The vertical structure of stratospheric planetary waves and its variability : theory and observations

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2000.Includes bibliographical references (p. 246-256).Observations of the vertical structure of stratospheric planetary waves reveal a large variety of structures, and a variability both on seasonal and on daily time scales. The extent to which linear wave theory explains these structures and their time evolution at a given time or season is not well known. The sensitivity of linear wave models to details of the basic state and model damping, both of which are not determined from observations in great accuracy, makes it hard to determine why the observations deviate from modeled waves in any given case. In addition, the ability of the observations to resolve the vertical structure of planetary waves is not obvious, given the low vertical resolution of satellite retrievals. The goal of this thesis is to understand the sources of observed variability of vertical wave structure, in particular, to determine whether linear wave theory can explain this variability, and whether the observations are capable of resolving it. We start by testing the ability of satellite retrievals to resolve the vertical structure of the waves. We calculate the radiances that a virtual satellite sitting at the top of our model atmosphere would see, and invert them to obtain retrieved temperature fields. The comparison to the model temperatures suggests that the retrievals are able to resolve their general features quite well, with a few exceptions. Above 1.5mb there is little observed information in the retrievals, and errors start growing above 5 mb. Also, small scale features are not resolvable, but most waves have large enough vertical wavelengths to be resolved. We also identify dynamic situations in the real atmosphere which are more prone to retrieval errors. These are mostly relevant to summer or to the breakup of the polar vortex, when the existence of critical surfaces may cause the waves to have sharp features. The next part consists of understanding the relation between vertical wave structure and the wave propagation characteristics of the basic state in a series of linear wave models, both steady state and time dependent. We study the normal modes on a one dimensional (vertical) troposphere-stratosphere system, using a framework of wave geometry, which allows us to generalize the results to many basic states. A large variety of vertical wave structures is found, similar to observed. This variety is due to the existence of stratospheric turning points. We extend these results to basic states that vary in latitude and height in a nonseparable way. The main problem is how to separate the wave propagation into the vertical and meridional directions. Our approach is diagnostic, where we calculate meridional and vertical wavenumbers from the steady state wave solution to a given basic state, and use them as a diagnostic of the basic state wave propagation characteristics. In particular, we are able to determine the location of turning surfaces for meridional and vertical propagation. By applying this wavenumber diagnostic to many model runs we show that the existence of a stratospheric waveguide renders the problem qualitatively one dimensional by determining the meridional wavenumber, regardless of the characteristics of the tropospheric forcing. In particular, the effects of damping and turning surfaces on the vertical structure are qualitatively as in the vertical propagation problem. In a complementary study, we regard the waves as consisting of many wave activity packets that propagate from the troposphere through the stratosphere, until they dissipate. A technique that follows a wave packet on its journey through the stratosphere, while keeping track of variations in wave activity that are due to refraction of the waves, is introduced and applied to the model runs. This allows us to separate between the contributions to the wave activity budget of damping, refraction, and time variations in the source of wave activity. Also, we can estimate the time scale for vertical propagation through the stratosphere of specific wave events. Finally, we use our diagnostics to study observed wave episodes. We show that the differences in vertical wave structure between middle and late winter episodes in the southern hemisphere can be explained as a linear response to the seasonal evolution of the basic state wave propagation characteristics. We also show that the occasional daily time scale variations of vertical wave structure within a given wave episode are qualitatively a linear response to time variations of the basic state wave propagation characteristics. Since the basic state variations are wave driven, the relevant theory is quasi-linear. Estimates of wave propagation time scales, obtained using our wave activity diagnostic, are also consistent with the theory. We take this is a qualitative assessment of the applicability of quasi-linear wave propagation theory on daily time scales, as well as an assessment of the observations of the waves and the basic state. The latter is not obvious since most of the relevant variations in the basic state occur above 5mb, where observations are less accurate.by Nili Harnik.Ph.D

