Quasi-stationary Planetary Waves Research Articles

Semidiurnal nonmigrating tides in low-latitude lower thermospheric NO: A climatology based on 20 years of Odin/SMR measurements

The Sub-Millimetre Radiometer (SMR) on board the Odin satellite provides almost 20 years of nitric oxide (NO) measurements in the mesosphere and lower thermosphere (MLT) at equatorial crossing local solar times (LSTs) of 6AM and 6PM. In this study, we use Odin/SMR observations to estimate how lower thermospheric NO mixing ratios at low latitudes are affected by solar nonmigrating tides. Most of the previous studies based on satellite data have focused on the signatures of diurnal tides in the MLT and above, while we concentrate here on nonmigrating semidiurnal tides. To study the contribution of these tides to NO mixing ratio variations, we average pairs of NO measurements along ascending and descending orbital tracks at 107 km altitude over latitudes between −40°and +40°. We consider monthly climatologies of these pair-averages and analyse residuals with respect to their zonal mean. In this way, it is possible to study the effect of nonmigrating even-numbered tidal components, albeit there is a non-tidal component arising largely from quasi-stationary planetary waves. Spectral wave amplitudes are extracted using a Fourier transform as function of (apparent) zonal wavenumber with a focus around −30°, −20°and 30°latitudes. From our analysis, it appears that the semidiurnal (apparent) zonal wavenumber 4 arising from the SW6 and SE2 tides is dominant close to the equator (e.g., at −20°), except during some boreal summer months (June, July, August). On the other hand, wave-1 plays a more prominent role at subtropical latitudes, especially in the southern hemisphere, where it surpasses wave-4 during 7 months (March and May-to-October) at −30°. There is little observational evidence to date documenting the presence of the semidiurnal nonmigrating tides in NO in the low-latitude MLT. Our results hence provide one of the first evidences of the climatological signature of these tides in NO, in an altitude range that remains poorly observed.

The Guiding Role of Rossby Wave Energy Dispersion Theory for Studying East Asian Monsoon System Dynamics

This paper is written to commemorate the 10th anniversary of academician Ye Duzheng (Yeh T.C.) pass away and his great contributions to the development of atmospheric dynamics. Under the inspiration and guidance of the theory of Rossby wave energy dispersion, remarkable progresses have been made in research on planetary wave dynamics and teleconnections of atmospheric circulation anomalies. This paper aims to make a brief review of the studies on the propagating characteristics of quasi-stationary planetary waves in a three-dimensional spherical atmosphere and the dynamic processes of the interannual and interdecadal variabilities of the East Asian summer and winter monsoon systems. Especially, this paper systematically reviews the progresses of the studies on the impacts of the interannual and interdecadal variabilities of the East Asia/Pacific (EAP) pattern teleconnection wave train propagating along the meridional direction over East Asia and the “Silk Road” pattern teleconnection wave train propagating along the zonal direction within the subtropical jet from West Asia to East Asia on the East Asian summer monsoon system and the summer precipitation variability in China, under the guidance of the theory of Rossby wave energy dispersion. Moreover, this paper reviews the dynamic processes of the impact of the interannual and interdecadal oscillations of the propagating waveguides of boreal quasi-stationary planetary waves on the variability of the East Asian winter monsoon system.

The global atmospheric energy transport analysed by a wavelength-based scale separation

