Weak Stratospheric Polar Vortex Research Articles

Classification of stratospheric extreme events according to their downward propagation to the troposphere

AbstractThis study presents a classification of stratospheric extreme events during northern winter into events with or without a consistent downward propagation of anomalies to the troposphere. Anomalous strong and weak stratospheric polar vortex events are detected from daily time series of the polar cap averaged (60°–90°N) geopotential height anomaly. The method is applied to chemistry‐climate model data (E39CA and WACCM3.5) and reanalyses data (ERA40). The analyses show that in about 80% of all events no significant tropospheric response can be detected. The stratospheric perturbation of both weak and strong events with a significant tropospheric response persists significantly longer throughout the stratosphere compared to the events without a tropospheric response. The strength of the stratospheric perturbation determines the strength of the tropospheric response only to a small degree. Results are consistent across all three data sets.

Temperature Variation over East Asia during the Lifecycle of Weak Stratospheric Polar Vortex

Abstract The authors investigate the circulation change during the life cycle of a weak stratospheric polar vortex (WSV) event and its impact on temperature variation over East Asia. The lower-tropospheric temperature over East Asia strongly fluctuates despite the slow decay of stratospheric circulation and the continuously negative Arctic Oscillation (AO) pattern during the WSV event. The temperature fluctuation is critically influenced by the variation of the East Asian upper-level coastal trough (EAT), which may be coupled to the stratospheric circulation during the WSV events. The EAT is deepened anomalously during the Peak phase (from lag −5 to lag 5 day) of the WSV, and East Asian temperature is lowest during this phase. During the next period (Decay-1 phase: from lag 6 to lag 16 day), in spite of the slowly decaying WSV condition, the cold temperature anomaly over East Asia is suddenly weakened; this change is caused by a westward-propagating signal of an anticyclonic anomaly from the North Pacific to East Asia. After about two weeks (Decay-2 phase: from lag 17 to lag 27 day), the cold conditions over East Asia are restrengthened by an intensification of EAT, which is related to the eastward propagation of a large-scale wave packet originating from a negative North Atlantic Oscillation (NAO)-type structure in the Decay-1 phase and its delayed influence on the East Asia region.

Connection between weak stratospheric vortex events and the Pacific Decadal Oscillation

We investigate the possible impacts of the Pacific Decadal Oscillation (PDO) on the occurrence of weak stratospheric polar vortex (WSV) events in the Northern Hemisphere winter. WSV events, which are defined when polar-cap geopotential height anomalies at 50 hPa fall below the 10th percentile in winter, are observed more frequently during positive PDO phases than during negative PDO phases. Additionally, tropospheric wave forcings that drive WSV events are remarkably different between the two phases of the PDO. During the positive PDO phase, the vertical propagation of wavenumber-one waves plays a predominant role with a rather minor contribution of wavenumber-two waves. This contrasts with the negative PDO phase when the WSV events are primarily caused by wavenumber-two waves. This difference is partly related to the PDO-induced tropospheric circulation anomalies over the North Pacific whose zonal wavenumber-one component constructively (destructively) interferes with climatological planetary-scale waves during positive (negative) PDO winters. The predominant wavenumber-two wave forcings during the negative PDO phase are likely related to the enhanced tropospheric eddy activity over Alaska that results from the poleward shift of the Pacific jet in response to the negative PDO.

Observational constraints on the tropospheric and near-surface winter signature of the Northern Hemisphere stratospheric polar vortex

