Stratospheric control of planetary waves Hitchcock, Peter; Haynes, Peter H.
Geophysical research letters,
28 November 2016, Letnik:
43, Številka:
22
Journal Article
Recenzirano
Odprti dostop
The effects of imposing at various altitudes in the stratosphere zonally symmetric circulation anomalies associated with a stratospheric sudden warming are investigated in a mechanistic circulation ...model. A shift of the tropospheric jet is found even when the anomalies are imposed only above 2 hPa. Their influence is communicated downward through the planetary wave field via three distinct mechanisms. First, a significant fraction of the amplification of the upward fluxes of wave activity prior to the central date of the warming is due to the coupled evolution of the stratospheric zonal mean state and the wave field throughout the column. Second, a downward propagating region of localized wave, mean‐flow interaction is active around the central date but does not penetrate the tropopause. Third, there is deep, vertically synchronous suppression of upward fluxes following the central date. The magnitude of this suppression correlates with that of the tropospheric jet shift.
Key Points
Coupled evolution of the stratospheric mean state and planetary waves drives half of their amplification prior to sudden warmings
Imposing sudden‐warming‐like anomalies creates a downward migrating region of local wave mean‐flow interaction confined to the stratosphere
The dependence of the equatorward shift of the tropospheric jet on the height of the imposed anomalies is quantified
To explore the mechanisms linking Arctic sea ice loss to changes in midlatitude surface temperatures, we conduct idealized modeling experiments using an intermediate general circulation model and ...with sea ice loss confined to the Atlantic or Pacific sectors of the Arctic (Barents‐Kara or Chukchi‐Bering Seas). Extending previous findings, there are opposite effects on the winter stratospheric polar vortex for both large‐magnitude (late 21st century) and moderate‐magnitude sea ice loss. Accordingly, there are opposite tropospheric Arctic Oscillation (AO) responses for moderate‐magnitude sea ice loss. However, there are similar strength negative AO responses for large‐magnitude sea ice loss, suggesting that tropospheric mechanisms become relatively more important than stratospheric mechanisms as the sea ice loss magnitude increases. The midlatitude surface temperature response for each loss region and magnitude can be understood as the combination of an “indirect” part induced by the large‐scale circulation (AO) response, and a residual “direct” part that is local to the loss region.
Key Points
Sea ice losses confined to the Atlantic or Pacific sector of the Arctic have opposite effects on the winter stratospheric polar vortex
Tropospheric Arctic Oscillation response is dependent on the sea ice loss region (Atlantic or Pacific sector of the Arctic) and magnitude
Broader Arctic Oscillation‐induced midlatitude surface temperature response is modulated by the local influence of the sea ice loss region
A configuration of an idealized general circulation model has been obtained in which a deep, stratospheric, equatorial, westerly jet is established that is spontaneously and quasi-periodically ...disrupted by shallow easterly jets. Similar to the disruption of the quasi-biennial oscillation (QBO) observed in early 2016, meridional fluxes of wave activity are found to play a central role. The possible relevance of two feedback mechanisms to these disruptions is considered. The first involves the secondary circulation produced in the shear zones on the upper and lower flanks of the easterly jet. This is found to play a role in maintaining the aspect ratio of the emerging easterly jet. The second involves the organization of the eddy fluxes by the mean flow: the presence of a weak easterly anomaly within a tall, tropical, westerly jet is demonstrated to produce enhanced and highly focused wave activity fluxes that reinforce and strengthen the easterly anomalies. The eddies appear to be organized by the formation of strong potential vorticity gradients on the subtropical flanks of the easterly anomaly. Similar wave activity and potential vorticity structures are found in the ERA-Interim for the observed QBO disruption, indicating this second feedback was active then.