Abstract. The atmosphere transports energy polewards by circulation cells and eddies. To the present day, there has been a knowledge gap regarding the preferred spatial scales and physical mechanisms of eddy energy transport. To fill the gap, we separate the meridional atmospheric energy transport in the ERA5 reanalysis by spatial scales and into quasi-stationary and transient flow patterns and latent and dry components. Baroclinic instability is the major instability mechanism in the transient synoptic scales and is responsible for forming cyclones, anticyclones, and small-scale Rossby waves. At the planetary scales, circulation patterns are often induced by other mechanisms such as flow interaction with orography and land–sea heating contrasts. However, a separation between circulation patterns at the synoptic and planetary scales has yet to be established. We find that both baroclinically induced and transient energy transport is predominantly associated with eddies at wavelengths between 2000 and 8000 km. The maxima in both types of transport occur at wavelengths around 5000 km, in good agreement with linear baroclinic theory. Since these results are independent of latitude, we adapt the scale separation of the energy transport to be based on the wavelength instead of the previously used wavenumber. We define the synoptic transport by the wavelength band between 2000 and 8000 km. We analyse the annual and seasonal mean in the energy transport components and their inter-annual variability. The scale-separated transport components are fairly similar in both hemispheres. Transport by synoptic waves is the largest contributor to extra-tropical energy and moisture transport, mainly of a transient character, and is influenced little by seasonality. In contrast, transport by planetary waves depends highly on the season and has two distinct characteristics. (1) In the extra-tropical winter, planetary waves are important due to a large transport of dry energy. This planetary transport features the largest inter-annual variability of all components and is mainly quasi-stationary in the Northern Hemisphere but transient in its southern counterpart. (2) In the sub-tropical summer, quasi-stationary planetary waves are the most important transport component, mainly due to moisture transport, presumably associated with monsoons. In contrast to transport by planetary and synoptic waves, only a negligible amount of energy is transported by mesoscale eddies (< 2000 km).

Central Asian Precipitation Extremes Affected by an Intraseasonal Planetary Wave Pattern

Abstract The characteristics and mechanisms of extreme precipitation events over central Asia are investigated based on daily data and percentile criteria. The composited extreme precipitation events have a life cycle of about 10 days. The precipitation signal is weak but significant from eight to two days before the peak. Then it amplifies rapidly and reaches its peak in two days. The earliest precipitation signal appears over the northwest of central Asia and moves southeastward. It maximizes over northern central Asia and the windward side of the mountainous region, suggesting the role of orography in the precipitation extremes. Diagnosis suggests that the precipitation extremes are closely tied to a quasi-stationary planetary wave train emanating from the North Atlantic and the resultant disturbed Asian subtropical jet. The sharpened and accelerated Atlantic jet 10 days before the precipitation peak feeds a cyclonic anomaly at its exit region, which disperses energy downstream and forms a well-defined barotropic Rossby wave train in the Asian subtropical jet, resulting in an anomalous cyclone to the west of central Asia. On the one hand, the cyclone-induced water vapor transport into central Asia enhances the moisture content and increases the convective instability in the lower troposphere, providing a favorable thermodynamical condition for deep convections. On the other hand, the vorticity and temperature advection induced by this cyclone and the mechanical lifting on the windward side of mountains provide a favorable dynamical condition for ascending motion, which triggers the release of convective instability and the occurrence of precipitation extremes in central Asia.

Dynamical evolution of a minor sudden stratospheric warming in the Southern Hemisphere in 2019

Abstract. A major strong sudden stratospheric warming (SSW) occurred in the Southern Hemisphere (SH) stratosphere in 2002 (hereafter referred to as SSW2002), which is one of the most unusual winters in the SH. Following several warmings, the polar vortex broke down in midwinter. Eastward-traveling waves and their interaction with quasi-stationary planetary waves played an important role during this event. This study analyzed the Japanese 55-year reanalysis (JRA-55) dataset to examine the SSW event that occurred in the SH in 2019 (hereafter referred to as SSW2019). In 2019, a rapid temperature increase and decelerated westerly winds were observed at the polar cap, but since there was no reversal of westerly winds to easterly winds at 60∘ S in the middle to lower stratosphere, the SSW2019 was classified as a minor warming event. The results showed that quasi-stationary planetary waves of zonal wavenumber 1 developed during the SSW2019. The strong vertical component of the Eliassen–Palm flux with zonal wavenumber 1 is indicative of pronounced propagation of planetary waves to the stratosphere. The wave driving in September 2019 was larger than that of the major SSW event in 2002. Major SSWs tend to accompany preceding minor warmings, preconditioning, which changes the zonal flow that weaken the polar night jet as seen in SSW2002. A similar preconditioning was hardly observed in SSW2019. The strong wave driving in SSW2019 occurred in high latitudes. Waveguides (i.e., positive values of the refractive index squared) were found at high latitudes in the upper stratosphere during the warming period, which provided favorable conditions for quasi-stationary planetary waves to propagate upward and poleward.

