sivakumar et al., 2004; Sridharan et al., 2010), airglow (French et al., 2005), radar (Espy et al., 2005; Dowdy et al., 2004), rockets (Johnson, 1969) and satellite-based (Fritz and Soules, 1970; Hu and Fu, 2009; Mbatha et al., 2010) observations. Unfortunately, most of the previous studies were carried out from northern hemisphere high, mid and low latitude stations and there are only a very few reports available so far related to the southern hemisphere. On the basis of observations during the September 2002 southern hemispheric major SSW, Kru¨ger et al. (2005) showed the role of interaction between eastward propagating waves and quasi-stationary planetary waves in the troposphere in causing preconditioning of the polar vortex breaking and subsequent warming in the stratosphere. Dowdy et al. (2007) carried out comparative studies of the SSW utilizing observations from two stations from the northern hemisphere and three stations from the southern hemisphere and they found weaker and earlier mesospheric zonal wind reversal, as compared to the stratosphere, with significant resemblance between the two hemispheres. Recently, Hu and Fu (2009) conducted an extensive study of the southern hemispheric SSW using a long-term database for 1979–2006 and reported strong warming during September–October. They inferred that an increase of sea surface temperature due to greenhouse gases could be responsible for such warming in recent decades. As has already been mentioned, very few investigations have been carried out so far from the southern hemisphere, especially from low latitudes, and our knowledge of such events is limited. Although a few studies are available on the September, 2002 SSW event which is believed to be the strongest warming episode to date in the southern hemisphere, so far, no report has been published on the signature of the SSW seen from a low latitude station. Therefore, in our present study we have looked into the planetary wave related dynamical response of the extratropical middle atmosphere to the September, 2002 major SSW utilizing meteor radar wind observations from Cachoeira Paulista (22.7°S, 45°W).
2. Observational database For the present work we have utilized various groundbased and space-based data sets during the period Day-
244 to Day-304, i.e. 1st September–31st October which are described below. 2.1. Meteor radar The meteor radar system at Cachoeira Paulista is a SKiYMET type which operates at a frequency of 35.24 MHz. It utilizes one three-element Yagi transmitting antenna and five two-element Yagi receiving antennas. Receiving antennas are arranged in an interferometric array of two orthogonal baselines with one antenna common to both. Details of the method used for estimating the horizontal wind from the backscattered meteor echoes can be found elsewhere (Hocking et al., 2001). The derived values of horizontal wind within the altitude range 81– 99 km with a vertical resolution of 3 km and temporal resolution of 1 h are used for further analysis in the present study. Short data gaps are filled up by linear interpolation. 2.2. ERA-interim database ERA-interim dataset provided by the European Centre for Medium-range Weather Forecasts (ECMWF) is basically reanalysis data from 1979 to the present time of various atmospheric parameters (Berrisford et al., 2009). For the present purpose we have used zonal wind, meridional wind, vertical wind and temperature at 37 pressure levels within the range 1000–1 hPa (0–48 km) with a latitudinal and longitudinal grid of 1° 1° and over Cachoeira Paulista the closest grid point is chosen as (23°S, 45°W). 2.3. TIMED/SABER data Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) is one of the four instruments onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite which measures limb emission from the atmosphere within the band 1.27– 17 lm using 10 channels. Kinetic temperature profiles are retrieved from the CO2 emission lines at 15 lm and 4.3 lm (Mertens et al., 2001). For our present study we have used V1.07 level 2A temperature dataset. The temperature profiles within the latitude-longitude grid of 10° 10° centered around the Cachoeira Paulista are considered for the present work. 2.4. NCEP OLR data Outgoing longwave radiation (OLR) recorded by the NOAA polar orbiting satellite based instrument Advanced Very High Resolution Radiometer – are provided by National Centers for Environmental Prediction (NCEP) (Kalnay et al., 1996). The data interpolated at 2.5° 2.5° latitude longitude grid are utilized as a proxy for convective activity. For the present work we consider OLR values less than 200 W m 2 as representative of deep convective
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clouds at the closest grid point coordinate (22.5°S, 45°W) over Cachoeira Paulista. 3. Results Differences between zonal mean zonal temperature and the temporal mean (July–October) of zonal mean zonal temperature at 10 hPa pressure level in the southern hemisphere, using the ECMWF dataset, are plotted during the interval 1 September to 31 October, 2002 (Day-244 = 1 September) in Fig. 1a. At high latitude (>60°S) the temperature difference is mostly negative during Day-244 to Day266 and becomes positive within a very short time, larger than 23 K around Day-270, indicating strong heating. From this point on the temperature difference remains almost positive. At lower latitudes the difference remains very low, except below 35°S latitude near Day-270 when a comparatively high negative difference ( 9° K) is observed, implying a temporary cooling. In the present latitude (23°S) the cooling lasts for approximately 3 days. The zonal mean zonal wind during the above-mentioned period is shown in Fig. 1b. Bold curve in the plot denotes zero value. At mid and high latitude the wind starts to turn easterly from Day-265 and remains unchanged for about 10 days before it turns to westerly again near Day-275. In general, at low latitude (<25°S) the zonal mean zonal wind is easterly most of the time. It can be noted that two
Fig. 1. (a) Difference of zonal mean zonal temperature and temporal mean of zonal mean zonal temperature during September–October 2002 (Day-244 1st September) and (b) zonal mean zonal wind plotted during same period at 10 hPa pressure level using ECMWF. Bold curve represents zero value in the plot. Region between two vertical lines shows peak warming period for the present and all the following figures.
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distinct patches of westerly wind exist near 20°S latitude just before and end of the warming period. Therefore both zonal wind and temperature indicate the impact of the major SSW at the present observing latitude. The peak warming period determined as per world meteorological organization definition, i.e. span of easterly wind near 60°S at 10 hPa level (Andrews et al., 1987), i.e. the region between the two vertical bold lines in all the plots of the present paper. Breaking of planetary waves causes mixing of air masses among adjacent regions of the atmosphere (Abatzoglou and Magnusdottir, 2006). Maps of potential vorticity (PV) can give important insights into the air mass mixing within various latitudes/longitudes. We have shown the PV evolution with ECMWF data at 10 hPa in the southern hemisphere over the temporal span centered about the peak warming (Day-270) in Fig. 2 taking reverse sign using the unit PVU (1 PVU = 10 6 K m2 kg 1 s 1). On Day-263 (Fig. 2a) the PV map shows well stratified layers of increasing PV magnitude with latitude and small scale low latitude to mid latitude intrusion represented by thin tongues. At high latitude the stretched highest (over all latitude range on this particular day) magnitude contour (yellow and deep green) is found to form a thin strip which is discontinuous near longitude 250–300°E, possibly signifying the onset of the split of the polar vortex. It can be noted that a tongue
Fig. 2. Evolution of the potential vorticity using ECMWF at 10 hPa (shown with opposite sign) in PVU (1 PVU = 10 6 K m2 kg 1 s 1) on (a) Day-263, (b) Day-267, (c) Day-270, (d) Day-273 and (e) Day-277.
