A new classification of the Arctic spring transition in the middle atmosphere

<p>In the middle atmosphere, spanning the stratosphere and mesosphere, spring transition is the time period where the zonal circulation reverses from winter westerly to summer easterly which has a strong impact on the vertical wave propagation influencing the tropospheric and ionospheric variability. The spring transition can be rapid in form of a final sudden stratospheric warming (SSW, mainly dynamically driven) or slow (mainly radiatively driven) but also intermediate stages can occur. In most studies spring transitions are classified either by their timing of occurrence or by their vertical structure. However, all these studies focus exclusively on the stratosphere and can give only tendencies under which pre-winter conditions an early or late spring transition takes place and how it takes place. Here we classify the spring transitions regarding their vertical-temporal development beginning in January and spanning the whole middle atmosphere in the core region of the polar vortex. This leads to five classes where the timing of the SSW in the preceding winter and a downward propagating Northern Annular Mode (NAM) plays a crucial role. The results show distinctive differences between the five classes in the months before the spring transition especially in the mesosphere allowing a certain prediction for some of the five spring transition classes which would not be possible considering the stratosphere only.</p>

Tropospheric response to stratospheric momentum torques

<p>An idealised model is used to examine the tropospheric response to stratospheric momentum torques with an emphasis on the response to high-latitude sudden stratospheric warmings (SSWs). Previous related studies have generally imposed such torques in models that lack a key element of realism; for instance, models that do not have a realistic stratosphere, models without stationary planetary waves (i.e., without topography), and models that do not have a troposphere and so precludes any investigation of a downward impact. The idealised moist model of an atmosphere (MiMA) used here overcomes these three shortcomings and is hence well-suited to study the downward impact of extreme events in the stratosphere in a more realistic setup. In particular, we impose transient zonally-symmetric momentum forcing to various latitude bands in the stratosphere, spun-off from a free-running control run (CTRL). In addition to varying the latitudinal location of the forcing, we vary the depth, duration and magnitude to examine the sensitivity of the tropospheric response. Preliminary results show that in contrast to thermally-forced SSWs for which the initial 'Eliassen adjustment' (i.e., the meridional circulation response during the forcing period) is opposite to that found during free-running SSWs, the momentum-forced events here, produce a meridional circulation that mimics that found in the free-running events. This meridional circulation immediately transfers the imposed momentum forcing to the surface, projecting onto the tropospheric Northern Annular Mode (NAM) and initiating a synoptic-wave feedback, a process that takes much longer to develop in the thermally-forced SSWs. Hence, a sudden and strong enough wave forcing (approximated here by an imposed momentum torque) can induce a meridional circulation that penetrates deep into the troposphere and immediately initiate a tropospheric NAM response. The applicability of these experiments to the real atmosphere will be discussed via comparing the evolution of the forced events to free-running SSWs identified in CTRL.</p>

Tropospheric eddy feedback to different stratospheric conditions in idealised baroclinic life cycles

A pronounced signature of stratosphere–troposphere coupling is a robust negative anomaly in the surface northern annular mode (NAM) following sudden stratospheric warming (SSW) events, consistent with an equatorward shift in the tropospheric jet. It has previously been pointed out that tropospheric synoptic-scale eddy feedbacks, mainly induced by anomalies in the lowermost extratropical stratosphere, play an important role in creating this surface NAM signal. Here, we use the basic set-up of idealised baroclinic life cycles to investigate the influence of stratospheric conditions on the behaviour of tropospheric synoptic-scale eddies. Particular attention is given to the enhancement of the tropospheric eddy response by surface friction and the sensitivity to wind anomalies in the lower stratosphere. We find systems that include a tropospheric jet only (modelling post-SSW conditions) to be characterised by an equatorward shift in the tropospheric jet in the final state of the life cycle, relative to systems that include a representation of the polar vortex (mimicking more undisturbed stratospheric wintertime conditions), consistent with the observed NAM response after SSWs. The corresponding relative surface NAM signal is increased if the system includes surface friction, presumably due to a direct coupling of the eddy field at tropopause level to the surface winds. We further show that the jet shift signal observed in our experiments is mainly caused by changes in the zonal wind structure of the lowermost stratosphere, while changes in the wind structure of the middle and upper stratosphere have almost no influence.