QUASI-STATIONARY PLANETARY WAVES IN MID-LATITUDE STRATOSPHERE–MESOSPHERE IN WINTER 2011-2020

The 10-year climatology (2011–2020) of quasi-stationary planetary waves in the mid-latitude stratosphere and mesosphere (40–50N, up to 90 km) has been analyzed. Longitude–altitude sections of geopotential height and ozone have been obtained using the Aura MLS satellite data. It is found that stationary wave 1 propagates into the mesosphere from the North American High and Icelandic Low, which are adjacent surface pressure anomalies in the structure of stationary wave 2. Unexpectedly, the strongest pressure anomaly in the Aleutian Low region does not contribute to the stationary wave 1 formation in the mesosphere. The vertical phase transformations of stationary waves in geopotential height and ozone show inconsistencies that should be studied separately.

Global characteristics of the westward-propagating quasi-16-day wave with zonal wavenumber 1 and the connection with the 2012/2013 SSW revealed by ERA-Interim

We analyze global characteristics of the westward-propagating quasi-16-day wave (Q16DW) with zonal wavenumber 1 (W1) in the troposphere and stratosphere using zonal wind, meridional wind, vertical velocity, temperature, geopotential, and potential vorticity data from the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis during one year from December 2012 to November 2013. The amplitudes of the W1 Q16DW are larger in the stratosphere than in the troposphere, and remarkable amplitudes are found at middle and high latitudes in the Northern Hemisphere (NH). More detailed analyses on the temporal variation in the W1 Q16DW show that this wave is significantly enhanced during the 2012/2013 Stratospheric Sudden Warming (SSW) event, and the strong wave most likely provides additional forcing on the splitting of the displaced polar vortex. Analysis of the Eliassen–Palm flux (EP flux) and its divergence of the interaction between W1 Q16DW and quasi-stationary planetary waves with wavenumber 1 during the 2012/2013 SSW event reveals that it causes an upward heat flux and exerts a westward acceleration on the background winds, indicating that this interaction plays an important role in the eastward stratospheric jet reversal. Moreover, the wave is amplified in the occurrence region of barotropic and/or baroclinic instability, suggesting a local source of the growing W1 Q16DW during this SSW event.

Dominant Characteristics of Early Autumn Arctic Sea Ice Variability and Its Impact on Winter Eurasian Climate

AbstractThe present study investigated dominant characteristics of autumn Arctic sea ice concentration (SIC) interannual variations and impacts of September–October (SO) mean SIC anomalies in the East Siberian–Chukchi–Beaufort (EsCB) Seas on winter Eurasian climate variability. Results showed that the decreased SO EsCB sea ice is favorable for tropospheric warming and positive geopotential height anomaly over the Arctic region one month later through transporting much more heat flux to the atmosphere from the open water. When entering the early winter (November–January), enhanced upward propagation of quasi-stationary planetary waves in the mid-high latitudes generates anomalous Eliassen–Palm flux convergence in the upper troposphere, which decelerates the westerly winds and maintains the positive geopotential height anomaly in the Arctic region. This anticyclonic anomaly extends southward into central-western Eurasia and leads to evident surface cooling there. Two months later, it further develops downstream accompanied by a deepened trough, making northeastern China experience a colder late winter (January–March). Meanwhile, an anticyclonic anomaly over the eastern North Pacific excites a horizontal eastward wave train and contributes to a positive (negative) geopotential height anomaly around Greenland (Europe), favoring a negative surface temperature anomaly over western Europe. In addition, the stratospheric polar vortex is also significantly weakened in the wintertime, which is attributed to a decreased meridional temperature gradient, and decelerated westerly winds provide a favorable condition for more quasi-stationary planetary waves propagating into the stratosphere. Some major features of atmospheric responses to EsCB sea ice loss are well reproduced in the CAM4 sensitivity experiments.

Future changes in stratospheric quasi-stationary wave-1 in the extratropical southern hemisphere spring and summer as simulated by ACCESS-CCM