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of low PV intrudes into the mid latitude obliquely near 20°S, 300°E (i.e. fairly close to our observing location). After 4 days, on Day-267 (Fig. 2b) the high PV strip has shortened its longitudinal length further concentrating over 30°E. A number of patches of low PV are visible at high latitudes which originate at low latitudes and spread to high latitudes due to large scale mixing. Hereafter, on Day-270 (Fig. 2c), the vortex core is found to be completely split into two segments with higher PV magnitude which appears narrow, rounded and shifted to low latitude, centered on about 40°E and 260°E separately. Asymmetry in the vortex structure is prominent as the vortex near 40°E longitude is broader than the other. Low PV from low latitude intrudes well across the high latitude regions. The previously mentioned tongue near 300°E has broadened and become oriented almost directly to the higher latitude nearing one of the split vortexes. Therefore a well-mixed state of air mass is evident in the last two cases. On Day273 (Fig. 2d) the broader vortex is found to shift eastward, the narrower vortex moves to lower latitude and a prominent connection is established between the two vortexes through a narrow strip of comparatively higher PV. Another low PV strip is observed to spiral around the narrower vortex near 260°E longitude. Subsequently, four days later, on Day-277 (Fig. 2e), the situation is quite similar to the initial state of a single vortex core represented by stretched high magnitude PV contour at high latitude, possibly due to diminution of the warming. A number of thin tongues are still visible during the recovery phase indicating weakening of the event.
Fig. 3. (a) Amplitude of planetary waves for various wavenumbers in ECMWF temperature (K) at 10 hPa height and 60°S latitude. The latitudinal variation of amplitude for (b) zonal wavenumber 1 and (c) zonal wavenumber 2.
In order to study planetary wave activity during this episode we have utilized ECMWF temperature at 10 hPa for calculating the amplitude of planetary waves of zonal wavenumbers 1–10 for the periods 2–20 days at 60°S which is plotted in Fig. 3a. To estimate it first, the temperature time series (1 day resolution) at each longitude grid for latitude of 60°S is undergone a band pass filter of cut offs at 2 and 20 days. Hereafter the filtered data is undergone least square sinusoidal fit (in longitude domain) for various zonal wavenumbers for each day to obtain amplitude corresponding to a particular day and wavenumber. The plot clearly shows significant dominance of zonal wavenumbers 1 and 2 during pre-warming tenure and diminishes remarkably during the warming period. To elaborate the role of the prevalent wave components we have shown evolution of the zonal wavenumbers 1 and 2 in Fig. 3b and c, respectively. Both planetary wavenumbers exhibit significantly high amplitudes at high latitudes before the onset of the warming event. It is interesting to note that the wavenumber 1 component diminishes very rapidly during the warming episode and remains sufficiently weak for several weeks after the event. Another important feature is the extension of the wave activity towards the low latitude during the major SSW, although the amplitude is smaller than high latitude. Following the warming episode the wave amplitude reduces at low latitude, recovering the pre-warming state. The zonal wavenumber 2 component remains active before and during the warming episode and subsequently decays at high latitude. The zonal wavenumber 2 wave also shows weak extension towards low latitude one week later (Day-277). The diminution of the wave amplitude after the warming event is slower in the case of wavenumber 2 than wavenumber, 1 indicating a stronger contribution of the wavenumber 1 component to the warming. To determine the periodicity of the planetary waves over Cachoeira Paulista, wavelet analysis has been carried out using Morlet as a mother wavelet. The wavelet power spectra for zonal wind (meteor radar), meridional wind (meteor radar) and temperature (SABER) at 90 km are shown in Fig. 4a–c and the same parameters at 10 hPa (ECMWF) are shown in Fig. 4d–f. Bold curves in each plot represent 90% significance level. Wave activity is larger (in terms of power) at 90 km (mesopause) as compared to 31 km (stratosphere). Although all the spectra in the mesopause exhibit several wave components in the 2–14-day band before and after the warming, wave activity is found to be significantly less during the warming episode. Stratospheric spectra, especially for the meridional wind, reveal prominent wave signatures in the 2–4-day band during the warming period although wave activity is generally weak most of the time. Dynamical condition over Cachoeira Paulista has been looked into in terms of temperature (ECMWF), vertical wind (ECMWF) and OLR which are plotted in Fig. 5a– c. In general, temperature shows well stratified layers at various altitudes in the stratosphere and troposphere. Above 40 km there is an evident signature of decrease of
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Fig. 4. Wavelet power spectra at 90 km for (a) zonal wind (m/s), (b) meridional wind (m/s) using meteor radar, (c) temperature (K) using SABER. Corresponding spectra at 10 hPa are shown for (d) zonal wind (m/s), (e) meridional wind (m/s), (f) temperature (K) over Cachoeira Paulista using ECMWF. Bold curves in each plot represent 90% significance level.
Fig. 5. ECMWF (a) Temperature (K), (b) vertical wind (Pa/s) in stratosphere and troposphere and (c) outgoing longwave radiation (W/ m2) over Cachoeira Paulista.
temperature which starts just before the onset of the warming and recovers its normal state by the end of the warming episode. Although cold-point tropopause (15–18 km) shows an increase of temperature just before the warming it recovers quickly and remains stable during the warming episode. The vertical wind has been plotted within the range 0–15 km as above 15 km both the value and variability are too small to be shown elaborately. The vertical wind ( ve values represent upward direction) is generally weak hovering around zero values most of the time except at the lowest altitude where upward wind is dominant most of the time. It can be seen from the plot that there is an upward vertical wind over the entire altitude range just before the onset of the warming event. Another instance of upward wind is visible near Day-275 (one week after the initiation of the warming). OLR shows strong convection (<200 W m 2) just before and after the warming event, but during the warming episode it shows lower convection (>230 W m 2). The background horizontal wind is an important factor in controlling the vertically propagating waves through wave-mean flow interaction. The wave can propagate through the atmosphere without dissipation if the background horizontal wind is opposite to the wave direction. If the wave propagation direction is the same as the horizontal wind direction the wave suffers critical layer
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interaction and deposits energy in the mean flow owing to dissipation. As Rossby (westward planetary) waves propagate along the easterly direction, zonal wind is expected to play an important role in filtering the waves propagating from the lower atmosphere. Daily mean zonal and meridional winds in the mesopause region (80–100 km using meteor radar) and the stratosphere–troposphere (0–50 km using ECMWF) over Cachoeira Paulista are shown in Fig. 6a–d. Zonal wind in the lower mesopause region is mostly easterly and at greater heights it changes its direction with time. During the warming period the zonal wind is mainly easterly over the whole altitude range. The meridional wind is mainly northerly at lower mesopause heights and in the upper mesosphere it is mostly southerly. During the initial warming period the meridional wind is generally weak and gets stronger at a later phase. The horizontal winds in the stratosphere–troposphere show some interesting features. The zonal wind shows a strong westerly jet near 12 km which continues till the onset of the warming episode. The wind weakens very rapidly as warming initiates and reverses to easterly within the next two weeks (Day-282) and continues for the next couple of days before recovery to westerly on Day-296. On the contrary, the zonal wind over the surface and upper stratosphere (30–45 km) remains easterly mostly. At the top of the stratosphere (45–50 km) the zonal wind is strongly westerly, lasting for a month centered around the warming period. Meridional wind near 12 km shows northerly flow for a couple of days before the onset of the warming. It
reverses to southerly shortly after the onset of warming and by the end of the episode it becomes stronger. Above 35 km the meridional wind remains northerly during the initial phase of the warming and turns to southerly at the final phase. 4. Discussion Our present study has illustrated some important dynamical aspects of the major SSW event from the southern hemisphere during September-2002 with observations from an extratropical station in the Brazilian sector. To a large extent, the present investigation owes its importance to the fact that this is the strongest major SSW from the southern hemisphere known so far. The major warming at the polar latitude corresponds to a cooling at the present observing latitude. Stratospheric cooling at tropical latitudes in the northern hemisphere was first reported by Fritz and Soules (1970) with NIMBUS III satellite observations. They concluded that the cooling was caused by heat transfer due to a change in meridional circulation and large-scale eddies. Upward propagating waves from the troposphere causes upwelling of the air and thus can produce adiabatic cooling (Yoshida and Yamazaki, 2011). Equatorward planetary waves traveling from high latitudes may dissipate in the tropical lower stratosphere and can drive upwelling and associated cooling (Yoshida and Yamazaki, 2011). Yoshida and Yamazaki (2011) also mentioned that convection related
Fig. 6. Daily mean wind (m/s) at Cachoeira Paulista (a) zonal, (b) meridional in the mesopause region using meteor radar and (c) zonal, (d) meridional in the troposphere–stratosphere using ECMWF.