Abstract
Overshoots are convective air parcels that rise beyond their level of neutral buoyancy. A giga-large-eddy simulation (100-m cubic resolution) of “Hector the Convector,” a deep convective ...system that regularly forms in northern Australia, is analyzed to identify overshoots and quantify the effect of hydration of the stratosphere. In the simulation, 1507 individual overshoots were identified, and 46 of them were tracked over more than 10 min. Hydration of the stratosphere occurs through a sequence of mechanisms: overshoot penetration into the stratosphere, followed by entrainment of stratospheric air and then by efficient turbulent mixing between the air in the overshoot and the entrained warmer air, leaving the subsequent mixed air at about the maximum overshooting altitude. The time scale of these mechanisms is about 1 min. Two categories of overshoots are distinguished: those that significantly hydrate the stratosphere and those that have little direct hydration effect. The former reach higher altitudes and hence entrain and mix with air that has higher potential temperatures. The resulting mixed air has higher temperatures and higher saturation mixing ratios. Therefore, a greater amount of the hydrometeors carried by the original overshoot sublimates to form a persistent vapor-enriched layer. This makes the maximum overshooting altitude the key prognostic for the parameterization of deep convection to represent the correct overshoot transport. One common convection parameterization is tested, and the results suggest that the overshoot downward acceleration due to negative buoyancy is too large relative to that predicted by the numerical simulations and needs to be reduced.
The history of observational studies regarding the influence of the stratospheric quasi-biennial oscillation (QBO) on the tropical and subtropical upper troposphere and lower stratosphere (UTLS) is ...reviewed. QBO phases of westerly (QBO W) and easterly (QBO E) winds are defined in the lower stratosphere. During 1960–1978, radiosonde data revealed QBO modulation of the UTLS with a warm anomaly during QBO W in the tropics and cool anomalies near 30°S and 30°N. This pattern agreed with theory of the QBO mean meridional circulation (MMC), which predicted a coherent, antiphased response between the tropics and subtropics. During 1978–1994, satellite observations of aerosol and temperature confirmed the existence of the QBO MMC. During 1994–2001, global data sets enabled analysis of zonal mean QBO variations in tropopause temperature. In 2001, National Centers for Environmental Prediction reanalyses for the 42-yr period 1958–2000 revealed seasonal and geographical variations in QBO W–E tropopause temperature, pressure, and zonal wind, which are presented here. An update using the 38-yr Modern-Era Retrospective Analysis for Research and Applications, Version 2, and the 40-yr European Centre for Medium Range Weather Forecasts Reanalysis (ERA-Interim) data sets provides a more complete view of seasonal and geographical variation.The QBO range in tropical tropopause values is ∼ 0.5–2 K, ∼ 100–300 m, and ∼ 1–3 hPa, being colder and higher during QBO E, especially during boreal winter and spring. The QBO temperature signal tends to be larger near regions where deep convection is common. The QBO signal in the southern subtropics is enhanced during austral winter. During QBO W, the subtropical westerly jet is enhanced, while the Walker circulation is weaker, especially during boreal spring. A new climatology of the QBO MMC is presented. QBO E may enhance convection by reducing both static stability and wind shear in the UTLS.
Volcanic eruptions that inject sulfur dioxide into the stratosphere have
the potential to alter large-scale circulation patterns, such as the
quasi-biennial oscillation (QBO), which can affect ...weather and transport of
chemical species. Here, we conduct simulations of tropical volcanic
eruptions using the UM-UKCA aerosol-climate model with an explicit
representation of the QBO. Eruptions emitting 60 Tg of SO2 (i.e. the magnitude of the 1815
Mt. Tambora eruption) and 15 Tg of SO2 (i.e. the magnitude of the 1991 Mt.
Pinatubo eruption) were initiated at the Equator during two
different QBO states. We show that tropical eruptions delay the progression
of the QBO phases, with the magnitude of the delay dependent on the initial
wind shear in the lower stratosphere and a much longer delay when the shear
is easterly than when it is westerly. The QBO response in our model is
driven by vertical advection of momentum by the stronger tropical upwelling
caused by heating due to the increased volcanic sulfate aerosol loading.
