2-day Wave Research Articles

Critical roles of convective momentum transfer in sustaining the multi-scale Madden–Julian oscillation

To understand the nature and role of multi-scale interaction involved in the Madden–Julian oscillation (MJO), a dynamical model is built based on two essential processes: the convective complex of the MJO modulates the strength and location of synoptic-scale motions, which in turn feed back to the MJO through the convective momentum transfer (CMT). Our results exhibit that: (1) The lower tropospheric easterly CMT coming from the 2-day waves slows down the MJO dramatically; (2) although the lower tropospheric westerly CMT coming from the superclusters can produce the horizontal quadrupole vortex and vertical westerly wind-burst structures of the MJO, it drives the large-scale motions to propagate eastward too fast; (3) the planetary boundary layer provides an instability source for the MJO and pulls the MJO to propagate eastward at a speed of 0∼10 ms−1; and (4) the optimal structure of the multi-scale MJO should be: the stronger superclusters/2-day waves prevail in the rear/front part of the MJO and produce lower tropospheric westerly/easterly CMT there. These theoretical results emphasize the role of CMT and encourage further observations in the multi-scale MJO.

A Model for Scale Interaction in the Madden–Julian Oscillation*

Abstract The Madden–Julian oscillation (MJO) is an equatorial planetary-scale circulation system coupled with a multiscale convective complex, and it moves eastward slowly (about 5 m s−1) with a horizontal quadrupole vortex and vertical rearward-tilted structure. The nature and role of scale interaction (SI) is one of the elusive aspects of the MJO dynamics. Here a prototype theoretical model is formulated to advance the current understanding of the nature of SI in MJO dynamics. The model integrates three essential physical elements: (a) large-scale equatorial wave dynamics driven by boundary layer frictional convergence instability (FCI), (b) effects of the upscale eddy momentum transfer (EMT) by vertically tilted synoptic systems resulting from boundary layer convergence and multicloud heating, and (c) interaction between planetary-scale wave motion and synoptic-scale systems (the eastward-propagating super cloud clusters and westward-propagating 2-day waves). It is shown that the EMT mechanism tends to yield a stationary mode with a quadrupole vortex structure (enhanced Rossby wave component), whereas the FCI yields a relatively fast eastward-moving and rearward-tilted Gill-like pattern (enhanced Kelvin wave response). The SI instability stems from corporative FCI or EMT mechanisms, and its property is a mixture of FCI and EMT modes. The properties of the unstable modes depend on the proportion of deep convective versus stratiform/congestus heating or the ratio of deep convective versus total amount of heating. With increasing stratiform/congestus heating, the FCI weakens while the EMT becomes more effective. A growing SI mode has a horizontal quadrupole vortex and rearward-tilted structure and prefers slow eastward propagation, which resembles the observed MJO. The FCI sets the rearward tilt and eastward propagation, while the EMT slows down the propagation speed. The theoretical results presented here point to the need to observe multicloud structure and vertical heating profiles within the MJO convective complex and to improve general circulation models’ capability to reproduce correct partitioning of cloud amounts between deep convective and stratiform/congestus clouds. Limitations and future work are also discussed.

Analysis of daytime ionosphere behavior between 2004 and 2008 in Antarctica

In this paper we present 5-year study of the lower ionosphere behavior obtained from VLF sounding done at Comandante Ferraz Brazilian Antarctic Station. The results suggested a strong influence of ‘meteorological processes’ especially during wintertime, when VLF measurements showed that the D-region was strongly affected by planetary waves in all years. This effect was superposed to the well known control by solar radiation. The 16-day wave was the most important PW modulating the lower ionosphere behavior. Since planetary waves are very predominantly of tropospheric origin, this result is an evidence of the vertical couplings in the atmosphere–ionosphere system.