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vertical heat flux convergence could be another potential factor for such cooling. Dunkerton and Delisi (1986) mentioned that temporal evolution of the size, shape and orientation of the circumpolar vortex is well represented by the PV field. The PV maps of the present study clearly show a split of the vortex into two separate segments far apart from each other during the major SSW. Shifting of a split vortex core towards sufficiently lower latitudes implies significant planetary wave propagation from high to low latitude. Considerable mixing of air mass between low and high latitudes is evident during the peak warming period which is believed to be caused by planetary wave breaking. Observed reversal of zonal mean zonal wind at the present latitude (Fig. 1b) can be attributed to the influence of the planetary waves generated at high latitude. Several tongues of low PV intrusion from the low latitude to higher latitude are observed. In this context it can be mentioned that Palmer (1981) found a change of direction of Eliassen–Palm (EP) flux several times during the warming. They reported that poleward and upward direction of the flux at midlatitudes is very important for large zonal flow deceleration to facilitate warming. Although both components of zonal wavenumbers 1 and 2 planetary waves exhibit significant variability in response to the major SSW, wavenumber 1 shows considerable diminution of its amplitude which persists for long intervals of time even after the end of the warming event. Shepherd (2000) explained that if the vortex goes away from the pole then the “wave-1” component should be responsible and if the vortex is split into two segments then one should expect a “wave-2” component. The present finding of vortex movement from polar to significantly lower latitude with two evident segments during peak warming as shown by the PV maps as well as wavenumbers 1 and 2 (responsible for driving waves 1 and 2) activity is consistent with the notion of Shepherd (2000). Guharay and Sekar (2012) reported propagation of wavenumbers 1 and 2 components from polar latitude to low latitude regions during February, 2007 northern hemispheric SSW similar to the present finding. According to the work of Labitzke (1977) it seems that for causing major warming, zonal wavenumber 1 should be very strong at least for a couple of days to facilitate the pre-warming condition. Labitzke (1977) also noted that zonal wavenumber 2 alone is not sufficient to cause a major warming. Palmer (1981) argued that zonal wavenumber 2 waves are not able to reverse the high latitude zonal wind through critical layer interaction. Johnson (1969) found a significant role of the zonal wavenumber 2 waves in nonlinear/vertical interaction during SSW. Hirota et al. (1990) pointed out the role of interaction of the eastward traveling wavenumber 2 and quasi-stationary forced wavenumber 1 which might lead to quasi-periodic amplification of wavenumber 1. Kru¨ger et al. (2005) mentioned the dominant impact of interaction between eastward traveling waves and quasi-stationary
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planetary waves on the polar vortex breakdown and subsequent warming. A wavelet periodogram for the present extratropical observing station shows weak planetary wave activity in the mesopause during the warming period. Although there is wave activity in the period range 2–14 days scattered over the spectra there are no evident wave components present during the warming period of the mesopause. Zonal wavenumber–frequency spectra of horizontal winds and temperature at 10 hPa (not shown here) revealed dominant westward components. Therefore planetary wave diminution in the mesopause during the warming episode can be attributed to strong dissipation in the stratosphere due to critical layer interaction between wave and mean flow because of easterly zonal wind. Using observations of multiple stations situated over both the hemispheres Dowdy et al. (2007) reported remarkably smaller spectral power of the mesospheric planetary waves as compared to the stratospheric ones during SSW and they explained such reduction in planetary wave power as being due to partial transmission of waves from the stratosphere. Using MF radar observations over three Antarctic stations Dowdy et al. (2004) found a 14 day planetary wave of zonal wavenumber 1 in the mesospheric meridional wind during the same warming period. Later, French et al. (2005) also reported a 14 day planetary wave of zonal wavenumber 1 during the same warming period from OH airglow temperatures over Davis, Antarctica. With radar meridional wind observations in the mesosphere over three Antarctic stations during the 2002 winter Espy et al. (2005) found a long period (43 days) planetary wave of zonal wavenumber 1 before the warming episode. However, after the warming episode they found a wave of much reduced period (14 days). Furthermore, Mbatha et al. (2010) reported a 14 day wave in the mesospheric zonal and meridional wind before the occurrence of the September 2002 SSW with observations from an Antarctic station. In the present scenario such planetary wave features are not observed at extratropical latitudes during or before the SSW, although a strong 12–14 day periodicity is found in the meridional wind after 2 weeks, which is most probably the same wave component as indicated by the contemporary studies carried out from Antarctic stations. This wave would originate at high latitude and reach low latitudes after warming. In the stratosphere we found a 2– 4 day wave during the warming period although its strength is low. In this context it can be mentioned that Sathishkumar and Sridharan (2011) observed an enhanced 2–4 day wave in the troposphere, mesosphere and ionosphere during a 2009 northern hemispheric SSW event, similar to the present finding. Recently, Lima et al. (2012) reported amplification of the 2-day wave activity in the equatorial southern hemisphere mesosphere during a northern hemisphere SSW and they concluded that cross-equatorial dynamical influence of the enhanced winter hemisphere waves in the stratosphere might be responsible for such an occurrence.