Direct aerosol-induced warming with subsequent thermal wind adjustment, as
proposed by previous studies, is found to only play a secondary role. This
interpretation of the response is supported by comparison with a simple
dynamical model. The dependence of the magnitude of the response on the
initial QBO state results from differences in the QBO secondary circulation.
In the easterly shear zone of the QBO, the vertical component of the
secondary circulation is upward and reinforces the anomalous upwelling
driven by volcanic aerosol heating, whereas in the westerly shear zone the
vertical component is downward and opposes the aerosol-induced upwelling. We
also find a change in the latitudinal structure of the QBO, with the
westerly phase of the QBO strengthening in the hemisphere with the lowest
sulfate aerosol burden. Overall, our study suggests that tropical eruptions
of Pinatubo magnitude or larger could force changes to the progression of
the QBO, with particularly disruptive outcomes for the QBO if the eruption
occurs during the easterly QBO shear.
Flow on a beta‐plane driven by a steady localised anticyclonic forcing of potential vorticity (or equivalently a mass source) is considered as a simple model of the Asian monsoon flow in the upper ...troposphere. Previous authors have noted that the response may be steady, or unsteady, according to the magnitude of the forcing, with the unsteadiness manifested as westward eddy shedding. A detailed study of the transition between steady and eddy‐shedding regimes reveals a third regime ('break up'), for intermediate forcing magnitude, where the flow is steady in the neighbourhood of the forcing, but the westward extending plume of low potential vorticity breaks up into isolated anticyclonic vortices some distance away from the forcing region. A related spatio‐temporal instability problem for flow on a beta plane is specified and analysed. The flow can be stable, convectively unstable or absolutely unstable. It is argued that these three stability regimes correspond to the steady, break‐up and eddy‐shedding regimes for the forced flow and good quantitative correspondence between the regimes is demonstrated by explicit solution of the spatio‐temporal stability problem.
Snapshots of the stream function and potential vorticity response in quasi‐geostrophic single‐layer experiments forced with a steady, localised mass source. The response transitions between different states (steady, break‐up and shedding), defined in terms of temporal and spatial variability, when varying the magnitude F0 or length‐scale r0 of the forcing. The different behaviours can be explained by the spatio‐temporal stability properties of the steady linear response and a transition of the system into an absolutely or convectively unstable regime.
Concentrations of water vapour entering the tropical lower stratosphere are primarily determined by conditions that air parcels encounter as they are transported through the tropical tropopause layer ...(TTL). Here we quantify the relative roles of variations in TTL temperatures and transport in determining seasonal and interannual variations of stratospheric water vapour. Following previous studies, we use trajectory calculations with the water vapour concentration set by the Lagrangian dry point (LDP) along trajectories. To assess the separate roles of transport and temperatures, the LDP calculations are modified by replacing either the winds or the temperatures with those from different years to investigate the wind or temperature sensitivity of water vapour to interannual variations and, correspondingly, with those from different months to investigate the wind or temperature sensitivity to seasonal variations. Both ERA-Interim reanalysis data for the 1999-2009 period and data generated by a chemistry-climate model (UM-UKCA) are investigated. Variations in temperatures, rather than transport, dominate interannual variability, typically explaining more than 70 % of variability, including individual events such as the 2000 stratospheric water vapour drop. Similarly seasonal variation of temperatures, rather than transport, is shown to be the dominant driver of the annual cycle in lower stratospheric water vapour concentrations in both the model and reanalysis, but it is also shown that seasonal variation of transport plays an important role in reducing the seasonal cycle maximum (reducing the annual range by about 30 %).
Stratospheric water vapor affects the Earth's radiative balance and stratospheric chemistry, yet its future changes are uncertain and not fully understood. The influence of deep convection on ...stratospheric water vapor remains subject to debate. This letter presents a detailed process‐based model study of the impact of convective ice sublimation on stratospheric water vapor in response to CO2 forced climate change. The influence of convective injection is found to be limited by the vertical profile of temperature and saturation vapor pressure in the tropical tropopause layer, not by the frequency of occurrence. Lagrangian trajectory analysis shows the relative contributions to stratospheric water vapor from sublimation and large‐scale transport are approximately unchanged when CO2 is increased. The results indicate the role of convective ice injection for stratospheric water vapor in a warmer climate remains constrained by large‐scale temperatures.