Variability of multi-site decadal running means arising from year-to-year fluctuations

Decadal mean variables are frequently used to characterise decadal climate variabilities. Decadal means are often calculated using yearly data which can represent variability at time scales from annual to centennial. Residuals from interannual fluctuations may contribute to the variability in decadal time series. Such variability is more difficult to be predicted at the long range. Removing it from the decadal variability means that the remaining variability is more likely to arise from slowly varying multi-decadal or longer time scale external forcing and internal climate dynamics which are more likely to be predicted.Here, a new approach is proposed to understand the uncertainty, potential predictability and drivers of decadal mean variables. The covariance matrix of multivariate decadal running means is decomposed into unpredictable fast decadal variability and the potentially predictable slow decadal variability. EOF analysis is then applied to the decomposed matrices to find the dominant modes which may be related to the drivers of the two types of variabilities in the multivariate decadal means.The methodology has been applied to 140 year datasets of North Pacific sea surface temperature and the Northern Hemisphere 1000hPa geopotential height. For sea surface temperature, the Pacific Decadal Oscillation is the major driver of the fast decadal variability, while the radiative forcing and the Atlantic Multi-decadal Oscillation are major drivers of the slow decadal variability. For the 1000hPa geopotential height, fast decadal variability is associated with the Northern Annular Mode, the East Atlantic Mode and the Pacific Decadal Oscillation. Slow decadal variability is associated with the Northern Annular Mode and the Atlantic Multi-decadal Oscillation.

Tropospheric eddy feedback to different stratospheric conditions in idealised baroclinic life cycles

A pronounced signature of stratosphere-troposphere coupling is a robust negative anomaly in the surface northern annular mode (NAM) following major sudden stratospheric warming (SSW) events, consistent with an equatorward shift of the tropospheric jet. It has previously been pointed out that tropospheric eddy feedbacks, mainly induced by anomalies in the lowermost extratropical stratosphere, play an important role in creating this surface NAM-signal. We use the basic setup of idealised baroclinic life cycles to investigate the influence of stratospheric conditions on the behaviour of tropospheric synoptic-scale eddies. Particular focus is hereby given on the enhancement of the tropospheric eddy response by surface friction, as well as the sensitivity to wind anomalies in the lower stratosphere. We find systems that include a tropospheric jet only (modelling post-SSW conditions) to be characterised by an equatorward shift of the tropospheric jet in the final state of the life cycle, relative to systems that include a representation of the polar vortex (mimicking more undisturbed winter-time conditions), consistent with the observed NAM-response after SSWs. The corresponding surface NAM-signal is increased if the system includes surface friction, presumably associated with a direct coupling of the eddy field at tropopause level to the surface winds. We further show that the jet shift signal observed in our experiments is mainly caused by changes in the zonal wind structure of the lowermost stratosphere, while changes in the wind structure of the middle and upper stratosphere have almost no influence.

The Corresponding Tropospheric Environments during Downward-Extending and Nondownward-Extending Events of Stratospheric Northern Annular Mode Anomalies