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Due to the enhanced planetary wave activity during SSW the stratospheric mean meridional circulation increases which in turn causes cooling in the lower stratosphere and upper troposphere in the tropics. Such cooling can modify the convective activity in the tropics. Our results show high convective activity before and after the warming episode which is supported by simultaneous high upward vertical wind, but during the warming convection is found to be very low. In this context it can be noted that Kodera and Yamada (2004) argued that stronger meridional circulation during SSW does not directly enhance convective activity, rather it triggers and destabilizes a balance between equatorial convective active regions. With northern hemispheric observations of multiple warming events Kodera (2006) noted a seesaw structure of enhanced convection in the southern tropics and reduced convection in the northern tropics due to a shift of equatorial convective activity towards the southern hemisphere following the warming. Therefore such a shift of convective activity to the opposite hemisphere may cause a reduced convection in the low latitude region which continues for several days after the warming as seen in the present case. Westward daily mean zonal wind in the mesopause (80– 100 km) during the warming period is possibly responsible for less planetary wave amplitude owing to critical layer interaction. Daily mean winds over Cachoeira Paulista reveal some notable features in the stratosphere and troposphere. Zonal wind shows strong westerly motion near 12 km before the onset of the SSW which weakens significantly during the peak warming period. Strong westerly wind helps the Rossby waves to propagate upward provided the wind speed is below certain limit known as the critical Rossby velocity (Charney and Drazin, 1961). Upper tropospheric westerly ducts are also favorable to the cross-equatorial wave propagation (Webster and Holton, 1982). Therefore they are important for tropical– extratropical interaction. Breaking of Rossby waves in the westerly wind can cause meridional transport of trace species as well as higher tropical convection (Waugh and Polvani, 2000). The zonal wind between 20 km and 45 km generally shows easterly or very weak westerly motions, indicating wave energy dissipation to the mean flow due to the proximity of a critical layer. An abrupt change of meridional wind direction and magnitude is conspicuous above 35 km and below 15 km during the warming event. It is generally northerly (poleward) during the initial, phase turning to southerly (equatorward) in the final phase of the SSW at these altitude ranges. Carrying out long-term composite analysis of northern hemispheric SSW, with a total of 39 events, Limpasuvan et al. (2004) concluded that modification of anomalous momentum flux due to increased wave activity is responsible for the change of residual mean meridional circulation from poleward under normal conditions to equatorward during the warming period, consistent with the present finding. Such meridional reversal can also be explained as follows. Under normal atmospheric conditions a negative temperature
gradient with respect to latitude causes poleward meridional flow of wind, but during warming periods the polar stratosphere becomes warmer and the tropical stratosphere and upper troposphere become cooler, leading to opposite flow of the meridional wind. Utilizing a long-term database of SSW in the southern hemisphere Hu and Fu (2009) found enhancement of the EP fluxes as well as EP flux convergence during SSW in extratropical latitudes, leading to strong meridional residual circulation associated downwelling and adiabatic heating in the high latitudes. 5. Summary and conclusions The present work points out some dynamical imprints of the rare event of September 2002 SSW observed from a southern hemispheric extratropical station. Although a few studies have investigated this event using high latitude observations, presently, there is hardly any literature related to its impact at low latitude. Planetary waves of zonal wavenumbers 1 and 2 are found to propagate to extratropical latitudes during the warming period. PV maps clearly show the split of the vortex into two segments, with significantly different sizes, and progression of the same toward midlatitudes, leading to significant latitudinal mixing of air mass. Planetary wave activity is considerably less in the mesopause region, as compared to the stratosphere, and a wave component of period 12–14 days is evident after 2 weeks of the warming period as reported by earlier investigators with observations carried out from high latitude stations during the same warming event. A 2– 4 day wave is found to be dominant during the warming period in the stratosphere. Convective activity at Cachoeira Paulista is found to decrease during the SSW, possibly due to a shift of the destabilized active convective regions towards equator. Acknowledgments The authors are grateful to the TIMED/SABER team for providing useful data for the present work. A. Guharay gratefully acknowledges the support from the Fundacßa˜o de Amparo a Pesquisa do Estado de Sa˜o Paulo (FAPESP). S. Sarkhel is supported by the National Science Foundation (NSF), USA grant ATM 07-21613 to The Pennsylvania State University. The ECMWF data is available at the website http://www.ecmwf.int/research/era/do/get/index. Wavelet software was provided by C. Torrence and G. Compo, and is available at URL: http://paos.colorado.edu/research/wavelets/. References Abatzoglou, J.T., Magnusdottir, G., 2006. Planetary wave breaking and nonlinear reflection: seasonal cycle and interannual variability. Journal of Climate 19, 6139–6152. Andrews, D.G., Holton, J.R., Leovy, C.B., 1987. Middle Atmosphere Dynamics. Academic, San Diego, California, pp. 129–130.
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to northern hemisphere phenomena. Journal of the Atmospheric Sciences 62, 565–566. Labitzke, K., 1977. Interannual variability of the winter stratosphere in northern hemisphere. Monthly Weather Review 105, 762–770. Lima, L.M., Alves, E.O., Batista, P.P., Clemesha, B.R., Medeiros, A.F., Buriti, R.A., 2012. Sudden stratospheric warming effects on the mesospheric tides and 2-day wave dynamics at 7°S. Journal of Atmospheric and Solar-Terrestrial Physics 78–79, 99–107. http:// dx.doi.org/10.1016/j.jastp.2011.02.013. Limpasuvan, V., Thompson, D.W.J., Hartmann, D.L., 2004. The life cycle of the northern hemisphere sudden stratospheric warmings. Journal of Climate 17, 2584–2596. Matsuno, T., 1971. A dynamical model of the stratospheric sudden warmings. Journal of the Atmospheric Sciences 28, 1479–1494. http:// dx.doi.org/10.1175/1520- 0469(1971. Mbatha, N., Sivakumar, V., Malinga, S.B., Bencherif, H., Pillay, S.R., 2010. Study on the impact of sudden stratosphere warming in the upper mesosphere–lower thermosphere regions using satellite and HF radar measurements. Atmospheric Chemistry and Physics 10, 3397– 3404. http://dx.doi.org/10.5194/acp-10-3397-2010. Mertens, C.J., Mlynczak, M.G., Lopez-Puertas, M., Wintersteiner, P.P., Picard, R.H., Winick, J.R., Gordley, L.L., Russell III, J.M., 2001. Retrieval of mesospheric and lower thermospheric kinetic temperature from measurements of CO2 15-mm Earth limb emission under nonLTE conditions. Geophysical Research Letters 28, 1391–1394. http:// dx.doi.org/10.1029/2000GL012189. Palmer, T.N., 1981. Diagnostic study of a wavenumber-2 stratospheric sudden warming in a transformed Eulerian-mean formalism. Journal of the Atmospheric Sciences 38, 844–855. Schoeberl, M.R., 1978. Stratospheric warmings: observations and theory. Reviews of Geophysics, Space Physics 16, 521–538. Shepherd, T.G., 2000. The middle atmosphere. Journal of Atmospheric and Solar-Terrestrial Physics 62, 1587–1601. Sathishkumar, S., Sridharan, S., 2011. Observations of 2–4 day inertiagravity waves from the equatorial troposphere to the F region during the sudden stratospheric warming event of 2009. Journal of Geophysical Research 116, A12320. http://dx.doi.org/10.1029/2011JA017096. Sivakumar, V., Morel, B., Bencherif, H., Baray, J.L., Baldy, S., Hauchecorne, A., Rao, P.B., 2004. Rayleigh Lidar observation of a warm stratopause over a tropical site, Gadanki (13.5°N, 79.2°E). Atmospheric Chemistry and Physics 4, 1989–1996. Sridharan, S., Raghunath, K., Sathishkumar, S., Nath, D., 2010. First results of warm mesospheric temperature over Gadanki (13.5°N, 79.2°E) during the sudden stratospheric warming of 2009. Journal of Atmospheric and Solar-Terrestrial Physics 72, 1139–1146. http:// dx.doi.org/10.1016/j.jastp.2010.06.003. Waugh, W.D., Polvani, L.M., 2000. Climatology of intrusions into the tropical upper troposphere. Geophysical Research Letters 27, 3857– 3860. Webster, P.J., Holton, J.R., 1982. Cross-equatorial response to middlelatitude forcing in a zonally varying basic state. Journal of the Atmospheric Sciences 39, 722–733. Yoshida, K., Yamazaki, K., 2011. Tropical cooling in the case of stratospheric sudden warming in January 2009: focus on the tropical tropopause layer. Atmospheric Chemistry and Physics 11, 6325–6336. http://dx.doi.org/10.5194/acp-11-6325-2011.