Steady-frequency waves at intradiurnal periods from simultaneous co-located microbarometer and seismometer measurements: a case study

Abstract. Simultaneous co-located microbarometer and vertical-pendulum seismometer measurements at St. Petersburg (59.9° N, 29.8° E) with total duration of 4 months are used to study atmospheric oscillations at steady frequencies at periods shorter than 8 h. The temporal behavior of the phase shift between oscillations detected simultaneously by both instruments is analyzed for oscillations of periods up to as short as ~0.5 h. Some of oscillations last up to several days. For the 42–90 min and 2.5–5 h period ranges, the temporal behavior of the power spectra are considered and correlated with the atmospheric angular momentum of zonal wind, tropical cyclones and large earthquakes. Signatures of sequences of subharmonics of the solar tide are revealed with periods up to day/30. There are indications of effects of the 5-day planetary waves and tropical cyclones on wave activity in the ~1–5 h period range. A weak increase of wave activity is observed when large earthquakes occur.

Aura MLS observations of the westward-propagating <i>s</i>=1, 16-day planetary wave in the stratosphere, mesosphere and lower thermosphere

Abstract. The Microwave Limb Sounder (MLS) on the Aura satellite has been used to measure temperatures in the stratosphere, mesosphere and lower thermosphere. The data used here are from August 2004 to December 2010 and latitudes 75° N to 75° S. The temperature data reveal the regular presence of a westward-propagating 16-day planetary wave with zonal wavenumber 1. The wave amplitudes maximise in winter at middle to high latitudes, where monthly-mean amplitudes can be as large as ~8 K. Significant wave amplitudes are also observed in the summer-time mesosphere and lower thermosphere (MLT) and at lower stratospheric heights of up to ~20 km at middle to high latitudes. Wave amplitudes in the Northern Hemisphere approach values twice as large as those in the Southern Hemisphere. Wave amplitudes are also closely related to mean zonal winds and are largest in regions of strongest eastward flow. There is a reduction in wave amplitudes at the stratopause. No significant wave amplitudes are observed near the equator or in the strongly westward background winds of the atmosphere in summer. This behaviour is interpreted as a consequence of wave/mean-flow interactions. Perturbations in wave amplitude summer MLT are compared to those simultaneously observed in the winter stratosphere of the opposite hemisphere and found to have a correlation coefficient of +0.22, suggesting a small degrees of inter-hemispheric coupling. We interpret this to mean that some of the summer-time MLT wave may originate in the winter stratosphere of the opposite hemisphere and have been ducted across the equator. We do not observe a significant QBO modulation of the 16-day wave amplitude in the polar summer-time MLT. Wave amplitudes were also observed to be suppressed during the major sudden stratospheric warming events of the Northern Hemisphere winters of 2006 and 2009.

Why do 2-day waves propagate westward?

In observations, the 2-day waves, identified as the convectively coupled equatorial inertio-gravity (IG) waves, only propagate westward. To understand this feature, a simple theoretical model is presented for the convectively coupled equatorial waves (CCEWs). Under the assumption that the convective heating is proportional to the vertical velocity on the first baroclinic mode, the nonlinear governing equation for the meridional velocity of the CCEWs can be derived. The optimal method is used to obtain the dispersion relation from this nonlinear equation, and the results show that the deep convection can slow down the IG waves by decreasing the mean state static stability, but the key leading to the westward propagation of the IG waves is the full meridional variation of the sea surface temperature (SST). The warm SST trapped near the equator excites long westward propagating IG waves, whereas the warm SST trapped near the ITCZ centered at 10° N excites short westward propagating IG waves. This theoretical model provides a simple tool to study the CCEWs in understanding the tropical circulation.

Sudden stratospheric warming effects on the mesospheric tides and 2-day wave dynamics at 7°S

The dynamics of the equatorial mesosphere have been investigated during austral summers of 2004–2005, 2005–2006 and 2006–2007 from meteor radar measurements obtained at São João do Cariri, Brazil (7.4°S, 36.5°W), together with stratospheric northern hemisphere polar parameters. Some recent studies have demonstrated coupling between high latitude northern hemisphere major Sudden Stratospheric Warming (SSW) events and mesospheric–ionospheric disturbances, including the equatorial region. Here we have analyzed the tides, quasi-two-day planetary wave and local mean zonal wind variability in the equatorial upper Mesosphere and Lower Thermosphere (MLT) region during three austral summers and found that upper mesospheric dynamics at 7°S has been affected when a remarkable major SSW event took place in January 2006. The more intense tides and quasi-two-day wave amplitudes during the major SSW event are suggestive of an association between them and high planetary wave activity in the northern stratospheric winter.