Using the NCEP–NCAR reanalysis dataset, this study classifies stratospheric northern annular mode (NAM) anomalies during the negative or positive phase into two categories—anomalies extending into the troposphere [trop event (TE); referred to as negative or positive TEs] and those not extending into the troposphere [nontrop event (NTE); referred to as negative or positive NTEs], and the corresponding tropospheric environments during the TEs and NTEs are identified. Compared with that for the negative NTEs, the upward wave fluxes entering the stratosphere are stronger and more persistent during the negative TEs. Furthermore, the stronger and more persistent upward wave fluxes during the negative TEs are due to more favorable conditions for upward wave propagation, which is manifested by fewer occurrences of negative refractive index squared in the mid- to high-latitude troposphere and stronger wave intensity in the mid- to high-latitude troposphere. However, the tropospheric wave intensity plays a more important role than the tropospheric conditions of planetary wave propagation in modulating the upward wave fluxes into the stratosphere. Stronger and more persistent upward wave fluxes in the negative TEs, particularly wave-1 fluxes, are closely related to the negative geopotential height anomalies over the North Pacific and positive geopotential height anomalies over the Euro-Atlantic sectors. These negative (positive) geopotential height anomalies over the North Pacific (Euro-Atlantic) are related to the positive (negative) diabatic heating anomalies and the decreased (increased) blocking activities in the mid- to high latitudes. The subtropical diabatic heating could also impact the strength of the mid- to high-latitude geopotential height anomalies through modulating horizontal wave fluxes. For positive NAM events, the results are roughly similar to those for negative NAM events, but with opposite signal.

Recent Breakdown of the Seasonal Linkage between the Winter North Atlantic Oscillation/Northern Annular Mode and Summer Northern Annular Mode

The summer northern annular mode (NAM) and the winter North Atlantic Oscillation (NAO)/winter NAM have a positive correlation from the mid-1960s to the 1980s. Namely, when the winter NAO/NAM is in a positive phase, the following summer NAM tended to be in a positive phase. During the period from the mid-1960s to the 1980s, the NAO/NAM signals extended to the stratosphere in winter. Also, the lower-tropospheric warm anomaly over northern Eurasia in winter associated with the positive phase of NAO/NAM continued into spring. In summer, the annular anomalies in the temperature and 500-hPa height fields appeared, and the high-latitude westerly wind was enhanced following the winter positive NAO/NAM. However, after circa 1990, the seasonal linkage was broken (i.e., the winter-to-summer correlation became insignificant). The stratospheric signal in the winter NAO/NAM became weak and summer signals associated with the winter NAO/NAM almost disappeared. Seasonal evolutions of atmospheric circulation and sea surface temperature (SST) anomalies associated with the winter NAO are examined for an early good-linkage period and a recent poor-linkage period. We discuss the possible causes of the linkage breakdown such as stratospheric ozone, North Atlantic SST, and Atlantic multidecadal oscillation, besides chaotic internal variability in the climate system. Simulations with the Community Earth System Model suggest that the ocean and/or sea ice with interseasonal memories possibly cause the linkage, besides large internal variability through which the linkage can take place by chance.

The Downward Influence of Sudden Stratospheric Warmings: Association with Tropospheric Precursors

Tropospheric features preceding sudden stratospheric warming events (SSWs) are identified using a large compendium of events obtained from a chemistry–climate model. In agreement with recent observational studies, it is found that approximately one-third of SSWs are preceded by extreme episodes of wave activity in the lower troposphere. The relationship becomes stronger in the lower stratosphere, where ~60% of SSWs are preceded by extreme wave activity at 100 hPa. Additional analysis characterizes events that do or do not appear to subsequently impact the troposphere, referred to as downward and non-downward propagating SSWs, respectively. On average, tropospheric wave activity is larger preceding downward-propagating SSWs compared to non-downward propagating events, and associated in particular with a doubly strengthened Siberian high. Of the SSWs that were preceded by extreme lower-tropospheric wave activity, ~2/3 propagated down to the troposphere, and hence the presence of extreme lower-tropospheric wave activity can only be used probabilistically to predict a slight increase or decrease at the onset, of the likelihood of tropospheric impacts to follow. However, a large number of downward and non-downward propagating SSWs must be considered (>35), before the difference becomes statistically significant. The precursors are also robust upon comparison with composites consisting of randomly selected tropospheric northern annular mode (NAM) events. The downward influence and precursors to split and displacement events are also examined. It is found that anomalous upward wave-1 fluxes precede both cases. Splits exhibit a near instantaneous, barotropic response in the stratosphere and troposphere, while displacements have a stronger long-term influence.