ScienceDirect Advances in Space Research 53 (2014) 257–265 www.elsevier.com/locate/asr
Response of the extratropical middle atmosphere to the September 2002 major stratospheric sudden warming A. Guharay a,⇑, P.P. Batista a, B.R. Clemesha a, S. Sarkhel b b
a National Institute for Space Research, INPE, Sa˜o Jose´ dos Campos, Sa˜o Paulo, Brazil Radar Space Sciences Laboratory, The Pennsylvania State University, 323 Electrical Engineering East, PA, USA
Received 14 June 2013; received in revised form 18 September 2013; accepted 1 November 2013 Available online 10 November 2013
Abstract The effects of a major stratospheric sudden warming (SSW) at extratropical latitudes have been investigated with wind and temperature observations over a Brazilian station, Cachoeira Paulista (22.7°S, 45°W) during September–October 2002. In response to the warming at polar latitudes a corresponding cooling at tropical and extratropical latitudes is prominent in the stratosphere. A conspicuous signature of latitudinal propagation of a planetary wave of zonal wavenumbers 1 and 2 from polar to low latitude has been observed during the warming period. The polar vortex which split into two parts of different size is found to travel considerably low latitude. Significant air mass mixing between low and high latitudes is caused by planetary wave breaking. The meridional wind exhibits oscillations of period 2–4 days during the warming period in the stratosphere. No wave feature is evident in the mesosphere during the warming period, although a 12–14 day periodicity is observed after 2 weeks of the warming event, indicating close resemblance to the results of other simultaneous investigations carried out from high latitude Antarctic stations. Convective activity over the present extratropical station diminishes remarkably during the warming period. This behavior is possibly due to destabilization and shift of equatorial convective active regions towards the opposite hemisphere in response to changes in the mean meridional circulation in concert with the SSW. Ó 2013 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Stratospheric sudden warming; Planetary waves; Convective activity
1. Introduction Major stratospheric sudden warming (SSW) is a dramatic feature of the winter stratosphere due to a split of the polar vortex leading to an increase of the temperatures by more than 15 K and reversal of the zonal mean zonal wind (Dowdy et al., 2007; Guharay and Sekar, 2012). Following the onset of the SSW event it takes about 4–6 weeks to restore the pre-warming zonal circulation state (Schoeberl, 1978). In general, major warming takes place once every 2 years in the northern hemisphere, but such an occurrence is not observed in the southern
⇑ Corresponding author. Tel.: +55 12302 87182; fax: +55 12302 86990.
E-mail address: [email protected] (A. Guharay).
hemisphere except for the warming event which occurred in September 2002. Such hemispheric disparity of the SSW can be attributed to the strength of the planetary wave activity. Strong forcing due to orography is supposed to cause greater planetary wave activity in the northern hemisphere (Andrews et al., 1987). It is theorized that planetary waves of zonal wavenumbers 1 and 2 from the lower atmosphere are mainly responsible by providing the necessary energy through wave mean flow interaction for sustaining such warming in the stratosphere (Matsuno, 1971). Matsuno (1971) explained that such interaction between planetary waves and mean flow decelerates the eastward winter stratospheric jet and forces downward flow in the stratosphere, thus creating adiabatic heating, and in the mesosphere it causes upward circulation leading to adiabatic cooling.
0273-1177/$36.00 Ó 2013 COSPAR. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.asr.2013.11.002
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The large amplitude Rossby waves (westward planetary waves) propagating through the eastward wind regime can break and cause intrusion of stratospheric wind of high potential vorticity (PV) into the tropical upper troposphere region of low PV (Waugh and Polvani, 2000). Waugh and Polvani (2000) also found a close relationship between PV and deep convection, with PV intrusion preceding the convection most of the time over the tropical eastern Pacific. For the last couple of decades a handful of investigations of the SSW event have been carried out using lidar (Sivakumar et al., 2004; Sridharan et al., 2010), airglow (French et al., 2005), radar (Espy et al., 2005; Dowdy et al., 2004), rockets (Johnson, 1969) and satellite-based (Fritz and Soules, 1970; Hu and Fu, 2009; Mbatha et al., 2010) observations. Unfortunately, most of the previous studies were carried out from northern hemisphere high, mid and low latitude stations and there are only a very few reports available so far related to the southern hemisphere. On the basis of observations during the September 2002 southern hemispheric major SSW, Kru¨ger et al. (2005) showed the role of interaction between eastward propagating waves and quasi-stationary planetary waves in the troposphere in causing preconditioning of the polar vortex breaking and subsequent warming in the stratosphere. Dowdy et al. (2007) carried out comparative studies of the SSW utilizing observations from two stations from the northern hemisphere and three stations from the southern hemisphere and they found weaker and earlier mesospheric zonal wind reversal, as compared to the stratosphere, with significant resemblance between the two hemispheres. Recently, Hu and Fu (2009) conducted an extensive study of the southern hemispheric SSW using a long-term database for 1979–2006 and reported strong warming during September–October. They inferred that an increase of sea surface temperature due to greenhouse gases could be responsible for such warming in recent decades. As has already been mentioned, very few investigations have been carried out so far from the southern hemisphere, especially from low latitudes, and our knowledge of such events is limited. Although a few studies are available on the September, 2002 SSW event which is believed to be the strongest warming episode to date in the southern hemisphere, so far, no report has been published on the signature of the SSW seen from a low latitude station. Therefore, in our present study we have looked into the planetary wave related dynamical response of the extratropical middle atmosphere to the September, 2002 major SSW utilizing meteor radar wind observations from Cachoeira Paulista (22.7°S, 45°W).