A comparison between ground-based observations of noctilucent clouds and Aura satellite data

A comparison is made between ground-based observations of noctilucent clouds (NLCs), obtained with a network of automated digital cameras, and Aura satellite data (the MLS instrument). The Aura data (water vapor and temperature) demonstrate reasonable values around the summer mesopause fostering NLC formation in June through August, when supersaturated air conditions occur. The temperature decrease leads, in general, to amplification of the NLC brightness. The 2- and 5-day planetary waves, extracted from the Aura temperature field, have definite influence on the brightness variations of NLCs. The temperature behavior around the summer mesopause at 60°N demonstrated a remarkable feature, namely, in 2007 the minimum of a Gaussian fitted seasonal temperature variation was observed, on average, 14 days earlier and was broader than the corresponding minimum in 2008. The different temperature climatology resulted in different seasonal variation of NLCs in 2007 and 2008; in particular, the maximum of a Gaussian fitted seasonal variation of the NLC brightness cycle in 2007 was advanced by 12–29 days relative to that in 2008.

Tropical connection to the polar stratospheric sudden warming through quasi 16-day planetary wave

Abstract. The Planetary Waves (PWs) are believed to have significant role in generating the wintertime warming over the polar stratosphere, known as Stratospheric Sudden Warming (SSW). However, the origin, characteristics and evolution of these waves are still speculative. The possibility that the PWs over the polar stratosphere, which play an important role in the generation of SSW, could also have contribution from the tropics has been indicated through many numerical simulations in the past, but due to the paucity of global measurements it could not be established unequivocally. The earlier numerical studies also indicated the presence of a zero-wind line (more general the critical layer, where the zonal wind amplitude becomes zero) whose real counterparts were not observed in the atmosphere. The present study based on the NCEP/NCAR reanalysis of stratospheric wind and temperatures of recent years clearly shows that (i) the zero-wind line appears over the tropics ~60 days prior to the major SSWs and progresses towards the Pole and (ii) an enhanced PW activity of quasi periodicity 16-days, which is also seen almost simultaneously with the zero-wind line, shows a propagation from equator to the Pole. This result is significant as it presents for the first time the connection between the tropics during the SSW events and the pole, through the quasi 16-day wave.

On the origin of mid-latitude mesospheric clouds: The July 2009 cloud outbreak

Mid-latitude mesospheric clouds (MCs) are a rare phenomenon and their existence is not well understood, as the mesosphere at these latitudes is, in general, too warm for clouds to form. During the 2009 northern hemisphere summer season an unusually high number of these clouds were reported over both central and southern Europe, and the western contiguous United States. In this paper we investigate the mesospheric temperature field utilizing data from the Microwave Limb Sounder (MLS) instrument. We find that the temperature occasionally is near the frost point temperature and that the presence of planetary waves with periods of 2-, 5-, and 16-days combine to provide temperature anomalies of 1–1.5K, lowering the temperature below the frost point for cloud formation and growth. Observed MCs are found to occur in close proximity to the 5-day wave anomaly. Model results show that the growth time to achieve visible particle sizes under the observed temperature and water vapor mixing ratio conditions are greater than ∼20h. Combined with climatological winds from a mid-latitude site, our study suggests that these clouds occur due to a combination of advection from higher and colder latitudes, and in situ wave growth.

Characteristics of 3–4- and 6–8-Day Period Disturbances Observed over the Tropical Indian Ocean