Plain Language Summary
Trends in stratospheric water vapor impact on both ozone depletion and the climate. Water vapor enters the stratosphere in the tropics and is tightly constrained by the cold temperatures around the tropopause (around 15–17 km altitude). For the present day climate the contribution from the direct injection of ice into the stratosphere by deep convection is thought to be relatively small, but it has been suggested these may increase in a warmer climate. If convection becomes more frequent and stronger under greenhouse gas induced climate change, the response of stratospheric water vapor might be different from that implied simply by changes to tropical tropopause temperatures. This study examines the roles of convection and large‐scale temperatures and transport in determining the water budget in the tropical tropopause region in a climate model. In response to increased atmospheric carbon dioxide the model simulates increased stratospheric water vapor with a substantial contribution coming from more convective injection. However, the relative contribution of convective injection to stratospheric water vapor remains roughly constant as carbon dioxide increases. Therefore, irrespective of whether convection becomes stronger or more frequent, the impact of convective injection of water is found to be constrained by large‐scale temperatures.
Key Points
We present an innovative method to quantify the processes that lead to a modeled increase in stratospheric water vapor from increased CO2
As well as changes in large‐scale temperatures and transport, we find convective injection of ice contributes to stratospheric moistening
The frequency of convective injection increases as CO2 increases, but its relative contribution to stratospheric water vapor does not
We present a novel probabilistic deep learning approach, the “stochastic latent transformer” (SLT), designed for the efficient reduced‐order modeling of stochastic partial differential equations. ...Stochastically driven flow models are pertinent to a diverse range of natural phenomena, including jets on giant planets, ocean circulation, and the variability of midlatitude weather. However, much of the recent progress in deep learning has predominantly focused on deterministic systems. The SLT comprises a stochastically‐forced transformer paired with a translation‐equivariant autoencoder, trained toward the Continuous Ranked Probability Score. We showcase its effectiveness by applying it to a well‐researched zonal jet system, where the interaction between stochastically forced eddies and the zonal mean flow results in a rich low‐frequency variability. The SLT accurately reproduces system dynamics across various integration periods, validated through quantitative diagnostics that include spectral properties and the rate of transitions between distinct states. The SLT achieves a five‐order‐of‐magnitude speedup in emulating the zonally‐averaged flow compared to direct numerical simulations. This acceleration facilitates the cost‐effective generation of large ensembles, enabling the exploration of statistical questions concerning the probabilities of spontaneous transition events.
Plain Language Summary
Stochastically driven systems are widespread in nature, from jets on giant planets to ocean circulation and the variability of weather. We describe a novel machine learning (ML) approach to the time evolving behavior of a well‐studied stochastically‐driven zonal jet system, serving as an analog for mid‐latitude weather. Our approach involves constructing a probabilistic ML model of our stochastic system, in contrast to existing research that predominantly focuses on ML techniques for deterministic systems or apply deterministic models to systems that are inherently probabilistic. The model is evaluated using various metrics, confirming its ability to accurately capture the statistical properties of the original system. Our approach significantly speeds up simulations compared to traditional methods, enabling the creation of large data sets highlighting specific events of interest, such as spontaneous transitions between different states characterized by varying numbers of observed jets, shedding light on aspects of the system's behavior and predictability. We demonstrate advantages of the proposed method over existing techniques and expect that these methods are highly transferable to other systems that exhibit stochasticity and inherent uncertainties.
Key Points
We present a probabilistic machine learning approach for modeling the time evolution of stochastically driven systems
Applying this approach to a well‐researched zonal jet system, we achieve a five‐order‐of‐magnitude speedup in emulating the zonally‐averaged flow
This enables us to efficiently generate large ensembles, facilitating the process of establishing accurate probabilities of rare events, such as transition rates between different long‐lived states
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