The role of tropical and extra-tropical waves in the Hadley circulation

The tropical overturning circulation is examined in a moist aquaplanet general circulation model forced using a non-interactive sea surface temperature (SST) distribution that varies between a present-day Earth-like profile and one that is globally uniform. A traditional Hadley Cell (HC)-like flow is observed in all experiments along with the poleward transport of heat and angular momentum. In simulations with non-zero SST gradients, latent heat released from organized convection near the equator sets up a deep tropical cell; midlatitude baroclinic Rossby waves flux heat and angular momentum poleward, reinforcing the thermally direct circulation. As the imposed SST gradient is weakened, the HC transitions from a thermally and eddy-driven regime to one that's completely eddy-driven. When the SST is globally uniform, equatorial waves concentrate precipitation in the tropics and facilitate the lower-level convergence necessary for the ascending branch of the HC. Conventional midlatitude Rossby waves become very weak, but upper-level baroclinicity generates waves that cause equatorward transport of heat and poleward transport of momentum. Moreover, these upper-level waves induce a circulation that opposes the time-mean HC, thus highlighting the role of tropical waves in driving a traditional overturning flow for uniform SSTs. In all cases, anomalies associated with the tropical waves closely resemble those that sum to give the upper-level zonal mean divergent outflow. Through their ability to modulate tropical rainfall and the related latent heating, equatorial waves cause considerable hemispheric asymmetry in the HC and impart synoptic and intraseasonal variability to the tropical overturning circulation.Comment: 30 pages, 12 figures, submitted to QJRM

Radiative effects of ozone waves on the Northern Hemisphere polar vortex and its modulation by the QBO

The radiative effects induced by the zonally asymmetric part of the ozone field have been shown to significantly change the temperature of the NH winter polar cap, and correspondingly the strength of the polar vortex. In this paper, we aim to understand the physical processes behind these effects using the National Center for Atmospheric Research (NCAR)'s Whole Atmosphere Community Climate Model, run with 1960s ozone-depleting substances and greenhouse gases. We find a mid-winter polar vortex influence only when considering the quasi-biennial oscillation (QBO) phases separately, since ozone waves affect the vortex in an opposite manner. Specifically, the emergence of a midlatitude QBO signal is delayed by 1–2 months when radiative ozone-wave effects are removed. The influence of ozone waves on the winter polar vortex, via their modulation of shortwave heating, is not obvious, given that shortwave heating is largest during fall, when planetary stratospheric waves are weakest. Using a novel diagnostic of wave 1 temperature amplitude tendencies and a synoptic analysis of upward planetary wave pulses, we are able to show the chain of events that lead from a direct radiative effect on weak early fall upward-propagating planetary waves to a winter polar vortex modulation. We show that an important stage of this amplification is the modulation of individual wave life cycles, which accumulate during fall and early winter, before being amplified by wave–mean flow feedbacks. We find that the evolution of these early winter upward planetary wave pulses and their induced stratospheric zonal mean flow deceleration is qualitatively different between QBO phases, providing a new mechanistic view of the extratropical QBO signal. We further show how these differences result in opposite radiative ozone-wave effects between east and west QBOs

Downward Wave Coupling between the Stratosphere and Troposphere under Future Anthropogenic Climate Change

Downward wave coupling (DWC) is an important process that characterizes the dynamical coupling between the stratosphere and troposphere via planetary wave reflection. A recent modeling study indicated that natural forcing factors, including sea-surface temperature variability and quasi-biennial oscillation, influence DWC and the associated surface impact in the Northern Hemisphere (NH). In light of this, we further investigate how DWC in the NH is affected by anthropogenic forcings, using a fully coupled chemistry-climate model CESM1 (WACCM). The results indicate that the occurrence of DWC is significantly suppressed in the future, starting later in the seasonal cycle, with more events concentrated in late winter (February-March). The future decrease in DWC events is associated with enhanced wave absorption in the stratosphere due to increased greenhouse gases. The enhanced wave absorption is manifest as more absorbing types of stratospheric sudden warmings, with more events concentrated in early winter. This early winter condition leads to a delay in the development of the upper stratospheric reflecting surface, resulting in a shift in the seasonal cycle of DWC towards late winter. The tropospheric responses to DWC events in the future exhibit different spatial patterns compared to those of the past. In the North Atlantic sector, DWC-induced circulation changes are characterized by a poleward shift and an eastward extension of the tropospheric jet, while in the North Pacific sector, the circulation changes are characterized by a weakening of the tropospheric jet. These responses are consistent with a change in the pattern of DWC-induced synoptic-scale eddy-mean flow interaction

Influence of the Quasi-Biennial Oscillation and Sea Surface Temperature Variability on Downward Wave Coupling in the Northern Hemisphere