A composite analysis of Northern Hemisphere’s mid-winter tropospheric anomalies under the conditions of strong and weak stratospheric polar vortex was performed on NCEP/NCAR reanalysis data from 1948 to 2013 considering, as additional grouping criteria, the coincidental states of major seasonally relevant climate phenomena, such as El Nino-Southern Oscillation (ENSO), Quasi Biennial Oscillation and strong volcanic eruptions. The analysis reveals that samples of strong polar vortex nearly exclusively occur during cold ENSO states, while a weak polar vortex is observed for both cold and warm ENSO. The strongest tropospheric and near-surface anomalies are found for warm ENSO and weak polar vortex conditions, suggesting that internal tropospheric circulation anomalies related to warm ENSO constructively superpose on dynamical effects from the stratosphere. Additionally, substantial differences are found between the continental winter warming patterns under strong polar vortex conditions in volcanically-disturbed and volcanically-undisturbed winters. However, the small-size samples obtained from the multi-compositing prevent conclusive statements about typical patterns, dominating effects and mechanisms of stratosphere-troposphere interaction on the seasonal time scale based on observational/reanalysis data alone. Hence, our analysis demonstrates that patterns derived from observational/reanalysis time series need to be taken with caution as they not always provide sufficiently robust constraints to the inferred mechanisms implicated with stratospheric polar vortex variability and its tropospheric and near-surface signature. Notwithstanding this argument, we propose a limited set of mechanisms that together may explain a relevant part of observed climate variability. These may serve to define future numerical model experiments minimizing the sample biases and, thus, improving process understanding.

Arctic Oscillation anomaly in winter 2009/2010 and its impacts on weather and climate

Using the Arctic Oscillation (AO) index, the exceptional winter (DJF) of 2009 has been analyzed. The middle-to-high latitudes of the Northern Hemisphere suffered from a nearly zonally symmetric anomaly of temperature and pressure. This situation revealed that two negative AO events occurred in the winter of 2009/2010, with unprecedented low values in January 2009 and February 2010. The negative AO event in January 2009 can be further divided into two stages: the first stage was mainly driven by enhanced upward-propagating planetary waves, which led to a weak stratospheric polar vortex associated with a downward-propagating negative AO signal; the second stage was caused by a lower tropospheric positive temperature anomaly in the high latitudes, which maintained the positive geopotential height anomaly of the first stage. The two successively occurring stages interacted and caused the lower troposphere to experience a strong and lengthy persistence of the negative AO event. We consider that the second event of negative AO in February 2010 is related to the downward-propagating negative AO after sudden stratospheric warming. Eleven long-persistence negative AO events were analyzed using reanalysis data. The results suggest that the negative AO in the troposphere might have been caused by stratospheric sudden warming, a downward-propagating weak stratospheric circulation anomaly or dynamic processes in the troposphere. Further study shows that the negative phase of the AO in the winter of 2009/2010 corresponded to a wide range of temperature and precipitation anomalies in the Northern Hemisphere. Therefore, to improve the accuracy of weather forecasting and climate prediction, more attention should be paid to the AO anomaly and its impact.

Characteristics of Baroclinic Wave Packets during Strong and Weak Stratospheric Polar Vortex Events

Abstract This study tested a numerical and theoretical prediction that the stratosphere and troposphere are coupled through the effect of stratospheric vertical wind shear on baroclinic waves. Wavelengths, phase speeds, and background quasigeostrophic potential vorticity gradients were analyzed over the Pacific and Atlantic during strong and weak stratospheric polar vortex events and interpreted in terms of the counterpropagating Rossby wave perspective on baroclinic instability. Effects of zonal variations in the background flow were included in the analysis of phase speeds. Observed changes in wave packet average wavelength and phase speed support the vertical shear hypothesis for stratosphere–troposphere coupling; however, changes in the intrinsic phase speed contradict the hypothesis. This inconsistency was resolved by considering the change in zonal wind speed in the lower stratosphere, which accounts for most of the change in phase speeds during strong and weak vortex events. Changes in the average wavelengths and meridional wave activity flux are also consistent with this modified hypothesis involving the stratospheric zonal wind. The results demonstrate that a simple mechanism for stratosphere–troposphere coupling can be found in the observational record.