2. Observational database For the present work we have utilized various groundbased and space-based data sets during the period Day-
244 to Day-304, i.e. 1st September–31st October which are described below. 2.1. Meteor radar The meteor radar system at Cachoeira Paulista is a SKiYMET type which operates at a frequency of 35.24 MHz. It utilizes one three-element Yagi transmitting antenna and five two-element Yagi receiving antennas. Receiving antennas are arranged in an interferometric array of two orthogonal baselines with one antenna common to both. Details of the method used for estimating the horizontal wind from the backscattered meteor echoes can be found elsewhere (Hocking et al., 2001). The derived values of horizontal wind within the altitude range 81– 99 km with a vertical resolution of 3 km and temporal resolution of 1 h are used for further analysis in the present study. Short data gaps are filled up by linear interpolation. 2.2. ERA-interim database ERA-interim dataset provided by the European Centre for Medium-range Weather Forecasts (ECMWF) is basically reanalysis data from 1979 to the present time of various atmospheric parameters (Berrisford et al., 2009). For the present purpose we have used zonal wind, meridional wind, vertical wind and temperature at 37 pressure levels within the range 1000–1 hPa (0–48 km) with a latitudinal and longitudinal grid of 1° 1° and over Cachoeira Paulista the closest grid point is chosen as (23°S, 45°W). 2.3. TIMED/SABER data Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) is one of the four instruments onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite which measures limb emission from the atmosphere within the band 1.27– 17 lm using 10 channels. Kinetic temperature profiles are retrieved from the CO2 emission lines at 15 lm and 4.3 lm (Mertens et al., 2001). For our present study we have used V1.07 level 2A temperature dataset. The temperature profiles within the latitude-longitude grid of 10° 10° centered around the Cachoeira Paulista are considered for the present work. 2.4. NCEP OLR data Outgoing longwave radiation (OLR) recorded by the NOAA polar orbiting satellite based instrument Advanced Very High Resolution Radiometer – are provided by National Centers for Environmental Prediction (NCEP) (Kalnay et al., 1996). The data interpolated at 2.5° 2.5° latitude longitude grid are utilized as a proxy for convective activity. For the present work we consider OLR values less than 200 W m 2 as representative of deep convective
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clouds at the closest grid point coordinate (22.5°S, 45°W) over Cachoeira Paulista. 3. Results Differences between zonal mean zonal temperature and the temporal mean (July–October) of zonal mean zonal temperature at 10 hPa pressure level in the southern hemisphere, using the ECMWF dataset, are plotted during the interval 1 September to 31 October, 2002 (Day-244 = 1 September) in Fig. 1a. At high latitude (>60°S) the temperature difference is mostly negative during Day-244 to Day266 and becomes positive within a very short time, larger than 23 K around Day-270, indicating strong heating. From this point on the temperature difference remains almost positive. At lower latitudes the difference remains very low, except below 35°S latitude near Day-270 when a comparatively high negative difference ( 9° K) is observed, implying a temporary cooling. In the present latitude (23°S) the cooling lasts for approximately 3 days. The zonal mean zonal wind during the above-mentioned period is shown in Fig. 1b. Bold curve in the plot denotes zero value. At mid and high latitude the wind starts to turn easterly from Day-265 and remains unchanged for about 10 days before it turns to westerly again near Day-275. In general, at low latitude (<25°S) the zonal mean zonal wind is easterly most of the time. It can be noted that two
Fig. 1. (a) Difference of zonal mean zonal temperature and temporal mean of zonal mean zonal temperature during September–October 2002 (Day-244 1st September) and (b) zonal mean zonal wind plotted during same period at 10 hPa pressure level using ECMWF. Bold curve represents zero value in the plot. Region between two vertical lines shows peak warming period for the present and all the following figures.
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distinct patches of westerly wind exist near 20°S latitude just before and end of the warming period. Therefore both zonal wind and temperature indicate the impact of the major SSW at the present observing latitude. The peak warming period determined as per world meteorological organization definition, i.e. span of easterly wind near 60°S at 10 hPa level (Andrews et al., 1987), i.e. the region between the two vertical bold lines in all the plots of the present paper. Breaking of planetary waves causes mixing of air masses among adjacent regions of the atmosphere (Abatzoglou and Magnusdottir, 2006). Maps of potential vorticity (PV) can give important insights into the air mass mixing within various latitudes/longitudes. We have shown the PV evolution with ECMWF data at 10 hPa in the southern hemisphere over the temporal span centered about the peak warming (Day-270) in Fig. 2 taking reverse sign using the unit PVU (1 PVU = 10 6 K m2 kg 1 s 1). On Day-263 (Fig. 2a) the PV map shows well stratified layers of increasing PV magnitude with latitude and small scale low latitude to mid latitude intrusion represented by thin tongues. At high latitude the stretched highest (over all latitude range on this particular day) magnitude contour (yellow and deep green) is found to form a thin strip which is discontinuous near longitude 250–300°E, possibly signifying the onset of the split of the polar vortex. It can be noted that a tongue
Fig. 2. Evolution of the potential vorticity using ECMWF at 10 hPa (shown with opposite sign) in PVU (1 PVU = 10 6 K m2 kg 1 s 1) on (a) Day-263, (b) Day-267, (c) Day-270, (d) Day-273 and (e) Day-277.
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of low PV intrudes into the mid latitude obliquely near 20°S, 300°E (i.e. fairly close to our observing location). After 4 days, on Day-267 (Fig. 2b) the high PV strip has shortened its longitudinal length further concentrating over 30°E. A number of patches of low PV are visible at high latitudes which originate at low latitudes and spread to high latitudes due to large scale mixing. Hereafter, on Day-270 (Fig. 2c), the vortex core is found to be completely split into two segments with higher PV magnitude which appears narrow, rounded and shifted to low latitude, centered on about 40°E and 260°E separately. Asymmetry in the vortex structure is prominent as the vortex near 40°E longitude is broader than the other. Low PV from low latitude intrudes well across the high latitude regions. The previously mentioned tongue near 300°E has broadened and become oriented almost directly to the higher latitude nearing one of the split vortexes. Therefore a well-mixed state of air mass is evident in the last two cases. On Day273 (Fig. 2d) the broader vortex is found to shift eastward, the narrower vortex moves to lower latitude and a prominent connection is established between the two vortexes through a narrow strip of comparatively higher PV. Another low PV strip is observed to spiral around the narrower vortex near 260°E longitude. Subsequently, four days later, on Day-277 (Fig. 2e), the situation is quite similar to the initial state of a single vortex core represented by stretched high magnitude PV contour at high latitude, possibly due to diminution of the warming. A number of thin tongues are still visible during the recovery phase indicating weakening of the event.
Fig. 3. (a) Amplitude of planetary waves for various wavenumbers in ECMWF temperature (K) at 10 hPa height and 60°S latitude. The latitudinal variation of amplitude for (b) zonal wavenumber 1 and (c) zonal wavenumber 2.
In order to study planetary wave activity during this episode we have utilized ECMWF temperature at 10 hPa for calculating the amplitude of planetary waves of zonal wavenumbers 1–10 for the periods 2–20 days at 60°S which is plotted in Fig. 3a. To estimate it first, the temperature time series (1 day resolution) at each longitude grid for latitude of 60°S is undergone a band pass filter of cut offs at 2 and 20 days. Hereafter the filtered data is undergone least square sinusoidal fit (in longitude domain) for various zonal wavenumbers for each day to obtain amplitude corresponding to a particular day and wavenumber. The plot clearly shows significant dominance of zonal wavenumbers 1 and 2 during pre-warming tenure and diminishes remarkably during the warming period. To elaborate the role of the prevalent wave components we have shown evolution of the zonal wavenumbers 1 and 2 in Fig. 3b and c, respectively. Both planetary wavenumbers exhibit significantly high amplitudes at high latitudes before the onset of the warming event. It is interesting to note that the wavenumber 1 component diminishes very rapidly during the warming episode and remains sufficiently weak for several weeks after the event. Another important feature is the extension of the wave activity towards the low latitude during the major SSW, although the amplitude is smaller than high latitude. Following the warming episode the wave amplitude reduces at low latitude, recovering the pre-warming state. The zonal wavenumber 2 component remains active before and during the warming episode and subsequently decays at high latitude. The zonal wavenumber 2 wave also shows weak extension towards low latitude one week later (Day-277). The diminution of the wave amplitude after the warming event is slower in the case of wavenumber 2 than wavenumber, 1 indicating a stronger contribution of the wavenumber 1 component to the warming. To determine the periodicity of the planetary waves over Cachoeira Paulista, wavelet analysis has been carried out using Morlet as a mother wavelet. The wavelet power spectra for zonal wind (meteor radar), meridional wind (meteor radar) and temperature (SABER) at 90 km are shown in Fig. 4a–c and the same parameters at 10 hPa (ECMWF) are shown in Fig. 4d–f. Bold curves in each plot represent 90% significance level. Wave activity is larger (in terms of power) at 90 km (mesopause) as compared to 31 km (stratosphere). Although all the spectra in the mesopause exhibit several wave components in the 2–14-day band before and after the warming, wave activity is found to be significantly less during the warming episode. Stratospheric spectra, especially for the meridional wind, reveal prominent wave signatures in the 2–4-day band during the warming period although wave activity is generally weak most of the time. Dynamical condition over Cachoeira Paulista has been looked into in terms of temperature (ECMWF), vertical wind (ECMWF) and OLR which are plotted in Fig. 5a– c. In general, temperature shows well stratified layers at various altitudes in the stratosphere and troposphere. Above 40 km there is an evident signature of decrease of
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Fig. 4. Wavelet power spectra at 90 km for (a) zonal wind (m/s), (b) meridional wind (m/s) using meteor radar, (c) temperature (K) using SABER. Corresponding spectra at 10 hPa are shown for (d) zonal wind (m/s), (e) meridional wind (m/s), (f) temperature (K) over Cachoeira Paulista using ECMWF. Bold curves in each plot represent 90% significance level.