Downward wave coupling occurs when an upward propagating planetary wave from the troposphere decelerates the flow in the upper stratosphere, and forms a downward reflecting surface that redirects waves back to the troposphere. To test this mechanism and potential factors influencing the downward wave coupling, three 145-year sensitivity simulations with NCAR’s Community Earth System Model (CESM-WACCM), a state-of-the-art high-top chemistry-climate model, are analyzed. The results show that the QBO and SST variability significantly impact downward wave coupling. Without the QBO, the occurrence of downward wave coupling is significantly suppressed. In contrast, stronger and more persistent downward wave coupling occurs when SST variability is excluded. The above influence on the occurrence of downward wave coupling is mostly due to a direct influence of the QBO and SST variability on stratospheric planetary wave source and propagation. The strengths of the tropospheric circulation and surface responses to a given downward wave coupling event, however, behave differently. The surface anomaly is significantly weaker (stronger) in the experiment with fixed SSTs (without QBO), even though the statistical signal of downward coupling is strongest (weakest) in this experiment. This apparent mismatch is explained by the differences in the strength of the synoptic-scale eddy-mean flow feedback and the possible contribution of SST anomalies in the North Atlantic during DWC event. The weaker synoptic-scale eddy-mean flow feedback, and the absence of the positive NAO-related SST-tripole pattern in the fixed SST experiment are consistent with a weaker tropospheric response in this experiment. The results highlight the importance of synoptic-scale eddies in setting the tropospheric response to downward wave coupling

Mechanisms of Hemispherically Symmetric Climate Variability

Inspired by paleoclimate evidence that much past climate change has been symmetric about the equator, the causes of hemispherically symmetric variability in the recent observational record are examined using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis dataset and numerical models. It was found that the dominant cause of hemispherically symmetric variability is the El Nino–Southern Oscillation. During an El Nino event the Tropics warm at all longitudes and the subtropical jets in both hemi- spheres strengthen on their equatorward flanks. Poleward of the tropical warming there are latitude belts of marked cooling, extending from the surface to the tropopause in both hemispheres, at all longitudes and in all seasons. The midlatitude cooling is caused by changes in the eddy-driven mean meridional circulation. Changes in the transient eddy momentum fluxes during an El Nino event force upper-tropospheric ascent in midlatitudes through a balance between the eddy fluxes and the Coriolis torque. The eddy-driven ascent causes anomalous adiabatic cooling, which is primarily balanced by anomalous diabatic heating. Using a quasigeostrophic spherical model, forced by an imposed surface eddy disturbance of chosen wave- number and frequency, it is shown that the anomalous eddy momentum fluxes are caused by the impact that the changes in the tropically forced subtropical jets have on the propagation in the latitude–height plane of transient eddies. Changes in zonal winds, and associated changes in the meridional gradient of potential vorticity, create an anomalous region of low meridional wavenumber in the midlatitudes that refracts waves away both poleward and equatorward. Tropical forcing of variability in the eddy-driven mean meridional circulation is another way, in addition to Rossby wave teleconnections, whereby the Tropics can influence extratropical climate. Unlike teleconnections this mechanism causes climate variability that has strong zonally and hemispherically symmetric components and operates throughout the seasonal cycle

How Does Downward Planetary Wave Coupling Affect Polar Stratospheric Ozone in the Arctic Winter Stratosphere?

It is well established that variable wintertime planetary wave forcing in the stratosphere controls the variability of Arctic stratospheric ozone through changes in the strength of the polar vortex and the residual circulation. While previous studies focused on the variations in upward wave flux entering the lower stratosphere, here the impact of downward planetary wave reflection on ozone is investigated for the first time. Utilizing the MERRA2 reanalysis and a fully coupled chemistry–climate simulation with the Community Earth System Model (CESM1(WACCM)) of the National Center for Atmospheric Research (NCAR), we find two downward wave reflection effects on ozone: (1) the direct effect in which the residual circulation is weakened during winter, reducing the typical increase of ozone due to upward planetary wave events and (2) the indirect effect in which the modification of polar temperature during winter affects the amount of ozone destruction in spring. Winter seasons dominated by downward wave reflection events (i.e., reflective winters) are characterized by lower Arctic ozone concentration, while seasons dominated by increased upward wave events (i.e., absorptive winters) are characterized by relatively higher ozone concentration. This behavior is consistent with the cumulative effects of downward and upward planetary wave events on polar stratospheric ozone via the residual circulation and the polar temperature in winter. The results establish a new perspective on dynamical processes controlling stratospheric ozone variability in the Arctic by highlighting the key role of wave reflection