Seasonally dependent quasi-stationary planetary wave activity in the southern hemisphere influences the distribution of ozone within and near the equatorward edge of the stratospheric polar vortex. Accurate representation of this zonal asymmetry in ozone is important in the characterisation of stratospheric circulation and climate and their associated effects at the surface. In this study, we used the Australian Community and Climate Earth System Simulator-Chemistry Climate Model to investigate the influence of greenhouse gases (GHGs) and ozone depleting substances (ODSs) on the zonal asymmetry of total column ozone (TCO) and 10 hPa zonal wind between 50 and 70°S. Sensitivity simulations were used from 1960 to 2100 with fixed ODSs and GHGs at 1960 levels and a regression model that uses equivalent effective stratospheric chlorine and carbon dioxide equivalent radiative forcing as the regressors. The model simulates the spring and summer zonal wave-1 reasonably well, albeit with a slight bias in the phase and amplitude compared to observations. An eastward shift in the TCO and 10 hPa zonal wave-1 is associated with both decreasing ozone and increasing GHGs. Amplitude increases are associated with ozone decline and amplitude decreases with GHG increases. The influence of ODSs typically outweigh those by GHGs, partly due to the GHG influence on TCO phase at 50°S likely being hampered by the Andes. Therefore, over the 21st century, influence from ozone recovery causes a westward shift and a decrease in amplitude. An exception is at 70°S during spring, where the GHG influence is larger than that of ozone recovery, causing a continued eastward trend throughout the 21st century. Also, GHGs have the largest influence on the 10 hPa zonal wave-1 phase, but still only induce a small change in the wave-1 amplitude. Different local longitudes also experience different rates of ozone recovery due to the changes in phase of the zonal wave-1. The results from this study have important implications for understanding future ozone layer distribution in the Southern Hemisphere under changing GHG and ODS concentrations. Important future work would involve conducting a similar study using a large ensemble of models to gain more statistically significant results.

Early indications of anomalous behaviour in the 2019 spring ozone hole over Antarctica

ABSTRACT The level of quasi-stationary planetary wave (QSW) activity in the Antarctic winter stratosphere provides insights into the likely behaviour of the ozone hole in the following spring months. Observation of anomalously large amplitude of the QSW in winter stratospheric temperatures is an indicator that strong disturbances to the polar vortex are likely to occur, and may lead to large reductions in both the area of the Antarctic ozone hole and the overall amount of stratospheric ozone that is depleted. In the sudden stratospheric warming (SSW) preconditions in 2019, the maximum QSW amplitude over Antarctica in August was approximately 12 K, which was only 2 K less than conditions prior to the unprecedented major SSW in 2002. The additional factors disturbing the Antarctic stratosphere in austral winter 2019 were anomalously warm sea surface temperatures in the central tropical Pacific Ocean and the western Indian Ocean, and the descending easterly phase of the Quasi-Biennial Oscillation. Under these preconditions, the Antarctic ozone hole in 2019 had the potential to demonstrate the early disruption and reduced level of the ozone depletion that has been confirmed by the satellite ozone observations. The anomalous ozone hole may also have important regional consequences for weather conditions in the Southern Hemisphere.

Local stratopause temperature variabilities and their embedding in the global context

Abstract. The stratopause is by definition the transition between the stratosphere and mesosphere. During winter the circulation at mid-latitudes and high latitudes in the stratosphere is mainly driven by quasi-stationary planetary waves (PWs), while the circulation in the mesosphere is mainly driven by gravity waves (GWs). The question arises of whether PWs or GWs dominate the variability of the stratopause. The most famous and dramatic variability of the middle atmosphere is a sudden stratospheric warming (SSW) generated by PWs interacting with the polar vortex. A similar phenomenon but smaller in magnitude and more regional is stratopause temperature enhancements (STEs) initially observed by local measurements and generated by breaking PWs. Thus it seems that PWs dominate the variability of the stratopause. In this study we want to quantify to which extent quasi-stationary PWs contribute to the stratopause variability. To do that we combine local lidar observations at Kühlungsborn (54∘ N, 11∘ E) and Andenes (69∘ N, 16∘ E) with global MERRA-2 reanalysis data bringing the local variability of the stratopause into the global context. Therefore we compare the temperature time series at Kühlungsborn and Andenes at 2 hPa, the altitude where STEs maximize, with characteristics (amplitude and phase) of PWs with wave numbers 1, 2 and 3. We found that for Kühlungsborn and Andenes 98 % of the local day-to-day variability of the stratopause can be explained by the variability of PWs with wave number 1, 2 and 3. Thus, the winter stratopause day-to-day variability is highly dominated by the variability of PWs.