Abstract A field observational campaign [i.e., the Mirai Indian Ocean cruise for the Study of the MJO-convection Onset (MISMO)] was conducted over the central equatorial Indian Ocean in October–December 2006. During MISMO, large-scale organized convection associated with a weak Madden–Julian oscillation (MJO) broke out, and some other notable variations were observed. Water vapor and precipitation data show a prominent 3–4-day-period cycle associated with meridional wind υ variations. Filtered υ anomalies at midlevels in reanalysis data [i.e., the Japan Meteorological Agency (JMA) Climate Data Assimilation System (JCDAS)] show westward phase velocities, and the structure is consistent with mixed Rossby–gravity waves. Estimated equivalent depths are a few tens of meters, typical of convectively coupled waves. In the more rainy part of MISMO (16–26 November), the 3–4-day waves were coherent through the lower and midtroposphere, while in the less active early November period midlevel υ fluctuations appear less connected to those at the surface. SST diurnal variations were enhanced in light-wind and clear conditions. These coincided with westerly anomalies in prominent 6–8-day zonal wind variations with a deep nearly barotropic structure through the troposphere. Westward propagation and structure of time-filtered winds suggest n = 1 equatorial Rossby waves, but with estimated equivalent depth greater than is common for convectively coupled waves, although sheared background flow complicates the estimation somewhat. An ensemble reanalysis [i.e., the AGCM for the Earth Simulator (AFES) Local Ensemble Transform Kalman Filter (LETKF) Experimental Reanalysis (ALERA)] shows enhanced spread among the ensemble members in the zonal confluence phase of these deep Rossby waves, suggesting that assimilating them excites rapidly growing differences among ensemble members.

Dominant winter-time mesospheric wave signatures over a low latitude station, Hawaii (20.8°N): An investigation

We utilize mesospheric O2 airglow emission intensity and temperature data collected during January–February 2003 on 17 consecutive nights from Maui, Hawaii (20.8°N, 156.2°W) to study the dominant and long period wave features at mesospheric altitudes. Apart from large day-to-day variability, it is found that nocturnal data for the period under consideration was dominated by a terdiurnal tide-like wave. Together, a quasi 5-day wave is also noticed with significant amplitude.

Radiosonde observations of high-latitude planetary waves in the lower atmosphere

The characteristics of high-latitude planetary waves (PWs) in the troposphere and lower stratosphere (TLS) are studied by using the data from radiosonde observations during 1998 to 2006 at three Alaskan stations in USA (Nome, 64.50°N, 165.43°W; McGrath, 62.97°N, 155.62°W; Fairbanks, 64.82°N, 147.87°W). It is found that strong PWs exist in two regions. One is around tropopause, and the other is in the polar night jet (PNJ) in winter. The PW activities are rather intermittent, and their lifetimes are no longer than two months. Among three perturbation components in zonal and meridional winds and temperature, the temperature disturbance amplitude is the smallest, and the amplitude for the meridional wind component the largest. Around the tropopause, quasi 5-, 10-, and 16-day PW activities can be observed simultaneously. Among these PW components, the quasi 5-day and 10-day PW are the weakest and strongest, respectively. Moreover, PWs around the tropopause are complex and no obvious season variability can be observed. However, in the PNJ, the higher region, only obvious quasi 10-day and 16-day PWs remain, with smaller amplitudes than those around the tropopause. And significant PWs in the PNJ occur only in winter. By calculating the refractive index for PWs, it is found that there is a persistent reflection layer around 11 km, which is thick in summer and becomes thin or even disappears in winter, revealing that PWs in the stratosphere can only occur in winter. PWs in the 2003/2004 winter at the three stations are analyzed in detail. It is found that for the focused observation duration, the quasi 10-day and quasi 16-day waves exist mainly in the troposphere and stratosphere, respectively. The quasi 10-day wave is a standing wave in the vertical direction, with vertical wavelength about 12 km in the temperature component and larger than 26 km in the meridional component. Moreover, the tropospheric quasi 10-day wave propagates westward with the zonal numbers between 2 and 4. The quasi 16-day wave is also a standing wave in the vertical direction with vertical wavelength larger than 20 km, and seems to be a stationary wave in the horizontal direction.

Weather regimes over Senegal during the summer monsoon season using self-organizing maps and hierarchical ascendant classification. Part I: synoptic time scale

The aim of this work is to define over the period 1979–2002 the main synoptic weather regimes relevant for understanding the daily variability of rainfall during the summer monsoon season over Senegal. “Pure” synoptic weather regimes are defined by removing the influence of seasonal and interannual time scales, in order to highlight the day by day variability of the atmospheric circulation. The Self-Organizing Maps (SOM) approach, a clustering methodology based on non-linear artificial neural network, is combined with a Hierarchical Ascendant Classification to compute these regimes. Nine weather regimes are identified using the mean sea level pressure and 850 hPa wind field as variables, and gathered into three classes. Two of these weather regimes represent the classical 3–5-day African easterly waves with a mean wavelength of about 3,000 km. Three others are characterized by a modulation of the semi-permanent trough located along the western coast of West Africa and might be interpreted in terms of the 6–9-day easterly waves. The last four weather regimes are characterized by a more or less strong north–south dipole of circulation. They can be interpreted as a northward/southward displacement of the Saharan Heat Low for two of them, and a filling/deepening of this depression for the other two. The circulation patterns of all these nine weather regimes are very consistent with the associated anomaly patterns of precipitable water, mid-troposphere vertical velocity, outgoing longwave radiation, and finally rainfall. Rainfall distribution is also highlighted over the southwestern area of Senegal.