Fig. 5. ECMWF (a) Temperature (K), (b) vertical wind (Pa/s) in stratosphere and troposphere and (c) outgoing longwave radiation (W/ m2) over Cachoeira Paulista.
temperature which starts just before the onset of the warming and recovers its normal state by the end of the warming episode. Although cold-point tropopause (15–18 km) shows an increase of temperature just before the warming it recovers quickly and remains stable during the warming episode. The vertical wind has been plotted within the range 0–15 km as above 15 km both the value and variability are too small to be shown elaborately. The vertical wind ( ve values represent upward direction) is generally weak hovering around zero values most of the time except at the lowest altitude where upward wind is dominant most of the time. It can be seen from the plot that there is an upward vertical wind over the entire altitude range just before the onset of the warming event. Another instance of upward wind is visible near Day-275 (one week after the initiation of the warming). OLR shows strong convection (<200 W m 2) just before and after the warming event, but during the warming episode it shows lower convection (>230 W m 2). The background horizontal wind is an important factor in controlling the vertically propagating waves through wave-mean flow interaction. The wave can propagate through the atmosphere without dissipation if the background horizontal wind is opposite to the wave direction. If the wave propagation direction is the same as the horizontal wind direction the wave suffers critical layer
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interaction and deposits energy in the mean flow owing to dissipation. As Rossby (westward planetary) waves propagate along the easterly direction, zonal wind is expected to play an important role in filtering the waves propagating from the lower atmosphere. Daily mean zonal and meridional winds in the mesopause region (80–100 km using meteor radar) and the stratosphere–troposphere (0–50 km using ECMWF) over Cachoeira Paulista are shown in Fig. 6a–d. Zonal wind in the lower mesopause region is mostly easterly and at greater heights it changes its direction with time. During the warming period the zonal wind is mainly easterly over the whole altitude range. The meridional wind is mainly northerly at lower mesopause heights and in the upper mesosphere it is mostly southerly. During the initial warming period the meridional wind is generally weak and gets stronger at a later phase. The horizontal winds in the stratosphere–troposphere show some interesting features. The zonal wind shows a strong westerly jet near 12 km which continues till the onset of the warming episode. The wind weakens very rapidly as warming initiates and reverses to easterly within the next two weeks (Day-282) and continues for the next couple of days before recovery to westerly on Day-296. On the contrary, the zonal wind over the surface and upper stratosphere (30–45 km) remains easterly mostly. At the top of the stratosphere (45–50 km) the zonal wind is strongly westerly, lasting for a month centered around the warming period. Meridional wind near 12 km shows northerly flow for a couple of days before the onset of the warming. It
reverses to southerly shortly after the onset of warming and by the end of the episode it becomes stronger. Above 35 km the meridional wind remains northerly during the initial phase of the warming and turns to southerly at the final phase. 4. Discussion Our present study has illustrated some important dynamical aspects of the major SSW event from the southern hemisphere during September-2002 with observations from an extratropical station in the Brazilian sector. To a large extent, the present investigation owes its importance to the fact that this is the strongest major SSW from the southern hemisphere known so far. The major warming at the polar latitude corresponds to a cooling at the present observing latitude. Stratospheric cooling at tropical latitudes in the northern hemisphere was first reported by Fritz and Soules (1970) with NIMBUS III satellite observations. They concluded that the cooling was caused by heat transfer due to a change in meridional circulation and large-scale eddies. Upward propagating waves from the troposphere causes upwelling of the air and thus can produce adiabatic cooling (Yoshida and Yamazaki, 2011). Equatorward planetary waves traveling from high latitudes may dissipate in the tropical lower stratosphere and can drive upwelling and associated cooling (Yoshida and Yamazaki, 2011). Yoshida and Yamazaki (2011) also mentioned that convection related
Fig. 6. Daily mean wind (m/s) at Cachoeira Paulista (a) zonal, (b) meridional in the mesopause region using meteor radar and (c) zonal, (d) meridional in the troposphere–stratosphere using ECMWF.
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vertical heat flux convergence could be another potential factor for such cooling. Dunkerton and Delisi (1986) mentioned that temporal evolution of the size, shape and orientation of the circumpolar vortex is well represented by the PV field. The PV maps of the present study clearly show a split of the vortex into two separate segments far apart from each other during the major SSW. Shifting of a split vortex core towards sufficiently lower latitudes implies significant planetary wave propagation from high to low latitude. Considerable mixing of air mass between low and high latitudes is evident during the peak warming period which is believed to be caused by planetary wave breaking. Observed reversal of zonal mean zonal wind at the present latitude (Fig. 1b) can be attributed to the influence of the planetary waves generated at high latitude. Several tongues of low PV intrusion from the low latitude to higher latitude are observed. In this context it can be mentioned that Palmer (1981) found a change of direction of Eliassen–Palm (EP) flux several times during the warming. They reported that poleward and upward direction of the flux at midlatitudes is very important for large zonal flow deceleration to facilitate warming. Although both components of zonal wavenumbers 1 and 2 planetary waves exhibit significant variability in response to the major SSW, wavenumber 1 shows considerable diminution of its amplitude which persists for long intervals of time even after the end of the warming event. Shepherd (2000) explained that if the vortex goes away from the pole then the “wave-1” component should be responsible and if the vortex is split into two segments then one should expect a “wave-2” component. The present finding of vortex movement from polar to significantly lower latitude with two evident segments during peak warming as shown by the PV maps as well as wavenumbers 1 and 2 (responsible for driving waves 1 and 2) activity is consistent with the notion of Shepherd (2000). Guharay and Sekar (2012) reported propagation of wavenumbers 1 and 2 components from polar latitude to low latitude regions during February, 2007 northern hemispheric SSW similar to the present finding. According to the work of Labitzke (1977) it seems that for causing major warming, zonal wavenumber 1 should be very strong at least for a couple of days to facilitate the pre-warming condition. Labitzke (1977) also noted that zonal wavenumber 2 alone is not sufficient to cause a major warming. Palmer (1981) argued that zonal wavenumber 2 waves are not able to reverse the high latitude zonal wind through critical layer interaction. Johnson (1969) found a significant role of the zonal wavenumber 2 waves in nonlinear/vertical interaction during SSW. Hirota et al. (1990) pointed out the role of interaction of the eastward traveling wavenumber 2 and quasi-stationary forced wavenumber 1 which might lead to quasi-periodic amplification of wavenumber 1. Kru¨ger et al. (2005) mentioned the dominant impact of interaction between eastward traveling waves and quasi-stationary