Adjustment of the atmospheric circulation to tropical Pacific SST anomalies: Variability of transient eddy propagation in the Pacific–North America sector

El Nin ̃o–Southern Oscillation (ENSO) related precipitation anomalies in North America are related to changes in the paths of storm systems across the Pacific Ocean, with a more southern route into southwestern North America during El Nin ̃os and a more northern route into the Pacific Northwest during La Ninas. Daily reanalysis data are analyzed to confirm these changes. Seasonal mean upper tropospheric eddy statistics show, for El Ninos (La Ninas), a pattern that is shifted southward (northward) compared with climatology. Paths of coherent phase propagation of transient eddies and of the propagation of wave packets are analyzed. A coherent path of propagation across the Pacific towards North America is identified that is more zonal during El Nin ̃o winters and, during La Ninas, has a dominant path heading northeastward to the Pacific Northwest. A second path heading southeastward from the central Pacific to the tropical east Pacific is more accentuated during La Ninas than El Ninos. These changes in wave propagation are reproduced in an ensemble of seasonal integrations of a general circulation model forced by a tropical Pacific sea-surface temperature pattern, confirming that the changes are forced by changes in the mean atmospheric state arising from changes in tropical sea-surface temperature. A simplified model with a specified basic state is used to model the storm tracks for El Nino and La Nina winters. The results suggest that the changes in transient eddy propagation and the eddy statistics can be understood in terms oftherefractionoftransienteddieswithindifferentbasicstates

Mechanisms of ENSO-forcing of hemispherically symmetric precipitation variability

The patterns of precipitation anomalies forced by the El Nin ̃o–Southern Oscillation during northern hemisphere winter and spring are remarkably hemispherically symmetric and, in the midlatitudes, have a prominent zonally symmetric component. Observations of global precipitation variability and the moisture budget within atmospheric reanalyses are examined to argue that the zonally symmetric component is caused by interactions between transient eddies and tropically-forced changes in the subtropical jets. During El Nino events the jets strengthen in each hemisphere and shift equatorward. Changes in the subtropical jet influence the transient-eddy momentum fluxes and the eddy-driven mean meridional circulation. During El Nino events, eddy-driven ascent in the midlatitudes of each hemisphere is accompanied by low-level convergence and brings increased precipitation. These changes in the transient-eddy and stationary-eddy moisture fluxes almost exactly cancel each other and, in sum, do not contribute to the zonal-mean precipitation anomalies. Propagation of anomalous stationary waves disrupts the zonal symmetry. Flow around the deeper Aleutian Low and the eastward extension of the Pacific jet stream supply the moisture for increased precipitation over the eastern North Pacific and the western seaboard of the United States, while transient-eddy moisture convergence supplies the moisture for increased precipitation over the southern United States. In each case, increased precipitation is fundamentally caused by anomalous ascent forced by anomalous heat and vorticity fluxes

100 years of progress in understanding the stratosphere and mesosphere

The stratosphere contains ~17% of Earth’s atmospheric mass, but its existence was unknown until 1902. In the following decades our knowledge grew gradually as more observations of the stratosphere were made. In 1913 the ozone layer, which protects life from harmful ultraviolet radiation, was discovered. From ozone and water vapor observations, a first basic idea of a stratospheric general circulation was put forward. Since the 1950s our knowledge of the stratosphere and mesosphere has expanded rapidly, and the importance of this region in the climate system has become clear. With more observations, several new stratospheric phenomena have been discovered: the quasi-biennial oscillation, sudden stratospheric warmings, the Southern Hemisphere ozone hole, and surface weather impacts of stratospheric variability. None of these phenomena were anticipated by theory. Advances in theory have more often than not been prompted by unexplained phenomena seen in new stratospheric observations. From the 1960s onward, the importance of dynamical processes and the coupled stratosphere–troposphere circulation was realized. Since approximately 2000, better representations of the stratosphere—and even the mesosphere—have been included in climate and weather forecasting models. We now know that in order to produce accurate seasonal weather forecasts, and to predict long-term changes in climate and the future evolution of the ozone layer, models with a well-resolved stratosphere with realistic dynamics and chemistry are necessary