The 16-day wave in the Arctic and Antarctic mesosphere and lower thermosphere

Abstract. The 16-day planetary wave in the polar mesosphere and lower thermosphere has been investigated using meteor radars at Esrange (68° N, 21° E) in the Arctic and Rothera (68° S, 68° W) in the Antarctic. The measurements span the 10-year interval from October 1999 to July 2009 and the 5-year interval February 2005 to July 2009, respectively. The height range covered is about 80–100 km. In both polar regions the wave is seen to occur in intermittent bursts, where wave amplitudes typically reach a maximum of about 15 m s−1, and never more than about 20 m s−1. Horizontal wind variance within a wave-period range of 12 to 20 days is used as a proxy for the activity of the 16-day wave. Wave activity is strong for 3 to 4 months in winter, where it is present across the entire height range observed and monthly wave variance reaches about 65 m2 s−2. Some weak and intermittent activity is observed throughout the other seasons including summer. However, there is a high degree of inter-annual variability and in some individual years wave activity is almost absent. The data are used to construct a representative climatology for the Arctic and Antarctic. The seasonal cycle of the 16-day wave is found to be very similar in both polar regions. The 16-day wave has slightly greater amplitudes in the zonal component of the winds than in the meridional. Mesospheric temperatures measured by the radars were used to further investigate the 16-day wave. The temperatures reveal a clear signature of the 16-day wave. Temperature amplitudes are generally only a few Kelvin but occasional bursts of up to 10 K have been observed. Observations of the wave in summer are sometimes consistent with the suggestion of ducting from the winter hemisphere.

Persistence of planetary wave type oscillations in the mid-latitude ionosphere

Planetary wave type oscillations have been observed in the lower and middle atmosphere but also in the iono- sphere, including the ionospheric F2 layer. Here we deal with the oscillations in foF2 analysed for two Japan- ese and two US stations over a solar cycle (1979-1989) with the use of the Morlet and Paul wavelet transforms. Waves with periods near 5, 10 and 16 days are studied. Only events of duration of three wave-cycles and more are considered. The results are compared with the results of a similar analysis made for foF2 and the lower ion- osphere over Europe (Lastovicka et al., 2003a,b). The 5-day period wave events display a typical duration of 4 cycles, while the 10- and 16-day wave events are less persistent with typical duration of about 3.5 cycles and rather 3 cycles, respectively, in all three geographic regions. The persistence pattern in terms of number of cy- cles and in terms of number of days is different. In terms of number of cycles, the typical persistence of oscilla- tions decreases with increasing period. On the other hand, in terms of number of days the typical persistence ev- idently increases with increasing period. The spectral distribution of event duration is too broad to allow for a reasonable prediction of event duration. Thus the predictability of the planetary wave type oscillations in foF2 seems to be very questionable. The longitudinal size of the planetary wave type events increases with increasing wave period. The persistence of the planetary wave type events in foF2 and the lower ionosphere is similar in Europe, but the similarity in occurrence of individual events in foF2 and the lower ionosphere is rather poor.

Global distribution and climatological features of the 5–6-day planetary waves seen in the SABER/TIMED temperatures (2002–2007)

The paper is focused on the global spatial structure, seasonal and interannual variability of the ∼5-day Rossby (W1) and ∼6-day Kelvin (E1) waves derived from the SABER/TIMED temperature measurements for 6 full years (January 2002–December 2007). The latitude structure of the ∼5-day W1 wave is related to the gravest symmetric wave number 1 Rossby wave. The vertical structure of the ∼5-day Rossby wave amplitude consists of double-peaked maxima centred at ∼80–90 km and ∼105–110 km. This wave has a vertically propagating phase structure from the stratosphere up to 120 km altitude with a mean vertical wavelength of ∼50–60 km. The ∼6-day E1 wave is an equatorially trapped wave symmetric about the equator and located between 20°N and 20°S. Its seasonal behaviour indicates some equinoctial and June solstice amplifications, while the vertical phase structure indicates that this is a vertically propagating wave between 20–100 km altitudes with a mean vertical wavelength of ∼25 km.