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planetary waves on the polar vortex breakdown and subsequent warming. A wavelet periodogram for the present extratropical observing station shows weak planetary wave activity in the mesopause during the warming period. Although there is wave activity in the period range 2–14 days scattered over the spectra there are no evident wave components present during the warming period of the mesopause. Zonal wavenumber–frequency spectra of horizontal winds and temperature at 10 hPa (not shown here) revealed dominant westward components. Therefore planetary wave diminution in the mesopause during the warming episode can be attributed to strong dissipation in the stratosphere due to critical layer interaction between wave and mean flow because of easterly zonal wind. Using observations of multiple stations situated over both the hemispheres Dowdy et al. (2007) reported remarkably smaller spectral power of the mesospheric planetary waves as compared to the stratospheric ones during SSW and they explained such reduction in planetary wave power as being due to partial transmission of waves from the stratosphere. Using MF radar observations over three Antarctic stations Dowdy et al. (2004) found a 14 day planetary wave of zonal wavenumber 1 in the mesospheric meridional wind during the same warming period. Later, French et al. (2005) also reported a 14 day planetary wave of zonal wavenumber 1 during the same warming period from OH airglow temperatures over Davis, Antarctica. With radar meridional wind observations in the mesosphere over three Antarctic stations during the 2002 winter Espy et al. (2005) found a long period (43 days) planetary wave of zonal wavenumber 1 before the warming episode. However, after the warming episode they found a wave of much reduced period (14 days). Furthermore, Mbatha et al. (2010) reported a 14 day wave in the mesospheric zonal and meridional wind before the occurrence of the September 2002 SSW with observations from an Antarctic station. In the present scenario such planetary wave features are not observed at extratropical latitudes during or before the SSW, although a strong 12–14 day periodicity is found in the meridional wind after 2 weeks, which is most probably the same wave component as indicated by the contemporary studies carried out from Antarctic stations. This wave would originate at high latitude and reach low latitudes after warming. In the stratosphere we found a 2– 4 day wave during the warming period although its strength is low. In this context it can be mentioned that Sathishkumar and Sridharan (2011) observed an enhanced 2–4 day wave in the troposphere, mesosphere and ionosphere during a 2009 northern hemispheric SSW event, similar to the present finding. Recently, Lima et al. (2012) reported amplification of the 2-day wave activity in the equatorial southern hemisphere mesosphere during a northern hemisphere SSW and they concluded that cross-equatorial dynamical influence of the enhanced winter hemisphere waves in the stratosphere might be responsible for such an occurrence.
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Due to the enhanced planetary wave activity during SSW the stratospheric mean meridional circulation increases which in turn causes cooling in the lower stratosphere and upper troposphere in the tropics. Such cooling can modify the convective activity in the tropics. Our results show high convective activity before and after the warming episode which is supported by simultaneous high upward vertical wind, but during the warming convection is found to be very low. In this context it can be noted that Kodera and Yamada (2004) argued that stronger meridional circulation during SSW does not directly enhance convective activity, rather it triggers and destabilizes a balance between equatorial convective active regions. With northern hemispheric observations of multiple warming events Kodera (2006) noted a seesaw structure of enhanced convection in the southern tropics and reduced convection in the northern tropics due to a shift of equatorial convective activity towards the southern hemisphere following the warming. Therefore such a shift of convective activity to the opposite hemisphere may cause a reduced convection in the low latitude region which continues for several days after the warming as seen in the present case. Westward daily mean zonal wind in the mesopause (80– 100 km) during the warming period is possibly responsible for less planetary wave amplitude owing to critical layer interaction. Daily mean winds over Cachoeira Paulista reveal some notable features in the stratosphere and troposphere. Zonal wind shows strong westerly motion near 12 km before the onset of the SSW which weakens significantly during the peak warming period. Strong westerly wind helps the Rossby waves to propagate upward provided the wind speed is below certain limit known as the critical Rossby velocity (Charney and Drazin, 1961). Upper tropospheric westerly ducts are also favorable to the cross-equatorial wave propagation (Webster and Holton, 1982). Therefore they are important for tropical– extratropical interaction. Breaking of Rossby waves in the westerly wind can cause meridional transport of trace species as well as higher tropical convection (Waugh and Polvani, 2000). The zonal wind between 20 km and 45 km generally shows easterly or very weak westerly motions, indicating wave energy dissipation to the mean flow due to the proximity of a critical layer. An abrupt change of meridional wind direction and magnitude is conspicuous above 35 km and below 15 km during the warming event. It is generally northerly (poleward) during the initial, phase turning to southerly (equatorward) in the final phase of the SSW at these altitude ranges. Carrying out long-term composite analysis of northern hemispheric SSW, with a total of 39 events, Limpasuvan et al. (2004) concluded that modification of anomalous momentum flux due to increased wave activity is responsible for the change of residual mean meridional circulation from poleward under normal conditions to equatorward during the warming period, consistent with the present finding. Such meridional reversal can also be explained as follows. Under normal atmospheric conditions a negative temperature
gradient with respect to latitude causes poleward meridional flow of wind, but during warming periods the polar stratosphere becomes warmer and the tropical stratosphere and upper troposphere become cooler, leading to opposite flow of the meridional wind. Utilizing a long-term database of SSW in the southern hemisphere Hu and Fu (2009) found enhancement of the EP fluxes as well as EP flux convergence during SSW in extratropical latitudes, leading to strong meridional residual circulation associated downwelling and adiabatic heating in the high latitudes. 5. Summary and conclusions The present work points out some dynamical imprints of the rare event of September 2002 SSW observed from a southern hemispheric extratropical station. Although a few studies have investigated this event using high latitude observations, presently, there is hardly any literature related to its impact at low latitude. Planetary waves of zonal wavenumbers 1 and 2 are found to propagate to extratropical latitudes during the warming period. PV maps clearly show the split of the vortex into two segments, with significantly different sizes, and progression of the same toward midlatitudes, leading to significant latitudinal mixing of air mass. Planetary wave activity is considerably less in the mesopause region, as compared to the stratosphere, and a wave component of period 12–14 days is evident after 2 weeks of the warming period as reported by earlier investigators with observations carried out from high latitude stations during the same warming event. A 2– 4 day wave is found to be dominant during the warming period in the stratosphere. Convective activity at Cachoeira Paulista is found to decrease during the SSW, possibly due to a shift of the destabilized active convective regions towards equator. Acknowledgments The authors are grateful to the TIMED/SABER team for providing useful data for the present work. A. Guharay gratefully acknowledges the support from the Fundacßa˜o de Amparo a Pesquisa do Estado de Sa˜o Paulo (FAPESP). S. Sarkhel is supported by the National Science Foundation (NSF), USA grant ATM 07-21613 to The Pennsylvania State University. The ECMWF data is available at the website http://www.ecmwf.int/research/era/do/get/index. Wavelet software was provided by C. Torrence and G. Compo, and is available at URL: http://paos.colorado.edu/research/wavelets/. References Abatzoglou, J.T., Magnusdottir, G., 2006. Planetary wave breaking and nonlinear reflection: seasonal cycle and interannual variability. Journal of Climate 19, 6139–6152. Andrews, D.G., Holton, J.R., Leovy, C.B., 1987. Middle Atmosphere Dynamics. Academic, San Diego, California, pp. 129–130.
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