Understanding the Anomalously Cold European Winter of 2005/06 Using Relaxation Experiments

Abstract Experiments with the atmospheric component of the ECMWF Integrated Forecasting System (IFS) have been carried out to study the origin of the atmospheric circulation anomalies that led to the unusually cold European winter of 2005/06. Experiments with prescribed sea surface temperature (SST) and sea ice fields fail to reproduce the observed atmospheric circulation anomalies suggesting that the role of SST and sea ice was either not very important or the atmospheric response to SST and sea ice was not very well captured by the ECMWF model. Additional experiments are carried out in which certain regions of the atmosphere are relaxed toward analysis data thereby artificially suppressing the development of forecast error. The relaxation experiments suggest that both tropospheric circulation anomalies in the Euro–Atlantic region and the anomalously weak stratospheric polar vortex can be explained by tropical circulation anomalies. Separate relaxation experiments for the tropical stratosphere and tropical troposphere highlight the role of the easterly phase of quasi-biennial oscillation (QBO) and, most importantly, tropospheric circulation anomalies, especially over South America and the tropical Atlantic. From the results presented in this study, it is argued that the relaxation technique is a powerful diagnostic tool to understand possible remote origins of seasonal-mean anomalies.

Downward Arctic Oscillation signal associated with moderate weak stratospheric polar vortex and the cold December 2009

The climatic anomalies and related mechanism for the cold December 2009 in Northern Hemisphere are analyzed preliminarily. It is revealed that both anomalously low monthly mean temperature and more extreme cold days occurred over many areas of mid‐latitude Northern Hemisphere in this December. This cold condition can be accounted for by the extreme negative Arctic Oscillation (AO) at surface level, which is the lowest one for the past 31 years. It is shown that the negative surface AO in December is closely associated with the downward propagating anomalies from the stratosphere, implying the involvement of stratospheric processes in this cold month. Since the preceding stratospheric signal is not as strong as those reported in previous studies, it suggests that some relatively weaker stratospheric anomalies can also propagate downward and influence the troposphere.

Stratospheric control of the extratropical circulation response to surface forcing

The transient atmospheric circulation response to fall season Siberia region snow forcing in an atmospheric general circulation model is investigated. The forcing generates a stratosphere‐troposphere interaction response whose coupling to the surface on multiple week timescales depends on the state of the stratosphere prior to the initiation of the forcing. An initially weak stratospheric polar vortex increases the likelihood of a negative Northern Annular Mode response at the surface. These results may provide useful insight for seasonal prediction of circulation anomalies arising from surface forcing in the extratropics.

Zonal wave numbers 1-5 in planetary waves from the TOMS total ozone at 65° S

Abstract. Planetary waves in the total ozone at the southern latitude of 65° S are studied to obtain the main characteristics of the zonal wave numbers 1–5. The TOMS total ozone data were used to analyze the amplitude and periodicity variations of the five spectral components during August-December of 1979–2003. A presence of the shorter period of waves 1–3 in 1996 (7 days) in comparison with 2002 (8–12 days) is revealed which can be attributed to the distinction in conditions of typical and anomalously weak stratospheric polar vortex, probably, a strong and weak mean zonal wind. The interannual variations of the monthly and 5-month mean amplitudes of the zonal wave numbers 1–5 are described. Wave 1 has the largest amplitude in October (up to 139 DU in 2000) and increasing amplitude trend (15 DU/decade for October 1979–2003). The 5-month mean amplitudes averaged over 1979–2003 are 53.6, 29.9, 15.5, 10.5, and 7.8 DU for the wave number sequence 1, 2, 3, 4 and 5, respectively. For the stationary components the amplitudes are 38.3, 4.8, 1.8, 1.2, 0.7 DU, respectively. Thus, the stationary component of wave 1 and the traveling one of waves 2–5 are predominant. The tendencies in a long-term change in the wave number amplitude can be explained by taking into account the degree of wave deformation of the stratospheric polar vortex edge, net meridional displacements of the lower stratosphere air, and the difference between the total ozone loss and negative trends in the polar and mid-latitude regions. Keywords. General circulation – Middle atmosphere dynamics – Waves and tides