An attempt to infer information on planetary wave by analyzing sporadic E layers observations

Abstract In this paper we presented two typical global 6-day planetary wave oscillations occurring in sporadic E layers. By analyzing the f o E s time series observed from several ionosonde stations on the basis of the methods suggested by Haldoupis and Pancheva (2002), we computed estimates for the PW propagation direction, zonal wave number and phase velocity. We obtained results that the 6-day PW, with zonal wave number about 1, propagates westward, which are in agreement with those reported from radar and satellite neutral wind MLT measurements. The results provide experimental evidence for a close relationship between PWs and midlatitude E s. In addition, the study proves the validity of E s observations measured routinely and rather reliably with a dense global network of digital ionosondes used as an alternative means of studying large-scale neutral atmospheric dynamics in the MLT region.

The two-day wave in the Antarctic and Arctic mesosphere and lower thermosphere

Abstract. There have been comparatively few studies reported of the 2-day planetary wave in the middle atmosphere at polar latitudes. Here we report on a study made using high-latitude meteor radars at Rothera in the Antarctic (68° S, 68° W) and Esrange in Arctic Sweden (68° N, 21° E). Observations from 2005–2008 are used for Rothera and from 1999–2008 for Esrange. Measurements were made of horizontal winds at heights of 80–100 km. The radar data revealed distinct summertime and wintertime 2-day waves. The Antarctic summertime wave occurs with significant amplitudes in January – February at heights between about 88–100 km. Horizontal wind monthly variances associated with the wave exceed 160 m2 s−2 and the zonal component has larger amplitudes than the meridional. In contrast, the Arctic summertime wave occurs for a longer duration, June–August and has meridional amplitudes larger than the zonal amplitudes. The Arctic summertime wave is weaker than that in the Antarctic and maximum monthly variances are typically 60 m2 s−2. In both hemispheres the summertime wave reaches largest amplitudes in the strongly sheared eastward zonal flow above the zero-wind line and is largely absent in the westward flow below. The observed differences in the summertime wave are probably due to the differences in the background zonal winds in the two hemispheres. The Antarctic and Arctic wintertime 2-day waves have very similar behaviour. The Antarctic wave has significant amplitudes in May–August and the Arctic wave in November–February. Both are evident across the full height range observed.

Variability of planetary waves as a signature of possible climatic changes

The long-term variability of stationary and traveling planetary waves in the lower stratosphere has been investigated using the data of NCEP/NCAR reanalysis. The results obtained show that during the last decades winter-mean amplitude of the stationary planetary wave with zonal wave number 1 (SPW1) increases at the higher middle latitudes of the Northern Hemisphere. It has been suggested that the observed increase in the SPW1 amplitude should be accompanied by the growth in the magnitude of the stratospheric vacillations. The analysis of the SPW1 behavior in the NCEP/NCAR data set supports this suggestion and shows a noticeable increase with time in the SPW1 intra-seasonal variability. The amplitudes of the long-period normal atmospheric modes, the so-called 5-, 10- and 16-day waves, diminish. It is supposed that one of the possible reasons for this decrease can be a growth of radiative damping rate caused, for instance, by the increase of CO 2 . To investigate a possible climatic change of the middle atmosphere dynamics caused by observed changes in the tropospheric temperature, two sets of runs (using zonally averaged temperature distributions in the troposphere typical for January 1960 and 2000) with the middle and upper atmosphere model (MUAM) have been performed. The results obtained show that on average the calculated amplitude of the SPW1 in the stratosphere increased in 2000 and there is also an increase of its intra-seasonal variability conditioned by nonlinear interaction with the mean flow. This increase in the amplitudes of stratospheric vacillations during the last four decades allows us to suggest that stratospheric dynamics becomes more stochastic.