Northern Annular Mode Variability Research Articles

Contrasting stratospheric–tropospheric multi-fractal behaviors in NAM variability

As a harbinger of anomalous weather regimes in the troposphere, the Northern Annular Mode (NAM) propagates from the stratosphere to the troposphere. This fact makes understanding and predicting NAM variability of great importance. In this study, the multi-fractal behaviors of NAM variability are investigated using extended self-similarity based, multi-fractal detrended fluctuation analysis (ESS-MF-DFA) and the NAM indices from 1000 to 10 hPa. The results show that there are contrasting multi-fractal behaviors between the stratosphere and the troposphere that have a transition band near 200 hPa. The stratospheric NAM variability is more complicated and has multiple multi-fractal regimes over different scales with marked contrasting warm–cold season features. To understand these contrasting stratospheric–tropospheric multi-fractal behaviors, three surrogate methods are adopted to show how temporal ordinal patterns over an annual scale contribute to these behaviors, whereas the distribution of NAM variability only plays a minor role. Further studies show that contrasting warm–cold variability may lead to these contrasting behaviors. Among them, warm–cold seasonal variations, power spectral density (PSD), and autocorrelation provide a similar conclusion. Results indicate that although predictions of the NAM index over the stratosphere are required and necessary, the complicated multi-fractal behaviors make linear prediction strategy difficult to obtain high realizable predictability of NAM variations over the stratosphere.

Key Role of the Ocean Western Boundary currents in shaping the Northern Hemisphere climate

The individual impact of North Atlantic and Pacific Ocean Western Boundary Currents (OWBCs) on the tropospheric circulation has recently been studied in depth. However, their simultaneous role in shaping the hemisphere-scale wintertime troposphere/stratosphere-coupled circulation and its variability have not been considered. Through semi-idealized Atmospheric General-Circulation-Model experiments, we show that the North Atlantic and Pacific OWBCs jointly maintain and shape the wintertime hemispheric circulation and its leading mode of variability Northern Annular Mode (NAM). The OWBCs energize baroclinic waves that reinforce quasi-annular hemispheric structure in the tropospheric eddy-driven jetstreams and NAM variability. Without the OWBCs, the wintertime NAM variability is much weaker and its impact on the continental and maritime surface climate is largely insignificant. Atmospheric energy redistribution caused by the OWBCs acts to damp the near-surface atmospheric baroclinicity and compensates the associated oceanic meridional energy transport. Furthermore, the OWBCs substantially weaken the wintertime stratospheric polar vortex by enhancing the upward planetary wave propagation, and thereby affecting both stratospheric and tropospheric NAM-annularity. Whereas the overall impact of the extra-tropical OWBCs on the stratosphere results mainly from the Pacific, the impact on the troposphere results from both the Pacific and Atlantic OWBCs.

Nonlinear features of Northern Annular Mode variability

Nonlinear features of daily Northern Annular Mode (NAM) variability at 17 pressure levels are quantified by two different measures. One is nonlinear correlation, and the other is time-irreversible symmetry. Both measures show that there are no significant nonlinear features in NAM variability at the higher pressure levels, however as the pressure level decreases, the strength of nonlinear features in NAM variability becomes predominant. This indicates that in order to reach better prediction of NAM variability in the lower pressure levels, nonlinear features must be taken into consideration to build suitable models.

Improvements in statistical forecasts of monthly and two‐monthly surface air temperatures using a stratospheric predictor

Anomalies in the extratropical stratospheric circulation are known to affect climate at the surface for up to 2 months during winter. Previously, several studies have proposed simple statistical models based on a stratospheric predictor to forecast surface climate; however, none of these studies considered a possibility, nor evaluated the skill, of categorical weather forecasts at individual sites. Here we evaluate the skill of the three‐category (below‐normal, near‐normal and above‐normal) monthly and two‐monthly mean surface air temperature statistical forecasts, both deterministic and probabilistic, which use, as predictors, normalized mean polar cap geopotential height at 150 hPa and mean surface air temperature over the preceding 30‐day period (persistence). We show that, during the period 1957–2013, at several sites influenced by the Northern Annular Mode variability, the monthly and two‐monthly forecasts based on the stratospheric predictor possess a moderate skill in December–March at lead times of several days, but in many cases they only marginally improve forecasts based on persistence only. Nevertheless at some sites in northeastern Europe and northwestern Russia the improvements indicate that statistical climate forecasting at these sites can benefit from addition of stratospheric information. The results of this study further demonstrate usefulness of simple statistical models based on stratospheric predictors, both as a complementary source of forecasts, and as providing benchmarks for evaluating the skill of more complex dynamical systems.