Upper-tropospheric bridging of wintertime surface climate variability in the Euro-Atlantic region and northern Asia

A remarkable feature of interannual climate variability is a robust link of wintertime anomalies of surface air temperature (SAT) in northern Asia to pan-Atlantic SAT variations associated with the North Atlantic Oscillation (NAO). Here statistical analyses of data from the era of satellite observations (1979–2017) are used to show that about 80% of the variance of the winter (December-March) mean area-averaged SAT anomalies in northern Asia can be explained by the anomalous surface circulation associated with an NAO-like mode of sea level pressure variability over extratropical Eurasia. These SAT anomalies are related equally strongly to the “Lake Baikal” vortex representing variations of the upper-tropospheric circulation over northern Asia. Support is given for the scenario that this vortex drives SAT anomalies in northern Asia via surface-reaching displacements of isentropic surfaces and that it is coupled to climate variability in the Euro-Atlantic sector via interactions between the North Atlantic storm track, quasi-stationary planetary waves, and zonal-mean zonal winds. The results underpin the importance of a lesser-known zonal wavenumber-3 structure of disturbances trapped over Eurasia by the polar front jet rather than the better-known zonal wavenumber-5 structure of disturbances trapped by the subtropical jet for NAO teleconnections.

Quasi-stationary extratropical wave trains associated with distinct tropical Pacific seasonal mean convection patterns: observational and AMIP model results

In this paper, mechanisms for the formation of quasi-stationary extratropical wave trains associated with four distinct winter patterns of seasonal mean tropical Pacific convection anomalies, including the western CP (W-CP) pattern, La Nina (LN) pattern, CP El Nino (CPEN) pattern and EP El Nino (EPEN) pattern, are investigated by utilizing observational and six Atmospheric Model Intercomparison Project Phase 5 (AMIP5) high-skilled models (ACCESS1.0, CanAM4, CCSM4, CMCC-CM, HadGEM2-A, MPI-ESM-LR) datasets. The observational results show that tropical Pacific forcing induces upper-level anomalous Rossby wave source that force vorticity over the central North Pacific through modulating the local Hadley circulation, then excite eastward propagation of quasi-stationary planetary waves penetrating into the North Atlantic, and finally establish the associated teleconnections in mid- and high-latitudes by local synoptic eddy–mean flow interaction. Different structures of wave trains may be explained by evident differences in intensity and scale of the extratropical vorticity forcing originating from distinct convection anomalies over the tropical Pacific. The MME results further confirm the possible physical processes obtained from observations, but there still exist some significant differences. These cover the North Pacific and North America and can possibly be attributed to the models’ simulation ability of extratropical vorticity forcing and local storm track intensity. In addition, the stratosphere–troposphere interaction plays an important role for extratropical atmospheric circulation anomalies. The stratospheric polar vortex not only modulates the underlying wave train, especially for the North Atlantic/Europe sector, but is also affected by the upper-tropospheric disturbance in high-latitudes through the upward wave activity flux.

The influence of large amplitude planetary waves on the Antarctic ozone hole of austral spring 2017

Quasi-stationary planetary wave activity in the lower Antarctic stratosphere in the late austral winter was an important contributor to the preconditioning of the ozone hole in spring 2017. Observations show that the ozone hole area (OHA) in spring 2017 was at the level of 1980s, that is, almost half the maximum size in 2000s. The observed OHA was close to that forecasted based on a least-squares linear regression between wave amplitude in August and OHA in September–November. We show that the key factor which contributed to the preconditioning of the Antarctic stratosphere for a relatively small ozone hole in the spring of 2017 was the development of large-amplitude stratospheric planetary waves of zonal wave numbers 1 and 2 in late winter. The waves likely originated from tropospheric wave trains and promoted the development of strong mid-latitude anticyclones in the lower stratosphere which interacted with the stratospheric polar vortex and strongly eroded the vortex in August and September, mitigating the overall level of ozone loss.