Signatures of naturally induced variability in the atmosphere using multiple reanalysis datasets

A multiple linear regression analysis of nine different reanalysis datasets has been performed to test the robustness of variability associated with volcanic eruptions, the El Niño Southern Oscillation, the Quasi‐Biennial Oscillation and with a specific focus on the 11‐year solar cycle. The analysis covers both the stratosphere and troposphere and extends over the period 1979–2009. The characteristic signals of all four sources of variability are remarkably consistent between the datasets and confirm the responses seen in previous analyses. In general, the solar signatures reported are primarily due to the assimilation of observations, rather than the underlying forecast model used in the reanalysis system. Analysis of the 11‐year solar response in the lower stratosphere confirms the existence of the equatorial temperature maximum, although there is less consistency in the upper stratosphere, probably reflecting the reduced level of assimilated data there. The solar modulation of the polar jet oscillation is also evident, but only significant during February. In the troposphere, vertically banded anomalies in zonal mean zonal winds are seen in all the reanalyses, with easterly anomalies at 30°N and 30°S suggesting a weaker and possibly broader Hadley circulation under solar maximum conditions. This structure is present in the annual signal and is particularly evident in NH wintertime. As well as the ‘top‐down’ solar contribution to Northern Annular Mode variability, we show the potential contribution from the surface conditions allowing for a ‘bottom‐up’ pathway. Finally, the reanalyses are compared with both observed global‐mean temperatures from the Stratospheric Sounding Unit (SSU) and from the latest general circulation models from CMIP‐5. The SSU samples the stratosphere over three different altitudes, and the 11‐year solar cycle fingerprint is identified in these observations using detection and attribution techniques.

Linear interference and the initiation of extratropical stratosphere‐troposphere interactions

Vertical fluxes of wave activity from the troposphere to the stratosphere correlate negatively with the Northern Annular Mode (NAM) index in the stratosphere and subsequently in the troposphere. Recent studies have shown that stratospheric NAM variability is also negatively correlated with the amplitude of the wave pattern coherent with the large‐scale climatological stationary wavefield; when the climatological stationary wavefield is amplified or attenuated, the stratospheric jet correspondingly weakens or strengthens. Here we quantify the importance of this linear interference effect in initiating stratosphere‐troposphere interactions by performing a decomposition of the vertical wave activity flux using reanalysis data. The interannual variability in vertical wave activity flux in both the Northern and Southern Hemisphere extratropics is dominated by linear interference of quasi‐stationary waves during the season of strongest stratosphere‐troposphere coupling. Composite analysis of anomalous vertical wave activity flux events reveals the significant role of linear interference and shows that “linear” and “nonlinear” events are essentially independent. Linear interference is the dominant contribution to the vertical wave activity flux anomalies preceding displacement stratospheric sudden warmings (SSWs) while split SSWs are preceded by nonlinear wave activity flux anomalies. Wave activity variability controls the timing of stratospheric final warmings, and this variability is shown to be dominated by linear interference, particularly in the Southern Hemisphere. The persistence of the linear interference component of the vertical wave activity flux, corresponding to persistent constructive or destructive interference between the wave‐1 component of climatological stationary wave and the wave anomaly, may help improve wintertime extratropical predictability.

The Role of Linear Interference in Northern Annular Mode Variability Associated with Eurasian Snow Cover Extent

AbstractOne of the outstanding questions regarding the observed relationship between October Eurasian snow cover anomalies and the boreal winter northern annular mode (NAM) is what causes the multiple-week lag between positive Eurasian snow cover anomalies in October and the associated peak in Rossby wave activity flux from the troposphere to the stratosphere in December. This study explores the following hypothesis about this lag: in order to achieve amplification of the wave activity, the vertically propagating Rossby wave train associated with the snow cover anomaly must reinforce the climatological stationary wave, which corresponds to constructive linear interference between the anomalous wave and the climatological wave. It is shown that the lag in peak wave activity flux arises because the Rossby wave train associated with the snow cover is in quadrature or out of phase with the climatological stationary wave from October to mid-November. Beginning in mid-November the associated wave anomaly migrates into a position that is in phase with the climatological wave, leading to constructive interference and anomalously positive upward wave activity fluxes until mid-January. Climate models from the Coupled Model Intercomparison Project 3 (CMIP3) do not capture this behavior. This linear interference effect is not only associated with stratospheric variability related to Eurasian snow cover anomalies but is a general feature of Northern Hemisphere troposphere–stratosphere interactions and, in particular, dominated the negative NAM events of the fall–winter of 2009/10.