High-Altitude (0–100 km) Global Atmospheric Reanalysis System: Description and Application to the 2014 Austral Winter of the Deep Propagating Gravity Wave Experiment (DEEPWAVE)

AbstractA data assimilation system (DAS) is described for global atmospheric reanalysis from 0- to 100-km altitude. We apply it to the 2014 austral winter of the Deep Propagating Gravity Wave Experiment (DEEPWAVE), an international field campaign focused on gravity wave dynamics from 0 to 100 km, where an absence of reanalysis above 60 km inhibits research. Four experiments were performed from April to September 2014 and assessed for reanalysis skill above 50 km. A four-dimensional variational (4DVAR) run specified initial background error covariances statically. A hybrid-4DVAR (HYBRID) run formed background error covariances from an 80-member forecast ensemble blended with a static estimate. Each configuration was run at low and high horizontal resolution. In addition to operational observations below 50 km, each experiment assimilated 105 observations of the mesosphere and lower thermosphere (MLT) every 6 h. While all MLT reanalyses show skill relative to independent wind and temperature measurements, HYBRID outperforms 4DVAR. MLT fields at 1-h resolution (6-h analysis and 1–5-h forecasts) outperform 6-h analysis alone due to a migrating semidiurnal (SW2) tide that dominates MLT dynamics and is temporally aliased in 6-h time series. MLT reanalyses reproduce observed SW2 winds and temperatures, including phase structures and 10–15-day amplitude vacillations. The 0–100-km reanalyses reveal quasi-stationary planetary waves splitting the stratopause jet in July over New Zealand, decaying from 50 to 80 km then reintensifying above 80 km, most likely via MLT forcing due to zonal asymmetries in stratospheric gravity wave filtering.

On the origin of the mesospheric quasi-stationary planetary waves in the unusual Arctic winter 2015/2016

Abstract. The midwinter 2015/2016 was characterized by an unusually strong polar night jet (PNJ) and extraordinarily large stationary planetary wave (SPW) amplitudes in the subtropical mesosphere. The aim of this study is, therefore, to find the origin of these mesospheric SPWs in the midwinter 2015/2016 study period. The study duration is split into two periods: the first period runs from late December 2015 until early January 2016 (Period I), and the second period from early January until mid-January 2016 (Period II). While the SPW 1 dominates in the subtropical mesosphere in Period I, it is the SPW 2 that dominates in Period II. There are three possibilities explaining how SPWs can occur in the mesosphere: (1) they propagate upward from the stratosphere, (2) they are generated in situ by longitudinally variable gravity wave (GW) drag, or (3) they are generated in situ by barotropic and/or baroclinic instabilities. Using global satellite observations from the Microwave Limb Sounder (MLS) and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) the origin of the mesospheric SPWs is investigated for both time periods. We find that due to the strong PNJ the SPWs were not able to propagate upward into the mesosphere northward of 50∘ N but were deflected upward and equatorward into the subtropical mesosphere. We show that the SPWs observed in the subtropical mesosphere are the same SPWs as in the mid-latitudinal stratosphere. Simultaneously, we find evidence that the mesospheric SPWs in polar latitudes were generated in situ by longitudinally variable GW drag and that there is a mixture of in situ generation by longitudinally variable GW drag and by instabilities at mid-latitudes. Our results, based on observations, show that the abovementioned three mechanisms can act at the same time which confirms earlier model studies. Additionally, the possible contribution from, or impact of, unusually strong SPWs in the subtropical mesosphere to the disruption of the quasi-biennial oscillation (QBO) in the same winter is discussed.

Impact of northern Eurasian snow cover in autumn on the warm Arctic–cold Eurasia pattern during the following January and its linkage to stationary planetary waves

The connection between Eurasian snow cover (SC) in autumn and Eurasian winter mean surface air temperature (SAT) has been identified by many studies. However, some recent observations indicate that early and late winter climate sometimes shows an out-of-phase relationship, suggesting that the winter mean situation might obscure the important relationships that are relevant for scientific research and applications. This study investigates the relationship between October northern Eurasian SC (NESC; 58°–68°N, 30°–90°E) and Eurasian SAT during the winter months and finds a significant relationship only exists in January. Generally, following reduced October NESC, the East Asian trough and Ural high are intensified in January, and anomalous northeasterly winds prevail in mid-latitudes, causing cold anomalies over Eurasia. Meanwhile, anomalous southwesterly winds along the northern fringe of the Ural high favor warm anomalies in the Arctic. The dynamical mechanism for the connection between NESC in October and the warm Arctic–cold Eurasia (WACE) anomaly in January is further investigated from the perspective of quasi-stationary planetary wave activity. It is found that planetary waves with zonal wavenumber-1 (ZWN1) play a dominant role in this process. Specifically, the ZWN1 pattern of planetary-scale waves concurrent with October NESC anomaly extends from the surface to the upper-stratosphere. It persists in the stratosphere through November–December and propagates downward to the surface by the following January, making the connection between October NESC and January climate possible. Additionally, the influence of October NESC on the January WACE pattern has intensified since the early-2000s.

Representation of the tropical stratospheric zonal wind in global atmospheric reanalyses

Abstract. This paper reports on a project to compare the representation of the monthly-mean zonal wind in the equatorial stratosphere among major global atmospheric reanalysis data sets. The degree of disagreement among the reanalyses is characterized by the standard deviation (SD) of the monthly-mean zonal wind and this depends on latitude, longitude, height, and the phase of the quasi-biennial oscillation (QBO). At each height the SD displays a prominent equatorial maximum, indicating the particularly challenging nature of the reanalysis problem in the low-latitude stratosphere. At 50–70 hPa the geographical distributions of SD are closely related to the density of radiosonde observations. The largest SD values are over the central Pacific, where few in situ observations are available. At 10–20 hPa the spread among the reanalyses and differences with in situ observations both depend significantly on the QBO phase. Notably the easterly-to-westerly phase transitions in all the reanalyses except MERRA are delayed relative to those directly observed in Singapore. In addition, the timing of the easterly-to-westerly phase transitions displays considerable variability among the different reanalyses and this spread is much larger than for the timing of the westerly-to-easterly phase changes. The eddy component in the monthly-mean zonal wind near the Equator is dominated by zonal wavenumber 1 and 2 quasi-stationary planetary waves propagating from midlatitudes in the westerly phase of the QBO. There generally is considerable disagreement among the reanalyses in the details of the quasi-stationary waves near the Equator. At each level, there is a tendency for the agreement to be best near the longitude of Singapore, suggesting that the Singapore observations act as a strong constraint on all the reanalyses. Our measures of the quality of the reanalysis clearly show systematic improvement over the period considered (1979–2012). The SD among the reanalysis declines significantly over the record, although the geographical pattern of SD remains nearly constant.

The influence of mid-latitude storm tracks on hot, cold, dry and wet extremes.

Changes in mid-latitude circulation can strongly affect the number and intensity of extreme weather events. In particular, high-amplitude quasi-stationary planetary waves have been linked to prolonged weather extremes at the surface. In contrast, analyses of fast-traveling synoptic-scale waves and their direct influence on heat and cold extremes are scarce though changes in such waves have been detected and are projected for the 21st century. Here we apply regression analyses of synoptic activity with surface temperature and precipitation in monthly gridded observational data. We show that over large parts of mid-latitude continental regions, summer heat extremes are associated with low storm track activity. In winter, the occurrence of cold spells is related to low storm track activity over parts of eastern North America, Europe, and central- to eastern Asia. Storm tracks thus have a moderating effect on continental temperatures. Pronounced storm track activity favors monthly rainfall extremes throughout the year, whereas dry spells are associated with a lack thereof. Trend analyses reveal significant regional changes in recent decades favoring the occurrence of cold spells in the eastern US, droughts in California and heat extremes over Eurasia.

Characterisation of quasi-stationary planetary waves in the Northern MLT during summer

Observations of planetary wave (PW) activity in the northern hemisphere, polar summer mesosphere and lower thermosphere (MLT) are presented. Meteor winds from a northern hemisphere chain of SuperDARN radars have been used to monitor the meridional wind along a latitude band (51–66°N) in the MLT. A stationary PW-like longitudinal structure with a strong zonal PW number 1 characteristic is persistently observed year-to-year during summer. Here we characterize the amplitude and the phase structure of this wave in the MLT. The Modern-Era Retrospective Analysis for Research and Application (MERRA) of the NASA Global Modelling and Assimilation Office has been used to evaluate possible sources of the observed longitudinal perturbation in the mesospheric meridional wind by investigating the amplitudes and phases of PWs in the underlying atmosphere. The investigation shows that neither gravity wave modulation by lower atmospheric PWs nor direct propagation of PWs from the lower atmosphere are a significant cause of the observed longitudinal perturbation. However, the data are not of sufficient scope to investigate longitudinal differences in gravity wave sources, or to separate the effects of instabilities and inter-hemispheric propagation as possible causes for the large PW present in the summer MLT.