Evidence for the chaotic origin of Northern Annular Mode variability

Exponential spectra are found to characterize variability of the Northern Annular Mode (NAM) for periods less than 36 days. This corresponds to the observed rounding of the autocorrelation function at lags of a few days. The characteristic persistence timescales during winter and summer is found to be ∼5 days for these high frequencies. Beyond periods of 36 days the characteristic decorrelation timescale is ∼20 days during winter and ∼6 days in summer. We conclude that the NAM cannot be described by autoregressive models for high frequencies; the spectra are more consistent with low‐order chaos. We also propose that the NAM exhibits regime behaviour, however the nature of this has yet to be identified.

Geomagnetic activity related NO<sub>x</sub> enhancements and polar surface air temperature variability in a chemistry climate model: modulation of the NAM index

Abstract. The atmospheric chemistry general circulation model ECHAM5/MESSy is used to simulate polar surface air temperature effects of geomagnetic activity variations. A transient model simulation was performed for the years 1960–2004 and is shown to develop polar surface air temperature patterns that depend on geomagnetic activity strength, similar to previous studies. In order to eliminate influencing factors such as sea surface temperatures (SST) or UV variations, two nine-year long simulations were carried out, with strong and weak geomagnetic activity, respectively, while all other boundary conditions were held to year 2000 levels. Statistically significant temperature effects that were observed in previous reanalysis and model results are also obtained from this set of simulations, suggesting that such patterns are indeed related to geomagnetic activity. In the model, strong geomagnetic activity and the associated NOx (= NO + NO2) enhancements lead to polar stratospheric ozone loss. Compared with the simulation with weak geomagnetic activity, the ozone loss causes a decrease in ozone radiative cooling and thus a temperature increase in the polar winter mesosphere. Similar to previous studies, a cooling is found below the stratopause, which other authors have attributed to a decrease in the mean meridional circulation. In the polar stratosphere this leads to a more stable vortex. A strong (weak) Northern Hemisphere vortex is known to be associated with a positive (negative) Northern Annular Mode (NAM) index; our simulations exhibit a positive NAM index for strong geomagnetic activity, and a negative NAM for weak geomagnetic activity. Such NAM anomalies have been shown to propagate to the surface, and this is also seen in the model simulations. NAM anomalies are known to lead to specific surface temperature anomalies: a positive NAM is associated with warmer than average northern Eurasia and colder than average eastern North Atlantic. This is also the case in our simulation. Our simulations suggest a link between geomagnetic activity, ozone loss, stratospheric cooling, the NAM, and surface temperature variability. Further work is required to identify the precise cause and effect of the coupling between these regions.

Meridional and vertical out‐of‐phase relationships of temperature anomalies associated with the Northern Annular Mode variability

Using the NCAR/NCEP reanalysis, we here present evidence suggesting that the out‐of‐phase relationships of temperature anomalies both between the low and high latitudes and between the stratosphere and troposphere are intimately related to the meridional and downward propagation of anomalies of both signs. The temperature anomalies propagate poleward and downward above the tropopause and propagate equatorward below the tropopause. The characteristic time scale for anomalies of one polarity to propagate from the equator to the pole (or the half period of the complete cycle) is about 55 days. The relatively slow meridional propagation helps to explain the well‐known seesaw oscillatory pattern between low and high latitudes found in monthly data. The equatorward propagation in the troposphere is synchronized with the poleward propagation of the stratospheric temperature anomalies of the opposite sign in both low and high latitudes, responsible for the out‐of‐phase relation between the stratospheric and tropospheric temperature anomalies in the polar region. Since it takes about 55 days for anomalies of one polarity to propagate from the tropics to the pole, such an intimate linkage between the anomalies in the deep tropics and high latitudes would imply a longer lead time for intra‐seasonal climate prediction in the extratropics.

40–70 day meridional propagation of global circulation anomalies

This paper reports a diagnostic study of circulation anomalies in a semi‐Lagrangian θ‐PVLAT coordinate by following contours of the daily potential vorticity (PV) field on isentropic (θ) surfaces using the NCEP/NCAR reanalysis II data set from 1979 to 2003. The leading EOF mode, which explains about 69% of the total variance of daily Northern Hemisphere PV anomalies in the θ‐PVLAT coordinate and is highly correlated with the stratosphere Northern Annular Mode (NAM) index, is used to construct a composite life cycle of the NAM variability. Composite circulation anomalies of both signs propagate poleward in the stratosphere and equatorward in the troposphere with an average time of about 44 days for warm anomalies and 72 days for cold anomalies to travel from the equator to the pole. Accompanying the meridional propagation, there exist a simultaneous downward propagation of stratospheric circulation anomalies in both the tropics and extratropics. A global mass circulation paradigm is proposed to explain the simultaneous meridional and vertical propagation of global circulation anomalies that appears responsible for the annular mode variability.